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MG MGB Technical - What causes a miss - update

For those that have not followed our on-line diagnostic process for a Triumph Spitfire I am refreshing. Here is the link to the original long thread

I replaced the HIF 4 original AAU needle with an AAA needle as recommended by Joe Curto (SU carb. specialist).

I also rechecked the float and cleaned the carb. Readjusted the carb to .065 below the bridge and started the car. No miss/pop in the exhaust.

Took the car today to a shop with a gas analyser/smog machine. The car ran great driving to the shop and came up really good on the gas analyser. We decided to go ahead and do a pre-smog check. Car passed. A few of the reading were close, but ok.

We then moved on to doing the actual smog test. Halfway through the 15 mile per hour test, the car started missing/popping.

Car failed the real smog test with a Carbon Monoxide reading of 1.54 (Max is 1.46. Pretest was 1.38)

So now I am back to square one. What would cause the car, that was running great to suddenly start missing/popping? Since the only thing that changed to fix it was a new needle and a carb cleaning, I am thinking it has to be a carb related issue.

I took the needle out, cleaned the jet with carb cleaner and put it back together. No change. Intermittent pop is still there.

I took the carb off, cleaned it, reset the float and reset the jet to .065 below the bridge, and reinstalled the carb. Now the car runs like crap.

One item to note. When I pulled the fuel line off to take the carb off, there was a lot of pressure in the hose. I have checked the fuel pump and it is putting out 1.2 pounds so that seems to be about right.

The fuel line pressure leads me to think the fuel needle is sticking.

I am going to read up on the HIF 4 and also order a new needle.

One question I have is how tight do you put the screw with the spring that holds the jet raising/lowering system on?

The car has a new fuel filter, but I am going to check this again. I did not find any dirt/material in the carb when I took it apart. Are there any passages in the HIF4 that I am missing to clean?
Bruce Cunha

When this happened to me it was intake manifold vacuum

Did they test hydrocarbons?

If that goes high it indicates unburnt fuel. If that's 'normal' it indicates combustion, so with CO high it sounds like over-rich.

With a working pump having fully charged the carbs and lines then there will always be a spurt of pressure if you disconnect a fuel pipe just after the ignition has been switched off. But 1.2psi is normal, and not 'a lot of pressure' as you seemed to think.

"the fuel needle is sticking"

Which needle do you mean? If a float valve is sticking open then the fuel level will rise in the float chamber and jet, which will upset the mixture. That should eventually result in an overflow from the carb, but it could take some time if it is only seeping. The usual check for that is if the electric fuel pump (is this electric or mechanical?) is clicking more than once every 30 secs. You can get very slight seepage, with clicks more than 30 secs apart, and after several minutes with the engine not running it will eventually overflow. But as that is less than the normal consumption at idle, and leaving the ignition with a stopped engine is not normal, it can be ignored.

Another thing you can do - with an electric pump - is disconnect it, leave it idling, and see what that does to the miss/pop before it cuts out altogether, i.e. does it get better or worse? You can also do this with a mechanical pump of course, by disconnecting the supply pipe from the pump.

If the float valve was sticking shut it would eventually empty the float chamber and stop.

If you mean the mixture needle on the piston, then you can observe that with the air cleaners removed. At idle it should be steady (although may fluctuate with each miss/pop), just off the bridge, be easy to nudge up or down and return immediately. Holding it very slightly down, or up, should change the mixture, so you could see what effect that had on the miss/pop.

"I took the carb off, cleaned it, reset the float and reset the jet to .065 below the bridge, and reinstalled the carb. Now the car runs like crap. "

You have obviously done something wrong, considering it's much worse than before, and one has to wonder if you have been doing something wrong all along, no offence intended.

Where does the .065" come from? If it's from a book then that is only a starting value, like two full turns down from the bridge, and you have to then adjust the jet for the highest idle as a coarse adjustment, then to the lifting pin as a fine adjustment.

One of the disadvantages with the HIF is that it has more passages to block, both for normal running and enrichment, and the enrichment valve can leak or stick.

There is no special tightness requirement for the jet adjuster screw.

Mark. 79 Spit does not have manifold vacuum. It runs a ported vacuum off the carb.

Thanks Paul. I never take offense to advice. This is the first time I have removed the jet, so I think I put the jet in wrong. Don't know if the bottom of the jet needs to go to the right or to the left. Will check that in the pictures on the web.

To clarify. When I took the fuel line off of the carb, it shot a good volume of fuel under pressure out. (engine off). I was thinking the fuel needle was sticking. I do note that it seems to have a very light spring in it. Have a new one ordered.

The spitfire runs a mechanical fuel pump that is putting out 1.2 pounds of pressure. I understand your point that with the car off, there could be pressure built up in the line from the pump to the carb. This just seemed to be a lot more pressure than I expected.

This is an HIF4,so any overflow is going to go into the hose going to the charcoal canister. I have not checked to see if there is fuel going out the carb. Worth checking thought.

This is the rear carb of what used to be a dual HIF 4 system. It has a solid throttle plate.

Your absolutely correct on the .065. That is the measurement I got when the carb was adjusted per the book and running at it's best. I use it as a starting point when I put the piston back in. It is pretty easy to measure the jet off the bridge.

Hydrocarbons during the real test were 22 ppm at 15 MPH and 58 ppm at 25 MPH. Max is 232/200

Hydrocarbons during the pre test (car running great) 13 and 32.

C0% was 1.54 at 15 MPH and 1.24 at 25 MPH, during the actual test and 1.38/1.23 during the pretest. Max is 1.48

%CO2 11.90 at 15 MPH and 11.89 at 25 MPH during the test and 12.62/11.89

As to the passages in the HIF. I dont see a lot. I will search the web to see if I can find a good diagram of the passages.

In regards to the screw I was referring to it is 24 and 25 in the picture below. Should the spring be pushed tight or ?

Would you think that some debris in a passage in the carb could be the issue? Really odd that when I changed the needle, I was able to get the pop/miss to go away, and it just started doing it after running fine for probably 50 minutes. Nothing touched on the car.

Bruce Cunha

Bruce, I'm not sure if you are using this website, but this is the best I have seen.

It will take you through the full dismantling, reassembly and tuning process. It's an invaluable source of information.

Andy Robinson

Bruce, As regards the jet and adjusting spring; it'e difficult to put the jet in it's "holder" incorrectly. The spring will be compressed as this is necessary to allow and hold the adjustment of the jet height. The one thing to check is that the pin end of the adjusting screw is properly located in the slot in the jet holder.
Another miss-assembly which is possible, is mixing up both the jet and the float between front and back carbs. The jet pick up must sit in that vaguely triangular section on the float chamber base.
Pressure in the line should subside as a little fuel in the float chamber evapourates and allows the valve to open.
As Paul pointed out and I remember pointing out previously, the 0.65" is an assembly measurement, not to be treated as a proper running measurement, a la fixed jet carbs, as this is adjusted to achieve the correct running mixture.
My opinion is that your problem is either electrical, head, ignition or valve related. Sticking valves are not unheard of especially if the guides are not reamed to give the correct working clearance, neither is a valve "picking up" a hardened seat when the engine is hot. A hairline crack opening up with heat and bleeding water into the combustion chamber. When the popping starts switch off and check the plugs. One obviously sooty plug would suggest incomplete combustion, an obviously clean one could indicate the presence of water.
Allan Reeling

Those figures are so close to a pass, and some of them are a pass, that really you would need to tweak the mixture weak and rich to see what the effect was.

"... I was thinking the fuel needle was sticking. I do note that it seems to have a very light spring in it. ..."

Still don't understand which which fuel needle you are referring to, or which spring. Float valves don't have a spring. Pumps will have at least one non-return valve if not two (disc valves in the SU), and they probably will have a light return spring just enough to close the valve when the pump diaphragm or piston pulls back ready for the next pulse to stop fuel flowing backwards. They will be pushed open against their spring pressure by the pump diaphragm. If they have any effect on fuel line pressure at all it would be to reduce pump pressure very slightly, but I doubt you would measure it. It is the diaphragm return spring which is pushing the fuel into the carbs, and giving the 1.2psi. Remember that 1.2 pounds per square inch in the 1/4" bore fuel line has proportionally more effect on how the fuel spurts out, like putting your thumb over the end of a hose.

The charcoal canister will eventually drop fuel onto the ground with a carb overflow, but it will take a lot longer. Disconnecting the hose from the float chamber port should not make any difference to running (or emissions for that matter) but would allow you to see any overflow relatively quickly.

As regards the screw I would expect it to bottom and tighten up before the spring is fully compressed. The spring allows the jet carrier 23 to pivot as the mixture screw 21 is screwed in and out, but stops it rattling around and varying the mixture when the engine is running.

I can't find them on that SU site but Haynes has drawings of the HIF passages and enrichment control, although my WSM doesn't. I'm attaching a copy which also includes the jet mounting arrangement. Should be good enough to read the text.

With seeming to work OK for a while after changes, then have the same problem, internal debris moving around is a possibility.



You may have already seen this John Twist YouTube on the HIF sensitivity to decaying debris from fuel lines, but if not.............


Larry C.
Lawrence Cordeiro

Andy. Thanks, That web seems to have a good description of how to take the HIF apart and clean it.

Paul. yours is also very helpful. Now I can assure I am getting all the passages clean.

Allen. Only running one HIF. Spitfire's in the US only had the single Zenith Stromberg carb (similar to the 79/80 MG. I replaced that with the single HIF as it fits perfectly. My ZS was really bad. Rebult twice (once by a British car shop) and we could not get it from running really rich.

I removed the float, fuel intake valve and the jet. Sprayed the passages I could see and put it back together. Had to set the float with the carb on its side because the fuel valve spring is pushed down by the weight of the float when the carp is upside down.

I did something wrong as it is running really bad and was not prior to taking it apart (Well at least not as bad as after I took it apart.)

In John Twists video on setting the float, there is some rebound when the float is pushed down. This is very minimal in this carb. Will see what the new valve and Jet tube do to it. Hopefully they will be here Saturday.

Paul, The needle valve for the fuel inlet that the float pushes on is the one. The valve has a spring in it that pushes the pin at the top up.

I did see the slit that the adjusting screw fits in on the jet adjuster. Think I got that correct.

Lawrence, another good lead. Thanks. Since the car was running perfectly and then reverted back to its intermittent miss , I am thinking it is a debris issue. Just have to find what it is affecting and clean it. There are rubber hoses from the fuel pump to the steel fuel line and from the steel fuel line to the carb, so particles are a potential.

Allan. We have ruled out electrical, by running a jumper from the battery directly to the coil. The car has a new coil, new wires (400 ohms in each wire), new plugs, new distributor cap, and is running a 25D distributor with all new points, condenser, and rotor from Advance Distributors. j

Compression is 132 to 136. Leakdown is nearly the same in each cylinder, and I am not seeing any coolant drop.

I pulled the plugs and all are caramel tan in color. #4 is the darkest, but not black.

Resprayed all the places where there could be a vacuum leak with starter fluid and did not get any increases in engine speed.

I did run a can of sea foam through the vacuum port on the carb just in case there is some carbon keeping a valve from seating. It did not change the pop/miss.

Only thing I do notice is that a few of the valve springs seem a bit weak. I can push the valve down a bit by pushing on these springs. Thinking that these may have been in the compressed state for the 7+ years the car has been sitting. But since the car was running very good for at least 1.5 hours running at the shop, I can't see how a weak spring would cause it to change from good running to popping/missing.

My feeling is the car needs to get a good run on the road. Unfortunately in California, I cant legally take it on the road without it being registered.

Bruce Cunha

"I can push the valve down a bit by pushing on these springs."

Crikey, either I wouldn't want you pushing me, or they are very weak. If weak, they could be sticking open slightly when hot, hence the popping in the exhaust which right from the start sounded like explosions more than misfires. You could possibly try aiding them by pulling up on the top turn of the existing spring, valve by valve, and seeing if that changes the popping.

"The needle valve for the fuel inlet that the float pushes on is the one."

Gotcha. The standard float valve does have a spring loaded pin at the opposite end to the conical valve tip, that seals to the seat to shut off the fuel supply when the float chamber is full. Don't know about Grosse 'Jets', except that they are currently held to be more of a problem than the current needle valves. Not really sure why it is needed, a solid pin would push the valve against the seat with exactly the same force as far as I can see, maybe it prevents the tip chattering against the seat and accelerating wear. No harm in fitting a new float valve assembly as it may be seeping, eventually to cause an overflow, or upset the mixture. But the change in readings from no popping to popping are so small, I can't really see it being purely a mixture problem.

Yea, at 20 bucks, replacing the valve springs is probably a somewhat easy fix.

I have read about using a soft cotton rope put in the cylinder to hold the valve up. Anyone done that? I think I do have an air valve with a spark plug adapter somewhere, so I can also just pressurize the cylinder to change the valve spring.

Got the new parts in today, so should have the Carb done by the end of the weekend.
Bruce Cunha

I'd rather do the rope trick - feed it in then wind the piston up. Better than air pressure, then knocking the valve ...

Bruce, you've gone full tilt on this project. I'm starting to think valve springs also. After all these cars are getting to 40 years old. Strange things wear out, especially when the engine has been laid up for so long. At this point, I too would go with new valve springs. It seems you've done everything else.

I've used the compressed air to hold the valves in place while replacing valve guide seals with no problem.


79 MGB
gary hansen

Hello Bruce,

Yes, the rope trick works fine for changing the valve springs. Some people use air, but I have not. A problem with air is that the pressure wants to push the piston down. If it succeeds, the piston is no longer high enough to prevent the valve from falling into the cylinder. It's not that likely to happen, but why take the chance.

C R Huff

If the rocker arm assembly is held in place by the head nuts, then I would not use compressed air to hold the valves in place. This could result in blowing the head gasket. So use the rope trick.


79 MGB
gary hansen

Frustrating day. Rebuilt the HIF4 with new fuel needle and seat. Also a new jet tube. Reset the float with 1mm clearance to a straight edge across the body, and the float just touching the valve.

Cleaned all passages with carb cleaner and air.

Car starts but only idles and that is rough. Try to accelerate it and it sputters and dies. Acts like it is not getting fuel.

Pulled the carb three times and rechecked everything and it does not change anything. Set the float with it upright and also on its side.

Took off the fuel line and put it in a container and cranked the car. Plenty of fuel pumping out

Only thing that has changed since getting it home from the smog shop has been with the carb.

I did run a can of seafoam through the carb vacuum port prior to pulling the carb off, but it was running and accelerating after that.

Checked the points, wires, rotor and cap. All are fine. Will do another compression test, but figure it has to be the carb.

Have not pulled the choke valve as it is moving and working. Is there any other reason I should pull the choke valve?

I will take some pictures of the internals of the carb tomorrow so someone can see if something does not look right.

Bruce Cunha

"Car starts but only idles and that is rough. Try to accelerate it and it sputters and dies. Acts like it is not getting fuel."

Are you using enrichment to start it? Have you tried pulling the choke control while running and seeing what difference that makes?

Hi Paul.

Actually, the car starts up pretty well. No choke. I did try applying the choke while it was running and it did increase the idle just a bit, but as soon as I lift the throttle, it sputters and dies. Even quickly pumping the accelerator does not get the motor to run faster, just dies. Idles, but sputtering and popping.

Thought it might be contaminated fuel, so I hooked up an external fuel tank to the fuel pump with fresh gas. Made no difference.

Did a compression test, 129, 130, 129, 135.

I rechecked the carb again, including pulling the choke valve. Cleaned all the passages again with pressurized carb cleaner.

Reset the float. Nothing is changing the running.

I have the float set 1mm below the body (with a strait edge across the body)

Going to try setting it so more fuel is in the bowl. All carb parts are in according to the SU web site, so I have no clue what else to do.

Bruce Cunha

I don't know how warm it is in California at the moment, but I would not expect a car with SU carbs to start from cold with no enrichment. However, given that it does start, the fact that it dies as soon as you lift the throttle looks like it needs an enriched mixture. As for pumping the pedal, it does precisely nothing on an SU as there is no accelerator pump.
Mike Howlett

As Mike says. The fact that the speed increases when you DO pull the choke indicates the mixture is weak.

As the engine appeared to be running well at the start (at least) of the emissions check, and is running so badly now, you have done something wrong in the reassembly.

Yep, came to that conclusion also Paul. Just can't figure out what I did wrong. Using the SU web page guide for disassembly/assembly.

I did write to Joe Curto, he has one of the leading SU shops in the US. Hoping he can direct me to what I an not doing correctly.

Here is a link to pictures of the carb last time I took it apart.
Bruce Cunha

That does sound rough! Almost like having the plug leads incorrect?

Unfortunately although I think I could just see the corner of the piston initially, as you lifted (presumably) the throttle quadrant up the camera moved and put it out of shot. Did the piston move at all?

And lifting that quadrant up by the amount indicated by the sheath should have made a lot more difference to the running than it appeared to. It seemed to make virtually no difference at all, so I'd have to question whether it was opening the butterfly. With the engine stopped lift the piston by hand and check the cable moves the butterfly by the full (more or less) 90 degrees.

In the video, I was lifting the choke, that is the sheath/wire you see moving. But your point is a good one. I will try lifting the piston and see what happens.

I know that the carb spacer gasket has a hole that I assume allows some vacuum for the piston. Could be that this is not providing enough vacuum for the HIF.

Also, there is a Rubber (?) plug with a hole in it on the face of the piston. What is this?

Bruce Cunha

Bruce, the piston transfer holes SHOULD NOT BE BLOCKED. They should only be temporarily blocked for a piston timing check.

The quotes below are from this website:

" There is a timing check that can be made. It involves removing the dashpot and piston, taking out the long spring inside, blocking the vacuum transfer holes in the base of the piston with small corks, and holding the unit upside down. Push the piston fully into the dashpot, and then time how long it takes for the dashpot to fall from the piston (into your spare hand). This should be around 8 10 seconds. If one is much faster than the other, perhaps a previous owner has got them mixed up."

Also -

"With HIF carburettors, there is a secondary jet operated by the choke mechanism. As you pull the choke knob, the jet rotates, so enriching the mixture. This secondary jet has an O ring seal, and if the O ring fails, the secondary jet then floods the inlet port, giving a very rich mixture. There is no indication from the overflow pipe, so it can be very confusing unless you know about it.

Have you checked the starter jet assembly o ring?


Andy Robinson

Try swapping plug leads 2 & 3
Chris at Octarine Services

I know some have had to alter those holes when they have superchargers fitted, as otherwise the piston doesn't rise properly, but ordinarily both holes should be open as Andy says.

"carb spacer gasket has a hole"

The gasket between the carb and the air-filter base should have two holes matching up with the those in the carb flange and the air filter base (arrowed in the attached). It allows the space under the widest part of the piston to be at atmospheric pressure at all times, otherwise the rise of the piston is restricted which causes a rich mixture. Later gaskets have two pairs of holes so that can't be fitted the wrong way round, but the air-filter base still can be. Of course, with no filter fitted that should not be a factor.

Thanks all. Not sure I made it clear on what hole I was talking about.

Here is a picture of the carb to manifold spacer. does the hole in this spacer provide vacuum to the piston?

In the picture I posted above, what is the black rubber looking plug?

Chris. Will recheck that.
Bruce Cunha

vacuum to raise the piston comes from the two holes in the base of the piston near the needle.

The two holes on the face of the carb allow air to enter under the outer rim of the piston so that when vacuum is applied to the inside of the piston chamber it is actually the normal air pressure under the rim that lifts the piston - with the two face holes blocked then the air cannot enter and the piston isn't lifted very much.

The hole in the manifold spacer and gaskets is used on later carbs like the KIF series - it has no function on the HIF carbs.
Chris at Octarine Services

If I lifted the choke on my HIF to the extent that the sheath moved as that video, my tickover would have risen dramatically. As Paul said the piston barely moved.

The SU Burlen tech rebuild site mentions that the enrichment O ring should be an easy fit when the assembly is put back in the body. Any force is likely to damage the O ring.

The text also implies that the assembly can be fitted more than one way "up". The spindle has "flats" that fit thru the cam, so it looks as tho this could be able to be misfitted.
The return spring (on mine) is not particularly strong-is your choke returning as it should? Tho to be fair I can push mine towards "off" a tad more than the spring does, but it has no additional effect on the mixture

Michael Beswick

Ah right I thought you meant the hole to the lower right of the needle as that looks like it is plugged with something blue.

It is shown on the attached SUCarbs drawing as a plugged blind hole (my blue arrows). It also seems to have a flange that sits against the base of the piston, which will be in line with the bridge, so I suspect it prevents the piston base going flat to the bridge, leaving a small air gap. Perhaps something to do with ensuring some mixture flow when cranking.

If it was the choke cam you were moving, that should also be opening the throttle via the fast-idle screw on the throttle cam.


Thanks Paul. when I pulled the choke out for cleaning, I noted that the choke idle screw was not touching. I have corrected that.

I did hear from my carb expert. He says all looks good in the pictures. He also said that the HIF does not use that hole in the spacer (ZS does) and that is why the gasket for the HIF to the spacer does not have the corresponding hole.
Bruce Cunha

Another day of working on the HIF. Disassembled it again and cleaned the passages again.

Regarding the Jet Bearing (16 in the attached), there are two holes across from each other in the jet bearing. In the reassembly instructions it just says to reinstall the jet bearing. Do those holes need to be in a specific position?

Also, the instructions say to mark the float bowl cover to assure correct re-positioning. My cover had a mark on it, but there are also two tabs on the inside that only allow the cover to be put on in one position. Just wondering if I am missing something on how the tabs go and am putting the cover on incorrectly.

Bruce Cunha

As long as the sealing washer is in place the jet rotational position doesn't matter. You are right the cover will only fit one way.
Allan Reeling

As long as the sealing washer is in place the jet rotational position doesn't matter. You are right the cover will only fit one way.
Allan Reeling

Thanks Allan. Going to try raising the float today as I think it has to be an issue of not enough fuel. Let you know how it goes today.
Bruce Cunha

Well, another day of working on the HIF, and another day of frustration.

I took the HIF off, and reset the float higher. checked that the fuel needle valve is working correctly. Put it back on and started it. Idles but roughly. same issue, as soon as I lift the throttle, it stalls.

I took it off again, and took it fully apart cleaned each passage again and used carb cleaner to assure each passage is clear.

Put it back together. Put it on and no change.

There is fuel in the carb, when I take it off, I can pour a good amount out the overflow.

This has me totally confused. Given the car was running prior to taking the carb off and checking the float, I still believe it is a fuel issue.

Compression is 129/130/129/135. Coil is new. Distributor is a 25D with new points, condenser, rotor and cap from Advance Distributors, new wires, new plugs.

Other than the SU's on mty 67 BGT and on my TD, I don't have another carb I can try. Anyone have a rear HIF 4 for sale?
Bruce Cunha

I have just looked at the video again and note, as Paul pointed out, that the piston doesn't seem to move. Are you sure that it is free to move? Take the damper out and lift the piston fully with a finger and let go. It should fall feeling under the effect of the piston spring and hit the bridge. It looks as though your piston has lost the part of the buffer which stops it sitting directly on the venturi bridge, so bottoming out will be a metallic click.
Another video clip showing the carb and piston movement when you open the throttle would be useful.
Allan Reeling


In my experience there are three things that cause the symptoms you have -

1. Plug leads 2 & 3 reversed;

2. The air balance holes on the front of the carb blocked; and

3. The overflow pipe from the float chamber blocked.
Chris at Octarine Services

"3. The overflow pipe from the float chamber blocked."

FWIW when this happens the float can't rise to shut the fuel supply off and the pump keeps pumping. On the MGB this is audible and fuel pumps up out of the jet, but with a mechanical pump you will only get the latter symptom of course, visible by looking in the carb intake.

Opening the throttle will stall an engine if the piston rises too quickly. The damper is supposed to delay that rise, you should feel this by trying to lift the piston manually with a stopped engine, but when released it should drop smartly. Alternatively if you unscrew the damper cap, lift it up a little, then press it down again you should feel the resistance of oil in the damper reservoir. If you can't feel both of those then the damper isn't functioning.

But as asked before, does the piston move at all when you start the engine, and try to move further when you open the throttle?

I'm convinced you are making a fundamental mistake somewhere, but until we get answers to fundamental question like these it's difficult to help further.

Thanks Paul. When I lift the throttle I can see the piston starting to move. I tried lifting the piston while lifting the throttle and it still dies.

Piston goes up slowly with resistance and drops smartly. Good resistance when I push the damper into the piston. I did check the drop rate per the SU website information and it is dropping in the appropriate time.

I agree that something is not correct, just can't fine it yet. Going to check the fuel needle again and set the float even higher
Bruce Cunha

" I tried lifting the piston while lifting the throttle and it still dies."

You do have to wait for the manifold vacuum to lift the piston. Lifting it manually at idle with the lifting pin is one of the adjustment checks and if the mixture is weak it will stall.

Before you got it running well enough to take it to the testing station I seem to recall you could not get the mixture rich enough, dismantled it, rebuilt it, it ran better hence the visit to the testing station.

What is the mixture situation now? I.e. can you richen and weaken either side of the point of highest idle? What did you use to set the mixture?

Hi Paul.

Started the car and ran the adjustment screw as rich as I could. This did allow me to get it to run with the throttle lifted, but it is running very bad. Here is a video of the car running with the jet set rich and a picture of the plugs after this run

Still sounds like it is starving for fuel.

The plugs are black. That would normally indicate it was running rich, but they are more of a sooty black rather than a wet black.

I also rechecked the points, coil, wire position. All appear to be normal.

This is an AAA needle. The original was an AAU. I also have an AAM that I have not tried yet.

Prior to it running poorly, I adjusted the mixture by moving towards rich until the motor reached the max rpm and then backed it off just a bit.

Also, I note the large hole on the back of the carb is open, but the gasket covers this. What does that passage go to?

Bruce Cunha

Hi Bruce,

This bit might be of some assistance. Last item on this diagnoses list; "Choke stuck open -HIF carbs - Jet stuck in bearing, not returning."

Too rich mixture even with jet all the way to the top each of these problems remedied in restored sets, included for reference only

On the H series, some after market kits include jet bearing washers which are too thick, preventing the jet from reaching its full height. The jet bearing gland nut may not be tightened sufficiently, again preventing the jet from reaching full height.
The shoulder of the needle should be even with the face of the piston place too deep into the piston will cause rich running.
Wrong or missing piston spring will allow the piston to rise farther than intended. Too little clearance in the piston to dashpot will raise the piston farther than intended.

Float level too high
Worn needle or jet incorrect needle or jet
Choke stuck open HIF carbs jet stuck in bearing, not returning


Larry C.
Lawrence Cordeiro

It sounds like you are back to where you were originally, not being able to adjust the mixture to be over-rich. What ever you did to cure that originally, has been reversed somehow.

That large hole isn't on my HS or HIF carbs, best if you investigate where it goes to, perhaps by pouring a little fuel in and seeing where it comes out.

I think you are at the point of sending it off to someone who knows these carbs very well.

Yea, I am getting to that point. Very frustrating given how simple this carb is.

Racking my brain to try and think of why this is not working. It was running so well.

I do appreciate all the thoughts and advise
the group has given.
Bruce Cunha

As regards your query and attached photograph.
On my dismantled HIF 4's, that passage leads nowhere, it is obstructed by the crankcase breather pipes. Put a wire through yours and check that this is so. If in doubt blank it off.
Allan Reeling

Decided to start over. Tomorrow, I am taking the carb off, taking apart again and soaking it in carburetor cleaner. Then I will blow out all the passages again and rebuilt it again.

There are so few parts on this carb, that it concerns me why it is acting up.

Bruce Cunha

Ok, Disassembled the HIF, soaked the body in Berrymans carb cleaner. Then flushed all passages with pressure can carb cleaner and then air.

Reassembled the carb per the instructions on page. I am reasonably sure this carb is assmbled correctly.

I did find one issue. The new fuel seat is taller than the one I took out. That would change the float set point. So, I put the old one back in.

Car started, but still running bad. But this time it is staying running. I put on a strobe light to check that the timing was ok and noted the strobe has a miss in it.

Now even if the cylinder were not getting fuel, there should be a pulse from the spark plug wire with each rotation. So, it is probable that I have an electrical issue.

Car has a 25D with new points, condensor, rotor and cap from Advance Distributors. New coil and wires.

Going to put the tach on the coil wire and see if the issue is there. If not, I will try another cap and also check the wires and ground on the distributor.

Bruce Cunha

Its Fixed.

First, I want to thank all of the folks that assisted me and helped me troubleshoot the issue.

This was one of the most frustrating issues I have had to deal with. It had me questioning my knowledge and my abilities.

After going back to the beginning and checking out the full electrical system, I found that the brand new condenser was failing (Not an Advance Distributors condenser). Swapped it out for an old one and the car runs fine. Readjusted the carb and took it for a run. Great pickup and acceleration. No miss.

Next week it will go in for smog and if all goes as planned, it will be up for sale shortly.

Just goes to show that sometimes, it is not the last thing you did that caused the issue.

Again. I want to thank all the members that offered assistance. Diagnostics via the internet are not easy, but I truly appreciate your input.

Oh and boy am I now very familiar with the HIF carb.
Bruce Cunha

Hi Bruce,

Glad to hear you have things sorted, and the car is running well. Let us know how the smog tests go next week.


Larry C.
Lawrence Cordeiro

Oops, I started to read this thread just now and thought after you mentioned the 50 minutes that it took to misfire: 'condensor'...

I'm glad you shared the solution.
Willem van der Veer

Just supports the old adage that "most carb problems are electrical!!
Allan Reeling

Ahh, the 10/10 repair... 10 weeks of diagnostics and 10 minutes to fix it.

I've had bad condensors right "out of the box" on several occasions. Some failed before I was able to get the engine started, others started to break up once they got "hot". Just because it's new, doesn't always mean it's better.

Good work on your part, Bruce.

Thanks for sharing the outcome.


79 MGB
gary hansen

I think I recall a few years ago that someone would answer "condenser" regardless of what the stated problem was. Maybe we take up that habit again.

C R Huff

Well, I used to say always start at the last thing you worked on when you have a new problem. I spent weeks figuring this was a carb problem. That, because everything electrical was brand new.

Not, I think electrical becomes the first thing to look at.
Bruce Cunha

This thread was discussed between 01/02/2017 and 13/03/2017

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