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MG MGB Technical - Can't find a cure for running rich - help!

hello all! I've read tons of archives, without finding an answer to my problem. My B runs too rich and I can't get it to improve, short of running the jet nut almost all the way up!

The car has rebuilt engine, new 45D distributor, carb bodies rebushed, all new jets, std. VB needles, etc, timing, points dwell all factory spec.

Mods are Delta camshaft, headers, Longflow air filters.

If I try to set the rich-lean mixture anywhere close to normal (e.g. 12 flats down), it runs way TOO rich (i.e. definite black exhaust). The idle is, of course, not so good. It drives, however, very well at that point.

If I weaken the mix (about 10 flats up!) to the point of getting a good idle, I then have poor acceleration (obviously).

I'm wondering if it's a needle problem. I have looked at the needle charts on Paul Hunts page, and using needle size found in car when I bought it (5, assume A5), seems I need something leaner. But, with cam and headers, I would have assumed I need richer??

I've also tried timing adjustments - no joy there.

Any ideas??
Thanks in advance. Dave
David Steverson

PS. Car is a '67 B with original engine, carbs.
David Steverson

David

I've just been through carb problems, opposite to you though, but if it helps.....sounds like too much fuel or timing is too advanced. Is the choke mechanism OK, that could give too much fuel. Perhaps one more go through the timing to ensure it is not too advanced. Hope it helps, good luck.
Richard Evans

You might check fuel pump pressure. If it's more than 1.5 - 2 psi, it may be overcoming the needle and seat valve. By the way, have you checked your float levels?
Matt Kulka

Thanks guys, I have thought of fuel pressure, as I have an aftermarket pump. I added an adjustable fuel pressure regulator - no joy on that either.

I have not checked float levels. I have those non-adjusting plastic floats from VB. I guess I was thinking if float level was off, I'd have fuel coming out the overflow pipes??

Dave
David Steverson

A certain close relative of mine (VERY close) screwed up once and used the wrong gaskets from the set to mount the carbs. One must use the gasket that does not cover the air holes.
Dan

Was there anything with the filters suggesting a change of jets?

A friend replaced stock with K&Ns, (and there was some posts on this) and he had to change carb jets to compensate for running rich.

Somebody out there has that knowlege I'm sure. -- I'm an old fuddy-dutty --- almost everything is stock for me.
glg

Dan,
I think your relative is closely related to me! Though I have already checked the gaskets..

glg,
Nothing with the filters about needle changes. I did not have stock filters with the car and I thought those Longflows looked good in the VB catalog. I replaced the needles with standard (VB only sells std and rich), as I was having the problem before then. Actually, I thought maybe the old needles were worn.

Thing is, I've got this car over at an MG shop here, guy was a BMC service manager, and he hasn't found the problem yet. At least that makes me feel better!

Dave
David Steverson

Dave. The idle mixture adjustment is set with the upper part of the needle (closest to the bottom of the piston), the cruise is set by the needle profile lower down. The first thing you need to do is to pick up a copy of the SU Tuning manual and read it. It covers all of this type of information and how the needles are described. Your problem sounds like this. When the idle mixture is adjusted correctly (start by using a dial caliper to set the jet .060" below the bridge height, then adjust for idle), the needles are too lean for your requirements. When the jet is adjusted lower, resulting in an excessively rich mixture at idle, the relationship between the needle profile and the jet height allow the car to perform more correctly. Thus, the proper solution is to correct the idle mixture and install a set of needles which have the correct, richer, profile for acceleration and cruise. Joe Curto in New York is one source of needles in the US and Joe stocks a much wider assortment than Moss or VB. The phone number I have for Joe is 718-762-7878.

glg. The HS-4 carbs only have one jet size, .090". It is the HS-6 series which has both the .090" and .100" jets. Hence, in this case, no larger jets are available. Les
Les Bengtson

Do yourself a favor and call Joe Curto and order the correct jets and needles. He can also help with performance needles. I have received kits with jets of different sizes anthough the needles were correct.
Jimmy P.

Are the needles correctly positioned with the shoulder flush with the bottom of the piston? I think solid (at least) needles can go higher than this, meaning that the jet has to be correspondingly higher to get the correct mixture. Are both the same? Have you checked that the jets are fully up when the choke is pushed home?
Paul Hunt

Paul,
Thanks for your input. Yes, the jets are set with the shoulders even with piston bottom. I have also checked that the jets are fully home when choke is off. I've sent a note off to Joe Curto to see if he has any input.
This is really getting frustating. I've had lots of B's and never had this issue.
Dave

PS I was over your way, in Erdington, last November.
David Steverson

Glad to hear you escaped ...

That really is a bit of a mystery with new jets and needles. Is it the same with the air cleaners off? Could they be too restrictive?
Paul Hunt

Paul,
Yes, it exactly the same with or without the air filters installed. Could the float level have this kind of effect?
Erdington's not so bad. Besides, it's hard to beat a good Balti. There's also a nice pub out that side of Brum, Dog and Doublet, sits on a canal.
Dave
David Steverson

I was having the same problem, and was reluctant to do my last resort of checking the float levels because on HIFs you have to take the whole stupid thig off the car, but of course that's what the problem was. I had a tiny leak in one, so the thing was actually a "sink" instead, and at idle it was sunk halfway down and the valve obviously was open and too much fuel was coming in, but at speed, the car ran fine.
So on a long trip at speed, it worked fine and burned off the plugs, but if I drove in traffic, it would foul up. Very frustrating trying to adjust the mixtures when the float was sabotaging whatever I tried! On your carbs it should be much easier to check the float levels!
Eric Welty

IME float levels have very little effect on mixture. I had a puncured float away from home, and rather than pump fuel out all the time I cross-connected the overdrive switch to the fuel pump. Turned on the pump till it started overflowing, then ran till it started cutting out and so on. It would run for quite some time between recharging the float chambers, with no apparent degredation in performance as it ran back down. Gotta be needles and jets, are you sure you don't have the larger jets and the smaller needles?
Paul Hunt

What type of fuel pressure regulator did you install, and have you added a fuel pressure gauge to verify that it is accurate? Some inexpensive regulators are less than acuurate and will have pressure surges at times of lower fuel consumption, such as idle and deceleration. A good unit such as the Holley HOL-12-804 is rated at 1-4 psi and will provide the consistency you need. Very few aftermarket fuel pumps are available in the pressure range our cars require, which makes a good regulator EXTREMELY important!!!
Jeff.

Jeff Schlemmer

Jeff,
The regulator is the type with an adjusting knob on top and numbers (1 - 6). I have been looking for a pressure gauge to see what I'm getting, but no success yet. I'm beginning to wonder if I need to just buy an SU fuel pump.
Dave
David Steverson

Paul, very clever. I drove all the way home across London with a punctured float once. Used nearly half a tank of petrol(!) Wish I'd thought of that one.
Incidently, in this condition it idled *very* badly with gobs of black smoke so, even though the fuel does pee out of the overflow pipe, I reckon that quite a bit extra finds its way down the carb throat as well....
T J C Cuthill

How are your needles and seats in the Float Bowl? I was getting leakage past old neeldes and it ran rich.

Bruce
Bruce S

TJ, I can well expect that the pump, if left pumping, will force fuel up through the jet making for the rich mixture you experienced, as well as out of the overflow pipe. And the check for leaking float valves is to leave the ignition on for a while with the engine stopped. An SU pump should not tick any more than once every 30 secs like this, any more indicates either bad non-return valves in the pump or a leaking float valve. The latter, of course, will be visible on the ground from the overflow! After filling up the charcoal canister, if fitted. The only check you have on continuous-running aftermarket pumps is the overflow.
Paul Hunt

David,

Minor point: Most any automotive vacuum guage will also measure fuel pump pressure. Check any auto parts store, they're not expensive. You can get a spare foot of 1/4" fuel line and a tee, to connect it into the system while you're there.

Please do let us know what the resolution of this is. It's become a very interesting thread.

Matt Kulka

Matt,
I had forgotten that about the vacuum gauge.
I may have found the problem(s), a combination of fuel pressure and choke settings. I readjusted my choke cable so that the choke knob stops just short of the dash bezel.
Also, I have some different metering needles coming from Joe Curto.
I'll do another post after I'm finished with all this.
Dave
David Steverson

I too got caught with a malfunctioning float. As suggested above the clue is when the fuel pump keeps clicking at a steady pace at idle. (I usually can't hear it at higher RPM due to the engine noise). Looking inside the carb I could see that the fuel was coming out of the HIF jet. The car ran only 5-10 minutes after giving it some 'rest'. And it ran best at wide open trottle.

I tried an alternate way to Paul's in order to 'get home'. Block off one of the carbs. After all you have two, one is a spare. For me the rear carb was at fault. I pinched the fuel line to the rear carb and then blocked the air intake with a piece of metal and duck tape. Runs fine this way, and even gets good gas milage.


regards
werner
werner haussmann

"I pinched the fuel line to the rear carb and then blocked the air intake with a piece of metal and duck tape. Runs fine this way, and even gets good gas milage."

Goddam P(resent) Os ...

But Werners post reminds me that if the overflow pipe is blocked air-pressure at the top of the float chamber prevents the fuel level and hence the float from rising, so the float valve remains open. Instead of pumping out of the overflow the fuel is pumped up through the jet.

Paul Hunt

FYI

Paul, in the US the HIF carb has the fuel bowl top connected to the carbon canister. It is a line going higher than the carb. It's at atmosphere pressure. Since the jet is lower it always comes out the jet first. I don't think there is any other place for the fuel to go; the jet or cannister.

Werner
Werner

All,
I seem to have this problem sorted.

1) Fuel pressure was wrong (actually too low). Bought a vacuum/fuel pressure gauge and set the pressure to 2.5 psi.

2) Choke linkage was out of sinc, one carb to another. Thanks to an email from Lawrie Alexander on that.

3) Installed different metering needles (from Joe Curto) and another set of jets.

I would like to thank everyone for their input on this. It's great to have a good running B now!

Dave
David Steverson

"Block off one of the carbs. After all you have two, one is a spare. For me the rear carb was at fault. I pinched the fuel line to the rear carb and then blocked the air intake with a piece of metal and duck tape. Runs fine this way, and even gets good gas milage."

You mean it realy ran ok this way??? And you drove it like this long enough to figure out your MPG??? Is it still like this???
John A

Yes, it ran OK.

Since it ran fine and it can't accelerate as hard as with two carbs I assumed the milage is as good or possibly better. The single carb would not idle the engine corretly so I had to use just a bit of choke to raise the idle to an acceptable level. I did not mention it, but I also disconnected the fuel bowl connection at the working carb. As I remember that was all that was needed. I was lucky to carry the tape and tool box.

No, I fixed it the next day. Turned out it was either a sticky float or needle valve. I took it to be the valve since banging on the carb did not clear it. But I replaced both to be sure. After installing the new float it kept changing the level and the carb had to be readjusted monthly. So, the float had to be replaced again. This time with a used one (original) from a junk yard.

werner
Werner

This thread was discussed between 26/07/2003 and 06/08/2003

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