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MG MGA - Missing at 3000 - 3500 rpm's

My '58 coupe has just over 190 miles on it's rebuild. The motor was rebuilt 6-7 years ago and has been started and run fairly regularly (clearly it is not driven very far). I've noticed when it warms up (160 degrees) it will begin to miss at 3000-3500 rpm's. I have never changed the spark plugs or wires since they have so few miles on them.

Could this be a timing issue?
gerard hutchinson

Sticking distributor advance?
Neil McG

Have you ensured the SU carbie dashpot bore and pistons are clean and dry? Over oiling of the dashpots can attract dirt and cause the pistons to hang which can lean the mixture.
Can we assume the carbs are adjusted correctly on a hot engine.
Could be the plugs, try some spares.
Pete
P N Tipping

My guess is distributor advance.
Steven Devine

Has it always done this or is it a recent happening?

Steve
Steve Gyles

"Missing" is ignition. I dont think timing causes an actual miss, but a spark leak will, as will poor points or a loose connection or a bad coil etc.
Art Pearse

Also check for weak fuel mixture at road speed.
Barney Gaylord

hi.try looking at the condenser first it is the one all the garages look at first Ian.
ip blain

Gerard, I had something similar to this a few years ago. Can you take your plugs out and have a look at their colour. See if the plugs on 1 and 2 are black and sooty whilst the other two look ok? Lindsay.
Lindsay Sampford

HA! Thank you for all the responses!

Steve - I only have 190 miles on the motor but I believe it is a relatively new development. It seems to happen once the motor has warmed up.

Neil & Steven - how does one check the distributor advance?

Barney & P N - The carbs have been adjusted by a local MG expert. And it ran well for the first 150 miles.

Lindsay - I have not check the ignition system in some time, but I have been thinking this is a plug and/or wire problem.
gerard hutchinson

Gerard, the easiest way to check the advance is with a timing light. Disconnect and plug the vacuum and watch the timing as the engine revs are increased.
Typically I would not describe the symptoms as a misfire, more that the engine doesn't want to rev. It seems to accelerate up to about 3,000 revs and then is reluctant to continue. I have seen this on several cars now over the past few years, replacing the distributor (initially from another car, to confirm) Immediately solves the problem. Another way to test is to advance the distributor by say 15 degrees and see if it revs easier. Makes it difficult to start and lumpy on tickover though...
Neil McG

Checking advance....I usually put a vacuum pump on the advance diaphram on the distributor...and see if its functioning. Timing light would be better to watch it under real time conditions.

Gerard was the distributor overhauled when you did the engine rebuild?
Steven Devine

Steven, I was referring to the mechanical advance, but yes if the vacuum advance is not working (either) it will not help!
Neil McG

Hey Neil,
I was thinking of both but usually start with the diaphram....

I notice with a lot of engine rebuilds no one wants to spend the money on the ancillaries....but if those parts are not up to the job you still get poor performance.
Steven Devine

Absolutely, and a brand new electronic distributor is only £55 delivered!

Neil McG

Lots of suggestions coming in. It would be interesting if Gerard could give us an update,

Steve
Steve Gyles

Sorry. Missed Gerard's response yesterday.

Steve
Steve Gyles

HA! You guys are the BEST!!

Yesterday I drove my car about 15 miles and it ran ok (not good but ok). And yes Neil, I would describe my problem as not wanting to rev, especially going up hill. By the afternoon the car would not rev past 1000 rpms. So today I got it started, loosened the distributor, and began to rotate until the idle smoothed out. That seemed to help a lot. I tightened the distributor down and took it for a drive. It drove a bit better than yesterday but after I returned to my garage idling became rough and the problem I experienced yesterday returned.

My distributor was not overhauled during the rebuild and I have no idea if the vacuum advance works at all. Should I replace the vacuum piece? I can't really afford the $500.00 electronic distributor.
gerard hutchinson

Does anyone have an opinion on this part?

http://www.ebay.com/itm/New-Complete-25D-Electronic-Distributor-MGA-MGB-1955-1980-/171126448757?pt=Motors_Car_Truck_Parts_Accessories&fits=Year%3A1958Make%3AMGModel%3AMGA&hash=item27d7ee6275&vxp=mtr
gerard hutchinson

Is it difficult to install this distributor?

http://www.ebay.com/itm/New-45D-Electronic-Ignition-Distributor-for-MGA-MGB-1955-1980-With-Sport-Coil-/170941850265?pt=Motors_Car_Truck_Parts_Accessories&fits=Year%3A1958Make%3AMGModel%3AMGA&hash=item27cceda299&vxp=mtr
gerard hutchinson

Has the missing stopped then? Or is it just constipated?
Are you sure the valve timing is correct?
Art Pearse

Hi Art. The "missing" stopped when I loosened and rotated the distributor. But later the problem returned. I tried adjusting the vacuum advance but it did not help. Yes I do believe the valve timing is right. I've been driving the car for a couple years and this is the first time I've had this problem.

gerard hutchinson

Hang on, hang on! If it is JUST a timing advance issue it should not get worse like that. You should also be able to get it to run at higher revs by turning to advance. Sounds like you might have some other issues.

I used these on a few cars now and so far they are all running perfectly:
http://www.accuspark.co.uk/distributors.html

A few weeks ago a friend acquired and MGB that had stood in a damp yard for nine years without ever being started or turned over. The engine had a weak spark and fresh fuel but was reluctant to run. The carb pistons moved freely. I have no doubt that with a bit of fiddling; cleaning and setting points, cleaning leads, plugs etc, etc. we should have been able to get it running ok. Instead my friend opted to go for the electrics pack. Electronic distributor with red rotor, cap, leads, plugs and coil all for £77 delivered. In a matter if minutes the parts were fitted and the car transformed. Fired up first time and runs like a dream. At that price you could have a spare set in the boot!
Neil McG

Yes Neil I agree!! Timing should not get worse. I am more than willing to buy a whole new unit. Are you suggesting I buy the unit without the vacuum advance? I am not sure how to find the proper distributor on the site you provided.
gerard hutchinson

Hi Gerard,
I had a similar issue several years ago which would respond to nothing I did. I finally replaced the distributor with a Petronix flame thrower electronic ignition and have never had a problem since. The actual change over was very easy (A lot easier than changing points etc).
I would be hesitant to purchase a used unit since older models were heat, dust and oil sensitive - once I took the old points distributor apart, I discovered that the vacuum advance plates had rubbed the insulation off the ground wire and slowly shorted the ignition out - but since going electronic I have never had to replace points, condensers, or anything else. Very pleased with the transition.
Good Luck
Bayard
Bayard DeNoie

Gerald, Have you checked the point gap?
Ray Ammeter

No one seems to have mentioned that well discussed issue - the poor quality of some types of rotor arms that plague us, the cause of so many ignition issues. Once they get hot they short out, causing misfiring etc.

Steve
Steve Gyles

Hope this isn't a "repeat" statement....But a lot of the black rotors will short out at high rpms....The red rotors , though more expensive, will solve that problem..Moss, and Schlemmer sell them.
It might solve this mystery.
Edward
Edward Wesson 60MGA

Gerard, you say you loosened and rotated the distributor, so you have changed the timing. Have you now reset it back to the correct position your MG expert set it to?
Is the points gap correct? to give the right dwell angle.
Have you checked the dashpots are clean and dry? Especially if you have the original wire type or K&N air filters. I've had this.
You should also, in turn, try replacing the condenser and coil, especially if it misfires when it gets hot. Been here too.
Pete
P N Tipping

My Y-type missed at higher revs and after trying almost everything I was tipped off by my father to measure the system voltage (at the higher revs). It turned out to be too low and after cleaning the contacts in the regulator the voltage was higher and the car runs fine now.

It's worth a try especially considering that the car stood for a while?
Willem van der Veer

UPDATE: I went with an electronic distributor from Accuspark. I positioned the number one piston to 7 degrees before TDC as instructed in the manual. I inserted the new distributor so that the rotor was pointed at the number one piston also as instructed. Then I rotated the distributor so the vacuum "bell" was vertical. I placed to wires on the cap in the order of 1, 3, 4, 2, as instructed (the manual is outside so I don't remember if its 4-2 or 2-4).

Then I tried to start the engine. As expected it did not start on the first try, so I proceeded to rotate clockwise slightly, then try, then rotate, then try, etc, etc, etc... There was a point where the engine tried to start -- backfires, grunting, little pops, etc. But it never idled even a little. I kept trying until the battery ran down. So now the battery is on a charger. I will have to try again tomorrow.

One final note: I dropped the distributor cap cracking it and breaking a piece/chip off. I put the chip back in and it held in place. I have place an order for a new one. Will the broken cap keep the car from starting?


gerard hutchinson

Gerard

Probably a silly question but when you put the number 1 piston to 7 degrees BTDC was it on its firing stroke? You may have set it up on its exhaust stroke, resulting in the problems you experienced.

Steve
Steve Gyles

Steve

I have no idea! I turned the crank pulley counter clockwise until the notch passed the TDC and stopped at 7D BTDC. And as I write my account it occurs to me that perhaps I should have rotated the crank pulley clockwise instead and actually stopped the notch BEFORE TDC!! hmmm...
gerard hutchinson

Gerard
I've been following this thread with some trepidation! In the nicest, most well-meaning possible way, I must say I think you should take some professional help for your engine problems. I just feel you could cause some damage soon by trial and error solutions applied, perhaps incorrectly, after taking each suggestion from well-meaning contributors. I do worry! Please forgive me!
Bruce
Bruce Mayo

Gerard

Bruce makes a valid point.

The No.1 piston comes to the top every crankshaft revolution, once on it compression/firing stroke and the next on its exhaust stroke. With what you have done you have a 50/50 chance of getting it wrong. As the engine failed to fire I would hazard a guess that you timed the distributor so that it was trying to fire on the exhaust stroke of each piston.

Worth a read of Barney's notes: http://mgaguru.com/mgtech/ignition/ig103.htm

Steve
Steve Gyles

Gerard, have you looked at the colour of your plugs yet? The colour of spark plugs tells you a lot about the way an engine is running. In my case, a few years ago, I had the same kind of mis-fire at 3000 rpm. It was caused by plugs 1 and 2 oiling up. This fault had me going for months, but to cut a long story short, it was caused by heavy-felt after-market air filters. These caused such a restriction to the air flow that the front carb was drawing oil mist from the rocker cover via the breather pipe. This clogged the air filter still more and the problem got worse and worse. The cure was a pair of K&Ns which transformed the performance of the car and solved the problem. Worth checking before you blow the budget on electronic ignition but still have the mis-fire!
Lindsay Sampford

Nice to hear from you again on the forum Lindsay.

Gerard, a really easy check on Lindsays suggestion is to simply remove the air filters, then do a test run the car without them.
You will soon see if there is any improvement.


If you want to find TDC on the firing stroke on number 1 cylinder you will have to take the rocker cover off so that you can see the valves opening and closing.

Its easier to turn the engine if you take out the spark plugs.

With a spanner on the crank nut, slowly rotate the engine in a clockwise direction (as viewed from the front of the car). Carefully observe the SECOND rocker from the front when it begins to push the valve down, this is the inlet valve on cylinder number 1.

When this rocker begins to lift, number 1 piston is coming up to TDC to begin its firing stroke.
So when this rocker is fully lifted you know that this is the correct position to check your ignition timing on the crankshaft pulley.

hope that helps

Colyn

PS I would always rotate the engine clockwise when setting the timing, as the free-play in the distributor gears (backlash) could affect the accuracy of the timing.
c firth

Steve Gyles may have diagnosed the problem with the new distributor. Another possibility is that the distributor drive gear is out 180 degrees but that is not likely since the engine ran with the old distributor setup. You did not say how much you rotated the distributor body to ge the vacuum advance mechanism vertical.
Don Carlberg

Further to my last comment, you can get an even better indication that you are approx at TDC you can put something soft like a drinking straw in the number 1 plug hole. It should be fully raised about half a crank rotation after the valve closes.
c firth

Gerard,

These guys can rebuild YOUR distributor. I think its about $100 american with the vacuum advance and the red rotor. Quick turn around all up to snuff.

Im not sure on your skill level with engines but do believe in working on them and learning. Reading the manuals spending time in the garage and getting good tips on the bbs from guys who have been there before. Changing your timing (rotating the distributor body clockwise) will advance the timing. Becarefull to much advance and you can ruin pistons.

My trick to finding tdc is remove #1 plug while engine is out of gear. I grab the starter switch tapping the engine a little at a time. I place my thumb over(not in over) the number 1 hole. compression stroke will push the compressed air passed your thumb. Look at timing mark on pully, you will have number one at tdc. Distributor cap of look at position of rotor that is the position for number 1wire.....rotation is counter clockwise 1 3 4 2 .


Hope this helps! :-)



http://advanceddistributors.com/
Steven Devine

Update: After follow the advice of ALL, the timing is sorted out and the car is running. However... it turns out it is still difficult to get the car to rev. I removed the felt air filters and I could watch the rear carburetor backfire when I tried to give the car fuel. So I'm back to not knowing what the hell is wrong!!
gerard hutchinson

Gerard

Did you remove the canisters or just the inner filters? If you still have the canister body in place it might be upside down on the rear carb, blocking the air passage hole for balancing the rear carb piston: http://www.mgaroadster.co.uk/odds_and_sods.htm

Steve
Steve Gyles

A backfire means its lean. As you have the filters off, does the rear dashpot piston lift and drop freely. As I have said before, there could still be muck around the piston causing it to hang. I have been here too.
Its a simple job, whip the piston out, keeping it upright to retain the damper oil and be careful to protect the needle. Wipe the piston and the dashpot's bore perfectly clean. Carb cleaner / petrol are ideal. Do the front one too while you are at it. Don't mix up the pistons and dashpots!
You are lean so, as you have the pistons and needles out, looking down into the jets, check for a similar fuel level. If different you will next check the float valve settings - see the manual for this.
As the rear carb backfired the fuel pump is probably OK.
Pete
P N Tipping

Have you tried a strobe light?
Art Pearse

Steve Gyles....Ive done that ha ha ha, Good call! Ive also had the inner felt suck its self in to the intake manifold causing a lean condition.

I wonder if it is a carb problem. Not trying to make more confusion....just pondering fuel delivery.
Steven Devine

a backfire through the carb can also be caused by a sticking valve. Timing can cause a backfire through the exhaust and poor revving.

Here it is a back to basics approach that is required. Simple tools like a compression tester and a strobe light are invaluable.



Neil McG

I and my British car mechanic friend concurs with the fuel/carb problem theory. I will follow his instructions to find out what is gumming up the works and clean it out.

Again, I would be totally lost without you guys. Thank you!!!
gerard hutchinson

I took a video of the carb backfires.

Copy and paste the link in the address bar of your browser: https://www.dropbox.com/sc/ofnb76suuxk7mfs/1MV11KW0j0

I have no idea what to do next...
gerard hutchinson

Hi Gerard, great video of your carb backfire problem.

Not sure if this helps but the video shows that your fuel separator bulb is incorrectly positioned horizontally. This will allow fuel vapour droplets to fall down to the distributor & cause the advance mechanism to activate incorrectly.

Clean out the vacuum advance at the distributor end & then reroute as per http://mgaguru.com/mgtech/ignition/ig121.htm

Don’t ask me how I know!

Richard

R A Evans

If you give it some choke to richen the mixture does it stop back-firing. Also have you tried the choke trick when out on a run to see it pulls better. Will help to identify if it is electrical or fuel
Andy
A J Dee

Have you checked your compression? That looks like one of your intake valves isn't seating properly, allowing the exhaust to escape back into the intake.
Gene Gillam

This motor is rebuilt and only has 223 miles on it. It was running perfectly until I put in some low octane fuel in last week.

How exactly is a compression test performed? May as well eliminate possible problems.
gerard hutchinson

After a can of carb cleaner my car idles without carb backfire. However, when I try to give it gas the backfire returns.

This is a link to a video of what is happening: https://www.dropbox.com/sc/jgkdrbf4p2pdbjk/rcXZvYOVwh

I think it is also important to note that when I put the car in gear and ease off the clutch, the carbs begin to backfire WITHOUT me trying to give the car gas. Why would putting the engine under pressure cause the carbs to backfire?
gerard hutchinson

This might be off....but just throwing it out there. My carb dashpot tops are brass (mga are....some one changed yours to later mgb ones baker lite.) Some carbs have a hole in the damper at the top some have a hole in the dashpot itself.

I think the rule is you have one or the other not both.

Mixed wrong parts can look right but function incorrectly.

Take a look at your carbs and check to rule out that issue.

http://www.mgaguru.com/mgtech/carbs/carb.htm

http://www.mgaguru.com/mgtech/carbs/cb105.htm

http://mgaguru.com/mgtech/carbs/cb105a.htm

Hope Im explaining it enough to help you...Hve seen it cause issues like yours in the past.
Steven Devine

Some more thoughts... If the distributor cap is leaking sparks then a stray spark can ignite fuel as inlet valve opens in wrong cylinder. Another possibility is inlet valve not seating properly. Bad tappet adjustment?
We'll get there...
Pete
P N Tipping

Are your plug leads connected in the right order 1, 3 , 4, 2?
Mike
Mike Ellsmore

Checking the symptoms on-line, the most popular cause is mentioned by others above: over lean mixture. Also check that the choke mechanism is functioning correctly.

On the electrical side whilst setting the ignition initially at about 7 degrees to get the engine going it is vital that you then refine the settings. I prefer dynamic timing with the strobe light. I usually aim for 32 to 34 degrees BTDC at 3500 RPM with the vacuum disconnected.

I believe changing fuel grades can also have some effect and may require a bit of timing tweaking to avoid pinking.

Steve
Steve Gyles

" It was running perfectly until I put in some low octane fuel in last week. "

Have you tried using a fuel additive to boost the octane value? I stopped using my fuel additive a couple of years back when I ran out of it -I found that the car runs perfectly without it most of the time - but I do get occasional spit back in the carbs.
Cam Cunningham

Bad gas with a little bit of water in it would cause some of this too.....Throw a bottle of Isopropyl alcohol in the tank. Dry gas. Cheap and cant hurt.

I have to put high test in my car and if theres a hint of water the car performs poorly as if one carb is shut down. It comes and goes till the water runs out.

Still might not solve it but good practise imho.
Steven Devine

As much as I would love to solve this riddle myself, I think my best decision is to get my car to my trusty local MG mechanic. I think if I can finally get it running properly and then drive it mire regularly these sort of mysteries can be reduced.
gerard hutchinson

Gerard

Make sure you get a full report from your mechanic and post your findings. It's always good archive material for the next generation of MGA owners.

Steve
Steve Gyles

Steve

I definitely will!!
gerard hutchinson

In an attempt to find a solution to my intake backfire mystery I changed the fuel filter and installed a clear filter. I have never seen how fuel looks as it flows from the tank to the carburators, so I'm not sure how "clean" fuel compares to "dirty" fuel. The attached photo show quit a bit of visible particals getting through the mesh filter.

Could this be causing my problem?


gerard hutchinson

This photo is an even closer shot.


gerard hutchinson

So long as it is getting fuel, that should not be a problem.

You still have something or a combination of things that are very wrong. Firing order, HT system, timing or carbs that are sticking or adjusted horribly out. You just have to work through the basics.

You have hopefully eliminated points, condenser and advance mechanism with the new distributor.

Also worth rechecking the tappet clearances.

Often it is not the actual check, but finding something else that has come loose or been disturbed that highlights the problem.

You know you are getting fuel and a spark, but you need to know why they are not happening at the right time and to the right mixture!
Neil McG

Sounds obvious but if it did only start with a new tank of low octane fuel I would hazard a guess it is the fuel. Empty it out and put in some fresh from another good supplier. I had a similar problem with my lotus elan. It was running perfectly, then filled up at a budget fuel station and it ran like a pig. Would not even tick over without some adjustment. New tank full of fuel from another station then back to running perfect. I know Lotus are a bit touchy but it was like a switch had been flicked from good to bad then back to good.
A J Dee

I certainly wouldn't rule it out, but less likely on a standard B series with SU carbs. We put a bit of fresh fuel in the carbs to start my friends MGB, but once it was going it still ran well enough on the 9 year old fuel in the tank!
Neil McG

My MGA has an early mgb motor with A carbs.....That much moisture in the fuel would reduce my car to a nonrunner. Put a bottle of dry gas in with octane boost or dump what you have and put in known good fuel.

You will see an improvement! I can tell right away when I get a tank of bad gas! It runs like poop!


Steven Devine

Did the dry gas strip the bonnet/hood? ;-)
Willem van der Veer

Gerard

Going back to square one, was this spitting an immediate effect of putting the lower grade 'suspect' fuel in the car? i.e. had you done anything else at all in the engine bay area since its previous 'good' run?

As an aside, I have always found these old engines aren't all that happy on lower grade fuels. They were designed around 98/99 octane. In the UK our usual fuel now is 95. Yes the engines run ok on it but it does not give dazzling performance and there is a tendency to 'pink' (knock) the engine under hard power unless you have the ignition timing backed off sufficiently. Personally, I accept the slightly higher cost of filling up and always put in 98 octane fuel (Super Unleaded over here). In my view the difference in performance is very noticeable (seat of the pants stuff, nothing technical).

Just my 10 cents worth.

Steve
Steve Gyles

Willem van der Veer

I bought the car 15 years ago from a restoration shop that it was abondoned at. Some kid must have painted it with a brush in the 1970s

There was a fair amount of putty on the bonnet and the paint peeled so I just stripped the rest of the paint off last year. Ive got new inner and outer rockers to install sooooo.... I just left it until I go hogg wild do the metal work and paint all at once.

Some people go out of thier way to tel me to leave it as is....but eventualy I want to get it back to OEW that it was when it was new.
Steven Devine

Gerard,
You haven't mentioned it, so I ask the question - have you checked that the air filter inner cans are fitted the correct way up (VOKES on top). If not, they will block the two holes in the carbs which might result in this condition.
You can't fit the front one upside down due to the breather pipe going to the valve cover, but you can on the rear one.
Cheers, peter.
P. Tilbury

Peter

I asked the same question back on the 4th April. However, Gerard's video of the spitting shows the canisters removed.

Steve
Steve Gyles

Since posting the video on April 5th I seem to have eliminated the constant Idle missfire/backfires with a fuel additive and carburator cleaner. The backfires from giving the engine gas remain. (Note: with the air filters off the engine idles much higher and I can't seem to lower it. 1200 rpm)

So yesterday I took Steve Devine's advice and added a gas dryer and octane booster. While the missfire/backfire problem remains there has been some improvement. (1) I am able to reach 2500-3500 rpms with some missfiring and sometimes with none. (2) I put the car in gear and released the clutch slowly with my foot on the brake to see if the missfires still occur under load - much improvement on this. I was able to back my car out of the garage and then back into the garage. This past Saturday I could not. I am still not able to drive my car smoothly but this problem does seem to be fuel related.

On April 4th, P N Tipping suggested that I check for hanging, mucked up pistons. I did not try this because I am not sure how to determine whether the piston moves up and down freely?!?!? Should there be zero resistance or should there just be a smooth movement? How does one "clean" the piston and dashpot bore perfectly? It sounds simple but I want to be sure first. Also, P N Tipping mentioned looking down into the jets. What should I hope/expect to see?
gerard hutchinson

To check the pistons in the carb, remove the air cleaners from the carbs, remove the dampers ( the brass nuts on top ) and lift the piston with your finger. Let it go and it should drop with a resounding "clunk" as it hits the bottom. If no "clunk", or the piston "hangs up" then it is sticking. You can try using the lift pins on the carb, if it has them before taking off the air cleaners, but remember the dampers have to be removed first. You may not be able to get the pistons to "clunk" using this method, as the pins only lift the piston a very little bit. Worth a try first before taking the cleaners off.

If both pistons drop with the resounding "clunk" as they hit bottom, they are free and then you don't have to go any further regarding this anyway.

Hope this helps.
CR Tyrell

CR Tyrell:

That does help! In fact I found a web page that described the check procedure in a similar way.

http://www.teglerizer.com/sucarbs/techtip6.htm#TROU
gerard hutchinson

Hi Gerard - Its all in that document your link goes to here. There's a section "Disassembly and assembly of suction piston and suction chamber". I've done it so many times, it seems impossible others haven't been there... Just be careful, one step at a time. Petrol is excellent for cleaning. The pistons can still appear to drop freely, but gritty dirt can be a bit hit and miss, clean it all up and you know where you are and remember to put the spring back on top of the piston! Use the right damper oil too. Thin SAE10 oil is too thin.
You should see fuel 1 - 2mm below the level of both jet's orifices when the pump has just run (and when ignition on) if not the float level valve settings may require adjustment. Should all be in your link.
Pete (P N if you must!)
P N Tipping

Ok Pete,

I performed the check and heard a distinct "clunk" as CR described but I'm not quite satisfed/convinced the mechanism is working optimally. So I want to clean these parts as you suggest but I am not sure what to expect when I start taking things apart. I understand how to clean the plunger rod and oil damper, but what should I expect once I've removed the suction chamber? Is the suction spring difficult put back in place? How many little bits will I have to be careful not to nudge out of alignment? Can I perform this operation with the carburators still attached to the engine?
gerard hutchinson

Heres something to look at:

http://www.youtube.com/watch?v=IfU47Oqq9wA

Twist has a lot of info on line you can look at.
Glad to see you are making some headway! :-)


Steven Devine

Leave the carbs right where they are Gerard..! Have a table or bench handy.
1. Undo and remove the damper on carb 1. Place upside down.
2. Undo and remove the three screws holding the dashpot down
3. Lift off the dashpot. Clean the bore and its mounting face and carb's mounting face. Use lint-free cloth or brush for cleaning.
4. Lift off the spring.
5. Carefully lift up and remove the piston c/w its needle. Tip updside down to drain damper oil. Clean piston. DO NOT BEND NEEDLE !
6. Repeat for carb 2. DO NOT MIX UP PARTS!!!
7. Check fuel levels in the jets - needs a good light. (no matches!!!) Switch on ignition, levels should remain a couple of millimeters below jet top. Ignition off..! If incorrect the float valves will need investigating later.
8. Carefully replace carb 1's needle and piston, aligning its groove with the guide in the carb's body. Add a spot of damper oil to the piston's steel shaft. Drop spring onto the piston, slide the dashpot back over the spring and piston (Clean and dry!) Rotate to align screw holes, refit screws and tighten evenly and not too tight.
9. Lift up piston with your finger and check it drops with a plop.
10. Refill with dashpot oil (SAE20) to below top of piston shaft. Refit damper just over hand tight.
11. Repeat with carb 2.
Probably quicker to do it than to read this...
Good luck. Pete
P N Tipping

Update:

I took apart and cleaned the "suction" portion of both carburetors and then put the bits back together. I also added some 93 octane fuel to my tank. The car started and ran as it has for the last couple days. The engine continues to miss as it accelerates from 500 rpms up to 2000 rpms. But as I slowly rev'd the engine up to 4000 rpms there were no misses. And as I slowly reduced rpms, misses began to occur again at and below 2000 rpms. The car continues to idle between 600-1000 rpms without missing at all.

I also discovered that when I engage the clutch the engine looses a significant amount of power. Rpms dropped from 1000 to 500 when the clutch was engaged and nearly stalled. What causes this loss of power?

Tomorrow I believe I should perform a compression test.

gerard hutchinson

Cleaned


gerard hutchinson

I wish I could upload more than one at a time.


gerard hutchinson

If idle speed drops significantly when clutch is depressed, this commonly means the dual carbs are out of balance, one running rich and the other running lean.

Misfire on low speed acceleration is commonly caused by faulty flex wires inside the distributor, the wires that have to flex every time the vacuum advance operates to rotate the breaker plate.
Barney Gaylord

Barney: Does the flex wire problem happen on both electronic and non-electronic distributors? I have both but currently using the electronic distributor. What is the fix?

Would a bad batch of fuel have a similar effect on out of balance carbs?

Results of compression test:
#1 piston - 110 psi
#2 piston - 118 psi
#3 piston - 112 psi
#4 piston - 110 psi
gerard hutchinson

Those are really low! Did you have the throttle wide open during the test?
Art Pearse

The flex wires are common to any distributor that has vacuum advance. They connect power (coil wire) and ground from the outer housing to the moving plate.

For testing these wires you need to disconnect one end, apply an ohm meter or test light, and wiggle the wire to see if it breaks the connection. Fix for a broken wire is to replace it. These are special wires with very fine multiple strands to be very flexible.
Barney Gaylord

I'm not sure why they are so low. I didn't touch the throttle. I also didn't take out all the spark plugs like john twist suggested. I'm thinking I need to run the test again.

Also, does anyone have a link to a detailed carburetor balancing procedure?
gerard hutchinson

Tune up the casual way:
http://mgaguru.com/mgtech/care/cf120.htm
Barney Gaylord

Gerard, if you didnt touch the throttle, then it will be closed and the intake suction pressure will be vacuum instead of near atmospheric, hence the low compression figures. Try it again with all plugs out (faster cranking) and throttle wide open. Best with a helper.
Art Pearse

Those compression figures almost have to be the result of a measurement error. Either in technique, or a gauge that is not properly calibrated. An engine that has problems will not generally be that consistent across all the cylinders.
Del Rawlins

Del,
I agree my technique must be the problem.

Art,
While I'm certain you are correct I don't understand the significance of the throttle being wide open. However, I will certainly do it your way! I don't have a helper so I have been pulling the starter cable from inside the engine bay. I believe I can pull the throttle cable as well. Would this solve my psi reading error?
gerard hutchinson

If your compression gauge screws in the plug hole, then your other 2 hands can do the job!
If it is a gauge with a rubber plug you need a 3rd hand to hold it on the hole.
The compression reading is the result of compressing what air is induced. If the throttle plate is closed, that air is limited and will cause a vacuum in the intake manifold, some air might reach it backwards from another cylinder intake valve, but again that supply is limited as its not open to the atmosphere. So you are compressing a low pressure gas by a certain ratio and the result will be low.
Art Pearse

I had a similar problem some two years ago -tried everything new 123; checked timing etc- the fault turned out to be the rubber grommet that fits on the banjo holding the front float chamber was perished/distorted and was blocking fuel supply causing hesitation at higher revs
Paul
P D Camp

So... car is running fine again but the orinigal mystery remains unsolved.

It turns out I installed the new distributor without also removing and realigning the distributor drive gear. My timing was/is 180 degrees out and as a result 2 ignition wires were reversed! UGH! This condition was discovered by John Mangles, the MG Doc at All British Car Repair.

I still have no idea why the car started running poorly to begin with but I also do not care! haha!! It is running better than ever now... You guys have been great and have taught me quite a bit about my car!!
gerard hutchinson

This thread was discussed between 24/03/2014 and 21/04/2014

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