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MG MGB Technical - What am I doing wrong with my carbs?

I have a 1969 GT, and one of my jets failed on a cross-country trip. I patched it with some generic o-rings and finished the trip, and installed a new jet when I got home. I also cleaned everything while I was at it. Installed everything, and found that the airflow on the front carb is a lot lower than on the rear. I figured that there must be a vaccuum leak somewhere, and found that the front carb had a loose throttle shaft. Ok, so that's been fixed. Holes in the crankcase breather hoses, so those get replaced too. The problem is, what was once a strong runner now barely goes. I can barely get her started, and she runs pretty rough while warming up. Once the engine gets warmed up its a little better, but the engine still threatens to die at low rpms, and i'm getting pretty frustrated with it. Idle fluctuates between 600 and 1500 rpm for no apparent reason. What should I be looking for here? Any help for a novice would be most appreciated.
Adam Birnbaum

Adam: The SU carb is a great little device, IMHO, and can be tuned to cover a wide range of faults that occur over a cars lifetime, such as worn throttle shafts, air leaks etc. Apparently your car, when it ran well, was tuned to those conditions. Now that you have cured these faults you need to retune.
I would suggest a comprehensive tune-up if one hasn't been done lately, i.e new plugs, points, etc. timing checked, and valves adjusted. Then tackle the carb settings.
Read and reread the procedure in Haynes. Obtain a Uni-syn device if possible. Adjust the mixture to the base setting on each carb (see the manual), and with the throttle shafts disconnected from each other begin the fine tuning. Adjust the mixture by raising or lowering the jet position, adjust the air flow for the same draw, adjust idle speed, and repeat the sequence until the carbs are neither too rich or lean, draw the same volume, and are at the correct idle speed. Use a dwell/tach meter to check the engine RPMs instead of the dash tach if possible.
When you re-install the air cleaners be sure that the holes in the gasket line up with holes in the carb. Sometimes the gasket is carelssly installed, which blocks the air holes and prevents the carb piston from raising as it should.
Andrew Blackley

I think that while you fixed the throttle shaft vacuum
leak on your front carb - you most likely knocked
the two carb settings out-of-even to each other.

You always have to re-synch the carbs to each other whenever the connecting shafts are disconnected or loosened; or when the carb(s)
are taken off of the manifold; or when adjusting
the idle speed, etc.

I'm with Andrew in that, anyone who wants to fiddle
with multiple carbs (of any sort) - must have a
Uni-Syn device (or similar) onhand.

The original (now, vintage collector) Uni-Syn, by
Edelbrock, are the best. I've read that the early
reproductions (Taiwan, etc.) of the Uni-Syn had
problems with the little red floater indicator in the
riser tube, but maybe this had been corrected
since.

I don't trust the 'ol "rubber hose in the ear" trick to
synch the carbs (one end of the hose next to ear
- other end of hose next to carb throttle mouth):

First, you have to listen to, and compare, the noise
coming from each carb mouth and adjust the
carbs until the sounds of the hissing gasses
are even to each other. This takes a certain amount of finesse and experience because you have to hold the hose evenly (at your ear and at the carb mouths)...while switching it back & forth
between the carbs...while fiddling with the throttle
screws. You'll be doing all this careful listening
- over all the clatter from the engine noise, too.

Also, if your carbs are very out of tune, or if there's
something goofed in the ignition or smog stuff
- all it's takes is one backfire, or a spit-back, or a
flaming belch through the carb mouth (...and into
the hose which you are holding next to your ear)
to completely ruin your day.
Daniel Wong

Ok, so I used my Uni-Syn to sync the air flow, and brought both of the jets out to twelve flats. Checked the timing, and it looks pretty good - 14 degrees (give or take one degree). New plugs, valve clearances set, oil change. I'm still suffering from an uneven exhaust note, and starting is still difficult. I've been fiddling with the jet adjustments for a long time now. From what I've read, the mixture is correct if you can raise the piston and have the engine speed up a little and then settle back down. I can _never_ get this to happen, and I'm starting to fear for my sanity. Is there an easier way to check this for anyone who doesn't have their own Colortune? I'm hoping that I'm just not getting the sync right and that there are no bigger problems there. Last time I did this it was so much simpler for some reason - damned thing started up first go, and one flat on each carb had me all set.
Adam Birnbaum

I am a relatively new MGB owner however I have been dealing with SUs for almost 30 years on my Datsuns. The problem with the 12 flats method is the relatively poor machining of the carbs especially when it comes to the position of the jets in relation to the bridge when the mixture nut is fully closed/tightened up. I recommend that you make sure the jets are even with the bridge and then turn the 12 flats. No two British SU carbs seem machined exactly alike and each needs to be referenced individually. I use the highest RPM method to get a decent idle. Keep turning the nuts to obtain the highest RPM. After that is done you'll need to balance the carbs. FWIW, the Jap licensed carbs (Hitachi SUs) are much better than the originals in terms of machining and construction. Flame me but it doesn't matter as the truth is the truth.
Mike MaGee

Have you checked your jet centering? Raising the piston and releasing it should be rewarded by a solid "clunk" as it drops to the end of it's travel. Moss's jet centering tool makes this a snap. It's a massive chunk of steel that will positively center your jets with minimal fuss.
R. L Carleen

Thanks for the suggestions and comments.

RL: My carbs have swinging needles. I was under the impression that carbs with this setup didn't require jet centering, and my pistons "clunk". Am I wrong about this?

Mike: I've heard similar stories about the Japanese produced SU's. Are those carbs at all interchangeable with their British counterparts? I've been told that the Japanese SU's are all-metric (I believe ours are Whitworth?), and thus can't be mated up to, for instance, MGB engines.

I did notice that my carbs are spitting feul. Would this indicate any particular tuning mistake?
Adam Birnbaum

Adam: The later spring loaded needles do not require centering, but they also are know to accelerate wear on the jet. A badly worn jet would be difficult to tune for and would cause rich running even with the jets fully raised.
Actually spewing fuel? I havent experienced this, but would think that a high float level could be a cause, along with worn jets. But you replaced at least one jet, correct?
When you do raise the piston ever so slightly does the engine speed decrease or increase?
Andrew Blackley

One thought. Do you know your dizzy #? Check the proper timing on Paul Hunt's mgb-stuff site or Paul Tegler's teglerizer site. I have 69 Roadster and proper timing for my dizzy/car year combination is 20 degrees btdc. Finally tuned it that way after 3+ years at 14 degrees. Marked improvement.

Also use a vacuum gauge or mity-vac to check your vacuum advance. Mine had a slow leak that would pull a vacuum then it would leak off and reguired me replacing the vacuum unit. Once you get it set up right you can check mechanical and vacuum advance per the charts above....then look elsewhere.

JTB
J.T. Bamford

Adam - you can't centre the jets with swinging needles as the needle moves to centre itself in the jet, wherever that may be. So as long as the pistons clunk the needles and jets are OK. I have also heard that parts from Hitachi 'SUs' are not interchangeable with the pukka item for the reason you mention. The 12 flats down thing is just a starting point for the jets, it doesn't need precise measurement of being flush before you start winding them down. Many people do seem to have problems hearing the rev change as the piston is lifted, and it is very subtle particularly on the HIFs, almost subliminal. An alternative way is to adjust the jet to give the highest idle for a given idle screw setting which should be easier to judge - but the air-flows must be balanced first, of course. Adjusting for max idle is one of the recognised methods in the workshop manual for early cars, for later cars you find that point then weaken until the revs just start to drop then richen only as much as is needed to get the highest idle back again. Another option is to use a vacuum gauge connected to the inlet manifold, but again they are only used to get the highest reading just like a rev counter, nothing more sophisticated. With my two cars I notice a very clear change in idle speed as I change the jets/mixture screws even a flat or two, the lifting pin gives a finer tune. I also back off the adjustment just a smidgen after reaching the ideal point to put it in the middle of the 'lost motion' space.

Paul Hunt

Unfortunately the Jap HS4 SUs won't fit w/o extensive modifications to the linkage and to the way they mount up to the manifold. The Brit SUs mount like / / and the Jap SUs mount like / /. The early Roadsters used SAE threads and the later ones used metric threads. I have 2 pairs of mint Jap SUs that are all polished and I thought it would be a snap to install them but I was wrong :(

One more thing. Make sure all your valve clearances are set correctly, the floats levels are correct and give your car a tuneup prior to messing with your carbs. All of the above effect your carbs.

BTW, your fluctuating idle speed indicates a vacuum leak more often than not. Are your throttle shafts nice and tight or is there play in them. If your throttle shafts are shot you're wasting your time.
Mike MaGee

The throttle shafts appear to be fine. I thought I would need to get them rebored and fit oversized shafts, but Jim at British Racing Green took a look at the carbs and said that almost all of the wear was on the shafts themselves. I've fit new standard sized shafts and there's almost no play in them, so I think I should be ok. I even let the carbs dry out completely and tried (I know, I know) sucking air through them - nothing.

JT - I discussed the backspitting with a friend who does racing engines, and he agreed that the timing might need to be retarded no matter what the book says. I'll check out the site that you mentioned.

Mike - I've done the valves, but I'll redo them and check the float levels as well. Hopefully I can find out what the problem is before I chuck the thing off of a cliff.
Adam Birnbaum

Unless your timing is advanced over 120 degrees, the timing shouldn't be the culprit on the carbs spitting back, Typically, the over-advanced engine will barely turn over because the pistons are forced down against the correct rotation and try to stop the starter. It really sounds as though you have reversed the leads to plugs two and three.
George B.

Adam,

Maybe not related, but do you have the impression that one carb is running very rich? Could be that the air vent of one float bowl is - slightly - blocked causing the fuel pump to pressurize the fuel into the carburettor. When that is the case your car will surely run very, very bad. You can check if all is OK by removing the carb pistons with the ignition on. The should be no fuel coming out of he jets.

The above happened to me this weekend and it looks like the problem you're describing.

Agree with others that the method described with lifting the pistons has for me never resulted in the "running right" situation. Just turning the jet hight until the highest RPM is found works better.

Bert
Bert Otten

Why don't you pull the plugs,that will show you what carb is rich or lean and adjust accordingly. You probably will have to do it more than once till you get it right
Jim Robertson

Adam,

Sounds like you got a ton of good advice, all sound and worth checking. A couple of thoughts. Your initial difficulty occurred on a "cross country" trip. If I understand you correctly, field repairs were made and the car performed well, with the problem now present, following renewal of the "temporary repair part"

I am into some wild guessing here, but are you sure the problem is the carbs? I also had some fun experiences with my 66 as to rough idle, poor running, dificult starting etc... After fooling with the carbs, and as yo say "find a big cliff", I turned to the ignition and specifically the plug wires. On removal of the dizzy cap and an ohmeter test for continuity, ( a solid current path ) I found two of the four leads to be defective. The leads were not the solid wire type and had stretched or worn so they did not provide the needed pathway for spark distribution to the plugs. I changed these over to the solid wire style and have had no further problem. From your address, I would guess the humidity is high there and moisture in any form associated with the high voltage low current spark system would be trouble.

Why would it occur now? Who knows, perhaps the long trip, perhaps in your recent repair activity. and more importantly, its an easy check at no expense absent a failure requiring parts. I have heard on the various threads that carbs, once set up are pretty stable. I agree with that and tend to look to ignition first, then carbs, but I understand why one would look there first given the trip events you had..

Good luck and don't through it off a cliff.
Jim Hanks

Jim,
I guess I should change my location in my profile. The original problem started on my trip from San Francisco to Wilmington, DE. Stayed overnight in a town in western Wyoming, and when I went to start the car the next morning I smelled gasoline. Popped the hood and found that fuel was leaking from one of the float bowls where the jet assembly plugs in. The seals were rotted out, and a new jet assembly wasn't forthcoming, so I improvised using some O-rings and LOTS of sealant spray. That got me all the way from Wyoming with absolutely no further problems. Get the car home, and decide that it might be a good idea to put in a replacement jet assembly. I wanted to fix some vacuum leaks while I was at it, so tore everything down on both carbs etc etc.
Lots of good advice all around. I haven't had a chance to devote more than a few moments at a time to the car in the last several days, but tomorrow is the day that I sit down with a few Sam Smith's Oatmeal Stouts and a printout of this thread. I still think it's the carbs, but new leads will go in just for good measure. I'll post my results here.
Adam Birnbaum

Any possibility there was a flip flop with carb gaskets to spacer/manifold?
glg

Had a similar case when I was rebuilding my B a few years ago. Try as I might I couldn't balance the damn carbs. I finally gave up on the original carbs that I had rebuilt and purchased a new set just to get the car going as we were in a rush to move.

Long story short, as I was installing the new carbs I found that I had inadvertantly left two washers behind the carbs on the intake. This had given me a nice huge leak and there was no way the carbs could work right. It was a wonder the car started at all. New carbs on, balanced in 15 min. and ran like clockwork.

Check for leaks w/ WD-40 around the carbs.
Luigi

Hi Adam and all,

You are not alone on this problem Adam,

I have recently bought a 1973 B GT which is fitted with an American spec cylinder head and am experiencing the same problems as you.

I have just bought some almost new HS4 carbs and thought they would be the answer to all my problems.

This was not the case and the problem is still evident.

Initially the car was holding back and spitting back through the carbs on acceleration giving the impression of a very weak mixture but the exhaust is very black and the plugs keep sooting up.

Since fitting the new carbs with perfect spindles the problem has got much worse.

Could there be a leak on the inlet manifold ? I will check that next !!

At the moment the car is fitted with AAU needles which were the right needles for an american spec head with the air pump rail system, although this has now been removed.

Does anyone know what needles and jets I should be fitting ??

Is there a correct way of fitting the jet feed tube into the bottom of the float chamber ??

There could be a couple of B's heading over the cliff if we can't solve this one ...

Cheers Steve

Stephen Farr

Stephen, I've fitted AAE needles for my 1969 and haven't had problems (before now). I understand that these were stock for several other years as well, including (from what I remember) 1973 and 1974.

So, today new leads went on, replaced one bent metering needle and noticed that the swinging setup wasn't biased in the correct direction, so I fixed this on both carbs. I also realised that I've been setting my valve clearances to the wrong spec, when for my year they're supposed to be 15 thou cold. Ok, done and done. I also took the opportunity as long as the pistons were out again to get both jets exactly flush with the bridge before bringing them down by 12 flats each - I think I was way too lean before, and this might explain why opening the throttle a little bit resulted in dropped rpms instead of what I'd expect.

The car is much better. The low end is back, driving it doesn't mean fearing stalls, etc. There are still problems - the front carb still isn't drawing as much air as the rear. If I tighten the throttle screw on the front to balance the air flow, the engine idles at 2000 rpm even with the rear throttle screw completely disengaged. I'm guessing that this means that there's still a vacuum leak there that I haven't found. I'm suspecting the valve seats, and a compression test will be the first order of business tomorrow. I think that was enough for one day!
Adam Birnbaum

I had a post on this BBS a couple of weeks ago with almost the same problems. Turned our that one of the butterflys was not completely seated in the bore. Hence the problem (impossibility) getting them syncronized and tuned. Hold your carb to the light and see if you can see any through the bore. You must hold the dashpot open while looking. There should be NO LIGHT through the butterfly (they are directional too). After that correction all they needed was a little adjusting and she runs like a top now.I have HIFs but I assume the same thing could happen to any carb type.

Just a thought,
Mark
73B "little purple car"
Mark Thomas

Adam; Have you checked the end plugs in the intake manifold for possible vacuum leaks. They sometimes will blow out when an engine backfires, one could be loose and cause a vacuum leak. Good luck, Clifton
Clifton Gordon

I recently overhauled a customers HS2's. Both throttle plate had not been correctly seated. All I had to do was loosen the screws, put the plate in the correct place, and tighten them down.
Idle is perfect. Nice to find a problem so quickly.
Safety Fast
Dwight
Dwight McCullough

Adam- Stuck open jet?????
vem myers

This thread was discussed between 16/09/2003 and 25/09/2003

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