SU carb. problem. Help,Help,Help!

[PAUL161]

72 MGB. I don't want to drag this out to far but I think you should know a little about the engine to help steer you in the right direction. The engine only had 500 miles on it when I put it to bed 15 years ago. Pulling it from the car I replaced all the seals, changed the oil,and after reinstalling in the car I fired it up by using a weber setup as I knew the weber was perfect. The engine fired right up and after a short time it ran great! Smooth as silk. Here comes the kicker. I rebuilt the SUs to factory specs using kits. New jets,needles(AAUs)Floats set to specs,etc,etc. After installing the SUs,and filling with fuel, the engine wouldn't start.I had to pull out the choke all the way just to get some life out of the engine.I had to drop the jets down to one turn from it's full limit to get the engine to run. Is this jet setting normal? Also while the engine is running and not smoothly,I removed the oil filler cap to look at the valve lifters to make sure they were getting oil pressure. (They were). But when I removed the cap,the engine dropped RPMs rapidly. Replacing the cap and the Rs came up. There is strong vaccume in the crank case. Is this normal? In my many years of working on everything from six carb. racing engines to family cars, I have never had a crank case with this much vaccume in it. For a test, I pulled the crank case vent hoses from the carbs, plugged the pipes. And the engine ran a little smoother but not near the way it should. I hope that there are some of you guys that might have an answer to this problem. I'm open to all ideas. Even though It runs great with the weber, I want the SUs on the car for originality. HELP! PJ /ubbthreads/images/graemlins/confused.gif

 

[mgbmedic]

15 years? old gas?, poor performing fuel pump?, etc... I'd troubleshoot at the beginning, try reinstalling the weber and if it runs great, then the problem is the S.U.s. If not, i'd look elsewhere. Did you sync the carbs?

 

[Banjo]

Something is not hooked up right. Any way you could post some pics? There should be about nutral pressure in the crank. It almost sounds like there is a major vacuume leak in the carbs. The crank vent hose is supposed to route to the air cleaners. Any chance it's plugged into the manifold, Possibly where the port for the brake booster is? Just some food for thought. Good luck.

 

[PAUL161]

The fuel tank was cleaned and the lines were purged. The fuel pump has 3.5 to 4 lbs pressure. This car is a total restoration.The car was totaly striped and everything was cleaned and checked before reassembly. I know it has something to do with the SUs but I am stumped. Thanks for the info. If you think of anything else, Please let me know.PJ

 

[PAUL161]

Thanks Banjo, Now I know that there isn't supposed to be a high vaccume in the crank case. I'll check the manafold and the carbs for a vaccume leak. If I find the problem, I'll let you know. Thanks again.PJ

 

[Tom]

Howdy Paul

I don't think this is the only problem but it could be a contributor:

With your air filters there are a pair of holes in the back plates that line up with holes either side of the main inlet on the Carbs.

While some back plates have holes above and below the mounting holes (so it doesn't matter which way up the filters are mounted) some don't, this would make the car run roughish, particularly if one air filter is correctly mounted and the other isn't.

Second possability is make sure you have damper oil in the dashpots (I'm sure you do but...)

When setting the carbs up on my 70 B I normally take the air filters off and wind the jets up until the top of the jet is flush with the cast in bridge of the SU inlet, I find a feeler guage helps for this. Then I mark the nuts (both mine have small hacksaw marks on the jet adjusters when the nuts are fully wound up) and wind down 1.5 turns or about 8-9 flats on both. Depending on the idle I will wind each up or down by the same amount to adjust.

You can heck if they are reasonably balanced by three tests:

1) There is an actuator under each of the carburettors which when pushed upwards will manually lift the piston.
The carbs are reasonably balanced if when lifted in turn each has the same effect on the idle once the car is warmed up (ie both nearly stall the car). if doing this test on one carb has less effect than on the other then the one which has the most effect is too rich relative to the other one

2) If you unscrew the dashpot pistons and leave them in place when you rev the car they should lift and drop by the same amount. As before if one reacts more than the other then that one is richer (and therefore doing more of the work)

3) Finally there you can set by ear, I find this method somewhat harder than the others, partially because you have to do it with the filters off which means further adjustment when they are reinstalled but mainly because I find it hard to quantify many old hands swear by this method. Effectively what you do is check the suction noise of each carb using a length of large diameter hose, if the sound is similar in both then you are close to being balanced.

Finally with the idle ensure both idling screws are set the same, there is a rigid linkage between the carbs but there is a little play in it at idle settings so both idle screws need to be set.

Not sure if this one will help but the other possability is if the crankcase breathers are not setup right, I suspect that US cars are different on this but on UK and Australian spec cars of that age there is a hose off the front crankcase cover which has a hose which should split in two and feed back to a fitting cast into the body of each carburettor. If this isn't fitted it will cause rough running.

Apart from that check the floats and float needles are working properly and that there are new filters fitted (which with a rebuild shouldn't be a problem)

SUs are simple beasts and there isn't much that can go wrong with them when they are set up correctly.

good luck with it!!

regards

 

[PAUL161]

Thanks for the info. Tom. It seems with the information I'm getting from everyone I am making progress. I did find out that one of the valves in one throttle plate was leaking. I corrected this problem and it made a great improvement on idling. I'm waiting for a syncronizer that I ordered to come through the mail. Hopefully it will come today. It's raining here today and the temps about 70F. I guess I'll have to take the wife shopping.The air filters are mounted properly, as I checked just checked them. Thanks again for all the info. PJ

 

[Roger]

Here's my elementary, First principles remote diagnosis.

A.You have a weak mixture. (Hence more choke setting)
B.It weakens more when you remove the oil filler cap.

I deduce you're breathing from inside the motor (hence the vacuum)so your breather is wrongly coupled to the manifold (full depression) rather that atmosphere side of your carbs. Fix that, then tune again.

 

[19MGA60]

I think the fuel pressure is to high. I thought it needed to be lower than that?

Tom

 

[PAUL161]

Tom, You might be right about the fuel pressure being to high. The fuel pump is an after market pump brand X. It was on the car when I bought it years ago. I have an old SU pump but it died. I think it would be more beneficial for me to buy a new SU pump from Moss than try an rebuild the old one I have. What do you think? PJ

 

[DrEntropy]

The pressure should be no more than 4 or 5 lb/psi. Most parts stores sell an aftermarket fuel pressure regulator that works well. If you're going for originality, the SU pump would be the way. As for the vacuum in the crankcase: Is the diaphragm in the "closed circuit breathing valve" intact? Sounds as if there's manifold vacuum straight to the crankcase and failure or misconfiguration of that "widget" is the only thing able to do that.

 

[Bruce74B]

I would try a pressure regulator from the local auto parts store before spending the money for a new pump if I was you and you think the pressure is too high. An adjustable type will only be a few bucks and might be a quick easy fix to the problem...worth a try in my book /ubbthreads/images/graemlins/cool.gif

Bruce /ubbthreads/images/graemlins/cheers.gif

 

[Guest]

I have a '72 and my fuel pump pressure is around 2 lbs and 4 or 5 is too much and will bypass the needles and hold the floats open. Also, when I adjust my carbs to specs I also end up lean in the mixture so I take the adjustments towards rich a very small adjustment at a time until it runs right by driving it. The adjustments are extremely sensitive and a little does a lot. With it set this way, I don't have to use my choke at all in the summer and it runs great.
Bob