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MG MGB Technical - Idle problem

Please accept my apologu for cross posting from another BBS

I have had my B for 2 years now and it is a real joy to drive. The previous owner had the engine completely redone, bored over I think 0.030 and about everything else in the engine bay was replaced new or rebuilt. It is a very strong driver and handles really well. I think I have put about 2000 miles on it during this time.

My question concerns idle, what is normal and what is abnormal and needs to be addressed.

Engine turns over first time, within seconds the oil pressure jumps right up to between 70-75. Engine cold and the rpm's a steady 900. I have noticed as soon as the temp needle begins to move, the rpms' start to rise. After a couple of mins of warmup and ready to go, the rpm's are up to about 1200. Down the road I go. Should I need to stop at lights etc, the rpm's are between 1200 and 1400.

Is this normal and does it represent a problem? Apart from this everything else is normal. I may have an issue with the choke cable as it does not lock into place. One has to hold the choke with one hand and turn the key with the other!.. Is it relatively simple to replace even with someone like me with relatively little mechanical skills! ?

When I bought my B my friends thought I was daft to take on such knowing me. Guess I must be trying to re-live my university days when I live in England. My friends had a 100/6, a Sunbean Alpine, Spitefire and not to forget the Fog-eye'd Sprite. What a joy driving over Ilkely Moore in the pouring rain with a leaking soft top and windscreen wipers not working LOL

Cheers

Gerry
GG Ginty

You might take a look at the throttle cable to see if it is binding. I had a similar problem. I couldn't get it to race in the garage but on the road the cable would stick. When I would try to figure out what was wrong I would manually work the throttle and it would snap back as designed. When I put my heavy foot on it the inner cable would move back into the sheath and get stuck there.

Hope this helps,

Jim
JMcHugh

If the throttle and choke cables are in good working order you may need to make some adjustments to the carbs. Adjust the idle stop screws on the carbs to get the correct idle when the engine is hot. Then adjust the set screws on the fast idle cams to give the proper idle speed when cold.
John H

Gerry
It may just be the idle screws on the carburettors that need adjusting a very easy job. The idle should be around 750rpm, 1200 to 1400rpm is way too high.
Rob
R J Collier

Thanks.

I'm assuming that all one needs to do is let the car warm up, turn it off and then lower the idle by turning the screw(the one nearest the carb cylinder), say a quarter of a turn anti-clock wise? and to repeat with the other carb?.

Cheers Gerry and Wigan!

GG Ginty

Gerry
Keep the engine running whilst adjusting the screws. The idle screws are verticle and the fast idle are inclined.
You may not need to adjust the fast idle as these only contact the cams when the choke is out.Fast idle screws should be set to give about 1500 rpm after the car is started and the choke is around halfway out.
Rob (10 miles fron Wigan!)
R J Collier

Gerry

What year B? Are you using the choke to start? Assuming that you have a manual choke it sounds like the fast idle setting is too high and as the engine warms up the speed increases. The HIF-4 SU (72-74) has both low and high speed idle adjusting screws.

HTH

Larry

Larry Hallanger

Larry,

It is a 74. How can one tell if I have HIF-4 SU's?
GG Ginty

Dual carbs, float chamber integrated into the carb body. Basically, each carb is a one-piece unit.

R.
Rick Stevens

There are a lot of things that can cause a too-high idle, and the idle and fast idle adjustment screws are only two of them! Others can be insufficient slack in the throttle or choke cables, incorrectly set-up interconnecting levers between the carbs, throttle butterflys not seating properly, butterfly poppet valves sticking open, or badly worn throttle spindle and bushings. Vacuum leak is often quoted as a cause, and whilst it does cause a higher idle *for a given idle screw setting* you should always be able to slow the engine to stalling point by backing off the idle screws alone. If you can't then you have one of the other problems. 1/4 turn is a huge amount for idle screws.

It isn't clear from your post whether the idle speeds you quote are with the choke out or in. 1200-1400 would be normal for the fast idle i.e. choke (assuming adjusted correctly) pulled 1/4" with a warm engine. It is also normal for the revs to rise as the engine warms, simply because it makes better use of the fuel. However with both HSs on my roadster and HIFs on my V8 I find the latter gives an uncomfortably high fast-idle if I use the factory settings, so I have mine back a bit from them.

It's also bad practice to let the engine warm up by idling, you should drive off immediately and use minimum choke. If you only need the choke for starting and can immediately let it go and the engine doesn't stall, theneither you have very warm ambient temperature or the mixture is way too rich. All cars are different, my roadster starts best with full choke, pushed half way back immediately it fires, then gradually the rest of the way as the temp gauge moves up towards N. The V8 needs full choke to start and for the first few seconds, gradually reducing as the temp gauge moves up, driving the cars in both cases. Only use gentle throttle while it *is* warming up, and that way you will need less choke which uses less fuel and less chance of washing oil off the bores which causes rapid wear, as well as protecting the engine from high temperature gradients.
Paul Hunt 2

Paul,

Thanks for your detailed reply. The 1200-1400 rpm I quoted was with the choke in. I normally use full choke all the way out while turning the key and pressing a couple of times on the gas peddle. I hold the choke out until the engine fires and let the choke back in slowly. If I let the choke go as soon as the engine fires, the engine usually stalls. So I hold the choke out for a few seconds, put the car into reverse and back out of the garage. I usually let it warm up a little while before driving off as it is sluggish for the first minute or so and then after that the car runs very strongly.
GG Ginty

Pressing on the gas pedal while cranking will have no positive effects as the SU carbs do not have accelerator pumps to provide extra fuel on taking off. Ray
RAY

This thread was discussed between 29/03/2007 and 02/04/2007

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