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MG MGB Technical - Inability to set low idle speed

I am hoping someone can give me some ideas as to what could be the cause of an inability to set an idle speed below 1000 rpm on an 80 MGB set up as follows:

Rebuilt 1800 with 289 degree lift elgin cam.
Dual HIF's (professionally rebuilt)
Air pump
Gulp valve (less than year old, Moss replacement)
Original 1980 electronic ignition
Vaccum advance connected off of intake manifold.
K&N conical air filters

Car has low CO level (283 ppm) but high hydrocarbons (883 ppm). Normally runs very well but now is running a little poorly since I have been trying to get it to pass Missouri emmissions test. All normal areas have been checked (ie plugs, new cap and rotor, timing, etc.)

Idle does seem to be somewhat temperature related. When cool the car will idle somewhat lower but the idle increases as the car warms up.

Thanks for any and all advice.
Rich

Rich

the high hco emissions are probably consistent with your long period cam - big overlap = big hco emissions

have u tried disconnecting the vacuum and seeing what happens - a mismatch between a distributor vacuum capsule requiring ported vacuum but attached to manifold may be one reason for too high an idle - just bcos dist is original does not mean capsule is standard

can't comment otherwise as not familiar with yr emission equipment

Chris
chris

It is normal for the idle to increase during warmup as the system gets more efficient, which is why all adjustments should be made when fully up to temp. You should be able to reduce the idle to nothing with the idle screws. If you cannot either one or both of the butterflies is not seating properly, or the carbs are unbalanced which means that one is fully closed but the linkage is then holding the other open. Also check the fast idle screws are clear of the cams. Slacken the clamps on the interconnecting rods so the carbs are independant, back off both idle screws and the engine should stop. If it doesn't the problem must be with the butterflies, listen to each intake to see which is sucking air. If they have poppet valves one or both may not be seating properly. When you have found and fixed the problem do a full setup of airflow and mixture balance. Manifold vacuum will always give a higher idle when connected to when disconnected, which is why you have to disconnect it and temporarily increase the idle screws to set the timing, then reconnect the vacuum and reset the idle screws back to normal afterwards.
Paul Hunt

Getting a bit off the original subject, I've heard that setting your valves looser than spec helps with HC for an emissions test. You are effectively reducing the overlap and lift. There may be some threads in the archives on this.

You need to get your idle under control first; I would follow Paul's thoughts there.
Tom G

Rich, get a Uni-syn to help figure out which is at fault. They're only about $30 IIRC. Should help immensely, and then follow Paul Hunt's excellent advice.
Paul K

I had an idle problem with my HIF's too. In my case it was the butterflies not seating properly. I could tell because when I applied extra pressure to the throttle linkage in the closing direction, I felt it close a little more but with resistance, and the idle dropped to a nearly normal level. The butterflies had a slight amount of adjustment in their positioning and that was enough to cause them to bind up when not centered properly. I loosened the securing screws just enough to let the butterflies moveed in the slots and then rotated them to the "closed" position so they centered themselves in the passage. Then I tightened the securing screws being careful not to move the butterflies and double checked the fit.
Craig Cootsona

A classic problem for the inability to obtain consistent idle speed is worn throttle shafts/bushes. Check with your rebuilder to see if these were, in fact, replaced during the carb rebuild.
Andrew Blackley

Further to Paul Hunt's comments: I found I had to pussh down on the throttle spindle to reduce the idle. On examination, I found that one carb butterfly was not quite centred - recentering fixed the problem.

BarryQ
Barry Quartermaine

Thanks for all the good info. To recap the throttle shafts were replaced in the rebuild. I am wondering if there is any way the gulp valve can cause this high idle RPM problem. When I installed the Moss replacement gulp valve I noticed that the the the RPMs did not go down immediately when you would lift off the gas. It was like something was keeping it open for a few seconds.

In any case I will see what happens when I disconnect the gulp valve. If that doesn't reveal the problem I will remove the carbs again (I am getting fairly good at that through so much practice) and check on the centering of the butterfly valves.

I will let you all know what I find out in order to help anyone else having this same type of problem.
Rich

Got a vacuum guage?

Normal vac at idle, with a stock cam, should = 21hg, with a steady needle indicator.

With a typical street cam, it should be around 16-18hg, with a slightly quivering needle indicator.

Anything lower than, say, 15hg...and I would snoop around for hose or gasket leaks, etc.

If you can, swap the gulp valve for another one.

Also check that one-way valve that is on the air injector manifold. You can perform a quick
"shadetree" test by pulling it off and blowing through it using your mouth. Air should go
only in one direction.
Daniel Wong

The other thing that can hold the butterflies open is if the throttle pedal stop is maladjusted. There should be a bit of free play in the cable. Nothing else but the carbs can feed fuel into the engine.
Paul Hunt

This thread was discussed between 13/12/2002 and 17/12/2002

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