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Car doesn't idle well

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drooartz

drooartz

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I'm having trouble with the idle on my MGB. I have some suspicions, but wanted to get your opinions.

So when I first start the car up, it doesn't want to idle (RPM drops). After driving for a while, it will usually idle okay, somewhere between 1500 and 1000 RPM. But sometimes as well it will do the same thing and the RPM will drop.

A corollary is that pulling away from a stop the car will bog down sometimes, seems to clear up with a couple stabs of the throttle.

My suspicion is that it's still really rich (I'm at nearly 6000' elevation). Current needles are richer than stock (#6 as installed by PO when the car was in Canada).

What else should I look at?
 
What's the best way to try and find a vacuum leak?
 
A piece of 1/4" fuel line a couple feet long and yer ear. Go around the shafts and carb mountings, listen for an increase in the "hiss". A spray-bottle with water in it and mist spray around the mountings and shafts. If you notice a change in RPM while spritzin' a spot, you've likely found a leak.
 
drooartz said:
What's the best way to try and find a vacuum leak?
Click to expand...

Start it. Let it idle.

Spray WD-40 near any areas where there may be a leak. I'd agree about checking the manifold to head gasket area. COmmon problem.
Otherwise, check carb to manifold gaskets, PCV connection, float needle/level....there are other areas but those are common.
 
What color is the inside of the tailpipe. Black is rich , gray is OK. Check plugs for over wet or sooty.
 
Tailpipe inside is black.

I'll try and get out tonight and play with the WD40 to see if I can spot any leaks. Need to pick up a couple of other needle options for the carbs as well to get them better tuned for my altitude.
 
Don't forget you can make some mixture change by adjusting the jet nuts.

Turn all the way "up" and then back down 12 flats (two turns). This is the basic setup.
To lean, turn both nuts up same number of flats.

Be sure you have good spark and ignition.
50% of all carb problems are ignition. :wink:
 
aeronca65t said:
Be sure you have good spark and ignition.
50% of all carb problems are ignition. wink
Click to expand...
Good point, Nial.

After it's dark outside, Drew, run the thing with the hood up and watch for fireworks around the dizzy and plugs/wires. In addition to your spray testing.
 
Just bought a set of new solid butterflys for the HIFs on the wifes 72. Never would idle down where I wanted it to. Has the valves in the butterflys in original carbs which have to go. Jeffs rebuilt dizzy is already on it. Too hot in the barn to work on it. Weather has to cool somewhat. Hit 112 a few days ago! PJ
 
Haven't had a chance to work on this yet, too much else going on.

My carbs should have the solid butterflies (UK spec). I need to check everything, but it's probably time to have the carbs gone over by one of the expert shops. No idea when they were last looked at, so there's probably some slop there. Dizzy too, time to send it to Jeff for him to work his magic.
 
Drew, the 72 ran terrible after it was rebuilt and no matter what I did, there was not much improvement, until, I had Jeff rebuild the dizzy as original with points and the improvement was tremendious! He set it up for normal driving. Money well spent. PJ
 
Drew checking for vacuum leak as described here is good idea. I've watch John Twist several times at the big MG gatherings tune people SU equipped cars, and the one he does most is with the air filters removed, and out of the way, is lift the piston with a straight screwdriver with the engine at idle, if the rpms raise, then it's too rich and he adjust the jet height nut lean until he can lift the piston and get no rpm change, you hardly ever hear anyone talk about this on the forums, but it great way to set fuel mixture on street car with SU carbs, and simple enough to do, as no special tools are required.
 
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