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MGB Air leaks and fast idle speed

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wkilleffer

wkilleffer

Jedi Knight
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Good evening,

I have a question about the fast idle speed on my 1974 MGB. It's equipped with an engine from around '71 or '72 with HS4 carbs.

The choke has always seemed a little tricky. I'll get it adjusted to the point that the car is idling around 1500RPM when cold with the choke pulled. I'll start driving and the engine will start to warm up a bit, though the pointer on the temp gauge will very much be at the left side of the gauge.

As the temp starts to creep up, the idle speed will rise as well.

I've been told that a rising fast idle speed is indicative of an intake-related air leak.

Is this true? If so, what's a good first step to take in finding and curing the leak?

Thank you,
-William
 
A good "low tech" method is to use a length of surgical tubing (or other hose of similar I.D.) and with one end near an ear, move the other end around all the gasketed intake areas. DO NOT stuff the thing into an ear.

My British pal calls it a "listenin' pipe."
 
DrEntropy said:
A good "low tech" method is to use a length of surgical tubing (or other hose of similar I.D.) and with one end near an ear, move the other end around all the gasketed intake areas. DO NOT stuff the thing into an ear.

My British pal calls it a "listenin' pipe."
Click to expand...

Might suck the brains right out of your head?
 
Can also use brake cleaner. When the leak is found, the idle will rise. Have used the pipe method for years, as well.
good luck.
 
But I was right in thinking that this issue is likely to be an air leak?
 
You can check for a vacuum leak easily enough, but I don't think you have a problem. It's normal for the idle speed to increase as the engine warms up. IMHO, a 1500 RPM cold idle is too high. Try setting it closer to 1000 RPM and see what happens...
 
:iagree: And I believe a vacuum (air) leak would show itself as a rough idle that would smooth out and increase when the mixture is richened, as when spraying fuel of some sort (like brake cleaner as suggested above) at the leak's source.

Let us know what you discover.
 
Joe Reed said:
You can check for a vacuum leak easily enough, but I don't think you have a problem. It's normal for the idle speed to increase as the engine warms up. IMHO, a 1500 RPM cold idle is too high. Try setting it closer to 1000 RPM and see what happens...
Click to expand...

And this just jogged my memory. It took me a couple of years to realize that I needed to set the idel when the engine was hot - not cold. Then the choke could take of cold starts. So now, it idles bang on at 800 rpm - only when warmed up.
 
:encouragement: Easy fixes are great! I once had a memory...I just can't remember where I put it...
 
Lol. Good stuff.
 
Joe Reed said:
You can check for a vacuum leak easily enough, but I don't think you have a problem. It's normal for the idle speed to increase as the engine warms up. IMHO, a 1500 RPM cold idle is too high. Try setting it closer to 1000 RPM and see what happens...
Click to expand...

Problem is that going too much lower than maybe 1200 is tough. 1200 almost seems the lowest I could go without a bunch of rich-running chugging. I know the mixture is going to be rich, but where do you draw the line with the choke?

Adjusting the choke is, frankly, a pain compared to adjusting the regular idle and mixture.

Where on the B's tach would you estimate 800RPM to be?
 
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