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TR2/3/3A vacuum leaks? How to check

PatGalvin

Jedi Warrior
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Hey all

In tuning my British Mistress, I am having some trouble keeping the idle dialed in. My goal is about 900 rpm, but it seems to just move around between 800 and around 1100 rpm, depending on her mood.
I have a semistock engine with a mild road cam (BPNW). Compression is increased from stock by using 87.5mm pistons/sleeves. Dizzy by Jeff at Advanced, with Pertronix. Last checked, compression was around 165 plus or minus 5 psi per cylinder. Engine has less than 1,000 miles on it. Carbs rebuilt by Paltech a few years ago. Valves set at appropriate clearances. Timing per Advanced Distributor's recc's. I've been worried about a vacuum leak on the distributor vacuum advance tube (as possible cause for erratic idle). Is there any way to either check or diagnose a vacuum leak on this line? Or do you guys have other tuning suggestions?

Thanks. She is running very strong, I think. And sounds great. This idle thing is bugging me.

And, of course, when my Spal fan kicks on, the idle drops a couple hundred RPM, due to alternator load. But that's for another thread.....

Pat
 

TexasKnucklehead

Jedi Knight
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I doubt that tiny vacuum advance line could leak enough to cause erratic idling. Be sure both throttle plates close all the way to the stop freely. It seems that the air moving at idle is enough to keep the throttle discs open a little. A short throttle blip usually gets them to close better. If anything is dragging or out of line, it may prevent normal idle. The biggest issue I had to resolve before my idle speed settled out, was the springs in the mechanical advance of the distributor. A little movement of the advance at low RPM will effect the RPM, so be sure the distributor plate/advance can move freely -it's supposed to get a drop of oil regularly.

If you suspect a vacuum leak, get a foot or two of hose and stick one end in your ear, and move the other end around the carbs and intake, you should be able to hear any significant air leaks. -If your ear is good enough, you can balance the carbs that way.

If you can't get a steady idle, try driving it around a couple thousand miles while not idling long enough to notice.
 

charleyf

Jedi Knight
Bronze
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My mechanic friend uses a can of carb cleaner with a fine spray tip( the long plastic tip). With the engine running spray the cleaner around the area you suspect a vacuum leak if the engine speeds up you know that the cleaner is getting into the carb via a vacuum leak. If it does not affect the engine speed --no leak.
Charley
 

M_Pied_Lourd

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My mechanic friend uses a can of carb cleaner with a fine spray tip( the long plastic tip). With the engine running spray the cleaner around the area you suspect a vacuum leak if the engine speeds up you know that the cleaner is getting into the carb via a vacuum leak. If it does not affect the engine speed --no leak.
Charley

I use a propane torch....unlit of course....:smile: but starter fluid or carb cleaner will work too. Pick your poison!
 

poolboy

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If the appropriate nipple on the carb is used for the Vacuum Advance module on your distributor...there would not be any vacuum at that nipple when the engine was at idle speed.
A jump of 300 rpms sounds like it might be related to a problem with a Vacuum Retard or with a Throttle Bypass Valve.
But having said that I admit I don't know which carbs or which distributor you have, so none of that may apply.
 
OP
PatGalvin

PatGalvin

Jedi Warrior
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59 TR3A
stock dual SUs. no throttle bypass valve on these old boys.
No vacuum retard either.

Pat
 

bnw

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Make sure you are using the proper (not stock) valve clearances for your cam. A quick call to BPNW will verify.
 

Brinkerhoff

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I agree with bnw, first thing is make sure the valve clearances are proper so the cylinders can pump air evenly, then adjust the carbs. Make sure the fast idle , choke , and throttle linkages allow the easy return to idle .
 
OP
PatGalvin

PatGalvin

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Will implement some of your good advice and let you know how it all turns out. Thanks for tips.

Pat
 

karls59tr

Darth Vader
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I've been told that when you run a non stock cam you you get a "lumpy" idle. I have mild street cam in my 3 and find that 900 rpm is too low for a steady idle.
 
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