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Self healing vacuum leak???

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Brosky

Brosky

Great Pumpkin
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Today was finally a play day with the TR6 carbs. Here's how it went. Is the old saying something like, "Idle hands are the Devil's workshop"? I had some time and made some interesting discoveries today. First I took all of my carb tops off and found that I have B1AF needles in all three. OK, now what?

In trying to identify them, I notice that the front and center carb numbers are barley readable, insofar as the downward movement of the needle, but the rear is completely clear with space above. OK again....

So I put everything back together and decide to adjust the needles, starting from scratch. I get my trusty two piece adjustment tool out, so as to not damage the diaphragms and turn each adjuster fully clockwise or bottomed out full rich. I back each off 2 1/2 turns counter-clockwise and start from there. Start it up and let it warm up and the stink is awful. Way too lean. Shut down and back each one off by 1/2 turn clockwise. OK, smoother idle no stink, road test time. After driving awhile I notice that the AF is about 10.9 to 11.1 at WOT at 3,500 RPM to red line. OK, getting better.

So now I start watching the meter at the 2,500 - 3,000 or high speed cruising range and I'm in the 13.5-14.2 range when on throttle. If I let off, I get a 15-16 reading on deceleration. Not bad, so I get it home and get ready to back the idle down, as it went up to about 11-1200 RPM after richening the mixture. I set it down to 850 and while I'm just cleaning up tools the car slowly starts to idle very smoothly. Just a purr, instead of sounding like it's got a hot cam as usual. I look at the gauge and I'm at 12.3 to 13.5 at idle. What the heck??? Where did the 15-16 go? I don't know, but I hope that it stays there. I had to do something for my wife, so I'm done for the night, but that's where I'm at right now.

Very weird, indeed. I'm sure that I have (had?) a vacuum leak, but where????? And why did it seal itself up after driving for a half an hour? Back to Sears tomorrow for a new vacuum gauge since the last clown to borrow never returned it and has moved away. More testing to do this week.

BTW, I went over that engine last Monday night. both hot and cold during warm-up and could not find a vacuum leak anywhere. I used half a can of carb cleaner going over every joint and fitting to try to find one.
 
Paul - from my *very* sparse realm of experience ... could it be a sticking throttle linkage? I've played that game many times with TeeYah: everything stable 'til it sits at idle - then a sudden idle speed (and mixture) change.

Tom
 
Tom,

That certainly has possibilities, but I did double check the throttles to make sure that they were opening AND closing completely, when I installed the Ratco cable kit. That was several months ago.

However, that doesn't mean that it can't be happening because I can't find the leak anywhere else and unless it's completely gone the next time that I run the car (doubtful), it's still hiding somewhere.

Thank you for reminding me to go back to a basic solution for this problem. After all of the GM carbs that I've seen do this, you'd think that I would have come up with that as part of my double check process, but I didn't.

Same problem, different set of eyes, different solution.
 
I agree,

I'd focus on the linkage.

Possible vacuum to the booster, but when the hose/valve/diaphragm start leaking the leak is usually very evident.
 
Paul - quick follow-up on the linkage. I replaced all my linkage from the firewall pivot forward. Of course, the problem is *before* the pivot - at the bushing where the linkage connects through the firewall (real PITA to work on).

Don't know the TR6 configuration, but see if you can reach down and shake the throttle linkage right at the firewall. That's where mine is binding. It holds for a few seconds, then releases and moves the linkage just a hair.

Tom
 
Not meaning to steal Pauls thread,

Tom,
If you do decide on replacing the throttle shaft bearings.

Also check the throttle shaft where it goes through the gas pedal throttle linkage arm. That's the arm with the return spring on it on the carb side mounted to the firewall. The pin that goes through the arm and attaches the arm to the gas pedal throttle shaft sometimes wears the inner shaft hole oblong but the hole in the arm still looks good. While having someone slightly depress the gas pedal you can see movement in the shaft but the arm stays stationary for a moment until the pin to inner shaft slack is taken up. If you notice this movement the fix is to remove the existing pin and drill out the holes slightly larger to accept a slightly larger pin.
 
I started the post on the replacement McMaster bushings for the TR6 firewall, so mine are done. That part is as tight fitting as possible. I'm going to run it until hot and loosen the flexible connectors between each carb. I have to do it to complete the carb adjustment anyhow, so maybe I'll see something binding while I'm there.

It won't be much as I have no sticking in the throttle causing the idle to ever stay high. I'm sure that something will show up when I have all of this apart.
 
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