When is a vacuum leak not a vacuum leak?

[Seth_O]

I have been working to get my 60 running. I pulled the manifolds and added a heat riser block-off plate and had the faces machined flat. Spot face didnt appear to be needed. I rebuilt my carb following the pinhead videos.

I reassembled with a remflex following their instructions on torque, in 4 passes (snug, 10lbs, 15lbs, 20lbs, with anti-sieze.)

Here is the result with a slight choke and about 10* advanced:



I am confident it's not a manifold leak as there is no reaction to spraying carb cleaner.

I does smell really lean, and I found that when I dial the IM screw on the carb all the way in to lightly bottomed out I get no change. Not sure if that could be related?

 

[g-man]

Maybe a vacuum leak inside the carb. Do you have a good carb you can try for reference? Easy to miss something on a carb rebuild.

You could go thru the steps in the FSM and do it over.

 

[2mountainfish]

Maybe a valve adjustment.

 

[Seth_O]

Maybe a valve adjustment.

Thought about that, compression test shows 140-142 across all 6. I think that indicates well-adjusted valves?

 

[Seth_O]

Maybe a vacuum leak inside the carb. Do you have a good carb you can try for reference? Easy to miss something on a carb rebuild.

You could go thru the steps in the FSM and do it over.

Can you help me understand this? The carb is sealed to the outside except for the intake at the top, what kind of an internal leak could cause a vacuum leak?

 

[John McVicker]

Could also be a leak around the carb insulator. You might need to pull the carb and really inspect the insulator on both sides for cracks...hairline cracks and/or damaged carb gaskets.

And what elevation are you at? You can expect to lose aprox 1in of vacuum for each 1k of elevation.

 

[jcj_78FJ40]

I would suggest a smoke test. The DIY paint can smoke tester works pretty well.

 

[Seth_O]

Could also be a leak around the carb insulator. You might need to pull the carb and really inspect the insulator on both sides for cracks...hairline cracks and/or damaged carb gaskets.

And what elevation are you at? You can expect to lose aprox 1in of vacuum for each 1k of elevation.

Brand new carb insulator plate. And spraying carb cleaner all around the intake/carb caused no change in idle.

 

[John McVicker]

Brand new carb insulator plate. And spraying carb cleaner all around the intake/carb caused no change in idle.

Ok good. What's your elevation?

 

[Seth_O]

I would suggest a smoke test. The DIY paint can smoke tester works pretty well.

Tks for this pointer, some pretty cool ones on the interwebs.

 

[Seth_O]

Ok good. What's your elevation?

<1000 ft, about 800 or so I think?

 

[fj80toyman]

Have you checked the PCV line? I can't see that in the video, is that in good shape? Mine tee's off just above the valve and goes to the air cleaner (HIC Valve) and I found that one to be loose and causing one (of many) of my leaks. There are so many places this could be leaking, and spraying carb cleaner may not provide the immediate result that the manifold leak did.
I would second the idea of doing a smoke test, but be ready for that, you don't want to do that over and over again, it's a little frustrating when you have a lot of smoke around and you're not quite sure where it's coming from. Be methodical.

Dan.

 

[Seth_O]

Have you checked the PCV line? I can't see that in the video, is that in good shape? Mine tee's off just above the valve and goes to the air cleaner (HIC Valve) and I found that one to be loose and causing one (of many) of my leaks. There are so many places this could be leaking, and spraying carb cleaner may not provide the immediate result that the manifold leak did.
I would second the idea of doing a smoke test, but be ready for that, you don't want to do that over and over again, it's a little frustrating when you have a lot of smoke around and you're not quite sure where it's coming from. Be methodical.

Dan.

I pulled and cleaned the PCV valve a few weeks ago, and replaced all the associated lines and clamps at that time.

 

[gt7058a]

Where are you measuring the vacuum?

 

[g-man]

Can you help me understand this? The carb is sealed to the outside except for the intake at the top, what kind of an internal leak could cause a vacuum leak?

There are vacuum passages inside the carb. When you adjust your IM screw you are letting more or less air to the bottom of the carb. Make sure you never crank down hard on that IM screw you can easily damage the tip rendering it inoperable. Easy to take it out and inspect for grooves or damaged tip. Also if you go back thru the carb check for body hairline cracks and check the threads on the vacuum passage bolt and the threads inside the hole. I think you should stall your motor if you turned the IM screw all the way down. For it to still run means air is still getting to the bottom of the carb. I suspect you left a part off. There is a small piece that screws into the bottom of the power valve make sure its there.

 

[Seth_O]

Where are you measuring the vacuum?

From the finned port that goes to the brake booster.

 

[Seth_O]

Really appreciate the responses. I think I'm going to build a smoker and try that next. If I dont get anything compelling there I'll pull the carb again.

 

[Seth_O]

Finished my DIY smoker today. Wish I would have done that a long time ago (like 1 or 2 manifold removals ago.....)

It works awesome - lots of smoke and was a cheap & easy build.

However - I found NO leaks anywhere. Like, none. I went through the engine compartment carefully for a good 20-30mins, the only smoke I found came from the top of the carb. I input from the PCV line.

So - back to my original question: what would cause it to behave like theres a vacuum leak, when there is no leak?

Am I down to the carb? Is there anything else that could be causing it? I dont have $400 to throw at a carb rebuild and have that not fix the problem.

 

[John McVicker]

I maybe stretching, but how about testing again with a different vacuum gauge?

 

[John McVicker]

I maybe stretching, but how about testing again with a different vacuum gauge?