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Old 09-20-2010, 10:10 AM
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Possible Vacuum Leak?!?

Hey all I hope I can get some insight because this is driving me crazy.

OK first of all I have a fresh rebuilt 355. First run everything ran great, started great, idled great. Ran a leak down test and found out I have a leaking exhaust valve. Pulled that head and had it repaired. Now that everything is patched back up when the engine is cold it will not idle at all. I have to hold it at 1500 rpm to keep her going. Once it reaches temp it will run excellent. I was just wondering if itS possible for the intake gasket to leak on the valley side enough to cause this but seal up once the engine comes to temp. Ive tested all exterior for leaks, checked timing..the engine ran fine, torn down repaired, put back together all in the same weekend so it cant be too much other than what i hope it isnt!

Has anyone heard of a leak sealing up once the manifold expands during the heat cycle?

please give me any info youve seen or heard...thanks!

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Old 09-20-2010, 12:35 PM
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How did you test for vacuum leaks? Did you spray starting fluid around the intake while its running?

This sounds more like a carb problem to me. Is the choke (and fast idle screw) adjusted properly? Have you gone through the usual carb tuning checks - float levels, butterflies .02 above the transfer slot, idle mixture set properly?



Quote:
Originally Posted by Cunferferth
Hey all I hope I can get some insight because this is driving me crazy.

OK first of all I have a fresh rebuilt 355. First run everything ran great, started great, idled great. Ran a leak down test and found out I have a leaking exhaust valve. Pulled that head and had it repaired. Now that everything is patched back up when the engine is cold it will not idle at all. I have to hold it at 1500 rpm to keep her going. Once it reaches temp it will run excellent. I was just wondering if itS possible for the intake gasket to leak on the valley side enough to cause this but seal up once the engine comes to temp. Ive tested all exterior for leaks, checked timing..the engine ran fine, torn down repaired, put back together all in the same weekend so it cant be too much other than what i hope it isnt!

Has anyone heard of a leak sealing up once the manifold expands during the heat cycle?

please give me any info youve seen or heard...thanks!
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Old 09-20-2010, 12:40 PM
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I used carb cleaner and also used unlit propane..I just don't see how the carb could have gotten out of adjustment but I'll mess with it..I also changed from shorty to long tube headers..would that throw the mixture off that much
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Old 09-20-2010, 01:10 PM
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Do you have an electric choke? If so, are you sure that you are getting current ot the coil? Furthermore, are you sure that the carb is properly grounded so that the choke can actually operate? Just some thoughts. Sounds like a choke issue to me.

Pat
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Old 09-20-2010, 01:11 PM
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Which carb are you running on it ? And did it do ok before ?


Cole
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Old 09-20-2010, 01:19 PM
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Eelbrock 1405..manual choke..ran excellent..pulled head for repairs, fixed the head..put it back together..now it won't run cold..got my total timing set to 36 as I did before. Carb wasn't touched. Only when assembling I used a different intake gasket fel pro 1206 and added larger primaried long tube headers..don't know how to test for an internal vacuum leak!
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Old 09-20-2010, 01:52 PM
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did you also check around the base of the carb for leaks with the carb cleaner/starting fluid?

I would still start by checking the basics on the carb making one change at a time and writing down what you change each time, so that you can always go back to your original setup. Turn the idle mix screws all the way in and back them both out 1.5 turns, turn the carby over and set your butterflies/transfer slot and see if it makes any diff


Quote:
Originally Posted by Cunferferth
Eelbrock 1405..manual choke..ran excellent..pulled head for repairs, fixed the head..put it back together..now it won't run cold..got my total timing set to 36 as I did before. Carb wasn't touched. Only when assembling I used a different intake gasket fel pro 1206 and added larger primaried long tube headers..don't know how to test for an internal vacuum leak!
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Old 09-20-2010, 02:04 PM
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Yes I tested the base...before my mixture screws were 2 turns out..now they are almost all the way out, figured it was a vacuum leak being I have to richen it up so much..once it warms I have to adjust them back in bc it's running so rich..I'm more than likely gonna swap out the gaskets and see what happens...the 1206's match my bowtie heads very well but they don't leave alot of room for error on placing the manifold down..I think I'll go to a smaller gasket and make it just a hair bigger than my manifold port to try and give myself for room to seal up..I never matched my intake to heads so I shouldn't have to worry about a step into the port..as long as my gasket is slightly larger than my manifold but smaller than the head port..correct?
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Old 09-20-2010, 04:07 PM
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Have you put a vacuum gauge on it to see what the vacuum is when cold. Even at 1500 it may tell you the story.
I would also check to make sure you didnt crack a line or something somewhere.
Everything you described point to a vacuum leak somewhere.
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Old 09-20-2010, 08:39 PM
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Quote:
Originally Posted by Cunferferth
don't know how to test for an internal vacuum leak!
Couple of ways to do it. You can make a plate that replaces the carb, drill and tap it for a 0-15 psi mechanical fuel pressure gauge an a Schraeder valve (tubeless tire valve). Disable all rockers arms and introduce 3-4 psi air into the intake manifold. Too much pressure could blow out what's left of the gasket(s). Listen for air at the dipstick tube. The other tip-off is that the volume under the plate will not hold any pressure at all on the gauge once you remove the compressed air source.

The other way, which I have not done before, is to use a smoke machine. Disable the rockers and introduce smoke into the crankcase either through the dipstick tube or drain the oil and blow it in through the drain plug hole. Wire the throttle blades wide open. Smoke coming out the carb will tell you that there is a passage between the crankcase and the intake ports at the manifold/head interface.

Here's a wiki article I wrote on how to deal with the problem once you find it....
http://www.crankshaftcoalition.com/w...al_vacuum_leak
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Old 09-21-2010, 08:08 AM
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Thanks for the replies..I'm going to just pull the intake and swap gaskets since I didn't previously have a problem..my intake hasn't been port matched so would it hurt to run a smaller port gasket that matches the Intake more than the heads?
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