How to find an internal vacuum leak - Team Camaro Tech
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post #1 of 13 (permalink) Old Dec 18th, 02, 02:26 PM Thread Starter
 
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I need to know how to check for a vacuum leak on the inner side of the intake.

I have had a problem with my car for some time now. It always wants to backfire through the carb when I first start it up. And takes about 5 minutes to warm up where it will idle. After it is warm it runs fine. I have tried changing the jets up and all of the different pump cams. I don't have the transfer slot uncovered at idle. My timing is 16 BTDC. I have a Comp 9000 mechanical advance dist with the advance starting around 1100. Also, if I adjust my idle after driving it the next time it will be too low. I have an electric choke that is working on the 850DP.


I just checked my antifreeze for the winter and the overflow bottle had none in it. I have not driven the car much lately due to being out of town. When I start it cold I get a little puff of two white smoke (probably valve guides) and water out of the exhaust.

Any suggestions or help?



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Kent
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post #2 of 13 (permalink) Old Dec 19th, 02, 12:36 AM
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I would believe the symptoms for a leaky intake gasket would be as follows:

1. Oil smoke when running, intake sucking oil from the underside into the ports.
2. Higher than idle speed, leaking air thru gasket would make the engine idle faster.
3. Low (numerically) vacuum gauge reading. Gauge reading lower than 15 inches would indicate a leak, therefore power valve might be coming into play.

Hopefully others here have suggestions.....

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post #3 of 13 (permalink) Old Dec 19th, 02, 03:43 AM
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Sounds just like a choke problem.

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post #4 of 13 (permalink) Old Dec 19th, 02, 09:29 AM
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I'm thinkn' choke too...
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post #5 of 13 (permalink) Old Dec 19th, 02, 02:20 PM Thread Starter
 
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I worked on the choke a little bit today. I set it at 2 notches to the rich side and set the fast idle to around 1200 on the second highest stop. I installed a 180 thermostat (was 160) since the motor was never getting hot. It would only get to about 175 in 100 degree heat at a stoplight. I also put in new plugs. The old ones were very black. No 1 was the worst. No 7 almost had a non fouled insulator. The plugs are R45TS gapped at .045.

I am going to check the choke operation tomorrow. Once I think the choke is working properly, I am going to work on the jetting. It seems to be rich. When I started it this afternoon, black smoke was coming out the tailpipes for a few seconds. You can smell fumes from the rich condition. My idle mixture screws are at 2.5 turns out. I will check this with the vacuum gauge after the choke works. I currently have a holley 4781 DP with 80 Jets in all corners. I will try some other jets tomorrow.

Thanks for the help Milan. I really appreciate it.

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post #6 of 13 (permalink) Old Dec 19th, 02, 03:43 PM
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Kent - Slow down and take a breath!! It sounds like you are going in too many directions at once with this. I am speaking from experience so I don't mean this as bad mouthing!! Jets have little to do with the choke or idle circuit or transition off idle. Also if your temp is running 175 with a 160 stat you have a strong cooling system. you might see 180 temps with a 180 stat but as soon as the stat opens the temp will drop to 175 again. Anyway 175 or so should be imaterial to your problem.

The first thing you need to do is put the carb back to factory specs. Just like it came out of the box. Next decide what it's doing that it shouldn't. Match the problem to the circuit that the problem falls under. Here are the basics;

Idle
Transition
Main Metering
Power
Secondaries
and Choke

If the car is idling rich it's an Idle Ckt problem or a blown power valve. Of course the choke could come to play in all this as well. Get the car warm so the choke is out of the picture and dial in the idle. Sec butterflys properly adjusted, floats properly adjusted, accl pump properly adjusted then lean out the idle mixture screws until the engine starts to stumble and then back out just enough so the engine recovers. With a vacuum guage adjust for highest vacuum and then lean it out a little, about 1/8th turn max though. Adjust idle if needed. At this point I like to balance all the idle mixture screws so I check how far out each is and find an average between them and set to it. If you still idle rich look to a blown power valve.

When it idles to your satifaction warm go for a drive and see how it runs. If it stumbles work on the accl pump (squirters and cams) remember the bigger squiter opening just dumps more gas all at once and the cam changes the duration of the squirt. I can't stress enough to do this with a warm engine... Next you can mess with the choke and then start jetting for performance. One step at a time though.

I don't have a choke hooked up on my carb and find without it it two pump starts on the coldest of days and after holding it at 1500 - 1800 for 45 sec or so it will idle. Mind you I'm in CA and coldest to me ia about 45 degs. I can drive off without sputtering or farting but unless it's up to temp I can't blip the gas WOT without a bit of stumble. with the choke you should be able to have a rich enough idle to over come a cold stumble.

If you don't have a Holley book I recomend getting one and check out their tech info on the web. www.holley.com It's a bit hard to find the tech stuff but look around you'll find it.




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...Dennis
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post #7 of 13 (permalink) Old Dec 19th, 02, 06:11 PM
 
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i'd check for that vacume leak at cab. base when the engine is cold and if it has back fired that will trash the power valve, all it takes is once, the water out the exaust is just condisation unless it taste like antifreeze aalso way to much advance cut it in half 8 degrees or pull it up to 3000 rpm an set it at 36 total degrees and see where the timing is at idle you may also have a problem with the mach. advance sticking this may also cause your backfire so fix the power valve and then check the manifold vacum the cam card should tell what the vacum should be or contact the cam co. for that info good luck
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post #8 of 13 (permalink) Old Dec 20th, 02, 01:59 AM
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I agree with Dennis to take this one step at a time, however Dennis I am not sure that Kent Is displease with the performance other than the cold backfire.

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post #9 of 13 (permalink) Old Dec 20th, 02, 08:03 AM
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But he has suggested a jet change to help resolve what sounds like a rich idle problem... Kent - I suggested what I would do and should have added record how it's set before you start so you can either go back if changes didn't work or compare how close you were after coming to resolve.

It's my opinion that starting at factory setting (like the manufacture publishes or was set by Chevy off the assembly line if it's factory) when you have reached an impass is the only way to go... I hear too many complaints it wouldn't run out of the box but that's because it wasn't set to spec's when assembled (always check new equipment just to be sure!!) I'm not saying factory settings are optimal but they are the best starting point!!
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post #10 of 13 (permalink) Old Dec 21st, 02, 04:17 AM Thread Starter
 
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Dennis,

The Carb is at the baseline as it was according to the Holley Books. The only thing that is new is the electric choke. I baselined the carb about 6 months ago when this problem started. It runs fine once it is warmed up. I think I have the choke figured out and will check it today if I have time.



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post #11 of 13 (permalink) Old Dec 21st, 02, 02:09 PM Thread Starter
 
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I tested the choke setup today. It was about 60 degrees. The choke worked and it idled around 1200 for a couple of minutes. I drove the car right after starting and it was driveable. It did not back fire. After it warmed up I set the idle mixture screws with the vacuum gauge. I ended up at 2-1/4 turns on each side with about 12" vacuum. It is running much better now. Thanks for all the help Milan and DJD.



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post #12 of 13 (permalink) Old Dec 23rd, 02, 07:59 AM
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Glad to hear the good news! wish it was in the 60s here!

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post #13 of 13 (permalink) Old Dec 24th, 02, 01:53 AM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by Milan:
Glad to hear the good news! wish it was in the 60s here!

<HR></BLOCKQUOTE>

The only thing in the 60's here is the wind . . . just kidding.
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