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272 won't idle unless choke is on

Posted By smd543 14 Years Ago
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272 won't idle unless choke is on

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smd543
Posted 14 Years Ago
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Yep, just put new gaskets on & the motor runs good! Thanks for the tips!! I'll try to post a couple pix of the project if anyone's interested.
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vacuum leak at the intake manifold?
yblockpinto312
Posted 14 Years Ago
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I saw one like that in my youth. The old mechanic just said "you`ll have that".

Greg Dietrich Mt.Morris Il
smd543
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Well guys, I think we've reached a turning point. take a look at these pictures of the left hand intake gasket. Talk about a vacuum leak!! I put compressed air in one of the cylinders to check for rings/head gasket leaking. The intake valve happened to be open, and I noticed the air blowing out the side.
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An engine that won't run without the choke "ON" is often the sign of weak spark. Take an old spark plug and bend the ground electrode back until you have a 3/16"-1/4" gap. Attach any spark plug wire to it and ground it good on the engine. Crank the engine over. If you don't see a bright blue spark jumping the gap you have a weak spark.
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Have you checked valve lash?  Way too tight lash could cause all of your symptoms.  Do that before you consider pulling the heads.
joey
Posted 14 Years Ago
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If the engine hasn't run for 5 years, it's not unlikely there could be a vacuum leak. Poor idle quality isn't necessarily caused by a vacuum leak; but a tangible vacuum leak will definitely affect idle quality. You can spray carb cleaner or wD-40 around the base of the carb, around the manifold, vacuum hose connections, etc. to try and spot it. If spraying into a crevice somewhere changes the way your engine idles, then you've found something. Do one area at a time and then wipe it clean before moving on because those fluids are combustible. But in any case, I would replace the carb/manifold gasket just on the basis of them having been sitting dormant for years.
Ted
Posted 14 Years Ago
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You might want to revisit the carb.  Double check the gasket seal between the main body and the throttle base assembly.  Any warpage in that area will disrupt the idle mixture fuel signal. 

Lorena, Texas (South of Waco)


Hoosier Hurricane
Posted 14 Years Ago
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An engine should run smoothly at 100 psi.  I don't think compression is the problem.  Is the idle speed set too low, so that it is the fast idle feature that keeps it running with the choke on?

John - "The Hoosier Hurricane"
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aussiebill
Posted 14 Years Ago
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smd543 (8/23/2011)
Paul - idle screws don't seem to affect it. They were definitely cleaned out. At this point, I'm quite certain that it's a vacuum / compression issue.

Ted - Carb was completely rebuilt with all new gaskets less than a week ago. I did retard & advance the timing to see if it helped. Engine sounded worse both ways, so I'm fairly certain the timing mark is correct.

Joey - Engine was hot. I also checked with throttle & choke held open. No change. I only took out 4 plugs at a time (left/right). I would have expected higher than 100psi, even with just one plug out though.

I did squirt a bit of oil in the cylinder to make sure the rings were sealed, and it didn't affect the PSI readings. At this point, given low compression and vacuum, I'm guessing it's intake / heads / valves / gaskets. Is there anything I should be looking for if I take the heads off? Also, since it's fairly clear that the engine was not rebuilt, should I take the oil pan off and check anything on the bottom half?

Just a few ideas to check, warm engine than hold hand over carb neck to choke it  as you bring revs up high, making engine suck real hard through carb and sometimes can clear things, try it a few times. And disconnect or block PCV if fitted and check for idle difference. good luck. regards bill. Smile

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia


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