joe, i am struggling with setting up this L72 she starts on a tap and idles like a dream and revs nice in the bay. very throaty and strong. it is all rebuilt to fact specs. k66 ign and holley 3247. i have converted to constant vacuum (which really smooths it out). when i take off in first it stumbles and i do not feel that legendary L72 power. runs good through 2nd and 3rd. i find advancing the timing helps. the more i advance the better it gets. i do not know if this is the fix or a band aid. i am hesitent to advance more with out advice. also i found adjusting the acc pump helped but do not know the correct proceedure for this. floats are set, idle mixture seems good and plugs are burning right. is fuel the issue? i did not want to add lead booster for fear of creating an octane addict as i was told it would. builder says the effective or net compression is 11:1. i am really at a loss as to what to do next.
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Re: others with advice would be apriciated as well
did not mean to imply only joe could help. i have just come to respect his advice,bill -
Re: others with advice would be apriciated as well
Bill -
The accelerator pump adjustment is covered in the '67 Chassis Overhaul Manual; hold the linkage at the wide-open throttle position, fully depress the little actuating lever at the accelerator pump, and adjust the spring-loaded screw on the linkage to achieve .015" clearance to the pump lever. A stout rubber band is helpful to hold the linkage at the wide-open position while you hold the pump lever all the way down and adjust the screw. This adjustment will ensure that you get fuel from the squirters instantly with the slightest movement of the throttle linkage.Comment
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Re: others with advice would be apriciated as well
thanks john i looked in my 66 chassis service manual and din not see, billComment
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Re: question for joe lucia
bill------
I think that getting the accelerator pump right is the next thing to try.
If the heat riser valve is still stuck closed, you MUST get that fixed before anything else.
I feel pretty sure that the problem is in carburetion or ignition and, most likely, carburetion.
The octane booster won't create an "octane addict" unless it creates combustion chamber deposits. Relatively short term use is not going to cause this. However, it's moot because the octane booster is NOT going to solve the sort of problem you have. The ONLY thing the octane booster will do for you is alleviate pre-ignition or detonation problems. If you don't have that sort of problem to any significant degree, the octane booster won't do a thing for you except cost you money.
You can advance the initial timing a few degrees more at a time until the engine experiences significant pre-ignition.In Appreciation of John HinckleyComment
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Re: question for joe lucia
fixed the riser today. bought the carb on ebay came with a whole list of work done for around a grand(got for $300) but so far found a bent acc pump lever broken high idle cam. i suspect it had a bad UPS ride, it was recolored and looks good so i do beleive the work was done. is there any thing inside that could have been damaged as well i can check? also what do you think about me advancing a little more? is this just making up for some thing else? one last thing, i do not know if it matters, but my intake had a full open plenum like on the L88 and showed no indication of being modified, i looked it over really good and is dated 12 65 thanks, billComment
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Re: question for joe lucia
I would swap the carb out with a known good one. Then see if it makes a difference. Make sure your harmonic balancer has not slipped either. Sounds like you are making it run better by taking it out of the normal adjustment range. IF the simple stuff don't get you going, you will have to sort of start double checking most everything. Just my humble opinion. TerryComment
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Re: question for joe lucia
bill-----
As far as I know, your L-72 application did not originally use a full open plenum version of the 3885069 manifold. The only version of this manifold that was full open plenum was the 67-68 L-88 application. I doubt that any of these manifolds were being manufactured as early as yours is dated. Consequently, I would say that this one has been modified whether it appears that way, or not.
The full open plenum configuration is used for better performance at high RPM. In my opinion, it's NOT what you want in a street engine. I don't know if it's the sole cause of your problems, but I would not be surprised if it's, at least, contributing to it. Personally, I would never use a full open plenum manifold for street operation.In Appreciation of John HinckleyComment
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a lot of the modified manifolds were machined
to completely remove the plenum divider and that was a mistake as there should be a some divider left after machining. to save a valuable manifold a divider can be welded back in. make sure you have the correct spring in the secondary vacuum diaphragm in the carb so it is not opening too quickly. use a piston stop the make sure you have the correct TDC mark and if you do power time the engine for a total 36 degrees at max advanceComment
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Re: question for joe lucia
OK a couple questions and comments if you will. You say the engine stumbles at takeoff. Do you mean miss or bog down some. It sounds like bog to me because you said it was fine in 2nd and 3rd. Now, you say the engine has been rebuilt to spec -- are you sure ?? For example - what cam is in the engine, are the timing marks on the timing gears lined up properly. What rear end ratio are you running, if you are running a 3:36 or higher like a 3:08 with the L72 it takes a little getting used to when pullng away from a stop light/sign because based on what cam you have, your car could tend to bog down. What RPM are you using for takeoff. The manifold should not be the open plenum type. I think the 8 degrees initial timing for an L72 is too low. The chassis service manual actually lists the timing for 10 degrees in one part of the book and 8 degrees in another. It has been my experience that the L72 loves lots of timing. Someone earlier suggested setting the engine for maximun advance using a dial back light and then see where your initial falls. That is a good idea.Comment
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Re: re jet carb?
what are the symptoms of an open plenum. when i bought it i was told they used both versions and when i ask the builder i was told it would be about the same. the L88 had higher comp and a larger carb. short of swapping it because it is such a nice manifold, could i rejet the carb? is the timing a fix or a band aid?Comment
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Different Manual...
The chassis overhaul manual that John refers to is a companion volume to the chassis service manual and is identical in appearance. The chassis overhaul manual covers only the rebuild of major assemblies such as the transmissions, differentials, carburetors, etc.
If you have the 66 Chassis Overhaul Manual, the procedure John described for adjusting the Holley 4150/4160 accelerator pump linkage is shown on page 6M-52...it shows the rubber band just as he said. (Just happened to have a copy of the old manual laying around. )Comment
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Re: characturistics of an open plenum?
it has a 3:55 but we swapped to a m20 to make up for the change. it was a 4:11 when i say fine in 2nd and third i have only gone about 2 miles with it but it did pill away. i think it is a combination of timing and the carb but i am installing a new fuel pump today as i have a tempoary 67 set up now. when you take off in first it acts flooded then clears up. i am swapping it with a 650 i have to rule out carb but will wait until i swap the pump . what are the characteristics of an open plenum?Comment
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Re: characturistics of an open plenum?
weak bottom endComment
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