Big block hesitation but not accelerator pump
The 1966 427 425 has not been driven much at all this year. There is a slight hesitation when you race up the motor, the kind you get when the accelerator pump is not working properly, which it is. I decided to give everything a quick once over and discovered that with the timing light hooked up the light would go off just for a second causing the hesitation. I don't get it. What would cause the ignition to fail at that point but work properly through the rest of the rpm range?? Any ideas????
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Re: Bog block hesitation but not accelerator pump
check your vacuum advance is working and follows the two inch rule -
Re: Bog block hesitation but not accelerator pump
Unless it's been modified, the L-72 has ported vacuum advance, and even if it's changed to full time the OE 360 12 VAC actually meets the Two-Inch Rule. If replacement is required a 12" B26 should be used. It has four more degrees advance, but this shouldn't be a problem.
For sure on a L-71 that also was OE with ported vacuum advance, the OE 201 15 VAC must be swapped out for a B26.
What I find odd here is that the OP says that the timing light actually stops flashing momentarily when the throttle is blipped. I can't think of any logical correction here, but I suggest trying a different timing light to see if it behaves the same. Also go through the idle speed/mixture adjustment procedure, and if the problem remains with the optimum mixture try going richer on the emulsion screws to see if that helps.
DukeComment
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Re: Bog block hesitation but not accelerator pump
Sorry but still doing things the ole fashioned way, disconnect the vacuum advance, plug the line and set the timing, then adjust the Holly carb to achieve the highest vacuum. The highest vacuum is 10 which seems low. Could this contribute to the hesitation and how do I fix it.
Different timing light same result, when the throttle is blipped the timing light misses a flash or two and there is a slight hesitation??? Something ain't right.Comment
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Re: Bog block hesitation but not accelerator pump
Ted - that is unexpected to hear the timing light missing a flash or two when you hit the throttle. Got a few questions to ask that might help us figure out what's going on. First off, which vacuum canister do you have? Can you verify the numbers stamped on it? What's your initial timing? When you say you got a 'high' vacuum reading of 10 inches, was it steady? Also, if you're still using the #3247 factory carb, where are you connecting the vacuum gauge? I looked up the average altitude in Tennessee and it's less than 1K feet so that shouldn't be a contributor to your somewhat low vacuum reading. I have a neighbor here in Prescott AZ that has a L72 and we've found around 11 inches steady on his Coupe and we're up over 5K feet altitude. When moving from sea level in Northern California to the hills of Prescott I found just about all my cars dropped a couple inches of vacuum. You did say the 66 hadn't been driven much in the last year, any issues with the carb, did the mixture screws make a difference when turning in or out? Just trying to get some background info. Lastly, other than the hesitation you've noticed, any other drivability issues?
Thanks,Mike T. - Prescott AZ.Comment
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Re: Bog block hesitation but not accelerator pump
Don't have the numbers on the vacuum canister since I have not been back to the car.
Not sure what the timing degrees are but the timing mark is at the top of the degree indicator. Bringing the timing closer to tdc and the engine stalls out.
The carb is the original 3247 Holly and I am connecting the vacuum gauge to the large port that goes to the pvc valve.
Regarding the carb, the mixture screws made some difference but not much from from all the way in to 3 turns out.. Also when the engine is turned off you can see some seepage in the primary throttle. Floats have been adjusted correctly and even a little on the low side.
Any thoughts are appreciated.Comment
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Re: Bog block hesitation but not accelerator pump
Normal L-72 idle behavior is 14" @ 900. You need to state idle speed, but 10" at around 900 indicates a higher overlap cam than OE. The OE carb VAC signal line is ported, so there is little of no vacuum advance at idle. This should be changed to full time. Get a couple of feet of 1/8" vacuum hose and a 1/8" tee, remove the hose to the choke vacuum break and make a new harness with a hose from the base of the tee to the VAC. Also, with only 10" idle vacuum you need a 8" B28 VAC, Airtex 4v1053. Do a Web search to find one. Rock Auto should be one source.
Look at your CSM/AMA specs for the centrifugal advance specs and use a dial back light to determine what it actually is on the car. Many have been modified over the years. I believe OE is start at 900, 30 @ 5000. This is very lazy and should be quickened with lighter springs. Set initial timing below your measured centrifugal start point. Of course this has to be done with VAC signal line disconnected and plugged. Then, converted to full time vacuum advance, total idle advance should be initial plug full vacuum, plus maybe a few degrees centrifugal if the centrifugal states below idle speed. Select and idle speed "as low as possible commensurate with acceptable idle quality". That's subjective and your choice, but with the OE cam I recommend 900.
The above may not solve you problem, however, set up as above the engine should have better idle quality, and lower fuel consumption, especially around town.
I can't think of any logical correlation between blipping the throttle and the ignition briefly cutting out because the fuel and ignition systems on these cars are completely independent of each other.
DukeComment
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Re: Bog block hesitation but not accelerator pump
Lots of good info which I will pursue attached is from the Archives.
December 11th, 2015, 11:13 PM
George Williams (18607)
Very Frequent User
Join DateAugust 1st, 1998Posts305
Re: Engine misfire/cutting outThe '65 396/425 should have TI; the "misfire and cutout on acceleration" issue sounds like a timing problem. I have seen the 396/425 TI system run good on slow/normal acceleration, but when you punch it the engine cuts out or misfires; this can mean you need more total advance. I would start with timing advance.
Get the engine to run and then look into the fuel concern. Good luck.
Dr. George
61 fuelie; 65 big block and
2013 60th Anniv convertible.
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