Re: 1966 L72 Idle and Stalling / Cold Starting
Any problem with your Holley with 91 octane gas? Do you have electronic ignition?
Any problem with your Holley with 91 octane gas? Do you have electronic ignition?
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Re: 1966 L72 Idle and Stalling / Cold Starting
Yes, it may well be a carb problem and that's where you go next.
You will hear lots of stories and lots of people blaming the fuel. It's one of the reasons some on this forum advocate wiring open (or removing) the heat risers and blocking the crossovers. Truth is, there are lots of cars in their oe trim running great on the fuel available today. And the fuel doesn't vary that much by locale. Octane ratings are certainly lower, but octane rating really only affects engine knock, or "pinging". It has no bearing on idle quality. You'll also hear stories about "heat soak" and "percolation", distilation charts and infrared thermometer readings of fuel lines. All kinds of theories and speculations.
So why do some cars have fuel problems and others don't?
Because the fuel is not the problem. If most can run great on the fuel, why can't all? Or at least, why can't yours? My theory is that fuel gets offered up as the culprit when the diagnostician runs out of viable options. That's not to say there aren't any more.
The other thing that has changed since the days of high octane fuel is the number of people that understand carburaters and the systems they depend on.
SteveComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
William,
Have you read Duke's papers? What I got from them was that gas today has about the same octane as it did in the 60's, it's just that the rating methods have changed. If the octane is too low, the engine will detonate. That's the only problem it will cause.
JoeComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Joe, that looks interesting. where do I find Duke's papers? I don't know what that is. thxComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Simply search the TDB, ported vacuum advance L-72 or L-71. Changing to full manifold vacuum advance may help , and there are other benefits as you will read. A more aggressive centrifugal advance curve may also help. Google duke williams san diego corvette and download the pdf.
Also search for a post started by me a couple of months ago, gasoline distillation curves. The higher mid range boiling points of today's ethanol laced gasoline can cause excess fuel vaporization from the bowls creating an overrich mixture, which can affect idle quality and hot starting.
Converting to full time vacuum advance reduces idle EGT, whilch reduces internal surface temperatures and heat rejection to the cooling system, which will help mitigate hot soak starting problems.
Also, as suggested, a slight reduction in float level may help along with insulating fuel lines. There have been threads on this with photos and before/after IR gun temperature measurments, which show clear reduction in fuel system component temperatures. Search the TDB, percolation.
The problems you are experiencing are relatively common, and there have been many threads discussing the reasons and solutions. It's not rocket science and you don't have to reinvent the wheel.
BTW, the problem has nothling to do with octane rating. The only problems that insufficient octane cause are detonation and shut down runon with ported vacuum advance.
DukeComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Sounds to me like your idle circuits may also be restricted.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Duke, thank you. What is "TDB" - Im new at this ...Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Never mind Technical Discussion Board ... had a long weekend. Will do , thanks for the adviceComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Thanks. Why then wouldn't that be a problem when it is warming up? It's only once it is warm that it shuts down on me at idle ... one theory is that the fuel is vaporized. other is that it is percolation and flooding the engine, others is that I'm just running too lean. Getting the scenarios down and will start trouble shootingComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
When it's warming up the choke is affecting/determining the air/fuel ratio. It doesn't tell you in which direction tho. You can have a lean fuel supply condition and the choke richening the mixture compensates for it. It can be a rich fuel condition and the choke is actually set lean, but it's all balancing out. The cold engine needs a richer mixture.
Yes, your fuel is vaporizing. It's supposed to. Liquid fuel doesn't burn. that's why we need the heat in the crossover and the heat riser.
Is the ambient temp that you're driving in hovering around 100*? Is everyone else in your area with carberated fuels experiencing the same problem? If not, the fuel's not percolating.
We could discuss forever all the possibilities. I prefer a methodical, systematic approach to the diagnosis. You have a cold start problem as well? You start with that because an improperly operating choke can also affect hot engine operation if it's not coming off properly.
So you start with the cold engine, check that the choke is coming on and off. Once you've followed it through it's operation and confirmed that it's coming off you next move to determinging if you have a lean or rich idle issue.
Cup your hand over the air horn and choke it slightly with your hand. If the engine speed picks up and the idle smooths out, you have a lean condition.
If no change or it gets worse pull the pcv hose off at it's vacuum source. Put your finger over the open vacuum pipe. Lift your finger slightly and slowly increase the amount of air you are letting past your finger. if the engine picks up speed and smooths out as you allow more air, you have a rich idle condition.
All this has take about 2 minutes after the choke has come off.
There's more.
SteveComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
The Nay Sayers against the real aspects of fuel percolation, heat soak, wired heat risers or blocked cross overs and ethanol laced fuel have spent most of their lives servicing cars in the brutal north winters. They should live in our flip-flops for awhile down here in the hot and humid south dodging hurricanes.
Stu FoxComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Stu,
Summers up here may not get as hot as a Phoenix summer, but they get every bit as hot as your Florida summers. 100* days are not foreign to us.
The difference is that us that trained in this climate are able to make them work in all temps. I've spent a lifetime listening to people blame problems they couldn't solve on the things outside their control. Then we fixed them.
Steve
Rather than let this thread get off track I'll start a new thread of the fuel distillation chart "science"Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
My 66 L36 with Holley would stall at stop lights only after extended highway driving. I found the locking gas cap was not truly vented, even though it had a hole in the top. After changing to an original vented cap, the stalling ceased, apparently due to vacuum condition in the tank being relieved. Sometimes the most obvious solutions are overlooked.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Thanks. I will check this out tomorrow.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
So what ever happened with this testing?Comment
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