I'm wondering if anyone has had any success minimizing the fuel percolation problem that seems common to this carb and aluminum manifold ('64 365 hp in my case)by switching to a 160 degree thermostat? This is certainly simpler than pulling the manifold and blocking the heat passage and should THEORETICALLY help.
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Re: Holley 2818 percolation problem - try 160 T/st
A 180 thermostat is not the answer. Does your heat riser operate properly? Even if it does try wiring the valve open and see if the situation improves.
These engines did not boil the fuel when new, so it's not the design. Also check you ignition map to see if it's in spec.
Duke -
Re: Holley 2818 percolation problem - try 160 T/st
I would like to second Duke's answer on this. Based on on of his many previous posts on this problem I wired the heat riser open on my 67 and all of the fuel percolation problems were gone for good.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
Normally, I'd agree with the approach of fixing the root cause and not patching over but from what I've seen in the archives, I get the impression that percolation has been a problem since these cars were new! My heat riser works correctly and both my vacuum and centrifugal advances seem as close to correct as I can tell with a timing light and masking tape. An insulated spacer is too visible, but I'm also thinking of trying two gaskets under the carb. Thanks for your help.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
If nothing else, wiring the heat riser valve open 100% of the time will give longer longivity to the muffler on that side.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
John Hinckley did an article on this in the Corvette Enthusiast I can't remember what issue. John please advise. Thanks BrianComment
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Re: Holley 2818 percolation problem - try 160 T/st
The issue I addressed in my "Tech Bench" column was plugging the "hot-slot", but the "hot-slot" design for small-blocks was only used for those with cast iron intakes (see pic).
A 160 thermostat will have no effect on the percolation issue, as the thermostat only controls the MINIMUM operating temperature, assuming the cooling system is efficient enough to cool the engine to that level (which is unlikely except in the winter). The thermostat will be fully open under normal warm weather conditions, and once it's fully open, maximum coolant temperature is solely a function of radiator efficiency and airflow volume.
Step one is to wire the heat riser open to reduce the differential pressure which determines exhaust gas flow through the manifold crossover passage. If that doesn't do it, step two is to block the driver's side crossover passage (you still need the one open on the passenger side, or there won't be any heat under the choke thermostat well, and the choke will take forever to open). If THAT doesn't do it, it's probably time for a phenolic insulator or the GM aluminum TSB-issue aluminum heat shield under the carb.
Comment
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Re: Holley 2818 percolation problem - try 160 T/st
John, You are correct about the T-stat. I hadn't really thought that through. I am not familiar with the GM aluminum TSB-issue heat shield. Please tell me more. ThanksComment
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Re: Holley 2818 percolation problem - try 160 T/st
Scott -
GM has had several aluminum heat shields over the years to deal with fuel percolation in the field over the years, one of which was #3969835 (for Holleys); they mount between the carb and the intake and shield the bottom of the float bowls from radiated manifold heat.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
John - great info from you as always - you may recall I have a L76 (Holley-equipped on top of an aluminum manifold) so your advice on this topic is of interest to me. Your reference to "the GM aluminum TSB-issue aluminum heat shield" caught my eye - by this do you mean it was only a service-issued item for addressing a fuel perc problem in service, and that such a part was not found on a car coming off of showroom floor? Meaning, where might someone obtain this possibly hard to find part, any advice on how long it was in service to support the TSB?65 MM Convertible, L76 (365 hp)Comment
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Re: Holley 2818 percolation problem - try 160 T/st
Scott:
Please further describe your problem. What do you mean by "fuel percolation"? There should be no insulator between your intake manifold, and your carb base plate, only a single layer gasket.
Does your engine load up while driving? Does it flood after shutdown? Did you verify that the float level is right, and needle-and-seat is serviceable? Just look at the primary side bowl---the secondary level is governed by the pri level.
JoeComment
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Re: Holley 2818 percolation problem - try 160 T/st
if GM does not still have them i have one on the shelf.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
I have a luck motorsports restored 2818 sitting on a complete factory 365 hp L76 setup...no perculation problem...you havent found the root cause yet, areas to check, timing, heat riser, carb setup, gaskets on intake and carb, and cooling systems effeciencyDino LannoComment
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Re: Holley 2818 percolation problem - try 160 T/st
Joe, I actually have both float levels set lower than spec. Just to the point where the car doesn't run out of fuel winding it up to 6500 rpm in 2nd gear. I have gone through the carb so the needle, seat, gaskets & power valve are OK. The carb body seems to be flat, having run across the metering block mounting surface with a file. I have no insulator under the carb. I have shot the water outlet with an IR gun and normal operating temperature agrees with the thermostat at 180. At 180, the engine occasionally loads up at idle but the most common symptom is that shortly after shutting it off, fuel sometimes dribbles out of the accelerator pump discharge nozzles. This obviously causes hard starting for anywhere from 10 to 60 minutes. If I understand the heat shield discussed earlier, it extends beyond the base of the carb and is therefore quite visable, something I'd hoped to avoid. Thanks to all for your input.Comment
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Re: Holley 2818 percolation problem - try 160 T/st
Scott:
FROM THE FACTORY, there was NO heat shield installed. I can say that my 2818 would "load up" once my L76 got hot. I tried my best to go after it, but just couldn't fix it. GENUINE HOLLEY rebuild kits didn't fix it. Finally had the carb restored/refurbished by a very reputable Holley rebuilder, and it has been very good ever since. Don't get me wrong, even if it works perfectly, it won't be as seamless as a modern F.I. system, but it should be driveable.
Holleys usually load up after hot shutdown. Thank G-d mine does not, at least not yet. You probably have to floor it, to open the unloader until it starts.
Best of luck,
JoeComment
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