blocking off intake manifold exhaust riser passage carb boiling percolating fuel prevention

I am thinking of blocking off the exhaust passage to intake manifold crossover to keep the intake manifold and thusly carb cooler. As an aide to help keep the fuel in the carb and not vaporizing out and flooding the engine. Also what is left when the gas recondenses inside the motor is likely missing its more volatile elements and I think is a likely contributer to sticky gummed up carnbonated intake valves.
Frankly todays gas boils at a lower temp and I notice the the bowls dry up after sitting on a hot engine. So I think lowering the temp will help a little bit.

Has anyone done this and have info to share on how it affected or not the fuel consumption?
Browsing other internet forums seems to suggest people who have done this have not noticed increased fuel usage. Mostly in the past, this was a trick way to cool the intake charge and get more power from the engine. Todays fuels vaporize well so I dont think it will hurt mileage.

Also I dont like the idea of fuel drying up in the bowls and leaving behind the sludge that clogs up the carb over time. The alcohols and other chemicals that dissolve the varnishes-sludges I suppose, evaporate and then this stuff which is insoluble in gasoline plates out on the inner surfaces of the carb.
The absolute best solution is fuel injection, no free air - gasoline surface to cause trouble.
Alternatively, I have also been thinking of turning off the fuel pressure to the carbs and running the engines till they die. I have an electric priming pump so it is no trouble to reprime. But that takes some thought to actually do it and I dont always remember.
The best thing I could imagine for a carb system would be one that automatically drains the bowls when the motor is shut off. And then it wont recondense in the motor or gum up things.

sdowney717 said:

Mostly in the past, this was a trick way to cool the intake charge and get more power from the engine. Todays fuels vaporize well so I dont think it will hurt mileage.

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It is true that the cooler air will allow more power, but that really means you can jet the carb richer to use those additional oxygen molecules. Otherwise, the engine will run lean, and likely not very well at low power settings, especially when cool. Racers aren't idling much, and don't care if the engine is rough at low power/idle settings in any case. I'd think docking a boat is a much different use case. Maybe fuel with alcohol does vaporize OK, not sure about that, but I do know that fuel injectors that do no spray well are still an issue with todays fuel. So, if I were tempted to go down this path, I would not do it without an exhaust oxygen sensor. Rigging that may be more work than installing the exhaust crossover restrictors in the intake manifold. Without the right sensors, you might well melt pistons.

I'd discourage the idea. Big reason is most carb'ed engines still have the cooler thermostats. It also takes a long time for the intake manifold to reach thermal equilibrium, at operating temperature. Getting rid of the crossover will make running the engine a major PITA until the manifold warms up.

The good news about today's "rapidly evaporating" fuels - when they evaporate, there is nothing left to yield the residue like in the last century. Those "insoluables" as you have labeled them will continue to be present (i believe most are introduced by the distribution chain) for a long time to come. They are readily removed once the carb is disassembled.

since I have twin engines, I will try it on one and compare the results.
It will take me a while to post back on this.

IF you don't run in cold weather, I think that would be a good idea (and I may try it myself). No way you need carb heat in warm weather, and it has to help economy. You'll need electric chokes since the mechanical ones won't work. Finally, it might even improve restarts after sitting a week.

Jeff

I'm not on the same page here. A cold intake will condense that nice atomized gas spray out of the carb. What you want is a fully mixed, atomized fuel air charge to enter thru the intake valves. A bunch of puddling gas in the intake runners is not how you get the desired effect. It's going to run like a cold carbed engine; in other words, lean, with unburned fuel exiting the exhaust. The carb can and should stay cool, using thermal isolating gaskets.

The intake is not very cold. Mine has hot coolant circulating through it and also around the carb base. It is quite hot to the touch, enough to burn your hand on the thermostat housing ( no exhaust gas there) after just a few minutes of run time. I dont know how much effect it will have on fuel boiling but it likely will help a little.

Even now I get small fuel puddling in the bottom of the intake with the exhaust passage open.
I can see it by opening the throttle and looking down inside. Plus you get that white vapor coming up which you know is vaporized (boiled) fuel which then cools and liquefies again inside.
Fuel is composed of many fractions which boil all at different temps. When the lightest fractions are gone your left with ones which are harder to vaporize, like when it sits and dries out the bowls. Plus as gas ages it forms sludgy sticky varnishes, due to oxidation reactions and losing the lighter alcohols benzenes etc... means those varnishes and gums which were soluble become insoluble and coat everything with the sticky varnishes. Over time they build up, I think on the intakes and carbs. And boat tanks tend to age fuel a long long time unlike a car where the fuel stays fresh all the time. Boat fuel just sits there and slowly goes bad with varnishes and gums levels and water building up in the fuel.
I just rebuilt some sticky carbs and only things that cut it are 90% isopropyl alcohol and or white vinegar.

The vinegar works very well cleaning out gunk, (also dissolves rust after 4 days soak it comes right off steel), you should neutralize it after cleaning. So far on the many carbs I have cleaned rebuilt, there has been no need for an expensive carb cleaner. Alcohol, vinegar, an ear bulb and air compressor to blow out passages and an occasional very fine wire.
White vinegar soaks on carbs wont need more than a few hours to get things cleaned up.
White vinegar is very mildly acidic. Sometimes, I will soak, examine it,scrub it with a toothbrush, blow it out, rinse it and soak some more.

Even now I get small fuel puddling in the bottom of the intake with the exhaust passage open.
I can see it by opening the throttle and looking down inside.

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Unless you remove the link that works the accelerator pump, I'd say its normal to see a small puddle under the primaries.

If the puddle exists elsewhere or is large, I'd think the carb has other issues that need to be addressed.

yeah, I dont think it is totally related to the accel pump. And this is a different carb model entirely rochester 4G to Q-jet.
However it is an IH 392 not a chevy motor so maybe they puddle a little in the depression under the carb.
Carb certainly does not leak down as I tested that. I think it is just one of that engines peculiar features