Carburetor Fuel Evaporation

michiganhotrod1

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I am scratching my head on this, suggestions welcome.
I have a pretty severe fuel evaporation problem on my 65 Chrysler 300 (383). It is stock, mechanically refreshed, with the stock AFB four barrell that has been rebuilt, new fuel pump, fresh filter, replaced pump push rod.
Overnight or even after parking for several hours after a trip I will have no apparent fuel in the carburetor bowl. Nothing comes out of the accelerator pump jets, making the car impossible to start.
Pour gas down the carb and it starts instantly, and it runs fine otherwise, no bogs, etc and the accelertor pump operates normally once running.
I know 10% ethanol fuel is volatile, but this is rediculous and really hurts use of the car.
Any suggestions what is wrong here?
Thanks.
Mark
 
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Trace 300 Hurst

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I'm good for about 24-30 hours after shutdown, and I attribute that to my very modern Street Demon carb. Beyond that timeframe, it's three full seconds of starting fluid or carb cleaner in one of the snorkels and it starts immediately.

That's how it's going to be with our "open to atmosphere" carbs. Gasoline has changed a lot in the past 57 years.
 

Big_John

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I am scratching my head on this, suggestions welcome.
I have a pretty severe fulle evaporation problem on my 65 Chrysler 300 (383). It is stock, mechanically refreshed, with the stock AFB four barrell that has been rebuilt, new fuel pump, frest flter, replaced pump push rod.
Overnight or even after parking for several hours after a trip I will have no apparent fuel in the carburetor bowl. Nothing comes out of the accelerator pump jets, making the car impossible to start.
Pour gas down the carb and it starts instantly, and it runs fine otherwise, no bogs, etc and the accelertor pump operates normally once running.
I know 10% ethanol fuel is volatile, but this is rediculous and really hurts use of the car.
Any suggestions what is wrong here?
Thanks.
Mark
That sounds excessive to me.

I would look for a internal leak in the carb.... Or something strange is happening to siphon the fuel out of the carb and back into the fuel line. My money is on an internal leak though.

My car (70 300 with a Thermoquad) will dry up after a couple days and my solution was to install a cheap flow through electric pump back by the tank with a momentary switch. Get in, hit the switch until the sound changes and get an almost immediate start with a couple pumps of the gas.

@Dana at Woodruff carbs could help with a pro rebuild that will work correctly.
 

Trace 300 Hurst

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Yeah, that does sound excessive to me, and I first thought about an internal leak, too. But that much of a leak would cause a "flooded" hard start....and maybe that is the problem. The OP didn't say anything about lots of smoke at startup, so I went with the evap theory. Or maybe it's some of both problems.
 

CBODY67

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The earlier carbs had a "Bowl Vent Valve" which is adjusted so that it is open at idle. It's on the outside of the nginecarb air horn AND is adjustable. Therefore, it can be adjusted to be closed at idle, as it is closed once the throttle linkage closes it at off-idle. Closing it at idle would mean any evaporation of fuel from the float bowl would be into the air cleaner rather than into the outside ambient air.

Chryslers of that vintage usually had a harder "after hot soak" re-start than GM or Ford vehicles did, for some reason. When you park the car after using it, is it in a closed building or out in the open air, where air can still blow into the engine compartment for cooling? Nosed into the prevailing winds or otherwise?

When the cars were designed, we had "summer" gas and "winter" gas. Winter gas was more volatile and easier to evaporate/vaporize in the cooler weather, but current E10 is even more volatile than that and we have it year-round. Whereas the old summer gas was a bit less volatile in nature.

Wondering if some upgrades to the Chrysler B/RB engine architecture might help the hot re-start issue? Like a metal plate to cover the "valley" and use normal, separate side gaskets (ala SBChevy) for the intake manifold, topped by an aluminum intake manifold? With the solid plate, no need for that "insulator" between the cast iron intake and the breat plate valley pan gasket, for more air circulation under there. Or on the cheap, just an aluminum intake and delete that insulator bag?

As large as those AFB/AVS and TQ float bowls tend to be, hard to imagine all of the fuel evaporating out, to me. A Rochester QJet with its small bowl capacity and being hidden from normal air flow, a different situation, I would think.

Just some thoughts,
CBODY67
 

michiganhotrod1

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Hi;
Thanks for the input.
No flooded hard start, no black smoke. Pour gas in the carb or use ether and it starts right up. Nothing comes out of the accelerator pump jets. Carb is an AFB, hard to have a leak in these. But I can try to find another one.
Mark
 

JMC85284

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I had similar symptoms on my 73 Imperial and thought I had carb evap going on, but then on closer look realized the line was pretty much dry at the fuel filter. So i replaced the fuel pump on theory that an internal FP diaphragm was bad and allowing fuel to run “downhill” from carb and voila, issue solved.
 

Big_John

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No flooded hard start, no black smoke. Pour gas in the carb or use ether and it starts right up.
I would say that there's no internal leak then.

So, it's either syphoning back or evaporating. Just to toss a couple ideas out there... Is the heat riser valve stuck closed by any chance (excess heat in the manifold). Float height? Fuel line against something hot?
 

Trace 300 Hurst

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So who is I84885 (new member) who had the audacity gave Big_John a "DISAGREE" but didn't have the cojones to offer his own highly regarded, learned opinion regarding the OP's carb problem?
 

Big_John

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So who is I84885 (new member) who had the audacity gave Big_John a "DISAGREE" but didn't have the cojones to offer his own highly regarded, learned opinion regarding the OP's carb problem?
Most of the time "disagree" is hit by accident. Not a big deal even if not. I've been wrong before.
 

Snotty

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It does sound excessive, but with today's gasoline the issue is vary common. I have the issue on both of my carbureted cars.
 

'66 Fury I

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Very interesting. Until this summer, I had been able to use alcohol free gas. Now, in thier "Wisdom" our dictators have forced us to use e-10. First problem- my 1 year old fuel pump failed. Valves not holding. Pump replaced and extra filter installed before pump. Second problem- Thermoquad flooding. Needle & seats replaced, along with new square rings in the jet wells, problem solved. Third problem- longer cranking on initial starts. Now, I guess you have provided the answer to the third problem. When I set up the carb, years ago, I blocked the hose nipple for the external vent. I like Big John's idea about the extra fuel pump. I'll add that to my "to do" list! Thanks to all!! Lindsay
 

polara10x500

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I'm using 100% straight unleaded with some octane boost, hoping I get close to 90 something. thinking bout trying some racing fuel. but I've had no probs so far with my mix, TN, 383 4brl,eldebrock, ele. choke, electronic ignition conversion, correct MOPAR spark plugs from factory made by champion and correct wires, correct dizzy cap and button, 10 to 12 BTDC initial, lots of things to give consideration to when dialing in to make these motors run right. Hopefully I'll be good to GO...4 a while. I miss that pungent hot gasoline from the back then. Seems like a lot of adjustment has to be made more and more today to keep yesterday here with us. Open to all suggestions on fuel and tuning especially thoughts on sea levels and where we drive these cars in North America today?
 

D Cluley

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Modern gas or not, that doesn't sound right. I have the same car, engine, carb & climate and mine will start fine unless it has been sitting for 4-5 days. I am usually running a mix of 89 and 93 from Speedway.
 

Toolmanmike

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Hi;
Thanks for the input.
No flooded hard start, no black smoke. Pour gas in the carb or use ether and it starts right up. Nothing comes out of the accelerator pump jets. Carb is an AFB, hard to have a leak in these. But I can try to find another one.
Mark
Pretty normal nowdays especially with a AFB and todays gasoline. E10 is even worse. I bet you can go for a cruise, shut the engine off, and you can hear the fuel boiling in the carb. A carb case spacer/insulator may help but you can't stop the evaporation just slow it down. Big John has some suggestions in post #3 and #8. The electric charge pumps work well and the carb/float adjustments and the heat riser operation are all very important.
 
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