Engine stumbles to a start.

[dustman_stx]

I have a built 390 and a 360 with a few mods that both have the same problem. When cold, you pump the gas a few times, hit the starter and they fire right off. However, when both are warmed up you have to crank for a few seconds and they stumble to a start. I have tried lightly tapping the gas then cranking, tapping the gas while turning over, a couple hard pumps then cranking- all of which make it even harder to start. Both act like they are flooded. I have adjusted the idle circuits on both according to the insructions with the carbs- one is a Holley and one is a Demon. I know for a fact the bowl levels in the Demon are fine because they have sight glasses. I am running initial timing at 15 on the 390 and 12 on the 360. Any thoughts?

[fordman]

my first thoughts are heat soak or timing too far.

[dustman_stx]

I don't completely understand heat soak, but I'm inclined to say no on that. From what I understand, fuel has to boil over out of the bowls, right? The 390 runs really cool. It has a 1" spacer, which at one point was a phenolic and it was doing the same thing then. I am wondering about the timing though. I've had it as far advanced as 20 degrees with no detonation. Should I maybe back it off to 10-12 initial?

[knightfire83]

I vote for heat soak too.

With today's fuel especially with ethanol blends, they boil at a much lower temperature than way back when.

Fuel will boil out the main jets and out the venturies, puddle in the intake below the carb, and it floods the engine on initial start up. As you discovered you will have to hold the throttle open while cranking to get'er going, and pumping the gas only adds to the problem.

Radically advanced timing will slow cranking, but it wont cause the engine to have flooding symptoms.

Back when my truck had a 2 barrel carb, I had to use a 1/2" phenolic spacer and 2 large 1/4" fiber gaskets on each side to solve heat soak.

Makes you wonder if people should get rid of their aluminum coolant heated carb spacers on our trucks, as those items weren't designed for today's fuel blends. That is unless the vehicle is operated in cold climates most the time.

[Redcap]

Sounds like heat soak to me too.

[ximy]

Mine does that a little bit.. you can actually see the wet venturies with a flashlight... after driving and the engine is warmed up.

[george worley]

When you shut the engine off open the hood and see if that helps it any. That lets cooler air get in to the carb. I have that same problem down here where I live and it is caused by high under hood temperatures and crappy fuel. It doesn't do it when the weather is cooler so I just live with it in the summer.

[dustman_stx]

I have an electric fan on it that rarely comes on- actually, it never has come on when the truck is running and the water is flowing. Maybe turn it down to come on at a lower temp? It's not even coming close to overheating, but maybe moving some air around under the hood would cool it down under there?

[cep62]

What can happen is the heat from the engine expands the fuel when it sits.
You get back in the truck and it shakes a sloshes the fuel so it floods .
I set my float level a "little" low and haven't had a problem.

[fastEdsel]

Try holding the gas pedal either half way or all the way down just before you crank and hold it there and they should flash up. I also would vote for heat soak and holding the throttle open is one way to start a flooded or flooding engine.

[dustman_stx]

Try holding the gas pedal either half way or all the way down just before you crank and hold it there and they should flash up. I also would vote for heat soak and holding the throttle open is one way to start a flooded or flooding engine.

Thanks for the responses so far. Maybe I'm being dense, but I'm really having a hard time with this idea of it being heat soak. I've got a 1" spacer, my bowls are set exactly as they should be, the truck runs really cool- as in 1/4 the way over at max. I've got coated headers. If this thing is getting heat soak, I don't see how this wouldn't be a consistent occurrance on every carbureted vehicle made. Running an aluminum intake should cut down on heat soak as well. I don't see how you'd ever get one fired off with a cast intake. I may have mentioned this before, but I have initial timing set at 15 degrees, I think. I did notice a slight amount of pinging/detonation the other night when the engine is lugged down. Maybe back the timing off just a hair? Don't know what I'm running total advance, but maybe around 10-12 would be more appropriate for initial? Any other possible explanations besides heat soak?

[knightfire83]

If this thing is getting heat soak, I don't see how this wouldn't be a consistent occurrance on every carbureted vehicle made.

It is dependent on engine / carburetor design, fuel line routing, and operating conditions. Other makes and models are effected more or less severely depending on their configuration. Rochester carbs (GM) especially 1 & 2 barrels will boil the fuel right out of the bowl vent, and yet doesn't end up in the intake. The problem is more severe for late 80's and earlier designs running in hot climates. Almost anything with fuel injection has a return line to the tank and there is a constant flow of cool fuel to the engine, thus eliminating heat soak.

Running an aluminum intake should cut down on heat soak as well. I don't see how you'd ever get one fired off with a cast intake.

Actually, aluminum transfers heat allot faster than other metals. So parts get just as hot... quicker.

I have an electric fan on it that rarely comes on- actually, it never has come on when the truck is running and the water is flowing. It's not even coming close to overheating, but maybe moving some air around under the hood would cool it down under there?

This is highly suspect. How are you measuring the engine temperature? If it the factory gauge, they are usually off by quite a bit, might want to double check that. And yes having a fan running will evacuate the hot under hood air, especially problematic with running headers.

It is not my intention to push the subject, it certainly can be related to other problems. The choke plate can malfunction, the carb can be cracked and leaking, or the thermostat can be stuck or in the wrong heat range, etc.

The truth is that your vehicle is flooding when hot and you try and restart. Heat soak is just the most common cause of the problem that I and others have identified on our vehicles.

[idaford]

Ive noticed that I have the same problem. I have a newly built 390 with a cam , performer intake and performer 600 carb man choke and hedders. It cold starts right up, but after it is warm I have to crank it for a bit to start it . If I hit the choke or touch the gas it wont start, unless I hold the peddle down. I have never heard of heat soak. So what is the best fix? Different carb, adjust float, ice pack on intake My buddy has no problem with his. It fires right up. He has a Street advenger carb and Pertronics. He says that Pertronics is what fixed his.
Todd

[dustman_stx]

Ive noticed that I have the same problem. I have a newly built 390 with a cam , performer intake and performer 600 carb man choke and hedders. It cold starts right up, but after it is warm I have to crank it for a bit to start it . If I hit the choke or touch the gas it wont start, unless I hold the peddle down. I have never heard of heat soak. So what is the best fix? Different carb, adjust float, ice pack on intake My buddy has no problem with his. It fires right up. He has a Street advenger carb and Pertronics. He says that Pertronics is what fixed his.
Todd

I'm running a Mallory Unilite (under the Summit brand name) with an MSD Digital 6 ignition, so lack of fire is not the cause, IMHO. This is also happening on my other truck with the 360 with a Pertronix Ignitor 2, 600cfm manual choke holley. That's what leads me to think it is something with my carb or timing settings. I'll list the basic setup on each of these engines, both of which exhibit almost identical symptoms.

360
Stock compression
Stock crank, rods, pistons
Rebuilt with new seals, rings, bearings, etc.
Performer RPM intake
Holley 600cfm manual choke
Pretty standard cam, slightly bigger- what some would refer to as an RV cam
Mech fuel pump
Pertronix Ignitor 2, Accell SuperStock coil
Autolite 45's and new plug wires
Timing at 12 degrees initial
Stock manifolds and exhaust
Standard water pump with mechanical fan

390
10:1 compression
Forged pistons and rods
Aggressive cam- 292S Comp Solid Roller
Roller lifters
C8AE-H heads ported and polished- CJ intake and exhaust valves
Hedman headers with 3" dual exhaust and H Pipe
Mallory Unilite ignition with MSD Digital 6
Holley Blue fuel pump, mounted correctly near the tank
Mighty Demon 750cfm mechanical secondary, no choke
1" spacer- currently aluminum but at one point was phenolic with same symptoms.
Performer RPM- had a Streetmaster 390 on it with same symptoms at one point.
High Flow water pump with electric fan
Timing at 15 degrees initial

[dustman_stx]

Update:

Played with timing last night. I thought my initial timing was at 15, but was actually at 18. Set it at 12 and decided to see what it was doing all in. Now, I'll start by saying that I'm not that educated about timing and carb tuning, so I'm learning as I go. My first mistake, I think, was assuming the distributor would have a usable setting from the factory. It has vacuum and mechanical advance. I revved the engine (couldn't see the tach, so I'm guesstimating here), and at around 2000-2500, I could see the vacuum advance kick in and pull the timing to around 25 degrees. I continued to rev and by I guess 4,000 this thing was advancing to over 60 degrees!!! I should be seeing all in by 3500 or so at somewhere around 36-40 degrees, right? I ended up blocking off the vacuum advance and right now it's running just mechanical. Ran it down the road and it seemed to do pretty good. I don't think I'm getting but about 32 degrees total right now, and it's coming in a little late. Also, I took off the air cleaner cover to see what was happening when I kill the truck hot. There was a LOT of vapor swirling around- lots of gas smell- this after sitting for a few minutes. Primary fuel bowl was sitting a little high, so I'm thinking I need to make an adjustment there. I am wondering if one of the K&N filter lids might alleviate this problem by allowing the heat to escape through the top of the air cleaner as opposed to holding it in? Any thoughts?