Aaargh- SBC, Hot start issue

[Mad Chemist]

Well, I'm about ready to yank my remaining hairs out. I've got a fresh rebuilt SBC, HEI distributor/coil, quadrajet carb. Starts like a champ when stone cold in the morning, but when I run it for while and get it up to operating temp, I have a hell of a time trying to get it started again if it's been shut off for a couple of minutes (ie. like going to the gas station). Let it cool for a bit, then it'll start right up again.

It turns over like a champ (until I crank long enough to drain the battery), so I don't think it's the starter. I thought it was a vapor lock issue, so I got rid of the mechanical pump (to give me more flexibility in running the lines), wrapped the fuel line in insulation and ran it away from hot components as much as possible. I mounted an electric pump reasonable close to the filter. This did not help.

I added a phenolic spacer between the manifold and the carb, thinking the carb was getting hot and percolating fuel out of the bowl and into the manifold, flooding it. This did not help.

I can smell fuel if I turn it long enough, so it seems to be getting flooded (from turning over so much, even without touching the gas pedal). I added a manual choke, to make sure the choke wasn't closed during a hot start. This did not help.

The one thing I noticed tonight was that the lights in the cab dim during my most recent start (while cranking), I'm wondering whether it may be a spark issue- maybe something in the starter is grounding out when it gets hot, so that too much juice is going through the starter and the coil is not getting a full charge to send to the plug? I replaced the coil, and that didn't make a difference.

Any thoughts on what might be going on, or things that I could try to diagnose it?

Thanks,

David

 

[splib]

I've had that happen with the 283 SBC in my '72 when I try to restart it after it's hot. If it doesn't start with a quick turn of the key, I hold the throttle to the floor and then turn the key; ususally fires right up. I've just learned to live with it after trying different timing and carb adjustments.

 

[Mad Chemist]

Wouldn't mashing the pedal and holding just dump more fuel into the carb?

 

[craptabulous]

it also lets in more air .

i have basically the same set up as you just i still run a mechanical fuel pump and no spacer . to start mine hot i give it a 1/4 throttle as soon as i start cranking and it fires .

if it floods hold the pedal all the way to the floor and crank it till it fires . don't pump it the accelerator pump will shoot fuel in the engine and make it worse .

do you still have the heat riser on your engine ? i took mine off and run a manual choke .

 

[Mad Chemist]

No heat riser, and also put a manual choke in.

I'll try the full peddle and hold trick the next time.

 

[dargreg]

Had same problem sbc 350 went to the Mallery muli spark end o f problem , much better fuel milage .Your engine will hardly only turn over once before it bounces to life .Also old or bad gas will do the same thing,however the mallery will burn that also.

 

[Mad Chemist]

Huh, it may be worth a try.

 

[dgangle]

I suspect something in the HEI distributor, Q-Jet or power to the distributor while cranking. Check to see what voltage is being supplied to the HEI while starting. If under a certian voltage, I understand the output can be greatly reduced.

I have the same set up as you but with a mechanical fuel pump and 10 or 110F, it fires to life in milliseconds, certailnly less than one revolution of the engine when turned by the starter.

 

[fj404life]

This sounds familiar to me. Does the starter crank slower when hot (as if under more of a load)?

 

[HAMBURGLAR11]

had this issue a while back. same exact symptoms. take off the distributor cap and beneath the rotor you will find a little gem called a control module it looks like a small matchbox that has terminals plugged in on both sides and should have 2 screws holding it down.. this module will cause the distributor not to spark. go to Vatozone and get a new one be sure to put down a base coat of diaelectric grease that should be included. pretty sure they can check them prior to purchase and it only cost bout 15 bucks. hope this helps

 

[oldracerguy]

HEI voltage

Had the same issue with my SBC. ended up being the 12 volt source to the HEI distributor was tied into the same line as the electric fan. ran a dedicated line and solved the issue. check your voltage when you are cranking with a digital voltmeter, you may be surprised. I think the HEI needs at least 11 volts to spark.

 

[Josie'sLandCruiser]

Hi All:

"MadChemist" appears to have figured out what the issue was. He just needed to get out in the mountains, breath some fresh air, etc. to clear his mind.

I'll let him elaborate.

Regards,

Alan

 

[Mad Chemist]

Well, I think I got it figured out. I had a mallory cap, and a pertronix rotor in there. When wheeling today, the "usual issue" occured, no starty after stopping for lunch. Pulled a plug and discovered I wasn't getting any spark. I checked the voltage, and ground for the distributor were fine. I the the metal tab on the rotor was just far enough out of spec between the two manufacturers to not be able to send the spark to the plug. I put a spare mallory rotor on there (to match the cap), immediately got spark and it fired right up. I think when it got hot, the cap expanded just enough so that contact couldn't be made? At any rate, have been able to shut down and restart since the mallory rotor was put in.

Thanks all for the input!

Special thanks to Andy and Alan (Josie's LC) for the patience on the trail to diagnose this!

 

[FJROME]

Thanks for the thread and all the info, I've been struggleing with the same issues. Hopefully not for much longer.

Jerome