No re-start when hot.

Jeff N

Jeff N

Your Mileage May Vary
So a new problem has suddenly appeared. The car starts up fine when cold, runs and drives fine and the temps stay at 190 or less. But the second I shut off the motor, it will not restart for at least a half hour. It doesn't matter what I do to the carb, choke open or closed, spray starter fluid in it, pump the gas, don't pump the gas, whatever, it just won't start. Not a cough or sputter, just cranks until the battery is done.

It seems clear that this is likely a heat related issue, but I can't figure out what. I have a 34 DAT carb, water temp actuated choke, no cold start solenoid, an after-market air cleaner and took out the carb fan years ago. I haven't done anything to the engine recently other than I just had it out to re-install the transmission. I checked the spark yesterday when it happened and there didn't seem to be any at one point. I have a Bosch electronic ignition that I've seen mention can go bad. Can they malfunction based on temperature?

Ideas?
 
I had a similar issue on 78, 1300. It turned out to be the coil. Cooked the coil when sitting idling in a traffic jam for 3 hours after a flash flood. Started fine when cold but as soon as the coil warmed up, no go. I guess the insulation broke down and it wouldn't restart.
 
The thingy

Someone can add a technical note to this ,but there is a little slider valve thingy on the carb. On mine ,I opened it and wire brushed some corrosion off the piston slider thingy and it stopped doing what yours is doing...hope this helps enough for you or someone else to know what I'm talking about:)
 
I ordered the GM HEI ignition module and will try Bob Brown's ignition module rebuild project. I figured that even if that's not the current issue, it will be an improvement on the electronic ignition. I may add a heat sink to the case, as heat seems to be an issue with the new module as well.
 
had this issue....

ever since I put a 34 whatever carb on my car, but it's random. Something I hope to work out this time around, since I've rebuilt the engine. Always starts cold, but warmed up, and sometimes I gotta pump it, or hold it down to the floor....frustrating

BTW, you'll love that HEI conversion, got it on my Pex, very reliable
 
The Bosch electronic ignition hasn't ever been an issue before, ever since I all but stole it on a no reserve eBay auction from someone here I can't remember for like $15. But the fact that it won't start or have any spark when hot seems to point right to it. I guess we'll see if the new module will cure it or not in a week or two when it arrives and I can get it put together. At least I can do all the prep work on it while I'm waiting.
 
Is it definitely a no-start?

Is there no spark at all? Or is it fuel-related? I find that it tends to be fuel-related on 90 to 100F days. If you touch the gas pedal after it is hot-soaked, it definitely will not restart. It will flood with one squirt. What I do is keep my foot off the pedal and crank, but have that foot ready to start gradually pushing down the pedal if the crank goes longer than 12 to 15 seconds, or ready to start pushing down on the pedal very gradually after the first tentative coughs of ignition. A good pump after it is hot and you will definitely be sitting there for a half hour!
 
I'd be putting my money on a carb issue. I had a similar issue with a 34 DMTR. Very frustrating! A full rebuild of the carb fixed it but my suspicion is that it was related to incorrect float levels.

Dom.
 
I KNOW, I KNOW, ME-ME-ME!

Being a '78 I was puzzled a bit until you said Bosch Electronic Ignition!

I can guarantee you that is the problem!

I ALWAYS CARRY A SPARE with me...

Prove it by accessing the ICM, unbolt it if you like... start the engine when cold and then heat up the ICM with a hair dryer. Shouldn't take but a minute to die at 1200 watts.

If what I said proves to be true... you will be the SECOND one I know of in 30 years that had this "soft failure".

Fixes include a replacement, or using a GM Module, or installing a GM module in a stripped-out Bosch case.

Here is the Schematic:



I don't know who to thank for the drawing but I believe a correction is needed.

The GM Module will probably need a good 12 volts that can be found on the BATTERY side of the Ballast Resister (BR). The Coil needs to operate on +9 volts though, and that is why the BR is there. You CAN run +12 volts to the coil (an old hot rodder's trick) but that is also why those old hot rods were on the side of the road with a burned-out coil. Good for 1/4 mile drags though...

So the drawing should show a SEPARATE wire coming from a +12 volt source (like the battery side of the BR) and NOT from the + side of the Coil.

Let us know how you do, Jeff... I'm almost POSITIVE this is the problem.
 
Don't bypass the ballast resistor

Unless you want a shorter life from the module and coil. :)
If you are using the stock coil, use the resistor.
The reduction in spark output is quite small and the components suffer when
you omit the resistor. Unless you change to the Bosch coil that does not require
the resistor, I would not recommend bypassing it.
 
Tear down of the unit to do the Bob Brown GM ignition module rebuild project has already started and the unit has been gutted, so I'm pretty much committed. I'll get the GM module in the mail next week and am hoping to have it all ready to just solder in and be done.

One of the times the car wouldn't restart I checked for spark at the plug, distributor and coil and found none, which is what led me to previous posts about the Bosch electronic ignition temporarily failing when hot. If it turns out not to be the real problem, at least I'm still ahead with an updated solid state ignition module for under $20.

When the car was hot, it would just crank until the battery failed with not so much as a sputter from the engine, no matter what I did or didn't do to the carb. When the motor had cooled, it would catch and start almost immediately, just like normal. It definitely felt more electrical than carb. But who knows, I've been wrong before and will be wrong again. Hoping next week to find out this wasn't one of those times.
 
I have had the exact same problem on my 81. A truck mechanic buddy suggested I take an ice pack and apply it to the ICM when I have this problem. The opposite of putting a hair dryer on a cold ICM but the same theory. Haven't tried it yet but I am following this thread with great interest.

Had a TR6 a long time ago that would not restart when hot and it was in fact the coil. Difference was it would actually die at speed and then fail to restart.

If anyone has the skills to build the ICM being discussed (I certainly don't) I would be happy to pay a premium to the constructor.
 
Greg, $10 vs about $40-$50 from what I could see at the parts stores, and I was apparently feeling far more patient than I usually am.
 
Felipe,
As Greg pointed to me earlier, an excellent tutorial by Bob Brown is HERE. If mine turns out satisfactory, send me yours and I'll rebuild it for something less than a "premium".
 
Thanks Jeff, keep me in the loop. The tutorial is wonderful but I know my limitations. Really interested to see how this works out. All the best and thanks in advance!
 
I'm with Larry...

what I gotta do sometimes, but all my woes started with the 34 model carb, and I have piddled with it a lot, trying to get it tweaked. Haven't got the car down on the ground yet, but starting it a lot, and just warming it up, and it seems to be better, after lots of reading and adjusting....kinda whats expected if you want a hot rod, carb was made for a car of somewhat larger displacement

as for that ignition system your building, as long as you bolt it to metal, you should have the heat thing conquered
 
Bob... are you saying the GM ICM operates on...

the Ballast Resister's +9 volts versus the +12 Battery Voltage via the ignition switch?

Is this YOUR drawing? If so on both points I stand corrected and the drawing is correct.
 
Tony, not my drawing ...

I don't know who did the drawing.
I've never actually made one, but if you look at my photo you will see that the photo below nearly matches the
schematic in your photo with 1 caveat: I leave the resistor in the coil circuit, and you'll notice the module get's
+12v in the same way the Bosch unit did.

ICM15.jpg



Black-Tooth said:
the Ballast Resister's +9 volts versus the +12 Battery Voltage via the ignition switch?

Is this YOUR drawing? If so on both points I stand corrected and the drawing is correct.
Click to expand...
 
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