Won't Start When Hot

I have a blown BAE in a pulling truck. I have ran into this issue twice now. The first time I ran not very far and they pulled my kill switch because of an issue with the sled. I took it back around to go last, which was maybe 30 minutes later. It wouldn't start. I put a spare points box on and it started right up. I sent the old one to MSD and it was the typical nothing wrong with it answer. Well on Saturday I ran it again with the spare still on, total running time was 2:20 and the actual run was 14 seconds. I was the last one to go and got into the pulloff. I fueled it and took it back around and it wouldn't start. It would turn over fine and seemed like it was spinning as fast as normal. After I got it home, it started right up without changing anything. What could be going on? My first thought was the points box, but then I thought maybe when it's warm it isn't spinning fast enough to fire on the crank trigger?

 

I've tried to start it a couple times after a run now with no luck. I hooked up a new points box the first time and it didn't help. Then I changed my kill solenoid and that wasn't it. One weird thing happened when I was trying to start it though. It hit one or two times just as I turn the mag off. I flipped it back on to see if it would start and the same happened when I shut it off again. It runs good, I always drive it off the track pull the fuel and shut it off. I think it's either down to the coil or cranking speed. Would the coil do that when hot even though I shut it off and there are no problems at the end of the run?

 

It is a blower mount starter on 36v. Crank trigger air gap is .050. Plug gap is .015 on 6061-10. I haven't tried just off the mag yet, that was going to be next on the list. I set the mag to 26 degrees and then set the crank trigger to match, so it should be phased in correctly.

 

I got the coil used with the motor, I have an old black one for a spare. Coil wire is new this year (~15 runs). The rest of the plug wires are on their 3rd season. It starts every time to warm it up in the pits and then starts fine to run after the warm up. It's just after a full run that it won't fire.

 

It is a cranking speed issue. After I ran it wouldn't start again. I switched over to the mag and it started right up.

 

What do you think it could be then?

 

I agree with Mike , a blower mounted starter does not normally have a cranking speed problem but as he said you could have weak magnets on the crank wheel. Been there! had 2 weak magnets on a wheel that was new, well a few years old but on the shelf as a spare never before installed on an engine and it would drop cylinders then quit all together.

 

Don't know as I have not run into this issue but am curious.
Once everything is warmed up from a run does the crank trigger gap change?

 

Just throwing ideas out there--- can a ground problem cause this ?

 

Not sure if you have found the problem on this by the recent posts....but I wanna tell a little story about a hot ignition system problem when I was a kid....

1967 Camaro with a big block and Mallory Uni-lite distributor (bought used at a local racer's garage sale)...ran fine around town for short distances....but when I went from Des Moines, Iowa to Car Craft Nationals in Indy 1979...the engine quit several times along I-74...no cell phone back then so we opened the hood to have a look....after a few minutes it would fire right up! Basically was a heat sink issue with the distributor or coil....high under hood temps created an "open" in the ignition system and when the temp came back down it would work fine...so I swapped to another ignition system and drove the car for many years...

Your coil and points box are probably mounted away from heat, but it sure sounds like a heat sink problem in the Mag, coil, switch or wiring to me...creating an open or ground...

 

I don't know, never checked that. I can see next time I run. I got a changeover switch on the way and we'll see how that does.

 

Change over switch should do it - I have one mounted in the cockpit as our car generally won't start on the trigger that runs through the command module. The batteries all test fine, same with trigger, new ignition components etc.. I'll have a new degree wheel on it so I suppose we can see if weak magnets are an issue. The switch setup it fires every time. As soon as the motor is running I flip the switch and then roll forward to the burnout box.

Brandon Booher
NHRA TAD #323
Torque MGMT. - Owner

 

I didn't see it mentioned, but remember the more OD the slower it cranks.

 

On 36 volts with a crank trigger and blower starter you get very close to cranking too slow. A weak battery , poor connection, marginal solenoid. or any other problem and although it may sound like it is cranking just as fast , it is just a little slower and won't start. At Lebanon valley we had to change a motor for the final and had thick cold oil, we went to start it as usual with 36v in the pits and it wouldn't fire Changed everything in the ignition (coil, box, grid, pickup, Mag, wires) and still no fire. Got our starting line 48 volt pack pluged it in and it fired on 1st revolution. beware of using 36volts.

 

Well it may not be a cranking issue after all. After I got my change over switch I tested the timing on both the mag and crank trigger to be sure it worked, it did no problem. Last night I got into a pulloff, so I started it on the mag and all was fine. As soon as it was switched over to the CT, it just died. I switched it back to the mag and it started back up. I just left it on the mag to make my run. There is an MSD 8168 in the loop as well. Could that be causing anything? When the changeover switch was flipped it was just like shutting the mag off, even though it worked when it was cool.