Ignition problems on blown alky combo

We are having intermittent ignition problems on our blown AJ 481
We run an MSD 44 mag with MSD points box ,coil and MSD digital 7 box
The engine will run perfect one time and refuse to start the next
We have changed the crossover switch , the coil, and checked the points box
This past weekend we had a new problem after switching every component except the mag the engine would fire run for a few seconds and backfire violently. Any suggestions as to what is causing this Thanks

 

Do you have the heads grouned back to the coil with wire? from head to head to coil.

 

I ran into two separate ignition problems in the last couple of weeks, you might want to check these out.

First one was a broken primary wire coming from the mag, it was broken inside the insulation so you couldn't see it. It would fire and run one time, then be stone dead the next.

The other one was a leaky coil wire, it was arcing between the boot and the insulation, I guess the insulation broke down. The motor would idle but as soon as you brought the rpm up, it would quit.

 

Another thing to check (on the no start at least, though I doubt it has anything to do with the backfire after startup) is to make sure you do not have any start retard programmed into the digital 7 box, the generators in the magnetos do not have enough steam at cranking rpm to tolerate the offset.

Richard G

 

Unplug the crossover switch and the Digital 7 and see if the motor will fire. You might even try running it off the mag and no crank, hope you figure it out.

Regards,
Brandon Booher
NHRA TAD #323

 

Thanks for all the replies
We have the heads grounded to chassis and all electronics are grounded to stop any stray voltages We replaced the coil wire before the weekend
We have tried running on mag only bypassing crank trigger and digital 7 This worked in the past showing us a problem in the crossover switch which we have replaced 4 times in two weekends
We think it had enough fuel as it was running out of the header and we followed the same startup routine that has worked in the past
We think we have a problem with the mag as it is the only part which was not new this season
Does this sound reasonable

 

Sometimes extreme basics need to be checked.I once had my timing wheel indicator on the crank hub one bolt hole off giving me 60 gegrees more timing than what I thought.Sometimes it would backfire a bit out the exhaust,and the next it would fire and run,but always started with difficulty.Luckey I caught it before WOT.Was the motor previously apart before this started?

 

No the engine hasn't been apart. All new plug wires and coil wire before this weekend The backfiring problem just started this past weekend.
The funny part of this is in previous days once we get it started it runs perfect
The problem mostly occurs on first start up.
Last weekend here is the sequence
Friday first attempt to start nothing Checked crank trigger OK Checked coil OKtried again started on mag wouldn't run on Digital 7 Changed crossover switch everything good
Made two passes everything good.
Saturday morning same thing again checked everything again new crossover switch seemed to solve the problem.Ran good qualified number 1
Sunday same senario changed crossover switch it fired ran for a few seconds mild backfire and quit. Again changed coil etc again finally fired ran a few secs and a violent backfire We checked everything can't find anything wrong. Maybe a bad wire or bad mag? Our season is over so everything will be checked and freshened for next season.Again thanks for the help

 

If you run a crank trigger, check the "wheel" on the crank. Maybe one or more of the manets has lost their magnatizem. I had a brand new RCD wheel that had two bad magnets, would not fire for shit. Check to see if all four magnets will pull and hold a small washer.

 

Thanks Mike.
It was well below 60 about 45 -50. We are using a mixture of 50 %Gasoline and alcohol. Next year when its cold we will try all gas

 

I had a problem a while back with mine when it wouldn't start at the line. Really embarrassing. Brought it back to the pits, disconnected the crank trigger and it started right up so we ran the rest of the weekend running it off the mag rather than the crank trigger. I called Dave Leahy and he told me if the motor cranks slow sometimes it won't fire off the crank trigger. We had let the starter batteries get low and it cranked slower. So just by charging the batteries to crank the motor faster we fixed the problem. Dave Germain

 

Up here in the cold north we are often in the low 60's or 50's. If we don't use gas to start it in the morning, it just backfires out of the pipes and sounds like the timing is 180 degrees out.