going crazy again with the msd box and no fire from the coiil

[Kevin71]

Had the msd box replaced.  It ran but had carb problems that have been resolved or so I think.  But in the mean time MSD box quit working.  Took it back to shop they said it had a blown fuse.  Ran for a day but was cutting out at about 2300 rpm to 2800 rpm.  Guy that worked on the carb suggested moving timing up a couple of degrees and see if that makes a difference.  Anyway left it in the garagae a week and tried to crank.  It is doing the same thing now.  It has power to the box, but no spark from the coil to ground.  When it did this before MSD said the box was bad so I had it replaced at a cost of $350.  I can't believe the box would go bad in a month.  It blow a fuse and replaced that it runs for a day then stops.  I am at a loss.  Ideas?

[Strawdawg]

First, I would check to see if the fuse blew again.

Second, I would check the voltage going into the box at about 3000 rpm to see if it is less than 15 volts-noting that you may need to remove the box from the equation as I asked you to previously and connect the distributor directly to the coil in order to get it going if the fuse is not blown.

If the voltage is less than 15 at 3000 rpm, or so and it runs okay when the box is not being used, I would probably try a different coil.

Only two things that normally would blow the fuse is possibly too much voltage or a coil that is pulling two much current.  An obvious third thing would be a short in the feed from the box to the coil.

I would also be talking to MSD about the issue instead of a shop.  If you bought a new box, it should have a warranty.  If not, MSD should be able to better test the box and verify it's function.  Hopefully, it is not one of the counterfeits that hit the market some time ago.  The miss between 2300-2800 makes me wonder about the integrity of the box, but it could be a bad coil as well.

[Kevin71]

I will have to bypass the box to get it running.  Then check the voltage from the power line that would have run to the box to see if it is to high at 3000 rpm?  Is that correct.  I am guessing that I would disconnect the box if it is not working while it is running with a direct line from the coil?

[Strawdawg]

yes, remove the box from the equation and run the engine with the distributor and the coil only.

You did this before as I recall.

If the engine runs, then check the voltage on the feed that goes to the box at 3000 rpm....I want to know what the voltage is at higher rpm to make sure the alternator output is being regulated correctly.

Also, I would like to know if the engine runs correctly with the box eliminated-using the distributor only-no misses

[Kevin71]

I am getting 14.5 volts at 3000 rpm.  Should I go ahead and replace the coil?  Then drive it to see if it has quit missing?  Also I should have mentioned the missing didn't start until the motor was warmed up.  Could the coil be getting hotter causing expansion in the wires and then loosing contact intermittently causing the missing..

[Strawdawg]

first drive it and see if it misses with the box not being used...just running with the distributor and the coil.

If it misses then, I would change the coil if you have one you can try, and see if that cures the missing problem.

[Kevin71]

Replaced the coil and it is still doing it.  I think I have other problems besides the msd box.  Car wouldn't crank when it was wired in.  I bypassed it and it runs.  But it still just starts sputtering and acting like it is going to just quit running.  If I pump the gas it will eventually rev up and smooth out.  If you floor it then let off and just drive normal it starts the sputtering and acting like it is going to shut off.  Could the carb have a little piece of trash that could cause this?  Linkage all seems to operate smoothly without binding.  It has a 3/8" gasket of some fiber material.  could it be that the fuel is boiling due to heat?

[Strawdawg]

The voltage was okay to the box when you checked it, and, changing the coil did not eliminate the miss...so I think it's safe to say the box needs to go back to MSD for service.

Now, I think you said you are in Florida so it's pretty hot.

If the car runs okay before under hood heat builds up, then I would kinda doubt that the carb is bad and it may well be a heat problem.  If it were me, I would monitor the fuel pressure when cold and when hot.  It needs to be around six psi at all times.

I use about an 1/2" wood composite spacer which has less heat transfer than the plastic versions, but plastic should work.  CP posted a link to a new spacer in one of the threads the other day that is supposed to be the best thing going.

Are you using a factory mechanical style pump, or an electric pump?  Return line?

When you turn the car off after it gets hot, is the bowl still filled with fuel to the correct level after a few minutes have passed?  Both bowls, I should have said.

[Strawdawg]

and be sure, after all the carb playing you did before, that the fuel level is set correctly in both bowls.

Also, if you let it idle with the hood up for awhile after it is cranked up, does it still run smoothly?  And if so, does it not run properly after it sits and runs with the hood closed?

[Kevin71]

The float level was a little too low.  I adjusted them up a little and it cured the problem.  It wasn't by much either.   

[Strawdawg]

You should be getting to be a pretty good troubleshooter by now.

[Kevin71]

I am definitely getting better.  But have a long way to go.  I couldn't do it without the help of everyone on this forum.  I can't thank everyone enough.