high rpm miss duraspark msd

bullseyeprec · #1 (**permalink**) 03-14-2004, 10:20 AM

I went to the dyno last week and got a high rpm miss. I dyno'd at different total advances, 32, 34, 36, and 38 and made most power at 32 degrees total. I am running a 392 crate motor with a duraspark dist and a msd 6al. Problem is I start missing at 4200. We swapped rev limiter chips 3 times and checked cap, rotor, plugs, and cap. Seperated plug wires and replaced coil. no change. We swwapped power valves and jetts to get 12:1 air fuel ratio but miss is still there. The dyno guy says its spark for sure since his equip reads spark discharge I just dont know whats wrong. He siad he doubts the msd is bad. I am not using the vacuume advance. Today I pulled the cap again, pulled the rotor, pulled the plate the cap screws to and looked at the plate that would rotate if the vacumme advance was hooked up. It rotates in a circular fashion as one would expect under diaphram resistance...BUT, It also wil lift up and down. There doesnt seem to be anything holding it down. Also, there doesnt seem to be anything to limit up and down travel so it doesnt look like anything is broken? Is that a problem? If not, any ideas? I sure appreciate your input. By the way..I dynoed 339 HP and 360 torque at about 4200.

vbdig · #2 (**permalink**) 03-14-2004, 12:33 PM

I am having a very similar problem with my new 392. I have changed jetting and carbs, I have a Barry Grant on order now, but I think my problem is ignition also. I have the msd 6al digital, and I am getting a miss under light acceleration at about 3000-3500, and a definet dropping of 1 or 2 cylinders under hard acceleration above 4000 rpm. I did weld on my car without disconnecting the msd unit, maybe my problem is there. I am going to install a new one this week and see if the problem is corrected. By the way, what jett size and carb size did you end up with on your car.

bullseyeprec · #3 (**permalink**) 03-14-2004, 12:51 PM

I went from 68 primary jets all the way tp 80's in the front and 86's in the rear. It ran best on the dyno and had best 02 readings with 70 in front and 76 in rear with a 8.5 power valve and 28 front squirter and 31 rear squiter. I am at about 450 feet asl. Ford said to use 80 and 86 jets but that was way rich for mine. They were right on the money as far as timing goes. Cant get this miss out of it though. I also had a bracket welded on the car before I got it but I thought MSD boxes either worked or didnt? I am at a loss....I am using a 750 holley by the way.

CSCHWER · #4 (**permalink**) 03-14-2004, 01:09 PM

If you are using the magnetic pickup lead, try rerouting the entire wire.

bullseyeprec · #5 (**permalink**) 03-14-2004, 02:03 PM

I did that too....the dyno guy suggested that and we did it right there on the dyno, no change. Keep the input coming...I am at your mercy!

Edbo · #6 (**permalink**) 03-14-2004, 02:17 PM

Neither of you with a similar problem mentioned what plug gap you were running.
As RPM increases coil saturation decreases. Try closing up the plug gap and see what happens.

My 393 will do something similar when the vacuum advance is hooked up. Revving it in nuetral will cause a slight misfire above 3K RPM because under no-load the advance goes to about 48 degrees. Not a problem while driving, unless I do it on purpose.

I run a Mallory coil and .045" gap on Autolite 3924's.

Ed

bullseyeprec · #7 (**permalink**) 03-14-2004, 02:41 PM

I am not using vacuum advance. Will verify plug gap again though. when using vacuum advance things really get bad...gets too much advance when vacuum peaks so I disconnected per fors tech advise

vbdig · #8 (**permalink**) 03-14-2004, 03:50 PM

I had a similar response when I increased jet size. I am at 3500 ft altitude, and went clear down to a 68 on primary, and a 67 on secondary. Also a 4.5 power valve. It ran pretty good but the Holley tech people said something must be wrong with the carb to need jets that small. I sent it to them and they said nothing wrong. I think either my msd is faulty or this engine needs far less fuel than everyone says. By the way, my plugs were gapped at 45 from ford racing, I have tried increasing gap to 55 with no improvement.

Bruce Edwards · #9 (**permalink**) 03-14-2004, 05:28 PM

What kind of plugs are you guys running? What compression? The more compression the smaller the gap. Try the old standard of 35 for a gap and see what happens. You may also want to stay away from exotic plugs. Try Champs or Autolites. Start in the middle of the heat range for your engine combo and move it up or down depending on how they look after running them.

bullseyeprec · #10 (**permalink**) 03-14-2004, 06:22 PM

Problem solved. I pulled all the plugs today and they were gapped at .040 and I found 1 of the plug wires cut in 2 when the wire ties were cut I guess the guys hit the wire. I replaced it and runs great. Back to the dyno tomorrow. If I got 339 HP on 7 cylinders I wonder what I will get on all 8 of them? My first coil was bad for sure, thats what mad us start trying other possibilities. You would think with new parts everything is good but sometimes its not the case. Hoping for 400 RWHP tomorrow but will post stats tomorrow pm. Thanks guys.

bullseyeprec · #11 (**permalink**) 03-14-2004, 06:24 PM

vbdig, I think you have the wrong powervalve....I ran my car at idle and it makes about 15" vacuum and running down the road it makes about 11 or more so I went with a larger number. With such a low number your engine may never drop that low to utilize the powervalve. If you have the same setup as mine, try a larger number powervalve.

vbdig · #12 (**permalink**) 03-15-2004, 07:50 AM

I'll try it, I am waiting for my new Barry Grant race demon, I talked to them and they recommended that model because of cam overlap. Do they take Holley jets and powervalves?

Edbo · #13 (**permalink**) 03-15-2004, 07:54 AM

Yes, the Demons use standard Holley jets and PV's.

Ed