Chasing major gremlin, hope this fixes it!

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RockinRobin

RockinRobin

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For about 6 months now my Duster race car has been plagued by missing/stumble/loss of power that starts at about 6000rpm. This problem only occurs when going down the track. I can do a burnout to 7000rpm and motor is smooth. (No, I DON'T usually burnout at that rpm)
This is a built 5.7 GenIII Hemi that is carbureted. The things I've done (that I can remember) so far are:
Jetted up/down
Fuel levels up/down (carbureted)
Advance/retard ignition
Clean out entire fuel system/filters
New fuel pump-22psi to the front, 6.5psi to the carb.
Relay for fuel pump.
New plugs
New coils
New Crank sensor
New Cam sensor
I gave up and sent the Holly 1050 Dominator back to Pro Systems. It should be returned to me this week so I can take it to the track again. Please cross your fingers for me, I used to win a lot of rounds before this happened.
 
I don’t know much about dominator size carbs, but what is you fuel tank vent like and what size needle and seat. It could be empting the bowls? or sucking them low enough to screw with the afr. Again not much experience with the big holleys
 
furrystump said:
I don’t know much about dominator size carbs, but what is you fuel tank vent like and what size needle and seat. It could be empting the bowls? or sucking them low enough to screw with the afr. Again not much experience with the big holleys
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I've accidentally run it down the track with the tank vent closed and it made no difference. If the bowls were emptying, raising the bowl level would have improved or eliminated the problem. If the setup was wrong (vent/needle seat size, etc) it wouldn't have run right since 2013.
 
Have you looked into possibility of a loose rocker arm? Not sure if you are running hydraulic lifters still or if you have adjustable rockers, but maybe valve springs worn or some valve float?
 
Has anything changed in your electrical system? Do you run an alternator? Is it working? Possibly low voltage causing misfiring of the coils? Is the battery healthy?
 
Bobzilla said:
Have you looked into possibility of a loose rocker arm? Not sure if you are running hydraulic lifters still or if you have adjustable rockers, but maybe valve springs worn or some valve float?
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Thanks, but put new valve springs in, so everything is tight. Revs to 7500 just fine, but not under full throttle load.
 
Sounds like the exact problem i had years ago with a stroker smallblock. Missed at the top of second and third gear going down the track under load.
Chased my tail nearly 1/2 a season
Turned out to be the spark box. In my case it was a 6al at the time.
 
B3422w5 said:
Sounds like the exact problem i had years ago with a stroker smallblock. Missed at the top of second and third gear going down the track under load.
Chased my tail nearly 1/2 a season
Turned out to be the spark box. In my case it was a 6al at the time.
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That's about the only thing I haven't replaced. Several calls to MSD and they are certain is isn't their 6Hemi controller. Come to think of it, I haven't replaced the ignition wiring harness either, but if they fire fine upto 7500rpm under light load I don't see how that could be it.
 
RockinRobin said:
That's about the only thing I haven't replaced. Several calls to MSD and they are certain is isn't their 6Hemi controller. Come to think of it, I haven't replaced the ignition wiring harness either, but if they fire fine upto 7500rpm under light load I don't see how that could be it.
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My 6al failure, car wouldnt miss in the water box, or even in first gear. Just from like 6k in second till i shifted it at 7 into high gear, then in the topof high gear too. Just when a real load was on it.
Popped enough to where it knocked about 4tenths out of the car.
Chased carb, plug, heat soak, fuel delivery issues before swapping out the box and problem never came back after that. I raced points with that car so it was raced most weeks
MSD always says nothng is wrong..... lol
 
RockinRobin said:
That's about the only thing I haven't replaced. Several calls to MSD and they are certain is isn't their 6Hemi controller. Come to think of it, I haven't replaced the ignition wiring harness either, but if they fire fine upto 7500rpm under light load I don't see how that could be it.
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It's not like MSD to lie. How the hell does MSD know the box is good without testing it?
 
Sorry to hear you having problems, Bob. Hope to get back out there with you if I can brave the heat in my garage long enough to get my junk back together.

Is it falling flat on it's face or surging?

20170707_202459.jpg


20170707_202444.jpg
 
RockinRobin said:
That's about the only thing I haven't replaced. Several calls to MSD and they are certain is isn't their 6Hemi controller. Come to think of it, I haven't replaced the ignition wiring harness either, but if they fire fine upto 7500rpm under light load I don't see how that could be it.
Click to expand...

Under heavy load, throttle wide open the cylinder pressures are higher. If t is going to misfire, that is when it will happen because it is hardest for the spark to "jump the gap" with higher cylinder pressure.

Part throttle doing the burnout is different, even if the rpm is the same.

What is your voltage when the misfire happens?
 
d55dave said:
Under heavy load, throttle wide open the cylinder pressures are higher. If t is going to misfire, that is when it will happen because it is hardest for the spark to "jump the gap" with higher cylinder pressure.

Part throttle doing the burnout is different, even if the rpm is the same.

What is your voltage when the misfire happens?
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Good question. If it continues I'll have to get a GoPro on the voltmeter and see. I suspected this but MSD assured me the 6Hemi Ignition controller works all the way down to 7.5 volts. I have 2 brand new Optima yellow tops in the trunk.
 
RockinRobin said:
Good question. If it continues I'll have to get a GoPro on the voltmeter and see. I suspected this but MSD assured me the 6Hemi Ignition controller works all the way down to 7.5 volts. I have 2 brand new Optima yellow tops in the trunk.
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I've heard that before too. I can tell you on the dyno if the system gets below 12 volts they lose power. Quick.
 
yellow rose said:
Nope. In fact, I could make the case that it's a bit on the small side.
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Agree. I had an ultra trick pump gas 318 with iron heads recently that ran best with a 950 on it.
Put several 750’s on it and they knocked ET and mph out of it
 
After the first sentence I was thinking MSD box. Very common problem in circle track cars. Most divisions even allow a spare box to be wired up as a back up.

In case you're not aware, FAST now sells boxes for hemis. I believe they are based on the now discontinued boxes from Crane. I have never experienced a failure with FAST or Crane. Great boxes.
 
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Pascamp said:
After the first sentence I was thinking MSD box. Very common problem in circle track cars. Most divisions even allow a spare box to be wired up as a back up.

In case you're not aware, FAST now sells boxes for hemis. I believe they are based on the now discontinued boxes from Crane. I have never experienced a failure with FAST or Crane. Great boxes.
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Can't find it on their website. All they show are boxes for EFI.
 
RockinRobin said:
Can't find it on their website. All they show are boxes for EFI.
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We had a worse, but similar problem on the hemi when we went higher compression. Turned out to be too weak of spark. But a MSD 6 fixed it >your results may vary . LOL
 
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