high rpm miss

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tchrislip

tchrislip

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We have a 360 in our 70 Duster that hit 383HP at 5,000 rpm on the dyno but flat lined at 5,200. Testing the valve springs showed they were weak and we changed them to match the specs per Comp Cams. After putting it in the car it still misses and flat lines at just over 5,000rpm. We have a Mopar performance electronic ignition and the engine was still pulling strong at 6,000rpm in the previous car. It has a 750 Edelbrock carb, and performer rpm heads and intake. The only change we made was to install Hughes 1.6 roller rockers in place of the stock 1.5 rockers. Any ideas?
Thanks
 
The ONLY change from the previous car (pulled to 6K) and being swapped to the Duster (5K) is the rockers? Is the fuel system the same? right now will it pull to 6K in first/second at the same load (in hg) that it misses at in 3rd at 5K? (When ign misses it will miss at the same RPM/load in all 3 gears whereas fuel starvation ususally takes getting into 3rd for the inadequate fuel delivery to finally catch up & make it miss) unless the fuel system is woefully adequate from the getgo for the horsepower you are making. Holler back
 
Are you getting full throttle at the carb? sometimes the pedal and the carpet makes contact before the carb is wide open,that will make the engine feel dead and missfire at higher rpm. yes i have chased that problem for a long time in the past.
 
You've got a Mopar electronic ignition? What ECU are you using?

I've had absolutley ZERO luck with the orange boxes over about 5k on two different cars now. Switched to a chrome box, no problems. The ECU's aren't what they used to be...
 
It misses or flat lines in any gear at just over 5k. It flat lined on the dyno too hooked to an MSD set up so don't think it's the orange box. Thanks for the ideas.
 
If you changed the rocker arms.......then you changed the pushrods and the lifter preload. Switch everything back and see what happens. If the problem corrects itself, look at the rockers, pushrods and preload. See what changes they have made.

If you change your engine timing and the car starts missing, you don't suspect the tire pressure.
 
Assuming the tach is accurate and its not a rev-limiter issue........Id check rotor phasing.
-My ancient Orange box goes to well past 7200 all the time.
-IQ52 may have nailed it though.
-When I set my engine up, I select fitted pushrods for about 1/4 turn preload on the lifters, using adjustable rocker arms set to the mfgs specs.This allows very little room for lifter pump-up.After having had to readjust the valves several times during the first summer(different valves each time), I increased the preload to 1/2 turn.It was quiet then for several years. This car was my DD for many years, and racked up 120,000 miles in the first 10 years. It currently has 125,000 on the Odo.
- Your engine build sounds very much like mine.Im using a Hughes 230*cam, 1.6s,Air gap,and 10.7 c/r.Once you get it sorted out, you will love it.
 
Miss turned out to be lifters pumping up. Backed preload from .04 to .015 and it runs great.
 
IQ52 said:
If you changed the rocker arms.......then you changed the pushrods and the lifter preload. Switch everything back and see what happens. If the problem corrects itself, look at the rockers, pushrods and preload. See what changes they have made.

If you change your engine timing and the car starts missing, you don't suspect the tire pressure.
Click to expand...

X2...


And have the rocker arms/push rods properly installed and adjusted???
 
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