340 11.6 to 1 compression engine misses

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topnotch66

topnotch66

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the engine is a 340 11.6 compression with big cam.400 + hp.msd dist.,msd blaster 2 coil, msd 6al rev limit box,msd wires, holley street avenger carb. starts and idles great sometimes. then next time you start it the engines misses, idles rough. Sometimes it runs smooth through exceleration and next time it misses through exceleration? i'm at my wits end. any ideas what to change next?
 
might not be helpful but did you thoroughly check for vacuum leaks. i had a carb with a bad throttle shaft seal that would leak some times and not others
 
Is the carb rebuild and has a filter before it? Make sure the MSD cap is in good shape the carbon ball in the center has been known to fall out.
 
A comeNgo issue is very nearly always in the fuel system.
But to save chasing my tail, I always check the ignition first. If you have flyweights in the dizzy, check them for smooth,non-sticking, snap back to park, operation. This is easiest to see with the timing lite.
1)The best time to check the fuel system is right when its acting up. The very first thing I would check for, is,water in the gas.It likes to accumulate in the bottom of the float bowl. When it gets high enough,it goes through the jet or jets, and causes anything from a momentary nose-dive while driving, to a stall-out at the stoplight, to intermittent rough-idling, to a nose-over during acceleration.
2)If the fuel is good, I would blow out the low speed circuits.
3)And lastly I would check the fuel level and prove it stable.
I think you will find the problem in one of these three.

FYI; a vacuum leak is usually a full time issue, as opposed to comes n goes. A sticking advance weight(s) usually just causes a high idle. A broken advance spring usually causes driveability issues when the advance comes in way to early.
 
Is the "intermittant" issue related to temperature?

Does it do it cold?

Or a warm restart?

Or both?

Watch the temp gauge and notice if it has any correlation to engine temp...
 
What that Street Avenger new or reman from Holley?

I had a 670 and was chasing my tail trying to get my car to run right. I tried everything that was suggested to try. Finally I swapped on a different carb and the car ran great!!!

I've been told by a few people that those street avengers are only good for stock engines. I tend to believe them since I had a problem with one. I've heard some people they work good for though. Might be worth a shot to see if you can get or borrow a different carb just to eliminate that as the problem.
 
The motor was bought as a fresh built balanced and blueprinted 340. checked for vacum leak nothing. the gas is fresh 106 octane ,the carb is new 670,it did it with the old carb too holley 850, the coil is new, it did it with the old coil too mpp,the wires are new, it did it with old wires also, the best advice so far is to check the dist. weights and springs and cap issues. if that dont help im gonna take it to a mopar guru down the road. i give up!! THANKS GUYS
 
I've been fighting the same EXACT problem for two years...changed and replaced many things....I have a 68 dart GTS 340 mild build such as yours...stopped by this old school mopar racers house one day( I didn't even know him) he was happy to take a look...took off the air cleaner looked at the 650 Holley and said " simple, the carb is too small...period" I'm like really? He said watch this...held his hand over the carb about 10" high...slowly moved it closer to the carb..At about one inch away....purred like a kitten in gear...for the first time EVER! Told me to put a 750 oh it and it would be fine...I did(that same day) and it is...needs to be richer he said....I was under the impression that that carb was way too big...try that out and report back... Just sayin...
 
Make sure your float level is such that gas just barely spills out when you take out the screws and jiggle the car...the 850 may have been a bit big....not sure...it worked for me with a 750.. Stock #72 primary....just how it came out of the box...go to ehow.com and search how to tune a Holley...good info for ya
 
I should add that with high compression, its easy to get out of the transfer sync with too much idle timing.Then the engine will hunt for fuel.

So what are your timingS?
And what are your cam specs?With a big enough cam,and aluminum heads, perhaps your Dcr is low enough that the 106 is overkill .Hi-octane fuels are slower burning.

I did a Dcr estimate for you:
To get 11.6 Static cr with a 340+.030 requires a TOTAL chamber volume of 66.6cc
If you put a cam in there with an intake closing event at 70*ABDC, the Dcr is 8.75.(at sealevel)
This may be a little high for iron heads, and lower than neccessary for aluminum heads.
Many of us are successfully running Dcrs of 8.6 with aluminum heads on as little as 87E10 gas. 8.75 might be fine with alum., and 93gas or even 91.
A 75*closing point will drop the Dcr to 8.35/sealevel, which would probably be fine on iron heads and 93 or less,gas. These fuels are faster burning.
Ive never run 106, so have no idea how it might affect idle quality.
 
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