Misfiring issue

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BBBCUDA

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Howdy guys, I have a bit of a puzzler. I decided way late in the season to get my long sitting Barracuda going. I put together a mild 383 for it, the engine is: 9.9:1 actual compression, ported closed chamber heads, Performer RPM intake, 750 DP Holley with Proform main body, (supposed to bump it to 780 cfm). 2" TTI headers, Mopar Performance ignition, chrome box, MSD Blaster 2 coil. Timing @ 36°, and it does NOT over advance.

I could only make 3 passes on the LAST weekend of the season, and my best was a 12.8, 102 and change. The car starts skipping at about 5,000 rpms, and I'm limping after about 1/2 track. The cam is the larger Summit grind, .465/.488 lift, 224/234° @ .050, and was degreed. I know the cam is small, but the Mopar Muscle article they did on the stock 383 showed the even smaller factory cam as making peak HP at 5,400 rpms. Shift point, (500 over peak power would be near 6,000). I can't come close to that. I thought fuel pressure, but I checked today, a constant 8 psi. My next thoughts are, the jets I put in are 70 front, 80 rear. Could it be lean enough to kill off the top end? Thanks in advance.
 
First question is, what is your definition of "skipping?"

What is your guess, missing (ignition?)

Is it possible you have a valve spring/ lifter pump-up / other valve train issue, and are floating some valves?

and WHAT DO the plugs look like?

I don't know why you say "supposed to bump to 780?) A 750 DP should be fine for now, at least until you get this straight.

How 'bout the ignition? Any chance the ignition is not getting adequate power? It could be something as simple as a bad coil, etc.
 
Is this the first time with this combo? What type of plug wires are on the engine? Did you ohm the wires for resistance? Any/all wires must be under 20K ohms! What is the pick-up coil to reluctor air gap? I do not like the Mopar Ign. Eventually you will choose an after market system. MSD is good. You choose! Although I have two MSD 6 AL ignitions to return for "missing" problems. But they were in the "cars" for 5 to 7 years w/no problems, then all of a sudden, miss fire from 4500 up to 6000! How big is the fuel line? 3/8 minimum. Number 70 jets front and 80 jets in the rear are a bit small, especially for fall. Even w/power valve you should be in the mid to high 70's front and mid to high 80's rear. Does the carb have jet extensions in the rear? Go "sparky" Go!
 
You did not say if your running vacuum advance. One thing I would do is to set your timing at 38 degrees, with full mechanic advance at 2600-3000 RPMs with no vacuum at all. If it was me I would drop the fuel pressure to 7 PSI and run it with the timing and fuel pressure adjusted. I always run the Mopar Electronic ignition systems and this set-up works for me. Just my opion.
 
Thanks for the help guys, I'll try and answer all your questions. The missing is a very audible backfiring out the exhaust, with a pronounced shudder through the car with each one. You have more time to notice it in high gear, and it's on it's face and popping not much after 5,000, and really struggling to get through the traps. The wires are just new Accels, I have not ohm tested them.

I did check the relucter air gap, and it was off a bit out of the box, but I corrected it. The charging system seems to be OK, (new alternator/regulator/ballast). The alternator puts out about 16.5+ volts, and it runs right around 14.5+ at the battery. I was so nervous at the track, I completely forgot to pull a plug when I got off the top end of the track. By the time I drove it all the way back to the pits, I didn't figure it would show me anything.

When I say the main body "bumped up" the carb, I mean the addition of the Proform main body makes a 750 DP into a 780 cfm. carb. I do not have jet extensions, but it will before the next trip out.

My fuel system is: Cell, 1/2" hard line to a 1/2" BG filter, Paxton 160 GPH pump, 1/2" hard line to a Holley 1/2" big port regulator, 3/8" hard line to the carb, each bowl gets it's own line.

I am not running the vacuum advance, and it has a very fast curve. It seems to be "all in" my not much more than 2,000 rpms., and it does not over advance. I think I heard that Mallory makes the new MP distributors. This one sure doesn't look like a stock MP electronic distributor inside. Thanks for all the help.
 
oldkimmer said:
.............worst in 3rd gear=low fuel pressure.............kim...................
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Checked yesterday with a good quality gauge with a bleed off valve so I could see if it dropped when it started flowing. Gauge read 8 Psi, and the bleed off shot a 20 foot stream of fuel out when I cracked it.

Would being way under jetted do the same thing? I am guessing fuel demand is highest under the most load, which I would think is in high gear. Thanks.
 
just curious but what size filter do you have between the pump and the thanks? The fuel cell doesn't have its own filter/strainer correct?
 
billytuffnuts said:
just curious but what size filter do you have between the pump and the thanks? The fuel cell doesn't have its own filter/strainer correct?
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The cell doesn't have a strainer that I am aware of. The filter is a Barry Grant item, about 2" in diameter X about 12" long, 1/2" inlet/outlet, and uses replaceable cartridges. I hope to get the car on a chassis dyno soon with some larger jets in it. I am running out of stuff to check.
 
BBBCUDA said:
The cell doesn't have a strainer that I am aware of. The filter is a Barry Grant item, about 2" in diameter X about 12" long, 1/2" inlet/outlet, and uses replaceable cartridges. I hope to get the car on a chassis dyno soon with some larger jets in it. I am running out of stuff to check.
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sorry I meant to ask what the filters micron rating was. There should be two filters/strainers on a fuel system...one before the pump and one after. The one before the pump (except in some gerotor type pumps), the micron rating really shouldn't be lower than about 75 or so. The reason being is that much smaller of a micron filtration can restrict flow to the fuel pump/carb and will actually make the pump work harder and not last as long. These type of strainers are generally placed inside the gas tank where the sending unit is.

There should also be a filter between the pump and the carb/fuel injectors with a smaller micron rating around ~30 or so for carburetors and even smaller for fuel injection as the injectors are more prone to clogging. Its easier for the pump to push the fuel than it is to pull it through a filter.

What I'm trying to say is theres a possibility your filter could be too restrictive, causing a lack of fuel volume and/or pressure at a high load, making your carb not get enough fuel and misfiring.
 
billytuffnuts said:
sorry I meant to ask what the filters micron rating was. There should be two filters/strainers on a fuel system...one before the pump and one after. The one before the pump (except in some gerotor type pumps), the micron rating really shouldn't be lower than about 75 or so. The reason being is that much smaller of a micron filtration can restrict flow to the fuel pump/carb and will actually make the pump work harder and not last as long. These type of strainers are generally placed inside the gas tank where the sending unit is.

There should also be a filter between the pump and the carb/fuel injectors with a smaller micron rating around ~30 or so for carburetors and even smaller for fuel injection as the injectors are more prone to clogging. Its easier for the pump to push the fuel than it is to pull it through a filter.

What I'm trying to say is theres a possibility your filter could be too restrictive, causing a lack of fuel volume and/or pressure at a high load, making your carb not get enough fuel and misfiring.
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I tried looking up the micron rating of the inline BG filter, but had no luck. As I recall it was in the 10 micron range though, and it IS before the pump. I can interchange the paper element for a mesh type, or just re-do the system to put it after the pump. Thanks for the help.
 
I had a misfire problem last weekend at the track, missing up in high gear at around 5grand.. re jetted carb and had no issues after that...
 
Another member with quite a bit more experience than me gave me a jet recommendation a good bit higher than I'm at now. I hope to do that, and get a few pulls on a chassis dyno before snow sets in. There is a place in Mass that supposedly gives you 3 pulls for $75. Thanks again fellas.
 
BBBCUDA said:
Another member with quite a bit more experience than me gave me a jet recommendation a good bit higher than I'm at now. I hope to do that, and get a few pulls on a chassis dyno before snow sets in. There is a place in Mass that supposedly gives you 3 pulls for $75. Thanks again fellas.
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it sounds like a lean issue, so a carb jet change could do the trick :iconbigg:
 
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