Small skip

[Lawrence S.]

Hi guys I was hoping I could get some ideas on what to do next to try and find my problem. I have a small skip/sputter that begins at above 1300 rpm. I have a 67 L-79 with 3500 miles on it and a Comp Cam modern grind L-79 cam. This is what I have done so far
Changed plugs
Changed distributor cap
Changed coil
Taken carb apart and cleaned it
The plug wires are new and looked at them during then evening looking for sparks found everything to appear normal
The ignition has a Mallory breaker less system

The miss is similar to when a plug is fouling but really is only apparent over 1300 rpm. A few nights ago it even seemed to go away but came back. I did not know if maybe the ethanol in the fuel has start to eat my fuel pump?

I would appreciate any suggestions

Thanks,

Lawrence

[Gene M.]

Re: Small skip

Lawrence,
New wires does not insure they are good. Change out each wire until you find it.

If you have solid wire (copper, stainless) this is most likely not your problem. Carbon core wires are more apt to give an issue with a miss.

[Timothy B.]

Re: Small skip

Lawrence,

Almost sounds like the Mallory could be causing the problem, is there any air gap that needs to be maintained when it's installed in the distributor, if not these things either work or not. I am not sure if the rotor phasing could be a problem, it's hard to tell when these things fire. One sure way to eliminate the Mallory is to install points and see if the problem corrects itself.

What are the camshaft specs, idle vacuum and what carburetor is on the engine? Also, how is the engine timed..

[Edward J.]

Re: Small skip

Lawrence, does the engine skip start at 1300 RPM's and continue up to higher RPM's ? or is it just a flat spot at 1300?

[Lawrence S.]

Re: Small skip

Lawrence,
New wires does not insure they are good. Change out each wire until you find it.

If you have solid wire (copper, stainless) this is most likely not your problem. Carbon core wires are more apt to give an issue with a miss.

The wires are stock style date coded for flite judging

[Lawrence S.]

Re: Small skip

Lawrence,

Almost sounds like the Mallory could be causing the problem, is there any air gap that needs to be maintained when it's installed in the distributor, if not these things either work or not. I am not sure if the rotor phasing could be a problem, it's hard to tell when these things fire. One sure way to eliminate the Mallory is to install points and see if the problem corrects itself.

What are the camshaft specs, idle vacuum and what carburetor is on the engine? Also, how is the engine timed..

The Mallory unit has been on for 3500 miles.
Not sure of the cam specs off the top of my head close to a stock L79 but vacuum is around 12 inches
Stock carb, 12 degrees of static timing at approx 800 rpm
The car is stock, top flite car. Except the modern L79 style cam

[Lawrence S.]

Re: Small skip

Lawrence, does the engine skip start at 1300 RPM's and continue up to higher RPM's ? or is it just a flat spot at 1300?

Skips all the way up the rpm scale and then popped at high rpms

[Patrick H.]

Re: Small skip

The wires are stock style date coded for flite judging

and here is your problem.

The dated repro wires are horrid. Great for the day of judging, terrible for daily performance.

Patrick

[Lawrence S.]

Re: Small skip

and here is your problem.

The dated repro wires are horrid. Great for the day of judging, terrible for daily performance.

Patrick

Yep I have always heard that and I am starting to think more and more that there lies the problem.

[William C.]

Re: Small skip

Despite the unknowns about "Modern L-79 cam" (it either IS an L-79 cam exactly, or it IS "something else" no "modern" about it) I would tend to agree that the wires are the most likely culprit. The technology of secondary ignition wiring is unique, and the manufacturer of the OEM stuff is long out of business now, so the quality of the aftermarket "dated" wiring is open to debate.

[Domenic T.]

Re: Small skip

Lawrence,
Just one other thing to think about is your Hydraulic lifter adjustment.

If one lifter is adjusted to far down it will cause a popping and a miss because it cannot bleed off in time as the oil pressure rises which hwill hold the valve open.

DOM

[Lawrence S.]

Re: Small skip

Despite the unknowns about "Modern L-79 cam" (it either IS an L-79 cam exactly, or it IS "something else" no "modern" about it) I would tend to agree that the wires are the most likely culprit. The technology of secondary ignition wiring is unique, and the manufacturer of the OEM stuff is long out of business now, so the quality of the aftermarket "dated" wiring is open to debate.

FYI.....

http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=CC&Product_Code=12-671-4&Category_Code=HFTNP

[Lawrence S.]

Re: Small skip

I put a new B28 vacuum canister in reset the timing put a new set of springs in the distributor and it is much better. The biggest improvement came when I put the new vacuum canister in. The distributor springs I replaced were old and just put a new set of the same stock rating in the distributor.

I did notice that the miss/sputter is now limited to just at around the point before the engine starts to have a load on it then the sputter disappears. I am probably splitting hairs at this point.


Thanks for all the help/input I always learn something from these posts...

[Gene M.]

Re: Small skip

After looking at your cam specs, it's quite a bit bigger than the L79 cam. It looks like it is designed to "come on" a little higher in the RPM range. The exhaust duration is real long. That may be the reason for the lag you feel.

[Duke W.]

Re: Small skip

...significantly "bigger" than the L-79 cam - more duration and, especially, overlap. It may be part of the problem because the carb is not calibrated for this much cam. It's gotta be a real dog down low!

Buy real built-to-print OE cams from Federal Mogul, Dana Corp., or Crane.

Duke