What is causing my L79 engine to miss?

[Jeffery F.]

Not long ago I posted about a problem I had with my 327 / 350 HP engine in my '66 Vette (4 speed) fouling the plugs and missing so bad the engine cut off. Since then I have put in fresh fuel (93 oct. / no eth), hotter NOS R45 AC plugs and changed the oil and filter. I drove to a friend's house an hour and a half away and upon arrival found that all 8 plugs were carbon-fouled, so the plugs were all pulled and cleaned up. The next day, on my way to a big car show about 45 minutes away, the engine starting missing again, very sporadic in the beginning but getting worse. When stopping, idle on the engine dropped to around 500 RPM (normally 7-800) and sometimes had to be given a little gas to stay lit. I finally made it to the car show but, as you can imagine, I was sweating about the trip home. I had a bottle of octane booster (Lucas +3 oct) with me so I added that to the 3/4 full gas tank. Upon starting it back up to leave, lots of black smoke from the rear exhausts and a gasoline smell. Engine was still missing and I found out if I kept the RPMs up, it would smooth out a bit so I only shifted into 4th gear one time keeping it mostly in 3rd on back country roads (which was around 45-50MPH and between 2500 - 3000 RPM). Finally made it home and have plans to pull and clean the plugs, change the points and condenser and replace the spark plug wires. Any other suggestions for what might be going on and how to fix this? Adjust carburetor? Rebuild it? Coil, distributor?

[Don L.]

Re: What is causing my L79 engine to miss?

I'll come out of the shadows and admit that I'm the friend that Jeff drove to on Friday and I'm the one that found his brand new (NOS) AC R45 plugs to be extremely carbon fouled upon his arrival at my house. This fouling happened after driving just 65 miles or so and most of these miles were at highway speed. I cleaned the plugs before heading to AutoFair at Charlotte Motor Speedway. As Jeff shares, the car sputtered its way into the raceway for the show. We limped the car back to my house and into my garage. The car exhibited no power and pinged in addition to the missing. In all, we put approx 80 miles to and from the show on Saturday. I pulled all 8 plugs today and found them to not be nearly as fouled as when Jeff drove to me on Friday, however, the car is running worse. Tomorrow, I will replace the Uniset ignition points and condenser with NOS parts. I'll check/set dwell and timing to specs. I'll also check ohms on coil. New plug wires arrive on Thursday.

This is a great car, recently/fully restored, so I'm confident that there are no corroded connections.

Any ideas or coaching is appreciated. Thanks.

[Rich G.]

[Duke W.]

Re: What is causing my L79 engine to miss?

I sounds like a carb issue, but certainly putting a dwell meter on the engine to check dwell and point resistance followed by a timing check is usually the first order of business with misfiring symptoms.

Since improperly assembled and/or installed distributors are a common problem, wire indexing should be checked. If proper and the distributor window is near normal to engine centerline dist. assembly and installation is probable okay.

If not the '66 CSM and COM are you friends, and I've started at least one thread on how to index the engine for distributor removal, overhaul, and proper installation.

The problem could be something as simple as improper float level or a leaky float. See the above docs on how to check and adjust.

Duke

[Don L.]

[David M.]

Re: What is causing my L79 engine to miss?

Basics:
History.... Did it just start this behavior? If yea what service did you perform?
Is the miss on a cold or hot engine?
Are all 8 plugs fouled?
Timing is set at?
Dwell is set at?
Verify VDC into the coil + static with key on, then cranking, then running.
Verify coil primary & secondary resistance HOT & COLD.

Cap, rotor, point set in good condition? Secondary(plug)Wires ok? Ohm them out.

Open the hood in the dark with engine running look for spark leak(shielding mostly removed).

Might be off on a tangent here: Pay close attention to the negative/black wire from the coil negative - to the distributor.
Its common for the internal copper strands to sever at the distributor base. The insulation may look OK but the strands are broken due to breaker plate movement, (back n forth with vacuum advance canister actuation). Ohm that wire while wiggling it. It will feel spongy if its bad.

This could cause a miss at idle and off idle. Whereas WOT may not be affected as the vacuum signal is not in the equation. Meaning it could be more noticeable at and off idle as the breaker plate is at max travel until the vac signal is lost.

To summarize all of this it sounds like a weak spark to me.

Just for fun hows the ground strap from the engine to the frame?

Eliminate ignition first...next is fuel.

[Don L.]

Re: What is causing my L79 engine to miss?

Basics:
History.... Did it just start this behavior? If yea what service did you perform?
Is the miss on a cold or hot engine?
Are all 8 plugs fouled?
Timing is set at?
Dwell is set at?
Verify VDC into the coil + static with key on, then cranking, then running.
Verify coil primary & secondary resistance HOT & COLD.

Cap, rotor, point set in good condition? Secondary(plug)Wires ok? Ohm them out.

Open the hood in the dark with engine running look for spark leak(shielding mostly removed).

Might be off on a tangent here: Pay close attention to the negative/black wire from the coil negative - to the distributor.
Its common for the internal copper strands to sever at the distributor base. The insulation may look OK but the strands are broken due to breaker plate movement, (back n forth with vacuum advance canister actuation). Ohm that wire while wiggling it. It will feel spongy if its bad.

This could cause a miss at idle and off idle. Whereas WOT may not be affected as the vacuum signal is not in the equation. Meaning it could be more noticeable at and off idle as the breaker plate is at max travel until the vac signal is lost.

To summarize all of this it sounds like a weak spark to me.

Just for fun hows the ground strap from the engine to the frame?

Eliminate ignition first...next is fuel.

Thanks to all for the advice so far.

David, I'm working a path that basically follows what you list above.

• The miss is at all speeds. The engine seems to dislike driving on low load vs high load. It also seems to be missing both cold and warm.
• Yes, all 8 plugs were fouled equally. That said, they were not nearly as fouled after driving it home from AutoFair and that's when Jeff drove at higher RPMs. Maybe that kept the plugs from fouling as much as when he drove to my house - not sure. The miss, however, was worse as the day/drive went on. I began to sputter, smoke (black) and miss as soon as Jeff started it.
• I'll set dwell and timing this morning as soon as the coffee does its job and wakes me up.
• I'll also check VDC this AM
• I did check coil resistance yesterday. The coil was cold at the time of measurement. Primary (+/-) = 1.9 ohms Secondary (0/+) = 11.37 ohms. I think(?) these values show the coil to be in spec.
  • I did not check the (-) wire to dist for resistance but will when I go the garage.
  
  

FWIW, I'll also add that this issue, at least in my mind, has been building for some time. Hindsight let's me see this. What started as a little and occasional mis has developed into a big miss plus ping, loss of acceleration, black smoke and more. Jeff has run several tanks of fuel through the car and even added octane booster. I don't think this is a bad fuel problem.

What else?

Thanks again!

[John S.]

Re: What is causing my L79 engine to miss?

+1 on the carb/float issue. I have always heard of Holley (stock 350 hp should have a Holley) floats getting pin holes in the brass float itself but until about 6 months ago had never experienced it myself. Primary float on my Holley 4053 carb on my 69 Z28 filled up and it was bad. So bad that the engine would not run at any speed as it was pumping raw fuel into the primary side of the carb. I doubt you have a filled up float as that would be extreme. Everything you talk about points to improper float setting on the carb. It could be that something has got stuck in the one of the needle and seat assemblies.
I know some people on the TDB have some negative views on the Holley carb but my experience has always been positive. They are very simple to work on and easy to adjust. MTCW

[Mark E.]

Re: What is causing my L79 engine to miss?

The challenge is the cause may be a myriad of things, some esoteric and difficult to find.

A short cut to eliminate a system as the cause is to replace it- install another carburetor and then a fully dressed distributor for example. Of course this requires a spare but can save a lot of time.

For cars I intend to keep, I acquire spares for this purpose. (They've also been a good investment- my spare vintage carburetors have appreciated more than my portfolio... I'll bet Joe Lucia has done well too.)

[Timothy B.]

Re: What is causing my L79 engine to miss?

After you set the dwell and check the float bowls for high fuel level if this engine has a reproduction coil that would be highly suspect in my mind.

[David M.]

Re: What is causing my L79 engine to miss?

I agree with fuel system being a path.
Fouled plugs can indicate several problems. Rich fuel is one. Floats are easy to check. How do the pump shots look?

I like to start simple and cover the basics one system at a time verses several. That can get overwhelming and drop ones self down the proverbial rabbit hole.
Eliminate one system then move to the next. But low hanging fruit (floats) is easy pickings too.

The miss is the only thing pointing me towards ignition first.

Hook up a vacuum gauge to a direct intake vac source and try to obtain the highest reading possible by adjusting the A/F set screws and twisting the distributor. Report the vac reading and if it flutters or is steady.

Engine history, mileage?

[William F.]

Re: What is causing my L79 engine to miss?

I've had pin holes in floats in 2 new Holley carbs in the past. If flooding, always check for this.

[John P.]

Re: What is causing my L79 engine to miss?

Hello Jeffrey, this is such a problem with Holley carburetors, especially if they sit over the winter months or in storage. The gaskets dry out and leak. I have two 1966 427/425Hp engines with Holley carburetors that I would have to rebuild after sitting for extended periods. When starting them they would miss, backfire and barely run. A solution to my issues was using VP110 racing fuel before storage. After discussing the problem with VP's tech department, I found out that their fuels remain stable for more than two years in the tank. After rebuilding the carburetors three years ago and using VP gasoline, I have had no problems. Due to its high price, I run high octane pump gas when driving the cars on a regular basis and return to VP for storage. It does not evaporate or degrade like the current available fuels. As recommended, check for float porosity and adjust to spec as required. When you rebuild your carburetor, make sure to properly clean all fuel and air passages. Make sure to purchase a quality Holley kit as many kits supply inferior gaskets and needle valves. Remember to change your oil and filter after repairs as it is almost certain the oil is contaminated with gas. John

[Jeffery F.]

Re: What is causing my L79 engine to miss?

Thanks everyone for the feedback and suggestions for a cure(s) to the problem. The carburetor has been rebuilt, fuel pump tested and oil and filter changed (we also suspected gas may have been in the oil). Once we fully test the "new" carb on a test run, we'll know if we need to make any changes to the electrical system.

[Don L.]