Bobchad

I've upgraded the stock HEI system; first with the MSD Pro-Billet distributor and then about 6 months later I added the MSD 6AL unit. Shortly after adding the distributor I developed a high speed intermittent miss.

While the car never missed like this before it did get a little rougher at speed when I had the intake manifold changed out about 18 months ago. I had mentioned that a while back and the suggestion was that I check to make sure that I don't have a vacuum leak. I've read to do that you spray a small amount of starter fluid around the base of the carb and the vacuum connections and wait to see if the engine idle picks up briefly.

The intermittenmt miss has happened 4 times in about 500 miles. All four times it starts during a freeway speed run. The first time was after about 50 miles with the new distributor. I had run about 15 miles at 65 - 70 mph and I started to get this rough running and misfiring sound that I thought was a rod bearing. I pulled off the freeway and came to a stop at a stop light. When I pulled away from the light, the miss was gone. I went home and checked out the system and found a plug wire that wasn't as tight as I would have liked it.

The next time it happened I had put 400 miles on the car over the weekend and was coming off a 65 - 70 mph run of about 25 - 30 miles. Same symptoms. Pulled off the freeway, came to a stop light and pulled away with no symptoms.

I changed out the spark plug wires with the MSD wires and checked each plug. All 8 are a nice even tan color.

Happened again, basically the same thing except the battery wasn't fully charged and I turned the car off and couldn't get it started again. Jumped it and once again no problems. Had the charging system checked and found that the regulator was shot. New alternator swapped in.

4th time was the same as the third at almost the same distance from my house in the same stretch of road.

The MSD distributor has 36 degrees of advance in at about 3200 rpm. The vacuum advance seems to bring it up to about 52 or 54 degrees at a steady throttle of 3200 rpm.

No temperature changes to the coolant or an oil pressure drop when it happens.

When I first put the distributor in and ran it I found some small carbon spots behind the exhaust. I have run some Techron through the fuel system to try and remove any carbon deposits.

I will change out the fuel filter as soon as I get back from Florida on Sunday. How would I go about checking fuel volume from the fuel pump?

I will also check the idle circuit to make sure that it isn't off but the plugs were a nice tan color when I pulled them recently.

I'll check the timing as well to make sure it has not changed.

Any other ideas?

Bob

mrvette

Toss a couple of quarts of alcahol in the tank, see if that helps....

lotsa water in the gas here in Florida...dunno about where you live...

GENE

stingry

I get water all the time. Also check your spark plug wires and see if you have two consequentive firing cylinder wires causing crossover. It is specifically mentioned in the MSD 6AL install guide.

I know you read the instructions....right...

Bobchad

Of course I read the instructions and checked to make sure I didn't have wires crossing over. Maybe I'll check again.

What does the alcohol in the fuel do? I suspect something to do with drying out any moisture in the fuel system. And I prefer Tequila but suspect the Corvette would appreciate another kind. Is there a specific type of alcohol to use?

1971corvette

The corvette will be happy with the Tequila also

wills670

alcohol mixes with water so it will mix with gas and then burn. gas and water will not mix but add alcohol and it will. also make sure you have good connections going to the msd box they do not like to run with under 12 volts.

Bobchad

Replaced the fuel filter. Expected to find some gas in their but there was none. Is that normal? Filter had a light black ring of stuff around the center of it filtered from the gas. I don't expect it was dirty enough to have had an impact.

Cleaned up some vacuum lines that were capped off and could potentially leak.

Checked idle adjustment and got a vacuum reading of 18 both at idle and at 3000 rpm under no load. No flickering.

Sprayed some starter fluid around the base of the distributor, vacuum lines, etc., with no impact on idle.

Wire placement seems ok.

Timing is 8 BTDC with no vacuum at idle, 22 with vacuum, 36 at 3,200 without vacuum and 54 with vacuum.

Does any of this mean anything to anyone?

The polarity of the distributor pickup is next on the list.

Haven't got the alcohol yet.

wills670

try running with out vac. advance 54 deg. seems to high i all ways thought 36-38 is about as much advance you could run. does it sound high to any one else?

BigBlockk

You say you had the intake manifold changed on this motor? Does the motor have headers on it? If so, you have changed the breathing characteristics of the engine. It is guite possible you have a lean surge going on here. What carburator are you running? You may need to richin it up a little.

Hope this helps.

BigBlockk

Later.....

Bobchad

Hopefully Lars will take a look at this at some point.

I've emailed MSD about this but haven't heard back. I may have to give them a call to discuss next week on the total advance and wire polarity.

Bobchad

I'm running the stock Edelbrock and have done both. The plug colors look good. Wouldn't they be off if it was too lean?

comp

only way to check it is ,new plug's fire it make a Hard pass and shut it off At the top end and pull plugs,,,, the low speed (rpm's ) will cover up
the wt on the tip's if it's lean just at top end

lars

Just a couple of comments on the discussion here:

First, at WOT and high rpm, vacuum leaks are virtually irrelevant. You can have a massive vacuum leak, and it will not cause a miss at elevated rpm and wide open throttle. Vacuum leaks tend to affect idle and part throttle operation, but get "drowned out" at WOT.

You timing numbers look good. 36 total mechanical and 54 with total combined centrifugal plus vacuum is right on the money. No issue there.

I would check a couple of things. First, set your plug gaps to .035". This will lower the resistance across the gaps and will reduce the possibility of high rpm misfire.

Water in the fuel is a common issue that will cause bizzarre problems. Run the tank close to empty and then drain it. Refill with new gas.

I didn't see what type of carb you're running, but you were talking about servicing the fuel filter - sounds like an in-line filter. Make sure you're not running 2 filters: All Q-Jets and some Holleys have in-carb filters. If you run an in-line filter in addition to the in-carb filter, you can easily run into high-rpm fuel starvation. This will appear as a "miss". If you are running an in-line filter without an in-carb filter, make sure the filter is not installed between the tank and the fuel pump: the filter needs to be installed between the pump and the carb.

A high-rpm lean-out condition from a lean jetted carb will also cause what you are seeing. Not knowing your carb type, make sure you are jetted no leaner than the stock jetting for that carb - you can even bump jetting up 2 sizes to be safe and then check your plugs for color.

If the problem persists, just yank the distributor out and install a stock HEI without the MSD system hooked up. If the problem goes away, you just found your culprit.

BigBlockk

Sounds to me like this engine was right on the edge of lean surge before you changed the intake. Adding the intake leaned it out enough to make it feel rough (lean surge). The new ignition made everything below the lean surge feel better so now the lean surge feels like a mis.

As far as plug color goes, in my experiance, it is only acurate in a small A/F range. Put another way, when you get to the very far rich or lean being able to discern color becomes very tough. A plug that is right on the edge of lean surge will look same as one that has slipped over.

Do what Lars said. Go up a couple jet sizes. I'll bet this problem goes away.

Hope this helps.

BigBlockk

Later....

Bobchad

Running a stock Q'jet. I just have the one filter.

I want to explore a couple of other less difficult possibilities but if they don't pan out I'll go ahead and have the jets increased two sizes. I posted on the MSD Board to see what they say. I purposely did not post the information about a possible carb jetting issue. While all of this makes perfect sense, it is easier for me to change a couple of wires than to take apart the carb, which I have no experience with. I guess I'm prioritizing based on ease of accomplishment rather than chances of success.

Plug gap for my car was stock at .045 according to the emission sticker but I can decrease it.

The tank is almost empty now. I'll run it a little longer, drain and replace the fuel.

Cleaning up the vacuum lines did smooth out the idle.

Thanks for the help. Let me know if anything else comes to mind.

Bobchad

I've been futzing around with the electrical stuff and am just about finished with that.

Getting ready to tackle the carb issue. I bought Doug Roe's book on Rochester carbs and am studying up on it and Lars' paper on basic carb tuning.

After going through them the first time, I took a look at the carb and it appears that I have a carb for a 1976 Chevy C10 350 cubic inch light duty with auto trans and federal emissions. The carb number is 17056208.

Of course I was told that when they replaced the carb it was the correct carb for the year, etc of my 77 Vette. The carb I'm supposed to have is a 17057204.

If I understand all the jets and rods relationships, the carb on the car is actually putting out a richer mixture than the stock carb would have put out. While both carbs should have #77 primary jets and secondary metering rod tip diameters of .057, the carb on the car has #48 primary metering rods where the original
carb had slightly larger primary rods at #52.

Assuming these carbs have the original sized jets and rods, do you think the car is still too lean or is this in line with the previous increases recommended in the primary circuit?

greyghost79

9 times out of 10 a high speed miss with MSD ignition installed is due to spark crossfiring to another cylinder because plug wires are touching. are you sure no wires (especially adjacent wires) are touching each other? MSD is very specific about this. especially if your plug wires are old or marginal anyway.

Bobchad

New MSD wires that they recommend using to prevent this problem. Purchased seperators to keep them apart.

Bangkok Dean

I have a friend here that told me some time back that he once had a fuel blader in his vette that was too near his fuel pickup and at high speed it would block the pickup then when he stopped it would be ok for a while. Now I don't know if you have blader but just passing on this info. If others have some idears on this please chime in as I am also interested.

EDDIEJ82

I had an edelbrock and msd and 6al on my old motor and it was fine. I doubt your problems coming from the carb.

I would check 2 main things first.

1. Fuel pump, if it ain't keeping the pressure up at high rpms. that will cause your symptoms

2. And I know this sound strange but your alternator, If your alt. isn't putting out enough amps to cover everything else and the msd, that will cause your symptoms too. I had to change to a 108 amp to keep mine from doing the same thing after I installed the dual spals. My stock alt. just couldn't keep up with everything.

If these aren't the culprit double check to make sure all the wires from the msd box are tight and getting good connections.

Wire seperators are a good idea too

Also tighten the plug gap up

With these and all the other ideas you should be able to trace it down