High Speed Intermittent Miss

[Bob Chadwick]

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.

It's happened 3 times in about 500 miles. All three 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 today, 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.

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

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.

When I first put the distribur 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 doubt it's mechanical due to the close proximity to the distributor change and the intermittent nature of the problem. I suspect a timing issue. Something to do with the advance system. Any ideas or thoughts.

 

[Vette/Berlina-coupe]

-don't you hate it when they do this...

:eyerole :crazy -naturally the first thing one would suspect is the sparkplugs, if you are running an expensive set, i would yank'em and pop-in a cheap $8-set of Autolights (the best plug's made, --maybe a step cooler running); --then, if problem persists it is probably an overload-breakdown of a transister in the microprocessor, a condition apparently almost impossible to analyze even by the factory; --as i had a similar problem with a new MSD/ECU-unit that they returned saying it checked-out OK, and they would'nt refund or anything so i gave it to a guy who reported back the same intermitent problem, -but he lived with it because it was free. Might try relocating the ECU if you suspect it is getting hot where positioned, otherwise throw it up against the wall... :eek ~Bob vH

 

[wishuwerehere82]

Bobchad,

I think you may have stated the problem already.
Battery Voltage.
I have had the same symptoms on my motorcycles whenever my battery was going south or the voltage regulator popped, or had a bad connection to the charging system.
It would start OK on a full charge, but after a long drive the voltage wasn't high enough to keep the spark hot enough to fire, especially at high RPM's. It will idle fine and go at lower thottle positions, but if you accelerate it starts to stumble and miss. Eventually the engine just goes slower and slower until it finally dies.
I also had the same problem on my buddy's '66 Pontiac 2+2.
Check your charging system while the motor is running, and ensure that your battery is at 13.7 volts at the terminals.
Maybe try running a test wire to a Voltmeter inside the car while you are driving so that you can monitor your voltage over time. Could be as simple as a loose alternator belt.

My .02

 

[Checklst]

Had a high performance boat once with a MDS system that developed a high speed, high rpm miss, turned out the control box was mounted to close to the distributor and the HF interfearence was jumping into the coil wire,



If your AL box is mounted close to the dist you might want to move it or shield the coil wire if it's not close disregard the thought.

Opps I forgot to say it turned out the HF interfearance was due to a poor grounding of the MSD box because of the fiberglass it was mounted to, I ran a ground wire the the Box and all was well at high speed.

 

[Bob Chadwick]

Bob and Checklst, by control unit, I think you are referring to the 6AL box. I don't think that is the problem because it developed before I put the unit in. The box is a good 6 - 7 feet away in the nose of the car in front of the radiator. It is grounded directly to one of the engine ground near the fuel pump. As I recall it grounded directly to the motor mount.

The voltage issue is an interesting avenue of pursuit. I suspect I have a bad battery. The volt meter in the car is always running a needle width right under the right hand side red line. It does this even after the car has been trickle charged all the way up and when I have run the car for a long time. Because the car has always started, that is until yesterday when I had to have it towed home, I have ignored it.

The battery is at least 3 years old; that's how long I have had the car, and has been run pretty dead a couple of time during the Winter while I was doing a couple of my major projects and the car couldn't be run. I put a good trickle charge on it last night and the charger indicates a full charge. A voltmeter read 17 volts. Should it read that high?

Drove it to the shop this morning. The whole way the voltmeter was a needle's width shy of the right side red line. Left it there to have the charging system checked out.

 

[Bob Chadwick]

Some interesting info from MSD Tech Support.

Also, make sure that you are supplying the distributor with no less than 11 volts and no more than 13 volts. This is crucial to the performance of the module.

 

[AKRAY4PLAY]

if the voltmeter is reading 17V, your voltage regulator is smoked. and that is probably what killed your battery. sounds like you found the problem though.

 

[Yoda]

Bob,

I remember GerryLP had issues with his HEI before and during the Auburn Run this Spring. Here is Gerry's Experience with a similar problem.. It may or may not help, but may give you a bit more insight

Bud

 

[curtis]

It's possible your fuel pump is getting weak and the carb runs out of gas after extended higher rpms. Or maybe partially plugged fuel filter.

 

[Bob Chadwick]

Bob,

I remember GerryLP had issues with his HEI before and during the Auburn Run this Spring. Here is Gerry's Experience with a similar problem.. It may or may not help, but may give you a bit more insight

Bud

I remember Gerry's problems. We went back and forth on them but I don't think I have the same issue because mine started before I put in the MSD box. Thanks for the info though Bud.

 

[Bob Chadwick]

It's possible your fuel pump is getting weak and the carb runs out of gas after extended higher rpms. Or maybe partially plugged fuel filter.

Fuel pump is brand new. The fuel filter could be an easy check if someone would tell me where it is. :L

 

[Bob Chadwick]

if the voltmeter is reading 17V, your voltage regulator is smoked. and that is probably what killed your battery. sounds like you found the problem though.

I wasn't clear. It was a hand held voltmeter with the car off.

The regulator is toast though and I'm in the process of replacing the alternator.

I'll post the results of that repair on the ignition.

 

[curtis]

I think the most common fuel filter is in the carburetor right where the fuel line connects. It is probably also the smallest one in the system.

 

[Bob Chadwick]

Well the new alternator did not take care of the problem. Left my house Saturday morning and got to about the same point that I was at the Satruday before and had the same miss. Pulled off the freeway and turned around and headed home. While I was off the freeway at slow speed the hard missing went away, though it was somewhat rough all the way home.

Curtis, thanks for pointing out the location of the fuel filter. I will change that as soon as I get back from Florida on Sunday. How would I go about checking fuel volume from the fuel pump?

I've been thinking about this for the last 600 miles. 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.

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 is not off.

Any other suggestions?

 

[Jack]

Maybe the pickup/trigger in the MSD pro billet distributor is bad ... or loose ... or air gap is wrong ... or pickup-to-box polarity is wrong?
JACK:gap

 

[curtis]

Bob, I don't know how to check for fuel pump volume, however, if you put a guage inline between the pump and the carb and run 3000rpm, it should readat least 5psi. If it is lower i would assume the pump output is low, even though it is a new one. I'm still betting on the filter in the carb. It is so small. If this is quadrajet, then it is a paper filter, be certain you put the new one in exactly as old was. they can get turned around, also watch for the little spring in there to pop out and get lost.

 

[Bob Chadwick]

Curtis,

I made it back from Thanksgiving with the in-laws. Unfortunately I brought a cold and will be taking a look these suggestions later this week. Fuel filter, vacuum leak and timing are the easiest and are first on the list.

Bob

 

[Bob Chadwick]

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.

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.

 

[wishuwerehere82]

Bobchad,

Your base timing looks right on the money.
No gas in the filter? Hmmmm. That might mean your check valve in the fuel pump is leaking back to the lowest point in the fuel line. This might be important information and it could indicate that your pump is not building up proper pressure to the carb. Is there any way you could put a pressure gauge in the fuel line before the filter to check the pressure?

54 is way too high for the advance with vacuum advance on. You may have lost an advance spring on your mechanical advance in the distributor. 54 would cause a lot of popping at high RPM's.
The max should be 38 with vacuum advance connected.

 

[wishuwerehere82]

Here's a link to tweak your advance curve.

http://www.msdignition.com/pdf/85551_85561_8547_dists.pdf

You probably need to put the heavier springs in the mechanical advance to get it back lower on the curve.