Miss/Misfire/surge on inline 6

[Improbcat]

Got a super weird one for your guys.

My truck had a 230 inline, which had a random misfire problem. It would not idle smoothly, and would under very light throttle seem to stumble & surge causing the truck to "bounce" like the stereotype of someone learning to drive a manual, but smooth out a lot under heavier throttle. It would also backfire a *lot* if I lifted off the throttle with the clutch out.

Put in a pertronix module a couple years ago, which helped a little. Put a fuel pump & a new carb in (old ones were leaking) which didn't help.

The engine was otherwise very worn out so I lived with it. Last month I swapped in a low-mileage 250 inline(which I saw run smoothly in the donor truck last year, and has been sitting in my garage since), and swapped over all my external accessories(carb, dist, coil, fuel pump). Clutch plate, pressure plate, throwout bearing & flywheel are all brand new so I know they aren't the culprit.

It runs exactly the same way. It wasn't as bad at first, but I did a 450 mile road trip and treasure-hunt rally and by the end of the day it was bucking violently when going down the road at idle or very light throttle.

I pulled the distributor and found massive play in the shaft, as seen in this youtube video.

https://www.youtube.com/watch?v=y3xRWlFDAHk

So a new distributor (swapped over the pertronix), and a new firepower coil went it. It got a lot better, and the truck is drivable again, but the issue is still there.

New plug wires were swapped on, which either did nothing , or possibly made it very slightly *worse*.

I'm running out of ideas. I know the wire to the coil is still the resistor wire because the old coil wasn't internally resisted (changing the wire as soon as it isn't a heat wave), but that shouldn't be causing it. I also wonder if the pertronix module has started going bad?

It seems like either I find something that is bad that "should" be the cause, but doesn't completely fix it(distributor), or have something where the list of symptoms of a bad parts is exactly my problem (plug wires) but that doesn't affect it.

[geezer#99]

First thing I'd do is park that pertronix on the shelf and swap in an hei.
Pick one. You'll need new wires too. So pick the one that comes complete with them.
Like this.
http://www.ebay.com/itm/CHEVY-Inline...1Vr7uE&vxp=mtr

Then put more timing in it. Start at 14 initial.

[Improbcat]

I'd kind of like to find the source of the problem before throwing more parts at it, as that hasn't worked so far.

[geezer#99]

So then put the points back in your distributor.

[nonstop]

Did you change the distributor?

[mud.man.rj]

Maybe it's fuel related, do a fuel pressure check to rule it out.
Mudman.

[68gmsee]

You say that it ran okay most of the day on a long trip and only acted up at the end of the day.

Is it an intermittent problem that comes and goes?

Does it act up only at higher speeds?

Can you duplicate the problem while idling or reving the engine?

[Improbcat]

Originally Posted by 68gmsee

You say that it ran okay most of the day on a long trip and only acted up at the end of the day.

Is it an intermittent problem that comes and goes?

Does it act up only at higher speeds?

Can you duplicate the problem while idling or reving the engine?

It seems to get worse as the engine warms up, but it is always there.

It was doing it all day, but was constantly degrading in performance. Tried to use the truck a day or two later and it barely would start and ran terrible. The new distributor(with swapped pertronix) and coil help this but didn't fix it.

It acts up at all speeds. It seems more throttle dependent than anything. If the engine is idling it twitches as it idles as the misfires happen. in gear and moving down the road at idle or light throttle the truck bucks a bit as it happens. If I rev the engine in gear or in neutral it feels like it smooths out a little.

[geezer#99]

What's your initial timing?

[68gmsee]

Originally Posted by Improbcat

It seems to get worse as the engine warms up, but it is always there.....

Since you can duplicate it I would go to the basics and work from there. A lot of times there may be more than one thing causing a problem.

As mentioned above, timing is important so you need to insure that timing is correct for you application. Also make sure you have full 12vdc going to distributor and are not using existing resistive wiring which drops the voltage down.

Double check all electrical connections and with a voltmeter across plus and negative at the battery insure the alternator is supplying at least 13-15 vdc at idle to the battery.

Make sure the ground cable to engine is good and battery terminal connections are also in good shape.

Check fuel pump delivery into a plastic container. One pint in less than 30 seconds of cranking. Disconnect power source to distributor before testing.

Another thing I would do is check for any sediment or contamination in the carb.

In case fuel in tank is contaminated, try fresh gas from a one gallon container placed under the fuel pump. Use a short piece of fuel hose to intake side of fuel pump for testing.

['68OrangeSunshine]

Originally Posted by Improbcat

Got a super weird one for your guys.

My truck had a 230 inline, which had a random misfire problem. It would not idle smoothly, and would under very light throttle seem to stumble & surge causing the truck to "bounce" like the stereotype of someone learning to drive a manual, but smooth out a lot under heavier throttle. It would also backfire a *lot* if I lifted off the throttle with the clutch out.

Put in a pertronix module a couple years ago, which helped a little. Put a fuel pump & a new carb in (old ones were leaking) which didn't help.

The engine was otherwise very worn out so I lived with it. Last month I swapped in a low-mileage 250 inline(which I saw run smoothly in the donor truck last year, and has been sitting in my garage since), and swapped over all my external accessories(carb, dist, coil, fuel pump). Clutch plate, pressure plate, throwout bearing & flywheel are all brand new so I know they aren't the culprit.

It runs exactly the same way. It wasn't as bad at first, but I did a 450 mile road trip and treasure-hunt rally and by the end of the day it was bucking violently when going down the road at idle or very light throttle.

I pulled the distributor and found massive play in the shaft, as seen in this youtube video.

https://www.youtube.com/watch?v=y3xRWlFDAHk

So a new distributor (swapped over the pertronix), and a new firepower coil went it. It got a lot better, and the truck is drivable again, but the issue is still there.

New plug wires were swapped on, which either did nothing , or possibly made it very slightly *worse*.

I'm running out of ideas. I know the wire to the coil is still the resistor wire because the old coil wasn't internally resisted (changing the wire as soon as it isn't a heat wave), but that shouldn't be causing it. I also wonder if the pertronix module has started going bad?

It seems like either I find something that is bad that "should" be the cause, but doesn't completely fix it(distributor), or have something where the list of symptoms of a bad parts is exactly my problem (plug wires) but that doesn't affect it.

Pertronix is still a high energy ignition, even if AC Delco didn't make it, and therefor it also needs full-time 12 volts, not 12 then 9-7 volts after heating up the resistor wire. So run a 12 or 10 gauge wire [red is a good choice] from the IGN Unfused terminal on the fuse block thru the firewall directly to the coil positive lead. Disconnect and tape off the OEM 20 Yellow lead on the coil. You can also disconnect the 20 Yellow lead from the starter.

How old is your Pertronix? I had one burn out on me after 7 years. It would ''black-out'' and shut down ignition if I went over 3000 RPM. Sometimes leaving my engine rotating cold at +60 MPH until I kicked in the clutch. I'd have to switch the key OFF then ON, slow to 50-55 then popstart in 4th. Wasn't good for the valve springs. I switched back to points for a year, then got an HEI set up for a '75 292, with a separate coil.

['68OrangeSunshine]

Nail down your timing first. You could also be a tooth off in your distributor alignment.

[Improbcat]

I finally got a chance to work on the truck after an embarrassingly long time.

Removed the old resistor wiring, and ran a fresh 12v wire to the coil.

Pulled the plugs, an all were normal looking and the same, so no bad cylinder.

Pulled the distributor, and confirmed TDC & that the balancer hadn't slipped. then I stabbed the distributor back in. None of this was wrong or off.

Hooked everything back up, and static set the timing enough to stat the motor. Once it started I grabbed the distributor to move it closer to correct timing, and it was arching from the (newish) plug wires to my hand, which was super weird. And the distributor felt greasy.

I was super confused at this point, so I let the motor run and warm up, because it always ran worse once it warmed up. Then I found what has probably been the source of the problem all along. The upper heater hose that goes into the thermostat housing on the 250 had a pinhole leak, and once warm was spraying water & coolant directly onto the top of the distributor & causing arcing.

I cut & shortened the hose to remove the leak(new hoses coming), and sprayed the distributor with W40, it already runs a hell of a lot better. I now need to scrub everything to get the coolant residue and re-assemble with dielectric grease and hopefully I'll fix this mess.

['68OrangeSunshine]

Are your plugs gapped to .045? They should be, for HEI and Pertronix.

[mud.man.rj]

Nice fine on the antifreeze leak, good for you.
Mudman.

[68gmsee]

Glad you were able to find the source of the misfiring, Improbcat.

One thing you might want to do now is to wait until night time and with a water spray bottle mist the cap and plug wires to make sure they're not arcing.

If you find arcing, get some good quality cap and wires.

[TJ's Chevy]

The pertronix is a decent unit so if it runs good don't waste money on another HEI. Run it until it dies. Set initial where the engine is happiest....in my cases somewhere between 10-14. And then put a new set of plug wires and cap on if you haven't already. A set of plug wires at least since you mentioned getting bitten by them while the engine was running. Napa premium plug wires are a great set.