two l20b's suffering from two cylinder ignition

[Velvetpants57]

so as the title says my 620 4x4 and now robs 510 both with l20s and matchbox dizzies are only running on the outer two cylinders. both engines will idle but very roughly, and have trouble with anything off idle. backfiring through the cab is present at idle and off idle. this was verified by, while idling, removing the two center plug wires with no change in idle condition. if one of the outer plugs is removed the engine will immediately die.

 

on the truck. a compression test yields average 165 psi on all cylinders. also with a new set of plugs and wires all 4 cylinders are getting a clean spark. the same is also true for gas. the two non running cylinders have gas on the plugs. timing both internal and ignition, has been verified, valve clearance was checked and adjusted.

 

robs car which was running beautifully. has now all the sudden been stricken with the same problem.

 

the only thing left to check is remove the matchbox and go back to a points to see if this long shot makes a difference.

[Stupid_fast]

Same thing happened with my toyota(running rough on 4 cylinders instead of 6), It was as simple as I switched around two of the plug wires. Ensure everything is correctly connected.

 

Or did this just happen randomly after running well for months? Then sounds like a points/distributor problem. :(

[Big Schlink]

I had electronic ignition on my VW bus, one day it randomly crapped out and wouldn't run. I really like points these days? :D

[Velvetpants57]

wires have been tripple checked for order. i have seen the motor that is now in the truck run like a top. as soon as it was swapped into this truck it was dead on two cylinders. only thing changed was the ei dizzy.

robs car was running great. it sat for some time (2months) and then had the same problem. like i said both cars are running ei dizzys and i think the next step is to swap points back onto the truck and see what happens.

[DanielC]

Try a different set of spark plug wires. Probably a good idea to try a different cap and rotor.

[datzenmike]

Blown head gasket between 3 & 4.

 

Valve lash set too tight on 3 and /or 4.

[HRH]

Yeah, if anything, try a different matchbox. I've had one go bad, but generally it's a high or low circuit. It will either not idle, or won't run up high, but one of the two settings will work. If it's not running for anything, there are other problems present.

[chester]

Mine is doing something like this. While I gad it running I pulled each plug wire one at a time. When I did 1 and 2 there wasnt much change. Then I did 3 and 4 and it made it run like shit. Then I went make through them and the second time around cylinders 1 and 2 when I pulled them made a difference but nothing like 3 and 4. I dont have the matchbox dizzy I have the 78 setup with the igintion bo ubder the dash on the passenger side

[mike]

Did you do the firing order backwards? Try swapping the middle two wires, or reverse the firing order from what you "know" is right. Ive done this before... and double/triple checked the firing order and adjusted the valves before I figured it out. The fact that its happening on both engines kind of makes me think that something like this is the case, matchboxes are pretty damn reliable.

 

Did you use the right pedestal for the matchbox?

[datzenmike]

RE:

 

Blown head gasket between 3 & 4.

 

Valve lash set too tight on 3 and /or 4.

 

Posted quickly this morning without reading more than the title. Was going out the door. The module can't be bad as some cylinders are firing so this leaves the rotor, cap or the wires. Firing order counter clockwise 1342

 

Seems that the firing order is good, valve clearance is good and you are running new plugs and wires but haven't changed the cap or the rotor. I would start with a new cap and rotor.

[rusty12ver]

Could the distributors suffer from shaft play or sloppiness?