small block firing on every other cylinder in firing order

[dlh760]

Hey , I tried searching but couldn't find anything on it. Has any one here run across a small block firing only every other cylinder in the firing order. 1,7,4 and 6 fire just fine.2,3,5 and 8 do not fire. Happened suddenly ran fine before hand.hei distributor.changed ignition module,cap ,rotor,coil,springs and weights on distributor and no change. Checked for spark on all wires and they all have spark .I pull the wires for 2,3,5,8 with it running and no change. Anybody have this issue and what was the cause. Any help is greatly appreciated.
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[luvbowties]

[QUOTE=dlh760;5707727]Hey , I tried searching but couldn't find anything on it. Has any one here run across a small block firing only every other cylinder in the firing order. 1,7,4 and 6 fire just fine.2,3,5 and 8 do not fire. Happened suddenly ran fine before hand.hei distributor.changed ignition module,cap ,rotor,coil,springs and weights on distributor and no change. Checked for spark on all wires and they all have spark .I pull the wires for 2,3,5,8 with it running and no change. Anybody have this issue and what was the cause. Any help is greatly appreciated.
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See if the firing cylinders are fed by runners on one side of intake, while the non-firing ones are fed by the other runners.

Not the best mechanic here, but I'd check for fuel or vacuum issues. For fuel: With air cleaner off carb, use something like a piece of cardboard, about 8"X8", w/a hole thru center to slowly slide down over air cleaner stud as if attempting to flood or choke engine out. As eng. begins to flood, speed up engine via the carb linkage to force it to keep running. Keep increasing both actions little by little, simultaneously making it flood more AND making engine keep running more. Maybe it will pull trash on thru jets and clear it up. **Some people swap plug wires and quickly open carb via linkage & let off several times, making the engine backfire thru carb & blowing trash the other way, but I don't really like that. It can BURN YOUR EYEBROWS, for one; and seems to me maybe cause a weak timing chain to break.**

To check for vacuum leak(s): just ease cardboard down over stud toward top of carb--but not harder flooding as above--and see if it just picks up by itself & fires better when carb begins running richer. If it runs noticeably better in this "choked condition", look for vacuum leak(s).
HTH,
sam

[68gmsee]

Never seen that before. Firing order should be 1,8,4,3,6,5,7,2 going clockwise.

How did you check for not firing if you're getting spark to the plugs?

[dlh760]

I pulled the plug wire off of the plug with it running it did not affect the way it ran on 2,3,5&8. It changed when I pulled them off on 1,4,6,&7. I have pulled the plugs and they all look good,I have swapped wires over same cylinders,I have checked compression in all cylinders,I have pulled valve covers and ran engine all valves opening and closing,I have pulled the plugs and attached them to the wire and started engine can see it sparking on plug.
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[truckster]

Crossed wires or cracked cap are the only things I can think of.

[luvbowties]

Originally Posted by dlh760

I pulled the plug wire off of the plug with it running it did not affect the way it ran on 2,3,5&8. It changed when I pulled them off on 1,4,6,&7. I have pulled the plugs and they all look good,I have swapped wires over same cylinders,I have checked compression in all cylinders,I have pulled valve covers and ran engine all valves opening and closing,I have pulled the plugs and attached them to the wire and started engine can see it sparking on plug.
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This info makes me think more than ever the carb is failing to feed the 2 middle cyl's on the driver's side or the 2 end cyl's on the passenger's side. I'd bet the intake runners feeding 2,3,5,& 8 come from the same side of the carb. It takes 3 things for a cyl to fire: compression, spark, and fuel. 2 of the 3 have been verified.
HTH,
sam

[68gmsee]

Originally Posted by dlh760

I pulled the plug wire off of the plug with it running it did not affect the way it ran on 2,3,5&8. It changed when I pulled them off on 1,4,6,&7. I have pulled the plugs and they all look good,I have swapped wires over same cylinders,I have checked compression in all cylinders,I have pulled valve covers and ran engine all valves opening and closing,I have pulled the plugs and attached them to the wire and started engine can see it sparking on plug.
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Really not a sure fire way to determine whether a cylinder is actually firing internally. First thing I would do is check the plugs to see if they look wet. Maybe go out and buy all new plugs if they're old and gummed up.

Also, as mentioned above, suspect a distributor cap problem or flooding.

[dlh760]

I had another set of plugs that I tried as well . The plugs in it have about 1500 or so miles on them. I have even swapped plugs from cylinder that fires to cylinders that don't and vice versa,no change same cylinders not firing.I have double and triple checked all vacuum lines,firing order.I sprayed all around the carb and intake with it running with starting fluid,no change in idle. Retorqued all intake bolts and then double checked everything again.I can see my primary squirter flowing fuel out both needles when I move throttle just slightly. I have pulled the carb off and looked inside the intake nothing obstructing anything there.I even completely disassembled my carb and I see absolutely no signs of anything out of the ordinary.I am running out of ideas on what it could be.
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[Daves70]

Bad cap or rotor or the cap is installed so that it is not sitting squarelly on the distributor. Bad wires or not pushed down all the way on the cap.No spark has nothing to do with the carb.Just take a REALLY close look should be a very simple issue.

[Daves70]

Please disregard my post. I reread and saw that you have spark on all 8.

[jd7]

what are your compression readings are they even ? By even I mean 10% of each other. In your initial post you stated the HEI distributor was changed was this after the problem or before ? Are you getting a good 12 volt signal to the distributor. I've seen cracked intakes cause similar problems. You also stated you checked for vacuum leaks, did you enrich the carb using propane to see if its fuel starvation ? It could also be your distributor grounding out. If there is no spark coming out of distributor it can only be two things not enough voltage coming in or a fault in the distributor

[dlh760]

My compression readings are 145-150 on all eight cylinders.I am getting spark from the wires and plugs. Just 4 dead cylinders.if I run it up to about 2000 or higher rpms it seems to start having all eight work but not before then.I didn't replace the distributor itself.I drove to my landlords house about 300 yards or so and truck ran fine,when I got back in maybe 2 minutes later I started it and it was running really crappy.drove it to work the next morning did not want to stay running under about 2000 rpms . Checked it out and thought it was either the ignition module or the coil. Bought an hei tuneup kit that includes ignition module,weights,springs,adjustable vacuum advance,cap,rotor,coil and installed everything on distributor started truck up and the only change was that it would actually stay running at idle albeit only on four cylinders. That's when I realized it was every other cylinder in the firing order was not working.
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[jd7]

Are you using a spark tester to see the actual spark gap ? Have
you done a leak down test?
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[jd7]

Did you enrichwith propane at idle to see if its fuel related
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[dlh760]

I used a spark tester and it would spark across even with it opened all the way up.I have done a leakdown test everything is good there.
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[dlh760]

I have not done the propane yet I will once I reassemble my carb and put it back on which I am getting ready to go to the shop to do now.I did spray it with starting fluid while running into the carb and my idle increased a little but not much..
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[Boog]

You have good compression, spark and air. I'm thinking fuel distribution. Even though you have opened the carb up and found nothing. It could still have blockage in there. Do you have another known good carb you could try on it?

[68gmsee]

Originally Posted by Boog

You have good compression, spark and air. I'm thinking fuel distribution. Even though you have opened the carb up and found nothing. It could still have blockage in there. Do you have another known good carb you could try on it?

Agree ^

I have never encountered a problem like this where only every other cylinder in the firing order is firing. I suspect that this is not the case but may seem like it. If the engine is already running rough and you remove spark plug cables it's very difficult to discern the difference.

My money is on something wrong with the carburetor since everything else has been eliminated.

[Hawghauler]

It seems to me that it must be the wires. You may be getting spark, but is it sparking when and where it's supposed to. Try isolating the wires from each other and any ground. If all the wires on that side of the engine are grounding or shorting against each other they won't work. Also look at them running in the dark and see if you see arcs off the wires in question. One trick I learned is to spray WD40 on the wires while it's running. This will often block any spark escaping the wires. The only time I've seen something like this was with a fuel injected motor having one fuel rail plugged, but your setup doesn't have rails. Hope this helps, good luck.

[dlh760]

The only other carb I have is the 2 barrel that was on the 305 that died.It is 2,3,5&8 that are not working.1-8-4-3-6-5-7-2. I have swapped wires from ones that were working to ones that weren't and the same cylinders regardless of wire don't fire.
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[Richard H.]

No one said timing so I will, 10 to 12 deg. Double check tdc also.

[PanelDeland]

If it ran fine before and all you did was swap to HEI,The first check would be TDC.IIRC a SBC will run 180 out but never quite right.One other thing I noticed in your post was a new Module,di you put dielectric grease under it?

[dlh760]

It had hei before I hadn't done anything to it when it started acting up . I then replaced everything on the hei and no change. I checked timing and it was exactly where it was before I had any issues,14btdc I tried going all the way down to 8 and up to 20 no real change as far as cylinders firing.I have triple checked Tdc .I did not pull the distributor to change parts I didit with the distributor still in.
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[Boog]

Since you already have a complete new ignition system all you lack is a carb rebuild and new filter to finish this thing up. Have you tried spraying carb cleaner or starting fluid at the base of the carb and the intake manifold gasket to head area? That will help rule out a vacumn leak.

[dlh760]

I just rebuilt the carb about 1500 miles ago. But I'm doing it again now since I tore it completely apart yesterday.I tried the starting fluid all around the intake and carb before pulling it off no change.
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