Firestone 7.5 ignition mystery

[twocyclemania]

Just picked up an early 50's 7.5 Firestone. It had no ignition. I 'dressed' the points ( and gapped them) and it started right up, ran nicely, and pumped well. However, it would not go to high rpms or idle. I found the lower cylinder was not firing even though I hooked up an in-line spark light and had great spark according to the light and manually by the exposed plug. I though maybe the coil was shorting. I replaced it with an OMC coil on the Firestone field and the same thing. Spark at the plug and by the indicator light but the cylinder is not firing. I then replaced the condenser with an OMC and once again had great spark at the plug and according to the indicator. The cylinder is not firing. By the way, I know the cylinder is not firing since I can pull the spark plug wire while running and there is no difference in it running and the cylinder is cool. The only thing I haven't replaced so far is the plug wire but why would I have spark at the plug but the cylinder is not firing at all? This is one I can't figure and hope someone has some ideas. I've never seen anything like it. Help!

 

[mrcrabs]

Re: Firestone 7.5 ignition mystery

why would I have spark at the plug but the cylinder is not firing at all

check your compression

 

[F_R]

Re: Firestone 7.5 ignition mystery

If you are confident that you have adequate spark, then obviously there must be something else wrong. I agree, first thing to do is check the compression. It probably has a blown head gasket, but could be something else.

If compression is good, and no water in the cylinder, then it may have a broken reed.

An orderly troubleshooting is needed.

 

[twocyclemania]

Re: Firestone 7.5 ignition mystery

Thanks for the replys! Compression on both cylinders is 90 so it's not a blown head gasket. The only other thing I can think of along with F_R is a reed valve. I was actually hoping for a bad head gasket after all the electrical work (eliminated that area) and the possibility of a reed valve. Is it possible a reed valve is stuck (this motor sat for ? many years) and if so how do you 'unstick' it? If it's broken does the engine have to come apart. I've accumulated and worked on many motors and I know there is the possibility of reed valve trouble but I have to say this will be a first. Any recommendations or advice is once again greatly appreciated; especially any shortcuts.

 

[F_R]

Re: Firestone 7.5 ignition mystery

Reeds don't "stick", but something may get under it and hold it open, or they can break.

Broken reeds are a rarity these days, but in years past it was not so uncommon. They are accessed by removing the carburetor and intake manifold/reed plate. No need to take the rest of the powerhead apart....Except that broken reed may still be floating around in there. It can cause damage if it gets jammed up between the piston and cylinder wall. Or it can go on out the exhaust without hurting anything. How is your luck today?

 

[twocyclemania]

Re: Firestone 7.5 ignition mystery

I pulled the intake manifold and all the reed valves look great. Unfortunately that leaves me with no answers. The only thing I haven't replaced is the ignition wire but I was getting great spark to the in-line indicator light and at the spark plug. I'm at a loss for anything else. I'm going to replace the wire but if anyone has any ideas please let me know. In summation I've replaced the coil, condenser, plug and I'm getting spark to the plug. The reed valves look good and compression is good. The bottom cylinder however will not fire?????

 

[MahtyMaht]

Re: Firestone 7.5 ignition mystery

This is driving me nuts. Unplug the top cylinder's wire, and squirt a taste of fuel in the bottom one, see if it pops. Flywheel key OK?

 

[twocyclemania]

Re: Firestone 7.5 ignition mystery

This is driving me nuts also. I'm glad someone else is sharing my frustration. The flywheel key is fine; in fact is a relatively large one. Your suggestion gave me an idea. I disconnected the upper plug wire (the cylinder that fires) and I managed to get it to start and (barely) run for a little while on the one cylinder that is giving me the trouble. Shortly afterward I connected the upper plug wire as she was running and she ran much better. I pulled that plug wire after a couple minutes and the motor died instantly. It would not start again with just the lower (problem) plug. Usually something works or it doesn't but this motor is 'possessed.' I'm going to pick up some metal core ignition wire from a local antique car repair shop. There's nothing left to try. By the way, all the time it was running I had great spark jumping from the wire boot of the problem cylinder to the plug. The only other thing I can think of is that the OMC coil I transplanted may be bad???? I'm going to try another coil also. I'll let you know. thanks

 

[mrcrabs]

Re: Firestone 7.5 ignition mystery

just try switching the coils to see if the problem changes cylinders

 

[F_R]

Re: Firestone 7.5 ignition mystery

So, you are telling us that it runs on just the bottom cylinder only, albeit poorly? That means it is firing, so it isn't a spark issue. It isn't a reed problem either, you tell us. You are running out of possibilities. Crankcase leak? Lower crankshaft seal shot? Water getting into cylinder? Scored piston skirt? It can have a trashed piston and still have good compression. Whatever it is, you can't cure a sick engine with a good spark.

 

[mrcrabs]

Re: Firestone 7.5 ignition mystery

FR it sounds to me that he has a sick coil, it sparks great when he test outside the engine but under compression its sickly, just my guess

 

[twocyclemania]

Re: Firestone 7.5 ignition mystery

To clarify: It runs great on the top cylinder. It did run a bit albeit weakly and for a short time on the bottom problem cylinder by itself. The spark to the lower cylinder is fine but running on it is just is poor or it eventually craps out. I'm beginning to believe it's the replacement coil I used and/or the ignition wire. I'm going to try another coil. Also, there is no water in the problem cylinder so I believe the head gasket is okay. It's driving me crazy and I'll get back to everyone (who's input I greatly appreciate) after I try the coil and new wire. thanks again.

 

[F_R]

Re: Firestone 7.5 ignition mystery

Well, change the coil if you want, but I say if it is sparking enough to fire the cylinder at all, that isn't it. Unless it is only firing intermittantly (missing). That would be obvious. Firing, but "weak" says a sick powerhead.

 

[mrcrabs]

Re: Firestone 7.5 ignition mystery

I think this situation calls for the stupied brother-in-law or dumb and pesky neighbor test, they say its the only way to tell for sure

 

[85 BOMBER]

Re: Firestone 7.5 ignition mystery

mrcrabs, would that be the same as the "little brother" test? You know, the one where you tell him to hold onto the little wire for you...

 

[mrcrabs]

Re: Firestone 7.5 ignition mystery

Yep little brothers will work just fine but remember you have to live with him and they can be vindictive little Sh!ts

 

[F_R]

Re: Firestone 7.5 ignition mystery

Remote posibility--carboned up exhaust ports. Rare, but possible. I just checked the parts book, and evidently it doesn't have a lower crank seal, so that isn't it. But it could have worn out main bearings. That'll do it.

 

[Jim Moffatt]

Re: Firestone 7.5 ignition mystery

Remote posibility--carboned up exhaust ports. Rare, but possible. I just checked the parts book, and evidently it doesn't have a lower crank seal, so that isn't it. But it could have worn out main bearings. That'll do it.

Check to see if the fuel pump diaphram is driven by he lower cylinder. If so the diaphram is probably torn letting raw fuel into the cylinder - flooding it so it doesnt run.

 

[tomhath]

Re: Firestone 7.5 ignition mystery

To clarify: It runs great on the top cylinder. It did run a bit albeit weakly and for a short time on the bottom problem cylinder by itself. The spark to the lower cylinder is fine but running on it is just is poor or it eventually craps out.

I'd try swapping the coils and condensors. They can work okay when cold but fail when they warm up after a few seconds of running.

 

[twocyclemania]

Re: Firestone 7.5 ignition mystery

Jim and tomhath: I'm going to try both of these ideas. The fuel pump diaphragm has not been looked at and since I've run out of possibilities this gives me something to try. If that doesn't work I'm going to swap the coils and condensers. I've never had a motor like this one. Unfortunately right now it's about 20 degrees out and the motor is buried behind a half dozen motors in the shed. I'll dig it out when the weather permits as I would like to know why a motor like this; apparently with few hours, is such a mystery. I thank everyone for their input and I'll be sure to let everyone know the resolution if I figure it out. In the meantime; any other suggestions are really welcome. Thanks!