HG Backfires???

[Mike Anderson]

Hi all,

I've got a problem with my HG backfiring and not starting.  It has been running fine, but slow to warm up.  It sat a few days and never quite seemed to fire right when I next started it.  It idled fine, but missed terribly when I increased rpm.  I checked the carb, then checked the magneto.  The points were barely opening, which seemed to make make sense for a cause for the missing and stuttering.  I gapped the points, put the mag back together and installed the carb.  

Now it won't start and backfires badly.  I rechecked timing with the mark on the flywheel and mag.  I've got spark on all four cylinders.  Not knowing what else to do I checked compression,  the #4 cylinder is only showing about 30 psi, the rest are fine.  This is a recently professionally rebuilt engine.  I suspect or guess that maybe some carbon is holding a valve open since it's been backfiring?

At any rate, I think it should run even with one weak cylinder.  It backfires through the carb the instant I try to start it and will periodically do this as I crank it.  I've tried retarding it as much as possible by rotating the mag, but no luck.  The spark plug wires haven't been switched.  I never removed them from the mag, and they're strapped together so can't go on any differently than they were before all this started happening.

The mag is a Prestolite, and my book only shows Wico. It looks like a Wico X.

What next?  I can't figure this one out, and my poor brain is just about worn out.

HELP!:(

Mike

[Blake Malkamaki]

I would double check the timing and make sure the wires are correct. I believe the firing order is 1-2-4-3. Make sure the wires are in the cap in the right order.

If timing is ok, I think it's probably the intake valve is sticking or has no clearance on that low cylinder. That would mess up the vacuum on all the other cylinders so they won't pull in fuel/air.

How long has it run since the rebuild? Has it ever run right since the rebuild? Did they put in hardened valve seats? Perhaps a seat has come loose?

First check the valve clearances before you pull the head off.

Let us know what you find.

Good luck,
Blake

[Crawler Boy]

Could you have turned the rotor inside the magneto when you adjusted the point

I dont know how how a prestolite mag works but since you says it looks like an wico itÃ,´s easy to touch it and it will turn a spot.
check if thereÃ,´s a mark on the rotor and the driving gear and turn the engine on the flywheelmark and check if they are inline

[Ronald Haines]

Try a new condenser in the mag. My HG was doing the same thing.
         Ron

[Mike Anderson]

I went through the whole thing again.....this time making sure I was on the compression stroke.  It seems that the rotor needs to be 180 degrees off from the little mark on it.  Does this make sense?  At any rate, it ended the back firing when I did this, and it sorta starts.  

It still won't really run, though.  Heavy missing....worse than when I started.  It won't idle smoothly now.   A strobe light fires a few times when cranking then it will miss a beat or two then fire for 5 or 6 times, then miss again.

Is this a symptom of a mag problem?  I also changed the condenser...didn't have any points, but they appear fine.

Arrrgh.

Mike

[Ray]

Mike,
Don't go by the marks on the flywheel, it can be put on 180 degrees off. We always take the #1 sparkplug out and physically check for #1 compression stroke. My guess is the flywheel was put on 180 degree off. Also check that litttle bronze bushing in the center of your governor cover to make sure it is not worn through, which could allow it to jump time.
Good luck,
Ray Z.

[mtncrawler]

Have you figured out the reason for the low compression reading on # 4 yet? A stuck or burned valve is going to effect more than just that one cylinder.

[R Fink]

mtncrawler would you explain that a little futher. Low compression on one cyl can affect others.????????:(

[R Fink]

Mike i am no expert by no means. i would say that you moved the rotor ahead or back when serviceing the mag. If it is OHV pull the cover and watch when that cyl comes up to fire and sit the timeing that way

[Bill Seal]

Back popping into the carburator could be caused by a stuck or burned intake valve, allowing the fire back into the intake manifold. Excessively lean mixure causes it too (hang fire). Backfiring into the exhaust means unburned fuel in the exhaust manifold or muffler (if you have one!), could be caused by fouled plugs. Could be crossed plug wires, but not likely on a four banger.
You could pull the side valve cover and see if one of the valves in the culprit cylinder has excessive clearance, indicating a stuck valve. You might even have a broken valve spring. (I struggled by on two or three cylinders for a long time, figuring I had a burned valve, finally pulled the cover and found a broken spring and no keeper. Talk about feeling stupid!)

[Mike Anderson]

Thanks for all the suggestions, folks.  I tried a few, and it became clear that the mag was the problem.  I took it in for a rebuild.  They said the cap and rotor were good, but points were bad and the condensor was the wrong kind.  They took it apart and cleaned it up, also, I guess.  

I also checked valve clearances, and the intake valve was tight.  I adjusted that and got the compression to 50 lbs.  Still less than the others, but I thought I'd run it with some Marvel Mystery oil in the gas and see what happens.  

I did quite a bit of work with it today and it performed like a champ!  Dang but that's a nice tractor!  I leveled a couple of areas for a neighbor and then did some work around here.  The neighbor is mighty jealous.  It ran quite, strong and smoothly.

Thanks again for all the input.

Mike

[Bill Seal]

Just noticed you're from hey-mule.
Now that you have your toys running, maybe you could use them to dig a toll tunnel to Tecate and use the receipts to finance your Cletrac addiction.

[Mike Anderson]

Not a bad plan, Bill.  Are you near enough to lend a hand or tractor?  

I seem to have enough projects on my little 10 acre slice of heaven to keep me busy, though.

I always thought it was Ha Mool, or at least that's what's on one local lic. plate.

I did some levelin' and cypherin' and came up with a center to center distance of about 7 3/4" between the upper pin and lower spring support pin for level flight on the HG.  A little lathe work and painting tomorrow and I should be ready to take off again this weekend.

Mike