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jrmorrill

Got fuel, air, spark but no start. Why?

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jrmorrill

I've struggled with this old 243431 engine for 20 years. The older it gets the more trouble it gives me. Once its running it does OK but it takes LOTS of cranking to get it started. Often running the battery flat, charging, and trying again. 

The plug shows great spark when I pull it and rest it against the block while cranking. I've got the MegaFire electronic ignition - no old points/condenser.

But when I pull the plug it shows wetness.

After lots of cranking the carb pukes gas and I walk away. 

Often during its cranking and cranking I can get a backfire or two or three from the exhaust.

I'm pretty well stumped - and about to switch to a 13HP Predator engine instead of continuing fighting with this old engine. I've repowered two other tractors with great success so I'm not intimidated by a repower.

But before heading down that road again ... any advice?

Thanks, Jason

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wwbragg
11 hours ago, fishnwiz said:

No compression.

If a valve were stuck open it probably wouldn't run at all.  But a valve that is burnt or poorly seated could result in low compression.  Head gasket, rings, or maybe it is sparking at the wrong time????  A sheared flywheel key can throw it out of time.

Maybe it is time for a rebuild anyway?????

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SmilinSam

Like fish indicated,  you need to do a compression test to see what the results are. I have a 15 here tht wont start. has fuel, air and spark, but only pumps up to about 50 lbs. Even when a little oil is dropped down the cylinder I only get about 50 lbs. I note that the fuel air mix is somewhat being blown back out the carb on the compression stroke, so I suspect a bad intake valve for one on mine.

Worst diagnosis problems I ever had , as Bragg hinted at above, was a partially sheared flywheel key. Drove me nuts trying to figure that one out. Was using a aftermarket "Atom" ignition module on it too.

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ShaunE
1 minute ago, SmilinSam said:

Worst diagnosis problems I ever had , as Bragg hinted at above, was a partially sheared flywheel key. Drove me nuts trying to figure that one out.

X-2!

Also if compression is low, leak down will pinpoint the problem area/areas.

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SimpleOrange

After 20 years time to readjust the valve clearances  Had an old Kohler that would start cold but not when warm after setting the valve lash problem went away.

Popping through the exhaust is a dead give away that your clearances are to tight.

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Wilbur643

Engine guys tell you, if it has fuel, air and spark, it should fire. Rarely do the mention the fourth component,- compression.

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SimpleOrange

Vacuum gauge can diagnose a lot of engine problems.

The gauge works just as well for single cylinder engines as those with multiple cylinders.

 

Edited by SimpleOrange

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Josiah deshong

My grandfather used to buy me tools at auctions when I was young I’ve had a vacuum gauge for years I’ve never took it out of the box. good post

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jrmorrill

Thanks everyone - I totally forgot to check compression.

Last year I lightly lapped the valves and seats and checked the head for any blow-by. All that looked good.

I did hook a compression tester to the engine last year and recall the values being below 50psi. I'll check again this week. I'm also going to try out the Restore product and see if that helps. 

Interestingly, I tried firing the engine this afternoon. On the second crank it fired and ran for about 2 seconds. It stalled out and then wouldn't restart - just cranked and cranked.

I'll check on compression, try Restore, and report my findings back here for everyone.

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SimpleOrange
33 minutes ago, jrmorrill said:

Thanks everyone - I totally forgot to check compression.

Last year I lightly lapped the valves and seats and checked the head for any blow-by. All that looked good.

I did hook a compression tester to the engine last year and recall the values being below 50psi. I'll check again this week. I'm also going to try out the Restore product and see if that helps. 

Interestingly, I tried firing the engine this afternoon. On the second crank it fired and ran for about 2 seconds. It stalled out and then wouldn't restart - just cranked and cranked.

I'll check on compression, try Restore, and report my findings back here for everyone.

Did you do both a wet and dry compression test.

Once had a 1966 Ford half ton with a 300 cid, engine had so much piston slap but it kept on running. That is if you started the engine a drove the truck at least once a day, if you left the truck for three days or more the oil on the cylinder walls would run off back into the crankcase.

When ever this happened you have to pull several spark plugs and give each cylinder a shot of oil, a light oil like WD40 worked fine to restore enough compression to start and run.

The old 300 six was still a runner when we pulled it then transplanted a chevy 350 under the hood. What a difference that extra 50 cubic inches made.

Edited by SimpleOrange
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lugnard

Even tho you have spark while holding the plug against the head...that does not mean that plug is firing under compression. Try a new one. Had a big go round with a kohler twin in a buddys skid steer. Lots of cursing and did the same thing with the plug grounded many..many times. Swapped carbs, pulled the flywheel, checked timing. Finally said lets try a new plug....purred like a kitten!!!!

Harry

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B10Dave

What Lugnard said. My 616 AC lawn tractor with the Briggs opposed engine had a miss at higher speed. Changed plugs when I bought it just in case.  Fiddled with the carb high speed jet but slight miss was always there. I knew the Champion plugs were new so thought that wasn't the problem. Last week just on spec I put in a new set of NGK plugs. Problem gone. One of the Champions must break down at high speed.

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MikeES

My experience with the ci B&S is that when they get below 60psi they are very hard to almost impossible to start ... seem to run fine and even not use much oil ...

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MikeES

Something in the wiring or the start button is grounding out the mag while the button is pushed.  I had a similar issue with a key start on a battery ignition Kohler.  I somehow connected the coil to a switch terminal that was powered with the key on, but unpowered in the key start mode.  (Probably for lights so they don't pull power while the starter is activated.)

Drove me nuts until I figured it out.  The tractor would start only just as I released the  key switch from the start position.

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jrmorrill

Following up on my original post ...

The engine tested with 90psi when cold and dry. It started and ran briefly. I shut it down. Then tested again and it resulted in 60 psi.

Weird ...

I've added some Engine Restore to the oil. Since then the tractor has sometimes started right up and other times cranked and cranked without start.

Tonight I ran the tractor for a while (after a really hard time getting it started) and tested the compression. When hot, after running for 10 minutes, the compression measured a little below 90psi.

I'll continue to tinker with this. 

Thanks for all the ideas and feedback. I'll continue struggling this this old hunk of iron. When it's running at 3600rpm it's great, but getting it started (and keeping the RPMs up) are turning to be quite a challenge.

 

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