Lawn tractor – 1 cylinder not firing?

Troubleshooting my dads Sabre lawn tractor with a twin cylinder 18hp B&S engine. It starts and runs on one cylinder. He pulled the plug on the bad cylinder and it’s getting decent spark just by visually looking at it, and the plugs are getting fuel as the plug is wet. New plugs have been installed which did not resolve the issue.I am thinking:1) Timing trigger on the fly wheel off or out of spec?2) Possibly not a strong enough spark, bad coil?3) Could the valves cause this is one was stuck or not in spec?The engine has a compression release feature, so at low rpms (such as starting RPM) you can’t easily or accurately check compression. I don’t think compression is the issue….but any ideas how I could test this?Any other thoughts. Tractor is probably 10 years old purchased from a dealership…not a big box store.

I’m seeing a conflict here, you say the dead cylinder appears to be getting good spark, but it’s also wet from fuel. I’m having trouble seeing how you can have both and still be dead on that cylinder unless the timing is way, way off.

Did you actually check there’s spark using a spark checker? I’m thinking you could be on the right track, it’s a weak coil so there’s spark but just not enough spark.

From there, I be wondering about the CDI brainbox, but no way would I try to start parts swapping just because I suspected it. I’d want to get that tested by a shop and know before I started down that route.

Interesting little issue.

Grouse

Low compression : due to a bad (Burnt) or stuck valve or bad piston or rings would cause this situation. I have even seen on my snowmobile the same symtoms because of a fuel pump with a leaky diaghram caused one cylinder to flood and not fire properly.

A very common thing on B & S engines is the key in the fly wheel gets worn ever so little it cuts the spark strength and usual symptom it makes the engine hard to start and loss of power, as it affects the ignition timing.
It is an easy fix for that, just use a wheel puller and put in a new square key.
This may not be the answer here, but boy a new key is like a real power booster as well as just and easy fix. Should be done periodically anyhow.They get worn just from the engine running.
Sudden stops cause these to wear faster.
They are a safey feature so as not to damage the flywheel due to sudden engine shut downs.

Hope this helps.

Iowa Roger

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A very common thing on B & S engines is the key in the fly wheel gets worn ever so little it cuts the spark strength and usual symptom it makes the engine hard to start and loss of power, as it affects the ignition timing.
It is an easy fix for that, just use a wheel puller and put in a new square key.
This may not be the answer here, but boy a new key is like a real power booster as well as just and easy fix. Should be done periodically anyhow.They get worn just from the engine running.
Sudden stops cause these to wear faster.
They are a safey feature so as not to damage the flywheel due to sudden engine shut downs.

Hope this helps.

Iowa Roger

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We are going to check this first as I think this could be the issue, but from there I will take a look at the valves and possibly buy a spark tester.

If that does not work, I will have to determine how to check compression or see if it needs a leak down test.

Sometimes the starter will turn the engine over fast enough so the weights that keep the valves slightly open, do release and the valves do close, but the starter has to turn the engine over fast enough. Make sure the battery has a full charge so it will turn over as fast as its going too, try the compression test first in both cylinders to compare, it may work, it should run atleast 90 pounds, if its down around 50 or 60 the valves aren’t closing.

I think both sparkplug wires come from the same coil under the flywheel and both plugs fire at the same time and if its blue spark that may not be the problem. I have two of those industrial Briggs, one on my wood splitter and one for parts. Pull both plugs and lay them on the block or use alligator clamps and ground both of them to the block and watch and see if they both fire at the same time. Is there a chance that one plug is fatiguing and the other is good.

I’ve never been inside of one of those Briggs and I’m not sure if their two cycle engines or not and don’t have valves. Call a small engine repair shop to see if their 2 or 4 cycle engines, I know they have oil in the crankcase so their probably 4 cycles but because I’ve never been inside one I don’t know how their engineered.

If both plugs are firing blue and the plug is wet my first guess is the rings are very worn in that cylinder or if theres valves one of them isn’t seating right. I’ve heard through the grapevine that those industrial Briggs don’t last a long time like Kohler and a couple others do. Briggs aren’t a bad engine but their not made like Kohlers are, to last probably twice to three times the length of time. If there’s a big difference in the cylinder compression, one compared to another its the valves or rings. You can get full rpm’s out of the starter to turn the engine over by fully charging the battery and by putting a good set of jumper cables on the battery from your auto, with it running. There’s a good chance that the starter will turn the engine over fast enough doing this that the centrifugal weights will swing out and let the valves close to get an accurate compression test, both should run within 10%. It may be the plugs fatiguing or its the rings or valves. Are both plugs the same number, they should be. Their not expensive and maybe its time too put in new plugs, its a cheap place to start, put in NGK’s or Bosch. Also check the plugwire while its running if you can, in the dark. Sometimes you can see where its arcing out dimly, and enough to change the temperature of the spark, maybe the plug wire on that cylinder has seen its better days.

Strong, leaning for Coil Pack, after some research…

I think the flywheel key is a longshot and it is not a normal item to replace like a spark plug But thats just my two cents worth.

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I think both sparkplug wires come from the same coil under the flywheel and both plugs fire at the same time and if its blue spark that may not be the problem.

I’ve never been inside of one of those Briggs and I’m not sure if their two cycle engines or not and don’t have valves.

Their not expensive and maybe its time too put in new plugs, its a cheap place to start, put in NGK’s or Bosch.

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I still have not had a chance to get over there yet to troubleshoot, but my dad did pick up a new timing key to try that first.

He has brand new plugs in it, and the plug wires look fine but may be worth checking in the dark. That helped me on an old Merc I had.

It’s a twin cylinder OHV, so both cylinders have their own sets of valves and separate coils. I have not had a chance to pull the valve covers off yet, but will be doing that if the key does not resolve the issue. I think I am going to try swapping the coils as well to see if the issue follows the coils.

I will let you know what I find.

Ya Hemi your on the right track and it sounds like you know what your doing. If the coils, plugs and wires are ok then it may be mechanical. It wouldn’t hurt to replace the key anyway while you have the flywheel off. Does it have points or just the flywheel magnent passing by the coils? I know nothing about any electronics it may have if its a newer motor. If you take a compression test and its ok then I would check the gasket on that side of the intake manifold then go to the electronics.

I just worked on a car and it was missing on one cylinder. Checked the plugs and found the cylinder. All the plugs were firing so I took off the valve cover and a rocker arm was off that pushed an intake valve. I retighhtened the nut on the rocker arm post and it ran for about 5 miles, then missed again. I took the valve cover off again and the nut was turning back up the post and it was a factory locking nut. I took the nut off and hit the top in three places with a 1/2″ extension and a two pound hammer. I put the nut back on and I immediately felt the nut grip the threads, so far its running good and should have fixed the problem. Maybe just maybe the rocker arm assembly has something wrong with it. If its a rocker arm problem then the compression will be lower on that cylinder because one of the valves isn’t opening and closing right. Let everyone know what it is, I’m curious too.

Well, I finally had some time to head over and look at the tractor with my dad. We got it running last night….but I can’t say exactly what we did that actually fixed it. It was one of a few things.

1) Timing key on flywheel was fine, so we put that back together.

2) Dad bought a new coil for the cylinder that was bad, so I installed that and gaped it to spec. Started the engine and still was running really rough like it was on one cylinder.

3) Pulled the covers off both sets of valves and checked them. Both were way out of spec. They were at around .010-.012, and spec is .004-.006. I adjusted them and put it all back together. It still was not running right.

4) Decided to pull the air filter and try it, and it started right away and ran great. The old oil filter was soaked with oil though so I am not sure if the valves being out of spec that much could cause that? Learning experience, always rule the most basic stuff fist. The air filter was brand new 2 months ago, and is used about once a week to cut the grass.

Something caused the air filter to get saturated with oil, could a bad coil or valves out of spec cause that?

Thats a difficult one to diagnose not seeing where the filter is at on the engine. I’m assuming its on a high point above everything else. Do you mow hill sides and did the motor backfire occasionally. Did you take a compression test too, just curious what the rings are like in that cylinder. Valve guides possibly but you said the engine is relatively new.

The only way it could soak the filter is through the intake manifold or if it has a vacume tube that comes out of the side of the motor and into the bottom housing of the air cleaner. On some motors theres a spring steel plate that keeps the vacume going one direction. Take the cover off and you’ll see a plate with maybe some type of filter screen, kinda like scotch bright. Behind that and against the motor block will be that spring steel plate and its thin, maybe .005 thick, check that to see if its got something wrong with it. That’s crank case ventilation and it comes in direct contact with the oil vapors, this may be where the oil is coming from, especially on steep hill sides where the oil runs to that side of the block. The vapors that come from the crankcase go through that spring steel plate, its a one direction valve, go up the tube to the filter housing, usually the bottom of the housing, through the filter and back through the intake. The vapors then can build up, Can’t think of anywhere else. Let us know!

Valves not sealing thus oil blowby to the airfilter

I have a craftsman 1991 18hp lawnmower but I runs on one but you file the sparkplug its fine for a while then it gets dusty and doesn’t run on both full time it hits 1 out of 3 hits on the other(hoping for easy fix parts are sparse for it)

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