318 backfired and quit!

It's a 1985 model with the B43 Onan. It hasn't made any mechanical sounds like a loose rod or anything like that. It has just under 2000 hours on it. It's been using a little oil, about a half pint after mowing 3.5 acres.

I was thinking it was running on 1 cylinder too but what would that have to do with the PTO not engaging?

I was mowing two weeks ago and the same thing happened to me. Turned out that one spark plug gave up the ghost while I was mowing. Mower still ran on one cylinder but would stall out immediately under load. Perhaps the same thing happened to yours (cheap fix) and just not enough power to run the pto. Does the pto try to engage?

BufordT said:

The PTO engages via electricity. Spark needs electricity. Backfiring can occur when the spark is weak or intermittent.

Let's be hopeful.

Charge and test your battery. Check the output of your charging system. Check the gap of your PTO clutch.

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Excellent logic and answer.

If that makes it run better and fixes the PTO then two things: take the battery to an automotive store and have it tested and if it is good then test to see if your voltage regulator is returning juice back to the battery like it should. I'm hoping it is one of those, they are way less expensive than my first thought.

My 318 did about same thing. Spitting sputtering all at once and would not take throttle. The coil failed. It was a old points type coil. Ordered a points type coil from flea bay, received a electronic ignition coil and it would not fire correctly. Finally got a points type ignition coil from amazon and all ok.

But in reference to yours possibly being electrical issue. My 318 has a on dash lamp to indicate battery not charging. Few days ago the BAT lamp on the dash panel came on while mowing. I found the slip on wiring plug of the voltage regulator fell of the regulator. Re-connected and used a zip tie to hold it in place. all ok.

Forest Gump said:

My 318 did about same thing. Spitting sputtering all at once and would not take throttle. The coil failed. It was a old points type coil. Ordered a points type coil from flea bay, received a electronic ignition coil and it would not fire correctly. Finally got a points type ignition coil from amazon and all ok.

But in reference to yours possibly being electrical issue. My 318 has a on dash lamp to indicate battery not charging. Few days ago the BAT lamp on the dash panel came on while mowing. I found the slip on wiring plug of the voltage regulator fell of the regulator. Re-connected and used a zip tie to hold it in place. all ok.

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So the dash light is triggered by system (alternator plus battery) voltage being below some threshold. I don't know off the top of my head what that is. The alternator could be fine, and trying like heck to charge a very low of failing battery, leaving few watts available for running the ECM, coil, and PTO.

I went out this morning to have a look and I couldn't find anything wrong. It started up and ran normal. I backed out both spark plugs to see what they looked like and they weren't terrible, a little black but both looked the same. I'm wondering if the fuel pump is acting up when it gets extremely hot like it was yesterday after I was finished mowing my 3 1/2 acres? I did notice the fuel filter had gas in it but it was far from being full. I wonder if that's normal? I'll have to wait until next week to really give it the acid test. It might only do it after the 318 gets stinky hot.

My old 1985 PTO switch itself did not en-gauge the PTO clutch and is/was intermittent sometimes. First time it happened, when I flipped the toggle handle down and it did not en-gauge I could go back up to off and back to on and get a en-gauge. (sometimes had to flip the switch handle two or three times) First time it happened I went to shop to test electrically and when I got to shop for a test it operated ok and I could not get a fail. I sprayed some contact cleaner around the bat handle of the switch and the switch operated easier by feel and has been ok ever since. If it fails again I will be looking for a replacement switch. You may have two problems, bad running engine and a intermittent PTO switch.

Also its normal for the fuel filter to not be full of fuel on a 318 with a pulse/vac fuel pump, but you can see some fuel flow through the filter when the engine is running.

I've seen condensers get erratic (usually they open with no capacitance) also and it's quite common on the old B43/B48 Onans and most generally fail when engine gets hot. If the engine is already running and the ignition points condenser fails the engine will run very rough and once the engine is killed or dies it's usually very hard to get a re-start. To get an idea if it's a condenser on a points type ignition system I take alligator clipped jumper leads and connect a test condenser on the neg post of the coil to ground. If it runs ok I rig the replacement condenser at the coil area and just cut the wire off the old condenser that is in the points box and remove the old bad condenser at a later time. The replacement condenser will be located (mounted) in the vicinity of the ignition coil instead of in the hard to get at points box. The wire lead lengths on a condenser is not critical, I usually have soldered on extended length wire leads on the condenser. Old burned points are also a source of problems but usually more consistently erratic than intermittent.

Also like the one guy also said sometimes a piece of carbon will break loose and hang under a valve and then eventually turn loose.

I would get ready for a test in future by having a condenser, a compression gauge and set of new spark plugs ready for a future test, trying to determine if the issue is ign, fuel or compression. I would also be ready to either pull a spark plug wire on each cylinder or ground the tip of the plugs to see if both cylinders are operating equally. (running on both cylinders) From experience I can detect by ear (exhaust sound) if a engine has lost a cylinder, but some people cannot.

I spent the better part of yesterday working on it. After reading all the posts here (thank you guys) and a few more threads on this forum it gave me some ideas. I replaced the spark plugs and the engine ran better but the problem was still there. I replaced the vacuum hose going to the fuel pump which was a bit of a trick and again the engine ran better but the problem was still there.

I replaced a rock hard piece of fuel line from the fuel filter to the carb and also blew through the filter (taste of gasoline) and it ran a little better but still backfiring and surging rpm. I tried a new condenser on the negative side of the coil and no change. I could pull one plug wire and the engine would lose rpm but still ran rough. I pulled the other plug wire and the same difference, problem still there.

So this morning I woke up with a clear head and after thinking about it in my sleep all night decided to take a different approach. I pulled the top off the carburetor, removed the slow idle jet being very careful not to cause any distortions in the brass. I couldn't blow any air through with my mouth. There's a hole in the bottom and on the side of the jet. I gave it a blast of carb cleaner and the hole it screws in to and put it all back together.

When I turned the key the engine popped right off. I've never seen it start that easy before and run that smooooth! There aren't enough o's in the word smoooooth to describe it. No more backfiring or rpm surging. It's running perfect at ALL engine speeds so the "Idle jet" must have something to do with the way the engine runs at all rpm's.

Without the help from this forum I would have never found the problem.

I really appreciate the great help!

I do that quite often with carbs, carefully take off the top and use carb cleaner or low pressure air and clean out the bow and jets. A set of Hollow ground gunsmith screwdriver adapters is good for removal of brass jets to reduce possible distorting a jet during removal. Some hollow ground screwdriver gunsmith sets can be found a wally world for less than $20.:tango_face_wink:

I also see this quite often.
Some of them Nikki's carbs that have a bowl drain will straighten up and run right if the bowl drain screw/bolt is removed and just let the bowl drain. Do this when the engine is cool. Suspect little bit of water gets in the bottom of the bowl.

AND some of them Nikki carbs with a low speed adjustable jet on a 318 with a B43 B48 engine is not user friendly to adjust in it's location (heat on the fingers, etc) AND the proper setting is not always 1 1/2 turns as stated in the service manual, more like 2 turns or more sometimes for a smoooth idle cold and hot engine, (mainly hot and the low speed jet adjustment may have an effect on the medium to high speed smoothness also, but wait for the engine to warm up before adjusting, then check it 24 hours later after cool. When properly adjusted it may have to warm u[p somewhat for good idle. Some of them Nikki's carb's are critical within 1/2 turn of the screw for good idle medium speeds and may have to just adjust for a good hot run and idle and let the engine warm up when first started cold to get a good smooth idle.

I use a 90 degree screwdriver type drill driver (termed offset drill driver) and install a nut driver handle on the driver and sometimes have to grind a proper size flat screwdriver driver. I have both a milwaukee offset drill driver and I think the other one which has a flex shaft is a HF.
The HF looks like this and the Milwaukee is shorter does not have the flex shaft and is more expensive. They use the interchangeable 1/4 drive shank screwdriver bits and you can use a 1/4 drive nut driver as the handle.

Really handy for hard to get at carb adjust screws when a angle drive or a short driver is needed and can keep hands in a safe place.

https://www.harborfreight.com/6-in-...sae-and-metric-fractional-readings-63731.html