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So.. I'm trying to figure out an issue on a twin cylinder 18hp tecumseh engine. When it's running it seems to run a bit rich and it pops and backfires a bit, mostly at idle speeds. But it will still do it at all speeds. If I take off the plug wire on the left head it almost makes no difference in the way it runs. But it does stop poping. If I pull the other side it will still run but a huge difference in how it runs but it will still run. It will also stop popping too. Spark to the left head is really good, nice blueish white. Compression when cranking is around 70psi. The right side is actually less at about 55psi. I have cleaned both carbs and no difference was made, I even replaced the left head carb but it stayed the same. When it's running I can spray carb clean into the left cylinder carb and no difference I'm run quality. If I do it to the other carb it will make a huge difference in how it runs. I've checked the valves and intake and everything looks good. No intake leaks and both rockers move fully and the same amount.

Any ideas? I'm totally stumped..

Engine numbers:

OV691EA-600824C​

your problem is always on the side with the smallest change in RPM and sound on a cylinder balance test. The reason is that the engine is relying mostly on the strong cylinder to run.

Take both the kill wires off the coils
If the problem goes away you need to replace ( or fit ) diodes to the kill wires .
The kill wires can pick up FR from the opposite plug causing the coil to fire at the wrong time or fire twice .

Thanks for the reply, I will try that tonight and report back.

Ok so... took the kill wires off both coils the lh head is still the weak cylinder, and almost no change in engine running. I swapped the ignition coils and it didnt change anything either..Could the cylinder head be bad somehow?

Cranking psi on LH head (weak cylinder) 90psi
Cranking psi on RH head: 70psi.

yes
Time for a leakdown test
Bad valves or blown head gaskets are high on the hit list .

what Bert said. It may be that the intake or exhaust valve is not properly opening.

I'll try to pull the head today and take a took. The only thing throwing me off a bit is that I can see both rockers on that side move the same amount down on the other cylinder does. It also builds more compression when cranking over too.

One valve could be bad or one head gasket could be leaking
They usually go between the cylinder & the pushrod tunnel

did you swap spark plugs?

I did swap swap plugs too, no change. I haven't had a chance to pull the head yet or do a leak down test. Hopefully today or tomorrow I'll be able to.

I know this will a pain but have swap the twin carburetor around?

Pull the carbs and "Bertsmobile1 boil clean" them. Sounds like either a vacuum leak or dirty carbs.
Valve clearance check would be nice.
Clean the cylinder cooling fins yearly.
Pull the ignition coils and clean slash polish the mounting surfaces. Provides proper ground for coils. Load test your coils.

So I did a leak down test, didn't really drop more than maybe 15%. Didn't really hear any air coming through the intake or exhaust valve. But I decided to pull the head anyway. I found that the exhaust valve guide is worn. But the valve seat and face look perfect. No abnormal wear or pittin, totally clean. I posted the link below of the guide wear. Could that still cause an issue with it?

Here is a picture of the seat

That valve guide is shot.

Is she drinking oil on the job?

It actually doesn't smoke at all, not even a tiny bit hah. So I did some more digging and found that the intake valve is leaking by the seat. It doesn't always do it but if I push the valve down and release it sometimes it will seal. I tried lapping it in several times and no difference so I'm gonna replace the head. I filled with intake chamber with WD40 and pressurized it. As you can see the bubbles. The valve seat in the head looks like it might be the culprit. There was a black stained area where it looks like compression was blowing by the seat on the inside where it's pressed in. Everywhere else around it was nice and clean.

She's a leek-a' ma-ma she's a leek-a'. Steve Lav on YouTube.

what valve guide? All I see is a big hole with a valve stuck in it,

So... finally put a new head on this engine. Made almost no difference in how it runs. I literally can't think of anything else. Compression is good, no leaking valves, perfect spark, swapped carbs, coils and plugs, all new gaskets. The cylinder bore looks good and has no scoring or abnormal wear marks.

Anyone else have an idea? I'm about to give up on it.

take a breath. Are you able to determine which side is now misfiring, or are both?

It seems like the LH head (of referenced from sitting on the seat) is the issue. But if I run it and clamp of the fuel to the RH carbutor it will stop popping and back firing once the fuel is burned out of it. It will run still but definitely a difference in power and sound. If I keep it running and remove the clamp it will go right back to popping and slight misfiring again through the exhaust, sometimes worse than it was. It does the exact same thing if I do it to the other carb but it won't drop off as much in speed or sound. If I pull the plug per cylinder it's pretty much the same result. LH head seems to make less of a difference when the plug wire or fuel is off. It seems like it misfires as soon as both cylinders are active, but pulling a plug wire or turning the fuel off to one of them will make it stop popping and missing.

It seems to go away when using the mower deck but the idle will start to surge once it gets hot. It seems like it's getting too much fuel maybe. Using the choke never seems to make it any better. It will start to run rougher when too much choke is on. I've cleaned and tried new carbs too with no change.

Also wondering.. can a bad muffler cause any of this? I only ask because I can hear a baffle or something rattling around when it's running and the exhaust seems to be louder than it should be.

tkos115 said:

But if I run it and clamp of the fuel to the RH carbutor it will stop popping and back firing once the fuel is burned out of it.

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Sounds like a leaking carb needle/seat. I would pressure test both carbs.



I would load test the ignition coils also. Confirm you have proper spark.

That was my thought too. Did that and it held pressure fine, as well as vacuum. No drop offs or leaks, I also put new carbs on with no change either.
I used a spark tester similar to the one in the video. It has an adjustable gap and it will jump the gap at almost the furthest setting. Spark color is whitish blue on both coils.

tkos115 said:

That was my thought too. Did that and it held pressure fine, as well as vacuum. No drop offs or leaks, I also put new carbs on with no change either.
I used a spark tester similar to the one in the video. It has an adjustable gap and it will jump the gap at almost the furthest setting. Spark color is whitish blue on both coils.

Click to expand...

How long did you test the carbs with press/vac?

So it sounds like you've done it all. New carbs, tested old ones. Is the fuel tank spotless in the bottom? Does this engine have a fuel pump? Should have good fuel flow AT the carbs. Carb linkages out of whack? Meaning one cyl set to 3k and the other just off idle?

You've missed something. All these new parts you've thrown at it. You will need to test all these new parts. New doesn't mean new anymore.

tkos115 said:

When it's running it seems to run a bit rich and it pops and backfires a bit, mostly at idle speeds.

Click to expand...

Lack of fuel, carb needle sticking open or vacuum leaks.

tkos115 said:

If I take off the plug wire on the left head it almost makes no difference in the way it runs. But it does stop poping.

Click to expand...

You just found a suspect cylinder.

tkos115 said:

Compression when cranking is around 70psi. The right side is actually less at about 55psi.

Click to expand...

This is a little low, still should run. Valves not set proper? Blown head gaskets? This you said you put a new head on.

Flywheel key just a touch off and about to shear? Flywheel torqued to proper spec?

Do what I do when I get a new to me used mower. Look everything over.

Are the block cooling fins clean and clear? As in looking like a new engine? Some engines have metal pans that run around the bottom of the block to funnel cooling air. Look under these pans for anything that shouldn't be there.

So I ended up replacing both cylinder head gaskets anyway as I had the other side from a kit. I lapped all the valves and set them to spec. I have noticed now, that even when its cold at idle and just above it will hunt/surge and pop still and it didn't used to do that. I can't find any vacuum leaks and replaced all intake seals. I will check the flywheel key again but when I looked before it seemed fine. All the cooling fins are clean and I had all the shrouds off and cleaned them as they were a little dirty. I tested both carbs for leaks and to make sure they are holding and seem good.

Will let you know when I check the flywheel again.

Thanks for your time.

tkos115 said:

that even when its cold at idle and just above it will hunt/surge and pop still

Click to expand...

Too much or not enough fuel. Possible vacuum leak/s. You ruled out weak spark right?

Long shot is flywheel key off just a touch creating improper ignition timing.

Slowly putting the choke on seems to make it worse or doesn't really correct it. It will just start to bog down more. It kind of fixes the popping and hunting, but the engine is at the point of running so rich it slows down in rpm and will smoke. So I think it's too rich personally. Spark looks good with a tester, it's bluish white and jumps a large gap on the tester with no issues.

As far as I can tell the flywheel key looks good.

So.. I was playing around with the carbs and I disconnected the linkage that connects the two together. This allowed the throttle shafts to be independent from each other. When I did that at idle it didn't seem to surge or pop if I kept them both at idle speeds. Basically what I did was allow the main carb to be controlled by the governor and throttle cable, and I held the other one at idle. The engine didn't seem to pop or surge at all. I didn't get a chance to run it long or at higher speeds since it was getting late and I didn't want to annoy the neighbors this late. Is it possable that somehow the carbs aren't synchronized correctly causing the other issues? I don't see a way to adjust anything other than idle unless the link between the carbs is bent? They seem to be at the same settings when looking at the throttle positions.

do you have a carb sync tool? Have you compared the flow on each carb?

I don't have a tool for synchronizing them. But I apprently missed a chapter in the repair book about doing it. It looks like it can be done with a feeler gauge and tweeking the rod between the two carbs. I don't have a flow tool either so I haven't been able to compare them, I actually haven't done that before to be honest.

tkos115 said:

As far as I can tell the flywheel key looks good.

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Is that the engine flywheel or a belt pulley? What am I looking at? Not a Tecumseeme pro.

That's the engine flywheel and crank

Why is that outer black ring loose in a couple spots? Recon the key looks good.

tkos115 said:

That's the engine flywheel and crank

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No it is not
That is the crankshaft PTO keyway
The flywheel is on the opposite end under the blower cover .

This is the flywheel side on top of the engine

OK
So you have a flywheel PTO
It is still not the timing key
The timing key is 1/2 in the crank shaft & 1/2 in the flywheel
The flywheel is the big round thing with the magneto magnets on the outside & the alternator magnets on the inside .

It can be the flywheel. Some engines the keyway is milled into the taper of the crank and all you see is the key in the flywheel and none in the crank

Not with a black V belt running around it .

Take some wider pictures of the flywheel.

Only difference between these pics and the orginal one is I have the nut and metal spacer in place in these pictures. I know the flywheel isn't under the engine where the belts attach.

I'm probably just gonna send this thing down the road. It was free and I'm pretty much done messing with it. I have set littlerly everything I can to spec, replaced everything, swapped everything, and tested everything and nothing has made a difference. Short of splitting the engine and replacing stuff, it's all in spec and adjusted by the book.

So assuming that key is 100% intact, I would say rule out the key and abnormal timing. Again I'm no Tecumseh expert like others on here.

WHICH, after rereading this one, takes me back to the carbs. Running rich and popping, adding choke makes it worse......... I would Bertsmobile1 boil clean those carbs. Use a bread tie (plastic cover removed) and rod out all holes both ways.

As sgkent is preaching on, sync those carbs.

so until the carbs are synced that issue cannot be ruled out. Was the exhaust cleaned out, meaning are there any screens or catalytic converters that can cause excessive back pressure?

Is there a fuel pump on this? If so do you have good fuel delivery AT the carbs?

Air, fuel, spark and compression.

I synchronized the carbs a little while ago today by the book. Made pretty much no difference. I did it 3 times to make sure and it didn't change anything. It does have a fuel pump and it is working correctly. I boiled the carbs a few days ago and no difference. I even put two brand new ones on and it didn't change anything. Compression is good, and so is spark. I put a new muffler on it as well since the old one was rotted out and rattling but other than the noise level the engine runs the same. There are just two exhaust pipes, one from each cylinder that go directly into the muffler. Both of those are clear and open.

I think this engine is just possessed or something.. I've never had an engine be this weird when littlerly everything is within spec that I can check without splitting the block. But the cam lift seems good since all pushrods move up and down the same amount.

tkos115 said:

Only difference between these pics and the orginal one is I have the nut and metal spacer in place in these pictures. I know the flywheel isn't under the engine where the belts attach.

I'm probably just gonna send this thing down the road. It was free and I'm pretty much done messing with it. I have set littlerly everything I can to spec, replaced everything, swapped everything, and tested everything and nothing has made a difference. Short of splitting the engine and replacing stuff, it's all in spec and adjusted by the book.

Click to expand...

I haven't read every post but have you taken the flywheel off and held the flywheel key in your hand? Bad fuel or trash in the fuel could of course affect both sides but I would doubt both ignition modules went bad at the same time. If you're still working on it check the flywheel key and you have to take the flywheel off to do that. If that is good I would check valve timing next. I won't go into that now since I don't know if you've already junked it but if you're still working on it, checking valve timing is pretty simple to do and only requires removing one valve cover.

I can only tell you what I would do right now. First I would put an inductive timing light on each cylinder and watch the flashes for a short while to see if there are any misfires in the ignition, or if the timing jumps around. If so I would find the cause. Timing should be about the same on both cylinders. Plug gap should be set to factory too.

Second I would cup my hands slowly around each carb throat to see if the misfire got better or worse. If cupping the hands causes the RPM to drop as the hands move in then the mixture is too rich already. If cupping the hands causes the RPM to rise then the mixture is too lean and the hands are acting like a choke. Either lean or rich will cause a misfire. If neither of these two tests yields an answer then go ahead and cut your losses. Valve issues can cause a misfire.

tkos115 said:

This is the flywheel side on top of the engine

Click to expand...

Iam in the same boat as you have done every thing you have. having a little success synchronize carbs did it with the feeler gauge then while running played with throotle stop screw seems to help. the only way to accurately check the flywheel key is to remove the fly wheel remove key and look for sheering it doesn.t take much to change the timing .Paul

So I realized that I never came back with an update on this mower. (Now that it's a year later). Here's how it played out.. and yes it's gonna sound weird, and not make much sense.. I ended up starting it up several days later to check stuff again and it was still running like it had been. I ended up cutting my hand on a sharp edge of something when it was idling. Of course my hand started to bleed and about 2 seconds after some blood dripped on the engine it littlerly started to run fine. I adjusted nothing and did nothing other than accidently cutting myself. I was completely dumbfounded, to this day it's still running totally fine. So either it required a blood sacrifice, or coincidently whatever was causing the issue worked itself out at that same moment. I have no clue, and I'm not sure if the engine is demonic but it's been running without issues since.