OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S V-Twin fouling one spark plug

Hi all,

I'm stumped on this one. I would be very grateful if anyone could offer suggestions.

So I have a Briggs and Stratton Vanguard 18HP vertical shaft engine. It is a 570cc V-Twin.
Model-type-code is 350777-1156-E1-0101...

The problem is that it misfires on cylinder 2 (the right hand side cylinder looking from the air filter side), at idle and at top no load speed, maybe a couple of times per second. It stops misfiring as soon as you put a load on it, and runs fine - as soon as I engage the blades it runs fine and doesn't miss a beat. It fouls the spark plug on the side that is misfiring. The other side doesn't foul the plug whatsoever, as illustrated in the attached photo. The fouled plug is black with soft carbon, but not at all oily.

It has separate mufflers for each cylinder and the exhaust from the misfiring cylinder smells slightly of unburnt fuel.

To try and troubleshoot this, I've done quite a number of things:
- Adjusted valves to 0.004", intake and exhaust, on both sides
- Done carburettor mixture adjustments, checked the choke isn't stuck on
- Cleaned the carburettor thoroughly, including both jets, the emulsion tube, and all passages
- Replaced spark plugs (Champion RC14YC)
- Checked the intake manifold for leaks
- Tried running it without the air filter
- Removed the cylinder head of cylinder 2. No evidence of a blown head gasket
- The valve seating surfaces and cylinder walls of the affected cylinder looked OK.
- Done a compression test - I get 100PSI on both sides with a cheap tester so at least they match
- Swapped ignition coils (includes spark plug wires)
- Swapped the spark plugs. It fouls the plug on cylinder 2, and cleans the plug on cylinder 1.
- Measured valve lift, it is almost identical between the two sides.
- Tried running it with the ignition kill wires disconnected

None of that makes any difference. If I adjust the idle mixture screw all the way in, it will eventually stop misfiring on cylinder 2 at idle but then misfires on cylinder 1 due to being too lean.

It seems like a mixture problem, to me. I did notice that the carburettor's idle mixture screw's tip is slightly bent. See the second photo. I can't understand how anything wrong with the carburettor would cause a problem on only one cylinder, given that it is a single barrel carburettor, but... perhaps if it was somehow letting un-atomised, raw/liquid fuel into the engine, it could be travelling towards cylinder 2 (which is positioned very slightly lower than cylinder 1), vaporising on contact with the hot intake, and causing an over-rich mixture? Let me know what you think. I would replace the mixture screw immediately but the cheapest I can find one is $45 so I'm a bit hesitant.



Thanks,
RLP

Personally, I would be having a closer look at cylinder 1, Even though its 2 that has the problem,

To me it seems like 2 is overfueling due to having the mixture rich enough to have cyl 1 run ok. The best test you can do to compare cylinders is a leak down test, make sure there isn't any problems internally, or checking the intake manifold. An old trick I learnt is while its running carefully spray a bit of carburettor cleaner around any gaskets like the intake manifold to head joint, while the engine is running. If the note changes you know you have a leak.

Only other thing i have come across is is there any play in the throttle shaft? Usually affects both cylinders but stranger things can happen.

I don't have the equipment for a leak down test, unfortunately. All I have to go on is the similarity of the compression test on both sides, and the fact that it runs well under load - this doesn't speak of any major internal issues to me.

It doesn't seem like there's much play in the throttle shaft, just a little bit. It is missing the throttle shaft seal though... same with the choke shaft.

I did take the intake manifold off altogether and didn't see any sign of leaks on either side. Now I have it re-sealed with RTV silicone which isn't ideal but I've observed no change in the behavior so I'm guessing it's not the issue.

When I adjust the carb so that cylinder 2 stops misfiring, the idle mixture screw is pretty much turned all the way in, and if I go much further both cylinders just die, so I can't imagine it's running too rich as a whole. Then again, the bent idle mixture screw could be messing with the adjustment...

RLP

Hi RLP and nath

I would remove the muffler and run the motor for 1 minute just to rule out the muffler being blocked on one side or the baffle being loose.

Cheers
Max.

Thanks for the suggestions so far.

I have tried running the engine with a different exhaust setup (a two into one exhaust manifold with a single muffler for both cylinders) and haven't noticed any improvement (still misfiring at no load and fouling on that side). This should eliminate the individual muffler as a culprit I think.

Still looking for ideas. I will definitely post back if I do figure out what it is!

RLP

A few other parts I would check are valve guide clearances for excessive clearance,cylinder head straightness with
a straight edge and feeler gauges ,bent push rods,loose rocker posts,sometimes the valve seats come loose.

I have heard of a few wiring faults that can cause intermittent running problems .The wire that goes to the fuel bowl solenoid can short out,
I would check the easiest problems first like the fuel bowl solenoid. You can Test the Fuel Shut-off Solenoid ,lots of You-tube clips.

https://www.smokstak.com/forum/showthread.php?t=107195

Cheers
Max

Since the main isn't in the fuel solenoid you can temporary replace it with a appropriate sized screw.

Also try increasing valve clearance to .006" as you might have an ACR problem. I know that not much but if problem clears up some then I would be looking at the camshaft for wear or an ACR problem. Just a thought...

AVB, that is very interesting regarding a possible camshaft/compression release problem. When I first got the engine, both spark plugs looked the same, and the exhaust valve on the problem side had about 0.012" clearance! The problem seemed to start right around the time I adjusted the valves back down to 0.004" clearance.

I'll have to wait until the weekend to test it with a larger valve clearance again and find out for sure. I'll try adjusting the exhaust valve on that side up to 0.006" or maybe a little more, and will watch for any improvement.


Max, I'll check the fuel shut off solenoid as well when I get back to it, that's an easy one to test for. I'll check pushrods and rocker posts while I have the valve cover off to adjust valves. If I do have to pull the head off again then I'll also take a closer look at the internal parts you mentioned.


Thanks,
RLP

Well, I adjusted the exhaust valve on that cylinder to 0.011" or thereabouts, and the behaviour was distinctly better on that cylinder. Not completely better, but definitely an improvement.

So... maybe it's an ACR issue. Am I doing any damage by running it with a large valve clearance like that? This engine calls for 0.004" to 0.006" so I'm kind of breaking the rules here. Obviously the proper fix would be to replace the camshaft, but it's quite usable now - do you guys think I can get away with this "band aid" fix for now, given that the engine has no problem starting (i.e. the reduced ACR action on that cylinder isn't a problem for starting)?

Thanks,
RLP

The only obvious problem would the possibility that push rod could dislodge and do some internal damage. Otherwise it sounds like you are narrowing the source of the problem.

If you have valve train noise from excessive clearance this will put a greater load on the valves and valve train components
and will eventually damage the camshaft lobes, rocker arms and the tips of the valves themselves.

Too much clearance causes a rattling sound and lackluster performance. Operating with too much clearance leads to less overall damage than operating with tight clearances.

The valve timing is also affected by loose valve clearance and will actually retard the cam timing to the valves with this issue along with not opening the valves as far and reducing overall power.

Max.

An update on this:

I thought I was onto something with the ACR, but upon further investigation I found that adjusting the exhaust valve clearance doesn't actually seem to make any difference. I think I must've imagined an improvement the first time I did it.

After some lengthy investigation and troubleshooting, I determined that it's actually a carburettor problem. If I block the idle jet with a small piece of wire carefully secured in place, the engine runs perfectly (not at idle though ) and both spark plugs come out clean. I think what must be happening is that the idle circuit is somehow dripping liquid fuel into the intake manifold while running, and that was making the mixture uneven between the two cylinders. Cutting it off means it runs leaner as a whole but I don't think it makes much difference to the mixture at full throttle.

It makes sense as I could always see a bit of liquid fuel sitting in the intake manifold after removing the carb. And it was using a LOT of fuel.

I still need to figure out why the idle circuit is causing this problem... but it's good I didn't spend money on a camshaft and open up the engine for no reason.

RLP

I guess if worst comes to worst you can always get one of these, not a lot of money and might solve the problem.
https://www.ebay.com.au/itm/14HP-15...y-799727-698620-Carburetor/112748721157?

Hi all,

Problem solved, finally - installing a different carburettor fixed it.

I bought a used genuine carb from eBay. After cleaning it up and installing it, the engine is now running as it should. Both spark plugs look the same, and I think they look about right.



I don't know what was wrong with the old carb, I went through it several times already, but I'm not going to waste any more time on it. Fuel consumption is a fair bit lower with the new carb as well, and I think it starts a bit more easily.

RLP

Glad to hear you got it sorted, hard to understand why some carbs do this sort of thing but they do and on the twins it can be frustrating when only one cylinder is affected, makes it so hard to sort out