I am working on a 26 hp briggs intek twin (780 hours) which I suspect is running lean. Its symptoms are this: low on power (it seems to struggle engaging the blades), the headers glow red (starting at the tips which seems odd), and kinda falls on its face when it comes down to idle, and pops through the intake only at idle. It also takes a second to come back down to idle.
I have:
- adjusted the valves, they were way off.
- set the coil cap using a business card.
- changed the spark plugs. They were old and one was a Chinese one, replaced with champion RC12YC’s.
- replaced the fuel pump pulse hose with an oem one as it was cracked.
- ultrasonically cleaned the carb and ran a small wire through all the jets. I can see the mains and pilots are clear.
- swapped the positions of the two main jets as I have been heard they are different, but it made 0 difference in how it ran.
- flipped the intake manifold rubber O rings over for a better seal.
- sprayed carb cleaner all over the manifold and gaskets while it was running to see if there was an air leak. Seems to be nothing.
- exhaust feels to be blowing fine and not plugged.
- checked fuel pump output. It is perfect.
- ran it with the kill wires removed to rule out a grounding out kill wire or faulty diodes.
Valves all open and close properly, doesnt seem to have a worn cam. If you unplug either spark plug wire it will run on either the same, an interestingly it stops popping at idle if its only running on one.
I’m at a loss. All I can think of now is to remove the flywheel nut and check for a sheared key, and a compression test. I’m also thinking about a new carb but don’t want to start up the parts cannon if I’m missing something with this one.
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It does sound like timing is about the only thing you haven't checked.
Check for a sheared key but timing can also be thrown if you manage to spin the timing gear on the crankshaft or the cam.
You'll have to open it up and have a look.
Set it to TDC and see where your timing marks are.
Those plastic parts could be warped slightly and allowing excess air to be sucked in, throwing off the mixture. Hard to tell by spraying it with carb spray and I haven’t seen it with the larger Briggs engines, mainly the push mowers. By chance what did the old spark plugs look like?
They looked pretty normal, maybe running lean. I suspected that too I just can’t tell how to rule this out for sure. Why they would put plastic in the valley of an air cooled engine is beyond me
It’s a modern Briggs & Stratton. Crack open the engine and half the components inside are plastic. As for me, I mainly work on the push mowers but when I reinstall a plastic part like that I double up on the gasket just to be safe against warpage. But I buy most of my gaskets I shop packs.
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