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I'm at a loss for what could be the problem. I refurbish or fix riders and yard equip. & tractors for a hobby. Briggs, Kohler, Tecumseh, Onan, etc. Singles, twins, V6/V8 autos yada yada.

I've got a Husqvarna YTH2148 with Briggs 21hp twin that won't stop surging at high throttle. I've tried almost everything.

It runs pretty much ok until 2800 rpms when it'll start surging with the govenor bouncing back and forth from low to high rpm with a cycle of about second or two aka surging. There might be a cough out the carb at idle or high speed once in a while as well. Adding choke reduces or eliminates the surging for a moment but usually winds up flooding the engine to low rpms then choke to off and black smoke till rpms up and starts surging again. A classic dirty or leaky carb or bad fuel scenario. I'm not unskilled at problem solving and have tried eliminating everything but the problem persists.

Here's what I have checked:

Gas seems clean. It's fresh. No contaminates seen. New fuel filter with visually 85% full of fuel. New fuel shutoff. No air filter in.place. Changed fuel pump. Appears adequate flow at carb intake. Tank cap removed no change. Vent seems ok. Fuel level seems adequate. Maybe pinhole in old fuel line but no significant evidence so far.

New oil and filter. Proper level and clean. Both ignition Coils swapped for other used ones. Spark plugs are new. Left cyl. might be fouling rich a bit more. Flywheel key ok. Charging ok. Battery ok.


Carb cleaned 3 times. No change. Did not replace seals etc as looks acceptable. Swapped with carb and intake manifold that had new rebuild no change. Throttle and choke linkage ok. Govenor- tried many adjustments to idle and wot springs and tabs. No positive effect. Internal govenor parts seem to operate to spec. Shaft turns about 10 or 20 degrees as normal. I don't suspect issues here. Idle ok about 1800 rpm or.lower. WOT set at between 3000 and 2600 rpm. Surging exacerbates with higher rpm.

Rider travels about the yard ok with occasional loss of power or cough whereas a bit of choke recovers after a rich moment. Usually doesn't have significant surge but can still tell not running smooth. Rpms vary up and down a couple hundred under slight.load. chassis doesn't seem to drag in any way.

Cyl. Compression of 125 left and 145 right psi. Seems a bit high for 400 hrs used machine but not excessively.

Valve lash set and reset again with intake about 5 thou and exhaust 8 ish. They're on the tight side. I'll try making them sloppy and recheck for the ships and giggles. Springs and guides seem ok. I'm thinking of pulling heads grinding valves etc on idea maybe a valve is sticky or fouled and cannot be seen on inspection yet.

Intake manifold not leaking. When running wot or idle no change when ether or brake cleaner or carb cleaner sprayed about in copious amounts.

Engine runs otherwise ok. Maybe slightly hot but there's no shroud so explainable due to poor airflow. I suspected over sensitive govenor but don't really think so. I'm pretty good at adjusting but haven't replaced Springs. Looks identical to surplus govenor assembly on hand.

What am I missing?
Any ideas would be appreciated.

I'm really at a loss. Feel like a noobie. I've got a new carb to try then I'll pull the heads and work them over otherwise swap the engine as last resort. After that I'll split the case and inspect govenor fly etc but don't think there's anything inside as a culprit.. yet. Cam seems to open valves ok too.

Last- surging is guaranteed if throttle opened quickly to 2800+ rpm. If opened slow surge may not occur as drastically at least until approaching higher than 2300 rpm. Applying some choke helps momentarily but will afterwards flood or continue surge. Even when not surging dramatically there's a perceived up and down rpm change but very subtle with occasional pop out the carb.

Physically holding throttle linkage at idle or speed smooths out rpms so again points to govenor but will still cough occasionally.

Sorry for the long dissertation but I'm looking for a clue before the dynamite option...

Thanks.
Dave

eaton9999 said:

Engine runs otherwise ok. Maybe slightly hot but there's no shroud so explainable due to poor airflow.

Click to expand...

Are you running it with no engine shroud or top engine cover? As in you can see the flywheel and flywheel fan blades? Big no-no if this is true.

Pull the fuel tank. Needs to be clean like a new one. Grass, bugs, dirt, leaves and such can plug up the tank outlet.

Install all new fuel lines from tank to carb. Filter and shutoff valve can be suspect as well. Take a glass jar. Pull fuel line off at the carb inlet. Make sure you have good solid flow into the carb.

After that possible main jet is plugged, choke stuck ON or leaky valves. Low carb float level. Dirt in the carb needle/seat. To me it sounds like lack of fuel delivery. Carb linkages out of tune.

Are the plugs black? Take a picture or a video of them. Also a video of it running and you trying to rev her to the moon would help us. Post it up on youtube.

Yes the shroud is off as needed to access carb etc. It'll be on once running ok.

Tried all your suggestions including swap of all parts and aux. Fuel tank to carb. Top rpm less than 3000 per spec.

Fuel flow type issue I agree. Maybe valves?

Thx.
Dave