Scag tiger cub running rough

[Chieftm2]

One of the things I've heard is that the pulse fuel pump on these if they've been replaced with an aftermarket, it doesn't provide enough fuel. It can idle okay, sometimes it requires a choke or high throttle to get started, but once you start trying to mow and get into the power, it just seems to bog. Might want to do a flow test on the pulse fuel pump...

I disconnected at the carb and cranked it and it’s pushing plenty of fuel. That pulse line is in rough shape though, not a great seal at the bottom? Thoughts?

 

[Chieftm2]

That pop is likely due to one of the mains being at least partially clogged. You can clean these externally much of the time in place. Remove the top plate w/ 3 screws, and carefully remove the gasket. Use a razor blade if needed. The gaskets aren't cheap and work well just by flipping and re using them. Once the mating area is clean and you're sure there are no little bits of gasket inside, a burst or two of carb spray with the straw down each main orifice will open them up. Quickly reassemble and test run. This cuts the cleaner with fuel so you don't have the cleaner soften the main rings or fuel pipe seals.

I'll also drain the bowl before this service and after running via the side bowl solenoid. If you decide the little red straw is needed to flush the bowl out, reinstall the solenoid and run it right away. Once a test run shows it's good, back on the shroud goes.

Base idle is 1300, only notes on top speed on your build is 3600 is the alternator test speed. Increasing your running speed 10% makes a nice difference.

Ok sprayed the carb out and the solenoid. No real obvious change. Any thoughts?

 

[WinterDeere]

I disconnected at the carb and cranked it and it’s pushing plenty of fuel. That pulse line is in rough shape though, not a great seal at the bottom? Thoughts?

They almost never are, but they don't really need to be perfect, either. It's using the percussive pressure created in the crankcase by pistons going up and down to drive a diaphram pump, the bleed-down rate of that hose mate is not a big problem, unless hose is completely torn at the end.

I hadn't commented on the rough sound in the video, both because I wasn't able to listen to it at work, but also because I know there are others here who know way more about diagnosing an engine that's running rough.

 

[Chieftm2]

They almost never are, but they don't really need to be perfect, either. It's using the percussive pressure created in the crankcase by pistons going up and down to drive a diaphram pump, the bleed-down rate of that hose mate is not a big problem, unless hose is completely torn at the end.

I hadn't commented on the rough sound in the video, both because I wasn't able to listen to it at work, but also because I know there are others here who know way more about diagnosing an engine that's running rough.

Really appreciate the help. The hose is actually jammed past both the little flanges because the flanges are in pretty rough shape. But like you said it doesn’t need to be air tight. It does appear to be pumping plenty of fuel too. Also seems to run a lot smoother at idea and medium throttle compared to wide open.

 

[WinterDeere]

I know a lot more about engine building than engine debug, as I've built quite a few, but don't get a whole heck of a lot of opportunity to chase down problems in misbehaving examples. Like you'd I'd check fuel delivery, and I'd check spark strength with a proper spark gap tester. I'd check compression and bleed down, maybe pop a valve cover and check valve clearance, and follow the manual procedure to check carb tuning, along with checking for vacuum and/or exhaust leaks. I also usually go to the technical manual for the mower, and follow thru the troubleshooting matrix, make sure I hit all of the "things to check" in that.

But exhausting that basic stuff... I'd probably be here, asking the same questions.

 

[tomplum]

It's hard to tell from the video. When you are there, you get pops from the airbox. Maybe listen again for that. A similar sound is made when you have a soft cam lobe, valve rockers or lash that is out of whack.

If you can get familiar with the proper valve procedure on these and know what to look for would be best. I can type it out for you, but it would be nice to have a good video to mentor you.

Basically, remove the spark plugs and valve covers. Check all 4 rockers to confirm they are secure in the head. Turn the engine via the fan in the normal running direction and observe how much each valve is opened. They should be relatively close. If one valve moves much less than the others, you have a soft cam lobe. Verify that the rocker is good on that cylinder to be sure. When the cylinders trip over one another to breath- you'll get popping and feeling that things just aren't right.

To set the valve lash, each cylinder is set when the piston travels 1/4" past top dead center when after the intake valve closes. .004" intake and .006" exhaust. Turn a full round , then determine the same position for the other cylinder. Rinse and repeat.

Clean the mating surfaces. You can use RTV to reseal the covers. It will bond to the old secure silicone which remains. Or use RightStuff. I use RS as you don't really have to wait. But I'll actually let it sit after the test run.

 

[RayIN]

Todays gasoline can go bad in as little as 30 days.
You'll be amazed at what a can of SeaFoam mixed into 5G gas can do to improve rough running.

 

[Fxfymn413]

First off go back to basics. Make sure you have new, clean fuel. Either drain the tank or use a temporary tank. Ensure the air filter is clean. If it has a fuel filter make sure that is new. Is the oil at the correct level and not contaminated by fuel. Do a compression test to make sure the valve train is working properly. What do the spark plugs look like? If one is clean and the other is not than the problem is in the dirty cylinder.

You said the engine is new, but the hoses are worn. How can that be?

Most problems that are hard to trouble shoot turn out to be something simple like a plugged fuel cap vent that gets overlooked because all of us convince ourselves it has to be something really bad.

 

[POGuy]

Thanks a bunch for the info. Did you get a chance to listen to the video? Does it sound rough to you? Just wondering if that’s contributing to what seems to be a power issue. I could be wrong but I feel like this thing should have tons of power for that little 48 inch deck? I don’t know if I mentioned it but there was a new carb put on last year. This is just a back up mower for my little farm IE why I don’t want to just pay someone else to figure it out. I have never messed with carburetors before so I’m slightly shy about taking it apart but I’m willing to try. Also the rpm’s were right at 3000. I turned it up to 3400. Not sure if the 400 is going to really make a noticeable difference. Thanks again

Have you checked that both coils are working?? Pull one plug at a time, if pulling one kills the engine, the OTHER one is not working. Just went through this with a Kawasaki feeling it surely was fuel related, not so!!