272xp got me not stumped.

[Okie294life]

Bought it with a scored top end, work done so far:
1. New crank seals
2. New piston/cylinder non oem
3. Rebuilt carb

- When i start it up it won’t idle at all, and only will start after choked. If will run at full throttle, but is screaming like a demon and not smoking at all. I cranked around on the hs jet some to see if it would calm down after cranking the hs jet open, didn’t really seem to make much impact. This is with both jets set one turn out. I tried to pressure test it but had cork in the intake as a spacer, and it was leaking. I got up to about 5psi and no leaks from the crank seals or the head so I feel pretty good about it at least beginning this process without any air leaks. Where to start? I’m bumfuggled.

 

[cscltd]

Fuel line, fuel filter?
Should pressure test before ripping it apart
Fixing it before knowing cause can result in same thing happening again or lots of unknowns.

 

[Gord404]

well if everthing else checks out ( vac test is very important and that wasn't done)- sounds like you have an air leak in carb block or carb gasket between the two or your carb has got other problems that just a rebuild won't solve it- another thing is the gaskets you use on either end of your carb block should be oem or at least not the chineese black ones, need I say they are not up to sealing properly,so do a by the book vac and pressure test- you can put a piece of innertube with a couple of bolt hles in it and put it between your carb and manifold- block off the exhaust the same way and pump through your spark plug hole

 

[singinwoodwackr]

Bought it with a scored top end, work done so far:
1. New crank seals
2. New piston/cylinder non oem
3. Rebuilt carb

- When i start it up it won’t idle at all, and only will start after choked. If will run at full throttle, but is screaming like a demon and not smoking at all. I cranked around on the hs jet some to see if it would calm down after cranking the hs jet open, didn’t really seem to make much impact. This is with both jets set one turn out. I tried to pressure test it but had cork in the intake as a spacer, and it was leaking. I got up to about 5psi and no leaks from the crank seals or the head so I feel pretty good about it at least beginning this process without any air leaks. Where to start? I’m bumfuggled.

Try 7-10psi. Sure sounds like an air leak.
You need better test plates to find air leaks than a cork…
Did you test the carb?

 

[Cantdog]

You didn't list the o-ring that seals the oil pump to the case. That would be my first guess.....as said ....a pressure and vac test will show the issue or at least rule out an air leak.

 

[singinwoodwackr]

You didn't list the o-ring that seals the oil pump to the case. That would be my first guess.....as said ....a pressure and vac test will show the issue or at least rule out an air leak.

And, you can’t do the test with the clutch removed.

Oops, was thinking 372

 

[Gord404]

And, you can’t do the test with the clutch removed.

272 it doesn't matteras long as the oil pump is on which contains the seal and big o ring- 372 it's a must- doing a 61 right now myself- most are the same setup as the 272

 

[J D]

Check impulse isn't leaking or partially blocked (saw & carb)... that would cause lack of fuel leading to the symptoms you describe.
A defective fuel pump diaphragm, fuel line, or fuel filter would cause similar symptoms too

 

[Okie294life]

You didn't list the o-ring that seals the oil pump to the case. That would be my first guess.....as said ....a pressure and vac test will show the issue or at least rule out an air leak.

Yeah I did that.

 

[Okie294life]

Try 7-10psi. Sure sounds like an air leak.
You need better test plates to find air leaks than a cork…
Did you test the carb?

I used sheet cork, but surprise it also sucks for this application, too coarse. I’ve got an exhaust plate just no intake plate. I’ve got a plug jack for the pressure tester just need to find a way to reliably block off the intake, guess I need to find some rubber somewhere to jam in front of the carb. I wonder if felpro coated in some hylomar or motoseal would work?

 

[pioneerguy600]

Check to make sure the throttle is actually closing ,linkage is free and not rubbing holding the butterfly partially open.

 

[JimR]

That sure does sound like a air leak. A Husqvarna 350 that I just did over had the upper crankshaft half not sealed. That saw idled around 4000 rpms. When I rebuilt it I found the leak. I thought the crank seals were gone. The previous owner took this apart and never sealed the case. Tire tube rubber makes a good gasket for the exhaust and intake. I hope you can find the leak easily. Have you sprayed carb or brake cleaner at the motor seals or intake manifold when running?

 

[pioneerguy600]

The hardware stores or plumbing outlets sell assortments of gasket materials, most come packaged around 6 X 6" ,in the assortment there is a sheet rubber piece, it will make many gaskets for blocking ports. Usually easier to acquire than tire tubes at least for me, both sides of the swatch are smooth with no ribbing. seals easily.

 

[Okie294life]

Fuel line, fuel filter?
Should pressure test before ripping it apart
Fixing it before knowing cause can result in same thing happening again or lots of unknowns.

I pulled it apart and surprise the fuel line was shooting fuel out like a water hose when I took the line off. I put another fuel vent on but still it won’t hold an idle at low 1.25 turns and high .75 turns. The carb doesn’t pass a leak down and when it’s running spraying crap tons of fuel. It has been kitted and everything back together on the carb is fine as far as I know. What would cause one not to pass a leak down test?

 

[user 201000]

Bought it with a scored top end, work done so far:
1. New crank seals
2. New piston/cylinder non oem
3. Rebuilt carb

- When i start it up it won’t idle at all, and only will start after choked. If will run at full throttle, but is screaming like a demon and not smoking at all. I cranked around on the hs jet some to see if it would calm down after cranking the hs jet open, didn’t really seem to make much impact. This is with both jets set one turn out. I tried to pressure test it but had cork in the intake as a spacer, and it was leaking. I got up to about 5psi and no leaks from the crank seals or the head so I feel pretty good about it at least beginning this process without any air leaks. Where to start? I’m bumfuggled.

I would start by adjusting the idle then the low jet then the hi. There's some pretty good videos on the YouTube. Hope this helps

 

[Okie294life]

Well it won’t stay running I’m going to try to tweak on the carb a little bit, but I had the filter off and when I did get it to run yesterday it was dumping and splattering fuel out the intake like crazy, and the carb won’t pass a leak down test so I’m assuming the needle valves pooched somehow.

 

[J D]

Likely it's the metering valve leaking. Take the cover off the metering side & remove the diaphragm then test it again... it should hold ~10psi on the fuel inlet & release pressure when you press the lever on top of the valve. If it is assembled correctly & doesn't seal inspect the valve seat & the needle, clean the valve seat with a q-tip or similar. If it does seal check your metering height & compare your metering diaphragm to the original one paying particular attention to the size of the little nubin in the middle (if its bigger it effectively alters the metering height). Some diaphragms also have a T shaped nubin that needs to slot into the metering arm

 

[Okie294life]

Thanks for the input when I get a chance I will. Just to save some time I ordered one of those cheap knockoff carbs…..surprise it starts and runs like a champ. I’d almost guarantee though that if the tillotson is setup correctly it will have more power as the carb listed covers everything from a 61 to a 272xp, so it’s probably a knockoff of the smaller 254B carb. We have an ultrasonic cleaner at work try to break it down and give it a bath at some point to see if that’s works.

 

[Old2stroke]

Thanks for the input when I get a chance I will. Just to save some time I ordered one of those cheap knockoff carbs…..surprise it starts and runs like a champ. I’d almost guarantee though that if the tillotson is setup correctly it will have more power as the carb listed covers everything from a 61 to a 272xp, so it’s probably a knockoff of the smaller 254B carb. We have an ultrasonic cleaner at work try to break it down and give it a bath at some point to see if that’s works.

Gasket in the right order? Between the diaphragm and carb body. On the wet side of the diaphragm.