272xp frustrations

[Trent Curtis]

272xp- new piston and rings (meteor). Base gasket delete and new crank seals. Rebuilt carb, new fuel line and fuel filter. New tank vent as well.

Afterwards the saw would high idle after running briefly at WOT. Swapped to a brand new carb and it’s doing the same thing.

Vac and pressure test held like a rock-

Saw idles great- but after running WOT the saw doesn’t come back to idle...... idles up and stays that way until fatten the low side. Settles down immediately- but the same symptom appears again immediately after another shot at WOT.

Sure seems like an air leak- but doesn’t make sense with the vac/psi test holding like they did. Impulse hole seems clear all the way to the crank.

Ideas?

 

[Huskybill]

Did you install new crankbearings?

 

[Trent Curtis]

Did you install new crankbearings?

No bearings looked fine. Seals did too for that matter . Just changed them while they were apart.

 

[Trent Curtis]

Tests were performed with piston at BDC

 

[Cease232]

You gotta turn the crank while under vacuum. Sometimes you won’t see a leak when it’s not moving.


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[Huskybill]

Exactly the static pressure leak test might not show up when the bearing is still.

I had this happen on 2t dirtbike engines the bearings felt tight till I split the case. Ever since then I do the top and bottom end together. But the question is first why did it fail in the first place. I’ve seen used bikes run into the ground with the crank bearings so loose the flywheel ripped the stator off the case. Actually the kid mixed the two stroke gas by eye.(color) the gas was still clear, no oil. Just an example what can happen.

 

[Cantdog]

I would suggest the o-ring that seals the oil pump to the case needs to be replaced if you didn't do it with the seals......but that, too, would show up in a press/vac test....??

Next I would ask if your throttle plate is not centered properly....but you changed carbs...???

Trigger or linkage hanging up somehow??

Certainly does sound like an air leak but everything I can think of would also not allow a stable pressure/vac test.......

I'll be interested in the outcome....

 

[Grande Dog]

Howdy,
I was thinking along the same path as Cantdog. I've seen throttle plates get worn enough from the side thrust of the throttle return spring to hang the plate open and not return to idle. You said a new carb replacement. Was it new, or just a different one? Also, when the saw is at normal idle on the ground being held down by the front handle, can you pull up on the rear handle and change the rpm? Soft mounts, and or linkage issues can do this.
Regards
Gregg

 

[Trent Curtis]

I would suggest the o-ring that seals the oil pump to the case needs to be replaced if you didn't do it with the seals......but that, too, would show up in a press/vac test....??

Next I would ask if your throttle plate is not centered properly....but you changed carbs...???

Trigger or linkage hanging up somehow??

Certainly does sound like an air leak but everything I can think of would also not allow a stable pressure/vac test.......

I'll be interested in the outcome....

All good thoughts.... throttle linkage is working properly..... I didn’t check the throttle plate. I’ll have to check that out.

I forgot to mention....... at WOT the saw starts to four stroke and then goes lean...... starts to scream and I get out of it immediately. Then the fast idle until i fatten the low jet to calm her down.

She’s pulling air from somewhere. I’ll roll the crank through another set of tests......

I’ll Let you know what I figure out........

 

[RedFir Down]

Is the throttle shaft good and snug in the carb body?

 

[Trent Curtis]

Howdy,
I was thinking along the same path as Cantdog. I've seen throttle plates get worn enough from the side thrust of the throttle return spring to hang the plate open and not return to idle. You said a new carb replacement. Was it new, or just a different one? Also, when the saw is at normal idle on the ground being held down by the front handle, can you pull up on the rear handle and change the rpm? Soft mounts, and or linkage issues can do this.
Regards
Gregg

Greg- I haven’t even gotten to the cut..... as it does it no load WOT.
Carb was a brand new peice of Tiwan scrap metal. Junk carb but brand new none the less..... with the same symptoms.

 

[bigbadbob]

Cracked carb block?

 

[Trent Curtis]

View attachment 662209
Cracked carb block?

I did psi test behind this block- but I suppose it’s possible that that the weather stripping i use to seal between this and the carb could seal off the crack.

Good idea- I have extras I’ll swap this out for the next round of tests

 

[Cantdog]

All good thoughts.... throttle linkage is working properly..... I didn’t check the throttle plate. I’ll have to check that out.

I forgot to mention....... at WOT the saw starts to four stroke and then goes lean...... starts to scream and I get out of it immediately. Then the fast idle until i fatten the low jet to calm her down.

She’s pulling air from somewhere. I’ll roll the crank through another set of tests......

I’ll Let you know what I figure out........

Yep roll her back and forth too and watch the gage.....

Did you use OEM seals? Or aftermarket?

I got some aftermarket seals from a well known reputable parts dealer but they really sucked......the way the outer lip was formed it looked like the two ends were joined or supposed to be joined but they didn't quite......may have worked.....but they got the circular file and on current saws I always go OEM.

If you find your throttle plate does not center, loosen the screw slightly, just enough to be able to move the plate, back the idle adjustment off until it no longer touches the throttle arm and close the throttle completely. This should center the plate in the bore.....tighten the screw and check if the gap as you crack the throttle is constant around the bottom edge.

 

[Cantdog]

View attachment 662209
Cracked carb block?

I though of that too....they have been known to crack.....but....again I would think that would show up in the pressure/vac test. It's hard to come up with an air leak that is not showing on the test.??? As spike60 would say...."A real puzzler"......

 

[bigbadbob]

Double check yer impulse, had a 361 have the same symptoms, and the impulse had come off.

 

[Cantdog]

Double check yer impulse, had a 361 have the same symptoms, and the impulse had come off.

The impulse is internal on these.....runs through the isolation block directly into the carb body.

 

[Cantdog]

I am most inclined to think it has to do with a gap in sealer on the base gasket delete.....but again if that was so it would not test good.....????

 

[Trent Curtis]

Double check yer impulse, had a 361 have the same symptoms, and the impulse had come off.

I just saw this.... got in from the shop.

You were on the right track! I tore it all apart..... shining a light in the cylinder impulse hole I found the problem. A piece of gasket was lodged in there...... really restricting flow.

Pulled it and tuned with the original carb..... ran like a champ!

Thank you for all the input guys.

No longer frustrated

 

[Trent Curtis]