Weird "Pffffffft.......pffffftttt sound

[jspbtown]

HS50 motor. Took the head off and cleaned things up. Everything looked real good. When I tried to get it running I can hear a real load "Pffft.....pffffft sound coming from the exhaust. I can get it running but not well. Almost sounds like it would be a cam timing issue in an old car motor.

Thoughts before I take it out of the bike and tear it apart?

 

[ole4]

Sounds like a head gasket leak

 

[jspbtown]

New head gasket. Just took the head off. No leaks. Gonna really inspect the valves but they look great. Got it to idle but no power at all. Seems to run slightly better with the main jet screwed almost all the way in, but still not enough RPMs to even engage the clutch. If I turn it out it starts puffing black smoke. New plug.

 

[jspbtown]

Also took the air cleaner off and it seemed like it was puffing fuel out as it was running. Intake valve leak?

 

[desert rat]

Do you have a tork wrench? What did you clean? What did you take of to clean? You got to give up the whole story to get the right answer. We might beat you up just a little but we don't leave marks.

 

[ssparx]

HS50 motor. Took the head off and cleaned things up. Everything looked real good. When I tried to get it running I can hear a real load "Pffft.....pffffft sound coming from the exhaust. I can get it running but not well. Almost sounds like it would be a cam timing issue in an old car motor.

Thoughts before I take it out of the bike and tear it apart?

I don't know, but I've sometimes got a
Weird "Pffffffft.......pffffftttt sound
from my exhaust too,
and I don't even have a mini bike.

 

[jspbtown]

Ok...so more details it is....

HS50
New:
Plug (used a CJ8 for clearance reasons
Head gasket (torqued to 200 in-lb)
Carb, filter, fuel lines
Intake & exhaust gasket
Muffler from Black Widow Motorsports
Oil


Mods:

Had to shorten stock air cleaner to clear down pipe on chassis.


Got it to idle but when I give it gas it shudders and falls flat. Almost seems like it missing. When I take the air cleaner off and try to rev it I can feel pressure coming out of carb. The main idle jet is adjusted about one turn out. Any more and it puffs black smoke and dies. Governor is adjusted per all the videos online. If I try to go opposite with the governor adjustment (for ha has) it dies.

Took the head off. The valves were very clean. Inspected them carefully and I could see no carbon build up on the seats. They all seat perfectly flush.

Ran compression test and had just over 80psi. Hooked up my compressor to the compression tester hose and pumped it full of air and no hissing sounds or leaks.

Stumped

 

[ole4]

did you have cam out or sidecover off? if you put the compressor in to the engine and it held air with no leaks cam must be out of time. once you hook up air to the engine like when doing a leak down test the piston will move to BDC and one of the valves will be open depending if it is on compression stroke of intake cycle.

 

[jspbtown]

No....no internals were messed with. Of course I got this motor from someone who said "It ran good" so nothing is off the table as to what might be wrong.

 

[jspbtown]

Coil issue maybe?

 

[ole4]

When you connected the compressor pressure to the engine did the piston move to BDC? Did it hold air at BDC?

 

[jspbtown]

I am not 100% sure where the piston was in its stroke. I had the small access panel off and I watched the valves and made sure they were both closed. The piston did not move when I applied the air. Of course it could have been at BDC.

 

[ole4]

Then that is your problem at BDC one of the valves will always be open. Redo that test and verify that piston is at BDC and it is holding air. If so you will need to verify the cam dots are aligned.

 

[desert rat]

If you put 80-100 pounds of air in the spark plug hole that piston had to get pushed to BDC. I don't think you could hold it back with your hand. At that point I do think it should start passing air, but I got to think about this, time to turn off all smoke detectors. I'm not sure how many deg after or before BDC the valve start to open.

 

[I74]

No....no internals were messed with. Of course I got this motor from someone who said "It ran good" so nothing is off the table as to what might be wrong.

Does it look like the side cover was taken off, & signs of gasket sealer ect?

What's your gaps on the valves when piston is at TDC ??

It's been years since I messed with one of those,, but am thinking that I set them up with valve clearances of around .006

Those are ''great'' engines,, & the only valve train problems on those that I remember,, was from the exhaust gasket burning up & then smoking the exhaust valve.

 

[jspbtown]

I will look again at the side cover. Nothing was glaring when I detailed it and put it in the bike but I was not specifically looking. I am going to check the valve clearance. I think it is .008 for the intake and .012 for the exhaust

 

[ole4]

If you put 80-100 pounds of air in the spark plug hole that piston had to get pushed to BDC. I don't think you could hold it back with your hand. At that point I do think it should start passing air, but I got to think about this, time to turn off all smoke detectors. I'm not sure how many deg after or before BDC the valve start to open.

This should help. Remember this is one rotation of the cam, so engine rotates twice. Every BDC one valve or the other is open.

 

[desert rat]

Thanks I was to tired to think last night. Most of us work on the top of the stoke not thinking about the bottom much.

 

[jspbtown]

"Every BDC one valve or the other is open. "

If this is true then what good does filling the cylinder with air do? Won't it just leak past the open valve?

 

[desert rat]

It should from the diagram be on an open valve at BDC