I was ridding today and she just stopped on me. After all the standard checks everything seemed good except for I wasn't getting any gas in the cylinder. Plenty of gas to the carb, bowl was full jet's were clear, I blew through them (Man gas tastes nasty). however no matter what I couldn't get any gas into the cylinder. I held a napkin over the plug hole and cranked it WOT no gas at all on the rag just a little black oil. I put a rag in the carb inlet, cranked it a few times at WOT and soaked the rag with gas. so now I know it isn't the carb.
I do a compression test and only get 25psi. Oh great I think I must have been running lean and burned a hole in my piston. So I pull the head off and everything including the piston, cylinder bore and cylinder head look brand new just like they did at rebuild time. So I am hoping like heck I blew the head gasket but I didn't have any coolant in my cylinder or gas in my coolant. So now my question is after I get my new head gasket tomorrow if my compression is still 25psi where the heck is all my compression going. There is no movement whatsoever side to side so I don't think I broke a ring. Also I don't have deep gashes in my cylinder that broken rings tend to leave. Any ideas at all guys. I need help with this one as I'm baffled. Even with a totally blown head gasket I'd expect more than 25 psi. I checked my reeds and they are perfect. I did the compression test with and without reeds with the same result 25 psi. btw this is a 400 scrambler so it doesn't have the automatic decompressor like the 4 strokes. Also one more thing I noticed a very small in the front upper half of the cylinder bore, I can't remember if it's supposed to be there or what it's for but it does look intentional and does not look like damage. I must say the engine is so clean inside you could eat off it.(if you like the taste the taste of gas and oil) no carbon build up at all since it is a relatively fresh rebuild. help help help

 

If you have no compression then you also have no vacum. It will not pull fuel from the carb.

Your rings are shot. I know they are supposed to be new but if your head gasket was not blown it has to be your rings.

 

when I put my had over the carb and crank it over it sucks pretty good, After I pulled the carb I could see it was pulling the reeds open but I have nothing to compare it too since I never put my hand over the carb to see how it sucked before it died. could very well be the rings but the piston is so tight. is there any way to check the rings without pulling the cylinder?

 

That little hole is supposed to be there. Its to make it easier to crank over.

I would take a look at the rings also.

 

DITTO onthe rings.Rings have been known to be bad from the start.

 

I'll be odd man out here and go with something else besides rings.

I think it will be crank case seal problems, cracked crank case? Could be my favorite, a plugged exhaust. No air in if it can't get the air out.
PS, I would have a look at the rings also, but if they look as good as you say, it won't be them. At 25 PSI it's going to be a hole in the piston, (you've already checked that) reed problems (same there) or seal problems.

PS, PS What happened to the great dog icon? That one was the best on the site!!!!

 

Yeah that was my dog daisy, I'll bring her back soon for you. I thought about the exhaust thing too since I recently replaced my stock silencer with an fmf power core. I stuck my hand back at the tail pipe while turning it over and was feeling flow. I'll check tomorrow by taking the silencer completely off. How can I check for crankcase leaks other than looking for holes are there any tell tale sings?

 

Doctor Turbo,
Thank you for the extra suggestions. I went out and pulled the fmf and got 37.5psi then I put the fmf back on and still got 37.5psi. I attribute the extra compression to the fact that I've had the battery charging for several hours and it was spinning much faster. Ok so now we've ruled out the exhaust. I didn't see any cracks in the case but that doesn't mean there isn't one. It happened when I was running full throttle on smooth dirt and it just died didn't hear anything break so I don't think I have a hole punched anywhere. Is there any way you can think of that I can test for case leakage when I have the cylinder off to check the rings? I'll tell ya what since you gave me so many good suggestions and Ideas I'll bring daisy back for ya.

 

Did the Piston ring gaps some how line up with each other?
I only say this cause I dont know if the pistons themselves have the little **** on opposite sides that keep them from aligning up with each other on the same side and also away from piston pin position.

 

Crank seals or the base gasket can cause a loss of vacum but not compression.

It HAS to be either rings or the head gasket.

Pull the cylinder and get it over with.