It has spark and fuel, not starting

[BrandonH]

Leakdown test came out good, compression test is low at 80psi so I will do a new top end soon before riding, right now just trying to start it. The carb is cleaned and has new reeds. and it has a good strong blue spark.

The only way the bike will even attempt to start is by giving it a squirt of gas in the carb, holding the throttle wide open and kicking it, it spins over a revolution or 2 then backfires out of the exhaust. Sounds very weak and weird when it is spinning over. I pulled the flywheel off thinking it could be a spun flywheel but the keyway was there and in good condition.

What could this be? Also how do I press the flywheel back on?

 

[tbrooks]

Might just be the low compression, have you cleaned the carb and set the float level?

 

[BrandonH]

Carb is spotless new jets and float was at 20.5mm

 

[Awk08]

in my experience, backfiring is usually one of 2 things, woodruff key or broken reed.

torquing the flywheel nut to spec presses it back onto the taper, don't forget the big washer under the nut

 

[BrandonH]

in my experience, backfiring is usually one of 2 things, woodruff key or broken reed.

torquing the flywheel nut to spec presses it back onto the taper, don't forget the big washer under the nut

That's what I had thought but the woodruff key was fine, and the reeds are brand new Boyesen.

 

[Herpderplol]

reeds can crack no matter how new. new is unlikely, but possible

 

[03blasterboy]

Is your exhaust plugged maybe, we had that happen on a golf cart once.

 

[Blaaster]

Flywheel nut is tensioned up to 53ft lb.

Is there any fuel getting to the float bowl, crack the drain screw open and see if any fuel comes out of the drain hose.

 

[BrandonH]

I can feel it puff air out of the silencer so I don't think it's clogged. I had the pipe off it looks fine. I just looked at the reeds they look fine. Fuel is in the bowl nothing is clogged up. If I loosen the drain screw it comes out.

Fresh plug, fresh. 93 with caster927 at 32:1 with oil delete.

Should it still at least start with 80psi? It attempts when we squirt a little fuel in the carb wide open. Sounds very very quiet and only rotates a time or 2 then backfires out the back.

 

[Bluesman]

The general concensus is that anything below 100PSI will only start by push starting it.

When you had the exhaust off, did you look into the motor? Hint: If the exhaust side of the piston skirt is broke off, it will not draw air into the bottom end, through the carb, even though it can still show decent compression with a compression gauge.

 

[Blaaster]

It will struggle to run with 80psi, it is surprising that it will start at all!

 

[speedyslyder]

name of compression gauge?
last rebuild?
what year is your blaster?
what reed stoppers are you using stock or dual stage plate?
it is to my belief if hose is to long on comp gauge it will lie.The one i got i need to remove tank to get it on properly,also should have a rubber o ring where threads are?
only time i've ever heard a blaster backfire was when tors was still there and parking brake was on

 

[BrandonH]

name of compression gauge?
last rebuild?
what year is your blaster?
what reed stoppers are you using stock or dual stage plate?
it is to my belief if hose is to long on comp gauge it will lie.The one i got i need to remove tank to get it on

properly,also should have a rubber o ring where threads are?
only time i've ever heard a blaster backfire was when tors was still there and parking brake was on

It's a (don't laugh) Harbor Freight compression tester. The hose is a foot long and I did it with tank off. I'm going to verify the gauge accuracy with my compressor tomorrow. Also I am missing the oring BUT I put grease on the head to seal it and I'm pretty sure it was not leaking.

2001 yamaha blaster bought it last month with no spark for $300. Did oil injection delete and removed everything from harness but the kill switch and it has spark now.

Using stock reed stopper plate with Boyesen super stock reeds, brand new.

Parking break was off, TORS is deleted, but the block still on carb

 

[Awk08]

you need the o-ring !

 

[BrandonH]

I'll buy one tomorrow and check it then. I used the tester on my drag car and it rotted the seal over time because of oil on the seal.

 

[Bluesman]

I have heard of a Harbor freight compression gauge reading 60 PSI on everything from a B&S lawnmower to a big block Chevy. Harbor Freight is an OK place to buy some things, but not diagnostic equipment that requires any amount of accuracy. If you're not going to step up for a Snap On or Mac, at least go to Sears.

 

[speedyslyder]

a-men everyone that has used harbor freight compression gauges have failed,this is why i even asked that question
back to your main concern...did you replace stator and if so is it oem?
or better yet what electrical parts did you change and there name brand names?
truth shall set you free lol

 

[BrandonH]

Stator, CDI and coil are factory from what I can tell. Stator has been untouched as far as advance mod goes.

It has good spark

 

[speedyslyder]

? how are you testing for spark?,you got it to spark with parts that other owner had no spark,when purchased
silly question but for a reason,what size main jet and pilot jet you got?

 

[Blaaster]

I really think that you are chasing your tail.

You need to know what the compression is, at 80 psi it is going to struggle to start, even if it has a good spark like you say it has.

Sqiurting fuel in the carb will momentarily raise the compression and possibly allow it to start, but when the excess fuel has been burnt 80 psi will generally not sustain life.

Bear in mind if the rings are worn to only allow low compression, they may not create enough vacuum to draw enough air through the Venturi to pick up enough fuel to keep the engine running.