Blaster Compression

[Taz100420]

Ok So I rebuilt my engine and it seems like its not getting compression. I hooked a compression tester up and it barely moves the needle. Im damn sure I installed the rings right and the piston also. It will run but very poorly IF I hold the throttle open just a hair.

I look around the head gasket and I dont see any seepage but would a slightly warped head make it loose THAT much compression w/o any seepage? Also what is the PSI it should be at when TDC? I have like 3 heads so I guess it wouldnt hurt to try. I work on four stroke vehicle engines so I mostly know my way around a motor lol. But this is stumping/agitating me a lil.

P.S.: No leak down tester atm. Imma try and work with a guy locally here and get his goin

 

[Blaaster]

If it is starting and you get no reading on the compression gauge, you must have fitted the tester wrong or done the test wrong.

A motor will not start with no compression.

A leak down test must be done on a fresh rebuild to check for the very common air leak problem.

 

[Taz100420]

Let me re-phrase that lol, its getting lil compression when I kick it over. But I can turn the flywheel by hand easily with lil resistance when its compressing. I shouldnt be able to turn it THAT easily should I? Just wondering b/c a 2-stroke is a whole new ballgame for me lol

Imma try and take a pic of what it reads when i kick it later today

 

[blasterboy32]

I think for a compression test your suppose to hold the throttle wide open and kick it over until the needle on the Guatemala stops going up. Someone correct me if I'm wrong. I also believe compression should be atleast 125 on a fresh rebuild.

 

[Taz100420]

So I must be doing it wrong then lol. It seems to be ALOT different than a 4-stroke:-[ I do compression tests on vehicles by bumping the ignition until its at its highest PSI and let it stay there to see if there is any leaks BUT that must be basically the point of a Leak down test, correct?

Imma 2-stroke newb! But only way I learn is hands on. And I wanna learn

 

[EZRider76]

Well, blasterboy32, you're wrong...you don't keep kicking until the "Guatemala" stops going...you keep kicking until the needle quits moving. 8-| Seriously, if you sent that from your phone, what have you been texting about???

You are correct on the at least 125psi though. I:I

 

[Blaaster]

Did you bathe the piston in 2 stroke oil after you fitted the rings.

A fresh rebuild will have a lower compression than a broken in motor, due to the rings not sealing to the bore.

What compression reading do you get in PSI, and not by turning by hand, but by kicking over 10 times with WOT.

If above 120 psi, proceed with a leak test, if compression is lower, you did summit wrong..

Do a leak down test before trying to start it.

 

[EZRider76]

So I must be doing it wrong then lol. It seems to be ALOT different than a 4-stroke:-[ I do compression tests on vehicles by bumping the ignition until its at its highest PSI and let it stay there to see if there is any leaks BUT that must be basically the point of a Leak down test, correct?

Imma 2-stroke newb! But only way I learn is hands on. And I wanna learn

Yes Taz, kick away until the needle quits moving. I like to do a cold and warm test and make sure they aren't too far apart. Cold because that's when you start the bike the most, and warm because you want to know how the compression is when it's running. Maybe I'm doing too much, but better more than less.

 

[Taz100420]

Oh yes the piston was well bathed lol. But I did kick it over like 10 times and I got it to 125 psi so I think thats good. So I guess I need to move onto the carb and maybe something is wrong in there.

But I will not kick the engine over any more till a leak down test has been done. TY for your help and educating me some on this subject fellas

 

[blasterboy32]

Well, blasterboy32, you're wrong...you don't keep kicking until the "Guatemala" stops going...you keep kicking until the needle quits moving. 8-| Seriously, if you sent that from your phone, what have you been texting about???

You are correct on the at least 125psi though. I:I

Ya it was from my phone i meant to say guage but needle is a better word. auto correct has done that to me twice on here now ill have to set guage as the defalt word.

Taz i hope you get it figured out just keep eliminating possibilities and youll find the problem

 

[EZRider76]

Ya it was from my phone i meant to say guage but needle is a better word. auto correct has done that to me twice on here now ill have to set guage as the defalt word.

Taz i hope you get it figured out just keep eliminating possibilities and youll find the problem

I know, I've had my **cough cough** smart phone since Christmas and I hate the auto correct...if I turn it off though, it gets worse because I have fat fingers and a hard time hitting the right letters.

Anyhow, Taz, do that leak down test. Did you happen to check out the plug when you had it out for the compression test? Did it look flooded, fouled or fine?

 

[Blaaster]

A leak down test is needed.

 

[Taz100420]

When I ran it before all my compression testing, I took out the plug and it was wet as hell. So I assumed it was flooded out. It got a lil better as it ran than just died out.

 

[Budget_Blaster]

Yes Taz, kick away until the needle quits moving. I like to do a cold and warm test and make sure they aren't too far apart. Cold because that's when you start the bike the most, and warm because you want to know how the compression is when it's running. Maybe I'm doing too much, but better more than less.

Thats called being a good tech and youre not a lone.

This is especially good to do on forged pistons that swell in the bore and also require looser tolerances than cast or hyper piston.I:I

You can never be too careful in my opinion when it comes to an engine especially if like most here you ride,build,maintain,and fund your own projects.

So i say good for you bro I:I

to the OP 120-140 psi anything less and id pull it apart to know why.

 

[Racer51]

Just outta curiosity 125 PSI is a stock bore compression reading after a rebuild? Cause that's what my superbee blaster is at.

 

[Taz100420]

Naw its only bored .020 over and I dont have the best gauge and it may be somewhere along the lines of 122-123 psi. Kinda hard to read it lol. The numbers are kinda close together.

 

[Blaaster]

125 PSI on a cold fresh, re build should be OK.

Compression can vary due to the amount of oil used on the rings during assembly, and the quality of the honing.

When the rings seat, after the heat cycles and break in ride, you can expect the compression to rise.

 

[Racer51]

cool thanks on that...so stock bore on that then (most likely) yea