backfire when try to start

[chip9399]

ok just put top end back together, it wont start, it jumps when trying but doesnt start, then a flame out exhuast backfire!
i rechecked timing was spot on, new plug also, good compression, throw some ideas out there guys...im aq lil stumped right now thx

[250rforlife]

what size is the pilot jet. whats the air fuel ratio. u sure timing is right? cam lobes down...

[tri5ron]

I'm not trying to be condesending here,...

but did you have a manual available when you re-assembled your top end?

I'm getting a sneaky feeling your cam and/or timing chain is not installed correctly.

I may be wrong here, (and I do hope I AM wrong), but I just felt it should be asked.

[jcs003]

sound to me like your timing is off.

[chip9399]

yea timing is right, guess i should check to see if flywheel spun?, i might try another carb just in case this one is messed up, what about spark? it is trying to fire just out exhaust? would pilot jet being cloged cause a backfire?

[tri5ron]

Just to make sure we are "on the same page " here, when you say "Backfire" that would indicate it is firing back through the carb.

A "Afterfire" is when it explodes through the exhaust.
I am understanding your description, as having a Afterfire issue ,...correct ?

Afterfires can sometimes be caused by a lean condition, allowing the headpipe to become hot enough to ignight unburned fuels in the exhaust system.
BUT,... the bike would FIRST need to be running, to get the headpipe that hot.

You had previously said that you had just got it back together, and I'm still wondering if by chance you might have put the cam in 180 out ?
Aren't you having this Afterfire condition on a COLD engine ?

If the cam was 180 out,...
wouldn't the exhaust valve be open when the ignition fired ? ,...hence providing a source of ignition, and resulting in a Afterfire condition ?

Or am I bass-ackwards in my thinking here ?

[Chuck75]

^ is correct, your cam is 180 out. unfortunatly its very easy to do.

[NacsMXer]

Originally posted by Chuck75
^ is correct, your cam is 180 out. unfortunatly its very easy to do.

It's not possible to install the cam 180 out and have it cause a timing problem since these motors have a redundant spark.

For example, installing the cam lobes down (makes reassembly easier), sets the motor to TDC on the compression stroke when the T mark on the flywheel is lined up with the notch on the cover.

Installing the cam lobes up (180 out), would simply set the motor to TDC on the exhaust stroke.

The only way you could mess up the timing is if you install the timing chain on the cam gear incorrectly, either advancing or retarding the timing. If you had the cam sprocket flange pressed off the cam and pressed back on incorrectly, this can also alter the timing (isn't supposed to be possible but happened to me...bad machine shop). You could also have sheared the flywheel key.

[Chuck75]

[QUOTE]Originally posted by NacsMXer
[B]It's not possible to install the cam 180 out and have it cause a timing problem since these motors have a redundant spark.

I stand corrected. sorry bout that cant say what I was thinking about at the time I posted that.

[NacsMXer]

Originally posted by Chuck75
[QUOTE]Originally posted by NacsMXer
[B]It's not possible to install the cam 180 out and have it cause a timing problem since these motors have a redundant spark.

I stand corrected. sorry bout that cant say what I was thinking about at the time I posted that.

All good man, just trying to prevent misinformation :)

[tri5ron]

thanks Nacs,
The last thing I want to do is create mis-info.

I was over thinking his problem I guess.

I have not changed the cam in mine yet, but I have been considering doing so, and therefore it has been on my mind.

Obviously, I had not done the proper homework yet, so I spoke without the hands-on experience.
My bad.

[NacsMXer]

Originally posted by tri5ron
thanks Nacs,
The last thing I want to do is create mis-info.

I was over thinking his problem I guess.

I have not changed the cam in mine yet, but I have been considering doing so, and therefore it has been on my mind.

Obviously, I had not done the proper homework yet, so I spoke without the hands-on experience.
My bad.

No worries :p

I used to be pretty mystified about setting the timing the first time I put a cam in. But it's actually very simple to get it right. As long as the motor is at TDC, the cam is installed lobes down, and the cam sprocket installed in the chain with the 2 horizontal lines flush with the head/single vertical line pointing straight up, the timing will be perfect.

Sorry for the hijack :ermm: