Backfire through carb?

Bean

New Member
1978 CT90 round bowl carb

Hey guys,

Just got my bike back from the shop (conveniently right as I have time to work on it since they dragged their feet doing it since September).

Carb was fully redone, it's the original from 1978. New plate and points. Timing set, allegedly, haven't checked if they did. New spark plug boot and plug. Still drains batteries overnight, will chase that issue later.

All my batteries have been eaten by this thing. Lucky I work for a dealership, they cost me less than $10. I got a battery up to the level of "enough" to at least ignite fuel a bit. I know these things need a fully charged battery to actually run, but I'm just working out issues until my battery gets here on Monday.

Here's the scoop though, when I try to start it obviously it won't go because of the battery situation, but if I have the throttle open while kicking it will backfire through the carb.

I know you're not supposed to kick with the throttle open, but it's just something I do as a last ditch effort if it won't work closed.

I've read this could be indicative of an air leak, timing gear being off a tooth, or timing installed backward. But the fact that it only happens with the throttle open makes me suspicious if any of that is the case or that the backfiring may be normal. Before I go undoing what I paid someone to do (timing), I'd like the expert opinion of you folks.

This is my first post and first project bike, but I lurk here a lot, I promise I'll be back to let you know if your advice works.
 

Bean

New Member
Yeah I got the bike but did not have time to work on it at all. Now that winter is basically here I have an excess amount of time and have a whole list of things and the parts to do it.

If the new battery does not fix it I will be pulling the covers and working on them. Mostly my long winded rant is a question of what causes backfiring through the carbs, and if simply opening the throttle while kicking can be the cause.
 

Midnightpromises

Active Member
So you picked it up from the folks that tuned it up without starting it upon picking it up????? Yes, timing 180 out will cause a backfire. Throttle position has zero effect on timing/ignition...
 

Bean

New Member
I'm back. Its running perfect.

There was a leak between the carb spacer and manifold causing the backfire. I set the points myself as well as the timing. I did it probably 10 times over the past two days, so I can do it with a blindfolded with my feet probably. Once I identified that the carb spacer was the issue, a quick Amazon order and sanding the intake flat fixed it right up, just in time for next spring.
 
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