Popping from carb, gpz900

I just picked up an ’85 gpz900.
Has been molested to some degree by a previous owner.
I’ve had the carbs apart and gave them a minor go through. They looked like they have been rejetted, with 138’s for mains.
So, I’ve gotten to run pretty good. But no. 1 carb is back firing. It appears to be the only one.
I’ve checked everything I can think of. Even swapped out the boot connecting it to the engine.
I have a spare set of carbs, I’m half tempted to swap out the bad one...

Sound like anything someone else has experienced?
Thanks for reading.
This year I’m at about 15 to 1, hours of wrenching vs. riding.
I know if you buy a project, you need to be willing to put in the time on the tools. But I’m really itching to ride.

Have you checked the valve clearness?

Backfire through carb is likely due to an intake valve failing to fully seal when supposedly closed.
Could result from such conditions as insufficient clearance, burnt or broken valve, or damaged valve seat.
Some of the combustion inside the cylinder escapes past the supposedly closed intake valve and shoots back through the intake port and on through the carb.



Hopefully, the mating surfaces between the valve and seat are undamaged and will again provide complete seal when closed, given proper clearance required to enable closure when seated.

Continued operation with insufficient clearance is one reason for burnt valve faces and valve seats.

Good Fortune!

This will be my next course of action.

Double check the plug/ignition wire on that cylinder too. If the gap is wrong or the plug is bad, it might cause puffing out through the carbs too. My bike did that on both cylinders when the points weren't set right, though since this is only happening on one cylinder for you I would suspect the plug first.

Quick and easy, plus cheap if it's faulty.

After the valve adjustment (and a fight with the cover gasket...) it seemed to run way better.
But, fuel started leaking from the carb.
I pulled it apart again, thinking it was a needle and seat issue.
Swapped out the needles with some I had that looked better.
Now it leaks fuel way worse and will barely start. So I obviously screwed up the float height.
Annoying, but sometimes I make my own luck.
Felt like a rocket ship for the drive I went on before discovering the fuel leak.
Going vacation for a few weeks before I can take another run at it.
I’m going to be a carb expert by the time this is over, either that or catch it on fire trying.
I Did find and replace one bad plug wire and already thought I’d do them all. And plugs as well considering how old they look.
The short ride I was able to take has me excited to get this worked out.

Don't worry about float height as separate issue.
Just assure that fuel level is within spec by performing the clear tube test.
So long as the float height produces the correct fuel level, the float height is correct.

Before removing the carbs to address float valve leakage, would first try with engine at idle, rapping smartly against the float bowls with the plastic handle of a screwdriver which sometimes helps seat the pointed end of the float needle into the float valve seat.

It's usually best not to mix and match needles and seats. And to consider the needles and seats married for life.

Good Fortune!

Patton wrote:
It's usually best not to mix and match needles and seats. And to consider the needles and seats married for life.

Good Fortune!

I wondered that after I did it. The bike didn’t even want to start. Figured it was flooding right away.

It’s good chance your issie is incorrect float high I can Recon. same symtoms.

Is it safe to assume that would make it hard to start? The starting issue appeared at the same time as the leak...

I would open up the carbs and checked the needle and seat for wear and tear and durt...

The floating «element» can also leak...

And then use clear tube test to adjust proper float height on all carbs.

I think it’s a fear chance it will go better after these

I did give the floats a little shake, to see if there was fuel inside them. There was none.
I will do a proper level set up first chance I get.