Backfiring

1981 KZ750 ltd H2

I picked this bike up from a buddy after my other was stolen.
Thought it'd make a great cafe racer project (always wanted one).
So some background on it.
A guy he worked with broke down right in front of his house and he had to have it so he bought it.
We did some work to it since it would run without spraying carb cleaner into the carbs.
I, at the time, was terrified of anything carb related so we picked up a Dyna S ignition for it and had no luck.
I told him to buy new carbs and he immediately wanted no more to do with it.
So I bought this thing about 2 years later.
First thing I did was tear down the carbs and clean them until they were new.
It had pod filters so I upgraded from 125 jets to 138's. still running the factory 62's.
New carb boots (holders), Vacuum boots look new. New plugs, new wires, checked and corrected valve clearances, checked compression (120+ on all cylinders). Did the clear tube test on the carb floats and corrected them. Full battery charge (been using my truck with some jumper cables). Pilot screws are two and a half turns out.
So here's the issue. I get a backfire that sounds like a fart coming out of the exhaust. And I suspect cylinder 4 since it isn't getting hot. the other three are. It runs great with that only exception (in the driveway, I'm not riding it until it is 100% ready).
The bike fires right up with the choke on. After a few seconds I can turn the choke off and it runs smoothly. Then a few seconds after that it starts to pop. Regardless of choke or not.
I have torn through this site, which is very informative, but have not found anything new to look at.
ANY suggestions are appreciated.
I'll still be looking over this site until I come to the solution.

What do the plugs look like? Sounds like you still have issues with carb #4.

I pulled them all and keep in mind these are very new.
Cylinder 1-3 are slightly brown. Cylinder 4 looks gray.

I've been reading over other posts and stumbled across one that mentioned that the exhaust gasket could be the problem. Any thoughts? I'm ordering them anyway because this bike has been sitting for at least 2 years and has around 45K on the odometer. The valves were out of spec and I have my doubts that any maintenance was ever kept up on it...

Grey is not a good color. Check for leaks, and always expect that maintenance was not updated.

Once running and warm, when the popping starts, spray some WD40 around the carb holder for #4. If the popping goes away, you have an intake leak. Just a quick test to see if this is the problem. Let us know your results...

Thanks !Seymour,

I've done that already without any results. Didn't idle up or down. Plus I went ahead and bought new carb holders and did it again. As I've said, this site has been EXTREMELY helpful but I just haven't found the problem. It's something small but I'm not the best at diagnosis (usually play it by ear).

As I said, I found another post that "Ed the all knowing", as I like to call him now, posted about exhaust gasket leaks possibly being the culprit. So that is my next step and I'll post the results. I've switched the plug wires between 1 and 4 on the coil and noticed that cylinder 1 and 4 both seemed to be missing so I'm going to replace the coils as well. I'm thinking it might be a combo of the two since the coils are the original. I was under the assumption that if one side of the coil was working the other shoudl be as well...? Maybe it's getting weak?

On that note, I've checked out z1 for the new coils and they were out of stock so as of now I'm going to purchase the Dyna. Unless anyone thinks it almost essential to find the originals?

I'm really pumped to get this bike on the road! Having owned sport bikes my whole life and reading from personal experiences on them and the power numbers I was shocked. These old bikes had some power!

Going out of town for the weekend so probably wont have any results posted for two weeks. So keep the suggestions coming!

What's the mileage on the engine? Have you checked the valve clearances? I wonder if one valve is getting tight and when everything warms up it stops sealing properly.

The valve clearances were checked and corrected.

Have you had the pipes off the bike? If so, what condition were the gaskets in? Could be gasket on 4 is MIA?

Also, thinking geeky... almost sounds like a heat related electrical problem(?) since it does it after warming up. Any cracks in the coils?

Have not had the pipes off yet. It's a cheap check so figured I'd just replace the gaskets.
Coils are definitely old. I see cracks around where they mount but thats it.
I know they are working, I just question how well. I'll be replacing them just for preventative maintenance.

Thanks for the imput !Seymore

You're quite welcome Brett...

Cracks can lead to a short to the frame. Electricity is an amazing thing... it can jump gaps small and large. If you have something to insulate them with... like a small, thicker rubber mat, might adapt that as an experiment to see if this is what's causing your problem. Yea, trial-n-error... but ya gotta start somewhere and just follow the bouncing ball...

If 1 is firing and 4 is not, you may have a bad secondary wire or boot. If you swap them and 4 works but 1 does not, then a new wire an;/or boot may be the solution.