Am I missing a part?

Hello Everyone!

I have recently purchased a fixer-uper motorcycle as a project to work on. As far as I can tell it is a stock (at least to the best of my knowledge) ZX 750 A1 1983. She has been beat up a bit and neglected but is in over all fair condition with very few miles.

Now along the way of fixing it up, I've gotten to the carbs and engine. I rebuilt the carbs so that is kinda on the back burner. What I am concerned about it the fact that it backfires and occasionally lights on fire. . .

I have removed the two reed valves on the top of the headers that have something to do with a normal pressure valve system that pumps fresh air into the exhaust system to keep the valves clear (again this is how i understand it at least from reading the klymer manual for the bike).

When I bought the bike it did not have this switch-valve that goes from the carbs to the exhaust. all it had were the reed valves and a tube connecting them. Now my question is this. It may seem simple but i cant seem to find the answer, how are the reed valves on top of the headers supposed to be set up? Do I need the switch valve that goes from the carbs to the reed valves for the bike to not spontaneously light on fire?

She starts and I can rev her up about three or four times and then she backfires on the deceleration and lights up. If i could get her to not light up and idle for a bit i could tune the carbs better, I think it is running too rich thus leading to some of the fire problems.

Any insight into this problem would be greatly appreciated!! Thanks in advance!!

1983 ZX 750 A1 - New Jersey

Have you checked compression, valve clearence, timing? You say you think it is running rich, what do the plugs look like. Are there any exhaust leaks, or carb holder leaks?

welcome..it is just a way to clean up the emissions and is not related in direct ways to your problem imo...you can just cap the breather plates,tubes off on the valve caps with a pastic cap and clamp...the carb issue is seperate..imo

Hello, and welcome.
I know jack about American Emissions stuff, they all came to Canada eh capped off with a metal plate and gasket over the openings.

To my mind backfiring through carbs and lighting up in flames suggest either a stuck open intake valve, or timing too far advanced. Have you done a compression test(should reveal a stuck open valve), and checked the timing advancer? Beyond that I'd have to say time to check valve clearances.

The backfiring and refusing to idle is what happens on that bike when you don't have air filters on it. So either hook up the stock air box (best idea) or put individual filters on the carbs.

Put the reed valves back in there and run a hose between the two nipples, or install block off plates where the two plates with nipples are.

I think I have a halfway decent seat and some other small parts left for a 83 GPz750 (Zx750A1).

I have other engine parts as well, look in the classifieds

start with the simple things.
change your spark plugs. you loaded them over.
bet you a dollar they are black as f#ck.

do that then post the results.

btw, how did you rebuild the carbs?
carb clean only or did you ensure all air passeges are clean?

we are picture whores so make with the pictures all ready! :woohoo:

Oh my! Thanks for the speedy plethora of responses. I will make pics a priority for you all the next chance I get.

O.K. The air-box problem that is amazing I will try the simple first. Thanks for the tip I was not trying with the air box on.

Carbs - Took them off, broke them down one at a time, bought a rebuild kit off my local shop and soaked them in carb-cleaner before replacing the parts, and putting them back on the bike.

Timing- Y'all aren't going to like me on this one, I think that is also a tiny problem. When I got the bike's point plate off the electronic advance pickup coils were missing and the cpu was fried. So what I did was place a mechanical advance on the shaft and then put some scavenged pickup coils (better than the ones it came with) on. After that what I did you use an almost magical fix, from this sight in fact. Using 2 GM HEI modules and a wiring schematic that is on this site I wired up the new replacement cpu and to my surprise it worked ^.^ So tweaking the timing on the mech-advance is a must do I would assume. I'm just happy it fires.

Spark plugs- Brand new =] first thing I replaced. The coils are good and fire in the correct order and what not, the outside and then the inside fire, I assume thats correct. and they pack one hell-ov-a wallop haha.

For the rest I will have to do some research into how to preform those tests. Thanks for all the insight, I will post pics and keep you all updated on the progress. Thanks for all the help Y'all. I really appreciate it!!!

~Chris
1983 Gpz 750 (ZX750 A1)

My friend did the same thing... put a mechanical advance and ignition from a KZ750 onto a ZX 750. Much cheaper and he said it ran great.

steell wrote:

Put the reed valves back in there and run a hose between the two nipples, or install block off plates where the two plates with nipples are.

My '82 kz750 has a similar anti-smog system setup. I was getting a lot of popping and backfire this summer so I tried removing the system and just have a hose running between the plates. Left the reed valves in. Runs like a champ.

Good Luck!