1985 Kawasaki GPZ900r Carb backfire

dyu1032 · 11-06-2012, 12:35 PM

Greeting, I just purchased a 1985 Kawasaki GPZ900r and the bike had been running well until yesterday. One of the carbs began to backfire at about 3-4k rpm. Performance doesn't seem to have dropped off. The seller also had the carbs professionally gone through, adjusted the valves, added a new Vance & Hines header and a new element for the original airbox, new Kaw. gas cap, new Kaw. fuel gauge sending unit, new Kaw. petcock, etc. etc. etc.

The threads I have read seem to indicate a possible leak at the intake manifold of the carb in question ( #2). I am not comfortable pulling the carbs apart and the suggestion to test this theory is to spray carb cleaner at the intake seals to see if the rev's increase. this would indicate a manifold leak. Other possiblities are gunk in the jets or a timing issue. I heard that if new headers were added the jets might be set to run too lean but that should be a problem accross all the carbs, no?

I plan to put the orginal headers back on next season so I want to discount all other possibilities before i make that move. I plan to test out the leak theory this weekend but is there anything else i should focus on?

Sequentialshift · 11-06-2012, 04:53 PM

I used to have one of those . Fantastic bike . Once you get it going properly it will be a great classic to own . Now I dont want to scare you .. but .... Mine started to do that . I did much ferkin around with carbs and plugs ,lead and coils . Then thought I would as I was in a tank off all exposed situatiuon ,whip off valve cover and check clearances .They were out , so I swapped some shims around and found another couple in the supply of sods and bits box .Put it back together gave the engine a few winds over on the starter ,checked em again .. it was out .. way out .Pretty much zero clearance on some of them ..Fast forward a couple of hours and I am lookin at valves much smaller than they should be . I was amazed it even ran atall . New valves ,gaskets and all was well again . Aparentlly its a common fault on the early ones once they get some miles on them . Mine had 50,000 on it .

singram · 11-06-2012, 05:12 PM

I feel the need...

dyu1032 · 11-06-2012, 09:42 PM

The bike only has 16k miles on it. I thought about a valve issue, but I am not that experienced enough to start playing with valve clearances. But I think owning a bike this old I going get experience real quick.

And yeah, that picture tells it all. I love the bike.

Sequentialshift · 11-07-2012, 07:45 AM

Just to check your valve clearance is no big deal . Limited skill will get you through that . Adjusting them is a step up . But done slowly and methodically can be learned by yourself .
Be sure to check the oil screen when you change the oil . They are quite fine on those and can clog up . They can run a bit funny on a highway in really cold weather as they do suffer a bit of carb ice issues in the cold . Just slow down or pull over let it warm up a bit and it will be fine again .If you do plan on riding through winter then partially blocking the rad can help keeps things at the right temprature .

dyu1032 · 11-07-2012, 11:35 AM

OK thanks for the advice. I'll let you know how it goes.

Tripped1 · 11-09-2012, 12:24 AM

Squentialshift beat me to it.

I had one of those up to 140,000 miles. Get used to checking the valves, Kawasaki made the seals out of toilet paper or something they ALWAYS need to be adjusted at maintenance intervals and usually a bit before.

...and yes, the intake boots are plain bastards. #2 and #4 on mine liked to get loose a LOT.

The 1986 GpZ900 was my first street bike. Those things are beast, but they are 25 years old now, and about 19 generations behind a modern sport bike.

Better question where in the hell did you find one that clean? That is a NICE looking bike for a machine that is 27 years old

dyu1032 · 11-09-2012, 08:36 AM

Thanks. Great looking isn't it? Found it on eBay after two years of looking. Almost bought a 1984 750 GPZ turbo but the mainenance stories scared me off. Its not about the technology for me, it about the look. I used to ride a 1984 GPZ 1100. It was an animal, but a little to long to be manuverable. This was a bike I have always wanted. And it rides great. too bad we have snow in the North East otherwise i'ld be out there everyday.