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Discussion Starter · #1 ·
Hello all, I have a 1984 GPz 900, with 40k. It has after market k&N air filters. Other than that its pretty much stock. I just got it, and had to do a little wiring to get it to start, but it does run. Once it warms up & the revs come down to idle, there is a puff(non technical term) that comes from the carbs back out towards the air filter, then it wants to stall unless I put the choke on.

Any suggestions on what might be causing this.

Thanks
 

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Navy Vet Search & Rescue
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Installing the pod filters makes it run lean and requires the carbs to be rejetted. The easiest solution would be to reinstall the factory airbox. Of course you can start the trial and error process of rejetting the carbs and eventually get it to run pretty decent with the pod filters too.
 

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Unfortunately for you, I don't believe you will be able to compensate enough by just adjusting the mixture screw. As a guideline of sort, if the mixture screw is located on the engine side of the carb it controls fuel, and if it's on the airbox side it controls air, out is more and in is less. Having said that I will also say this rule is not true 100% of the time. If you'd like to dig in to the subject of jetting the carbs here is a link that might be helpful to you.
CV Carb Tuning Procedures
 

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Discussion Starter · #9 ·
OK, I took the spark plugs out, and #2,3 & 4 look good. #1 looks a little black & moist. #1 is the only carb backfiring as well. What can I adjust to lean it a little. Or should I rejet just that carb.
 

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itching to ride
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Yes other things can cause it. But if your clearances are off you will not get the carbs right. They need to be checked even when the bike is running well if it has been awhile since it has been serviced.
At least you do not have to buy shims since you have the screw type adjusters.
 

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Discussion Starter · #13 ·
Yes other things can cause it. But if your clearances are off you will not get the carbs right. They need to be checked even when the bike is running well if it has been awhile since it has been serviced.
At least you do not have to buy shims since you have the screw type adjusters.
Just got my new jets, I bought 3 sizes to test with, just my luck it snowed last night, so I might have to install snow tires to test them.
 

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Navy Vet Search & Rescue
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As mentioned previously, check/adjust the valves before you do the carb work. If the valves are off and you go through all the trouble to try and get the carbs right, then you will have to go back and adjust/sync the carbs again after you adjust the valves. It's a good time to do the valves while you wait on the snow to clear.
 

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Discussion Starter · #15 ·
I checked the valve clearances & they were in tolerance.
I installed a 135 main jet, the #2,3 & 4 plugs look very good now, used to have a little carbon on them. But #1 still has carbon build up. #1 carb is also spitting out fuel and backfiring. Could it be a bad coil not fire all the time or something plugged in the carb. I tested the float bowl height, and it seems right.
Not sure where to go from here.
 

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I'd try a compression check to see how #1 compares to the other cyls. The only time I've had gas spitting back out the carb on mine was when that cyl was too lean but I'm sure a burnt valve could cause it too.
 

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GHOSTRIDER
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+1 on the compression check and post your readings. I would have liked to known what the valve clearances were too. And yes, something can be wrong with #1 carb causing it to act up differently than the other three. Cam chains stretch with time, tensioners and guides wear as well as rings all causing a naturally cold blooded engine to really cough and spit alot until theyre good and hot. The no.1 cylinder is usually first to show any signs of valve and valve guide and seat wear because oil inherently runs down the valve stems while the bikes on the kickstand. That oil gets cooked on and around the edge of both or all four valves when the engine gets refired causing #1 cylinder to lose its compression and faulter before the other three. A poor mechanical carb sync will also cause one or two carbs to act up through start up and low rpm use. Id start with a new plug in no.1 cylinder and note any, if any changes along with a compression check taken with the coils unplugged and the throttle held wide open. note and post your readings and a diagnosis with a prognosis will be easier to pinpoint and give. My first thought when I read this thread was a poor mechanical sync, or low compression on no.1 ie the valves over no.1. It can also be as simple as a bad plug wire, but its not the coil other wise two cylinders would misfire. Always verify that fat blue spark and 12 volts to the coils before delving into the carbs! Then compression checks followed by the valve clearance checks and corrections to any tight valves and only then, tune the carbs. And theres way too much ado about getting the carbs vacuum synced and thats wrong. If your compression is good and nearly even across the four, and the valves are good, then a good mechanical carb sync will be adequate. I use the vacuum sync in conjunction with air fuel mixture screw adjustment to simply squeeze out the max vacuum at each carb during idle only after everything else is done. I hope this helps and hasnt confused you or dashed your hopes.Switch no1 plug and wire first, then note compressions, thats where you start. Good luck!
 

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Discussion Starter · #18 ·
Thanks for your responce, however, after checking a few things, #1 & #4 carbs pop & backfire into the air filter. The left coil connects the leads to the #1 & 4 sparkplugs. I'll try changing the plug & wire. Valve clearances were .15 intake & .2 exhaust.
 

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GHOSTRIDER
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I dont know what the specs are for your bikes valve clearances but those numbers indicate that the dreaded tight valve condition does not exist for you and thats good! 1&4? That indicates a possible ignition coil problem and/ or a low voltage supply to that coil causing a weak spark off that coil. If you switch plug wires from one coil to the other and see if the weak spark goes to 2&3. You also might check the input voltage at each coil to verify whether or not the weak spark is due to a weak input rather than a bad coil, coils need maximum input voltage to produce the fat blue spark necessary to fire a cold engine. The service manual for your bike will outline the procedure to conduct the resistance tests for the primary and secondary windings in the coils. Coils usually dont just stop working , they die a slow hard to detect death. Now then, with that understood, were still dealing with the knowledge that #4 plug looked good and #1 plug looks bad, Right? Stick a new plug in #1 and note any differences, It can be as simple as a fouled plug! A wet carbon fouled plug will will pass high tension voltage without even sparking! That in itself can cause the problem youve described. Ignition timing and/or a loose cam chain will also create those "pops" or hickups through the carbs youve described. My bike does it (sporadically) until shes warmed up to operating temps, which is normal for the older big fours.Stick some new plugs in her and see if that dont improve the hick ups. Post those compression numbers too, that can explain much as to why no.1 is darker than the other three.;) :hohoho: PS I think you can all but rule out the carbs causing your problems so dont go carb crazy yet.
 

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New sparkplug caps(I think z1 enterprises sells them) along with new sparkplug caps can make a big difference in performance.

The caps have resistor designed to reduce RFI in radio's and TV's and will go bad over time. Unscrew the old ones and new ones will screw onto the sparkplug wires.
 
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