85 GPz 750 Cafe Project

Re; fuel t's. I can't say with certainty but I don't think so. They look old and stock to me. Rubber coated with the 'o-rings' being part of the rubberized coating. Ancient and rusty inside until I cleaned them. Out to try raising the needles to the highest position and see what happens per Pablo's suggestion on KZ650. If there isn't any change I may do a carb swap with the bank from the 82 GPz and see how it behaves.

C-clips in the bottom slot so needles raised all the way up. The bike now runs up to 6K rpm, versus 5K yesterday. I think this suggests my mains are too small since no other adjustment was made but I got an extra 1K rpms.

I chose to go with the larger set of jets to make any change more dramatic, meaning the 117.5 jets. No change. Freaks me out. I put the gas tank on the bench and a 5/16 drain hose, moved the manual petcock to on and reserve. Both flow easily and fully diameter stream going out of the petcock with the gas cap closed. It ain't the tank or petcock. I took the bike out, it happily starts now, burbles along gracefully as I leave the neighborhood as all the Kawi fours do. Accelerates nicely past 4K rpms. After it warmed up on some country roads I rolled on the throttle. Bogs and runs out of fuel at 6K rpm. Same symptom as yesterday even with the jet change. I still believe the fuel bowls aren't filling properly. If I roll along at 2K rpms in first gear, stab the throttle, it leaps forward and runs up to 6,500 rpms, then slides back to about 5,700 rpms as it depletes the gas in the fuel bowls. This is repeatable. I'm fed up for today. I've taken the carbs back off. I'll re-recheck the fuel levels. If they are in spec 4mm - 1mm below the carb casting edge, I'm going to put the other bank of carbs on it after checking the fuel level on the alternate set of carbs. I'll see what happens tomorrow.

Update: Installed the tank and air filters. Fired up, let the bike come to full warm. After holding it on 3K and 4K rpms after warm up I let it return to idle for 10 seconds, then cracked the throttle to 8K rpms. Waited a few seconds, 9K rpms. I did notice a slight breakup at 7K but the rpms continued to rise. KEY: repeated high rpm throttle ups results in lower and lower top rpms achieved. From 9K down to 6,700 rpms. Let it idle for 30 seconds, it will throttle up to 9K rpms...once. Repeatedly cracking the throttle results in ever lower rpms until it gets into the mid 6k rpm range.

Also of note I broke out the timing light. Timing looks good at idle, on the F mark, full advance at about 3,500 rpms. The timing marks are steady at idle and above 3K rpms, but the timing marks jitter off idle up to 3K.

So these tests have all been in neutral, or just this latest one? Probably not a fuel issue if there's no load on the engine yet it still won't rev. I'm thinking an ignition problem. The electrical load is strictly a function of RPM (spark firing frequency); just about everything else is dependent upon engine power output (which is essentially zero at any RPM while in neutral). Maybe repeat the experiment but this time connect a battery charger connected in parallel with the battery.

The bike bogs at 6K rpm while riding. Flat hits a wall. Sounds of the exhaust is an odd hollow sounds. In neutral it will tach up to 9K in a test this morning on a full warm engine. But if you crack the throttle in back to back events the max rpm drops each time you crack the throttle. Give the engine 30 seconds of so called recovered time and idle and the process can be repeated. High rpm but lowers each time you crack the throttle in close succession in neutral.

Because coils take time to charge, and they get less time to charge as RPMs go up, ignition electrical load decreases as RPMs go up. The duty cycle stays about the same (after a certain RPM regarding the factory electronic ignition), so the coils are charging for the same percentage of time and resting the same percentage of time regardless of RPM. But as the RPMs increase, even though the time percentages stay about the same, the actual time of charge decreases, so the average level of charge reduces.

So it's not really an issue of electrical load, but the decreased dwell time can cause the coils to have insufficient charge.
I have only seen this be a factor when there is a low voltage issue, or when the spark plug gap is much larger than stock spec, or if the coils have too high resistance for the given dwell.

Does the timing light drop out when the RPMs hit the "wall"?

Also, is it possible the vacuum slides are not moving up, as if they might have a tear in the diaphragm? With the airbox, that is almost impossible to check for movement, I guess.

Are you using an inline fuel filter designed for cars? That is the see-through type with a pleated paper element. I've had the RPM "wall" issue with those filters.

The fact that it drops with every rev would imply a fuel issue, either too much or too little. If the plugs are coming out white, I guess too little.

The bike has new Dyna coils and wires, stock pickups. Great question on the timing light, I have no idea but I'll check tomorrow. Pablo had me check the slides. I took 20lbs of compressed air and blew it in the intake port on top of the intake throat and each of the sliders moved up and down nicely, evenly.

I had a clear inline pancake filter with the round screen in it, but I took that out because I too sense a fueling issue. Yes the spark plugs come out white. Part 2 of why I think I have a fuel issue. I've got one more bank of carbs I can swap in as a test, I'll do that tomorrow.

I pulled plug #1 and #4, both are white and powdery.

I also use the pancake type see-through filter with a screen, and it seems to flow a lot without any issues.

Sorry if this has been done/asked already, but does it make a difference if the choke is on?

I removed the inline filter, It is now a one piece clear 5/16 inch fuel line. Choke is off when riding and testing in neutral. I fire it up with the choke on, work the choke down until it will idle on it's own with the choke off. Give it another couple/three minutes to heat soak, Then start taching it up in stages, 3K, return to idle for a few seconds, 4K, etc.

loudhvx wrote: Because coils take time to charge, and they get less time to charge as RPMs go up, ignition electrical load decreases as RPMs go up. The duty cycle stays about the same (after a certain RPM regarding the factory electronic ignition), so the coils are charging for the same percentage of time and resting the same percentage of time regardless of RPM. But as the RPMs increase, even though the time percentages stay about the same, the actual time of charge decreases, so the average level of charge reduces.

Hmmm, I might have to concede your point there. Makes sense if the inductance of the coils is relatively high. I should probably break out the scope...