kz900 engine missing under load in 5th gear 3K rpm - runs good in neutral

I am unable to get my bike to run right and am stumped.
Perhaps someone could give me a suggestion how to troubleshoot because I am out of ideas. While I have always worked on my own bikes I would not say I am an ace mechanic.
History:
The bike was completely refurbished. (1976 kz900 bored to 1015cc) the bike was already punched out to 1015 and well broke in when I bought it 3 years ago. It now has just about everything new. I completely disassembled the motor to renovate the castings. I inspected everything and reassembled it. When I first put the bike back together it had no power in the 3,000 rpm range and would not idle under around 1800 rpms. Once you got up in the higher revs it would pull your arms off. That was when it had high lift cams and VM33 carbs. I assumed the problem was the carbs. After talking to a carb expert I was convinced the vm33s were not the best for cruising which is what I wanted the bike for so I had a set of VM26s profesionally refurbished and bench tuned and tested. When I got the carbs I put them on, and the bike had the same problem. At this point I had verified the cam timing and the timing advance mechanism. All was all working correctly. All the easy things to check, plug gaps (the plugs looked pretty good as far as rich lean mixture goes). I figured it had to be the cams at that point and put the stock ones back in. Now the bike idles great at 1,000 and in neutral it revs up fine and seems to run good. However, on the highway under load in forth or fifth gear it runs like heck. As if the plugs are fouling or it is missing (it is not backfiring however). Its almost as if it is running out of gas. The plugs still look pretty good, 2 and 3 are a little black but dry. Just going up my street in first and second gear it seems fine. the problem doesnt show up till I get up to speed in high gear.
I have new dyna coils (the green ones 3 ohms).
New Dyna S ignition.
New plug wires and caps and I have double checked them for their connections.
I re-checked the cam installation (timing marks at TDC) and valve clearances, they are correct.
Compression seems good (although I have not checked it with a gauge) and the bike does not smoke.
I checked the timing with a timing light it is correct and advances properly. What baffles me is it only does it under a load (or it seems this way as much as I can tell). I pulled the plugs and laid them on the engine to observe the spark in the dark. When I crank the engine with starter the sparks are thin bluish white but I wouldn’t describe it as FAT blue.
any ideas?
How do I check if all carbs are not running out of gas?

Regards
Don Browning

76kz9004ever wrote:

Now the bike idles great at 1,000 and in neutral it revs up fine and seems to run good. However, on the highway under load in forth or fifth gear it runs like heck.

What baffles me is it only does it under a load (or it seems this way as much as I can tell).

You said the spark is good and the advancer is working OK. It sounds like carburetion. I would check the fuel levels to make sure they are all right. Drain the bowls to make sure the gas is clean (check fuel filters).

That range of operation is pretty much controlled by the needle position. As you open the throttle maybe 1/4 to 3/4 in mid RPM, the float is raising. The position of the needle in the float has a big effect on how much fuel you are getting.

How does it run up at high RPM under full throttle? If it's OK, you have the correct main jet size. If not, you need to fix that first. Then maybe try adjusting the needle height if possible to get more gas.

As Bountyhunter said, check the fuel filter (on the petcock in the tank). Also, make sure there are no kinks in the fuel line (you may even want to replace it just to make sure it's not plugged up.) Another thing to check is the gas cap vent. If it doesn't vent properly fuel won't flow very well. Does you petcock have a diaphram? It could be that it's not opening the petcock properly if it's damaged. Ed

It is possible to get bad plugs new out of the box, so try new plugs and see what happens.

How do you check the gas cap venting?

76kz9004ever wrote:

How do you check the gas cap venting?

easy way is to let ride with the cap unlatched so a little air can get in and see if it runs any different.

have you checked the voltage at the coils?
and maybe look for any bad connections going to the ignition system.

I am going to hazard a guess and say that it is in the carbs. If it is running fine every other time except under a load like that, i would say your needle position may be the problem. I have had this problem on my 650 and again on my 900 and simply changing the needle position and rejetting fixed the problem and they both ran smoothly after. The way that i understand it to work is; at part throttle (while cruising) the carbs are recieving fuel from the pilot jets, but when you crack the throttle open the carbs switch over to the main jets and there may be a lean spot in the transition area. I dont know for sure if that is how it works but it was explained to me like that.

check the main jet size it should be 115. sounds like its running waay rich. It will sound fine at idle but when u get to about 5grand it sputters> too much gas. With the 1015kit it should have a 132jet. With black plugs it cant be running out of gas. May want to check compression also.

kzdev1360 wrote:

I am going to hazard a guess and say that it is in the carbs. If it is running fine every other time except under a load like that, i would say your needle position may be the problem. I have had this problem on my 650 and again on my 900 and simply changing the needle position and rejetting fixed the problem and they both ran smoothly after. The way that i understand it to work is; at part throttle (while cruising) the carbs are recieving fuel from the pilot jets, but when you crack the throttle open the carbs switch over to the main jets and there may be a lean spot in the transition area. I dont know for sure if that is how it works but it was explained to me like that.

From my service manual:

Pilot jets are for light throttle opening (under 1/4) and lower RPM (on mine under about 2500). Main jets control everything at throttle openings above 3/4 at higher RPM. In between 1/4 and 3/4 throttle, the piston is rising and the needle controls the flow for the most part. There is transition overlap between all the systems.