Hello from Finland
I bought VN750 1995 in the september when someone near by me sold it insanely cheap because of there were some minor problems in this kawasaki. I didn't know anything about vn750 but I can fix things with my hands and I figured out that this might be easy and cheap step for me to buy my very first kawasaki. I have had many bikes before but mostly Honda CX500, GL1100, Shadow models etc.
I towed it to my home and it started but it could only keep running if choke was fully open AND idle screw was turned totally in (max position). Anything other and it died immediately. So first I had to get it running properly.
At this point sorry for my long post but I want to be precise and tell everything. I try to tacle all the problems one by one.
At first I removed all those vacuum tubes from cylinders etc. All those unnecessary vacuum stuff to the trash can. I plugged those nipples on headers with silicone caps with hose clamps. Also carburetor vacuum port with silicone cap to avoid vacuum leak.
Now it idle a little bit better. No full choke but still max idle screw. I had to remove carburetors and that was one pita job to do. Took me something like two hours. I have original air surge box and original air filters which I tend to keep. I like the way they look.
I did the carb cleaning good. Disassembled it into a pieces. Gave ultransonic bath to every single part. Finally rebuilt it with full rebuild kit. New needles, jets, acv valves, needle jets, pilot screw etc... The big diaphragm was in perfect condition in both carbs. Now it is perfect but there was many thing wrong with these carbs. In both carbs there were identical needles which I replaced those different ones which were thinner than those which was inside and other one is little longer and thinner than the other. They were markes N53A and N53B.
N53A = F -> Front -> thinner and a little bit longer
N53B = R -> Rear -> the thicker one
Maybe the thinner goes to rear cylinder carb to compensate fuel-air ratio. Not sure but I think because rear cylinder is not in direct wind it will be always a little bit hotter. And as we know also too lean fuel-air ratio will make cylinder run hotter. Maybe this thin needle gives a tad more fuel to rear cylinder to prevent it run so hot. Just a minor modification maybe from Kawasaki? I'm not really sure about this but just something to think.
Original needles which I found from carbs were both N31F mode. Much thicker. I figured out that Kawasaki used these needles in two first year models in vn750. 1986 to 1987 or something as you can order those needles as replacement parts for those year models.
Also there were #110 main jet (also first year models I think) which I replaced for the stock size #132 (year model 1995).
Pilot jet was size #38 which it is for every year model.
Air cut off valves (ACV) were little old and I decided to change both too.
Maybe this carb was rebuilt before with wrong year model carb kit.
The most weird one was pilot scews. There was missing washers and both o rings were broken and did not look o ring at all anymore. Even the sharp tip of another pilot screw was little bent. So everything was replaced with new parts.
Now the carbs are in perfect condition. Even the diaphragms make this woosh woosh sound when lifted up with finger and let drop down.
After this full carb rebuild I put them back to bike. Again it was pain in the ass. What a design. Mr. Kawasaki must have been drunk when designed those. But when I got them back and tried to start the bike it started perfectly at first try. Actually now it starts everytime at first try. It idles perfect. No more full choke and no more max idle screw setting. Works like charm.
Except I when to ride and noticed this common 5000 rpm sputter on load. When wide open throttle fast it even sputters somewhere near 3000-4000 rpm. If I jump off from bike and there is no load then it revs to 7000-7500rpm which not even near the red line. I noticed the vent hose was not in place in that round hole behind right side air filter box. I cut it's head to 45 degree angle as I read from here it should be done and put the vent hose back where it belongs. No any help. Same problems and symptoms.
I decided to do full petcock rebuild if petcock did not give enough fuel to carbs. Now it is like a new. Ultrasonic bath for parts and new rubber parts. Also new 8mm I/D fuel lines without any blocking fuel filters.
I also checked air filters. They look good but ordered new ones which has not arrived yet.
I checked fuel tank ventilation all the way from tank cap to the rubber tube which is connected to the rear part of the tank. Air moves freely.
I have checked all the electrical contacts and cleaned them all.
I also checked the fuel height in float bowls with transparent tube and fuel height was 100% perfect.
When bike idle I tried to spray carb cleaner in everywhere near carbs, inlet boots, airfilter tubes, surge box around and everywhere. I tried to find leak if revs gets any higher. Nope... they didn't. No leaks anywhere.
I checked ignition coild they are like a new and no cracks. I changed new spark plugs with right gap on them.
I changed new fuel without help.
Everything seems to be as they should and still this same problem.
Today I decided to try to take a look again. Somehow I tried to take the carb vacuum hose off from the airbox and plugged it with my thumb when same time gave good wide open throttle revving and BOOM!
Suddenly it revved up to 10 000 rpm like no hesitation at all. I scared because it revved so much faster than before. Same time I was so excited. I removed my thumb and it same stumbling again.
I plugged vent hose several times and gave a good rev and always it went easily to 10 000 rpm
Happy happy joy joy
What should I do now?
I have read tens or hundreds topics here related to this 5000 rpm stumbling and every time in some point someone writes this same comment "vent hose should not be blocked and it should be in neutral air" + "cut it's head to 45 degree angle and push it back to where it belongs behind the right side air box". Why mine is working as opposite?
It is now winter and slippery here in Finland so I can't test ride it before april/may of nex year.
Has anyone else had same experience where revs goes up to the redline when you block the vent hose?
I don't know what to do next
Any help ideas would be great