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Discussion Starter #1
Hello from Finland :smile2:

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 :grin2:

BUT!!!
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 :doh: Any help ideas would be great :smiley_th
 

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Try this: Warm it up, then see what happens if you set the petcock to 'off'.

Before it runs out of gas, does it clear up and run right?

Only thing I can think of right now, is maybe the fuel level is too high in the bowl(s) and blocking the vent stops fuel from flowing in. Do the plugs and exhaust tips look sooty and black?

"Max idle screw setting" - Is it running on both cylinders? Could also be because it's flooding.

Nice clean looking bike, wheels look very clean (and everything else), nice color combo, forward controls, and it's got a chrome coolant tank cover too, those are rare. I still have chrome covers for the coolant tank cap.
 

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Hi, Jalaty, welcome to the forum!

Sounds like you've done all the right things so far. Can you tell if the vent hose is sucking or releasing air? Can you do a leakdown test?
Does anyone know exactly why a breeze on the vent hose/pressure in the crankcase causes such a drastic performance issue on the vn750?
 

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Does anyone know exactly why a breeze on the vent hose/pressure in the crankcase causes such a drastic performance issue on the vn750?
Air pressure against the vent hose opening pressurizes the fuel bowl, reducing fuel flow into the bowls. If we had a fuel pump instead of gravity feed, it probably wouldn't matter where the vent is placed.
 

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Discussion Starter #5
Try this: Warm it up, then see what happens if you set the petcock to 'off'.

Before it runs out of gas, does it clear up and run right?

Only thing I can think of right now, is maybe the fuel level is too high in the bowl(s) and blocking the vent stops fuel from flowing in. Do the plugs and exhaust tips look sooty and black?
I actually ride it two times with this problem before winter came. First time I run it so far with petcock ON position when it finally stopped. It run good and suddenly just stopped because of fuel ended. I set petcock to reserve and it started without problems and I ride to home.
So no weird symptoms before fuel ends.

Plugs looks good. Exhaust tips are black because I have not washed them and previous owner run it like it was.


"Max idle screw setting" - Is it running on both cylinders? Could also be because it's flooding.

Nice clean looking bike, wheels look very clean (and everything else), nice color combo, forward controls, and it's got a chrome coolant tank cover too, those are rare. I still have chrome covers for the coolant tank cap.
At first it was not running on both cylinders. But after my fixes it now runs great with both cylinders. It is not flooding and the fuel level has been checked to be exactly what the manual says. Clear hose attached to the carb drain nipple and the fuel level was as it should be.

I like the look too. Great color combo and I have not seen here in Finland those forward controls or chrome coolant tank cover. I have a bad knee and I need to ride with forward controls or knee starts to hurt after a while.
 

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Discussion Starter #6
Sounds like you've done all the right things so far. Can you tell if the vent hose is sucking or releasing air? Can you do a leakdown test?
Does anyone know exactly why a breeze on the vent hose/pressure in the crankcase causes such a drastic performance issue on the vn750?
I have no idea if it is sucking or releasing the air?
I'll go out to do some more testings and take a video.
Forum needs 5 postings before I can link any urls so I need to write couple of nonsense chapters :D
 

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Discussion Starter #7
Air pressure against the vent hose opening pressurizes the fuel bowl, reducing fuel flow into the bowls. If we had a fuel pump instead of gravity feed, it probably wouldn't matter where the vent is placed.
This is something what I was amazed too. Why these carbs are so sensitive of ventilation?
For example Honda CX500 carbs had small ventilation ports and no tubes at all. Not any kind of problems ever read realted to those ports.
 

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Discussion Starter #8
ok this is my fifth posting here in this forum so from now on I can post urls too.
I'll go out and take a couple of videos and will post them here.
 

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My guess is you've misidentified the hose in question. (I think it may be a vacuum port). But I've been wrong once or twice.
Post a carb diagram and call out the number on the "vent" hose you are dealing with.


When I deleted all the extraneous hoses I tied the F/R vacuum ports together with one hose as a balance tube.

A damaged mix screw seat is reason to replace a carb body. No way around that one...

Maybe the F/R needles plus big mainjets are giving you grief. Did you change them under recommendation?
What's your elevation over there?
Looks like you are running the stock exhaust yet...

One change at a time makes for smart tuning.

Once you are able to ride it again...
A simple test and tune technique is to engage the enrichener (choke) while riding at the stumbling RPM.
If it is indeed a fuel problem, and you are too lean it will improve. Too rich, it won't help.
The affected rpm range (under load) tells you which carb curcuit needs tweaked...

Also, did you disable the air injection reed valves?
 

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This is something what I was amazed too. Why these carbs are so sensitive of ventilation?
For example Honda CX500 carbs had small ventilation ports and no tubes at all. Not any kind of problems ever read realted to those ports.
It must be in the design and layout of the ports on the carb.

I was going to suggest checking that all hoses are in the correct location. There was one poster with a bad running engine, and the fuel/vent lines were mixed up, it was getting fuel through the vent ports.. He said it ran fine after straightening the hoses out ,,, if I recall.

The choke can help ID the problem sometimes, but when my main jets were clogging, the choke had no effect. Probably because there's no air restriction (choke plate) with it, just an enrichment circuit.
 

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Discussion Starter #11 (Edited)
My guess is you've misidentified the hose in question. (I think it may be a vacuum port). But I've been wrong once or twice. Post a carb diagram and call out the number on the "vent" hose you are dealing with.
There are four nipples pointing up in the middle part of carbs. The outer ones are black plastic and for fuel tubes. The middle ones are ventilation nipples right? Here in this picture of mine there are only ventilation tubes attached.



I blocked the right carb vacuum port with silicone cap and hose clamp as you see in this picture left



A damaged mix screw seat is reason to replace a carb body. No way around that one...
That's true. But it should affect more in lower rpm range? This problem only occurs at high rpm. mix screw seat is in cood condition. The tip of the screw was a little curve and I changed it.

Here is the picture and you can see the o-ring which was just piece of rubber. The black one in the picture. Rubber propably caused the tip to goe curve when screwed in. Now there are new parts.




Maybe the F/R needles plus big mainjets are giving you grief. Did you change them under recommendation?
What's your elevation over there?
Looks like you are running the stock exhaust yet...

A simple test and tune technique is to engage the enrichener (choke) while riding at the stumbling RPM.
If it is indeed a fuel problem, and you are too lean it will improve. Too rich, it won't help.
The affected rpm range (under load) tells you which carb curcuit needs tweaked...

Also, did you disable the air injection reed valves?
I disabled all the vacuum tubes related to reed valves and taped the hole under air box.

When I run the bike I tried to run with tank cap open. No effect. I tried to give choke when stumbling like you adviced. rpm dropped little more. So no good effect either. Otherwise choke is working when idling and it will increase rpm as it should.

I changed the jets because the full rebuild kit contained #132 main jet which should be stock jet size for 1995 model. #110 jet sounded way too small for this.
 

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Discussion Starter #12 (Edited)
Last evening I decided to go thru all this again since I got it stop (only once) when spraying carb cleaner to fron intake manifold. But just once it stopped. I thought there might be an air leak in the manifold or the manifold clamp is not tight enough. If there was a leak it was very small since I got it shut down just once and I tried several times.

I removed carbs again and double checked that all the vent tubes and holes are open. They are. I checked the intake manifolds are in good condition. No leaking cracks or other. Then put all together again and screw all intakes clamps and air box snorkel clamps tight to make sure no leaks will be.

I even changed air filters to new ones. Emgo foam filters. The original like which I oiled and with paper dried the extra oil away. Like foam air filters should do.

No effect. Still the same freaking stumbling at high rpm.

List what I have done this far:
- full carb rebuild (#132 main jet, #38 idle jet, acv, pilot screws, all...)
- 45 degree cut for carb ventilation tube and vent tubes and ports are open
- new foam air filters
- no leaks in airbox anywhere
- pilot screws open 2.5
- checked tank breather all the way and it's open
- full rebuild for petcock
- new fuel lines
- new vacuum line from left carb vacuum port to petcock
- all electrical connectors cleaned
- even side stand kill swich is cleaned and moving freely

I have no more ideas what to do? Should I change back those #110 main jets which came with the bike? Can main jets be changed without removing carbs?

Some previous owner have changed the stator and regulator rectifier. It is charging good solid 14.5V. There are some pickup coils or something next to flywheel if I have read correctly. could those be somehow in wrong position and cause this kind of symptons? Or could failing CDI cause this? Hard to believe since it takes high rpm if vent tube blocked.

Here is youtube video (4 minutes) which explains how it works now. Video taken today.


And another


This last video is taken yeasterday before latest carb removal. In place 1 minute 30 second you can see left intake manifold leaked and bike almost stopped. That's why I checked manifold rubbers and they are good and re-tightened clamps. It is not leaking anymore as I tried this same spray method to find leaks.

https://youtu.be/R8EAQb_WmY4?t=88
 

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Discussion Starter #13
How about this. My bike stumbles when vent hose is open. If I block it then it takes good rpm which means in that case the fuel air mix is perfect.

If vent is blocked it restricts fuel flow to the carb because there will be vacuum in float chamber.

Doesn't this mean that by restricting fuel flow it works better so by restricting fuel flow with smaller main jet instead of blocking vent tube it it would probably work better too.

So maybe that's why there has been #110 main jets before I changed those #132 jets. (I changed jets and did full carb rebuild before I ride first time, because bike was not running good)

Does this sound good? Should I try those smaller main jets? Can main jets be changed without removing carbs?
 

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That's my thinking, the carb is rich for some reason, blocking the vent leans it out.

110 is very small for the main jet. The previous owner may have to tried to correct the rich mixture by installing that size jet.

Reasons for being rich that are not easily detected: clogged air bleeds, float height too high, slides sticking.

1) Air bleeds are hard to check, the carb body needs to be soaked and the bleeds blown out with compressed air if a problem is suspected. It's trial and error. Is the throat of each carb wet before the bike is started?

2) Float height actually isn't hard to check on these carbs, but more work to adjust. A piece of clear hose is all it takes to check. (see manual)

3) Have you checked the action of the slides and condition of the diaphragms?

Is there any black smoke when you're revving the engine near redline?

What color are the spark plug tips?

Is the vacuum hose to the petcock dry on the inside?

Jets can be changed with carbs in place, but it's tricky and tedious. Would be best to correct the problem vs. masking it.
 

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Discussion Starter #15
Oh boy! It's working :)

I found this text somewhere here in the forum
"All the VN700's had a 135 main jet as standard. The 86 VN750 had a 110 main jet for US & Canada & general market models. Only exception was the German & Swedish market models which had a 108 main jet. The pilot jets & starter jets are the same on all models"

Then I remembered that for Honda CX500 the ones sold in Finland are the "General market models".
So I guess this is too.

I changed the main jets back to #110. Took someting like 20 minutes to change them. I don't know if there are some differences in carbs between US/Canada and general market models but with #110 main jets it's working good now :)
I did not change back those original needles. I change them in next summer if it does not work good when driving.

I want to thank you all in this forum who had similar kind of symptons. I have tackled all those possible issues one by one.
Because of this now I have much more knowledge of this bike model :smile2:

I also want to give special thanks for those who have been helping me with their ideas and comments. It's always great to find forums where people are willing to help each other no matter which country or continent they are from.

Thank you all :grin2: I hope one day this topic helps someone else too.

Finally I want to show you the result
 

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Had no idea there was that much variation in stock jetting.

Now you won't have to rev it in neutral any more :) .
 

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Discussion Starter #17
I felt so bad to rev it on neutral. But I always let it idle first few minutes. I even had to apologise my neighbor for this revving. He is really old man and does not like motorcycle sound at all and there am I revving this one many nights :grin2:
 
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