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Discussion Starter #1
I have had problems since last summer with a poor running(if it even starts) VN750 -86.
After some suggestions from this forum I took the Carbs apart and cleaned everything but it didn't help.
The last suggestion I got was to change the float valve, which I have been planning to do.
Now I have the oppurtunity to buy a complete carb assembly from a scrapped VN750 from 1993, so I'm thinking why take my apart again if I can buy a newer one and hopefully get rid of my problems?:rockon:

The question is: Will a carb assembly from a 1993 fit my 1986 VN without mods?
My petcock has PRI-ON-RES if that affect the carbs in any way.
The bike is probably American from the beginning and has since then moved to Belgium and now finally Sweden.

/Tony
 

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The carb assembly part numbers are different at bikebandit.com, but I have no personal knowledge about interchangeability. Those more familiar will probably chime in.

Do you know how the carbs were performing on the other bike, and how long since it ran? Carb problems are very common on bikes that haven't run for a while. Those carbs may need to be cleaned too! Too bad they don't sell Sea Foam in Sweden. That might solve your problem without removing the carbs for cleaning. Have you tried using a locally available carb cleaner that can be added to your fuel? If you haven't, I'd try that first; just follow the instructions on this site about adding Sea Foam to the carbs and fuel. Ask around about what is the best product available to you.
 

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Discussion Starter #3
The carbs haven't changed!

I have finally had the time to change carbs on my bike.
I can now say that a carb from a 1993 VN750 fits a 1986 VN750 without any problems! So as a info to anyone that of some reason needs to change their carb, it seems as they should be interchangeable between the different modelyears.
 

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Discussion Starter #5
It seems like it solved the problem.
I haven't been able to ride it yet, still to much sand on the roads here.

After changing the carbs and the battery I started it and the engine runs better than before, no backfire and misfire.
 

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The Professor
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Good to hear you got her fixed, carbs haven't changed since first produced! :beerchug:
 

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Linkmeister Supreme
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It`s good to hear you got her up and running good again. Did you just get the bike last summer? The first post sounds like you have been riding for some time and the bike then started acting up. It would be interesting to know why you didn`t find anything wrong with the original carbs when you pulled and cleaned them. Enjoy the new riding season and stay safe.
 

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Discussion Starter #8
Problem not solved

It still doesn't work!!! :doh:

It works at idle but as soon as I turn the handle it dies..
I had a nice 15min walk home when I took a test ride the other day..

I have started to suspect the RR or alternator but don't know what is supposed to be normal.
When the bike idles it reads 13.5v on the battery but I can hear it's not idling alright. Then from time to time it "jumps up" to a higher rpm(3000rpm) and runs ok, then it's around 14v instead.
What is a correct volatage for a idling bike?

OR is it maybe the "new" carbs I put on that are stuck somehow, they have been lying around totally dry for a while before I put them on.

It feel like the bike gets closer and closer to a dealer or the scrapyard...

HELP!


I got the bike 4 years ago for my wife.
It has been OK for a while but started acting strange when we came home from the summer vacation last year.
 

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Simple Solutions
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when the bike died the and u had to walk home was the bat dead? what makes u think its fuel? do you have spark ? how are the plugs and wires? air filters ? just trying to get info, where did your carbs come from? was the 93 from the same elevation ? or did the old bike have an ear shave? did u happen to run her out of gas or lower then "normal"
bad gas caused my bike to die on me once i blew out all the passages and and she fired up ... im thinking your on the right track with fuel but what about the other variables the screen on your petcock is it clean, was there rust in your tank or in the carbs that were clear when u fired it up but when the rust re settled it got pumped into a jet. If she fires try this try pulling a boot off and rev the engine to 3-4k and plug the carb (with your hand) for a half a second. Seafoam (http://www.seafoamsales.com/motorTuneUpTechGas.htm) i dont know what the equivalent is there but there has to be something like it ...can you identify if your issues are both cylinders or just one?
 

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The Professor
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Normal voltage is 12.5, your regulator is working so that is not the problem. I also believe you problem lies with fuel valve "petcock/filter assembly" since you have the same problem with two different carbs. It could also be a clogged fuel cap vent.

Unlock and open the fuel cap with the bike on the center stand. Start the engine and let run for a few minutes, twist the throttle and see if there is any change. If not then proceed below.

Did you drain the tank and put in fresh fuel?
Is the carb vent hose clear and venting to atmosphere?
Have you replaced the vacuum line going to the fuel tap?
Is there fuel leaking from the fuel tap into the vacuum line?
What position do you have the fuel tap lever set on?

If the vacuum diaphram in the fuel tap is bad it will allow little to no fuel to flow with the lever in the "ON" or "RES" position. Open the drains on the bottom of both carbs to see if you have fuel and if it is contaminated.

Set the fuel tap lever to "PRI" which is prime and it will bypass the vacuum diaphram and let fuel flow into the carbs. With Carb drains open and lever on "PRI" fuel should flow until you switch lever to "ON" and carb bowls drain. If fuel does flow you have a problem with the fuel tap "filter/petcock assembly"
 

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Discussion Starter #11
I drained the tank and cleaned the filter on the petcock before changing the carbs. It has fresh gas. As far as I can see it behaves the same no matter what position I put the fuel tap in. PRI-ON-RES is all the same.

Shouldn't there be any problems with idling if there were problems with the petcock/filter? It seems to be able to idle for a long time without any problems.

I have a theory, if one or both of the vaccumpistons in the carbs are stuck could this cause my problems?

The battery wasn't completely dead when I had to walk it home.
The carb assembly looked identical.
 

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The Professor
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I drained the tank and cleaned the filter on the petcock before changing the carbs. It has fresh gas. As far as I can see it behaves the same no matter what position I put the fuel tap in. PRI-ON-RES is all the same.

Shouldn't there be any problems with idling if there were problems with the petcock/filter? It seems to be able to idle for a long time without any problems.

I have a theory, if one or both of the vaccumpistons in the carbs are stuck could this cause my problems?

The battery wasn't completely dead when I had to walk it home.
The carb assembly looked identical.
The petcock could be only letting a small amount of fuel to pass and the bike will idle with no problems, as soon as you twist the throttle it will starve for fuel.

Let's go to the battery for a minute. You said it was not completely dead, this is not good. a battery with a weak or bad cell will cause charging and ignition problems.

Do you have a standard wet cell battery or an AGM MF battery? How old is it?
If it is a standard wet cell close to year old it is probably giong bad.
You may just need to replace the battery with an AGM MF battery.

A weak battery will also let the bike idle with no problems but as soon as you twist the throttle the ignition breaks down because there is not enough volts to feed the coils.
Our ignition sysyems run off the battery and the stator and regulator/rectifier charge the battery.
Our bikes will run with no problems without a stator or R/R untill the battery is almost dead. You could probably go 100-120km on a good fully chared battery.

Have the battery load tested and or replace.
Check the fuel cap vent.
Check carb vent hose.
Drain carbs and set fuel tap to PRI and see if you get good fuel flow.

I doubt if two sets of carbs will have the same problem. Either you are not getting enough fuel or you have a weak or bad battery.
 

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Discussion Starter #13
The battery is a brand new MF battery.
I moved one of the fuelpipes yesterday as it may have obstructed the fuelflow. I also moved the carbvent hose and tested it by plugging it(when I plugged it the engine died).
For a moment, this seemed to help.
I took it for a short testdrive and soon it was back to "normal".
It "chokes" when i try to get it past 3-4krpm and after a while I can't get i past idle, dies a soon as a I turn the handle even a little bit.
Then after a while it won't even idle correctly.
Strange thing is, it seems to be something that comes and goes when it's idling. It runs uneven and then it's like it's firing up as it should and the rpm goes up by 1-2krpm and it runs even for a while. then its back again to running rough.
It may be running a little bit better in PRI mode, but I'm not sure.
 

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Simple Solutions
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The petcock could be only letting a small
Let's go to the battery for a minute. You said it was not completely dead, this is not good. a battery with a weak or bad cell will cause charging and ignition problems....
A weak battery will also let the bike idle with no problems but as soon as you twist the throttle the ignition breaks down because there is not enough volts to feed the coils.
Our ignition sysyems run off the battery and the stator and regulator/rectifier charge the battery....
i want to throw a couple things out there on these statments... i had lost the connection to the frame from my batt and the bike ran completely off the charging system in fact i went close to 200 miles this way.... one i didnt realize it till i set up camp in the middle of no where and tried to start it the next day pushed started it and mad it to my destination .... took a while and found the issue the connection broke from the frame....

and when my batt failed it would crank and not fire... (could smell gas....)

...
Pull the plugs and crank , crank some more can u smell gas and feel compresion comming off the sylinder.... touch the plugs to the frame do u have spark,

I'm thinking its a chunk of rust inside the petcock... or the vac line going to the petcock is being obstructed... get a couple hoses apply vac on the middle line and see if u have flow from both leads off the petcock... (on on and res)

With the engine running pull the vac line off the petcock and plug with thumb see if 1 u feel suction and 2 see if the bike reacts to both operations...


With whats been checked double checkd and reevaluated whats goin on in the air boxes? is there enough air to sustain air fuel mixture ? is there a leaky vac line?(or are both vac lines in place?
 

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The Professor
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The battery is a brand new MF battery.
I moved one of the fuelpipes yesterday as it may have obstructed the fuelflow. I also moved the carbvent hose and tested it by plugging it(when I plugged it the engine died).
For a moment, this seemed to help.
I took it for a short testdrive and soon it was back to "normal".
It "chokes" when i try to get it past 3-4krpm and after a while I can't get i past idle, dies a soon as a I turn the handle even a little bit.
Then after a while it won't even idle correctly.
Strange thing is, it seems to be something that comes and goes when it's idling. It runs uneven and then it's like it's firing up as it should and the rpm goes up by 1-2krpm and it runs even for a while. then its back again to running rough.
It may be running a little bit better in PRI mode, but I'm not sure.
This is good, this is the kind of feedback we need to help correct the problem.

Did you open the fuel cap and see if it made any difference?
Have you checked or replaced the vacuum lines on the carbs?
With the key on test the voltage of the red coil wires.
To rule out the petcock you can make a temporary fuel supply. Remove the fuel tank connect a tee to the fuel lines, connect fuel hose to the tee and a gear oil bottle. Cut a hole in the bottom of the bottle and add some fuel, start the bibi and see how it runs.
 

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I had similar problems with my '89 last year. Turned out to be a vacuum problem. In my case there was a very slight crack in the vacuum line where it bends just before connecting to the carbs. When you replaced the carbs, did you replace the holders/boots? Spray some cleaner on/around the boots while it is running. If it smooths out, they could be cracked or not seated properly. Just out of curiosity, are the carbs jetted the same as your originals? Have you checked/adjusted the pilot screws?
 

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Discussion Starter #18
I think the problem is finally solved..
I took the bike to a mecanic.
The carbs were cleaned and they replaced the jets and now the bike finally runs as it should!

A tip to everyone out there, if you have the money and have tried to solve a problem once to no success, Take it to a profesional!!:doh:

At least they can tell you what is wrong so you don't try to solve problems you don't have, leaving the true source unfixed..
 

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1992 VN 750
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109 Posts
So...what was the final diagnosis? Simply clogged jets and dirty carbs? Were the jets the wrong size? No vacuum issues or petcock problems? Jet sizes in Europe are different than they are in the US for this bike. Thanks!
 

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The Reanimater
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847 Posts
maybe the "new" carbs I put on that are stuck somehow, they have been lying around totally dry for a while before I put them on.
I think the problem is finally solved..
I took the bike to a mecanic.
The carbs were cleaned and they replaced the jets and now the bike finally runs as it should!

A tip to everyone out there, if you have the money and have tried to solve a problem once to no success, Take it to a profesional!!:doh:

At least they can tell you what is wrong so you don't try to solve problems you don't have, leaving the true source unfixed..
Or you could of Cleaned & blew out the Carbs before you put them in.
You just paid a Shop to do what you could of done......fixed it yourself.

Oh and I doubt they changed the jets just blew them out.
 
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