84 vf500f running on 3 cylinders

Hey Guys I am new here so please be gentle

My vf500f was put back on the road after a 10 year rest. Had to clean the gum out of the carbs but other than that she has run well for the last 12 months.

I run premium to keep the carbs as clean as possible but have run into a problem.

No2 has developed a distinct miss at idle and appears to fail completely under load, thus three cylinders.

These are the steps I have taken so far but have run out of ideas.

I pulled the carbs down and cleaned the small amount of buildup from the bowls, checked all jets were clean but did not give all passages a blow through (possible oversight :redface

I have checked the compression and it is at 160psi on all cylinders

I have fitted new plugs and checked for good spark at all plugs, I even swapped the coils between front and rear to confirm the problem stays with number 2.

I am a little loathed to pull the carbies again but if that is all that I have left I will happily strip them and check every passage to try and find the problem.

If anyone has any ideas I am open to comments. I love ridding this little bike back and forth to work, it makes the trip so much fun.

Thanks

 

Check valve clearances ?

Did all the plugs you replaced look alike or was there anything different on that #2 ?

 

Well I had actually ridden the bike twice to try and figure the problem through so the plug on #2 was wet and black, no oil deposits or carbon build up like I expected for a worn out cylinder.

I did not mention this before but she has 97000 KM on it (not good old miles) and most of those were hard with a previous owner, so I dont really know how much is left on the motor.

But the compression is good, she doe not use any oil and before this had been a relative stable bike.:smile:

I am going to order the carbie insulators this week and give the carbies a more thorough going over. I will let you know if it makes any differance.

I was wondering if it could be the ignition pickup, but I dont have access to a spare set to test this.

 

Just take a inductive timing light and clip it on ANY of the spark plug wires and you will know if it is firing. EASY!!

I would bet money that the idle jets are clogged up. You have to take them out and make sure you can see daylight through them. A VERY common problem that will NOT be cured by using a spray can of anything. You have to take them out and clean them.
DO NOT USE a paper clip to unclog them. That will ruin them. They have very small holes in them. Just soak them in carb cleaner overnight and then use compressed air on them. Take precautions to protect your eyes.

 

Yes....I caught that too, but the hardest thing for any engine to do is to idle. That would be the real test of any electrical system. To idle smoothly. Just a hunch.

Almost never see a main jet clogged up except on lawn mower engines.

 

Hey Guys

To get the bike to run originally I had to pull the carbies down and soak in carb cleaner as suggested, this was 12 months ago though, I did not do this when I had them off last but I did take the idle jets out and make sure I could see through them.

I could be they have some build up that needs the overnight soak again, the thing that throws me is that at idle it is at least trying to fire, missing every second or third firing, however take it off idle say 2K RPM with no load and it seems to run better, if I then take her for a run the cylinder sounds decidedly dead and the bike is definitely down on power.

The timing light is a good idea that I have used before and just did not think of- thanks for the reminder Donald.:wink:

If the light does not show anything I will wait for the insulators to come in and pull the carbies all the way down for a soak and clean.

I will also drop the tappet covers and check for any suspect valve springs, I would really hate to put a valve through a piston as parts for this vehicle in Australia and near impossible to come by.

Thanks for the help guys, I will let you know how I get on.

 

When you say insulators which part are you talking about? Are you talking about the little metal fuse looking thing in the spark plug cap? If so, that is to insulate radios from noise (static). Those are condensers. About 5-6 ohms .

Check this out:
http://vfrworld.com/forums/mechanics-garage/29212-making-new-plug-wires.html

 

sounds like a plugged pilot circuit. there's more to it than just the pilot jet. the carbs and jets should be thrown into a vat and soaked to ensure a complete cleaning.

 

if the circuit is plugged with rust or some sort of debri how is the cylinder going to get fuel? Is no. 2 plug fouled? the bike will idle and run poorly with 1 cylinder dead but it will run and you can rev it. Just like having a fouled plug. With a bad vacum diaphram it will idle as well but opening the throttle will cause it to stumble or stall unless you rev the shit out of it. then the engine vacum will eventually pull the slide up. i know this first hand had a VF1000R did the same thing. would idle on all 4 but twisting the throttle just a little bit would stall it out. Twist the throttle hard and it would pick it up and run good. Had a torn diaphram.

 

A torn vacuum diaphram will work but it is very irratic and depends on the size of the tear or hole.
Question? Did the clamp on DC timing light show that all four plugs are firing???

 

The first 1/4 throttle is controlled by the air pilot screw and pilot/idle jet.

It will still run well midrange on the needle and slide, and the upper end on the main jet.

 

Hi Guys

OK just got home from work and dug out the old timing light, the spark on th rear cylinders is nice and constant, the spark on the font appeared to be iratic in comparison, so as with all motor cycles i disconnected all the connectors in the ignition circuit and gave them a clean. Because of where they are located they get a lot of road and chain grim in that area so they did need a clean,

The result is a spark pattern that is a lot more stable.

However the miss is still there and it does not seem to correspond to the occasional miss fire in the spark, I will probably have to see if I can get hold of a replacement set of cdi's but that is not easy over here.

Anyway the suggestions on the carbie slides is a good one, but it will have to wait until the weekend as the neighbors get up set with me running the bike up and down the revs past 7 at night.

Any way thanks for the input and I will let you know how I get on with the rest of your suggestions.

 

The timing light shows a steady flashing with no missing on the rear cylinders.

The timing light shows that the spark is not consistent on the front cylinders.

The spark has to be steady with no missing no matter what the carb is doing.
If the front two cylinders get the spark from the same coil then I would test that coil AND the ends that plug into that coil .
You can unscrew the fitting at the coil and remove the coil wire and clip off about 1/4 inch of the cable and re insert the cable. This often helps because the ends of the cable get burned and degraded.

As far as the CDI they hardly ever fail. They are all electronic. If they get over heated it can get damaged .

So first I would check the coil for the front cylinders.

Remember the timing light has to show a steady flashing. This problem has to be solved first.

One more simple quick test.
When it is completely dark - lights in the garage off. Start the bike and look for sparks coming from the plug wires to the frame or other metal parts of the bike to make sure the spark is not grounding due to the plug wires covering being degraded.

And one more thing ....While you are looking at the coil, make sure the two 12 volt wires going to the coil have a tight clean connection.

 

If you go back and read post #8 you get a good clue.

You say if you try to accelerate the engine it sounds like it is missing.
Then you say if you let it idle it is better.

To me this still sounds like a clogged idle jet. Very common.

This bike sat for 10 years!!!
Every time I hear this story on this forum it is always the same problem.

And do not forget that the fuel filter and gas tank can be full of junk.

No one wants to get to the carbs and clean the idle jet.
It is difficult on these bikes. That is not your fault.

Every one wants to find almost any other reason - like some small plug and play part to change to make things better.