1986 VFR750 Problems

Hi, I own a 1986 VFR750. Lots of fun and sounds awesome, however it has not been without it’s problems.

Problem #1

After I bought the bike it would start fine, however after riding for 5 or so minutes then engine would stop as if it was running out of fuel. I would wait 2-3 minutes and bike would start and have no further issues all day. It would mostly do this when it as wet or a cold humid day.

Problem #2

Burning voltage regulators, blowing up batteries and causing fuse holders to melt.

Problem #3

After a carburetor rebuild the bike would rev to 5000-6000rpm then the engine would break down. I’d apply the choke and the bike would run fine revving to redline no worries.

Problem #4

Starting the bike when cold with full choke, the bike runs at high idle as it should for 30 seconds, then starts to slow down until it stalls. Once warm the bike runs fine. Catch it before it stalls and give it a few revs, back up to high idle then slows and stalls.


Problem #1 was getting progressively worse, I went to a local mechanic and we concluded that it could be carbi ice, as it happened on wet, foggy, cold, or humid days. I did a carburetor rebuild to as the needles were worn, and they needed a birthday anyway, and I thought it couldn’t hurt.

After I rebuilt the carburetors problem #2 started. Now I had 2 problems. I pulled the carbs off and apart 5 times one Saturday and cleaned a rechecked everything, but it did not fix any of the problems.

In the end the bike would stop every 2-3 km and would not rev past 5000rpm unless the choke was on. So frustrating.

I did some research on forums, but no one was having these problems…except the voltage regulators.

Problem #1. I did read some threads about the fuel stop relay with bikes not starting, so I thought I would start here. I bypassed the fuel stop relay, and the bike would no longer stop every few KM, however it still would not rev past 5000rpm. The fuel stop relay was getting hot and failing.

Problem #2 fixed. I bought a genuine SHINDEGEN MOSFET voltage regulator/rectifier ($240AUD) and mounted it above the rear foot peg to get better air flow. Fixed the problem.

Problem #3 I spoke to a mate of mine, who just happened to work on these bikes back in the 80s, about problem #3. He said, “It sounds like an ignition problem. The spark it too weak to ignite the lean mixture at high rpm. Pull the choke on and it richens the mixture and the weak spark could ignite the richer mixture.” I disconnected all the ignition system and CDI plugs and cleaned them with contact cleaner. I then connected all the plugs, and hey presto, the bike would rev past 5000rpm. It was just a coincidence that after I did a carbi rebuild the ignition problem started. I may have dislodged a plug when taking the carbies off, who knows.

Now problem #4 has started. Pulled carbies off again and cleaned choke circuit, choke plungers, and everything else, bench balanced the cabs and installed them with no change to problem #4

Has anyone had a similar or the same issue as problem #4?

 

This is just a knee jerk observation...

Are you sure you need "full choke"? Typically when I start mine, I start at full and dial it back pretty soon so it's not revving to the moon. Maybe you are too rich with full choke to let it run on its own.

My bikes all run bigger pilots and can come off of choke completely pretty soon. If I Iet it go too long on choke, it burbles down and dies.

 

dija confirm that choke circuit pickup tubes were clean and that you could poke a .012" steel wire in to a depth of about 35mm ?



you can't be sure idle jets are clear until you put a .010" steel wire through them.




you need to double check the float level settings too and make sure the small filters just above the float needle seats are clean.

 

Yeh, never mess with float level height if you know what good fer yah! kidding aside, there is a measurement in your manual for taking this, there is a slight tilt and its either taken on the body of the carburetor or the gasket material. When I have worked on other peoples problems, I would check this - My personal bikes except the 2 stroke, all have stock Honda jetting and require choke just to start cold, once started, you can literally thumb off enricher 20 seconds and bike will idle ok, I still warm them up a minute/minute and half before snicking into gear. Kawasaki no choke and idles perfectly but use it sometimes when it has not been ridden in a few weeks just to aid starter clutch ease.

Make sure your slides are going up by blowing air into your air intake, I could tell you where it was on Gen3 bikes, but not on yours, its a t-fitting close to the over flow t fitting.

 

My '86 wouldn't run over 5000 RPM either. That's why it got pushed to the back of the project list. I never thought about the CDI contacts......... that sounds like a good place to start.

 

Thank you for all your input and sorry for not replying.
I have just revisited the choke problem. I looked under the tank and could see a kinked fuel hose. I re-routed the fuel lines to remove kink. Still no change. Removed Carbs and checked. They were all good. Then I thought back to My son's CRF250X and how it was hard to start then didn't start. It was because the exhaust valves clearances had closed up due to the titanium valves wearing. So I thought while everything was off I would check the valve clearances on the VFR. The clearances were all tight. I reset the clearances and away it went