1990 model running rough intermittently.

[R89B]

Hi all, I've been a long time lurker on the forum but this is my first post. 

 

I've got a 1990 model VFR750 that has a bit of an odd issue. 

 

Back last year I was out for a ride and about an hour in it started running really rough, almost like a misfire. A few weeks later I gave it a full service making sure I checked the spark etc and everything seemed ok. 

 

After the service it run really well and I've done several rides since lasting up to an hour each and they all went well. 

 

This week I went out for a longer ride and after 90 mins the problem came back. It struggles to rev under load, mainly between idle and 5k rpm and then slightly better above this. 

 

I can let the bike go cold and try again and the problem is still there. 

 

My first thought was an issue with the carbs but in all honestly I don't really know. 

 

Any help would be much appreciated. 

 

Ps. To top it off, the ignition switch also failed on the last ride so I had to do a temp fix to bypass the ignition switch. 

[jefferson]

Might check and see if there is water in your fuel. Did it run the same after the ignition switch bypass?

[R89B]

Hi.

 

Thanks for the reply. 

 

I'll have to check about water in the fuel. It's on its second tank of petrol since my original issue a few months ago but I'll pull the line on the fuel tap and drain some off. 

 

Yeah it ran exactly the same unfortunately. I first thought it might have been the switch causing the issue but no such luck. 

 

 

[Thumbs]

I’d drain the float bowls and see what comes out .... sticking float?

[R89B]

Ok thanks. I'll have to dig out the manual and see what I can do. 

 

The bike has probably only done a 1000 miles in the last 5 years so it could be all gunky etc in the fuel system. 

 

Thanks for your advice. 

[DannoXYZ]

vacuum leak

[R89B]

Thanks for the reply. I'll spend sometime working through everything this week. 

[R89B]

Hi all. I've only just had a chance to look at the bike and after working through the flow chart in the Honda service manual we diagnosed it to the Spark Plug caps. The coils etc all seem to be operating correctly but the rear 2 plug caps failed the resistance test with nothing passing through. 

 

I've just ordered 4 new caps and will also replace the leads. Fingers crossed. 

 

Thanks for all your help.