So I have this problem with my 86 vf500f. It runs fine but I can't go over 65mph. I dont think its anything about the speed, just the rpm. When I get up to that its at 5000 rpm and giving it more gas makes it buck and surge a little but doesn't give me anymore power. Its a lot like the fuel is not getting to the engine fast enough. The vacumn and fuel lines aren't kinked at all but when I'm at speed, removing the vacumn line doesn't change anything at all.. so maybe its that? Does the 86 have a fuel pump? I don't think so but I could be wrong Maybe you all can set me right on this but here is what I might be wrong 1 Carbs out of sync? 2 Fuel vacumn line problem? 3 Fuel pump? I haven't done a huge amount of work here but I was just wondering if Im missing something.. Thanks!
If your bike has 100,000 km then the problem could be the carby needles and needle jets (sometimes called atomizers or emulsion tubes). The needle is about 50 - 60 mm long and tapered too a point at it's lower end. Under normal riding conditions it will wear in the middle section gradually becoming thinner and therefore allowing more fuel than the engine requires at your usual cruise speed. The needle jet will also have ovalised and therefore the engine will run slightly rich right thru the entire rev range. Major symptom is slowly declining fuel consumption and hard to get passed 60 - 70 mph. Try coaxing the bike into a higher speed / rev range by using the gears or down hill run. If you can easily maintain 80 - 100 mph on a level road once up there then this is problem solved. The tip of the needle would never get the use of the middle portion. This problem eventuated when lead was removed from fuel. I've done Ducati's @ 40,000km and ZZR 250 @ 95,000km. Bigger induction pulses suck the needle harder against the inlet side of the jet causing accelerated wear. Use upper cylinder lube once repaired, or buy a new bike. In Australia the basic parts can cost $100 per carby.
Dejapong, I missed your original comment on removing the vacum line has no effect. As this would allow air to be sucked directly into a single inlet manifold it should at least cause one cylinder to lean out, unless you blocked it off. Can you start and ride the bike with vac line removed and plugged. What about setting the tap to "prime"? Have you tried my ride test yet? Let me know
hey waynej, I've been working on it and this is what I've come up with so far. The previous owner had obviously worked on the bike, The petcock valve would let fuel out in the on position. I took it apart and found it was missing a spring that closed the vacumn diaphram and the diaphram was on incorrectly. I fixed that and now the engine will eventually shut off when the vacumn is removed. That changes nothing with the performance. I tried what you said going downhill, The thing is, It will go only up to 70 mph and then stop accelerating. If I pull in the clutch I can rev it further but once I release it, It won't go. As I go uphill, the speed drops to 55mph and It just doesn't have enough power to go. Pulling and releasing the choke does nothing. The pilot jet valves have all been readjusted by the previous owner so that could have something to do with it maybe? The brakes are not dragging and the chain is fine. I need to check compression, I guess, but the spark is fine. How about a vacumn leak? The carb boot's look good but then you never know. Thanks for your help! Deja
Quick and easy check for vacumn leaks between carbs and engine. Use any spray can of WD40,CRC, Selleys RP7 or spray degreaser. With the engine idling spray all around the inlet manifold rubbers and joins etc, if the engine suddenly dies or increases its speed, it indicates there is a vacumn leak and the spray has been sucked into the engine. If you use a combustable spray the revs will increase, and a non-combustable spray will cause the motor to stall.
I finally found out what was wrong with it. The guy who took it apart before me forgot to put one of the main jets in! I guess I was running on 3 cylinders the whole time. I put the jet back in and its running a lot better now.
