Hi guys, new addition to VFRWorld so go easy.....but I do need some help please! Recently my 91 VFR750 has been getting 'stuck' at the top end of teh revs. After about 7krpm the bike doesn't want to go any higher, you can feel it struggling and every now and then it will suddenyl burst through and on to the red line. Any ideas what could be wrong? I'm guessing its either fuel or air and haven't done any diagnostics yet. I have recently had to change the starter clutch, could I have put something back wrong to cause this? Thanks for any help you can offer
I would first check the air filter for obstruction, costs you nothing but time. Then Spark plugs and connections. Then fuel filter (if there is one, I don't know)and after that it would be time to pull the carbs and check for debris in the float bowls. Has the bike sat for any period of time? It could be that the main jets need cleaning or replacement. I have never had that generation of VFR but I have brought plenty of stumbling motorcycles back from the brink.
Sounds like varnish on the main jet needles or a pinhole/tear in the rubber diaphragms of the main needle slides located in the vacuum chamber. Both can be inspected without removing the carburators when you go in to look at the air filter. Push the slide up with a bamboo chop stick from your favorite chinese take out, then inspect for varnish on the needle. Then push up the slide and release and see if the slide returns quickly or in a damped manner as on a cushion of air. The too quick return is an indication of possible tear or hole in the rubbers. Do a search here on 'cleaning carbs' for a good pictoral on the process.
........"getting 'stuck' at the top end of teh revs. After about 7krpm the bike doesn't want to go any higher, you can feel it struggling and every now and then it will suddenly burst through and on to the red line." Misfire in one cylinder. Pull plugs check sparks and for bad color showing as either white or black, clean and regap plugs, install, try again. If that don't fixit, clean carbs until it runs right, starting first by cleaning idle and main jets, which fixes most problems fairly easily and relatively quickly. Drain float bowls into a paper towel checking for red specks or dark goo. Inspect tank for any sign of rust. When you first start you'll need a well-charged battery, choke on according to local temps, start, then begin carefully touching exhaust headers checking heat. Cold pipe isn't firing, find out why. If it won't idle without choke after about 1 /1/2 min, carbs may need cleaning. Like KB sez look at slides moving up equally while engine revs increase (with air filter off). Got another bike to ride ?? We don't know how long you've owned the bike or the mileage ?? For any first-time poster :welcome: this may prove a helpful reminder and useful disclaimer: CAUTION :TOP END ON THIS BIKE IS NOT SOMETHING FOR NEWER RIDERS TO PLAY WITH for more than about 3 or 4 seconds, ever imho. Pulls like mad in any lower gear and still has awesome pull at 125 with no hesitation to keep climbing if you're so inclined. Cruisin along at about 120 years ago on an interstate @ just about 1/2 throttle, i remember throwing caution to the winds and openerup full throttle. That moment had got to be the engine's biggest test ever and a great moment of Hondaexcellence. Had no desire to go faster, just wanted to feel that dizzying PULL. Treat the bike easy for awhile before attempting to find out what it (or you) can do. Unless you're already an experienced rider, stay away from that TOP END !!! Between about 3500-7000 rpm is the best cruising speed for the V4.
