1991 750 struggles to get past 4k rpm

Discussion in 'Mechanics Garage' started by Kevin Andrew, May 22, 2018.

  1. Kevin Andrew

    Kevin Andrew New Member

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    Hi guys, I did some searching but didn't really see anything so hopefully this can get some attention and I can get pointed in the right direction.

    Bought a 1991 Vfr750 and it sat for years with 27k on it. The seller told me it wouldn't run, but would with starting fluid. He was right, so I completely tore down the carbs after watching hours of carb rebuilds. I soaked the lower end of the carbs, put new stock sized jets in, cleaned the needles and passage ways....it looks spotless on the inside.

    I put everything back together and it sprang to life, I was so excited! I took it for a spin, and although it was a little rough around the edges, I got it to hustle pretty effortlessly, granted, it did bog just a little bit, but it sounded beautiful and clear, I even took a vid. But fast forward to the next day, I put new fuel in, charged the battery, and when I went to turn it on to see what needed work for the day, it struggled to start. She hated the choke, and refused to start with any bit of choke, but once I got it warmed up(which was a struggle) I got it to idle around 1100rpm. I took it for a spin, but had much more hesitation and bogging than less than 24 hours before. The longer I rode it, the worse things got. It hardly wanted to idle, struggled to get above 2k in gear, and was backfiring out of the carbs doing anything. Full throttle would hardly get me to 4k in neutral, almost like a limiter, and would usually die as soon as I let the clutch out.

    Any vfr veterans have any idea what my problem might be? Before posting I tore the carbs down again and they're spotless. I cleaned two float valves that seemed to stick just a little more than the others, and examined all of my jets-crystal clear. I have a brand new air filter, two new spark plugs (planning on installing the other two tonight), and no idea what to do. I think I'm going to bypass the fuel pump later, which I don't entirely know how I'll do, but any help or recommendations for anything I might need to do would be greatly appreciated! Thanks guys!

    Just an additional thought-I used gunk to submerge the bottoms of the carbs and then I used carb cleaner to spray it off. Once it started it ran real rough throwing out white smoke, but after a few minutes it ran better and better with no smoke. Today there's still no smoke but runs terribly .
     
    Last edited: May 22, 2018
  2. squirrelman

    squirrelman Member

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    dija remove the 4 idle mixture screws and blast cleaner +compressed air through the holes ?? small filters above the float needle seats were cleaned ??

    main fuel filter clean ?? test that fuel flow from the tank is OK, a good solid flow without dribbling. try running with fuel filler cap open to eliminate a possibly clogged tank vent.

    remove air filter and inspect to check that all vacuum slides rise and fall evenly in response to throttle inputs with engine running.
     
    Last edited: May 22, 2018
  3. Kevin Andrew

    Kevin Andrew New Member

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    You know, I was a little too nervous to do that the first time, but seeing that it doesn't even run, I'll be as careful as can be and take care of those screws tomorrow. As for the filters, I didn't clean them because they were clear of any debris when I looked at them. I'll have to take a closer look at the fuel filter as well, but I think it's ok-we'll see. Once I clear the filter I'll check to see if it runs with the fuel just in the lines...should knock out the vent possibility, but I did try it with my current situation. The needles in the vacuum slides are clean, and feel pretty even in resistance and rebound speed when I moved them with my finger, but I'll keep an eye out next time I fire it up. Thanks for the tips, I'll comment again with the results!
     
  4. Kevin Andrew

    Kevin Andrew New Member

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    So one thing's for sure-the carbs are as clean as a whistle now, and I made sure to count my rotations with my needles. Also checked the fuel filter, and put in two new spark plugs as planned. Still, was unable to turn her on initially. I didn't attempt any starting fluid because I figured that if I had solved the problem, I wouldn't be having such a hard time starting. I watched the fuel filter have fresh fuel pulled in and through it, and it filled quickly. When conducting a spark test on my new plug, it sounded like it wanted to start so I put it back in. Fired right up and idled great but hated the throttle and threw out white smoke like a smoke machine. It didn't smell sweet or smell like burning oil, but burned my lungs when I was in the garage. Any thoughts?
     
    Last edited: May 23, 2018
  5. Brian Rodgers

    Brian Rodgers New Member

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    White smoke - is it sucking radiator fluid into the cylinders? Check radiator fluid level.
    Coils/spark plugs checked out? Curious.....
    Wonder what would happen if you swapped on some carbs from another bike; just to see if it IS the carburetors...
     
  6. squirrelman

    squirrelman Member

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    Nothing since mid-May, so maybe he gave up ?
     
  7. Kevin Andrew

    Kevin Andrew New Member

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    Totally forgot about this forum-my bad. After trial and error I found that it wouldn't go past 4k because the rubber vacuum seals around the needle valves were letting air slip past. After reseating them it worked hundreds of times better. First time working on carbs. Put me through a lot of trouble, but sure learned a lot. Smoke was from incorrect fuel/air ratio. Exhaust smelled a bit like fuel so it was just running rich.

    Currently trying to fix my speedo and odometer. The odometer is counting up miles a lot faster than actual miles traveled, and as I have read around the internet as of recently, the speedos aren't all that accurate anyways-and mine isn't. Any threads or links to getting the odometer to count miles correctly would be greatly appreciated if you know of any.
     
    Last edited: Mar 16, 2019
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