93 VFR750 RC36 Posted August 4, 2012 Share Posted August 4, 2012 All, I got my bike back from having the valve stem seals replaced and cam shims checked. It went away running like a top and pulling like a train (99.5hp at the rear wheel). It came back not idling properly. Hangs up when rolling off the throttle and takes a couple of seconds to settle back down. Though maybe I had a float valve leaking so stripped the carbs. Red Herring, nothing to be found. Checked carb balance and found the front carbs are completely out of synch with the rear. Tried to adjust them and ran out of adjustment. I've never run into this before but the rear butterflies are closing and preventing the fronts from closing fully. On the carb tune, the front two are balanced within 1cm/Hg. The right rear is 14 cm/Hg higher and the left rear is 10 cm/Hg up. Effectively, the front throttles aren't closing completely and I'm completely lost as I've wound the two rear adjustment screws right out and run out of adjustment. What have I done wrong? What could have gone wrong when it was apart in the shop? Bent linkage between front/rear? I've only touched the adjustment screws on the linkage. 1 screw balances the front right carb with the front left (master with throttle cable). A link arm goes from the front left to the rear and both rear carbs have an adjustment screw to synch with the master. Confused and awaiting some wisdom. Stew Link to comment Share on other sites More sharing options...
Member Contributer YoshiHNS Posted August 4, 2012 Member Contributer Share Posted August 4, 2012 Just have to keep playing with it. Maybe take them off and bench sync them, then toss them back on the bike and finish the sync. Don't forget to use the idle adjustment if you need it. Can't really offer any good solid advice. Link to comment Share on other sites More sharing options...
93 VFR750 RC36 Posted August 4, 2012 Author Share Posted August 4, 2012 Way ahead of you. Did the bench sync or thought so anyway. Adjustment seemed strange so I pulled the adjustment screws and cleaned/polished the linkages to make sure there was no build-up of gunk in the way. Dropped them back in on the assumption that I'd missed something and "it was OK last time so a quick adjustment should work". I can wind the idle adjustment off to leave a gap at the stop with the rear throttles closed and the fronts are still a bit open. Link to comment Share on other sites More sharing options...
NakedViffer Posted August 4, 2012 Share Posted August 4, 2012 I would pull them off and go over them looking at the manual or a diagram to double check and make sure you aren't missing any pieces in the linkage. After that you should put all the adjustments back in the middle and make sure your idle adjustment is all the way backed out. Use a light on the opposite side of the carb bore to see how well that plates are closing, and how much they are opened once you start adjusting them. From that point, if nothing is missing or bent, you should be able to get them back into sync. Link to comment Share on other sites More sharing options...
Member Contributer COS_VFR Posted August 4, 2012 Member Contributer Share Posted August 4, 2012 How confident are you thar fhe stem seals and shim check were done correctly? Link to comment Share on other sites More sharing options...
93 VFR750 RC36 Posted August 4, 2012 Author Share Posted August 4, 2012 COS_VFR, Fairly confident. The fact that I can't get the front throttle butterflies to close fully leads me to think the problem is in the mechanical linkage on the carbs, not the beast below. Naked, I do believe that I'll have to follow that line. Carbs have been out twice in 36 hrs, why not once more? Stew Link to comment Share on other sites More sharing options...
samaki Posted August 5, 2012 Share Posted August 5, 2012 When properly synched the butterflies do not close fully anyway. It sounds like two things are possible that likely happened while in for the valve adjustment: 1) you have an intake manifold leak somewhere between the carb and the head. 2) The valves on one or more cylinders have been adjusted too tight as this will cause the idle to hang like you are describing. Link to comment Share on other sites More sharing options...
93 VFR750 RC36 Posted August 5, 2012 Author Share Posted August 5, 2012 I'm winding off the idle stop completely to be able to bench sync the carbs as closely as possible, hence the full closure. The carb to head rubbers were pretty hard ('93 era, original vintage) so I've ordered a new set. I'm going to put in RC36a style permanent vacuum port adapters (brass) and put in the new rubbers before I try again. I've found the linkage between the front and rear carbs has taken a few knocks over the years but I've fettled the clearances on the adjustment screw tabs with a dremel and I've now got all the carbs opening at the same time on the bench. Wait a couple of days, I'll report back. Link to comment Share on other sites More sharing options...
93 VFR750 RC36 Posted August 22, 2012 Author Share Posted August 22, 2012 Problem solved. I fitted new carb rubbers, balanced the carbs to within 2 mm.Hg and 'presto' the smooth howl of a V4 returns. Idle good, not hanging up and throttle response is once again sharp. Vacuum leaks . . . what a pain! Link to comment Share on other sites More sharing options...
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