Topic: '75, CB750F - Increasing Vibrations at Speed - Issue or Just Normal?

[RSchaefer]

So rebuilt top end with no bore - but - ALL of HondaMan's low cost upgrades per his book.  Specifically reworking the intake ports, adding a bowl in the intake just under the valve and removing obstructions in the combustion chambers.  All of this with consultations & pictures sent/received with Mark.

New wheel bearings, All Balls triple clamp, trued wheels, "o" ring chain, etc., etc., etc.

I was always concerned that I may have changed the volume of one intake/combustion chamber over another during this process but that somehow carb adjustment would mitigate any differences?

Anyway I have 150+ miles on her and she seems to run great, starts right up, idles smooth at 1,200 RPM.  Little noise from the drive train but I'm thinking that's normal for a 45 year old engine.  Pulls hard past 80 MPH at 7K RPM.  Have not redlined her or max speed yet.  Mostly reminds me of the '70, CB750 I drove in the Air Force back in the day.

Vibrations - so maybe its me having spent the last 10-years on my GL1800 wing, but it seems like maybe I could make some kind of adjustments to the carb's to reduce the increasing vibrations at speed?  Or maybe it is what it is!

Mirrors start to become useless at 50+ MPH and generally the vibrations increase with speed, not major vibrations, just wondering if that's normal or unbalanced carb's would contribute to the vibrations?  I seem to remember the solid handle bar mounted mirrors no longer being effective at speed from my youth.

I have the old world mercury carb sticks and adjusted for nearly identical vacuum at idle and would then bump the throttle to see the mercury pick up the same in all 4 cylinders.  Didn't hold it at say 3K RPM and attempt to balance, not sure if that's even possible.  Maybe the vibrations will reduce somewhat as she breaks in.

So wondering what everyone thinks, possibly someone who has done HondaMan's intake and chamber mods?

[ekpent]

 Do not really have a lot to offer for suggestions but I have several 75-76 750F's and all of them have good clear mirrors no matter what speed or rpm I am riding at. The mirrors are stock Honda, are yours also ? Do not notice much vibration in the foot pegs and hand grips either.

[RSchaefer]

Yeah so I may have an issue.  Yes they are the stock Honda mirrors!

[jaytee-nz]

I noticed a vibration increase on my '74 CB750 a while ago - turned out that some of my engine mount bolts were loose.
Tightened them up and I could see out my mirrors again !

[RSchaefer]

Not motor mounts, I have the doubled up mounts Mark recommended and they are tight

[BenelliSEI]

Have to ask the obvious: tires are centred on the rims and really well balanced? My ‘75 750F (SOLD) was incredibly smooth at any speed!

[Jerry Rxman Griffin aka MuthaF'er]

Mine only slightly blurs the mirrors with some speed. Carbs synched, balanced crank

[seanbarney41]

My 75f is about the smiothest bike i have ever ridden, gets even smoother at about 85mph

[RSchaefer]

I trued the rims and had a mechanic do the balance with new tires so I think I'm good there.

I went back to HondaMan's section in the book regarding removing rough areas in the intakes and he does start with a caution regarding removing more material in one intake verses the other and this could lead to vibrations so this might be the issue, that said I tried to be consistent.

I have noticed a hanging idle issue, not real bad compared to some other members videos.  So maybe its a carb sync issue.  I'm also up at 6,300 FT in the summer so maybe altitude is playing an issue.

I'm going to go back and look at everything again, ignition timing, throttle cable routing, and most importantly carb sync.  HondaMan says in his book that you will have a difficult (maybe means impossible) time sync'ing the carbs if you have created differences in the intake capacities due to porting.

Will see!

[Primus]

I get a buzzing feel in the footpegs and bar grips on my 750K7 at motorway speeds, but everything feels smooth after riding a Harley where the mirrors are a waste of time!  As for my BSA, well after 40mph all my fillings fall out.

Paul

[RSchaefer]

So I went back and checked lifter clearances and I seemed to be maybe .001 or so loose and I readjusted all to .002 for intake and .003 for exhaust.  Don't think that would have anything to do with the vibrations.

I checked my points gap with a dwell meter and appear to be right on, I'm using Mark's electronic ignition.

I had an old cheap strobe from like '63 that I really can't see anymore at my age and bought a new Xeon battery powered unit that I can see clearly without making the room dark.

In checking 1-4 my "F" mark was off maybe 3 to 4 degrees, passed the "F" not between the "F" & "T" so I think that's firing early?  In trying to move my TEK plate I'm unable to rotate it enough to get the "F" under the mark, maybe 1 to 2 degrees away now.  Funny the TEK plate has a wider arc at the top screw for adjustment but the arc's are less on the two lower screws.  I've used a file/grinder and made the arc's wider on the two lower screws to pickup the adjustment of the upper arc.

I have the "F" model CB750, not the "K" and in talking with Mark he recommended an adjustable cam sprocket (or drill the stock one) and as I remember it, advance the cam 5 degrees, which I did.  Not sure if this would then effect the adjustability of the TEK plate.  Anyway I will be able to get there now.

After getting everything put back together and warming up the engine I attempted to do air screw adjustments to dial in the carb's but am not really "hearing" the changes in the engine much.  I also noted that I should do the carb sync 1st, before attempting air screw changes.  I think the carb sync process will show if there is a problem with me having thrown off the intake volumes with my port modifications per Mark's book.  I did the carb sync a few years ago when I 1st got it running but I may have not been as accurate as I should have been.  Will be doing the carb sync tomorrow.