Topic: 78 CB750K Help PLEASE:-(

[750]

So I have talked a bit about this before, and I have rectified a number of problems but I'm still having a small issue.  When I am riding along at a steady throttle position around 4900RPM and up and I open the throttle I get a vary slight hiccup and then it caries on threw the rpms like nothing happened.  If it were human I'd say it's heart skipped a beat.  However, if I'm winding it out from 1100rpms redline it seams to do fine in 1st, 2ND and 3rd gear but it surges a bit in 4Th and 5Th gear starting around 7000rpm to 8000rpm, almost like it's getting too much fuel because if I let off a on the throttle a bit and vary slowly role back on the surge almost disappears.  I have meticulously cleaned the carbs twice with a complete disassemble and chem dip the first time.  I have replaced 1-3 carb boots with new ones.  I couldn't get #4 so I siliconed the old number 4 back on to prevent air leaks and a lean condition.  This made a vary big problem into a vary small problem.  It really helped.  I also jetted up to 117.5  as per many peoples recommendations on from this group.  Other than that, the bike is as bone stock as you can get.  Could this be an electrical problem?  Is this something that electronic ignition would take care of.  I feel like I'm chasing my tail.  I know I'm close but I don't know where to turn next. 
Please help if you can
Thanks.

[KB02]

If the bike is a bone stock as you can get, then why did you re-jet? That might be causing some of the issue (might not, too - I'm just throwing out ideas). I would start by putting the stock jets back in and seeing if that stumble at 7-8 goes away or not.

When you cleaned out the carbs, did you make sure that your pumper jets were working?

[Inigo Montoya]

Yeah there should be no reason to go that high on the mains even with a k&n filter. That could certainly be part of the issue. Easy and short term test would be to remove the filter and ride for a bit and see what happens. If it goes away, you know you are too rich.
The accel pump only affects initial throttle twist, it has no effect higher up.

[HondaMan]

What color are the spark plugs? On the 78 750, they should be white to very light tan when run at hiway speeds. If they are darker than that, and your accel pump is working, you may be drowning it when you're pulling open the throttle that way.

This symptom is common on the earlier 750, especially with an 18T front sprocket and 120/90 rear tire, when running at about 4500 RPM in high gear: pulling open the throttle would produce a slight hesitation and then the bike would take off. It's an overgearing situation on those bikes and changing to the 17T sprocket (or 110/90 rear, or both) helps a lot. I believe the 78 has the same gearing as a 17T front on the older bikes, though, so I doubt it's gearing on yours, unless the rear tire is just too big.

Make sure your points are pristine and the condensors are young. The condensors, in particular, last only about 4 years if they have the rubber top where the wire enters: about 6 years max if they have the epoxied tops (those will turn yellow when old, giving a clue). After the points pit and turn grey, or if they are blackened, they need to be cleaned, reshaped, and smoothed well. Points files seem to be hard to come by these days, though!

[BobbyR]

I am running 120 mains on my 78K and I have no issues at high rpms. In fact I have no issues at all. Stock airbox and all.
I am suspecting some fuel starvation, gas is moving like hell out of the float bowls at those speeds.

[750]

Accel pump is working great.  I rejeted  because the K7 had 115s and the K8 only had 110s.  Threw this site it always seams to be the general concensus that the K7s and K8s were jetted way lean to meet the emissions standards of the 70s.  The plugs seam to look great, that golden brown that we are always looking for.  I will however take your advice and re-jet back to the original 110s and see if that fixes my problems.
Thanks

[754]

Maybe needles need to be shimmed? not sure
 77 had adjutable , 78, you got to use shims I think..

[Inigo Montoya]

See my k7 has the k8 carbs, virtually identical except the smaller jets and no clip slots. I have a k&N and still have the 110s. I have not had any issues with starvation. Like I said, you can always remove the filter and ride a bit and see what happens. Unless you are on a dirt road, 10 minutes will not hurt anything.

[BobbyR]

You can get stavation with incorrect float height also. You can check the float heights without removing the bowls.

[Inigo Montoya]

The old clear tube trick!
He just has to determine if he is getting too much or too little fuel.

[TwoTired]

Accel pump is working great.  

There is working great, and then there is working properly.
Whenever you twist the throttle,  there should be a very strong stream of fuel from each of the squirter posts.  There is also an adjustment between actuator rod and linkage.  See Chapter 24 of the Honda Shop manual.

I had a 78F model (same style carbs) where the accell pump worked.  Still stumbled on throttle twist.  The pump diaphragm membrane has two "ears" with holes in them, where prior over-tightening had squashed the membrane and closed off the holes in the "ears".  This restricted pump flow, even though the pump still worked.  By matching the hole sizes in the ears to the hole sizes they sealed (Drilled them out).  The good stream from the squirters turned into very strong stream, (seemingly enough to squirt all the way to the intake valve), the throttle stumble problem was then eliminated.

I rejeted  because the K7 had 115s and the K8 only had 110s.  Threw this site it always seams to be the general concensus that the K7s and K8s were jetted way lean to meet the emissions standards of the 70s.  

The emission testing of the era involved idle and low speed exhaust sniffing.  Main jets don't dominate fuel mixture until 3/4 or higher throttle positions.  Emissions testing wasn't concerned about full power operation back in those days.

What float height did you adjust to when you worked on the carbs?

[Inigo Montoya]

The adjustment should be .008 in between the pump rod and the lever that pushes it.

[BobbyR]

but it surges a bit in 4Th and 5Th gear starting around 7000rpm to 8000rpm, almost like it's getting too much fuel because if I let off a on the throttle a bit and vary slowly role back on the surge almost disappears. 

This leads me to believe that you may have fuel stravation. My K8 at speed will exhibit the same symptoms just before I need to switch to reserve.

[Inigo Montoya]

Hard to say. My 77 does not do that at all and that is with a k&n and no jet change.

[750]

You guys won't believe this!!  I found the problem.  Never would have suspected it.  There is a mesh filter that fits up into the tank above the petcock.  It didn't look clogged at all but air didn't seam to flow threw it well (Me trying to blow threw the mesh).  I think the materials may have just broken down over the years.  The reason I never would have suspected it is the tank is in mint condition inside.  Anyway, for whatever reason it was slowing fuel enough so the boles couldn't fill fast enough.  I took the filter out temporarily and put the tank back on, filled it with gas and went out for a quick put.  Sure enough, the old girl runs great perfect.  There is still some hesitation but it is nearly gone, and the surging is completely gone.  The carb boots definitely were a problem but I think this was even bigger.  Now I just have to buy a new filter and I'm on my way (I HOPE).  Thanks for all of your help and all of your recommendations. 

[Inigo Montoya]

Some like the intank filter, some like the inline. Either works fine.