Topic: Carb overhaul

[Keith]

Lately my K2 CB750 has been having symptoms.....slight hesitation off idle, "8 stroking" when trying to accelerate, then clearing out, plugs black after 400 mile run. specs are good, went ahead and took the carbs apart today. #1 was pissing out of the overflow, # 2 and 3 floats were WAY high...the adjusting tabs were bent so they weren't touching the float valve; the float frame was contacting the top of the seat. I'm surprised it ran as good as it did, but had to always make sure the petcock was turned off. Pulling the emulsion tubes and main jets, etc. and found quite a bit of crud. Here's the plan: New float valve assemblies, genuine Honda ($) New carb dust covers. I have new O-rings, gaskets, etc. Will clean all existing jets (genuine Keihin parts) Soda blast the bodies, reassemble with new K and N filter, enlarge the air intake a bit  on the original airbox. I am running with no fuel filters, new screen in petcock, NOS fuel tank. I've had this bike about 14 years, never had the carbs apart (looks like somebody did at some point) Any suggestions or ideas welcome.

[evinrude7]

3000 maintenance also.  float height.  bench sync. 

[ekpent]

  Keep the  float bowl gaskets in place if they are original and do not leak. The new rubber ones are crap if you use aftermarket and will swell up,fall out and do other bad things. I have done many,many 750's over the years and the only float bowl gasket I ever replaced was an aftermarket and that was with an old stocker    They can handle carb dip also so I just keep them in place and let them rest comfortably.

[Keith]

Thank you!  3k service done. I threw out the old bowl gaskets, they were glued in and ripped when trying to remove. Thinking of new floats too.(?) I appreciate all help!

[thebearded1]

Not sure how similar to a 550 the 750 is but i'd replace the o rings between the carbs at the fuel tees and throw on some new fuel lines since you'll have everything apart. I just did that on my 550 due to a leak at the tees

[Keith]

Yep, got all the o-rings. Fuel lines new last year. Thanks!

[Bankerdanny]

Not sure how similar to a 550 the 750 is but i'd replace the o rings between the carbs at the fuel tees and throw on some new fuel lines since you'll have everything apart. I just did that on my 550 due to a leak at the tees

Fortunately the '76 and earlier 750 carbs are WAY easier to work on than the 550 carbs. Individual carbs can easily be removed from the 750 rack, no shaft running through the carb body. Main jets screw in and the float bowls are held in place by clips, so jet swaps with the carbs on is a snap. Add in the super easy to remove airbox and working on 750 carbs is a pleasure compared to the torture that is 500/550 carbs with the OEM airbox.

[ekpent]

Not sure how similar to a 550 the 750 is but i'd replace the o rings between the carbs at the fuel tees and throw on some new fuel lines since you'll have everything apart. I just did that on my 550 due to a leak at the tees

Fortunately the '76 and earlier 750 carbs are WAY easier to work on than the 550 carbs. Individual carbs can easily be removed from the 750 rack, no shaft running through the carb body. Main jets screw in and the float bowls are held in place by clips, so jet swaps with the carbs on is a snap. Add in the super easy to remove airbox and working on 750 carbs is a pleasure compared to the torture that is 500/550 carbs with the OEM airbox.

   Amen brother Danny !!

[Keith]

Thanks everyone. The concern I have now is #1 float bowl was dripping when I pulled the drain hoses off. That was the only float that was adjusted anywhere near correctly. Now I'm thinking I should buy new floats as well. The floats were the black plastic type, should I buy the brass floats, or does it matter. I want to use only genuine Honda parts if possible. Do the plastic floats become porous over time?

[RAFster122s]

Could have a dirty float needle seat. Drop float needle and either qtips with some carb cleaner or paper rolled into stick to shove up the opening and spin wiping the seat...  If carbs off you can look into seat to see if it appears clean and undamaged...  Floats, even brass ones, can spring a leak. Brass ones can sometimes be solder repaired. Odds are more towards the needle either binding and not seating or something on seat keeping it from sealing. Presuming new float needles...

[Keith]

I can see the shiny ring around the tip where it contacts the seat, and it was clean. All the debris was when I pulled the jets out. As a former auto tech/mechanic? I only want to do this once, so I ordered the floats and valve assemblies.

[ekpent]

  Only 750's to have brass floats were the 69-70 and 71's. I swap them out on riders with the foam/plastic ones from later model bikes as the brass one do become fragile and leak from pinholes etc.. Only thing to wear out on the foam ones is maybe getting a divot from the float needle on the tab.

[Bodi]

I've never found a bad plastic float, the brass ones will also go almost forever unless the bike is parked for years with fuel left in the bowl (or, much worse, fuel in the tank and petcock left on)... this also attacks the body alloy and you'll also be dealing with that. The thin brass can corrode from whatever evil stuff is left once the fuel evaporates, and it develops pinholes - easy to spot by shaking them (no liquid should be in there) or holding under hot water - bubbles means holes.
The little tang on the float that pushes the float valve button can get a tiny divot where it rubs, that can cause the mechanism to stick. A bit of careful filing will make it flat again. The pivot pin can get glued to the float and posts with fuel varnish, it should be free to turn in the float particularly. Cleaning and a bit of polishing the pin fixes that.
The float valves do wear out and develop leaks, just barely dripping from the overflow with petcock on when parked is typical. I strongly recommend biting the bullet and getting the rather expensive OEM replacements rather than any aftermarket ones if they are leaking.
Also intermittent dribbling can be from small dirt particles in the fuel. They can stick in the float valve seat and hold it open a bit. The stock fuel filter screen in the petcock should stop them but installing a good inline fuel filter can be helpful - of course that's against the abhorrence for these filters felt by a few members here. Personally I've not seen problems caused by inline filters. YMMV.

[disco]

I will add to what Bodi said about the float valve (ie needle & seat). I was having very similar symptoms to what you described ie fouling plugs, pissing fuel occasionally etc. Turns out my needle & seats were leaking fuel past them. The bike was literally drowning on the first 25% of throttle. Bike ran great once on the main jet, but was a pig when driving around town.

Btw the needle & seats were brand new, but aftermarket. Ripped ‘em out & re-installed some used Keihin that were good, and problem solved.

[thebearded1]

Fortunately the '76 and earlier 750 carbs are WAY easier to work on than the 550 carbs. Individual carbs can easily be removed from the 750 rack, no shaft running through the carb body. Main jets screw in and the float bowls are held in place by clips, so jet swaps with the carbs on is a snap. Add in the super easy to remove airbox and working on 750 carbs is a pleasure compared to the torture that is 500/550 carbs with the OEM airbox.

Geez that sounds like a dream  compared to wrestling the rack out and breaking it down to replace a few o rings haha

[Bankerdanny]

Fortunately the '76 and earlier 750 carbs are WAY easier to work on than the 550 carbs. Individual carbs can easily be removed from the 750 rack, no shaft running through the carb body. Main jets screw in and the float bowls are held in place by clips, so jet swaps with the carbs on is a snap. Add in the super easy to remove airbox and working on 750 carbs is a pleasure compared to the torture that is 500/550 carbs with the OEM airbox.

Geez that sounds like a dream  compared to wrestling the rack out and breaking it down to replace a few o rings haha

The only positive thing I can say about the pods that came on my 550F were that they made pulling the carbs very simple. I was fighting sticking floats during my 2017 bike trip and pulled the carbs at a camp ground outside Monument Valley, UT to work on them. If I had been running the stock airbox at the time it would have been a huge pain in the ass. But since the pods were on I was able to pull the carbs in about 15 minutes.

[PeWe]

I changed float valves/needles when carbs were on the CB750K2 with K6 carbs+ airbox, (probably same).
I glued the needles in place with vaseline. Floaded it off when floats were back on, by open petcock and let it flow, moving the floats up- down helped. Paper under to soak it up.
 
I noticed that airbox had some fuel inside after an overflow so I drilled small holes (1mm) in each corner to let it out. Probably after forced fuel fill after carb work by blowing into tank fill hole. (by mouth).
I have not seen any fuel dropping out from box so it is OK now.
Edit:
Forgot to mention that the float bowl gaskets need some small amounts of glue here and there so you can remove bowls without gasket falling off which is pita to get right when assembly the bowl with carbs on bike. I used Permatex aviation no3. I removed all overflow with acetone soaked ear cotton stick. Test mounted bowl and cleaned again.

[Keith]

Carb update: I purchased a Harbor Freight soda blaster....the thing works pretty well! Things I discovered; the needles were set on the lowest position, (that's another reason for running so rich) and the rod that lifts the carb slide on #2 carb was bent. I noticed the plating on that rod was worn through on one side down to copper, and when installed and tightened, there was a tight spot when manually lifting the slide. I pirated one from a spare set of carbs, smooth now like the others. Luckily, the P/O only had this bike long enough to get it running and then sell it.....seems everything he touched was a hack job. Well, shame on me for not taking the carbs apart sooner.