Carb problems: choppy idle, bogging on quick throttle. What to do, what to do?

Question

Just did a full tune-up and rebuilt the carbs on my baby: a '78 Honda CB750K SOHC 4.

Carb rebuild was proper: separated bodies off the rack, soaked every metal part, replaced main jet, needle, gaskets, etc. Finally, bench-synched the slides.

It runs, but definitely not as well as it should be.

Symptoms: Bike starts easily with chokes closed, but revs are erratic--starting high (~3.5 - 4k rpms) and then dropping (all the way down to ~1.2k rpms) either by itself or with a little help by pushing in / closing the throttle.

With the choke in or out, any quick pull on the throttle (from idle to 1/4 throttle) causes some hesitation before finally 'kicking in.' Sometimes it just dies outright. This is only up to ~1/4 throttle, after that, the bike responds pretty well, though not perfect. I was able to improve this hesitation a bit by enrichening the mixture, but it's way too rich: intake rubbers are wet, plugs are fouled.

Help. Please. :)

Answer 1

A bike that old will need more than just a bench carb sync.
Some cylinders ware out at a faster rate than the others.
So the carbs must be synced while the bike is running, to compensate for the indifference's.
Electrical isn't the problem, but it couldn't hurt to adjust the points & timing.

If you want to go the "poor man's" way of syncing the carbs without gauges -
While the bike is running, feel the exhaust pressure coming out of the #1 & 2 mufflers and sync those 2 carbs.
Then feel the pressure at #3 & 4 and sync those.
Then sync the left 2, to the right 2.

Answer 2

Soaked every part...did you use some of the squirt stuff in a can or the high powered stuff parts stores sell by the gallon? I used the latter on some very varnished up carbs, had to let one soak for one day and the other for two days. After that, it ran very rich on the 2 day job and rich on the one day but normally on the other two that didn't require soaking. I finally found that it wasn't the needles that were worn, but rather the soak eroded the needle jet, not the main jet, but the brass jet the tapered needle slides into. If you did some serious soaking, that might be the cause of the richness.

Answer 3

feels like a lean undertaking. attempt going lower back to previous carb and notice if concern nevertheless exists. If it runs ok on previous carb, it certainly may well be jet length concern in new carb. while re-jetting, i take advantage of trial and mistake. in case you're careful, jets could be drilled to next greatest length(s) utilising small drill contraptions accessible at MSC business furnish. look at drill sizes #a million via #eighty. There are charts that time out how large a hollow those small bits make. to illustrate: # fifty 8 bit will make a nil.0420 hollow. understanding what length your jet is will help as a commencing factor. you will possibly desire some thing in or around #fifty 5. Use appropriate pin vise as jets could desire to be intently drilled out with the aid of hand and burr loose. Use a jewelers loupe to envision.

Answer 4

http://www.hondachopper.com/

Try here...these guys are some of the best old Honda wrenches I know. They'll set you straight.

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Sounds like you have an adjustment off somewhere, especially if you simply did a rebuild....i.e. not increasing jet size or anything like that.

Answer 5

Look in your gas tank and see if there's rust bits in there. If so, you need to get an in-line fuel filter. Might also try lubing the throttle cables.