Yamiya CB750 Carb kits

[ashimotok0]

Has anyone used these 750 carb kits from Yamiya in Japan. They say they have sold 2,500 kits so I guess they are OK. We need them for a '69 Diecast. They come with 3 different sized of main jet.

http://www.yamiya750.com/index.php?main_page=product_info&cPath=246_21_181_143&products_id=153

Cheers ... Ash (UK)

[Steve Swan]

I've used the yamiya kits before.  More often than not the float needle and seat have to be lapped in so gas doesn't leak past the valve.

Based on my experience,  i don't use jet needles unless i replace the needle jets and needle jet holders.  Unfortunately,  the kits don't come with the needle jet holders.

[ashimotok0]

I've used the yamiya kits before.  More often than not the float needle and seat have to be lapped in so gas doesn't leak past the valve.

Based on my experience,  i don't use needles unless i replace the needle jets.  Unfortunately,  the kits don't come with the needle jets.

Thanks for reply Steve... These are for my friends '69 diecast and one of the main reasons he wants to buy is mainly for the needles as he has fitted a Cruzinimage kit and the needles measure wrong. Yamiya do sell genuine needle kits but I fear the other Cruzinimage parts may also be of poor quality so a needle kit alone may not solve all of his carb  issues.

looking at the Genuine Honda needle kit, the Yamiya full kit would appear to contain all three components. Am I incorrect?  The float needle looks like the later part with the wire clip and I assumed they would be therefore Viton tipped. Any thoughts please?

Best wishes ... Ash

[Steve Swan]

cruzinimage gives me the creeps just thinking of the name, much less typing it.

the 3 different parts i am referring to are on page 56, cb750 parts book, print date 69.4:

1.  16151-300-004, NEEDLE, jet
2.  16133-300-004, JET, needle
3.  16139-300-004, HOLDER, needle jet

myself, i cannot imagine replacing only one of these parts, because all three are wear items and they wear together.  replacing one or two of 3 worn parts with new parts, makes no sense to me, but then, there are certainly times i am known to not be the brightest bulb in the package.  

all i know is EVERY needle i have ever seen in these non-oem kits is not like an original needle (either taper or length is different) and the one time i replaced the stock needles (for no good reason other than replacing the needles because they were new replacements, the initial throttle opening response was beyond a major hesitation when turning the throttle open.)  Going back to oem honda needles took the problem away.

another story, E2241......  on first running of the engine, i had one hellish stagger, "sort of" off idle when motoring off in 1st, especially when shifting into 2nd or even at low rpm in 3rd gear.  In 4th and 5th (faster rolling speed and slightly higher rpm's) i could not create the stagger (but there was a sort of "dead spot" in these upper gears.)  "VERY ODD," i thought to myself.  initially i tried very unsuccessfully to tune the stagger out with the pilot needle screw.  then i tried lowering the jet needle to get less gas, because i live at 5,000 feet altitude and thought i might need more air.  made stagger worse.  so, not knowing any different, i raised the needle, of course more gas, just as bad.  and further more puzzling was the fact the stagger really was occurring ONLY between 1/4 and 3/8's throttle open and not with greater throttle openings, so my logic and understanding of carburation relative to air/fuel passageway circuitry at various throttle valve openings told me raising or lowering needle could not correct this condition, because the 3 above parts control AFR's between the entire range of 1/8th to 3/4's throttle valve openings.  and the stagger was puzzling because i had not ever encountered a stagger so profound as this, basically when opening the throttle at "take off" speeds, especially profound in the first 2 gears.  so bad was this stagger, if too much throttle was given taking off from a stop and opening throttle and even more especially so, when slowing down in a sharp 1st or 2nd gear turn, the engine would "fall" flat on it's face (as if engine dying) when given throttle, then upon slightly closing throttle, the engine would "catch" and the motorcycle lurch forward.  (not a good thing if you have a car following directly behind.)

after some thought about the roles each circuit plays separately as well as in concert with each other and what conditions the stagger was occuring and when, i tried replacing the jet needle in all 4 carbs with the after market jet needles from what vendor i cannot remember.  the stagger was even worse than ever before, so i KNEW i was on to something.  so i replaced the old needle jets with the aftermarket and kept the new replacement jet needles and the stagger was no better.  hmmm, i thought.....  "that leaves only one other wear part and that is the needle jet holder," which OF COURSE does NOT come in any of the aftermarket kits i am aware of......

so.......  i sucked it up and bought Honda nos oem, needle jets, jet needles and needle jet holders, each set running around $45 each, so you can do the math for cost of 4 sets.  thought to myself, "this had better be the problem or i'm gonna really be stumped (and ANNOYED).........."

so........  installed the new oem jet needles, needle jets and needle jet holders in each carb and...... NO MORE STAGGER OFF ANY GEAR AT ANY LOWER THROTTLE OPENING.

you can form your own opinion off my experience.

and, of course you know the main jet only comes mostly in to play when the throttle valve is 7/8's to fully open.

[kp]

That's a very interesting story Mr Swan. I liked it 

[ashimotok0]

That's a very interesting story Mr Swan. I liked it 

Yes Steve .. I am most grateful for your comprehensive replies (as always !).

I have asked ChrisR how his carb brass components have fared over the monumental mileage that his 'rider' sandcast has covered. Do you think it would be better to ultrasonically clean original Keihin parts, inspect for wear and run with those rather than use aftermarket parts because I constantly read horror stories about aftermarket kits. I was hoping that because Yamiya have been supplying their kits for so long and sold 2500 kits, that they would be OK to use and that any 'bugs' in quality/correct sizing of components would have been ironed out by now. They claim in their listing for the kit  that their parts are as good quality as OEM Honda.  Point taken that the kits always miss out the vital part, which is a real shame.

Yamiya do list Genuine kits  but David Silver only lists the needle itself, as special order, usually meaning he gets them in from CMS in Holland but CMS only have one in stock.

Best wishes ... Ash

[Steve Swan]

good to hear from you Ash, your posts are always some of the best.  imho, you answered your own question, "it would be better to ultrasonically clean original Keihin parts, inspect for wear and run with those rather than use aftermarket parts."  of course, i always keep the parts from each carb together in a container, so they don't get mixed up.  i would imagine it takes quite a few thousands of miles  to wear out the jet, the needle and the holder.

i've bought several sets of Yamiya kits for the different gaskets, have never used the hardware.

why did he change out original Kehin for aftermarket parts ?  were the original parts showing incorrect carburation symptoms, then if so what were these symptoms ? 

if there were no symptoms with original Kehin parts and if he still has the original oem Kehin parts and has kept them separate respective to each carb, i would take the carbs back to original and see how the engine responds to the original carb parts. going back to stock setup is a good baseline starting point.

i'm sure your friend knows to change only one thing at a time when "tuning" carbs.

[ashimotok0]

Kind words Steve,Thank you.

The bike before and after is here. It was an import from the USA. It had 15k miles showing when he bought it, as shown with blue/green tank but he was unsure of its validity. He was told by an engine rebuilder  that Cruzinimage carb kits are fine so he fitted them ... I think he realises now that they are not very good ! I have suggested that he cleans the original Keihin parts   thoroughly in a friends ultrasonic cleaner and try them out. I am not so sure if he has kept all of the mating parts together though. I will post when he's tried them. He is a very talented patternmaker and has repro'd the top triple tree (yoke in the UK) and carb alloy plate for the early CB 750.

Best wishes .... Ash

[Steve Swan]

nice looking bike.  if he has not run carb bodies through ultrasonic, he should, then squirt WD40 or some comparable penetrant through all passageways to make certain they are all open.

[ashimotok0]

My friend has refitted his original needles etc and the bike is running 100% better so sends you a big thank you Steve for your comprehensive replies. All of Cruzinimage's products worry me. Many people seem to assume that because they are supplied from Japan that they are good quality but that is clearly not the case. I am from the old school of thought that I was taught as a kid "You get what you pay for son. "

Cheers again ... Ash