Topic: Yoshimura Exhaust, Cam, and Carbs

[Bigmant]

Hey gang!

First time poster here! I am rebuilding my fathers 74 CB750K4 and my goal will be to keep it mostly stock looking with a mild performance increase.

I had the privilege of living in Japan for the last few years and I was able to go visit the Yoshimura branch over there and place an order for their exhaust, cam and carbs.

My question to you all, is how the cam and carbs will work with a big bore kit from the likes of one of the popular builders such as Cycle X or RP Engines. I have built some harley engines in the past, but typically when using a big bore kit, the cam and fuel injection timing usually is specific to the kit. So will I be good with an 836 kit and run the Yoshimura Daytona cam and their carberators?

[MRieck]

That combination should work well.

[Bigmant]

Is there any rejetting of the Yoshimura carbs that will need to be done? I guess I just want to make sure before I drop some money on all the parts that they don’t just work with a stock size 750 engine

[Jerry Rxman Griffin aka MuthaF'er]

Yoshimura carbs? Which carbs are these?

I ran the Yoshi 812 kit for many years along with the Daytona cam. You will like the better low end from the 836 and the MUCH better top end starting strong at 7000 rpm from the Daytona. Be ready for it's strong surge. Upgrade your valve springs.

[Bigmant]

The carbs are these bad boys:

https://shop.yoshimura-jp.com/en/product/result.php?maker_id=2&car_id=970

They look awesome, and from reading on this forum, the throttle response is awesome, although I’m going to pay dearly for it.

[754]

Mikuni TMR ××× 32 I think some on here have experience with these.

[dealer3100]

Hi there all...so just chiming in here for the first time as well...I did a mild build on a 1978 cb750 k in like 79 or 80.... back then all i did was put in a stock what was called then cb750 pre k cam...Dont know what the grind was on that but wish i did!... 29mm smooth bore mikuni carbs and a 4 into 1 RC pipe...kept the stock flat top pistons so it ran good on #$%*ty gas! lol.. but the question now is where do i go to find performance parts for that same motor? Just walked into that exact bike and i want tobuid a little street rod so cal style  to putt on.... Can anyone point me in the right direction?         

[Old Scrambler]

You found the correct place to start.............after a little more research on this site.........you will be ready to talk to Ken at Cycle-X for the most complete set of GO FAST parts to fit your budget

[PeWe]

Mikuni TMR ××× 32 I think some on here have experience with these.

My K6 with Mikes billet block has the base model Mikuni TMR32. (Cheap version )

Yoshimura has improved them with their special Multiple Jet Nozzle. A hollow needle with holes that spray fuel in the open venturi area.

[Bigmant]

Getting the thread back on topic, I understand the Yoshimura carbs multi jet needle was and is an improvement over the standard jet needle, but will that setup need any rejetting to make work with an 836 kit? I’d hate to spend all the money for those carbs only to find they won’t work.

It’s been pretty tough to find any information about them, even on this forum. Probably because they are so expensive.

[PeWe]

Then you need to get in touch with people that use that very carb if you can't do it yourself to get a start point.
Filtered or stacks?

My TMRs got most of the jets changed despite sold for CB750.
I wrote a thread about it, double and triple check variants of fuel screws, pilot jets, needle jets, needle heights and main jets. (Main jet std Mikuni same as for VM29 and similar)

I had no guideline written by others, own road tests only.
My carbs have adaptors for filters.

I doubt that racers will bother how it act below 3500 rpm. It took a lot of tests to get that area OK with smooth take offs, not sooting while waiting (idling) for green light.

[MRieck]

Getting the thread back on topic, I understand the Yoshimura carbs multi jet needle was and is an improvement over the standard jet needle, but will that setup need any rejetting to make work with an 836 kit? I’d hate to spend all the money for those carbs only to find they won’t work.

It’s been pretty tough to find any information about them, even on this forum. Probably because they are so expensive.

I think you will have to figure out the jetting. I would highly recommend dyno work so the A/F can be monitored. I messed around with the MJN systen about 25 years ago on a CBR600 used for roadracing. If I remember correctly "needle" height had a notable effect.

[Jerry Rxman Griffin aka MuthaF'er]

When replacing carbs, plan on re-jetting

[Finnigan]

I've been researching this exact setup for a few weeks and haven't so much been able to find a manual in Japanese to translate.  I was planning on ordering various sizes in jets to play with since I assume they'll take a few weeks to reach the states.  Glad you're starting this thread, I'll be watching closely and contributing any knowledge I find as well.

[Old Scrambler]

I would pay attention to the Yosh idle recommendation when jetting.........

[Bigmant]

Do you have a link or anything to the Yoshimura recommendation? Curious to know what you’ve heard!

I’m wondering if the Yoshimura  carbs either come with, or if you have to order different nozzles. According to some of the stuff I’ve read, the nozzles are hand build to very tight tolerances, so I feel like it’s not something you can pick up from anywhere.

[Finnigan]

This website says they recommend 65mm piston kit which I believe is 836cc: https://japan.webike.net/products/19986983.html
I may have been looking at the cam when I thought it said std pistons work...

[Bigmant]

Well that’s good news, that’s more info than I’ve been able to find. My next step is to call them I think, and get some info from the source. I’ll report back to you all when I find out

[Old Scrambler]

Regarding revs on start-up with a performance cam.............the first few minutes are critical to keep inertia and oil on the valve-train to assure well-oiled surfaces and reduce stress on the opening-event areas of the cam and rockers/adjusters. On start-up of a fresh motor build and/or after oil changes or extended weeks of not running the motor, I hold the idle on my Racing Motor at 2000 rpms until I feel heat, then back off to 1700.............On a mild street-tune I would not drop below 1400.........

BTW.......I like the Yosh hardened adjusters and will try to get a set.

[PeWe]

The Yoshi carbs have the same choke as the std TMR, right?
The idle is high when choke is on, not below 2000 rpm, after a short while 3000 rpm.

The modern Yoshi cam is very modest with relatively low lift.

Not close to the old Daytona which had numbers like RC295
Where In lift is around 9.8mm, Ex 9.15mm

- Is Yoshi's data really correct?

DAYTONA CAM2
MAX LIFT(mm)
IN 8.40mm/EX 7.80mm

DURATION(°) @ 1mm (0.04")
IN 240°/EX 234°

Standard Tappet Clearance(mm)
IN 0.05mm/EX 0.05mm
(This is even tighter than stock 0.05/0.08mm)

Even the well known tame Web Cam 41 is hotter.

Lift
0.360" (9.14mm)

Duration @.05"
250*
"Broad power range for street or touring. Bolts in to stock engines."

32mm carbs on a CB750 need a cam with higher lift and a good ported head.

[Bigmant]

I’m fairly sure that information is correct, but that’s super interesting when you compare the numbers. It’ll be something I ask about when they call me back. It makes me wonder if the cam is designed for a stock 750.

A quick 5min search on here told me that stock these are somewhere in the realm of .315 220degrees (intake) .313 235 degrees exhaust. So by that, the Yoshimura seems a little hotter than stock, but not as hot as some of the 836 cam sets being offered these days.

[PeWe]

Check this site, lots of cam specs and how to interpret the numbers.

 See upper left.
http://satanicmechanic.org/cams.shtml

A hot and not hot cam can have similar numbers which is explained.
High lift with relatively low duration and small overlap give power a street bike can enjoy.

Megacycle and Webcam has made many profiles.
Old Yoshimura is there too.

Cams is a favourite topic on this forum

All cams had a function and purpose when designed.
CycleX,  Dynoman and APE sells new cams.

Megacycle to be ordered separate.

Long overlap decrease the real compression, higher CR pistons needed.
It works the other way too.

[Jerry Rxman Griffin aka MuthaF'er]

I can dig out my original Yoshimura Daytona cam card if needed.

[scottly]

Getting the thread back on topic, I understand the Yoshimura carbs multi jet needle was and is an improvement over the standard jet needle, but will that setup need any rejetting to make work with an 836 kit? I’d hate to spend all the money for those carbs only to find they won’t work.

It’s been pretty tough to find any information about them, even on this forum. Probably because they are so expensive.

How expensive are they? Not much point in spending a lot of money on bigger carbs if the head is the limiting factor. Most vintage cams were "crutches" for poor flowing heads. Spend your money on head work first, and you can always upgrade carbs later..

[Bigmant]

They’re gonna be about $2600. There are cheaper options for sure, but these aren’t vintage options. Head work may definitely be in the cards! The plan is to tear the engine down and rebuild it anyway, so porting the head might as well be done while everything is apart. The tricky part will be to find a reputable guy to do in souther California that someone can vouch for