Topic: Mikuni RS34

[luckylefty]

Hello all,

I just got my 74 cb750 recently. I am considering to replace my carbs to the RS34 from cyclexchange. My engine is stock and i am not planning to upgrade it. will there be any issues?

thank you.

[newday777]

Welcome aboard the forum luckylefty

I think you'll find that the RS34 might be too big for a stock motor.
Did you call CycleX and ask them?

[Tracksnblades1]

At least they’re like a big ole Holley. Their pumpers are adjustable. 😁

[PeWe]

34 mm must be way too big for stock. 29mm is a closer match.
Better bang for the buck  to go 836, restore head that can  have bad guides and valves on the exhaust side to start with. Use stock carbs.

My K6 has 970cc with 32mm carbs. Runs fine with very good response.
I doubt it will run better with 34mm carbs.
Maybe in the very upper range. I do not want to miss anything below 6000rpm.

Same bike ran well with 836cc and stock carbs jetted for the setup. Ported head and cam. It had smoothbore 29 but boots cracked.
Not much lost with stock except for midrange throttle response. Same top speed.

[luckylefty]

my 74 cb750 came with a PD41B carbs that's off a k7. I feel it's choking and need to replace the carbs. so I am looking at less expensive options. getting a used stock and restoring would be a challenge where I live.

[PeWe]

Stock carbs  can be found for not that much if lucky.
I got a set (K6 carbs) for $100US a few years ago, 2020. Taken apart and cleaned. Still apart.

Local swap meets or private persons. Not gold diggers on eBay. I guess you can find a good set for max $200US.
Then new set of carb boots. Honda stock, not cheap cracking aftermarket.

[Tracksnblades1]

What ?

https://mikunioz.com/shop/mikuni-rs-34mm-carb-kit-honda-sohc-cb550f-four/?v=13b249c5dfa9

[Tracksnblades1]

I think the 1980 nighthawk 750 ran 34mm cv carbs. I’ve read on here some have installed them on sohc 750 with good results. CV carbs have butterflies that the throttle operates and the vacuum/spring actuator operates the slides..They won’t bog like the round top with no accelerator pumps do below 3000 on snap wide open throttle openings..

[PeWe]

Very optimistic!
I wonder how the dyno curve will look like. A huge concave curve instead of convex between 3000-6000rpm?
On a tuned  engine.

More modern bikes have bigger carbs. They must have a better head design and flow.

If replaceable throat rings were a part of the design as Weber has a different thing. Just adjust the venturi with rings.

I read somewhere that CB750 heads flow were limited by the inlet spigots. My ported head I checked had 29-30mm inner.

[Tracksnblades1]

Very optimistic!
I wonder how the dyno curve will look like. A huge concave curve instead of convex between 3000-6000rpm?
On a tuned  engine.

More modern bikes have bigger carbs. They must have a better head design and flow.

If replaceable throat rings were a part of the design as Weber has a different thing. Just adjust the venturi with rings.

I read somewhere that CB750 heads flow were limited by the inlet spigots. My ported head I checked had 29-30mm inner.

Indeed….

The Mikuni RS advertisement reads the same 25% increase but doesn’t identify the application(s) the gains were achieved.

When comparing accelerator pump less mechanical carbs, I think the smaller mechanical carbs provide the best drivability with the expense of some top rpm dyno hp. Believing in most stock applications the majority of riders will be happier with the smaller ones, especially at lower rpm’s. When larger CV carbs came out on dohc Honda’s 750, hp went up along with the dual cams and larger cv carbs.

The CV carbs were a good compromise for all rider and riding types. Providing good street performance and adequate high rpm operation..The CV carbs remind me of a well prepared vacuum secondaries 4 barrel, anyone could drive them with good results, as compared to their mechanical 850 double pump cousins. However swapping from a 650 vacuum secondaries to a 850 or 830 annular on a well prepared small block would result in 50hp and 40lb ft instantly. And before the debate, a well prepared 800+ cfm mechanical carb will provide excellent performance and Drive ability too. However in ET racing some still prefer the larger vacuum secondaries carbs over their mechanical size equals. Experiencing more consistency depending on the driver and the engine’s requirements.

Talking two strokes, it’s interesting Honda chose the same Mikuni 38mm for their later 125 & 250CRs. While some of the earlier 125, 250, and 500crs wore 38 keihins..

[PeWe]

CV carbs must have a smoother response than flat slide carbs, right?

Mikuni TMR32 flat slides have a very direct and quick response. A little bit too good.
It would be fine with less at take offs and low speed.
I have trimmed down the first half of the throttle roll.

When carbs had a lean pilot circuit even worse.
A button to choose either street or race.

Honda 250  2-stroke was single cylinder engine?
If yes, 4 will make 1000cc so that size of carbs not that strange.

Later bukes with carbs around 40mm had a higher rpm range? When revving 12000-14000 rpm fuel and air need to be delivered very quick and often

I'm just speculating, no expert

[Tracksnblades1]

CV carbs must have a smoother response than flat slide carbs, right?

Mikuni TMR32 flat slides have a very direct and quick response. A little bit too good.
It would be fine with less at take offs and low speed.
I have trimmed down the first half of the throttle roll.

When carbs had a lean pilot circuit even worse.
A button to choose either street or race.

Honda 250  2-stroke was single cylinder engine?
If yes, 4 will make 1000cc so that size of carbs not that strange.

Later bukes with carbs around 40mm had a higher rpm range? When revving 12000-14000 rpm fuel and air need to be delivered very quick and often

I'm just speculating, no expert

Yeh we’re on the same page. The CVs are like most all carbs they can be turned  for the different engine modifications too.
Flat slides, I agree need proportional throttle cam that move the flat slide from idle to 1/8 really slow, 1/8 to 1/4 just a little quicker, 1/4 to 1/3 a little quicker, then from 1/2 throttle to full just light switch it. Because there’s not a lot of difference, especially on the two strokes..

Yes 250cr are single thumpers. My point was on two strokes with the doubling of the displacement twice, 125/250/500 the flat slide or semi flat side carb size remains the same.. Riding the 125 it’s Mikuni TMX 38 has the best proportional throttle control. Meaning the more you open the throttle the fast you go. However the 250 with the same TMX carb, in the first 1/2 turn of the throttle lies the majority of the hit. Where you need a greater degree of proportioning in throttle movement to engine output. From 1/2-2/3 throttle to wide open has less of an effect on the 250 that the 125 carbs being the same. The 500 is a whole different animal. With its intense vacuum signal, From the idle to 1/3 throttle finesse will determine if you’re still upright or on your lying on your back. I’ve experimented with different and modifying the throttle cams to give up slide movement per throttle movement.  Then light switching anything from 1/3 throttle to wide open. It helps but the throttle finesse required to tame the hit in the first 1/3 throttle can be upset by just hitting bump or tiring physically or mentally a little. But it the second most exciting thing I’ve had between my legs..🤪

I’m a fan of big carbs but proper tuning is required to get good response and drivability from them especially on the smaller engines. It’s easier just to run the smaller one..

I’ve included the attachment for PeWe. the same has been written and observed for years. Smokey championed it for years as the optimum velocity and intake resonance timing. Especially with his SmokeyRam or his designed z28 dual 780vacumm secondaries cross ram Chevrolet offered for the 302.

https://www.motortrend.com/how-to/148-0403-seven-holley-carbs-test/

https://www.motortrend.com/how-to/148-0403-seven-holley-carbs-test/

[luckylefty]

Welcome aboard the forum luckylefty

I think you'll find that the RS34 might be too big for a stock motor.
Did you call CycleX and ask them?

cycleX told me it's fine to fit it into a stock engine. I am still thinking which will be a better option as I am not looking to upgrade my engine in the near future.

[Tracksnblades1]

Welcome aboard the forum luckylefty

I think you'll find that the RS34 might be too big for a stock motor.
Did you call CycleX and ask them?

cycleX told me it's fine to fit it into a stock engine. I am still thinking which will be a better option as I am not looking to upgrade my engine in the near future.

Lucky lefty,

It’s hard to make recommendations without knowing one background.

Most of the older guys tuning their own bikes grew up watching their grand dads  adjusting the carb on the push mower to get It perfect before using it. Learning to start engines with point ignition s, hand chokes,  then stove and electric automatic chokes with vacuum pull off and mechanical unloaders. Lived through the Carter, quadrajets, Holley, bendix, motorcraft, fomoco, strombergs and the likes. It’s hard to make a recommendation without know another’s background. If you’re a tuner and like jetting, accelerator pumps, needles float levels etc and then do it again, again, again, you’ll be fine. If your best buddy or your uncle is, you’ll be fine. If you’re paying a professional tuner you’ll be fine.

If this is your first time owning a carburetor or tuning one, it can become overwhelming conveying the actual symptoms to forum members. Especially without their ability to see, hear what you trying to explain.

I’d ask the question if anyone has or knows anyone running the rs’s on a stocker or smaller displacement engine like the 550..

Cycle x  will have already tried it…they’re tuners and builders…😇

[PeWe]

Really interesting reading about the carb tests.

This is needed for CB750 too:
"......
This equation assumes 100 percent volumetric efficiency (VE) and can be worked out using the following equation: VE = (displacement x rpm)/3,456. For example, if we were to run a 350ci engine at 6,000 rpm through this equation, the formula would tell us that a 350ci engine requires a 608-cfm carburetor. "

There are flow numbers for various ported heads on this forum. Plus a comment by Mike Rieck I guess that inlet spigots are the restriction.

The cost to get a better V8 is relatively low compared to  CB750
They provide some goodies for both Chevy and Ford
Chevy page:
https://smedingperformance.com/collections/chevy-engines

I need a car!!!

Where to find a similar test for CB750?
Covering various bores like 760, 836, 900 and 1000cc would be perfect.
Or only 760cc (62mm) and 970cc (70mm) which are common.

The dyno charts I have seen with RS34 have all a bog down at 4000-6000 rpm running rich.
See this thread, search RS34.
http://forums.sohc4.net/index.php/topic,83748.0.html

I can be the needles that are too rich. (Thin taper).

But, these are all tuned engines with bigger bores, higher compression, ported heads, bigger in valves and hotter cams.
A stock engine is completely different.

Going for RS34 is when engine is modified with all above improvements. Maybe using 836 today but plan 970/1000cc in the future.

I have recently jetted my TMR32 to work fine from idle to 9500rpm.
Carbs had wrong needles from delivery. I found a jetting chart for TMR carbs at Webike homepage. To find those needles for sale AND in stock another thing.

I had tried go change ALL other jets in many combinations tested on dyno and road.

Not possible to go around the fact of too rich needles. Always too rich and too lean somewhere.

My Mikuni TMR32 were less demanding with less flowing exhaust.
A huge difference when going from rather open Yamiya Lotus Root 4-4 to Hindle 4-2-1.

All this about RS34 get me ideas to try a set

I have another engine + bike build ongoing. Planned to get 890cc + ported head 34/28mm valves with a good cam. I'll might try different. All parts waiting to be assembled.
That engine also milled for Billet block. Its pistons have got horisontal gasporting so more to expect.

Franks frame kit make cam change + timing quick on bike.

The first I should try with RS34 is leaner needles.
This based on dynos on this forum. Dyno thread.

My TMR32 will most likely fit the 890cc build direct. Maybe one step smaller main jets.

RS34 to my 970cc build that work very fine.
107.5whp on dyno last year. 33.5/28.5mm valves. 6.5mm stem, not thin 5mm.
Result on above Dyno thread.
Then too rich needles so it might be a better midrange today.
I have a Mikuni needle comparing Excel chart where there are plenty of different needles to compare the taper with flow.
TMR needles are 9 family which means 90 mm something.

RS34 another lenght.
I'll update with that Excel and RS34 needle family.

[PeWe]

I had to search about RS34 and its jets
https://mikunioz.com/shop/mikuni-rs-34mm-carb-kit-honda-cb750-four-k-models/?v=13b249c5dfa9

Their jetting:
Pilot Jet: VM28/486-17.5, Needle: 9DZH1
Needle Jet: P-4 (568 Series), Main Jet: N100.604-120/130

Needle jet P4 rather big.
TMR32 use P1
Needle very close to my TMR32 that were impossible rich.

Interesting which needle CycleX kits have or what needles used in the other guys carbs with dyno charts.
Needle here the 90mm series.

Here they deliver a kit with throttle assembly. My TMR had same recommendation.
I did not want to change stock look so I used CB750 stock throttle with cables which was possible by adding a nut where cable sit on carb throttle mechanism.
Nut cut with saw blade so it could enter the wire. It should only use as a stopper.

This Mikuni needle comparing Excel chart show that the stock RS34 needle is very close TMR32 needle I got in my carbs that were way too rich! So not strange that the dyno charts look like they do if using same needle.

I have used 4 needles in the chart to compare the taper.

That chart show so much more as main jet and other stuff too when browsing around on that tab.
TMR34 has same needle as TMR32 in the chart Webike has.

If I should have a set of RS34 I should try leaner needle closer to TMR after check which needles the other guys used. If as delivered.

# Purple-9E11-52 deliv with TMR32
# Blue - RS34 as link above
# Yellow- Correct for TMR32
# Turquoise- Alternative like TMR32
When using chart see how it will change when changing clip needle height.
TMR32 clip 4 from top.

RS34 and TMR too rich clip clip 3


Below how my TMR32 looked like when using its delivered needles at its delivered height 4:6. Here with 836 and Megacycle 125-20 cam.

Reminds me about dyno charts with RS34
Note! Torque in Nm


Correct needle I finally found years after at same height should have fixed it properly. Back then changed to 2:6 from top.

[Tracksnblades1]

I had to search about RS34 and its jets
https://mikunioz.com/shop/mikuni-rs-34mm-carb-kit-honda-cb750-four-k-models/?v=13b249c5dfa9

Their jetting:
Pilot Jet: VM28/486-17.5, Needle: 9DZH1
Needle Jet: P-4 (568 Series), Main Jet: N100.604-120/130

Needle jet P4 rather big.
TMR32 use P1
Needle very close to my TMR32 that were impossible rich.

Interesting which needle CycleX kits have or what needles used in the other guys carbs with dyno charts.
Needle here the 90mm series.

Here they deliver a kit with throttle assembly. My TMR had same recommendation.
I did not want to change stock look so I used CB750 stock throttle with cables which was possible by adding a nut where cable sit on carb throttle mechanism.
Nut cut with saw blade so it could enter the wire. It should only use as a stopper.

This Mikuni needle comparing Excel chart show that the stock RS34 needle is very close TMR32 needle I got in my carbs that were way too rich! So not strange that the dyno charts look like they do if using same needle.

I have used 4 needles in the chart to compare the taper.

That chart show so much more as main jet and other stuff too when browsing around on that tab.
TMR34 has same needle as TMR32 in the chart Webike has.

If I should have a set of RS34 I should try leaner needle closer to TMR after check which needles the other guys used. If as delivered.

# Purple-9E11-52 deliv with TMR32
# Blue - RS34 as link above
# Yellow- Correct for TMR32
# Turquoise- Alternative like TMR32
When using chart see how it will change when changing clip needle height.
TMR32 clip 4 from top.

RS34 and TMR too rich clip clip 3


Below how my TMR32 looked like when using its delivered needles at its delivered height 4:6. Here with 836 and Megacycle 125-20 cam.

Reminds me about dyno charts with RS34
Note! Torque in Nm


Correct needle I finally found years after at same height should have fixed it properly. Back then changed to 2:6 from top.

That’s some good info in your posts. I’ve screen shot those for future 34s or bigger 😁. 10.5 : 1 man if I was following you and could keep up. I’m sure I could see some black smoke coming out of her. It read 16:1 was the best power AF befor the big dip..

My son and I had a similar “fat” problem in the middle with our Honda CR250. Prior to buying a Keihin air striker for it, we changed to a Yamaha YZ250’s S8 slide and needle. Before the idle, pilot circuit, and wfo was good but it 4 stroked and blew sponge (2 stroke lingo) out the silencer in the middle. Now that we are satisfied with it we notice a lot of CR 250s that have the spooge coving the side of the rear fender.

Good info PeWe. 😇

[PeWe]

We all help eachother to search for info.
That Excel sheet has lots of info, another thing to understand all details.

Comparing needle taper is the only thing I have used so far.
2 close needles can get same profile with different height. Just click clip number.

EDIT:
I have tried to find where I found that Excel file around 2019.
Must have been a german site like https://www.mikuni-topham.de
But I could not find it.