Topic: Mikuni VM29 adapted to CB750 OEM inlet rubbers

[PeWe]

VM29 - CB750 inlet rubbers were really bad, too flexible and prone to crack so I began the project end of the 80's. OK today some years later.
The most important was to cut the rack,  reweld it which was done in the days. A friend work at a company that are aluminium specialists. No problem to weld.
Last week:
- Only drill 3 new holes displaced 10mm in the throttle shaft for throttle linkage and carb 3,4. cut it 10mm
- Shorten t-pipe for gas (5mm on each side)
- shorten choke axis and bend the choke link for carb2
- Fasten the bracket for throttle return spring in new holes. Cut a piece from it so it fits between carbs 2-3.

[dusterdude]

Cool

[Retro Rocket]

Did you know that you can buy rubber manifolds that fit these carbs straight on to the 750..? I have a set  and i think Mark at M3 racing sells them, saves all the cutting...

[dusterdude]

Cool too

[PeWe]

Did you know that you can buy rubber manifolds that fit these carbs straight on to the 750..? I have a set  and i think Mark at M3 racing sells them, saves all the cutting...

Yes, I know too well.
The rubbers I have used were too soft. The rubbers flexed constantly when the engine was running which must have had a negative effect on the performance. Rubbers collapsed which must have reduced the under pressure sucking air into the engine.
It was very visible.
My first set of rubbers got cracks rather fast. I tried to seal them with extra layer of silicone and tape around. Did not help. 2 Pistons melted after appr 800km. EDIT: 8000km All top ends of conn rods were blue... pistons 2-3 melted nr 2 got a hole. Maybe late ignition on 2-3 might have been a factor too.

I found my second attempt to use Mikuni rubbers from 1988 (found the receipt too). cyl 2-3 rubbers have deep cracks were they grip the head spigots.
That's why I do not want to use the crappy Mikuni rubbers that have too soft rubber. Maybe they have redesigned them since the mid/late 80's.
I'm convinced that the CB750 OEM will work better since cyl 2-3 rubbers do not have the sharp curves as the Mikuni rubbers have.
Stiffer rubber will ensure air/petrol mix in higher volume than the flexible Mikuni rubbers. No more fluttering inlet rubbers.

I had problem with my car when the turbo inlet hose was too flexible, the engine power went up and down like a kangaroo when WOT from 3500rpm-5000rpm (surging) . Ordered a metal pipe with small rubber hose connections from USA, Evolution Racewerks that solved it. Turbo engine with better program loaded +50% boost pressure and more abrupt need of air.

[MRieck]

Did you know that you can buy rubber manifolds that fit these carbs straight on to the 750..? I have a set  and i think Mark at M3 racing sells them, saves all the cutting...

Yes, I know too well.
The rubbers I have used were too soft. The rubbers flexed constantly when the engine was running which must have had a negative effect on the performance. Rubbers collapsed which must have reduced the under pressure sucking air into the engine.
It was very visible.
My first set of rubbers got cracks rather fast. I tried to seal them with extra layer of silicone and tape around. Did not help. 2 Pistons melted after appr 800km. All top ends of conn rods were blue... pistons 2-3 melted nr 2 got a hole. Maybe late ignition on 2-3 might have been a factor too.

I found my second attempt to use Mikuni rubbers from 1988 (found the receipt too). cyl 2-3 rubbers have deep cracks were they grip the head spigots.
That's why I do not want to use the crappy Mikuni rubbers that have too soft rubber. Maybe they have redesigned them since the mid/late 80's.
I'm convinced that the CB750 OEM will work better since cyl 2-3 rubbers do not have the sharp curves as the Mikuni rubbers have.
Stiffer rubber will ensure air/petrol mix in higher volume than the flexible Mikuni rubbers. No more fluttering inlet rubbers.

I had problem with my car when the turbo inlet hose was too flexible, the engine power went up and down like a kangaroo when WOT from 3500rpm-5000rpm (surging) . Ordered a metal pipe with small rubber hose connections from USA, Evolution Racewerks that solved it. Turbo engine with better program loaded +50% boost pressure and more abrupt need of air.

Yes....I have seen the same thing with the newer aftermarket insulators. Lots of air leaks.
 I forgot to add....nice work!

[PeWe]

More photos. CB750 OEM rubbers vs Mikuni CB750 rubbers.
(OEM are wrong type, K0 16211/1612-300-305. I have ordered for the longer head spigots in later K heads 16211/1612-300-030. Hopefully rigid rubbers and not bad soft copies)

Cracks....

Test with OEM rubbers, carbs not completely pressed to bottom. I'll preheat the final OEM rubbers for smoother assembly and easier fitment.

Maybe the jetting will be different. I remember that my Local tuner reacted on my need of larger main jets and pilot.
Main jets: 127.5
(I have ordered some jets (120, 122.5, 125, 130) when I cannot find my old collection, nice price on eBay £2,49/each)
Pilot: 25 (I think that it was 20 first)
Air screws. 3/4 turn out. (should be 1.5 turns according to Sudco)
Everything else as Sudco writes about CB750.

I also found that my old KN filters had shaped to OEM Keihin. My bike is worth a better set...
Another nice surprice, found KN RC-2454,  good price on eBay.co.uk where I aslo ordered  Stainless Steel Braided hose for front brake. - Who want to drive fast and trust over 35 years old rubber hose for the front brake?

I have to work fast with my engine before my wish for better conrods affect my economy... new super rods need to be balanced which make it to a good idea to find a complete set, lighter crank with rods on + alternator, balanced and ready.  Crank that fits BBBBBB block and new green bearings will help
I have already bought new reinforced primary chains from Germany (only one company that have this), camellia cam chain and APE sprocket from US despite I had planned to use my old not that much used chains... New cam is worth new parts for better timing I suppose...

[PeWe]

...........
........ I forgot to add....nice work!

Thank you very much! Approvement from a CB750 pro like you feels very good!

I hope I'll soon can show the completed result... This weekend I need to remove all parts from the frame that I'll leave for sandblast and new 2k primer + 2k black paint.

[PeWe]

If I should  start doing this today...Not cut the rack.
Find short pipe bends of aluminium or stainless steel that are connected to head spigots with short silcone hoses, same on the other side to the carbs.
Double amount of clamps, but should work

I'll have my carbs attached to the frame with brackets so they do not hang in the inlet rubbers only. The mikuni carbs have a hole on outer side beside MIC, perfect for threaded piece of metal. Mine look a little bit corroded today,

[lwahples]

I have two sets,one has aluminum adapters held on with a tiny set screws. Making them fit a CB750 with Honda rubbers. The other set looks like yours.

http://www.nooffswitch.net/lwbike0309/parts01.jpg

This picture doesn't show the adapters to good.How much of a built motor,will it take to run these?
                                               Larry

[PeWe]

I have two sets,one has aluminum adapters held on with a tiny set screws. Making them fit a CB750 with Honda rubbers. The other set looks like yours.

http://www.nooffswitch.net/lwbike0309/parts01.jpg

This picture doesn't show the adapters to good.How much of a built motor,will it take to run these?
                                               Larry

I could make your photo a little bit more visible.
The aluminum adapters might be for Kawasaki? Mikunis were very common on Z1 bikes. VM26, VM28 from factory, VM29 as after market tuning.

VM29 seems to be possible on a std CB750 according to Sudco.
836 + ported head + cam will probably be a good combination with VM29.

Parts list with jetting examples:
*** Update 6 July 2023 ***
Actual Sudco catalog found on www.sudco.com
http://sudco.com/38pdf/38.pdf
Page 166 with parts list and jetting.


VM29 is not described here, but the tuning hints are very informative.
 http://www.mikuni.com/pdf/vmmanual.pdf

Inlet adapters must have a smooth inside which make it tricky in the transitions between rubber and metal when making own. Thick rubbers that can be carved inside to match carb spigot, pipe bend, spigot on head....

[lwahples]

The link to Sudco is pretty cool! Thanks for posting it.
                                                                               Larry

[PeWe]

Update:
Got correct rubbers today that fit the head spigot better. Complete set found on eBay.de (sandguss)
There are a lot of nice parts for CB750 to purchase, ebay as plenty of small shops with new produced parts and NOS.
My bike will also get green tank with side covers in OEM style with all emblems..... Found on eBay... finally like a nice CB750

[Don R]

  Somehow, I stumbled onto this thread. I found new sudco "plastic" rubbers for my 29 smoothbores. This appears to be the last set around though.
https://andrewsmotorsports.mybigcommerce.com/honda-cb750k1-k5-cb750-76-cb750f76-cb750-sohc-vm29-manifold-kit-005-026/

[PeWe]

I think forum member BenelliSEI in Canada bought them or very similar to his chopper he bought with the Sandcast he restore.

Dynoman had for some years ago.  But I had already got the rack cut and welded back then.

It will be fun to use these again to my CB750 build. All jetting changes in my TMR carbs  using various sizes of the needle jets and how that affect the needles and pilots will help.

I understand why I had to increase pilot jets back in the 80's when they had Kawasaki 900 O-6 sizes instead of Sudco's recommendation of O-8.
Wrong size (here too small) will make problems that will never go away.  Larger pilots/needles to compensate a specific area will cause problems in another throttle lift.
Needle jet size also work together with the needle taper.

Then a question why Kawasaki 900/1000 need smaller needle jets with same needles. Maybe the displacement make higher underpressure?
I'll see.
My last use was with 836. I have notified all jettings with comments in an Excelfile .
Different pilots, needle jets, air jet, needle heights and main jets. Airscrews very often at 1/2 turn out, also where others have written.

I'll see, maybe next year

[BenelliSEI]

I think forum member BenelliSEI in Canada bought them or very similar to his chopper he bought with the Sandcast he restore.

Dynoman had for some years ago.  But I had already got the rack cut and welded back then.

It will be fun to use these again to my CB750 build. All jetting changes in my TMR carbs  using various sizes of the needle jets and how that affect the needles and pilots will help.

I understand why I had to increase pilot jets back in the 80's when they had Kawasaki 900 O-6 sizes instead of Sudco's recommendation of O-8.
Wrong size (here too small) will make problems that will never go away.  Larger pilots/needles to compensate a specific area will cause problems in another throttle lift.
Needle jet size also work together with the needle taper.

Then a question why Kawasaki 900/1000 need smaller needle jets with same needles. Maybe the displacement make higher underpressure?
I'll see.
My last use was with 836. I have notified all jettings with comments in an Excelfile .
Different pilots, needle jets, air jet, needle heights and main jets. Airscrews very often at 1/2 turn out, also where others have written.

I'll see, maybe next year

Per..... I did find a set for the chopper, but truthfully, they had a horrible bend in them! Your solution looks 10X better and likely works 20X better!

[BenelliSEI]

Per.... here’s the SUDCO units I used, last ones on the page. J.

[Don R]

 You can google them and find ads for them but you can't buy any. I'm convinced I got the last set in the wild. There is also some info about Mikunis in the other bikes thread in a sticky at the top.
 I think the UPP brand are what Sudco was selling and are urethane rather than rubber.

[Don R]

 I found them in case anyone is interested.
https://www.z1enterprises.com/carb-holders-4-cb750-for-vm29-vm28-vm26-carbs.html

[Don R]

 Update; I stumbled across this thread again, my Sudco/UPP urethane carb adapters are still intact. The bike only has a hundred or so miles on it due to a top end rebuild and miscellaneous other issues so they haven't been subjected to a lot of heat.
 I did run the bike with a wide band O2 sensor probe in the 4-1 exhaust, the 29's appear to be properly jetted. full throttle a/f is 12.8 to 13-1 part throttle cruise it leans up a bit. I tried a video but the digital camera doesn't see the LED readout very well.

[grcamna2]

Update; I stumbled across this thread again, my Sudco/UPP urethane carb adapters are still intact. The bike only has a hundred or so miles on it due to a top end rebuild and miscellaneous other issues so they haven't been subjected to a lot of heat.
 I did run the bike with a wide band O2 sensor probe in the 4-1 exhaust, the 29's appear to be properly jetted. full throttle a/f is 12.8 to 13-1 part throttle cruise it leans up a bit. I tried a video but the digital camera doesn't see the LED readout very well.

Good to see z1enterprises still has them.

[willbird]

Update; I stumbled across this thread again, my Sudco/UPP urethane carb adapters are still intact. The bike only has a hundred or so miles on it due to a top end rebuild and miscellaneous other issues so they haven't been subjected to a lot of heat.
 I did run the bike with a wide band O2 sensor probe in the 4-1 exhaust, the 29's appear to be properly jetted. full throttle a/f is 12.8 to 13-1 part throttle cruise it leans up a bit. I tried a video but the digital camera doesn't see the LED readout very well.

I have noticed in a lot of Youtube video that capturing wideband reading on video is pretty erratic. The refresh rate must be close whatever a common video frame rate is. Where did you mount the O2 sensor ?? Which wideband did you choose ?

Bill

[seanbarney41]

I found them in case anyone is interested.
https://www.z1enterprises.com/carb-holders-4-cb750-for-vm29-vm28-vm26-carbs.html

Wow, I own a set of these boots that I bought here used years ago along with a rack of vm26's.  I still have not tried to run a set of Mikuni's on a cb750.  My boots are certainly used but I have no idea for how long or for how hard.  They still are soft enough to use and  not cracked.  I am tempted to grab a new set from Z1 Enterprises as the viability of running vm's on a cb is gonna go away forever probably when these are gone.  But wow!  They are sure proud of 4 rubber boots.

PeWe that link to the Sudco base jetting chart is very useful!  I would love to read their tuning tips but the link no longer works for me.  If you still have access to that page, please update the link!

[Don R]

 I had racing contingency money from Autometer and spent it on a wide band gauge with memory. I made a probe with some chrome moly tube I had left over from a funnycar repair. It flows pretty good and goes way up in the muffler, the vertical tube is to mount the sensor and provide a drip chamber for condensation.
 On the green bike the tubes weren't fully welded yet and leaked some air causing the lean idle readings.

[grcamna2]

Don,the bottom pic you mentioned with "the green bike" and the gauge;is that a K0? you still have it? Love that color