Mikuni TM-36 Questions for GT 750

[tz375]

FYI, I ran a simple set of simulations through MOTA for the same 80HP motor with Jemco pipes. I ran it for 30mm, 32mm,43mm and 36mm crabs to see what the quick and dirty answer was.

At peak HP (6500) highest HP was 79.3 and lowest was 78.4 - no real difference at teh top end from 30 through 36mm.

At 4,000 the difference was huge. Lowest 16, highest 25HP. The lowest was the 36mm carb but the highest was not the small carb, but 34mm. I don't have time to work out why, but the difference between 32 and 34 was small at that HP level - all the way up the rev range.

At 7,000 all lines were at the same point +/- 0.1hp All very interesting. The probable answer is that smaller carbs are just the wrong size for the ports at low flow levels but at peak revs, gas velocity is high enough to cancel most of that out.

In the real world we usually are using less than full throttle at 4,000, so in the real world the difference is likely to be 3/5 of 5/8 of SFA at most revs. Pickup will probably be different though, favoring smaller carbs.

Moral of the story is that Big is not always better.

[jeff kushner]

"At 4,000 the difference was huge. Lowest 16, highest 25HP. The lowest was the 36mm carb but the highest was not the small carb, but 34mm. I don't have time to work out why, but the difference between 32 and 34 was small at that HP level - all the way up the rev range.
"

We did all the experiments years ago and found the 34 to be the best size using the round tech for these engines so I'm glad to see the modern software backing that up. Another thing we found was when Zuk switched to the dreaded CV's, we'd rip them off for smaller rounds and had better response and more top end. They just needed the much larger 40's (CV) to compete with what 32 rounds could do. Their goal was all about emissions, not power. I've switched many CV'ed bikes to rounds and other than a little tuning effort, an easy and worthwhile switch.

Some of the guys bored their 34's to true 35mm....did you run any sims with 35's? We told them they were wasting their time & money.



jeff

[tz375]

Those BS40s are interesting. They are different to other BS series carbs used on bikes like the XS650 Yamaha in that they have a raised floor to allow for the additional primary air flow. That reduces the effective area to a lot less that 40mm. Add in the vertical fuel tube and the fat throttle shaft which restrict flow even more and the flow is further reduced to somewhere closer to the equivalent of a 32-34mm carb.

Back to back tests conducted by a racer back in the day showed that the BS carbs had enough flow capacity to support more HP than most bikes will ever make and at the drag strip they reportedly gave a couple of MPH additional at the top end compared to those even heavier old "VM32" slide carbs off a J or K.

I have not tested any or all of the above on a dyno or at the track, so I can't comment. I also didn't run the sims for a basically stock bike. I was testing on a ported high compression motor with Jemco pipes, which may or may not create the same results.

35mm are similar to 36 but slightly better at most RPMs, but in this case 34 seemed to work best. What that suggests is that for a mildly tuned motor, BS40 carbs are probably the best choice. YMMV.

[Suzukidave]

So if i do convert the FI project back to carbs 34mm should be the size to go to and i did happen to bring along that set of 34s that came off the 440 snowmobile we had talked about yrs ago . I do remember you ( TZ375) saying they might be hard to tune for a reason that escapes me .

[jeff kushner]

But for those of us that remember, or have worked on a lot of vintage bikes....the CV's were touted as the next best thing then sliced bread when they came out mostly in the early 80's however almost to a bike, they were cold-blooded and took forever to warm up. While this is the fault of the settings more than the carb itself, we all saw that they had to fit much larger carbs then had been the case on bikes with histories like the GT750, CB750 Hondas etc. Most "riders" loved them because there were hardly no adjustments and most motorcyclist's hated them for the same reason....kind of ironic. Through in the fact that every command had to be recognized then adjusted for as the diaphragm moved the slide after receiving the input from the throttle causing the delta in pressure....I'm no where close to the fence, I hated the danged things then and still do today<LOL>.

My turbo has 4 very special Mic CV carbs.... made only for the Yamaha 650 turbo they are the only turbo carbs built to be pressurized. The system is such that the 22 pounds of possible boost(I run 18PSI) doesn't force the fuel back into the tank...it's a slick application but I paid heck when I rebuilt them while restoring the bike a couple years back. Lot's of seals....like 40 some as I remember!

jeff

[tz375]

So if i do convert the FI project back to carbs 34mm should be the size to go to and i did happen to bring along that set of 34s that came off the 440 snowmobile we had talked about yrs ago . I do remember you ( TZ375) saying they might be hard to tune for a reason that escapes me .

Not necessarily. All software makes assumptions. In this case it's all about wave dynamics but in a piston ported motor. When I was looking at a reed configuration I needed larger carbs to match the reed box shapes and section changes.

What was the thought I had some years ago? Not sure but probably that a 440 twin might have very different settings to a street bike. I fitted some sled carbs to my bike last year just to see what happened and it was barely runnable at the time. I didn't have time that evening at the drag strip to play with settings. Typically sleds are set up much richer than bikes because of the colder air that they run in. That and the fact that they run really only at full throttle and high rpms. Are teh 440 carbs 34mm small bodies or regular large body I cannot remember

On yours we need to get the EFI hooked up and running and see what the EGTs look like. Having had very limited success with a heated UEGO sensor on mine, I think I'll go back to EGTS.

[tz375]

Jeff, the nice thing about CV carbs is that they are like a modern fly by wire system - but analog. Crack open the throttle and as long as the slide is taking its own sweet time to rise, there's no bog that you get with slide carbs.

The problem with them though is that they do not flow as well as a slide carb at WOT, so they need to be larger to get teh same performance. GT750 40mm CV carbs or 32mm slides. Of course there are BS38s that flow more air than a BS40 but that's a whole other story and more modern BST CV carbs flow even better.

The advantage of a CV carb is smooth street riding and they are good at that. On a race bike they are not always a detriment and Suzuki were racing GSXRs with CV carbs. So did Yamaha and Honda before they went to EFI. Throttle response is still adequate because a bike does not accelerate as fast as the slide can rise. What they lack is that fine sharp pickup as the throttle is rolled on. There's a slight smoothing going on. Perfect for the street.

[pull the wire]

I definitely agree with TZ that CV carbs are much more rider friendly and not as sensitive to weather changes and altitude. When their right their right.

When Harley went with the cv carb in the 90's, most folks hated them cause they didn't know how they worked and puzzled on how to tune them. After a few years most folks loved them for their trouble free reliability and great fuel mileage. It wasn't un common at all to get well over 50 mpg with a stock-ish 1340 HD engine. As mentioned the biggest reason was for emissions. The incredibly un even pressure pulses in the intake of the HD V-Twin didn't seem to bother the 40 mm Keihin CV. Big Bonus

Just like all other manufacturers, Harley had teething pains with the first generation EFI introduced in 94 and upgraded to a Delphi software EFI in 2001. The Delphi system is infinitely tune able (with additional software and a dyno) and ultra reliable but to complicated to retro to a 3cyl 2 stroke. (Sorry guys, I love Harley's too cause I'm an enthusiast of powersports and they been paying my bills for 20 odd years)

I still have my stock CV's but Like Jeff mentioned, too many things in the way and good parts are getting expensive to get. I can buy a big handful of jets for my VM 34's for the price of 1 CV diaphragm.

[jeff kushner]

Gosh PTW, you reminded me of the early Marieli FI system from Harley. I spent hours and hours on a friends but without the correct electronics, it was very difficult to diagnose and repair. We would get it running right then it would start to misfire. Sometimes it was clogged injectors(very sensitive to debris), sometimes the heat caused the misfires........they are STILL haunting my dreams! Earlier this summer, a guy broke down in front of my house. When I walked out front to offer help, he told me that he thought his FI was the culprit...his bike, an early MM system. I apologized that I couldn't do more and offered my garage till he could pick it up but he pushed it down the street....not sure exactly where he lived.

It's good that we can have differing likes and dislikes so there's no real argument, just different preferences.

In learning how to work on a wide variety of systems, you learn the "tricks" of how to keep them in top performing condition. On my CV equipped turbo, after replacing everything inside but the diaphragms which were in good condition, I sanded the slides and the inside where they reside with 1500 or 2000 wet/dry till they passed the "clunk test"; when you drop them, they go straight to the bottom with a clunk and don't gently slide down the bore, taking their sweet time. I also sync them with a manometer after setting the initial opening with a set of small dowels. Of course this only works if you have the rack off the bike (the dowel part). And don't overlook sync'ing the chokes! A lot of guy just connect them but don't realize that the cable adjusters are there for a reason. You really want them raising the choke (enrichener) plunger the same amount to fully open the passage. If you set them up like that, it makes an amazing difference in performance for you CV lovers!!!!

jeff