Flash III PhaseII: Tuning MadDog's Worst (Again!) Setup

[Wheelman-111]

Greetings:

Doffed the stock manifold and carb assembly tonight. From this:




To this:




The designer of the CT manifold evidently wasn't first in his class of industrial design. That rear outside bolt is a pain to install one sixth turn at a time with a chopped-down Allen wrench. This was made even more interesting by a couple of semi-munched case threads - not sure how this happened because the mani evidently had never been removed. Eventually guided all 4 Allen bolts into their tight quarters.

Purchased all the components to fab my own custom cable, as I had done with the CV24 Keihin. You can get all that stuff here:

http://www.dratv.com/mayoowncuthc.html

However my soldering skills suck so bad I couldn't trust those little 3x6mm ends to stay on the 1.5mm cable and gave up tryying. Instead I modded the stock cable to add a half-inch of free pull by cutting off a bit of the casing with a Roto-Zip 1" cutoff wheel. I added a hair too much, so Mik throttle cable opens only 75%. An easier cable adjustment would be nice. I'll have to pull the twist grip to fine-tune.

The pump was plumbed to the manifold port with a T connector for vacuum. Starting up was painless once I remembered it has a manual choke. It turned over rather fast. Then I remembered I'd removed the spark plug. Low speed jetting and uptake was just fine. It idled for 4-5 minutes without issue. Too noisy and too late to test ride tonight. High hopes have I.

Finally can see the new Logo! Looks a lot like Flash!

[Wheelman-111]

Greetings:

MikuniUni Install and first commute ride completed this morning.


The choke knob is hiding there ahead of and behind the slide tower:

Starts promptly on the manual choke, which I can (barely) reach ahead of the shock. Only needs 10 seconds or so at 70 degrees this morning.

The Mik carburets at least as smoothly as stock on the throttle, all the way through the range. Unbelievably, with no attention paid to the jets as received (130 main, 15 slow), it's about as perfect a match for this 78cc Nikasil as I could have imagined. It takes remarkably longer to warm up, but settles to operating temps about 25 degrees cooler than it ran with the stock carb. Perhaps the 109 in the stock carb was a bit lean after all. Notably, there's no trailing-throttle burble - at all. It's disconcerting, as when slowing down instead of the typical ring-ding-ding it just hums as if I'd hit the kill switch.

Performance has yet to be measured. A note here about perceived performance:
The scooter responds like a much faster bike at incredibly small throttle openings. That's no surprise when you consider that at 1/4 throttle, this carb exposes the same access to intake as the stock carb wide open. What remains to be seen is the power increase, if any, at the 24mm's WOT. Based on the midrange response, RPM and gearing will still be the limiting factors for top speed, but it sure jumps ahead harder than the stock ever did, even wide open. The question is whether the very much-enlarged 24mm throat does the engine any good when it is opened up all the way. If the engine can't use all of that volume/area, the vacuum at the venturi goes south and the mixture ratio goes all to *.

[Bear45-70]

Greetings:

MikuniUni Install and first commute ride completed this morning.


The choke knob is hiding there ahead of and behind the slide tower:

Starts promptly on the manual choke, which I can (barely) reach ahead of the shock. Only needs 10 seconds or so at 70 degrees this morning.

The Mik carburets at least as smoothly as stock on the throttle, all the way through the range. Unbelievably, with no attention paid to the jets as received (130 main, 15 slow), it's about as perfect a match for this 78cc Nikasil as I could have imagined. It takes remarkably longer to warm up, but settles to operating temps about 25 degrees cooler than it ran with the stock carb. Perhaps the 109 in the stock carb was a bit lean after all. Notably, there's no trailing-throttle burble - at all. It's disconcerting, as when slowing down instead of the typical ring-ding-ding it just hums as if I'd hit the kill switch.

Performance has yet to be measured. A note here about perceived performance:
The scooter responds like a much faster bike at incredibly small throttle openings. That's no surprise when you consider that at 1/4 throttle, this carb exposes the same access to intake as the stock carb wide open. What remains to be seen is the power increase, if any, at the 24mm's WOT. Based on the midrange response, RPM and gearing will still be the limiting factors for top speed, but it sure jumps ahead harder than the stock ever did, even wide open. The question is whether the very much-enlarged 24mm throat does the engine any good when it is opened up all the way. If the engine can't use all of that volume/area, the vacuum at the venturi goes south and the mixture ratio goes all to *.

The major down fall of carburetors is that they are only efficient at idle and WOT. Slide carburetors are better but the basic premise still holds true. This basic fact is why EFI is such and improvement over the carburetor, it is efficient across the board and is why fuel economy is better, but rarely is the a power improvement. The only why for a carburetor to be inefficient at WOT is for air flow to be so low as so dot draw enough air thru the venturi, thus screwing up the fuel flow. But from your description of throttle response I don't think this will be and issue for you.

[Wheelman-111]

Greetings:

Thanks Bear! Yeah, how hard is it to keep a mixture straight when you have a computer anticipating every stroke with input from temperature, pressure, throttle position and O2 residuum sensors. There's no art to that, unless you're designing the computer.

But you're right, I think the 24mm is the right size - or close enough - for this 78cc bore.

[Wheelman-111]

Greetings:

Curse this Deepinnaharta weather. Rain every day, and so much testing to do. Weather be damned, sez I, donning Dry Rider for a shot at top-speed MikuniUni testing.

It was gusty too. Crosswinds on Major Drive, my preferred Proving Ground. Close by, 4 lanes, light traffic after dark, and a 60-MPH speed limit to keep me from trouble with the local Constabulary. Nik Plus Mik still takes longer to warm up than it did with the stock carb jetted to a true 1.09mm. Runs a lot cooler too, except the obligatory brief jump when I slam the throttle closed after prolonged WOT. However it hasn't exceeded 375 since the Mik went on.

Crosswinds in my favorite part of the test road, so I limited my run-up to 3/8ths of a mile, carrying GPS snug inside the rainsuit. No hill, crosswind from the right, no cars, let's go WOT! It shoots away remarkably quicker than the stock carb ever did, but...


The GPS don't lie. Given the conditions, call it a wash with my previous best of 56.8.

What's harder to convey on the computer screen is how quick it feels compared to before. With the mushy auto clutch and 9:1s, it doesn't actually wheelie (unless I "Help") but the front wheel feels light for the first 30 yards after launch. I wish I'd put a stopwatch to its 0-40 MPH time prior to the Mikuni installation. Maybe I will now and in the future.

So how can a scooter that I say has a lot more power be no faster, you axt? Tire circumference, 9:1 gears, and maxed variator are fixed ratios. If the engine has the power to pull the variator all the way to its limits, the only determinant of top speed will be RPM. Adding a much larger intake and a carb with six (6) times larger aperture area than stock gave it more torque and midrange, but evidently didn't eliminate the engine's breathing choke points, which I believe are the ports and the V8 exhaust. VeeDubh20 is on the record stating that the V8 still limits peak RPM compared to some other pipes, and I now believe his assessment is correct. Around 96-9800 RPM, the torque is gone and the engine simply won't spin any faster.

Going back to my original goals in Flash III Scooter Builds, a quick, reliable 55 is what I wanted. I guess I got it. Part of me wishes I could boast of the 70MPH tops that some enjoy. For now the Interstate is not an option. If it were, I wonder how long an SA/16 with stock cooling system would last. I'll stick to terrorizing the surface streets. For now.

[Bear45-70]

More power (power is a poor term in this reference) does not alway equal a higher RPM and without those additional rpm you will not be faster unless; You change the overall gear ratio to use that horsepower at the rpm it is available at. Also a quicker acceleration, AKA a better torque curve, will make you think you have more horsepower when in fact you have more usable torque. Fast and quick mean different things with engines, top speed (mph) is horsepower (rpm limited) and acceleration (Elapsed Time or ET to drag racers) is the torque curve.

Two strokes even unload, I'm talking just crank the throttle wide open on the stand, will only rev up so high. This is caused by the ability of the engine to pump only so much air. When you reach that limit the engine stops adding rpm, regardless of load. 4 strokes are different because of the extra revolution to complete the combustion cycle.

[Wheelman-111]

Greetings:

All is exactly as The Bear said. Flash pulls like a freight train @8800 RPM until just after the variator maxes. Then the revs climb pretty decisively until 92-9300. After that, you can feel it tumbling off the far side of the torque curve. All powered up and noplace to go. I don't plan on holding 9500 for too long ever again. That's the only time I'm seeing the temp gauge get much above 325. Evidently skirting safe piston Speed Limits. I hear the Gendarmes in that neighborhood are unfriendly.

[maddog]

dude you maxed out your stock build and did it the right way- congrads. the larger carb works on minarelli oddball scoots like my italjet or gtrdou to the gear ratio, you can try the dio trans conversion which nobody has done on a stock setup, vt cycles has it for about $90 cut the cases and trans covers and i believe your stock scoot will do 60mph= thats hot!!! wheelman has the wrong gear ratio for his 78cc setup ooops! now he knows that 9:1:1 is only for a stock setup.

[Wheelman-111]

Greetings:

I agree with you 'Dog. This bore and Mik+V8 setup could munch on 8.4 for sure. With the 9:1, it already ran 49.2 on my GPS with the stock 50cc bore kit. But 7MPH faster doesn't begin to describe how much stronger it got with the 78cc Nikasil.

It's always give-and-take with gearing, isn't it? If you go for the top speed, you lose some accel. I ride almost exclusively around town. 4 blocks then a light. 2 blocks then a stop. 9:1s make more sense, even if I can't enter the Top Speed Hall of Fame.

Still. a bit more meat on top, or at least avoiding a meltdown if I get stupid, is attractive. I dunno, maybe the ZX trans or maybe just a bigger Ruck vario. I have no complaint about the low-end/accel now, so maybe not worth cutting the back of the case.

BTW I still have the ball-bearing spring shim... Thanks, 'Dog!

[808BMW]

I say it all the time, I live on a hill with an 18% grade, have 6.9:1 gearing, and no problems whatsoever.
ZX trans is definately helping the bottom end, but even with the stock honda trans I thought it was fine.

Having gearing that is "too long" for my hills was always my biggest worry about building a bike, but as long you have a bore kit and good tuning you are fine. 8.4 is perfect for you since you want to keep as much bottom as possible, but maddog is totally right, 9:1 is for stock bikes

A lot of motocross teams switching to EFI have noted at their level of performance, there is surprisingly no difference in gas milage.

[Wheelman-111]

Greetings:

Living with Flash III this past week has been pretty darn good, I have to say. I haven't so much as removed a side skirt in 10 days, unheard-of in the OCD Wheel-world.

The starting ritual had a step added. I must activate the manual enrichener, which I could just barely reach under the side skirt. Now I just keep Melman, my trained spider monkey in the big underseat storage bin. I send him down there to pull the choke knob. It needs about 20 seconds on the enrichener before it starts letting me know to turn it off - 4 stroking at a fast idle.

After that, it just... well, uh, Scoots! The engine really has loosened up and occasionally threatens to wheelie, especially when the clutch is cold. Buzz up to 50 in practically no time. I do have to resist the temptation to keep WOT above that, for fear of prolonged over-rev, but I have the tach and temp gauge to remind me. Otherwise, it does what I need it to do, and rather well.

ZX Trans and/or 8.4:1 gears are tempting, but I'm enjoying having clean fingernails for a change. I'll think about it before heading to Utah. I will have to install the vacuum port (Thanks again to ZR50Blank! ) once I figure out what size/thread of tap it requires. Squirting the oil into the vacuum port via a T-fitting hasn't seemed to be a problem, but it just ain't right.

[evilone]

I called VTCycles recently and Keoni said they don't carry ZX trans parts anymore and they are getting hard to come by. Thanks to a lead from zr50-blank i have ZX rear pulleys on the way.

[SpyderMike]

hey guy...we were talking about homemade cables and your lack of soldering skills

well i found something for ya

http://www.sip-scootershop.com/EN/Produ ... ottle.aspx

[Wheelman-111]

Greetings:

Great find, Spyder!

[maddog]

Well its late and i can only laugh at your setup= your piston must not have any skirts to clear the case= i would just run you for pink slips and beat your sorry * low budget piece of crap