Is Engine Braking Throttling My Throttle?

[Minimichael]

So, I have a pretty much stock 5hp B&S, but with a Vm22-type chikuni, custom rubber intake manifold, connected to a basic twist throttle set up. It's all on my no frills arctic cat ramrod frame on 10 and 12" wheels and geared nice enough for fun top speeds near 3O (@4200rpm?) while still delivering a good degree of torque off the line, too. The centrifugal clutch is standard as well and seems to work fine. The bike pulls strong all along the twist, up and down the rpm band, with no struggly spots.

My problem is the throttle seems so touchy and ready to accelerate or decelerate at the slightest blip-change of the wrist, and it's the decel that is particularly noticable and gets tiring because the first and only gear is still engaged by the clutch and PTO but the PTO was just told to reduce work and so my wrist letting go of any of the throttle tension immediately translates into my wheels slowing down.

Anybody else suffer from unintended engine braking on their mini setups? Is this a good case for a manual clutch? I'd like your comments. Thanks.

 

[revnexis]

It sounds like the issue is mainly coming from the sensitive throttle response combined with the centrifugal clutch remaining engaged during deceleration, which creates noticeable engine braking whenever the throttle is released. A manual clutch could certainly improve control, but before going that route, I would recommend trying a softer throttle return spring, adding a small amount of throttle free play, or adjusting the clutch engagement RPM if possible. Reducing the throttle sensitivity should make the bike feel much smoother and less tiring to ride.

 

[Skipp]

So, I have a pretty much stock 5hp B&S, but with a Vm22-type chikuni, custom rubber intake manifold, connected to a basic twist throttle set up. It's all on my no frills arctic cat ramrod frame on 10 and 12" wheels and geared nice enough for fun top speeds near 3O (@4200rpm?) while still delivering a good degree of torque off the line, too. The centrifugal clutch is standard as well and seems to work fine. The bike pulls strong all along the twist, up and down the rpm band, with no struggly spots.

My problem is the throttle seems so touchy and ready to accelerate or decelerate at the slightest blip-change of the wrist, and it's the decel that is particularly noticable and gets tiring because the first and only gear is still engaged by the clutch and PTO but the PTO was just told to reduce work and so my wrist letting go of any of the throttle tension immediately translates into my wheels slowing down.

Anybody else suffer from unintended engine braking on their mini setups? Is this a good case for a manual clutch? I'd like your comments. Thanks.

To me it sounds like you don’t have enough throw on your cable….

 

[Minimichael]

Thanks guys. You all make good points and it seems I might benefit from following up on each one. I'll update this thread as that happens.

 

[Minimichael]

To me it sounds like you don’t have enough throw on your cable….

By throw do you mean slack? I had thought about slack, but a blip is still a blip once the cable is engaged. Or do you mean range of motion? I had wondered whether that throttle's internal ramp wheel could be a culprit.

 

[Minimichael]

Get a stiffer garter spring. It should more readily unlock the clutch on slight decelleration.

It's probably time I opened a clutch anyway. Thanks.

 

[Minimichael]

It sounds like the issue is mainly coming from the sensitive throttle response combined with the centrifugal clutch remaining engaged during deceleration, which creates noticeable engine braking whenever the throttle is released. A manual clutch could certainly improve control, but before going that route, I would recommend trying a softer throttle return spring, adding a small amount of throttle free play, or adjusting the clutch engagement RPM if possible. Reducing the throttle sensitivity should make the bike feel much smoother and less tiring to ride.

Thanks I'll start with the softer return spring idea. This would be the one inside the vm22's throttle slide. I'll post again on this.

 

[Skipp]

By throw do you mean slack? I had thought about slack, but a blip is still a blip once the cable is engaged. Or do you mean range of motion? I had wondered whether that throttle's internal ramp wheel could be a culprit.

It’s what ill
Call throttle play….how much twist. before engagement…,when I put mikunis on a bike usually make a cable….i personally have found I like 2-5/8” play (throw) at the carb end….thats yje amount of cable sticking out from the vinyl sleeve…just my opinion

 

[Minimichael]

Update ...some overview pics of the clutch, carb, cable end, twist throttle pulley housing, the underside of the carb slider cap and some slider return springs.

Clutch: I can't id the clutch but believe engagement happens about 2300rpm. Looking into alternatives like 2600 and 3100 engagements. I may have one laying around and will update.

 

[Minimichael]

Carb: The slide carb is a vm22 (jetted and metered rich to run on this bike's extended radiator hose intake).

Cable end: The slider cable extends out from under the cap just far enough to allow the slider to bottom out within the carb (with another sixteenth of an inch to play with if need be, but I don't since the idle screw controls slider height and therefore cable length). The distance between WOT and bottomed is just about 3/4".

Slider springs: My vm22's spring is the shorter one on the right. I note that with slider installed this short spring is still much longer than necessary, when all that appears necessary is enough spring length to fully return the slider to bottomed out when the throttle is released. So I cut the long one to be even shorter than the short one (but still long enough), put it back together and test rode it and ... While the engine braking is not fully resolved, I found the shorter custom spring did relieve a lot of tension and made easier to hold the twister steady, which reduced the likelihood of my mis-blipping into engine braking in the first place. I believe this one will stick. Thanks for those tips above.

 

[Minimichael]

Twist throttle: At rest, the fully backed off throttle is marked at about 12 o'clock on the grip. There is no slack, as I see it, since the slightest turn toward 11 o'clock starts unleashing the hounds. (I'll look into adding a touch of slack here). As the pic shows, I marked the massive pulley housing with tape and also marked the tape with many increments, as I expected to make many recordings (note my handy tape arrows on the bar). But as it turns out, all of the wrist action for riding this bike happens within the first two ink marks on the tape (ie: 11 and 10 o'clock, or about 7/16"). In fact, you can throttle from zero through about 3200rpm before even crossing the first tic mark (so mostly between 12 and 11). 10 o'clock is pushing over 4500rpm. So, I've determined that my pulley may be too big. I think with the slider only raising up about 5/8" inch to reach WOT, and with the huge grooved pulley in there getting that job done in such short order, that it steals all the range out of the wrist work. In later posts, I'll be trying a spare throttle or two and updating again on this.

 

[Minimichael]

You have a quick twist throttle. That is contributing to, if not all, of your problem. They're designed for racing where you're mostly at WOT constantly. A lot of hobbiest have purchased them for their price and looks.

Your spring looked fine. I've ran cable play at varying length with no issues. I make my own, so I am able to adjust to whatever. I wouldn't have cut it. The springs have crimped ends and they serve a purpose. This was not your problem. It only seems less of a problem because you have removed some return spring tension. The tension on these is good for solid returns on that slide. I'll argue it with Dave off line, LOL. Your problem is 100% your throttle. Get a new twist throttle, and a new spring please. You'll have to rebuild the cable. I can help with that.

Ok. I'm familiar with making cables, but thanks. The spring I cut was a spare and I'll just put the old one back in. Will order a new throttle, as the only non-racing one I have is for 1" bars.

 

[Bonanza MX]

To me it sounds like you don’t have enough throw on your cable….

That is exactly what I’m thinking. When I raced motocross we had quick turn throttles. They were maybe a 1/4 turn to full wide open. It was all or none which is perfect for racing. In hare scrambles riding through the woods you would wear your wrist out if you had a normal throttle

 

[Havasu Dave]

It’s what ill
Call throttle play….how much twist. before engagement…,when I put mikunis on a bike usually make a cable….i personally have found I like 2-5/8” play (throw) at the carb end….thats yje amount of cable sticking out from the vinyl sleeve…just my opinion

Well, knowing it's 2 5/8" is probably worth a cookie. Works on real Miks though. Not on Chiks.

 

[Minimichael]

Update: I just got a cheap throttle with variable twist speed options via 3 interchangable cam shapes. I'm learning that my throttle was not only quick, but also progressively so. In its own way, each shape in the kit is a stable circle, but my original twister's arc bulges toward elliptical, and so increases throttle opening speed as I'm nearing WOT.

 

[Minimichael]

So this cheapo kit's swappable cams are essentially slow, medium and fast, and the medium one roughly tracks with the speed range of my original throttle's pulley/cam.

 

[Minimichael]

I test rode the bike with the slow cam. It was a longer wrist twist to get up to top speed range, but not too long. It's also a smoother acceleration than it was before. But the original problem is still sorta there. Let up on the twist as you're approaching speed and that feeling of engine braking remains noticeable enough that I'm not done. Plus, now I miss a little of the low rpm grunt that used to be there – with the slow cam, anyway.

To be continued..

 

[Mark Brooks]

Keep us in the loop minimichael great info here for those of us that hv no clue, like me. I didn't come from motorcycle world. Not allowed as a kid.

So question lurking in the shadows...

Thumb Throttles, is this a viable option? Pros vs Cons thots?

 

[sparkwizard]

I believe Havasu Dave suggested stiffer garter spring in your clutch to disengage at a slightly higher RPM. Have you looked into that option?
That would allow smoother decel at part throttle.

 

[Minimichael]

Just picked up the rough equivalent of a blank canvas. Time to attempt some custom mods..