mini-stroke advantages

Does your Spree/Elite already run great, and you're trying to make it quicker/faster? Need a monster motor swap? Discuss your ideas here.

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evilone
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mini-stroke advantages

Post by evilone »

So what exactly are the advantages of a mini-stroke crank other than it holds more volume?
I was thinking about some disadvantages.

You need to keep the compression low so crank does not get damaged and to keep from blowing a hole in the piston. So we add as many as 4 base gaskets to achieve this, but for every base gasket you add it raises cylinder possibly having an effect on the port timing. Raising cylinder also increases crankcase volume which sooner or later will be "too" much volume. I may be wrong but can having all this extra crankcase volume make it harder to tune a carburetor and make it a lot more sensitive?

Just some thoughts. Last year i ran an R1 pipe, stock crank, and i had 7.9:1 gears. I can't help but think the PG Long and 8.4:1 gears with stock crank would perform just as good as using the mini-stroke. I can hit 70 mph now but getting there is not much quicker than last years setup, i'm thinking the stock crank with port timing where Malossi intended it to be along with PG Long and the 8.4:1 gears will also hit 70 mph. The R1 was holding me back and the 7.9:1 gears made motor labor more. Just some brain-storming...any thoughts? :)
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Re: mini-stroke advantages

Post by Wheelman-111 »

Greetings:

Yes, more displacement is the "advantage". More fuel burned per stroke can produce more power, if the setup is adjusted correctly in terms of port timing and compression. I reached for too much (150PSI) and paid the price in durability. (150 miles... :( ) But it had power. The crank itself is fine. Even the cylinder looks like it's restorable. Only the piston couldn't handle the combustion chamber heat escalation during that fateful run.

The crankcase volume doesn't change with a stroker crank. The movement of the piston up inhales a fraction of the cylinder's total volume into the crank. ( I say a fraction because the incoming charge is under negative pressure...) Then the case charge swaps places through the transfer ports and is fed to the cylinder. One disadvantage of gaining displacement through longer stroke is higher piston speed, but going from 41.4 to 44.1 is hardly a game-changer. Strokers - all else being equal - also make more torque at lower RPM than an engine that uses a bigger bore diameter, so RPM isn't as much of an issue.

The final issue is of build quality and precision. Stock Honda cranks are reputed to be hard to beat there. I used Honda bearings instead of the Ruima-supplied ones for this reason. New piston's on its way - thanks Taz!
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Re: mini-stroke advantages

Post by evilone »

HMMMMMMMM, i wonder why the heck my carbs are so tempermental then. Last summer when using stock crank i could go up or down in jet size no problem, now it seems both the 26mm and 28mm want to run fine one day and like crap the next day. It's more at idle and 1/4 throttle when they get choppy (kinda like pulling 1 plug wire off a 4 cylinder motor in a car), that's the best way i can think to descibe it. So then i go to change the pilot and the d*** float bowl gasket gets fuel soaked and swells up so i can't put carb back together till gasket shrinks. It's getting old really fast and wondered if mini-stroke might be causing it....i didn't think it would be but that's all i changed other than exhaust. Another thing, last summer with stock crank i didn't have the lag up to 15 mph before it took off like it should and that has me wondering too. I'll try it with 4 base gaskets and if it's still hard to tune carb i might try stock crank just to rule the mini-stroke out. I just want to ride and the wife says "let's go for a ride" and i reply "it's running like s*** again". :evil:
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Re: mini-stroke advantages

Post by bakaracer »

As the saying goes"no replacement for displacement" and that the longer stroke gave you more torque to get past the wall that the stock stroke couldn't do.As for the carb tuning,weather change will be the reason for the tempermental 2 stroke engine.I have a winter jet setup and summer jet setup.I usually jet in october when the air temp drops to the high 40's low 50's and change it to the winter jet setting.april is when I change to the summer jet setting when the air is above high 50's.
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Re: mini-stroke advantages

Post by Keoni »

I'm personally against mini-stroker cranks. I've seen to many issues with them from build quality to not working very well but when they did work they ran pretty decent. Port timing and compression is always an issue with them as well as making sure you have enough of a spacer/gaskets to make whatever cylinder you are using work well with it.

If you are a technical person than you might have fun with it because it is a lot of work getting it to work properly but if you like to bolt things on and go I'd stay away from it.
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Re: mini-stroke advantages

Post by evilone »

Keoni wrote:I'm personally against mini-stroker cranks. I've seen to many issues with them from build quality to not working very well but when they did work they ran pretty decent. Port timing and compression is always an issue with them as well as making sure you have enough of a spacer/gaskets to make whatever cylinder you are using work well with it.

If you are a technical person than you might have fun with it because it is a lot of work getting it to work properly but if you like to bolt things on and go I'd stay away from it.
That's the thing, it seems like everyone with the mini-stroker have polini or other brand big bore. I'm the only one i know of with a mini-stroked Malossi, sanjuro has a Malossi but he's using a stock crank. That's what makes me wonder if it does affect the Malossi kit, it can run great and an hour later run like crap. Last year the carb made it run smooth all summer long with no jetting changes but i ran the stock crank. So....any other Malossi 72cc mini-strokers out there who have/had same concern? I can tune a carb but this is more irritating than it is fun.
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Re: mini-stroke advantages

Post by bakaracer »

I think its just a coincidence that your runing into jetting issues.Mini stroke setup and stroker setups are more sensitive to jetting because of the amount of air its sucking in and exhaust and case psi pressure increase.I think your runing into jetting issues is because of the seasonal changes in the air temp and berometric pressure changes which will mess with your jetting.I hawaii it doesn't change much there so they don't run into those issues.for them if the air gets colder,all you have to do is adjust the air fuel screw a bit and its good.Here in the states depends where you live,I found a complete jet setup change is needed.
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Re: mini-stroke advantages

Post by evilone »

I'm going to try figuring this one out till i'm at wits end, thanks for all the help and replies from everyone, i really appreciate it. I wish them dang pilot jets were easy access like the main jets. I don't mind changing jets but when i'm in the middle of tuning and the gasket for the bowl is swollen and too big to re-install is when it gets irritating.
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Re: mini-stroke advantages

Post by bakaracer »

are you trying to tune the 28mm carb now?
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Re: mini-stroke advantages

Post by evilone »

Yes, i'm trying to get both dialed in to do a comparison of throttle response and top end between the two. It got too cold to ride here again so i haven't tried the 138 with the 52 pilot like you suggested yet. Same setup as last year only i was at 138 and 55, that should get me close. I took out the 145 that was suggested and checked plug and it was very wet, obviously toooo fat.

So....am i apparently the only Malossi mini-stroker here?
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Re: mini-stroke advantages

Post by evilone »

Below is the way i try to tune my carbs, Does this sound right for our small motors?

As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. If your engine doesnÔÇÖt respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet.

Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.

The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply.

Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.

Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isnÔÇÖt one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting.
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called shotgun.
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Re: mini-stroke advantages

Post by Wheelman-111 »

Greetings:

Nice write-up, Evil.
I'll copy and Sticky in Tech next time I can get to a proper Magic Electric TV Typing Machine.
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Re: mini-stroke advantages

Post by evilone »

I got that off another website so i don't know who to give credit for the write-up but it deserves stickied. I just copied it and pasted it to Microsoft Word for personal use.
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Re: mini-stroke advantages

Post by bakaracer »

yup thats about right!what you just posted is what I was explaining to you earlier.
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Re: mini-stroke advantages

Post by trains420 »

what's an insulator. the white part. so the threads at the end of the spark plug like when your turning it in, the last ones, to go into the threads. near the white part.
and
If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly

bike bogs, sounds rich, so lean the engine, by lowering the clip, raising the needle. and bike sputter and sound rough, sounds lean, and raise the clip and lower the needs to make it richer.
i though you lower the needle and it clogs up the hole and you can't get as much gas, leaner, and raise the needle, let more gas in, it runs richer. which way is it. does anyone know.
and on the stock carburetor, when you take the top off with the cable going in it, and the needle is under it, there's a spring going around it. i don't want to tear anything up, and i don't know how it, well, i don't know what's in there, and how to get it back together, just what do i turn to get it off. pull, unhook. big mess in there just looking. anything to break. is it cool to just start using pliers and pulling stuff off.
is there a picture of the parts. and the clip is a cir clip ive moved them before, forgot which was was rich.
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