Sorta-stuffed crank in AV7

spencerrrrr r /

Assembling a stockish AV7 this week from some parts I have laying around. I’m considering the following set up:

This crank:

https://www.treatland.tv/original-motobecane-crankshaft-p/mbk-racers-crank.htm

With this cylinder:

https://www.treatland.tv/AV7-airsal-50cc-kit-p/av7-airsal-kit-02140239.htm

If I case match the airsal kit to stock AV7 cases will there be enough volume to preserve my primary compression?

The airsal transfers are a little bigger than stock and I see that the crank isn’t “fully stuffed” but I’m not convinced that’ll leave enough volume. If not, I’ve got a stock crank around here somewhere...

Re: Sorta-stuffed crank in AV7

you'll be fine, the difference in primary compression isn't worth worrying about.

you could probably get away with a stock crank, they usually do pretty good on your stock ported type builds but i have seen a couple of them burn up the big end bearing on kitted builds so i like to replace them when possible.

plus with a moby its always nice to do those crank seals, since you cant get to them after the engine is put back together.

Re: Sorta-stuffed crank in AV7

Unless you are turning more than 9k then the stock crank can take it. That full circle crank has RH threads on the magneto side, so you also need a right hand M11 nut for the flywheel. The difference in primary will not make a noticeable difference on a full circle crank.

In my opinion, always on a build (stock crank or full circle) you need to open the mouth of the oil journal with a drill bit so it is funnel shape. That will feed more residual oil to the mains where it is needed. Best practice is to change the spi seals as Graham mentioned.

Also match port the transfer lead ins and index your cylinder to get the best bang for the buck, whether you are staying completely stock, porting or kitting.

Re: Sorta-stuffed crank in AV7

spencerrrrr r /

Ah it looks like it’s actually this guy:

https://www.treatland.tv/motobecane-doppler-crankshaft-p/mbk-doppler-crankshaft-263985.htm

But good to know that I won’t loose much primary with a full circle.

Transfers are matched and I’ll funnel up those oil journals.

Think there will be more vibration with this full circle? Seems like the offset stock crank would act as balancer to the piston.

Thanks everybody

Re: Sorta-stuffed crank in AV7

the balance factor should be close, its just how they achieved the weight reduction and the manufacturing method.

i have seen 2 airsal kitted (slap on kit job) big end bearings go out at well under 9k rpm so i'm leery of them.

oh yeah one other thing i forgot to mention- because of the stock airfilter on AV7's and them being piston port, they like to accumulate water in the bottom end. If they are left outside, or even sometimes if they are shedded... maybe something with condensation, i dunno. Either way i've worked on a lot of AV7's with rusty bottom ends, way more than other types of bikes.

I think in both of those failures mentioned above, there was corrosion in the needle bearing that caused them to fail early. Nothing you could feel, mind you. I was just thinking about it and forgot about that factor. So something to be wary of.

Re: Sorta-stuffed crank in AV7

spencerrrrr r /

Funny you should mention the rusty bottom end. I freshened up a stocker a few years back and parked it in a fairly nice shed for a year, came back and the whole bottom end was rusted up. I’ve seen the that ‘high-water mark’ on a few AV7s

Re: Sorta-stuffed crank in AV7

yeah its bizarre. my brother had a hobbit that got left in a damp garage a few yars ago that the crank turned into a solid rusty mass, but nothing else was affected at all .... super odd. usually with the two stroke oil coating things, even setting up, engines tend to stay pretty clean.

i dunno if its more or less common with a dellorto on a shorty intake vs. the stock bendy has anything to do with it? Maybe something with the exhaust? who knows.

Re: Sorta-stuffed crank in AV7

Those Doppler cranks as of late haven't been great, lobe hitting cases and other issues...

The stock(ish) crank you linked to has been treating my AV7 Malossi (60cc) cylinder just fine, truth be told I really like the Airsal cranks, but either will work great for what your going for

as the God Father of Mobys, Rebel said above.

Re: Sorta-stuffed crank in AV7

Rusty cranks and rusty gudgeon pins happen a lot. If you store a bike in a unheated building best to cocoon the bike in plastic and tape it up goodly. Throw a bag of kitty litter (Pestel beads is best but any clay litter will do as second choice) sliced open onto the floor of the cocoon before you close it up and tape the heck out of it. The litter will wick up any moisture in the plastic bag of bike.

Re: Sorta-stuffed crank in AV7

spencerrrrr r /

Little update with this setup regarding the full circle crank:

Since the rebuild this bike was ridden maybe ~200 miles. The cheapo silent blocks from treats wore out fairly quickly. This weekend I swapped out the thrashed blocks for some of the nice $14 ones as well as the counter spring with a 30# McMaster gas strut.

The new blocks are much harder than the cheapos and the vibrations are considerable. I had to cut some weather stripping to fit behind the headlight bezel just so it would rattle apart at idle.

The problem I’m having now is even with the carb clamped down, at ~6K it will rotate back and forth and at around ~7K fuel foams and spurts out of the top around black piece.

I’m going to swap out a bottom end with a stock crank and see if the vibrations become manageable

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account