Vertical Minarelli BBK install

[twinjogs]

I have an old 89 Yam Jog with Vertical Minarelli, It has modded 50cc (machined head) typical racey 6-reed intake (just fits in front of rear tire) and a Technigas pipe??, Im not really sure...Chromed expansion chamber, greenish carbon fibre silencer. Scooter ran reasonably but hot 325-350C. Might of soft seized at part throttle if I recall, was running a stock carb from a vertical or a horizontal, I dont remember...its been a while. Weird but it has a 21mm/15spline crank but only a 10mm pin....oddball?

Anyways, the desire to put it back on the road has me finding an Athena 70cc sport kit for $80, I couldn't say no.

Reading the instructions carefully as I never do I see it says 15.3:1 compression (really?) I thought this was a street head, what kind of AV gas is going to be needed for this? Any thoughts about a carb replacement size for this combo?

Thanks

[harleyracer59]

compression causes heat. so milling the head on your old 50cc is probably what caused your excessive heat. your new motor should run on pump gas 91*. if not put another head gasket or a thicker one to drop compression ratio
carb id run 19-24mm.  

[twinjogs]

I turned the head on a lathe to surface and add a squish band mostly. I can't really remember how much I decked it. I didnt have a tach for tuning just a temp gauge epoxied into head. Ill try and do better this round.


These are pictures of the exhaust, Im pretty sure it is a Tecnigas RSii






I found a Mikuni 26 from a KDX80 that runs well, Its a bit big but I thought I could make a bushing to reduce the outlet ID to 20mm and give it a try. OR is that hopeless??

I ordered an OE airbox so when I get that and if it fits well I will look for a 17.5/19mm to try an keep it more OE. I may have to ditch the 6 petal intake. I don't really like the fitment\noises of the PODs I have had in the past

[2stroked]

A 26mm carb on a 70? IMHO that's a bit over kill, and won't give better performance than a 19-21 mm carb. Bigger carb will likely cause more issues. But,that's just my ¢2.

[twinjogs]

Too busy over the summer to get this done, but on it know...for spring

Added a '05 Zuma font end narrowed with a custom lower clamp to fit the narrow little jog. Added a diagonal brace after seeing the pretzel Zuma donor cycle. And a pair of 18spoke jog wheels with Michelin S1 3.5 tires. Safe now I think so time to make it faster.

Original Technigas RS70 pipe - probably crap but its what I have
19mm Carb
Athena 70 BBK

The Flywheel wobbles on the OE crank so I tossed it and bought a 42mm stuffer type. Not 100% sure the crank fits the case but I can bore it if required...what kind of clearance should I be looking for?

Is there a good thread to read about how to fit piston/stroker/port heights etc? I know the basics but info from more knowledgeable folks would be best here, I only want to do this once or twice. Thanks

[Lucass2T]

Tecnigas rs70 isnt that bad they say. Only read good stories about it. I have one too, but haven't tested it. Your setup looks nice, carb is a good size for the cylinder/exhaust combo.

[twinjogs]

Hey All

Ive asked these questions before but didnt really get an answer from the xsperts.

Try again:

I am building\rebuilding a vertical Minarelli in a Yamaha jog.

I noticed the flywheel wobbles on the crank and one end of the crank is bent. It runs true on V-blocks but the flywheel end is out so maybe it got dropped?? Its one of 2 cores I have that I do not know the history of sobeit.

I tossed the flywheel and got greedy and bought an Asian 2mm stroker crank. The crank has arrived and it is not a bolt in. The cases need a little hogging out with the boring head.

Q: I bored the case halves to counterweight size +0.4mm, is that enough clearance?


Cylinder/Head fitment:

Athena recommends 0.9mm squish, I think that is a max figure, so I can work towards that but, I realize my piston dome at BDC should be even with the bottom of the port windows for best cooling... what if its not?

If the piston is above, you can add base shims to raise the cylinder. If the piston is below, then run no shims/gasket...then what? Mill/Turn base of cylinder? and then what happens to your squish? Is it ok for the piston dome to be lower than the port window...what does that do?

It was all close to reasonable before the stroker crank, so Im thinking now it will be off. I have not yet mocked up with the stroker so stay tuned.

Q: Anyone know of a good thread\resource that details setting up cylinder port heights and squish?

[niz76]

Yep you can add base gaskets/shims to raise cylinder to get piston crown under the port windows but of course that opens your squish gap up top. Welcometo 2t tuning! lol. Of course in a best case scenario you'd take the time to get piston below the windows and also close up the squish gap but it takes effort- especially on a stroker many times... There's tons of good write-ups in the Tech Library on these subjects.