Project Cafe Rocket 686 begins. Finally!

[edfmaniac]

They even make a TM36/40 - B70 and have it listed under Yamaha XTZ660 / SZR 660 / MUZ 660. That may be a bit much though unless I send the head back for more aggressive porting.

[basser23]

Think the 36/40 would be a bit much like ya said....maybe able to find some FCR 33 or 35's on evilbay for less than the Miki's

[Srinath]

edfmaniac: Your piston looked OK in the pics on page 2. Have you sold/got rid of it. I am looking for something that I can use to measure the thing. I am also really worried I may have burnt it by having it too lean. I have to likely bore the thing so I cant use the 100mm. But alteast I can get weight and height and so on from an intact piston.

Anyway I'll jet the living daylights out of it ... to the point its so rich it wont even run, then try backing it a hair. The crazy thing is ... on GS500's I must have jetted about 500 carbs over the last 10 years. I have personally run a gs to 48k with it consuming more and more oil but never showing any signs of it threatening to eat a piston ...

Cool.
Srinath.

[edfmaniac]

Don't have it and it wouldn't do you any good anyway. Just inspect your cylinder. If it doesn't need more than a clean up and hone, you''ll want to measure it and buy a piston that is just a couple of hundredths of an inch bigger, then have the cylinder bored to match the piston at your local machine shop. They should want the piston and/or the manufacturer's specs for it when you drop it off. I made the mistake of pretending like I knew what I was talking about and asking them to match it with another cylinder that I brought in, only to have to make another trip down there with the hi comp piston so they could get an exact measurement.

[Srinath]

Yea I read that part too, made me cringe, both then and again when I read it now.

I suspect my piston has rings stuck, not a hole, I am thinking of putting 20/50 wt oil in the chamber and seeing how fast it runs out. I suspect it will almost hold that. There is no way this guy has a hole, it still puffs quite pronouncedly in the exhaust ...

What ever, distinction without a difference.

Cool.
Srinath.

[Srinath]

You dont think if the piston and ring has fused together but ntohing else has been damaged -

Somehting like this and a good stock piston will work ?

http://www.ebay.com/itm/01-06-YFM660-Ra ... 05&vxp=mtr

Thanks.
Srinath.

[Srinath]

OK my un startable bike had a 102 piston in it. The piston is by JE and it said 102m on the under side.
And the rings were broken and stuck, the piston was cracked from the rod to the top and there was just 1 base gasket.
Is it possible that it had a squish height issue ?

I am taking it to mechanic to find out why ... but I may not know why for a while ...
I will put up a pic, the piston seems very nice, except for the crack in it ... as in, no pock marks, no sign it was melting ... cyl wall also OK.
The bike was harder and harder to restart for the last year and when it ran it had decent power.
I also seem to have cams with a extra layer on the lobe - a grey layer. Pics coming up of that too.

Cool.
Srinath.

[edfmaniac]

The only way I know to determine if the squish was too tight is to measure it. I can't believe a bike that you were riding regularly got that bad without you noticing something first. Scary!

[Srinath]

I'll tell you what is worse ...
I consider myself a carburetion expert. I've jetted and run about 25 GS500 for local GS riders and my own ... all of which I have tested like crazy. Worse yet, I've done probably 350-400 sets of GS carbs. People send it over and I jet it, adjust it and send it back, and they almost toss it in and it works nearly perfect. I actually tested all the setups on it with an O2 sensor in the exhaust though. I didn't do that on the MZ.
Anyway this MZ has pretty much scrambled my brain ... I have to get it set right, cos I have 2 of em now. Cant have the other thing go this way too.

Anyway, is it a problem to run the original airbox and filter not to mention the stock carbs that have been jetted with a 102 high compression bore ?

I'll get the thing checked today, maybe I can use the cyl etc and just replace a piston. Of course the option is that I buy a stock cyl and piston and call it a day.

Cool.
Srinath.

[Srinath]

I guess I need to stop sounding so much like "woe is me" "why me" "Its not fair" ...

Seems like someone did a 686 build, did some 1/2 assed work around it and tossed it in my lap.
It did run and run like the dickens ...
I guess I should try to save the cyl, put another 102 piston in it and get all of it put back right and see if the carbs work ok.

I'll measure the valve dia etc and post. And the cams etc etc pics all around coming up.
One thing I am is ... patient.

Cool.
Srinath.

[Srinath]

edfmaniac: I looked and I could not find it - what compression is that piston - I know you had an extra base gasket etc, but what was its spec'ed compression ratio ?

Thanks.
Srinath.

[edfmaniac]

I'm running 11:1 but still haven't figured out why the engine starts to break up above the factory cut off range. I've stopped trying to resolve that issue and am just running the OEM ignition until I can afford the Mikuni's. If the engine still won't pull all the way to 9000 without losing combustion, I'm going to fabricate a super bad ass air box. Big singles are supposed to be a pain in the ass once you start putting out real horse power.

[Srinath]

I'm running 11:1 but still haven't figured out why the engine starts to break up above the factory cut off range. I've stopped trying to resolve that issue and am just running the OEM ignition until I can afford the Mikuni's. If the engine still won't pull all the way to 9000 without losing combustion, I'm going to fabricate a super bad ass air box. Big singles are supposed to be a pain in the ass once you start putting out real horse power.

Thanks, I am thinking I'd just do 10:1. The cams may be modded, but valves are stock. The cams measured 1/2mm larger on lobe size, dunno if they are modded, also they had a steep opening ramp and a more gentle close side. Not like my GS500 that has a symmetric cam.

The possibility is that you may be too rich ... you know in the old days they used to put bigger carbs and bigger jets on the 750 as opposed to the 1100 GSXR. A bigger chamber and longer stroke = more intake velocity, and combines wiht smaller carbs meant it would need smaller jets.
Cool.
Srinath.

[DAVID THOMPSON]

this is a very short stroke motor...one of the problems is the time to burn the fuel your getting in there...
like a Diesel there may not be time to get it all burned before the next stroke...so you may reach a rev limit you can
not get past because it just does not have enough time to get the job done..

there are limits mine was a mac10 go kart motor in the 1960's the piston top welded its self to the head
when the rod stretched to far from the rpm and the heat

[basser23]

Hey David, I had a Mac 7 and a 9 that did the same....it was fun before that