Single point seizure

[Mekanix]

How about RAVE Valve Clearance ?? Were the valves fitted to the new bore??

Yup. Installed the Raves on the cylinders and then put the Pistons in it to measure the actual clearance between the piston and valve even when the Piston is rocking side to side and the valve is moving side to side.

I used the actual piston with a piece of sandpaper wrapped around it to shave the Raves down and make sure they were properly rounded to fit the cylinder.

 

[etemplet]

.020" clearance on the raves and the piston ?? If not I'll bet they are making contact. The rings extend beyond the piston then you have to add the clearance in the bore. .020" is about as close as I would feel comfortable.

 

[Mekanix]

Can't remember exactly.
I made this whole top end kit for my very first 787 as a backup and I was going with what it said in the manual.

 

[etemplet]

I bored my cylinders 1mm over and I know I went .020" clearance on the raves. I read the procedure somewhere. Someone actually machined the arc on the rave in a milling machine. I used my old cylinders and placed the raves in position, shimmed around the piston to get the bore exact and trimmed the rave to make sure I had at least .020" clearance between the rave blade and the bore. That could be your problem or a possible problem. I'd make sure.... .020" check the manual just to make sure.

 

[Mekanix]

I bored my cylinders 1mm over and I know I went .020" clearance on the raves. I read the procedure somewhere. Someone actually machined the arc on the rave in a milling machine. I used my old cylinders and placed the raves in position, shimmed around the piston to get the bore exact and trimmed the rave to make sure I had at least .020" clearance between the rave blade and the bore. That could be your problem or a possible problem. I'd make sure.... .020" check the manual just to make sure.

That's a good idea and I'll check it out. My Rave caps also have holes in them so I could see when they open and also feel the Rave caps to see if a specific cylinder is misfiring. The very first engine I did I went overboard on The Rave and took way too much material out. But the engine ran very smooth in the mid-range.
When I change the valves to brand new ones and clearance them properly it was a totally different feel. There was a new mid-range misfire or it just feels like the cylinder was hitting harder instead of smooth like before. I guess if the raves are clearanced properly that makes the engine a lot more hard hitting in the midrange.


With the machine I'm working on now I have even tried holding the Raves all the way open by hand and the throttle at 3/8.
It was still misfiring or felt like it was 4 stroking.
I'm hesitant to think that it's the Rave valves only because leaning it out momentarily made it perfectly smooth in the mid-range.

 

[Mekanix]

This is why this one has me stumped.
Everything I improved makes things worse

 

[etemplet]

On the engine I did, which is in my wife's ski... the one that beats the hell out of mine... I built the engine and hers has a lot more punch. I was thinking her raves were tighter fit to the bore. I like what you are doing to check the raves performance. I was thinking of doing that. I was/am thinking I had bellows that were too stiff and not letting them open enough. You gotta take some pictures and hook me up with your procedure.

I'll see what I can find on the raves for you.

 

[etemplet]

This is why this one has me stumped.
Everything I improved makes things worse

Ahhhhh..... the chase. That's why I love it. When you finally get it... you learned so much. What matters, what doesn't..... Muhaahahaa !!!

 

[Matt Braley]

Some thoughts..... Whoever punched out and honed the cylinders did not leave enough piston to cylinder clearance in my opinion. What happened to the owner/rider is a "cold seize" where the piston was swelling faster then the cylinder. I've seen recommendations for as low as .045 but with your pistons .75 over when OEM does not recommend more then .25 over I'd shoot for about .006. Wiseco needs .007 to keep from doing this because they swell a little more.

When your looking at the burn pattern on the top of a piston clean is lean. Carbon indicates rich/safe and the look most are shooting for is some carbon in the center with clean piston around the squish band. A perfectly tuned race ski would have no carbon but still not be too lean.

Yours looks too lean. I don't really trust the carb feeding it. either that or the O-ring at the top of the cylinder is letting a little water by.

 

[etemplet]

Thanks for clearing the rich/lean Matt. Good info on cylinder boring too !! I am still suspicious of the rave valve clearance. I removed my valves and filed them to get the proper clearance. I don't see how sand paper can get it done. They recommend modifying the rave valves on anything over .25mm Overbore.