Your absolutely right outlaw. When the piston is traveling down it creates positive pressure in the crank case (unless you have a 102mm diameter crank case vent). Even if the pressure above the piston was only atmospheric, the pressure differntial would push oil into the combustion chamber.
During intake stroke a negative pressure is created in the cylinder and intake track (again, unless you have a 102mm diameter intake and carb)
The carburator requires a pressure drop to function. The air is accelerated through the ventury and this acceleration creates a lower pressure in the carb. This lower pressure in the ventury lets the higher (atmospheric) pressure in the carb bowl force fuel into the ventury.
If you map the pressure drop through the intake it increases as you move closer to cylinder (unless the port is very poorly designed)
If the engine didn't smoke (especially during decel) it was probably because the crack was to small or possibly was flexing as outlaw stated.

PS: for people that don't really know for sure what they are talking about, you should say that your suggestions are speculation or best guess. Don't let on that you KNOW what the cause of a problem is if you are guessing.
It just makes it hard for someone like Wshrdskin that is trying to find the cause of his problem.
Don't mean to be rude
Well that was lunch, now back to the shop, only 3 more race engines to assemble and dyno this week, LOL
Ray

 

I did use a poor choice of words. I won't argue that there is negitive pressure in the chamber. But it is closer to neutral than one might think. The breather is a big hole and vents most of that air out.
If the carb slide has a bigger hole in it than the leak in the piston, it will draw from the least resistance, and that will be the carb unless the hole in the piston is bigger than the carb opening.

This is true for a one cylinder. You have to remember when you have two cylinders in a common case the other piston is going up at the same time, which causes an equal pressure balance.

wshrdskin PM me your email address, I have the info you wanted. I will be glad to email it to you when I get your address.

 

Done. [img]i/expressions/face-icon-small-smile.gif[/img]

We'll all get that chance. I just recieved 1badRaptors 12:1 JE that he used for a few rides before building his quad up again. I will be going through the timing and Ignition system before I put this together and fix anything I might find.

Time will tell, although it won't be a 11:1 but if there is a problem on my end and I can't find it I would assume this 12:1 will fail eventualy also.

The manual is fairly expalnatory on checking the ignition system so I shouldn't have any problems going through that. The only thing it doesn't explain how to check is the CDI. Anybody have any info on that? TIA

 

I think the ignition timing is suppose to max out at 38* you could check that with a timing light. What else on the CDI did you want to check?

 

Don't know what I would want to check. The book does not cover it at all. I'd assume the correct voltages coming out of it at the correct times I'd guess. Couldn't say.

 

I think alot of you guys know alot more than me, on some engine parts. But I still think this is a jetting issue, last post on this also. Oh well, if you look at each part one at a time only. What would you say about each one by it self.
Then put them together. I think we are missing to much of what went on. Some of us can see what happen but why, that's were the facts you have at hand. But at the same time you have to draw on other options.


later

 

Hey Doctor, a Rap is only one cyl, I know iv'e been working very late but I believe that's what we are still talking about. LOL
You are sort of right about a multiple cylinders, Crank case pressure is less but "blow by" still creates a positive pressure in the crank case. If there was no leakage by the rings, an even number cylinder engine would have a neutral crank case pressure. But ufortunately, even with gapless rings, we still have "Blow by".
You can't eliminate crankcase pressure unless the crank case vent is as large as the cylinder bore (on a single cylinder aplication)
We did intake pressure mapping when I was doing engineering on a high performance aerospace engine (never had a need to do it in the racing industry) and the negative pressure in a properly tuned intake port is MUCH higher then you think.
The rules of physics apply here. If you have a 102mm piston traveling downward and any thing less then a 102mm diameter intake you WILL have negative pressure abover the piston!! I'm soory, but that's just physics.
If you don't have negative pressure you don't have velocity, if you don't have velocity you don't have anything!!
There is a reason why our engines dominate every race they run in, it's not because we grasp a straw's. It's because we do the engineering and TEST, TEST and TEST some more.
We have two dyno's that run steady, with customers engines or testing new designs.
I am supposed to be retired now (was on the freedom 33 package) now I own a shop building ATV, Motrorcycle and Snowmobile race engines (work as much now as I did building stock car engines, but what the heck)
Ray

 

If there were only a place for a belt drive vacuum pump for PCV [img]i/expressions/face-icon-small-smile.gif[/img][img]i/expressions/face-icon-small-tongue.gif[/img]

 

After reading thru this a third time and gathering info from other places here's my thoughts again.[img]i/expressions/face-icon-small-smile.gif[/img]

The crack comes first- I've seen and heard of engines that have been torn down with cracks in the pistons and no holes and by the looks of yours and Sam's that looks to me what happened.

Octane- We've seen or heard of them running on 91/93 with and without a problem all the way to 107 and still cracks develope. I don't think detonation from low octane is the problem and see none of the classic signs of detonation.

Ignition/Timing- It could be but with the failures on a wide range of octane levels which will effect the timing of max combustion pressure I kind of doubt that as the main culprit. Could be a contributing factor.

Lurication/Cooling- None that I know of had high water temps but oil temps we don't know. I think the bottom of the piston lack enough oil for good lubrication and cooling. Could also be a contributing factor.

Fuel- I know the jetting was on the rich side on a few of the falures and needle valves were larger than stock. The only thing left would be the petcock. I know two of the failures were using stock petcocks and single feeds to the carbs but a third might have been a Pingle dual feed to the carbs. It's possible the carb bowls could have been running low on a few but not all. I don't know, it may have something to do with the failures but definitely not the main cause.

What's left? The only thing I can think of is the piston itself. Maybe a bad batch maybe a not so good of a design for the raptor that can't handle not so perfect conditions brought about by one or more of the things listed above.

I feel the top of the piston is to thin and lacks enough oil to cool the bottom of the piston and starts flexing.

 

I stand corrected about what engine we were dealing with ( For some reason I was thinking two cyl). Thanks for the correction.

About the intake vacuum. If you have a hole hole in you intake slide lets say 15mm and have a hole in the piston, lets say, 1/8mm, what path do you think the air will take? As you say, it's physics, for anything to take the path of least resistance. Same thing with the hole in the piston and the vent tube. The vent tube is bigger, so it wins the pushing contest. Sure, some air and oil will get past the hole in the piston, but not much.

That's why very little oil got up on top of the piston. It takes very little oil to make a smoke screen come out of the pipe. No one saw very much smoke out the pipe. If there was much of a positive pressure in the case he would have seen a ton of smoke.
At idle, I would be willing to bet this thing smoked pretty good because of less disparaty between the two hole sizes.

I totally agree with you on the eliminating pressure in the case, without having a hole as big on the other end.

As far as intake vacuum goes. The type of racing I do, the vacuum gage says 0 (or next too it) from the start of the race to the end, so I can't comment on high(er) intake track vacuum.