Rebuilding The Rotax 399cc

[rushking19]

Is that a new or used piston? It looks like its been used and the seller has cleaned the top, also there is no crosshatching visible in the cylinder which would normally mean that it has been well used.

As far as for denny saying that fan cooled sled engines not running well above 30*, well it not the heat that does it. It the resulting lower density air that is causing the engine to run pig rich. Dropping a few jet sizes will fix that problem as long as you don't go to low and cause it to go lean.

Yeah it does look kinda used but it looks like it wasnt used much, and he cleaned it well so it looks like its still useable.

And I wasn't worried about over heating issues because if already knew I had to re jet it

 

[exenos]

Usable and being good are two different things. If you use that piston and cylinder your just taking a crap shoot unless you measure the ring gap and piston to cylinder clearance even then its no guarantee if those two measurements check out. Its your choice whether you want to use that piston or use a new oversize piston, however I'd consider $50 for the new piston and $20 to get the cylinder bored to be cheap insurance. When I rebuilt my sled last year I replaced all the fuel lines and most of the coolant lines. The one line that I didn't replace because I thought it looked fine was the return line from the coolant overflow/reservoir. On the first ride it split and emptied the entire engine of coolant. It overheated and ate two of the three head gaskets back into the coolant passages. Ended up costing me another $400 because I thought that something looked fine.

tl;dr Dont gamble with the used piston.

 

[rushking19]

Usable and being good are two different things. If you use that piston and cylinder your just taking a crap shoot unless you measure the ring gap and piston to cylinder clearance even then its no guarantee if those two measurements check out. Its your choice whether you want to use that piston or use a new oversize piston, however I'd consider $50 for the new piston and $20 to get the cylinder bored to be cheap insurance. When I rebuilt my sled last year I replaced all the fuel lines and most of the coolant lines. The one line that I didn't replace because I thought it looked fine was the return line from the coolant overflow/reservoir. On the first ride it split and emptied the entire engine of coolant. It overheated and ate two of the three head gaskets back into the coolant passages. Ended up costing me another $400 because I thought that something looked fine.

tl;dr Dont gamble with the used piston.

Then I'll just ask the seller if he could just buy the cylinder and then buy the other over sized pistons I saw, and then see if I could take it to my automotive class to get the cylinders bored

 

[rushking19]

And what are the down sides to just buying standard size pistons and rings and not boring The cylinder out, just wowondering ����

 

[rushking19]

Will it do any real harm?

 

[exenos]

It wouldn't do any harm directly but it would be a waste to not go one size over and bore it because you are only fixing half of the problem. The cylinder wears as well and will not allow a standard size piston to seal properly.

 

[rushking19]

It wouldn't do any harm directly but it would be a waste to not go one size over and bore it because you are only fixing half of the problem. The cylinder wears as well and will not allow a standard size piston to seal properly.

That's a lot exenos, and don't you get more HP with a bigger bore

 

[exenos]

Technically yes you do but would it be enough to notice? No. A .010 overbore will gain you a few cc's which really mean nothing in the long run. You would get more noticeable performance from the engine by simply cleaning up any burs and irregularities from casting in the ports.

 

[firemanjim]

.01-.02.-.03-.06 are the most common over bore sizes in a small block Chevy.
Not sure about a kart engine, but .030 don't sound like too much.....

 

[exenos]

10, 20 and 40 thou are the common sizes for overboring a sled engine. No one seems to make a .030 overbore for some reason. A .040 over on my sled only bumped it from 649cc to 670cc and the math says I gained 2.3hp...

 

[rushking19]

Thanks Alot I'll look around do see who can bore it for me and for how much, I'll check to see if there in factory spec before that

 

[rushking19]

And iif there in factory spec do I still have to bore it?

 

[exenos]

Its your choice then. If they are well within spec then you are good to go with standard pistons. If they are the right on the edge of being out of spec then personally would go one size up but that's because I want every last bit of power I can get. As long as the cylinder is not ovaled then you could probably get away without.

 

[rushking19]

Its your choice then. If they are well within spec then you are good to go with standard pistons. If they are the right on the edge of being out of spec then personally would go one size up but that's because I want every last bit of power I can get. As long as the cylinder is not ovaled then you could probably get away without.

My pto side cylinder ,I inspected it and it's not ovel and looks within spec, but I'll have to wait till I get the mag side cylinder I'm ordering to see if its within spec, if it is I'll just hone both of them out a little, but if it's not I'll get both Of them bored out

 

[rushking19]

While waiting to order parts I decided to tear down the Tillotson Carburator,so I can properly clean it up, so it would run without a problem.

 

[BirdFanatic]

Nice looking carb are you using dual carbs or single ?

 

[rushking19]

Nice looking carb are you using dual carbs or single ?

Single

 

[rushking19]

You guys were telling me to jet down the carburetor, so which kinda jet size should I get

 

[exenos]

You may have an adjustable jet in that Tilly, I'm not sure. I've only really messed around with mikuni's. The general procedure for re-jeting is to check the plugs after going wot for a little bit then cutting the ignition while still keeping the throttle open. You keep going down one jet size until they look good.

One thing to keep in mind is that you will have to re- jet it back up when it gets colder out.

 

[rushking19]

You may have an adjustable jet in that Tilly, I'm not sure. I've only really messed around with mikuni's. The general procedure for re-jeting is to check the plugs after going wot for a little bit then cutting the ignition while still keeping the throttle open. You keep going down one jet size until they look good.

One thing to keep in mind is that you will have to re- jet it back up when it gets colder out.

Arent the adjustable ones the ones that have a high and low mixture screws