RM125C piston

[pmc57]

OK all you two stroke guru's. What would happen if the lube / cooling holes in the skirt of a RM125C piston were drilled a little off line and were exposed to the open exhaust port? Would it have a detrimental effect on the performance of the motor and or lubrication of the exhaust bridge??

[Lozza]

What do you think the result would be?

[PERM250]

i,ll play lozza what will happen

[Lozza]

I know what will happen let's see if you can work it out eh? I don't think someone would ask the question if they were a 100% sure the exposed holes wouldn't be a problem.

[pmc57]

Lozza, thanks for your informative reply, if I wanted to play games I would have asked my kids rather than bothering all the helpful and friendly like minded people on the forum.
Here's my thoughts anyway on what may happen.
Apart from loosing the lube effect at the bridge which is an important issue on it's own, you'd also probably loose some, or more of your fuel mix straight out the exhaust rather than being transferred up into the chamber resulting in lack of potential performance. I was thinking it may also cause a loss of compression, but on second thoughts I think not, transfer would be the main issue.
My main reason for posting the question was to ask without any prompts of possible answers and to get a second, third, fourth... opinions and whether I should just lash out and get a new wiseco and drill the holes myself in the right position. Unfortunately the piston I have now has the holes already pre-drillled.
 

[Lozza]

Why not just put your thoughts down to start with?You had obviously given the problem some thought and your thought were actualy very very close to the mark.Nobody will cruel you for coming up with your own thoughts/theory on why something might happen.
Cooling lubing the brigde is the main reason for the holes,when the bulk of the intake flow happens the exhaust port is open.When the holes are exposed to the exhaust port it is when the crancase is filling, result would be sucking some spent gases back into the crankcase from the exhaust(piston ascending). ATDC will lose some charge to the exhaust but piston speed is accelerating so not much lost on decent.Most of which happens in a fraction of a second with 125cc revs on board, biggest loss will be low speed, starting will be difficult.
Has the barrel been resleeved?

[pmc57]

Lozza, No it hasn't been resleeved or from I can tell ported, but it is at 2.00 mm o/s. I guess you're looking at the port timing?
Another question not related to the piston issue but still causes me some concern. When the motor is running, there seems to be a lot of blow back through the carby? I had the bike up on the stand without the air filter element in the airbox and fuel / air mix was just going everywhere. Is this a normal occurance as I don't usually get a chance to view what's happening in the airbox without the filter in place?
Is excessive blow back through the carby and airbox normal??? 

[Lozza]

 
How wide is the EX bridge < or> 3mm?With the resleeve sometimes when the sleeve goes in it 'sprials'(I dunno why but) a bit and the bridges/septums end up slightly askew from the bridge in the cylinder.If it hasn't been sleeved, the piston is OEM, hasn't been ported and the bridge holes don't line up with the bridge that is odd, very odd.
Not completely familiar with the C model is it only piston port or power reed?With any piston port you will get blow back when not on pipe/in phase but those holes could be making that much worse.I have had 'some' but never 'going everywhere'.PICS please.

[pmc57]

Lozza, Here's what's happening.
It hasn't been re-sleeved or ported. It has been freshly rebored to 2mm o/s with a new, non-OEM piston and rings (hence the incorrect hole line up). The 125C (1978) has a reed valve but no power valve.
On initial start up it ran / runs very sluggish at low revs and lots of smoke as if it's loading up and difficult to get up into the higher rev range. This may be because it's just been rebuilt and needs time to 'clean itself out' of the assembly oils etc.
In a process of elimination i'm trying to strike off all possible causes for it not wanting to run as crisp as it should. All up it's only been run for approx 5 minutes so come this weekend we may have a better feeling on how it's looking once we get to give it a better run and possibly a ride.
Possible causes I thought are:
1. The pilot jet,  needle and slide cut-away are not as per the parts book, I'm working on getting this right but would this explain the sluggish low rev range? You may expect to feel it when riding, but not while it's on the stand i thought.
2.The holes in the skirt? Going by feed back here this may or may not be the problem.
Other than these two factors I'm not sure. Any other suggestions to consider would be appreciated.

 

[Lozza]

OK got any JB Weld or Devcon?You could plug the holes and run to see if it made any difference but I would be starting at the standard jet sizes first for the blow back issue(but still don't think the holes are helping any).You will eventually have to replace the piston and redrill the holes properly, wondering the brand of piston.Wisecos you drill your own holes.....

[pmc57]

Leave it till after this weekend, and I'll give you an update
pmc57

[Doc]

Ed, you going up to nudgee practice on saturday? if yes I'll see you up there and talk to you then 

[GMC]

I wouldn't have thought you would get any blow back with reeds fitted. I'm guessing your reeds are damaged

[Lozza]

I'm thinking it's a power reed model with the famous 'half pregnant' half reed/ half piston port intake

[Doc]

Famous simply because it works Loz The 125C is the case inducted ' Suzuki Power Reed' setup like 99.8% of the aircooled RM's (exceptions being the 125M and the 125S) and it works well. RM's would not have been so successful without it as it gives the engine added punch down low without forsaking top end power. Typical hi-perf engines without this are the RM125S and the CR125 Elsinores, both go really well and are super fast when they are screaming their tits off but both have absolutely no power down low needing a busy clutch hand and left foot to keep them singing on the tighter tracks. They are a deadset prick to get off the line without max revs and novices had lots of trouble riding them. With the case reed setup fitted from the A onwards the RM became much more rideable and user friendly by the masses and everyone was happy. Someone on a forgiving bike with decent roll on though less outright HP will normally be quicker than someone on a weapon of mass destruction if he/she can't keep the thing on the pipe. Why be it for you not like the case reed setup Loz?