OzVMX Forum
Clubroom => Bike Talk => Topic started by: pmc57 on May 06, 2008, 09:28:45 pm
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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??
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What do you think the result would be? ???
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i,ll play lozza what will happen
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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. ;D
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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.
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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?
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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???
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;D
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.
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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.
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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.....
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Leave it till after this weekend, and I'll give you an update
pmc57
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Ed, you going up to nudgee practice on saturday? if yes I'll see you up there and talk to you then ;)
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I wouldn't have thought you would get any blow back with reeds fitted. I'm guessing your reeds are damaged
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I'm thinking it's a power reed model with the famous 'half pregnant' half reed/ half piston port intake
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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?
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Lozza, We ran the 125C at Nudgee today with the correct jetting and it ran great, It seems a mixture of the incorrect pilot, needle and slide cut-away we enough to stuff things well and truely.
Thanks to all for their very constructive input.
pmc57
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Ran OK with the holes out of alignment?
No no big fan of the power reed set up Doc it's way inferior to a full reed set up, but yes better than a piston port.Feel it's more an $$ thing that kept it going for so long.
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hey doc the c went great finaly got it on the track give you a ride on it at the next meet .it was a little scary.ed
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Ed, yeah I was to be up there myself as I'm really hanging for a ride but work once again dictated what I did with my life. I hate that! >:( glad to hear she's all go. I was figuring it being in the carb as I've had much the same happen with my TM when I first built it and didn't have the correct jetting or correct carb for that matter. I feel the out of alignment relief holes in the piston will still work ok even if off centre to the bridge. The holes will effectively still relieve the hot spot on the piston and it will add a little lube to the cylinder wal even if not in the correct place, in my view it will work but obviously not as effectively. At 56mm bore you could buy a std TM125/CR125 piston, drill the holes in the absolute correct spot and run with that (cheap cheap!) but you must run it as a single ring piston by omitting the lower ring that would/will otherwise snag or hang up on the ports. ;)
Loz, again you're talking about outright HP figures. This is where we differ, I talk of good power being had from the stock motors with minor tweaking and you talk of full blown race engines. Suzuki had enough faith in the system to start using it in another guise on production bikes way back in '71,(enter the rotary valve case reeded induction) Eventually the half piston port half reed version found it's way to 'all' RM's, PE's, TS's, TF's 50cc thru 400cc from '78 to '81 and none of these bikes were down on power at the time comparatively speaking. I still think there is more gain to be had setting up the chassis properly but each to their own of course ;) I like the case inducted RM styled motors, I like the way they deliver the power and I love the reliability that came with them..but then, maybe I am just a little biased :P
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I used to race a 125C back in 79.
Case reeds were the best then and Suzi went back to them in 84 after trying full reed in 82 83.
Casereeds are much better in my opinion.
The "power reed" name was given because the standard reeds were manufacture by boyesen and licensed to suzi. In other words all new suzi's came with boyesen reeds standard.
Back to that piston. Have you tried contacting Crooks suzuki in the UK. They will more than likely have a correct standard fit one although I'm not sure if suzi went as far as 2mm.