Piston, o-ring gaps, engine

Hello,

In the past I posted some messages related to a engine block thread being stripped, I was finally able to fix it, and presently the stud is withholding 10 foot pounds which seems good for me. But I having some doubts and maybe issues assembling the engine: piston, circlips, o-rings.

After having put together the piston, pin, plain bearing, circlips on the connecting rod (I still have to perform several tests and assemble the whole thing to ride it but I after lots attempts with ether and new spark plug I was able to ignite the engine) I checked out the compression and it is showing approx. 120 PSI, which sounds good. But I have a few doubts about the engine reassembling that I have done so far: I connected the piston to the connecting rod first rather than installing the piston into the cylinder bore and from there connected the piston already installed inside the cylinder bore to the connecting

rod.; so I did no have a good angle view to align of the piston ring gaps and their respective pins located on the grooves of the piston. Basically I could see that the ring gaps were close be aligned or aligned with the pins located on the grooves of the piston. Before that I embedded the piston and walls of the cylinder with 2 strokes oil and after a little bit of effort the piston went through the cylinder bore; but it is not clear for me how important is such alignment: when I was fiddling with the piston in order to get it through the cylinder the rings may have moved and get somewhat misplaced. I haven't done it before so I don't have a lot assurance, but I believe that it was done right. If not, what would be the implactions? Since I am having 120PSI would it indicate that the rings are properly aligned? I also cleaned the carb with carb cleaner and a thread of wire (but I don't have an air-compressor ). Besides I noticed that even with the idle screw very accelerated the engine is having a low idle (the throttle is very responsive and I can have high

rpms with the throtle though); what could the causes of this (anytihnge related to the piston?)?

Another doubt I have is with respect to the cir-clips installation, it was awkward and took me a long time to install them and they seem to be properly installed; I tried to force them out and them did not come easily: I also tried to align the gap of the cir-clips away form the concaved part of the insertion area on the piston . Another thing is that the engine was covered with paper towel in order to protect it from dust and the elements and it was also deprived of any lubrication for some time (I believe that such 2 stroke engine use plain bearings)

and besides there is no oil tank (since I had to remove it), so in

order to lubricate it a little bit I squeezed a little bit of WD-40

inside the crankshaft.

Any insights and help would be very appreciated.

Thanks,

Pedro

Re: Piston, o-ring gaps, engine

Jason Luther /

as far as the rings go, you assembled the engine correctly, by putting the piston on the con-rod first. if the rings ends were not over the guide pins on the piston, you never would have been able to fit the piston into the cylinder, the rings would not have compressed. so everthing sounds fine.

idle can be effected by many things, like float adjustment, ignition timing throttle cable tension etc. a good starting point for the idle control needle is 1 1/2 turns out.

and the circlips, as long as they are seated inside the groove on either isde fo the wrist pin you should be fine.

engines use ball and roller bearings (a plain bearing is usually used in the clutch, but it depends on the engine, however the wd-40 certainly wont hurt anything

Re: Piston, o-ring gaps, engine

Thanks for your response; good to know that everything sounds good! But it is strange because even when the idle screw is all the way down the engine is idling at low weak idle. Could it be related to the reed valves? But I am not sure that it is the reed valves because I can accelerated it to high rpm when I use the throttle.

Re: Piston, o-ring gaps, engine

Jason Luther /

what kind of bike is it? some carbs, like on japanese bikes, have an idle mixture swrew which is used for fine tuning the idle, and an idle screw which moves a tapered pin in and out which raises the slide to increase the idle.

Re: Piston, o-ring gaps, engine

It is a 2 strokes 49cc scooter. It is made in China, the engine is a Suzuki Turbo. The brand is Marshin Power (well known in Australia). As far as chinese bikes goes, it has a good construction overall. I believe that I am familiar with its carb, but I don't have specs.

The carb has a mix air-fuel screw and a idle screw that acts on a groove located on the cylinder of the throtle cable that connects to the carb. In the past whenever I regulated it I would have high rpms if I turned the idle screw all the way down which is not happened by now, it is all the down and apparently it is working fine, but it should have much higher rpms at that point (usually the mix screw does not have a lot of influence at that point as far as it is turned 1 and 1/2 turns clockwise). I would go from 1 and 1/2 turns clockwsie and from that try to have more fine adjustments; but with the current setup once the idle screw is turned all the way down I should have high rpms. The problem is that it is turned all the way down and it is having low but stable rpms.

The funny part is that even the kick start is working now (which did not happen before). But the the idle screw is turned all the way down and that is the only way that it works, otherwise it dies out.

Re: Piston, o-ring gaps, engine

Jason Luther /

thats weird, i bet you have an air leak somewhere. spray a little carb cleaner arond connection points (carb, intake, head etc) and see if the rpms change, thatll tell you where a leak is

Re: Piston, o-ring gaps, engine

Hmmm... I was thinking about that too, and yesterday I sprayed wd-40 around the manifold and I could not notice any clear difference. I used wd-40 because it is less volitile, if there were air leaking it should reduce the rpm I guess. I tested compression of the engine before and the reading was around 110PSI (I will test again today though). I also notice that it is fouling the spark plug easily (I will have to buy more spark plugs today). I will try the air leak test again in other parts too. If there were some leakage on the cylinder head portion I believe that my compression would not be 110PSI; am I right?

Thanks a lot

Pedro

Re: Piston, o-ring gaps, engine

Jason Luther /

check your float bowl fuel level. if the float doesnt shut the fuel off soon enough that will definatly effect idle. i had two old brass floats for puchs that had pin holes in them, letting fuel inside. thus they didnt float as they should therefor not letting the engine idle at all

Re: Piston, o-ring gaps, engine

joe schuitema /

How did the drilling and tapping go? Oh, how did the new headbolt work out? PS: I was the one that made the headbolt(debjays@chartermi.net)

Re: Piston, o-ring gaps, engine

I sure know who you are, Joe. Well, the tapping and everything else went fine, but I ended up with some other problems that has nothing to do with the thread. Thanks a lot for you help! I will mail your tools this week. Read my next post to know about the other problem that cropped up.

Pedro

Re: Piston, o-ring gaps, engine-> Things went A

Looks like my worst fears were confirmed. My rush in putting the o-rings and pistion together without proper angle view rather than inserting the piston with its rings inside the cylinder bore first and then connecting both as one piece to the connecting rod proved to be my downfall. First there were some tingling metal noise coming from the engine some times which caused me some distress; afterwards when I measured the engine compression I obtained a lower measure than I would normally (100PSI rather than 120PSI) which was disturbing. A few minutes ago I took the cylinder bore, piston, etc, apart and out just to find out that the first o-ring was completly distroed (there were only small sections of what used to be the first o-ring in the first groove the pistion and also a small portion of the pistion was distroyed (a very small portion on the upper edge of the pistion maybe measuring 8mm). My question is what implication does it has for the engine as whole (the crankshaft).

Thanks,

Pedroa

Re: Piston, o-ring gaps, engine-> Things went A

Jason Luther /

you assembled your engine correctly. unless you broke the ring while putting the piston in, you never would have gotten the piston in the cylinder unless the rings were located in the groove correctly. but since the piston moved freely and it ran fine, everything was okay. my guess is you put the piston in upside down so the ring end gaps were located over a port, and they got snagged and broke. as far as damage to the crank, i doubt it. see if there is any excess play, check your cylinder walls though, they may have been gouged.

Re: Piston, o-ring gaps, engine-> Things went A

I have the feeling that I put the pistion upside down as you said. For some reason I could swear I have seen a pic where the two big orifices on the skirt of the pistion should face the exahust port. That would explain everything. But it is not big deal as far the crank is ok and it seems to be working ok; besides the ring that broke dow in small parts and the fragments of the small section of the upper face of the pistion that also broke down may have found its way to the crank blocked by the lower ring that was intact.

I am going up upgrade it to a 70cc anyway. But I can detect some scoop out on a small upper area of the wall of the cylinder; does that mean I would have to fix the cylinder in case I want to use it again? I am looking around for upgrade kits, do you believe that aluminum cylinders are better than the cast iron ones?

Thanks,

Pedro

Re: Piston, o-ring gaps, engine-> Things went A

Jason Luther /

from my experience, a little gouge wont really hurt things. take the good ring and put it on top then take a psi reading, thatll tell you if the cylinder is at least usable. if the cylinder you have now is cast iron then you can hone it out, but if the gouge is more than a few thousandths of an inch deep, youll exceed the clearance limit.

as far as aluminum vs cast iron, aluminum cylinder (such as puchs) tend to have much tighter clearances since the aluminum piston and cylinder will expand the same, thus higher compression and more power. however i would think the overall quality of the top end is more important. crappy casting and port schematics will play a much bigger role. use this site and others to take a poll about the different kits available

Re: Piston, o-ring gaps, engine-> Things went A

The scooped out section on the walls of the cylinder is very limited and shallow but I can feel it with my fingers. Would it be possible and safe to use it with only one ring (once I transfer the the good oring to the groove locate on the front of the piston). Another thing is that a tiny small portion on the edge of the piston was destroyed too (would I believe woul affect compression). Is there any easy way to fill that small portion of the piston located on the edge of the frontal face that was destroyed (some compound maybe)?

Thanks a lof for your enlightment

Re: Piston, o-ring gaps, engine-> Things went A

Jason Luther /

i have run a few engines with only one ring, not for extended periods of time (months) mind you, but they ran just as they would with two rings. i would not try to repair the piston, as it would probably just break off again and then really screw things up. again i have used pistons with chips missing from the top of them without any problems. just take some fine sand paper and _lightly_ sand the chipped area to knock down any high spots first. i would lightly sand (220 grit) by hand the area of the scratches to make sure nothing is protruding.

once everything is together take a psi reading, i bet itll be right near where you left off

Re: Piston, o-ring gaps, engine-> Things went A

Hello,

Sorry about the delay in my response. But it seems as if the damage on the pistion did impact compression negatively. Recapitulating, I posted messages indicating that I had ordered a 72cc kit for my 2 Strokes 49cc engine. I also pointed out that I inserted the pistion upside down which virtually destroyed the first o-ring; what I noticed because of the brittle noise coming from the engine and the reduced compression of the engine thereafter (100PSI rather than 120PSI). Even thought there was a small portion of the wall of cylinder bore that has been scooped out (I could feel it with my fingers, but it was kind of shallow) coupled with the edge of the upper frontal section of the pistion face which was destroyed; I put the lower o-ring on the first groove and checked out the compression... but I was out of luck and the compression was in an all time low: only 60 PSI. Which means that the I will have to wait until the upgrade kit comes... Hope that the upgrade kit will work out for me.

But I was wondering bout a few things: What compression should I expect of the engine once it is upgraded (same compression, higher compression?)? What spark plug should I used (right now I am utilizing BPR6HS)?

Thanks,

Pedro

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