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I just bought a 1990 yamaha RT 180, it ran when i brought it home but the compression was pretty low. I bored and honed the cylinders, fitted it with a new piston and ring set and it ran like new... for a few hours. Yesterday while out trail riding I was decelerating to a stop and the bike completely shut off, I haven't been able to get it to kick over again and I even tried push starting it. Today so far, I have checked the spark plug (which is good) I am getting good spark, I made sure the exhaust wasnt obstructed or filled with oil, I checked all the wiring, the air filter element and the carburetor (air/fuel, idle screw and jets)...basically, I am getting gas, spark, air, and compression is good.....and ideas? P.S. I am a factory certified harley and yamaha technician, I am just at a loss, I am guessing it's something simple that I am over looking.
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When you remove the cylinder, you should have the bore checked by a machine shop. The bore wears on the front and rear of the bore and can wear slightly oval. In which case it should be rebored and an oversize piston and rings used. The cylinder should also be honed so that the new rings and the bore wear in together and make a good seal. The standard is 56mm.
most piston /ring kits come in .010" , .020",.030",.040" oversize
most engine re-conditioners use cnc borers set to an over size and bore to that
after honing to get the correct cross hatch pattern in the bore , that is it
the ring gap is what is important and it should be around .003" per ever 1" of bore
so for a 3" bore the gap should be around .010"
experience has proved that if the cylinder is bored to say .020" oversize then when the gap for .020" oversize rings is checked ,it generally comes in around .015 and even as large as .020"
max specs for ring gaps is .025" which is for a worn cylinder
just honing a bore is a waste of effort as bores wear in a taper ( larger at the top than the bottom and this taper increases the wear of the new rings and can lead to broken rings
the ridge at the top of the bore has to be removed to allow the piston to come out easily and to prevent piston failure when the new rings are fitted
have a talk with engine reconditioning shops and then make an informed decision
remember that if the cylinder can be separated from the crankcase , you can buy standard or oversize piston/ring/cylinder sets as assemblies so there is no need to bore out or hone.
Many piston compression (rather than oil) rings are uni-directional, and need to be installed with the proper side upward. The rings will have a "T" or "TOP" etched on the surface that needs to face upwards.
So my first question is, did you install the rings upside down?
Second, why did you replace the rings?
Did you measure the cylinder bore? Was it oversized, or out of round?
Is the bore worn enough that the piston is now too loose a fit?
Did you get the correct size rings? If you bored the cylinder, did you get the proper oversize rings?
If you can turn your engine over by hand easily by pushing down the kick lever, and your not Arnold Schwarzenegger, You have a compression problem. Do a compression check. If your bike has under 100 PSI you need to disassemble the top end , bore or hone the cylinder (depending on condition) and install a new piston, rings, and gaskets. This should fix low compression. If the lever is going down but the motor is not turning over then you need to remove the side cover and check for broken kick gears. Reed valves have nothing to do with this problem.
Hi,
Most late modle MX & enduro (from about 1990 on plus some even earlier)
have what is called an electrofusion bore.
This is a very thin plating on the alloy cylinder. They use this so as to allow heat to dissipate quicker & can run tighter piston cylinder tolerances for improved performance.
To answer your questions,
No, they can't be re-bored(like the older style steel sleeved bore bikes could)
Yes, they can be honed but it requires a special type of hone(finger type bottle brush style hone).
Pistons are available increasing in size by much smaller incriments than the old style & are usually marked A,B,C,D(as the coating is much thinner than steel sleeve).
If it is scored very badly & you can see alluminum spots/scores anywhere on the cylinder surface there are several ways to deal with this.
Here in Australia there is a company(electrocill) that can re-cote the cylinder which is much cheaper(plus thicker & stronger) than replacing the barrel
If you have a cylinder re-coated It has to have all studs, seals & power valve assemblys removed.
You would also need to supply(or have them supply) a new piston in order for them to set the correct clearance.
Also here in Australia you can have most of the common models fitted with a sleeve & this gives you the abillity to rebore in the future.
In the U.S this company is known as Wisco(serco in Aust.)
It is worthy of noting that the piston rings are not interchangable between an electrofusion & steel type cylinder.(so if you opt for a re-sleeveing any spare rings from your spares kit ar of no use).
If you need a lead on how & if these services are available in your country(if your not in Australia) I suggest taking a look in the classified section of your local dirt bike magazine.
The trick is to undertake regular srevicing of your air filter(this is A MUST),oiling it properly with propper foam filter oil, useing a top quality 2stroke oil(motul,syn-lube for eg)
& regularly inspect & replace rings/piston as needed to get the best life from electrofusion or steel type bores
Hope this explains it all for you
Regards Andrew Porrelli
Unless you have experience with Braking Systems you might be best take the master cylinder off and take it to a mechanic. If you understand what your dong then you will need a Master Cylinder kit, some circlip pliers and a small bore hone and a drill. Dismantle the master cylinder and clean, hone the bore until it looks pretty smooth all the way down, use some brake fluid to reassemble all the rubbers on the piston shaft and slide it down the bore and refit the Circlip, boot and refit. Bleed the Master Cylinder first and then on each calliper to remove all the air from the system.
A word of warning, if the Master cylinder bore is pitted; it might need to be re-sleeved this IS a job for a Brake Mechanic or Engineering shop.
What is the compression on your scoot?
Honing is for small imperfections.If you can run your finger nail over Imperfection and it clicks or hangs up you need a BORE job..Honing is for very small issues. Seconhand parts also must be inspected and checked for wear.Mic the cylinder for proper specs. Please get back at me and Ill try to be of further service.
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