Force 125 #2 cylinder low compression (replaced piston and rings...still low)????????

[JonathanSShaw21]

I have recently acquired a nice boat with an engine that didn't want to start and idle. so i took the head off and noticed my #2 piston second down from the top had low compression. I took intake off (top) and got my 1/4 12 pt to take cap bolts off... yada yada... i replaced piston with a standard size piston and rings from weisco and i'm now getting only 10 more pounds of pressure. Rings seemed good and they were labeled top so i put that side up badda bing badda boom never replaced head gasket but i did copper coat and install. I have now ordered a new one and I'm going to take it back apart... any suggestions as to what else could cause it to remain low on compression about 70 psi??? Any help... I can tear down a Harley including a v rod motors in my sleep... this backwards 2 stroke vintage machine is killing me i just want it to run me through the summer than it can crap... Please anyone?

 

[JonathanSShaw21]

Old piston had obvious broken ring and ring land. Thus making me think a good hone new std piston and rings we would be good to go.

 

[JonathanSShaw21]

Old piston had obvious broken ring and ring land. Thus making me think a good hone new std piston and rings we would be good to go.

 

[glenn property of pam]

check the casing for cracks, leaks and also check out the reed valves. A 2 stroke has to have a sealed crank case to draw in the air then the case has to be able to hold pressure as the piston retracts so the air/fuel mix is forced into the cylinder. Im not a expert but I know if you knock a small hole in a motocross bikes crank case it wont work.

 

[JonathanSShaw21]

Im taking it back apart and removing piston again for measurement and re check of ring and rehone if remeasuring comes out ok... Im scratching my head... other things i should check for?? My carbs are flooding and it only starts with the engine tilted up and gas pours out of intake into carb ends... Im thinking all unburnt fuel but im clueless with the two strokes.

 

[JonathanSShaw21]

Im taking it back apart and removing piston again for measurement and re check of ring and rehone if remeasuring comes out ok... Im scratching my head... other things i should check for?? My carbs are flooding and it only starts with the engine tilted up and gas pours out of intake into carb ends... Im thinking all unburnt fuel but im clueless with the two strokes.

 

[JonathanSShaw21]

I have to admit... stupidly I drilled a hole in case ot get to carrier bolts and managed to pull it off... i plugged and sealed it with Teflon. seems to have sealed.

 

[G-Force]

Check for damaged or broken reed valves if they are broken and missing don't start the engine until you recover the pieces

 

[JonathanSShaw21]

is that going to effect the compression? I felt that new piston is in there nice and should hold proper compression. See where we are tomorrow afternoon

 

[glenn property of pam]

reed valves and a sealed crank to a 2 stroke is a bit like intake valves to a 4 stroke. If you don't get any air in the piston chamber then there's not much to compress.
p.s fuel should not be pouring out of the carbs and that will need to be fixed.

 

[Jiggz]

If you can take pics and post them (head and gasket) it will give us a better idea why the compression is still low despite new rings and piston. Did you verify the old piston if there is any marking on its head? It could already been an oversized piston and replacing it with a smaller standard size will not help. A pic of the old piston can also help in diagnosing.

 

[jerryjerry05]

Reeds will NOT affect compression.
The head gasket what condition?
The piston, is it the right size?
The head, is it flat? A warped head can cause the problem.
Fuel coming out of the carbs, the floats need setting, the carbs dirty, full of crud, install a filter between the carb and the fuel pump.

 

[Frank Acampora]

Since a 2 cycle engine combustion chamber always goes to atmospheric pressure at the bottom of the stroke when both exhaust and bypass ports are open, during a compression check, compression ratio will remain substantially the same under most conditions. Carbs can be open or closed, on or off the engine, manifolds and reed valves can be off the engine---compression ratio is basically the same. It is a function of air in the cylinder at atmospheric pressure being compressed. Since nature abhors a vacuum, as I said, when both ports are open at the bottom of the stroke, the cylinder will fill to atmospheric pressure (14 PSI) no matter what.

Low compression CAN be a function of a bad head gasket, a badly scored cylinder, or a standard sized piston and ring set installed in an oversized cylinder. Standard bore on the 125 is 3.3125 (3 5/16) and oversize would be .010, .020, .030, or .040.

 

[JonathanSShaw21]

Ok so its apart YAY! why is my brand new weisco piston measuring 2.96 and my cylinder wall is 3.125? is that not the right one? standard piston from weisco for a 3.125 cylinder... am I confusing myself. When i measure from top of wrist pin hole to top of wrist pin hole its 2.96 on the same caliper I use a T gauge to measure cylinder in three spots on and X and Y axis... When i put the piston rings in the cylinder without the piston the ring end gap on both rings were .045 n i hear they are suppose to be .013 from a weisco rep? I ordered a stock piston off ebay with stock ring sin good shape thinking it will solve the top cylinder and piston problem which come to find out after taking head back off top cylinder was holding compression at 120 but was missing a little chuck and part of the ring. Soooo Ill have anew piston and rings for the 2nd cylinder hoping they are good fitment this time and ill have a good used piston in top hole with good rings... i really like the stock ring set up seems more durable than weisco.

 

[JonathanSShaw21]

What is the piston to cylinder wall fitment? .005 right? Id love to learn a lot more about clearances and fitment in motors to help asses this type of stuff.

 

[Frank Acampora]

Chrysler and Force never published a piston to cylinder wall clearance. Suffice to say that a Wiseco piston will be correct, although Wiesco does recommend additional ring end gap for high performance or racing engines. Ring end gap should be no greater than .016 so if you were getting .045 you can be reasonably certain the rings (and thus piston) were too small. Your numbers are wrong: STANDARD bore is 3.3125 not 3.125. 010 over would be 3.3225 and 030 over would be 3.3425. Are you sure the piston diameter is not 3.296 not 2.96? 3.296 would give approximately the correct wall clearance in a 3.3125 bore. BUT--Don't overthink it. measure the bore correctly and buy a piston to match--don't worry about the piston diameter. Let the manufacturer do that.

Again, with .045 ring end gap, not only are you getting excessive leaking through the gaps BUT--the rings will not be circular and will not seat correctly against the cylinder wall, decreasing sealing and compression ratio.

As I said, If the cylinder is bored to the correct oversize or if it is standard and not worn beyond specifications, the stock Wiseco piston will fit with correct clearance.
(Standard wear tolerance is .002 for wear, taper, or barrel shape, measured at three places inside the cylinder.) In fact, Wiseco quality control is so good that in the large number of pistons I have installed, it was never necessary to adjust end gap of the rings.

Pistons are cam ground to account for uneven heating and expansion and have a major and minor diameter. They are also tapered with a lot more clearance at the top ring than at the skirt since the top of the piston expands much more than the skirt.

 

[Jiggz]

No Title

Here's a table for the 85 and 125 HP motors from the owners manual. Take note of the bore and stroke data as reference.

 

[jerryjerry05]

The ring end gap is adjusted by you.
Use a fine file.
Emery board or sand paper.
Just be careful when you do.
The piston you removed, did it have any #s engraved on the top of the piston?
Post a pic?
A manual, a factory manual will have all the info you need.
Wiseco stuff is good.
They might take it back if it's wrong.
I need to get my manual out to get the specifications
What year?.

 

[JonathanSShaw21]

OK so i will post pictures but i found that around the ports on my Y axis measurement its 3.341 which is really close to not usable anymore if i go .030 over correct? So i guess im going to have to completely disassemble and do all holes. Ugh... i'm capable of it just really didn't want to do this but i love the old girl so im hoping .030 in all cylinders Ill be good to go for a while after reassembly. Top hole could probably just be hones and put new standard piston it measures 3.314 at it widest point but the compression ring was missing a piece and had a little gouge in piston on cyl 1 top hole. I will post pics later. Is there such thing as to boar the #2 hole with some odd machine without disassembling the whole motor and just covering up the crank and cleaning well. Worth a shot?

 

[JonathanSShaw21]

I have a friend that has in the past honest the wall out to an oversize... takes a while and puts a new oversized piston in and it hold and runs fine... I do understand at that point it is not balanced but will it last the rest of the summer that way until im on budget to do all cylinders and other damage will occure before that... Im capable of tearing completely apart and doing all holes but can i get by for a little while until i can put all the money together and do it the right way? any thoughts?