engine rebuild low compression

[David Young]

You 'might' not be getting fuel (with the oil) to the cylinders. Could be the reason your cylinder walls are messed up.

 

[Silvertip]

The crankshaft doesn't contact the cylinders so no -- it is not the problem. If the cylinders were not scored before and they are now you did something wrong during assembly. You honed the cylinders which may have left a very sharp edge on one or more of the ports in the cylinder. These can catch a ring as it passes and obviously do damage. The sides of the ports are not much of a problem but the top and bottom edge can be. Improper ring installation or wrong ring size can also cause problems.

 

[emdsapmgr]

When you installed the 3 new pistons, were they factory pistons, going into a factory bore. Or an off-brand piston going into a factory bore. Were the piston-to-liner measurements to tolerance or were the pistons tight in the bore when you installed them? Prior to re-starting your new block, was a new impeller installed and were the carbs completely torn down (including the removal and inspection of the carb jets) prior to running the engine?

 

[moey]

with the rebuild i changed the impeller
cleaned the carbs twice as it was running rough after first time
as for the pistons would it matter if one con rod was a different type of con rod compared to the other 2 the wrist pin that goes through the con rod was a larger type one i think from the newer model evinrude 70

do the pistons have to be of the exact same design e.g wrist pin size cond ride size and width

im just stumped as it is all three the scoring was differently not there when i put it together

i also added a picture of the crankshaft before it went in
if you look closer you might notice abit of pitting where the con rods attach i had someone trying to tell me that crankshaft could have screwed eveyrthing

the oil used was 25:1 Litres measurment

2 pistons were #319295
1 was 387284
all standard
as for the rings my brother put them in but to be honest no measurements his mechanic amate measured by ring being able to slide into the cylinder with gap closed but had enoguh for expansion
could the piston rings be the problem they all scored in the same position in each of the bottom of the cylinder

ill upload a couple pictures of the cylinder tomorrow

 

[emdsapmgr]

I don't see a legit piston part number 319295 in the Bombardier database. It does seem to be a part number from the OMC database-the way it is 6 digits, starting with a 3. Perhaps that is a piston casting number-not the final machined part number. It's not a consumer part number. 387284 is a factory piston part number. The fact that all pistons are (hopefully) standard, factory pistons is good. They all should weigh the same-important for long-term durability. Not sure that the weight of the different con rod is the same, even though they may be factory rods. It is important to make sure the rotating assembly components all weigh the same. They need to be when the rotating assembly is running a 5500 rpm's. The fact that you've mixed them may cause long-term problems but is probably not be the reason you've experienced piston scuffing after this rebuild. When you cleaned the carbs, did you pull all the jets out for inspection? To be visually sure they are not restricted for fuel flow?

 

[moey]

i blew each and every jet
i also blew out the carbies with my compressor
this engine started and everything but just one day on the muffs while it was idilling called it quits
heres a picture of the scoring in one of the cylinders its not very clear but you will get the idea its very light but enough to cause the problem i guess

 

[emdsapmgr]

Hard to see the scoring in the pic. Not unusual to see a few shadows after it's first run. Could have had a pc of dirt in the ring groove when first started. That can cause shadows on the liner. You lightly honed all the cyls before fitting. If you've still got good cross-hatching that's visble, you should still get some reliable time out of it.

 

[flyingscott]

Check the piston looks like there are dings in it if so something went through it. Also when you honed the block did you clean out the ports oil and shavings build up in them during the honing process.

 

[Faztbullet]

Was the replacement pistons used??? The cylinder bore looks like crap, what did they 'mike' out at??? That cylinder looks worn as it has a shadow where ring haven't been contacting at end of cylinder

 

[flyingscott]

????---------What honing tool / method did you use.----Pictures of the honing equipment ?--------What oil mixture are you running ?

I agree I see no hone marks for only running on muffs

 

[moey]

That piston pictured is the one being piston again with new rings
when the rebuild was put together it was at 120 psi
now 100 10 psi if I'm lucky
I cant give up on this rebuild I've put to much effort
do you guys recommend to hone the cylinders and put new piston and piston rings acordingly
or bore cylinders and get over sized piston?

 

[scout-j-m]

Did you ever confirm how or if you honed that cylinder? I think the pros on here will need that info before they can offer too much more advice.

 

[emdsapmgr]

When overhauling, you have to hone the cyls to insure that the rings will seat. Also, I never rebuild, but what I put in new ringsets. Rings make compression and that's everything to a 2-stroke. The put the engine through a "new" engine break in process to be sure the new rings seat properly.

 

[Faztbullet]

What you did was a refresh as a rebuild brings everything back to a factory spec. 1st 'mike' the cylinders to see if past wear limit, new rings in worn/tapered cylinder result in excessive ring gaps=compression and idle problems.

 

[moey]

Im going to get the cylinders "miked"
What measurment should i be looking to get roughly

Im going to have to pull this thing apart
But by the sound of things from you guys i dosnt seem like its bad scoring
But what can i do to fix this problem do i have to bore out the cylinders or whats my options
Thanks

 

[moey]

As for the postin in the picture that was the piston tbat was in it as a mechanic said its fine so i changed the con rod bearings and rings on it

 

[Faztbullet]

Stock bore is 3.00, if its 3.003-3.005 or larger it need bored