low compression on #1

S

stang33

New member
Joined
Feb 24, 2004
Messages
33
Location
Toledo, Ohio area
Engine guys need some help here:

My wifes sled is a '98 Vmax 700 XTC. ALL STOCK and original with 2500 miles on it.
The #1 spark plug looked black, 2 & 3 where perfect cardboard brown color. So I did a compression check.

The compression on the number one cylinder was 10 psi lower the 2 & 3.

I pulled the head & cylinders off the engine to have a look.
I found brown coloring on #1 piston's side to be all the way down to the wrist pin. The other two where just below the rings. So yes it does indicate excessive blow by.

The tops of pistons 2 and 3 have carbon built up on them. #1 did to EXCEPT for an area around the whole top edge of the piston. So the top of the piston is clean, no carbon build up, from the edge to about a 1/4" from the edge. It looks like an "O".

Why is it like this? I don't see any scoring or damage or anything. Is it like this because of its lower compression, raw fuel cleaning it???

I put a new piston and rings and bearing in it and it now shows 122 psi on ALL cylinders.

I am just worried why it had less carbon on it compared to the other 2. Can someone shed some light on this for me. :dunno:
 
thats from a too rich(lots of wash) condition,the excessive gas is washing
the top of the piston..you want some but not that excessive.

Read!!!
 
It was 110 on #1, other two had 122 psi. I put a new top end in it now. Its dead even all across em now. I might of been able to get away with just the rings but since I had it torn down I figured I might as well do it all.

Would the cylinder running a tad rich cause it to wear out quicker to? To much wash, clearing oil off the walls, more wear??

I could just lower the needle then?

The carb looked clean, I sprayed, and looked through the jets.

I did not check the float level though.
 
I just took apart a zr900 with a twisted crank by 10 Degrees off... One piston has carbon and the other has maybe 1/4 full or carbon and like a circle....... When a crank is off it will fire way differnt and make the sled run like shit low rpm. But i dont think that your cause... I was just saying u rebiult it and now its fine Probbly your piston were wore.. Were your piston shiny on the side? If it was that means u needed a piston anyways.
 
stang33 said:
Would the cylinder running a tad rich cause it to wear out quicker to? To much wash, clearing oil off the walls, more wear??

I could just lower the needle then.
Click to expand...
I would say yes, the extra gas was washing off more of the oil in that cyl.

check tech section for your stock carb set up and set to spec..
 
Could you explain that to me daman, if the oil and gas are mixed in the fuel pump, how could it get more gas to wash off the oil?


Stang33, its very likely you just had more blow by on that cylinder, 10psi wouldnt really make a differance in the way the sled runs. Sometimes rings just dont seat perfectly. Those engines were notoriously rich from the factory, low compression head set stock didnt help either. They can really wake up from just simply running 142.5 mains in all 3 cylinders(no stagger jetting needed) and dropping the needle down a half of a shim, meaning simply leave the needle in the stock 3rd clip, and place both of the palstic shims on top of the clip, this will make the sled much crisper revving and is good to 20 below zero temps. This has been tested in Canada where it gets cold!
 
thanks guys for all the help! I feel a lot better now.

I was afraid the clean area on the piston was from a lean condition, even though the plug looked a bit rich. Its hard to tell with so much oil/carbon on that one plug. It just bothered me that only the #1 was like that.

I'll try moving both shims on top of the needle. And check the plugs again when we head up north.

Can you get an ideal reading from the plugs while running the sled on a stand to check the low, and mid range??

Its hard to tune the sleds when the only time we get to ride is on our yearly tripe to the U.P. of Mi. And who wants to work on them then??
 
stang33 said:
Can you get an ideal reading from the plugs while running the sled on a stand to check the low, and mid range??
Click to expand...
No not really,motor should to be under some sorta load and held in that
carb position/circuit for a wile..
 
it was rich... stang33, thats why there wasnt carbon on the edge of the piston, the extra fuel creates extra wash. The pto they run a larger main init for the added clutch heat, but like I said before those engines are pig rich stock and you can swap in 142.5 mians in all 3 and lower the needles down a half with the shims, you can take that set up to the bank!


Daman, I asked you to explain it, simple question and I see you cant, hey if you cant answer it with facts POSE your way thru it, seems to work for ya....LOL :wel:
 
You must log in or register to reply here.


Back
Top