Compression change after decarb?

[jbjennings]

I was just wondering....I know there are lots of recommendations of decarbing old engines which I have followed since you guys are obvious experts with these oldies. I am curious---do you usually see a compression increase after decarb? How about the HIGHEST compression gain seen after decarb? What is usually a sign that it needs a decarb?
Thanks,
JBJ

 

[DJ]

Re: Compression change after decarb?

Decarb is just maintenance. Sometimes it fixes a low compression engine, sometimes it doesn't. That all depends on what's in there to begin with.

I have seen 20% increases.

A typical sign of needing a decarb is:

1. Low compression.

2. Oily gunk running out the exhaust ports.

 

[samo_ott]

Re: Compression change after decarb?

DJ, I dont think I've seen a 2 stroke that does not have oily gunk running out the exhaust!

 

[DJ]

Re: Compression change after decarb?

samo,

You'd be surprised at the difference there can be between a clean one and a gunked up one.

 

[HighTrim]

Re: Compression change after decarb?

That is one of the many items that I do when my lower is dropped for servicing the water pump, I pull out the exhaust and clean it up with deep creep or the like. I have seen decarbing work well on 2 cylinders where the bottom cylinder is a tad low. #$*& flows downhill you know

 

[Texasmark]

Re: Compression change after decarb?

I keep Sea Foam in my engine ('02 90 Merc) and have no gunk coming out and little carbon. To get carbon on your finger from the inside of the prop hub (where the exhaust comes out) you have to rub and there's not much there to rub.

Mark

 

[Silvertip]

Re: Compression change after decarb?

Ditto on what Mark said (75HP Merc 2-stroke).

 

[jbjennings]

Re: Compression change after decarb?

Very interesting.......I may start using a little sea foam in mine all the time, also, after I finish the decarb procedure TD discussed on all of my motors. The first one I decarbed didn't change the compression at all, but it did get rid of a lot of gunk.
THanks for the info.
JBJ

 

[Texasmark]

Re: Compression change after decarb?

Couple of comments jb:

1. Maybe over time your compression will increase

2. If you don't have a looper with wedge rings, it may not make a lot of difference. Wedge rings have to be loose so that compression pressures can force them to expand and improve the seal to the cylinder walls. Conventional rings have no mechanism for doing that, except for slight natural spring pressure.

Mark