Low compression?

[screaminta]

I have previously posted about a knocking sound that I claimed not to be piston slap because of a new top end....Today I wised up and did a compression test because I feel that I have properly broken it in....But the compression is only around 155~160 ish i did this warm so that may be why, and also I have a cometic head head that seemed kinda thick..should I switch to a stock honda gasket? What do you all think about the compression? is it too low? I didnt bore it, I had someone hone it and the put the same size piston in it...I may also try a different compression tester..I dont know gimmea some advice...

[screaminta]

anybody?

[baseballplaya23]

i would try a diff compression tester before you start changing stuff let us know what you get after you try a diff one.

[2nd sucks]

kick it a few times when its hot before you test

[beerock]

155 160 sounds good

but, honing a worn cylinder and putting the same size piston in it definately makes me feel like you have piston slap.

i would run it for a few hours like 10-20 and suck it up and take the jug off and have it bored to the next size up and buy a new piston. if you dont your gonna end up breaking the skirt and wrecking the top end AND bottom end.

[screaminta]

hey 2nd sucks why do you say kick it a few times before i test when hot?

[2nd sucks]

builds compression to running compression...not sitting there compression

[wilkin250r]

There's nothing wrong with honing a cylinder and putting in the same size piston, provided you go through all the proper steps.

Did you verify the bore clearance? Did you use a new piston?

Overbores are valuable. I know I certainly can't afford to be overboring every time I rebuild, or I'd be wasting a fortune in re-sleeves and porting. A single bore should be capable of going through several pistons before boring to the next size, provided it is rebuilt in a timely manner.

160 is a little low, but not unreasonable. My first suspect is your compression guage is reading low, and you're actual compression is higher. My next suspect is your rebuild, did you check your deck height, squish, and all those associated parameters?

[screaminta]

ok so I tried it with another gauge and i got 175~180 ish so now I ma very happy. The only thing is that this other gauge didnt have the little extension on it and my gauge did so then I went and took the little extension off of my gauge and got 170~175 ish. Now my question is that should the little extension be on the end of the hose? An extension is really the only way that I can describe it....it had the same threads inside and out and just threads over the .5" of thread on the end of the hose so that you get about 1-1/4" of threads...so what do you all think? And yes the machine shop checked all the piston clearances and said that they were good.....let me know what you guys think

[wilkin250r]

I can't type a long post like I did the last time I covered compression testing, I just don't have it in me. Let me try to cut it down for you.

Let's say your engine has a 20cc dome, and in the compression reading is 190psi. Now, add a compression tester with a long hose, and you are actually reading the compression inside the dome AND the hose.

Get me?

So, let's say the volume of that hose is 3cc, so now you are reading 23cc (the dome and the hose) instead of just 20cc.

Think about it. You have the same amount of air, only now it's being put into a larger space. Well, it doesn't take a rocket scientist to figure out that a larger volume will have a lower pressure reading.

From 20cc to 23cc is about a 15% change. Oddly enough, if you knock 15% off 190psi, you'll get about 160.

[screaminta]

i understand exactly what your sayin wilkin...my point is that is it possible for that little 2" extansion cause a 10-15psi difference? if so then I guess that I am pretty happy with my rebuild considering it was my first also...thanks for the info though

[machwon]

Also, be sure and open the carb to full throttle. This can also affect the static psi reading.