Logo

Compression test results opinion pls

ants

Member
"Hi, Currently got engine out

"Hi, Currently got engine out of boat I am restoring, and just done compression test on it. Results are as follows on a mid-late 70s 3l 4cyl 130hp for which specs says compression should be 140PSI:

1) 130
2) 113
3) 122
4) 118

I then put a bit of oil into the cylinders (about 10ml), and retested, got the following results:

1) 140
2) 113
3) 128
4) 126

So def somthing odd with cyl 2! I suspect that its valves, actually I presume that 2,3, and 4 need valves grinding as they are so much lower than one even with oil in cylinder, although that helped at bit.

I only had the engine briefly idle (roughly) when I got it, before I then removed it as it had other problems (broken fuel pump etc). The boat had sat for a few years unused prior to me getting it.

I am debating whether to do any further work on engine before putting it back in, as I really just want to get the boat finished and go use it for this summer, and maybe pull the engine and do valve job and maybe rings over next winter.

Based on those results do you think its worth putting the engine back in and seeing how it goes, or is that a dream and its obvious that it needs some major work on it now and so dont waste time refitting it only to have to remove it straight away when it doesnt run very well?

At the end of the day its only supposed to be a cheap propject boat (since when did those two words go together?!?
lol.gif
), so I am not bothered about having it factory spec, just reliable and useable.

Many thanks,

Anthony"
 
"Update:

just done the test


"Update:

just done the test again:

1) 140
2) 128
3) 135
4) 126

so I guess that maybe it took a while for the oil to flow around the pistons, as they have come u a bit, esp #2.

So as a general interpretiation it seems adding oil into the cylinders has increased the pressure in each cylinder by around 10-15psi. so def appears to be some cylinder / ring wear. Plus maybe some valve wear / depositing in 2,3 & 4 as they are all lower than 1. Question is how acceptable is this, and is it likley to have a noticable effect on the running, espically the difference between the different cylinders?

Thanks,

Anthony"
 
"Anthony--
IIRC, generally


"Anthony--
IIRC, generally accepted deviation across cylinders is 15%(if im offbase hopefully someone will correct me), and you're (barely) within that. If the engine has sat for along time it could just be that the rings and seals dried out. I'd actually try flowing some more oil and see if things improve more. This is why the engine manuals always talk about "priming" an engine that hasn't been run in a while. If you flow more oil and it gets better, i'd almost be tempted to put it back in and run it for the season(maybe testing it through the season to keep an eye on it) sometimes the best thing that you can do for an engine is to run it. good luck

--chris"
 
"Compression will not be the c

"Compression will not be the cause of any running problem you have. Those numbers will also be collectively higher when you check compression with the engine warmed up. You're mostly after consistency in the numbers, btw.

I would put it together and not compound your project by adding a rebuild. Easy enough to pluck the engine out in a couple years if necessary. Thats my opinion.

Dave"
 
"Anthony
I'm with Mistaha


"Anthony
I'm with Mistahays. Run it as is for the summer and then take a look at the compression, after the engine is warmed up.
Bert"
 
"quote [Plus maybe some valve

"quote [Plus maybe some valve wear / depositing in 2,3 & 4 as they are all lower than 1.] unquote

It is not uncommon for front-most cylinders on carbureted engines to run a little higher compression than the other more rearward cylinders (nearer the carbureter)when the motors have some age. Has to do with wear from fuel distribution pattern over time, with gravity also being a factor."
 
"Hi,

Many thanks for all yo


"Hi,

Many thanks for all your replies, I will stop worrying about it for now then, and get on with the huge amount of jobs to be done to just get the boat back on the water!

Thanks Anthony"
 
Back
Top