From what I've learned in the short time I've been here, is that gauges can vary a lot. A 75 reading on one gauge may be a 90 on another. And 100 on yet another. The screw-in type of gauge is much more accurate than the press-on, etc.
If you're checking out a new motor, that 75 might be a bit on the low side and maybe you want to look for something with a little better numbers.
If you're wondering about how long the one you have might last . . . If it's working fine, it should keep working fine for quite some time longer as far as compression is concerned. You might try decarbing and taking another reading too. Your reading may come up quite a bit.
I'm sure someone will disagree. And as is par for course on this forum there'll be good reason for it.
The reason I was wondering about the compression is it is a little hard starting but runs great after that. I have worked in snow mobile engines for 30 years and they don't recommend a tune up with less than 100 psi. It takes four good pulls to get 75 psi with throttle open, and it is a screw in gauge. Thanks guys
It takes seven or so pulls to start it has new plugs and good spark. The choke is closing OK but I am wondering if it is not getting fuel when choked. I use it for a trolling motor on my 16' Bayliner once it is started it will idle all day. That's why I was wondering about the compression on start up.
As for fuel. I'm assuming you're pumping the ball until hard and all that. If you're doing that, and it's idling properly afterwards it's not likely you've got a fuel delivery issue, choke or otherwise.
I just bought this this summer so I don't think it has been decarbed. The guy that I bought it from said he had a problem starting it also. The fuel ball is new and pumps up hard so I know the carb is full. One other thing that I have been reading here is that I should be using a 40 to 1 mix. I am using a 50 to 1 mix a 40 to1 would leave more oil on the piston and give you more compression.
A 40:1 vs. 50:1 mix isn't going to change your compression any. Compression is all about rings and cylinder bore. A carbon build up can stick the rings inside their grooves so they don't seal as well.
Your compression sounds perfectly normal for a 4hp with some use. I manage to get 80 on mine ('72 Johnson variety) after four pulls. It is in average mechanical shape.
Normally it takes 3 or 4 pulls to start the engine from dead cold depending on how quick I am. With the choke closed, I'll pull 2 or 3 times until the engine starts. If I'm not quick enough to get the choke open before it floods, that means a further 3 or 4 pulls with the choke open before it clears.
I think the biggest mistake people make is to set the carburetor idle mix too lean. It'll idle really well with it set lean but it's a bear to start. Try richening that up 1/8-1/4 turn first. If that isn't it, make sure the choke plate isn't bent or something preventing it from closing fully.
40:1 won't affect compression but it really helps with the plain bearings on the crankshaft and connecting rods.
I will defentally check the idle jet setting. I usually set it to speed up by leaning it down and then richen it to where it slows down and runs a bit rich. Does any body know how many turns it should be open?
The start position is 1 and 1/2 turns out from lightly seated (remove the knob first). My one sits at about 1 turn out on the low speed and 1/2 of a turn out on the high speed.
I have 6 small outboards from 1.5 to 7.5 HP. All were bought used. 1.5 HP=105 psi; 3.5=100; 4=98,85; 6=90,90; 6=85,90;
7.5=109,110. The 3.5 and 7.5 start with 1 or 2 pulls; the others with 4 to 6 pulls. Also the high psi's seem to smoke less.
The 7.5 always starts with 1 pull (choke out, fuel dial rich).
I have 6 small outboards from 1.5 to 7.5 HP. All were bought used. 1.5 HP=105 psi; 3.5=100; 4=98,85; 6=90,90; 6=85,90;
7.5=109,110. The 3.5 and 7.5 start with 1 or 2 pulls; the others with 4 to 6 pulls. Also the high psi's seem to smoke less.
The 7.5 always starts with 1 pull (choke out, fuel dial rich).
