90 pounds of compression in a johnson 30 hp?

Brandon Yuchasz

Brandon Yuchasz

Well-known member
Guys I have been playing around with a 1987 Johnson 30hp. It runs fine at a low RPM but stutters at full throttle. Almost likes its missing. I did a compression check and I have 90 pounds in each cylinder.

Do you think that is acceptable compression on that motor or could it be the cause? I can't do anymore trouble shooting till tomorrow AM.
 
There are some low compression motors that can run on 80 psi. I was always told as a rule that once it drops below 100 psi you'll start to have trouble. As long as the cylinders are within 10 psi of each other, that is usually ok. Try a leakdown test to further see if you have some problems with the scoring of the cylinder, worn rings and valves etc.
 
Mike thanks for the quick reply. The compression in both holds solid at 90 not leaking over time. Wet testing does not increase the compression either.
 
Brandon, Before you panic on the compression I would check out the carb and ignition, not neccesaraly in that order. I hate to even say it but did you pop for a new set of plugs? The compression should be good since they are the same. Try another gas tank and line also, no special reason except that is good troubleshooting. Eliminate some of the easy cheap possibilities. Could be running low on fuel at high RPMs. Good Luck and keep us up to speed on your progress.
 
I saw that the numbers were basically the same betwenn the cylinders.

I second everything that Tom said.

K.I.S.S. Keep it simple stanley(didn't want to call you stupid haha)
 
Thanks Tom,
I did go through two sets of new plugs to be sure on them and put a new gas line and tank on it with the same results. I did not change the gas though because I could not get fresh gas tonight the stations were closed so I was running older gas in it with a 50 to 1 mix in it instead of the 100 to 1 its calling for. So in full disclosure I am running older gas that is to rich for the motor..


I do plan to put another set of clean plugs in it in the AM with fresh 100/1 gas.
 
BRANDON- DO NOT AND I REPEAT DO NOT USE 100/1 OMC CHANGED THEIR MIND ON THAT ONE. IT SIMPLY IS NOT ENOUGH OIL.. Maybe some synthetics recomend 100/1 but DO NOT do it. The freshness issue is another thing. Get some new gas and try again. Some motors will do OK on stale gas but it is one of those things that is easy to check. And I use a good oil for the same reason- cheap and good insuraance. Good Luck.
 
Brandon, I have been finding that this new gas has been fouling the pick up tubes inside of the fuel tanks if it sits for to long and is not treated. Try someone elses tank with your new fuel line or pull the pickup tube out of the tank and clean and check same before you start with the other suggestions.
 
Brandon, I have been finding that this new gas has been fouling the pick up tubes inside of the fuel tanks if it sits for to long and is not treated. Try someone elses tank with your new fuel line or pull the pickup tube out of the tank and clean and check same before you start with the other suggestions.
Click to expand...


I actually had a pickup tube fall off when the fuel got switched, it ate the glue that held it on, ethanol deteriorates alot quicker than the older blends of gas, you can actually loose as much as 1 -2 percent a month in octane as it sits. It is very hard on rubber parts (hoses, o rings, etc.) I would definitely change the fuel and see if you can borrow a good tank. It sounds like you actually may be sucking air somewhere at high speeds. Check all your fuel connections. Chris
 
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