Runs Better at Lower Altitudes

kmk1012 · 09-02-2009, 09:41 PM

I have a 1989 Johnson 48SPL on my boat, it runs perfect on Yaquina Bay. The higher in altitude I get the worse it runs. It won't idle at Detroit, but runs out fine at higher rpm's. At Paulina Lake-idleing isn't in the picture and it seems way under powered-won't plane out with normal load. Do you think it could be carbs, spark, or linkage? The motor has 138.4 hours on it, according to my meter.

bvs · 09-03-2009, 06:06 AM

Your problem is the lower air pressure at higher elevation. You loose 3% hp per 1000 feet just because of lack of oxygen. In addition, your carburetors keep spitting out fuel at about the same rate, so you start to run a lot richer and lose additional hp as well. The reason you can't idle is because it's like you've got the choke continually pulled.

You can get a lot of performance back by dropping your prop pitch 2" for every 2500 feet in elevation (e.g. if you run a 19-inch prop at sea level, you should run a 15-inch prop at 6,000 feet). You'll loose top-end speed, but at least you'll be able to make it over the hump and get on plane.

If you plan to do all of your boating above 5000' then you should re-jet the carbs. You'll idle a lot better, foul fewer plugs, and burn less fuel. However, if high elevation operation is only occasional, it isn't worth the hassle of re-jetting. Make sure that if you do re-jet your carbs for higher elevation that you change them back for sea-level operation. Otherwise you'll run really lean/hot and melt your engine.

kmk1012 · 09-03-2009, 10:55 PM

I will primarily be fishing/crabbing near sea level for now. Your advice is well informed-if my problem continues to persist, I will take it to a dealer. Thank you very much!

CRD4Liberty · 09-04-2009, 05:49 AM

BVS gave some good advise. Another interim thing to try, that doesn't cost a lot, is to go to a hotter set of plugs. All plugs have a heat range and this can help them fun fine down low and be a bit better at not fouling at higher elevations.

kmk1012 · 09-04-2009, 02:13 PM

Right now there are some newer (less than 5months old) factory spark plugs in it. I'll try to find some hotter ones. The motor always seems to be running cold, this may help-thanks.

ondarvr · 09-05-2009, 06:08 AM

Running cold has nothing to with the spark plugs, its a cooling issue, so if it is running cold also, then that's a separate problem and don't use hotter plugs to compensate.

BVS gave an excellent answer on what your issue is, so if your motor is running correctly now at sea level, the dealer won't be able to help at all unless you're having them change the jets for you (very pricy).

On some years the 48spl (same as 50 hp) had an adjustable low speed jet, so fine tuning the idle might be possible when you're at higher elevation. You can access the screw through the front of the carb with long skinny flat blade screw driver.