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1988 johnson gt 150 low hp problem

C

Chuckd

Member
I have a 1988 johnson gt 150 on a 1989 nitro 1800tf (17ftbass boat). The idle is a little low but it sounds like it runs good through all rpm ranges. I have tried 2 props. First one is a 14 1/2 24p raker. When I throttle , i hear the the engine rev up nice and fast, but the rpms on the tach barely move up until im on plane which takes about 8 seconds. The boats bow rises high ,revs high then planes out. Then it runs good to a wot of 4400 rpms at 53 GPS mph. Its tilted and trimmed properly . Way too low rpm, so I tried a second prop.

Second prop is a 14 3/4 19p viper. With this prop the boat jumps out of hole in about 3 seconds and runs strong to wot of 5200rpm 45 gps mph. This motor should be a lot stronger. Compression is within 5% of 80 on all cylinders. I have replaced all fuel lines. The bulb feels firm. Tach is new. I put new champion ql77jc4 plugs in. What gives? Could it be a coil or cylinder not firing causing it to have lower hp? It sounds like it runs good, just not on performance.
 
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Compression is way to low.----try another gauge first.----Could be running on 5 cylinders, so check for strong spark.
 
Your topic says " low hp problems "------So hurry to a shop to have the motor output tested.----------This can be done with a test propeller or a dynomometer.
 
racerone said:
Your topic says " low hp problems "------So hurry to a shop to have the motor output tested.----------This can be done with a test propeller or a dynomometer.
Click to expand...

Shops around here get 100 bucks an hour, so that's why I'm on the forums. For the do it yourself and help.
 
With the engine in the water and not on a hose, hook up a timing light to each cylinder and rev the motor to 2000 rpms and see if all the cylinders are firing. If the light is erratic or does not light...could be a power pack, coil pack, plug wire or yes...even a bad spark plug...seen new ones bad. Make sure all you electrical connections are good and clean. CDI website has a troubleshooting guide for your ignition system. Also check the timing. Carbs clean? rebuilt?. Do the carbs open fully at WOT? To be honest...your motor sounds like it still has plenty of life in it. I'm not sure the prop is the right selection...my guess would be a 15x17 or 15.25x15. Im sure there are those who know better about the prop selection
 
I just did a test with a ignition spark tester , followed an old forum member by the name Joe Reeves advice. I set the spark gap to 7/16ths and found all but one had strong spark , one had none. Purchased a coil online and it will be here monday.
 
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