1972 Johnson 125hp no full throttle

[t2sunquan]

Hey guys, new to this forum and also new to boating, i've messed with jetskis but never a boat until now. I bought this 17' tri hull with a 1972 johnson 125hp "starflite" engine on. I cannot get full throttle, my rpm's are at 3900-4000 with my lever engaged fully in forward. I've taken apart the throttle assembly and messed with the throttle cable, and cannot get anything above 4000rpm, It went to 7000rpm for a few seconds then dropped back down to 4000 at one point, I'm only doing like now 25mph if that. Any help would be greatly appreciated because i'm at wits end. It did have old gas when i bought it but i replaced most of it with fresh gas running at 50:1 ratio. I've also checked to make sure the plates were opening which they are.

 

[Daviet]

Re: 1972 Johnson 125hp no full throttle

Is it firing on all cylinders?

 

[boobie]

Re: 1972 Johnson 125hp no full throttle

Back to basics. Do a compression and spark test on it. Spark should jump a 3/8 " open air gap.

 

[t2sunquan]

Re: 1972 Johnson 125hp no full throttle

Compression is 121psi across the board, checked the spark gap and it gives a nice blue at 3/8".

 

[boobie]

Re: 1972 Johnson 125hp no full throttle

When you have it running, pull one plug wire at a time and see if the drop in rpm is equal on all cyls.

 

[t2sunquan]

Re: 1972 Johnson 125hp no full throttle

I'm going to rebuild the carbs, when i took them apart i found the needle sticking in both of them. But i will try disconnecting one plug at a time.

 

[boobie]

Re: 1972 Johnson 125hp no full throttle

Do the plug trick after the carbs are repaired. It just proves to see if it's hitting on all cyl's or not.

 

[t2sunquan]

Re: 1972 Johnson 125hp no full throttle

Okay did everything has said, there was a piece of astro-turf floor sticking in the needle (figure that one out), i did not find a fuel filter ANYWHERE so i added one. Thing runs great now, firing on all cylinders. Now to my next question, it has a 4x12.8x17 prop on it. At wide open throttle i'm hitting 7000+rpms, is this the right prop for this? The recommended op. range is 4500-5500. this is in smooth river water.

 

[t2sunquan]

Re: 1972 Johnson 125hp no full throttle

Anyone?

 

[orbanp]

Re: 1972 Johnson 125hp no full throttle

Okay did everything has said, there was a piece of astro-turf floor sticking in the needle (figure that one out), i did not find a fuel filter ANYWHERE so i added one. Thing runs great now, firing on all cylinders. Now to my next question, it has a 4x12.8x17 prop on it. At wide open throttle i'm hitting 7000+rpms, is this the right prop for this? The recommended op. range is 4500-5500. this is in smooth river water.

You need a bigger prop. The WOT RPM should be in the recommended range. You will damage your engine if you over rev it.

Peter

 

[boobie]

Re: 1972 Johnson 125hp no full throttle

The first thing I would do is take it to a shop that has a shop tach and verify your tach is correct. For every inch of pitch you only change your rpm appox. 200-250 rpm. Figure it out.