1971 Johnson 125hp, low speed problems

[Old Head]

I thought I lucked out finding this motor over the winter in an auto junkyard. After rebuilding carbs, water pump, installing plugs, new starter, new fuel pump and a few other things, it seemed to run well enough to install on my vintage Boston Whaler. After much fooling with the carbs, I still can't get it to run right at low speed. Runs great over 3,000 rpm, but won't idle right and runs at low rpms like it's not hitting on all four cylinders. I'm starting to think maybe it's not a carb problem, but it sure straightens out as soon as those high-speed jets kick in. Any suggestions? Thanks in advance.

 

[jim dozier]

Re: 1971 Johnson 125hp, low speed problems

From your description, it sounds like a problem with the mixture in your low speed and/or idle circuits. The fact that it runs well at higher speeds would suggest that your ignition and fueltank and fuel pump are OK. I would recheck your carbs with specific attention to the floats, float valve, idle jets, and mid range jets. Perhaps your cleaning and/or rebuild was insufficient in those areas. How exactly did you do the carbs?<br /><br />What happens if you choke it at low rpms, does it pick up a little? <br /><br />Also, what are the compression numbers. If you have poor compression I think that will affect your idle, someone with more background here would know.

 

[Old Head]

Re: 1971 Johnson 125hp, low speed problems

Thanks for the quick reply. Can't recall the exact compression numbers. Tested right after I got it. But the numbers were all within 5 lbs. As for the carbs, I installed the standard OMC kit parts and blew everything out with compressed air.

 

[jim dozier]

Re: 1971 Johnson 125hp, low speed problems

Did you gain access and visualize the low and mid range jets, soak them overnight and then blow them out with compressed air or did you just install the new parts. My guess is that you missed some crud. You may not be able to see it without a microscope so you must be scrupulous with your cleaning. If you didn't get all the way into the jet area there is probably some crud still in there. Do a search on this board and I think if you read enough you will find this to be the most common problem with carb rebuilds.

 

[Old Head]

Re: 1971 Johnson 125hp, low speed problems

You're right. I didn't get all the way in. Sounds like an excellent next step. <br />Thanks very much.

 

[Old Head]

Re: 1971 Johnson 125hp, low speed problems

After consulting my manuals, it looks like those low-speed jets are underneath core plugs. Is that right? There were four plugs that came with the carb kit. The Clymer book only appears to show one plug on each carb, on the outside, so I must be missing one.<br /><br />By the way, I see references on this forum to OMC crossflow engines. Is this '71 a crossflow? And what does crossflow mean?

 

[thadias]

Re: 1971 Johnson 125hp, low speed problems

I thought I lucked out finding this motor over the winter in an auto junkyard. After rebuilding carbs, water pump, installing plugs, new starter, new fuel pump and a few other things, it seemed to run well enough to install on my vintage Boston Whaler. After much fooling with the carbs, I still can't get it to run right at low speed. Runs great over 3,000 rpm, but won't idle right and runs at low rpms like it's not hitting on all four cylinders. I'm starting to think maybe it's not a carb problem, but it sure straightens out as soon as those high-speed jets kick in. Any suggestions? Thanks in advance.

have a question for you! I have the same motor on a 16' glastron and I'm getting speeds at about 48mph(at 5200rpm). Seems maybe it should be going a little faster, what do you experience? Also I am having the same low idle type miss which seems to be gone at higher rpm's.

 

[emdsapmgr]

Re: 1971 Johnson 125hp, low speed problems

Your engine is a crossflow. The good news is that it runs well at high speed. Probably a simple adjustment. These early models don't use the typical slow speed jet. They use an adjustable needle valve instead. Two per carb. (one per carb throat.) You have not mentioned adjusting these during idling and what the results are. I'd make sure the idle screws are set according to the manual and then adjust from there for proper idling characteristics before drilling out the core plugs. There are no mid range jets. Did you do a sync and link? How about the idle timing? All impact idling.