carb conversions?

[octofoil]

I have a 77 Johnson 55hp (55EL77D). The boat is new to me and the motor was rebuilt 10 - 15 years ago (maybe 60 to 100 hours on it so far). The thing is that according to everything I have read, the carb numbers do not match and the original carbs are no longer available. I believe it supposed to have carb #'s 387917 & 387918. What is on the motor is two of 321700. I have given this motor a complete tune-up and it runs SMOOTH over 1000rpm. The problem is at idle up to about the 1000rpm range. I rebuilt the carbs but was a little reluctant to pull the brass straws out because they are peened in with lead (sorry, not sure the proper name for this part?, it is a brass tube that fits into the idle passage and is about 2? long). Before I'd pull that tube out, I'd want to be shown how or have another carb body available just in case. The problem appears to be whoever rebuilt these previous. One of the carbs had the brass straw pushed further into the casting and the other one was bent at the other end). Before I rebuilt the carbs, I tried to spray a little carb cleaner in the idle passage to see what it would do. When I sprayed it in one carb, it would bog down the motor. When I sprayed it into the other carb, there wasn't much difference. The rebuilt helped over the entire rpm band, but the same thing happened when I sprayed the carb cleaner in the idle passage. The idle circuit is fixed on these so there is no way for me to adjust (or at least no way that I know how to adjust).

So, the different carb #'s are making me wonder what should be on that motor, and what could be on that motor. I don't have issues with rebuilding linkages if necessary, I'd be more concerned with a good fit to the intake and sized properly.

I'd take any advice I can get. This motor seems to be a gem, if I could just smooth out the idle.

 

[Daviet]

Are you still running the surface gap plugs, convert it to the center electrode type, it should improve the idle.

What do you have the idle speed set at?

 

[octofoil]

Are you still running the surface gap plugs, convert it to the center electrode type, it should improve the idle.

What do you have the idle speed set at?

Thanks for the reply. I'm using standard plugs. I was always of the belief that that the other plugs were designed for high compression at higher rpm's to keep from "blowing out" the spark. But I am willing to give it a try. My idle is set at about 600rpm.

Its probably beneficial to say that my objective is to troll as slow as possible. Right now, at 600 rpm, its moving about 2.8 - 3.0 mph. Even though prop size doesn't make as much difference at low rpm's, I have this sized right so top end is at 5700-5800 rpm. Its an 11 pitch prop so I am not putting much demand on the motor, but like I said, once I am over 1000 rpm, it is unbelievably smooth.

I have what I assume to be the stock jets, but I am assuming they are interchangeable with the later year carbs, but I hate to toy with it too much without knowing. It looks like its the same carb kit from mid-70's through '97 and the same carb/intake gasket for most of those years too.

 

[WernerF]

To me 600 rpm seems a little low for a 2cyl looper. The fact that it never stalls at 600 rpm in gear makes me think it's alright.

 

[octofoil]

To me 600 rpm seems a little low for a 2cyl looper. The fact that it never stalls at 600 rpm in gear makes me think it's alright.

No it doesn't stall, and I bet i could get it lower than that but I don't want to stress the motor that much. Maybe this is just the inherent characteristic of the design? After a little research, what you are saying makes what Daviet said make much more sense.

It be nice to know if the different jet sizes in the later models made a difference and if they are interchangeable.

1. New center fire plugs
2. try different jets if interchangeable (????)
3. learn a little more about fuel mixtures to see if there is a more optimum mixture for idling (synthetic, octane, etc...)
4. trolling plate.... if I don't lose too much with steering control to include R at the boat ramp, it might able me to run a little higher rpm, and move a little slower too.