55hp idle orifice size?

[scott_maier]

Hello,

I have a 1978 Johnson 55hp that has some problems stalling when shifting into gear. I've done a link and sync and the timing is spot on. It will run fine wide open, but does hesitate when initially giving her the gas, and sometimes it will sputter and stall when slowing down to an idle... for example when pulling up to the dock. I'm wondering if the carbs have the wrong size idle orifice plugs installed. We are thinking it is running rich. The other morning it was very cold outside and she fired right up and ran fine with not much choking. The boat has only been in the water a few times since rebuilding the carbs and the area below the two small exhaust ports is now stained with black soot. Can someone tell me what the diameter of the idle orifice holes should be? I looked on the web and there are four to choose from.

Thanks in advance,

 

[Joe Reeves]

Re: 55hp idle orifice size?

Very easy to install them incorrectly also.

15" shaft engine: Top carb=322776 .029 - - - Bottom carb=322936 .028

20" shaft engine: Top carb=318823 .031 - - - Bottom carb=317473 .030

 

[scott_maier]

Re: 55hp idle orifice size?

Ok. Thanks for the part numbers. Can you look at the following picture and tell if it is a long shaft or short? I don't have the model number here in front of me and I'm not sure if it is a long shaft or short. The boat is about 1.5 hours away, at my folks house.


https://picasaweb.google.com/102157156913470682912/McKeeCraft#5632916252419896242

 

[Joe Reeves]

Re: 55hp idle orifice size?

No, sorry. Give your folks a call and have them measure the center portion of the transom from the top to the bottom. That'll tell you which you have. My guess is 20" as it seems that nobody wants a 15" model anymore.

 

[scott_maier]

Re: 55hp idle orifice size?

Ok. Thanks anyway.

Questions...

1)What does the shaft length have to do with the carburetor idle orifice size? Is it because of more back pressure on the exhaust with a longer shaft?


2)Does a .002" difference in orifice diameter really make a difference in idle?

 

[Joe Reeves]

Re: 55hp idle orifice size?

Ok. Thanks anyway.Questions...

1)What does the shaft length have to do with the carburetor idle orifice size? Is it because of more back pressure on the exhaust with a longer shaft?

2)Does a .002" difference in orifice diameter really make a difference in idle?

Shaft length affects exhaust back pressure which affects the engine's intake etc.

Yes, .002 would make a difference. For example, from experience, many of the older V4 engines needed to be enlarged from .030 to .031.... going to .032 or .033 resulted in flooding the engine, too rich at idle.

 

[scott_maier]

Re: 55hp idle orifice size?

Shaft length affects exhaust back pressure which affects the engine's intake etc.

Yes, .002 would make a difference. For example, from experience, many of the older V4 engines needed to be enlarged from .030 to .031.... going to .032 or .033 resulted in flooding the engine, too rich at idle.

Okay. Thanks again.

 

[scott_maier]

Re: 55hp idle orifice size?

It's been a few months so I'm not sure if I'm hijacking my own thread here, but thought I'd post a comment.

I pulled out the orifices Saturday and found the top one was a .031" and the bottom .032". I didn't have much choice so I just switched them and took the boat for a spin.It fired right up and ran pretty good, but it's still running rich.... I ordered a .030" (and another .031") so hopefully I'll see a difference with the right size orifices.

I'm also hoping that having the right orifice in the bottom carb will help with the fuel consumption. This motor is very thirsty right now. If i take a 45 minute ride at 3/4 throttle it'll burn close to 3 gallons. Anyone know what the normal burn rate would be?