Low Max RPMs on 1978 55HP Johnson OB

[vaughneric2000]

Greetings All!<br /><br />I have a 1978 55HP Johnson OB. It is Model# 55EL78S. The motor has always been very dependable, but it wouldn't start after winter storage this year. My son assured my that he ran a fuel stabilizer through the lines and motor before he drained it of fuel completely. I suspect he might have bought a six-pack of Bud instead. I have been working on it for a few weeks (the motor, not the Bud). I worked my way to the Carbs, and after rebuilding them, I was quickly back on the water.<br /><br />Now, after seeing the motor perform this weekend, I have two questions:<br /><br />Question 1: <br />The motor doesn't idle well (shakes quite a lot more than it used to in idle). I have a Clymer manual for my motor, but when I looked up my motor's carbs, I got a little confused. When I rebuilt the carbs, I removed two orifices, cleared/cleaned them, then screwed them back in after cleaning the rest of the carb and installing the various other parts of the kit. I replaced the float, needle and seat, and adjusted the float level. My manual shows a long needle valve in its diagram on model A of my carbs, and it shows no needle but does show a fuel/air mixture screw on model B of my carbs. There are only two models for my motor, and my carbs don't seem to exactly match either of them. Is there a needle I have not found that could be causing the motor to idle poorly?<br /><br />Question 2:<br /><br />My motor seems to be running fine above idle. But I notice that my Tach is reading only about 4,000 RPMs at WOT. I checked the WOT set screw, and the linkage is indeed reaching the WOT screw stop. Could something else be sapping my RPMs?

 

[walleyehed]

Re: Low Max RPMs on 1978 55HP Johnson OB

at WOT you are drawing fuel from the high-speed, AND low-speed circuts. an Idle jet, or air-bleed orfice could cause some problems but I'm not sure it would drop your RPM 1500 or more.<br />Do you know what wot rpm was BEFORE the Bud incident?????

 

[vaughneric2000]

Re: Low Max RPMs on 1978 55HP Johnson OB

Thanks for the quick reply, Walleyehed:<br /><br />Last year, before putting the boat away for the winter, the RPMs at WOT were pretty steady at about 5300. Some loss of power is noticable (obviously) from last year as well. When I cleaned and rebuilt the carbs last week (first time I had to do that, by the way), I didn't find any needle valves of any kind (other than the float needle, and that was replaced), and the two orifices did not come as a kit part. I just unscrewed them, cleaned them and put them back were they were. The orifices were screwd in tight, which I believe is correct. Is that correct? Or do either of these orifices have to be unscrewed to adjust them? <br /><br />If these brass orifices don't adjust, then I essentially have a carburetor that isn't adjustable at all. <br /><br />Oh, and one other thing I wanted to ask:<br /><br />In the carbs's mouth, a brass tube runs from the float-bowel, through the throat of the carb, and is imbedded and fixed somewhere out of sight after it passes through the throat. A second, smaller tube is inside of the larger one, running the same path as its brother. I noticed that the inner, smaller tube isn't centered within the larger tube; it is pushed against the wall of the larger tube, and it stays there. I used carb cleaner to make sure I cleared any blockage that might have been in the holes of the larger tube, but the center tub will not come out. Are there any design specs within that tube assembly that I should have addressed as far as cleaning goes? The carb kits came with a handful of small misc. parts.It came with some o-rings, another variation of the float-bowl gasket, a very small white plasic fitting of some sort, and some metallic discs that I would guess are used to replace discs if the carb is leaking at all. I didn't do anything with these, as my Clymer manual didn't mention them. Are there any needle valves under any of the carb body's original discs? <br /><br />In closing, first let me just apologize for the lengthy questions, but I often see posts where much of the info you need to help diagnose, ends up coming to you in later posts. I'm just trying to get all the info I have to you up front.<br /><br />Recap: <br />1. Where's the needle valve? Or is there a needle valve in my carbs at all?<br /><br />2. Do you think the carb could be causing both the rough idle as well as the lowered RPMs?<br /><br />3. If there is no needle valve (other than the float bowl valve), then how could a person adjust one of these typs of carbs?<br /><br />Thanks again.

 

[walleyehed]

Re: Low Max RPMs on 1978 55HP Johnson OB

OK, first off, the inner brass tube not being centered is normal-you're ok there.<br />The only needle you have will be the float needle.<br />Your carbs are "fixed-jet" and changing jets is the major adjustment.<br />Some of the late 70's engines had an idle bleed screw behind the BACK part of the intake cover, which had to be removed to adjust them. Beings you had the carbs off, obviously you don't have the adjustable idle bleed screws or you would have noticed them.<br />What you have told me so far, is leading me to the thoughts of a compression check to eliminate a few other potential issues.<br />The Clymer manual is very hard to follow, and I feel for ya there...OEM is, in this case, always best.

 

[steviecops]

Re: Low Max RPMs on 1978 55HP Johnson OB

Hi<br />There should be 3 orifice plugs on your carbs. The high speed orifice is at the bottom of the float bowl, in the centre as you look at the carbs. That orifice plug has a nut shaped screw. The off idle orifice is at the top of the carb, in the centre as you look at the carbs. This orifice plug is a normal screw set into a keyhole shaped recess. The idle orifice is on the top of the carbs on the left hand side, as you look at the carbs from inside the boat. They are at an angle of about 45 degrees and have a nut shaped screw, the same as the high speed orifice plug.<br /><br />As well as cleaning the fixed jets, you need to clean the metering passages in the carbs. There will be two tiny holes in the carb throat, one in front of the butterfly and one behind it. These get plugged easily and have to be cleaned thoroughly.<br /><br />Good luck<br />Steve