Attn: Johnson gurus, idle problem

My '78 70 HP runs great except for idle. Carbs are clean, timing correct, spark and compression are excellent. After idling in gear or neutral, the motor slowly begins to decrease RPMs from 1000 down to 500 in about 10 seconds. It will die unless in bump the choke. It then will run fine for about 10 seconds and the do it again. Off the idle circuit it is fine.<br />

Re: Attn: Johnson gurus, idle problem

Scott,<br /><br />1000 rpm sounds a bit high for idle.<br /><br />I would suspect a lean condition. Check all the hoses under the hood thoroughly and check the integrity of yhe fuel line (from tank) and the connections. A lean mixture will give a strange idle.

Re: Attn: Johnson gurus, idle problem

fittings, fuel line, fuel lines under cowl, squeeze ball, all new. Fuel pump rebuilt.

Re: Attn: Johnson gurus, idle problem

Scott,<br /><br />Keep looking for an air leak. Around the base of the carbs, Upper and lower crankcase seals. You can find one by spraying some WD-40 around seams while the engine is running.

Re: Attn: Johnson gurus, idle problem

I've been wondering when this would pop up. There is a strange situation that shows up every once in a while with the 49 cid motor. Let me see if this is your scenario;<br /><br />As the motor is at idle it is set slightly high to try and keep it running but it slowly "fades" and quits. It doesn't "spit" and quit suddenly. It will start right up whether cold or hot, accelerates and runs fine above about 1200 RPM. The carbs are clean, the gas is fresh, the plugs are Champion, the compression is good, you've played with the idle timing and adjustments, maybe even changed some idle jetting and it probably won't do this on the hose out of the water. Or if it does its not as bad. Someone has told you the rings are stuck so you did the Engine Tune treatment and it does the exact same thing. <br /><br />Sound about right?

Re: Attn: Johnson gurus, idle problem

You are right DHadley. Did you fix yours?

Re: Attn: Johnson gurus, idle problem

I never owned one that did it but we've fixed many that did. Most of the time the guy has went so far as to rebuild it. Sometimes twice. With no improvement.<br /><br />This may sound a little strange (and I'm sure someone will point that out) but just try it. It's not expensive and can only help. <br /><br />Change the reeds. I usually switch to number 133 Boyesen.<br /><br />The 3 cylinder reeds are the only ones I've seen do this. It seems they "stiffen" up for some reason. The reed plates (valves) have become too stiff to open enough at idle and it slowly passes out. More RPM = more vacuume = the motor runs. Most conventional thinking tells us the motor has lost the base compression (the ability to pull fuel in) or the labryth seals are shot. Hence the need to rebuild. The problem is that if it doesnt work you've spent a bunch of money for no gain. I know you said the compression is excellent but you gave no numbers. If the compression is good and within 15 lbs high to low that would lead us to believe the motor is basically in fine shape.<br /><br />Let me know how you make out. <br /><br />Good luck!

Re: Attn: Johnson gurus, idle problem

Dhadley,<br /><br />Now that is a HOT tip.

Re: Attn: Johnson gurus, idle problem

Thanks Djohns19, I hope it helps Scott. And anyone else. I know it sounds wierd but it's worked many times. <br /><br />Stranger things have happened!

Re: Attn: Johnson gurus, idle problem

Very interesting. One thing I don't understand though, if stiff reads is the reason then the cylinders are starving for air and fuel. Why would bumping the choke on the starter key raise the idle back to normal. If the reeds are stiff then the shot of extra fuel and closed choke plates should not force air/fuel in or increase the vacuum. The mechanic said compression was great and about equal among all 3 cylinders. Don't remember the number. One other thing, if I put my hand over the center carb it has much less effect on the idle than # 1 or 3 carbs. Maybe an air leak? Not at the carb mounting gasket. Maybe the throttle shaft???? I did ensure that all 3 carbs had fully closed throttle plates before I tuned the motor.

Re: Attn: Johnson gurus, idle problem

btt

Re: Attn: Johnson gurus, idle problem

There were some carbuators changes to solve this, I think, but cannot find information. Still looking.

Re: Attn: Johnson gurus, idle problem

Clanton - there were a bunch of bulletins out on the carb rejetting from the mid to late 70's. It seems there were some that seemed to contradict others. I have always tried to correct this problem with jetting first but if the motor does not respond to big changes in idle jets we usually end up looking at the reeds. Once the reeds are changed the motor then responds to the jet changes.<br /><br />If you can't find the bulletins, let me know and I'll see if I can dig them up somewhere.