Diagnosing Idling Problems

[nabeel.alsalam]

Why is it idling so low?<br /><br />From my search through past threads I can see that idle problems are very common. I’ve read many of those and followed almost all the suggestions to try to find a solution to my own “doesn’t want to idle problem”. So instead of boring you with everything I’ve done and asking what to try next, I’d like to try to get your help on how to read the symptoms or narrow the source of the problem. <br /><br />I’ve tested the spark and checked the timing, so I think I need to focus on the fuel supply side. (By the way, it is 1991 Johnson 40 hp.) <br /><br />After starting the engine, I can keep it going by periodically pushing the primer. Sounds like not enough fuel caused by clogged carb jets or passages.<br /><br />Pumping the bulb during this process doesn’t help, so it doesn’t sound like a bad fuel pump or supply from the fuel tank.<br /><br />I can get the engine to operating temperature and slowly drop the fast idle lever by this “occasional primer push method.” But to get the lever all the way down, I have to back off on the idle mixture screw several turns beyond the usual 2.25 turns out. Even then, the idle speed is very very low. This is richening the mixture, so this also points to the engine not getting enough fuel up from the bowl and through those passages on the top of the carb. <br /><br />When the engine dies and I take the plugs out, they look wet. That seems to point to having too much fuel. <br /><br />I’ve heard the sneezing symptom on other engines which my 1991 Johnson never does. So it doesn’t seem to be running lean. It isn’t getting too much air.<br /><br />If I’m lucky and am able to get it into gear and take off, the engine seems to run great. So, at least, at higher rpm the fuel/air supply system including the VRO pump seems to be doing its job. <br /><br />I saw on one of the threads that a “worn out engine” will not suck enough air and fuel to idle fast enough. A friend and I checked the compression and that rises to about 130 lbs after a few seconds of cranking. Is there something else that would cause the engine not to suck in enough air and fuel? <br /><br />I saw on one of the threads that an engine will idle faster with muffs on because of reduced back pressure. Is that right? I haven’t tried that. <br /><br />Are there any tests I can do to help narrow the source of my problem? <br /><br />You guys are very generous with your time and expertise. Thanks.

 

[ob]

Re: Diagnosing Idling Problems

It will run a bit higher rpms on the muffs.I believe your plugs probably appeared wet in the wake of having to initiate the primer button so many times to keep the engine running.Normal plugs will appear 'slightly' wet anyway.I think you're on the right track in suspecting restricted low speed circuit in one or both carbs.Did the engine set out of use for some time?

 

[nabeel.alsalam]

Re: Diagnosing Idling Problems

Yes, the engine had sat for a year and a half. I did rebuild the carbs but I did not remove the welch plugs. It looked very clean to me. However, I may rebuild them again.

 

[Solittle]

Re: Diagnosing Idling Problems

Do you have a thermostat? A cold motor will not idle well.

 

[fireman57]

Re: Diagnosing Idling Problems

I would pull my carb again and make sure that all the passages are clear and that you didn't miss a gasket somewhere. Also check your fuel pump for any tears or waviness in the diaphragm. I do not have a manual for your engine but just offering suggestions on stuff that has happened to me.

 

[alcan]

Re: Diagnosing Idling Problems

Here is a list of things to check.<br />Poor quality or old fuel.<br />cloged fuel filter.<br />fuel leaking thru primer solenoid.<br />damaged VRO unit.<br />throttle linkage misadjusted.<br />throttle cam pick-up point misadjusted.<br />idle speed to slow.<br />throttle arm stop screw misadjusted.<br />throttle cable misadjusted.<br />float valve/s stuck.<br />incorrect float/s setting.<br />Carb base gasket leaking.<br />warped manifold. <br />Manifold gasket leaking. leaf plate gaskets leaking.<br />leaf plates bent or broke. leaf plate screws loose. I am confused. How can you ritchen the idle mixture. These carbs use fixed high and low speed orifaces. Blow carb cleaner thru the low speed orifaces. If they are clear they 'er clear. I would suspect fuel or air leaking thru the primer soleniod. At any rate, the best bet is to obtain a manual. This way you will be able to systematicly test and eliminate each detail step by step. This makes things easy. One other detail on these models. There is a plastic cover to the rear of the power head, covering electrical stuff. Under this cover is a bus bar to which wires are attached. There is also a plug held together by a wire keeper. Inspect all wires routed behind the bus bar. The screws that attach the wires to the bar protrude thru the bar and rub the wires bare or short out on the wire plug keeper if it is routed in the area.

 

[nabeel.alsalam]

Re: Diagnosing Idling Problems

Solittle, I believe I have a thermostat, but I'll check. The water (and air) is cold around here. <br /><br />Fireman57, OK, I'll pull the carb again. This time, I'll remove the core plug and inspect the passages. Carb cleaner did flow through those 3 tiny holes in the top but being inexperienced I don't know if it was fast enough. I'll work extra hard to make sure those passages are clear.<br /><br />Also, the OEM service manual says to check for a leak between the the tube that goes up through the venturi where it meets the top of the venturi. They say to add a drop of what I believe is LocTite if it does leak. I didn't check for that leak the first time. <br /><br />Alcan, on the starboard side of the carb (up top) is a needle screw with spring and o-rings on either end. Screwing it in leans the idle mixture. Screwing it out richens it. This and the timomg seems to be the only two adjustments that affect idle speed. My engine, however, is very insensitive to changes in that screw.<br /><br />I did rebuild the primer solendoid to eliminate a leak that was there. <br /><br />I'll check the wires under the busbar. I think the plastic cover is missing on my engine. Not good, I suppose.