Thermostat and idling

[nabeel.alsalam]

I'm still looking for the reason my 1991 Johnson 40 hp won't idle. <br /><br />I can keep it running by keeping the fast idle lever up and pushing the primer every 2 or 3 seconds. I've tried richening the mixture and that seemed to work when I was running the engine on the muffs, but when I put it in the river yesterday, that did not help.<br /><br />There is a thermostat in the engine, but can this behavior be caused by a thermostat that is stuck open? How often does that happen? The water is 40 degrees right now.<br /><br />I have the carbs off again and this time I've remove the core plug. The passages and clean and clear. I've followed the link and sync procedure in the OMC manual to the T. I've checked that the spark will jump 7/16". The VRO seems to pump lots of gas and the pulse limiter was fine.<br /><br />Thanks.

 

[rodbolt]

Re: Thermostat and idling

if afdter all else is tested and known good have a look at the reeds. if on is open,chipped or damaged idle will be adversleyaffected. if all tests good you may have some collapesd piston skirts or a plown out crank seal. but before a teardown insure all else is to spec. by bumping the engine primer your forcing a very rich fuel mix. so either there is not enough engine vacume to allow the carbs to operate or something is restricing the carbs. if your model carbs have the black plastic top covers remove them and check for warpage with a striaght edge. the have to be flat. if they bow in the center they can leak and cause idle issues.

 

[nabeel.alsalam]

Re: Thermostat and idling

I'll take a closer look at the reeds. With the carbs off, I did peer in at them at them and they are very shiny with no apparent damage, but I was just quickly peering in.<br /><br />The compressions seems fine -- about 130 after 4 or 5 cranks. <br /><br />So I take it from your response that I'm barking up the wrong tree in worrying about the thermastat.

 

[Basscat 1]

Re: Thermostat and idling

The only way to really inspect the reeds is to remove them and hold them up to the light. If you see any light around the edges, then they will have to be replaced. Also check the reed block for warpage or damage. Boyesen reeds fixed my idle problem.<br />A stuck open thermostat may not be the cause, but it will certainly have an affect. I would check it, especially in water that cold.

 

[ob15]

Re: Thermostat and idling

What rpm is the motor running on the muff in your driveway? This should be higher than when the motor is in water. If you set the idle to spec in the driveway it will probably not run in the river. One thing I noticed on an older motor I had was the upper exhaust relief was blocked. When I set the idle in the water, the idle in the driveway was real high.<br /><br />I don't mean to change the subject or lead you in an different direction than the experts are taking you. If I'm off base, someone please speak up.

 

[CaptainPuget]

Re: Thermostat and idling

A thermostat stuck open will cause poor idling, especially in cold water. It is very easy to check, if it is stuck open it is usually apparent from a visual inspection. They often corrode such that the spring will no longer close them.

 

[nabeel.alsalam]

Re: Thermostat and idling

Dave F., I tried to maximize the rpm when running it in the driveway by adjusting the mixture screws back and forth. I don't have a tac but the idle speed sounded "right". <br /><br />Once I put it in the water, I simply could not get it to run even with the idle lever up without pushing the primer every few seconds. <br /><br />I just went to the manual to find out what the upper exhaust relief is. I see there is an exhaust relief baffle in the midsection of the engine. I've not been into that part of the motor.

 

[ob15]

Re: Thermostat and idling

Something else & correct me again if offbase, can you adjust the idle screw? Not the mixture screw.<br /><br />If your motor is similar to my 89 50, the exhuast relief is located on the rear of the engine on the lower motor cover. It's a little black piece with 2 rectangular holes in it. A small amount of water may drip out of it in the driveway and much more will come out when it's in the water.<br /><br />I hope I'm not leading you in the wrong direction. I'm not as experienced as the "commanders". I'm just a lonely old petty officer.

 

[ob]

Re: Thermostat and idling

Check the fuel hoses for any leaks from the tank to carburetors.Make sure the fuel primer bulb will pump up firm and 'hold' before starting engine.If no external leaks are present,my guess is either and air leak on the carbs as rodbolt mentioned or fuel restricted carburetor/s.<br /><br />Where are the idle mix screws set now?Where were they before?

 

[nabeel.alsalam]

Re: Thermostat and idling

Dave F., the idle screw is set during the link and sync procedure. It appear to affect the timing at idle. I did check that with a timing light (when it was idling in the driveway) and it was close to the specified 3 degrees ATDC.<br /><br />I appreciate that an engine is a complete system and its ability to idle can be affected by almost any part of the system including, I guess, exhaust back pressure. When the motor is in the water, that back pressure is greater which, I guess, is the reason it idles faster with muffs than in the water. <br /><br />All thoughts are appreciated.

 

[nabeel.alsalam]

Re: Thermostat and idling

ob & rodbolt,<br /><br />I do not believe there are any restrictions in the carb from junk in the carbs. I suppose I should check to make sure the air bleed orifice and the main jet orifice are the correct ones???<br /><br />The bulb pumps up hard and there are currently no fuel leaks (I did rebuild the primer solenoid to eliminate a leak there). <br /><br />I do suspect insufficient vaccuum, so I will remove and inspect the leaf plate assembly. That does affect vaccum, doesn't it? <br /><br />By the way, a friend who is helping me did two things while we were working on it in the driveway. Do they tell you anything? First, he feed some propane into the airbox. That seems to increase the idle a bit. Second, he covered the holes into the airbox. That also seems to improve the idle a bit.

 

[nabeel.alsalam]

Re: Thermostat and idling

ob, In the driveway, we got it to run pretty well with the idle mixture screws about 3 turns out. The starting point the manual mentions is 2.25 turns out. <br /><br />Once in the water, we tried going out another .5 to 1 turn, but it would not idle.

 

[ob]

Re: Thermostat and idling

If this engine has ever run fine ,I'd have no reason to believe that air or fuel orifices are improperly sized.Did this engine set out of use for a spell before this issue?Restrictions can be caused by gummy deposits as well as particulates.It doesn't take much.Also just a small(as in tiny) air leak on carb body or fuel connector will cause the same symptom.

 

[nabeel.alsalam]

Re: Thermostat and idling

I bought the boat/engine from a friend. <br /><br />He bought the engine, had a mechanic put it on the boat and tune it. He then took it for one ride. He said it ran fine but that it would not idle -- he jammed it into gear and went. <br /><br />He then let it sit for 18 months and sold it to me. That's why I rebuilt the carbs right away. However, I did not see any gummy deposits. <br /><br />So, unfortunately, I have no baseline of experience to compare to. <br /><br />Where are the spots for airleaks on a carb body? I did replace the O-rings around the idle mixture screw. It has new gaskets for the bowl body and around the high speed jet. I did put fingernail polish on the one core plug.

 

[BillP]

Re: Thermostat and idling

I had a 140 omc without tstats. It didn't idle worth squat in 75 degree water without them. I can't imagine what 40 degree water does if your tstats aren't working...but I'll bet its worse than mine was.

 

[CaptainPuget]

Re: Thermostat and idling

It is better to adjust the mixture with the engine in the water. When it’s on muffs there is no exhaust back pressure and it will tend to run lean and at a higher rmp. You should be able to get a garbage can under it and fill it with water (removing the prop first helps). Also, you want to do this adjustment with the engine at the proper operating temperature i.e. with a working thermostat. You also don’t want to maximize the rpm with this adjustment, that will be on the lean side. You want to find the max rpm, then turn it the other way (richening the mixture) until it starts to run a bit “lumpy”. The proper setting is about halfway between the two cheating a bit toward the rich side.<br /><br />You should also be able to check the relief ports with the garbage can as well. At low speeds the water pressure should prevent the exhaust from exiting through the prop and it should exit freely from the relief ports. If the ports are blocked it won’t run at low speeds because the exhaust has no place to go. As the engine speed increases the exhaust pressure will also increase enough to overcome the water pressure and exit through the prop.<br /><br />Idle is the last thing that you set and you want to do the final adjustment with the boat it the water, in gear and putting across the water. You should be able to increase the idle quite a bit before the cam picks up the throttle. The pickup point is where the low speed timing should match the spec. Typically the pickup point rpm is quite a bit higher than the spec idle rpm.

 

[reeldutch]

Re: Thermostat and idling

so what rpms is it idling in the driveway?

 

[nabeel.alsalam]

Re: Thermostat and idling

The thermostat is stuck open! I couldn't tell it was open until I compared it to a new one at the OMC dealer<br /><br />I've taken off the carbs and taken them apart again and removed the core plug at the top this time. The passage over the 3 little holes appear to be clear. I've sprayed carb cleaner into the passageway and it flows through those little holes. I've run a thin wire through the little holes and up through the tube the supplies fuel to that passageway. I'm as confident as I can be that the carb is in good shape.<br /><br />The air bleed orifice is #33 (OMC 320015) and the main jet orifice is #47D (OMC 328542). Does anyone know if those are the correct ones for a my 1991 40 hp Johnson operating at sea level? The model number is VJ40TLEIA and the carbs do not have the plastic tops.<br /><br />I've taken the manifold off to examin the leaf plate assemblies. I don't see any chips. Holding them up to the light, I don't see any leaks. There is some rust on the bar that holds the stops in place. That worries me but ...<br /><br />I've had to special order the gasket so I'm waiting. <br /><br />Captain Puget, thanks for the advice on setting the idle speed. Once I get everything back together, I'll follow your procedure. <br /><br />reeldutch, I'd say it idled at about 700 (just my ear) in the driveway. <br /><br />Thanks everybody for your help.

 

[reeldutch]

Re: Thermostat and idling

get a rpm gage

 

[OBJ]

Re: Thermostat and idling

Nabeel...if you've been trying to set the idle without a tach, your fighting a losing war. Take reeldutch's advice and get a gage or timing light with a tach built in.<br /><br />700rpm, even by ear, out of the water is too low. Any engine that idles properly at the correct rpm in the water will pick up 4 to 500 rpm when out of the water. The reason was stated above by CaptainPuget, no back pressure on the cylinders so the engine dosen't have to work as hard.<br /><br />It sounds like the best thing you can do is set the boat in the water and then set the idle. Once you can get it in gear and the engine warms up, set your low speed needles.<br /><br />Good luck and let us know how you make out.