Old OMC motor - new fuel & ignition parts but won't idle!

[Swell]

My 1968 Johnson 6hp starts ok and runs fine with plenty of power, but won't idle.

It has low-ish compression - around the 70-80 mark - and there is an imbalance between the 2 cylinders, pretty much bang on 10%. It runs well at speed but gets rougher at low speed and idles for a while but then stops.

Could the compression be the reason for this?

I did a pretty major overhaul recently but stopped short of opening up the crankcase and replacing the piston rings. I'm thinking perhaps I should have, but could the cause for the lack of idle be something else? I fitted a carb service kit and replaced ALL ignition parts, and it has a great spark.

Is it the compression or could it be something else?

 

[Chris1956]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I would not think compression is the source of the bad idle. I would check points, ign coils and carb idle mixture. Have you cleaned the carb recently?

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I completely overhauled the carb, including idle jets etc. and have set the slow speed jet carefully. I've replaced all ignition parts with original OMC parts and have spent a lot of time setting it all up to provide the maximum performance. The spark jumps a 1/4" gap no problem. I'm using an equivalent spark plug though - could that be the problem?

It's been consistent since I did the work - starting ok, running ok at speed but won't idle. As I say, it idles for a while but shakes really badly until it just seems to shudder and die. Fair enough it could be shaking because of poor ignition say in one cylinder, but that does all seem ok. Hence why I suspect the compression - anything else to check?

 

[the machinist]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I agree with Chris on the compression for idle. You have to remember that these rope starter motors do not have a lot of cranking power, so your actual reading may be low. However it surely will not hurt & may raise your compression by doing a SeaFoam decarb job.

I would pull the low speed jet (idle needle) & look at the straightness of the needle's small end. If it is bent slightly you may not be getting a true idle fuel mixture. Also while you have it out check for how good it seals on the threads, it may be pulling air in there. If nothing else, smear some chassis grease on the outer threads.

Also check your timing plate, is there any sideways wobbly type slop? If so, you could be getting variable point gapping if the timing plate moves

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Thanks - I'll check the idle jet, though it is something I have done previously, and it looked to be nice & straight, but I'll re-check it anyway. I have replaced the seals along the threads and tested it for air leaks by temporarily applying 'plumber's mait' to the outside of the needle housing. It made no difference so I'm assuming there's no air leaks there.

One question though is regarding the source of the fuel for the idle jet. It follows a fairly complex route all the way from the base of the float chamber via a very narrow duct. I checked it and it seemed to be clear but it struck me that it could be a potential issue as blockages would be hard to detect and to clear. Anyone have any experience of that?

I'm in the UK so used Redex as opposed to Seafoam to decarb the engine. I would like to try Seafoam though as I was expecting a lot more smoke than I got! Is it available in the UK?

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Also check your timing plate, is there any sideways wobbly type slop? If so, you could be getting variable point gapping if the timing plate moves

Sorry - missed your last point. I'll be sure to check that. I'll pull the magneto tonight if I get the time.

 

[kfa4303]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I'm thinking Sea foam and carb cleaning. These links may help.

http://forums.iboats.com/showthread.php?t=167352
http://www.outboard-boat-motor-repa...on 3 HP 1952-1967 Ignition System Tune-up.htm
http://www.outboard-boat-motor-repa...hnson 5.5 HP 1954-1964 Carburetor Tune-UP.htm
http://www.sschapterpsa.com/ramblings/johnson_QD.htm

 

[Sea18Horse]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Sounds like you need to do some more specific troubleshooting. How does it respond to supplemental fuel? If you have a propane torch you can add some fuel in the carb throat that way. Or you could use a spray bottle of premix. Propane is safer around a running engine but premix is better for the motor.

As it's dieing disconnect one plug wire at a time. Minimal change on one vs. instant kill on the other? Maybe you have an upper crank seal leaking? Mag area getting messy?

Remove the plugs and look inside the combustion chambers. One cylinder washed out and clean? Maybe you have a small head gasket leak. Good luck!

Cheers.........................Todd

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Cheers for the links KFA. Lots of very useful info there - especially the slow speed setting. I didn't do it anything like as carefully as that.

I'll try all those tests Sea18horse. I presume the propane test either eliminates the carb (no change) or otherwise (problem goes away).

Again with the plug wires - instant elimination or highlights the cause.

How would I identify an upper crankcase leak?

I'm pretty sure the mag area is clean. All looked good in there.

I re-surfaced the cylinder head and had to fix a sheared bolt. I haven't checked it inside visually after re-fitting though so that's a definite check. The compression variation did improve though after that job so I'm hoping it's ok.

 

[lindy46]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Did you remove the core plug from the top of the carb and clean the idle circuit beneath?

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Did you remove the core plug from the top of the carb and clean the idle circuit beneath?

Yes I did that.

Does anyone have any experience of checking and cleaning the fuel feed to the idle jet?

 

[Haffiman]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

On that oldie I would suspect the crank case compression starting to get low. Any oil in center under the flywheel indicates a leakage and bad seal and even the lower seal is probably not in best of condition. The most common reason for older engines having trouble with idle, provided carb and ignition works, is low crank case compression of which there are very few way of testing. Low crank case compression gives too low air speed through the carb throat to pull fuel into the engine.

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

On that oldie I would suspect the crank case compression starting to get low. Any oil in center under the flywheel indicates a leakage and bad seal and even the lower seal is probably not in best of condition. The most common reason for older engines having trouble with idle, provided carb and ignition works, is low crank case compression of which there are very few way of testing. Low crank case compression gives too low air speed through the carb throat to pull fuel into the engine.

I replaced the driveshaft seals - gasket, o ring etc. (and had to actually fit a driveshaft pin to compress the seal as there wasn't one present, nor was there a hole in the driveshaft to take the pin! - I suspect a replacement driveshaft at some point in the engine's history.) So that should be ok.

I can't be sure about the top seal at the magneto / flywheel, so that's something to check.

I notice there's an oil drain valve underneath the leaf valves which returns excess crankcase oil to the exhaust. I had this apart to clean it and it seemed ok, but I'm not sure now if it needs to be fitted a certain way round. Could this be an issue? Presumably it is designed to open under a certain pressure in the crankcase. I'm just thinking that if it is permanently slightly open that could be reducing the crankcase pressure.

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I've checked a few things

Removed both spark plug leads when engine was dying and the engine immediately stopped both times - so the spark is ok.

There's no oil seeping from the flywheel base seal.

Both cylinders are the same inside and pretty clean.

I tried readjusting the slow idle jet with no improvement.

It does have a tendency to flood quite easily. It always starts on one or two pulls, except when flooded. Could that be an indication of a carb issue? Not sure why it would affect idle though. Could this be the cause of the poor idle?

 

[Sea18Horse]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

I've checked a few things
It does have a tendency to flood quite easily. It always starts on one or two pulls, except when flooded. Could that be an indication of a carb issue? Not sure why it would affect idle though. Could this be the cause of the poor idle?

It could certainly, But it sounds like you were pretty thorough in your carb overhaul so it seems less likely. Plus assuming it is dieing because of too much fuel you should be able to shut down the flow with the idle mixture adjustment. You could try clamping off the fuel line leading to the carb and see if it idles better just as the carb begins to run out of fuel.

Did this problem exist prior to your overhaul? Why did you go through it originally just out of curiosity?

I wonder if you have a fuel pump diaphram leaking. Is it remote mounted? If so you could clamp off the pulse line and see if things improve. Or if not you could clamp off the supply line from the tank.

Cheers.....................Toidd

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

It could certainly, But it sounds like you were pretty thorough in your carb overhaul so it seems less likely. Plus assuming it is dieing because of too much fuel you should be able to shut down the flow with the idle mixture adjustment. You could try clamping off the fuel line leading to the carb and see if it idles better just as the carb begins to run out of fuel.

Did this problem exist prior to your overhaul? Why did you go through it originally just out of curiosity?

I wonder if you have a fuel pump diaphram leaking. Is it remote mounted? If so you could clamp off the pulse line and see if things improve. Or if not you could clamp off the supply line from the tank.

Cheers.....................Toidd

I did the overhaul because it was a nightmare to start and kept cutting out on one cylinder. Coils were cracked, and carb was leaking. Also a sheared cylinder head bolt. So just went for it. It is frustrating the lack of idle and flooding.

I do wonder about the fuel pump. Are you suggesting a leaking diaphragm could cause a low crankcase pressure?

Could it also be the case that the fuel pump isn't performing at low revs? It might be idling until the float chamber is empty. If I pump the bulb at idle would that test that?

Is there a way of testing the fuel pump? I have tried blowing through it. Air only flows one way.

 

[nwcove]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

yes pump the bulb when you first see the symptoms start, it will help narrow things down.

 

[AlTn]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

you can remove the fuel pump leaving both fuel lines on, or take the one off going to the carb....put some nuts on the 2 mounting screws inorder to hold the pump together..block the outlet barb or hose...then pump the primer bulb and look for any leaks, particularly out the hole that the pulse feeds...any leak indicates a bad diaphram..also...you can gravity feed the carb and with the pump to carb hose unattached, determine the fuel flow of the pump at idle....it should show a pretty strong pulse of fuel as the motor idles, no interruptions, no dribbles ..if the flow is weak, the diaphram may have become brittle and hardened and is no longer efficent

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

All makes sense. I'll try those suggestions.

Just another thought. If crankcase pressure is low could that also result in a poor fuel feed due to a lower pulse to the fuel pump?

Could worn piston rings also contribute to a low crankcase pressure?

Any other potential reasons for low crankcase pressure? Are there any particular gaskets which might be to blame - besides the driveshaft seal which I've replaced and the flywheel seal which seems ok?

Just one other thing - don't know if its relevant - there is no thermostat fitted so the engine will be running colder than it should be.

 

[Swell]

Re: Old OMC motor - new fuel & ignition parts but won't idle!

Tried pumping the bulb when idling and it still died. Maybe lasted a little longer but hard to tell. Will try other fuel pump tests when I can.

I wonder if crankcase pressure can be measured by putting a pressure gauge on the fuel pump pulse duct and using a gravity feed for fuel? Sounds feasible and it would enable a comparison to be made.