6HP Johnson won't idle, runs rough and misfires - help!

[Swell]

Hi,

My 6HP 1968 Johnson will not idle, and intermittently runs rough and misfires, and is pretty intermittent at starting. As far as I know I've eliminated any fuel pump / carburettor issues - largely through help from this forum. The idle jet is clear with no air leaks.

With plenty of revs it is pretty stable, but invariably, as soon as the power drops off it just starts suddenly losing revs, then suddenly gaining revs, and at idle speed it seems to want to idle, but just can't sustain it - as I say, I'm convinced the slow speed fuel flow is fine. At times I'm sure it's only running on one cylinder. At other times it runs ok for a while at medium / low revs but will invariably cut out eventually.

I did a compression test and it's not too bad - 90 in one and 78 in the other cylinder, which is just outside what is generally thought to be ok i.e. +/- 10 psi, but obviously pretty close. I did find out today that it has the wrong sparkplugs, and have ordered a replacement set, but there is a spark from both. Don't know if that could be the issue, but I suspect it's more than just that.

Any ideas? I'm thinking it could be ignition but don't really know where to start. I've ordered a flywheel puller either way!

 

[HighTrim]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Your compression is a bit more of a problem than you think. Should be within 10% of each other, not 10 psi. I would start with that before dumping money into the ignition system. You could try a decarb, see the FAQ, but not sure if youll even up over 12 psi difference, but it definately wont hurt.

If not may need to investigate further by pulling the head.

 

[Daviet]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Check the spark with an air gap tester, it should jump a 3/8" gap and be bright blue.
Does it help to pump the primer bulb? If you have good compression and spark, the only thing left is fuel delivery. Did you overhaul the carb, did you remove the core plug on top of the carb and clean the passages under the plug?

 

[hawkf250]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

The compression split will cause the rough idle the 10 psi split is the max, however, I have seen motors run with as much as 15 psi split, the were just rough as hell. I would recoment a full carb clean, take it completely apart including removing the jets soak it in carb clean for at least a couple hours if not a full day, then blow through every orfice with compressed air, I just had an issue where I thought that I had the carbs sparkling clean and there was still a restriction in one of them causing all kinds of problems. Just cleaning on section of the carb will not do the job. Also to eliminate and fuel system problems, try running on an external tank that is clean and known to be good.

 

[hawkf250]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

High trim is right on the money about the compression, also a blown head gasket can cause the symptoms that youre having as well.

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

The compression split will cause the rough idle the 10 psi split is the max, however, I have seen motors run with as much as 15 psi split, the were just rough as hell. I would recoment a full carb clean, take it completely apart including removing the jets soak it in carb clean for at least a couple hours if not a full day, then blow through every orfice with compressed air, I just had an issue where I thought that I had the carbs sparkling clean and there was still a restriction in one of them causing all kinds of problems. Just cleaning on section of the carb will not do the job. Also to eliminate and fuel system problems, try running on an external tank that is clean and known to be good.

I did a full carb overhaul using a kit and replaced everything I could including the float. I didn't soak the carb though, but did use plenty of carb cleaner on all the jets etc., including the idle jets under the core plug and replaced the core plug.

I get the feeling though that the carb is not the issue - that there is a problem somewhere beyond the carb. It's the fact that the running issues are intermittent which worries me. If it was just rough all the time, ok, but as I say I'm convinced one cylinder is cutting out intermittently. It will be running fine then suddenly lose power and lose that even running sound and instead sound wheezy, like it's pulling fuel but it's not ingiting - or intermittently losing compression, I'm not sure which.

 

[Tim Frank]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Have you had the motor its entire life?
If the coils are original they may be at the end of their useful life.
If you get the flywheel off, do a full ignition troubleshooting sequence....these mag ignitions are covered in FAQ.

New points and condensors might be all you need.

BTW, what plugs are you running?

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Have you had the motor its entire life?
If the coils are original they may be at the end of their useful life.
If you get the flywheel off, do a full ignition troubleshooting sequence....these mag ignitions are covered in FAQ.

New points and condensors might be all you need.

BTW, what plugs are you running?

I've just bought the motor, so I've no idea whether or not anything's been replaced. The plugs in there are NGK B6S - should be B7S which is the equivalent to the Champion J4J listed in the manual.

I'll pull the flywheel and do like you say as it's probably needed either way.

Could an ignition problem give rise to the symptoms I've listed - suddenly not firing on one cylinder - or is head gasket the more likely culprit?

 

[HighTrim]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

IF you decide to proceed, get rid of the NGKs and put Champions in her, that is what she was designed to run on.

Ill throw this out there again, look into the low compression first BEFORE throwing money into ignition components. Just my humble opinion, but is opinion from experience. May just be a bad head gasket, rings, etc... to even out the compression.\

Then, if you still feel like there is a spark issue, test it with an inductive timing light while underway to determine if you are losing a cylinder.

 

[BigB9000]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

could be bad wires, arcing somewhere. different throttle positions move the wires, causing it to run ok again.

I have a video of this somewhere...

 

[Tim Frank]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

I've just bought the motor, so I've no idea whether or not anything's been replaced. The plugs in there are NGK B6S - should be B7S which is the equivalent to the Champion J4J listed in the manual.

I'll pull the flywheel and do like you say as it's probably needed either way.

Could an ignition problem give rise to the symptoms I've listed - suddenly not firing on one cylinder - or is head gasket the more likely culprit?

IMO, an ignition problem could definitely cause these symptoms.

I would have the same red flag on the compression as HT, but since it is new to you, I'd want to see what is under the flywheel. A visual inspection will tell you a lot.
Cracked coils, pitted points would be easy to spot.

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Cheers for all the replies. I'll post back when I've tried something - not sure exactly what as yet but I'll check out the ignition for sure!

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

One question - I've checked in the maintenance manual about removal of the head gasket, but it just describes removal and disassembly of the whole power head.

If that's what's needed fine, but is it possible to change the head gasket without all that? It does just look like the cylinder head should come off quite easily.

 

[AlTn]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

yes...it's as simple as it looks...if a bolt gets stubborn...soak it with a good penetrant and wait..might try the ol "screw it back in and back out" a little at a time...hopefully yours comes out easily

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

I've removed the cylinder head - which was easy enough - and found one of the cylinder head bolts has sheared. It's the bottom right hand one, on the spark plug side. The bottom cylinder was the one with the lower compression so could this be the cause?

The head gasket is old and needs replacing, but there are no obvious breakages across the various apertures. Th cylinder bores are clean with no scoring, though the pistons are coked up on their top surfaces. Difficult to say about the rings as they can't be seen from the top of the cylinders.

Could a new gasket and new lower right cylinder head bolt solve the compression issue?

 

[AlTn]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

along with a good decarb, yes...make sure your head is perfectly flat before reinstalling it...you can accomplish this using 180 grit wet/dry sand paper or emory cloth attached to a perfectly flat surface ie., glass or polished granite, marble, etc. and moving the head in a figure 8 pattern over it....you can finish with 400 grit if you feel it necessary

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

I removed the power head and tried to get the sheared cylinder head bolt out. Problem is there was about 3mm of the bolt left protruding and it was pretty jagged. I tried mole grips, but no good, so I tried drilling it for a screw extractor and ended up with a sheared screw extractor! Also, I tried hacksawing the bolt square and despite trying not to managed to very slightly score the face of the engine block AND the end of the cylinder bore! Aaaaaagggghhhhh!

My plan now is to forget trying to remove the sheared bolt and drill and tap a new hole between the sheared bolt hole and the adjacent cooling duct. There's less material to work with so I'll go for a size down. I reckon it'll work just fine. My only worry is the scoring - especially on the end of the cylinder bore! As I say, it is only very slight, but it's there. I have some gasket adhesive which is specifically designed for worn cylinder heads - Hermetite Red - which is supposed to be really good stuff, so I'm hoping that will do the trick. Hope I haven't fixed one issue and in so doing created another!

Does anyone know how much of an issue this could be and if there is any specific sealant I can use other than the hermetite red? Or do I need to grind/smooth it over?

 

[AlTn]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

seems to me you'd be better off taking the powerhead to a machine shop and having the head bolt removed...it'll probably not be the first mangaled bolt they've removed

 

[Daviet]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Heat can be your best friend. Heat the bolt up and expand it then let it cool off, sometimes that's all it takes.

 

[Swell]

Re: 6HP Johnson won't idle, runs rough and misfires - help!

Fair comment!

How much of a big deal is the scoring of the end of the cylinder bore? It's the bit which would be in contact with the head gasket. Would sealant do it or do I need to use grit to grind it flat?