1991 520h P220G spark but rough running?

[Duramax7man7 505]

Both cylinders have spark. Runs better with rear wire disconnected. But with both cylinders firing its like they are working against each other. 

 

So I did an experiment. If I removed the rear cylinder spark wire and plug the engine fired up great obviously on one cylinder. If I left the plug in and the wire off of that rear cylinder the engine ran like a single piston engine. If I unplugged the front cylinder wire the engine wouldnt start at all. I switched the wire on the rear cylinder with my 1990 520h and the 1990 520 ran fine. The 1991 still the same thing. 

 

The smoke is black coming out. Not blue or white. Is this a valve issue possibly from sitting so long? Either way I will be tearing it down to look at it, de carbon and set valve lash. 

Edited April 13, 2017 by Mastiffman

[WVHillbilly520H 10,369]

Black smoke means it's running "rich" with too much fuel or a weak spark not burning all of the fuel, does the coil look original? or a replacement? some people try cheap aftermarket coils and they don't perform as intended or the original is just weak as is sets on the Onans they're subjected to lots of heat and vibration vs the old Kohlers mounted externally, also older spark plugs will degrade over time and may not be the correct heat range or gap, hope this helps, Jeff.

[Duramax7man7 505]

Always a big help Jeff! I Appreciate you.

 The coil seems like the original but I guess I wouldn't know until I took it off. I could test the coil couldn't I. Anyone know off hand what the Ohm reads are supposed to be for the Primary and Secondary windings? I'll try and find it in one of the manuals as well.

[GMan 82]

Mastiffman,

Use the following:

 

Onan Coil Test Procedure

 

IGNITION COIL
 

To test primary and secondary windings within the
ignition coil first make sure the ignition power is off and
coil is at room temperature of 70°F (21 "C).
 

1. Use a Simpson 260 VOM or equivalent.
 

2. Place a black lead on negative (-) coil terminal and
red lead to positive (+) coil terminal. Primary
resistance should read between 2.90-3.60 ohms.
 

3. Change resistance setting on ohmmeter. Place
ohmmeter leads inside of spark plug cable holes
(Figure 3). Secondary resistance should read
between 14,500 - 19,800 ohms.
 

4. If either of the above resistances are not within specifications,

replace the coil.

 

Hope this helps............................Gary

 

[Duramax7man7 505]

NICE! Thanks a bunch! Will do that later and report back.

[squonk 38,937]

Do you even have compression in the rear cylinder? 

[Duramax7man7 505]

There is compression. How much Im not sure. But would it cause the engine to better with the plug out and wire off it that cylinder had low compression? 

 

 

EDIT: Coil Primary is good @ 3.1ohm but the secondary is registering @ 20,510???

 

Also the long 18" spark wire is at 1.54ohms and the new 18" on my 1990 520h is 4.59 ohms.

 

The short one is 1.89ohms and the 1990 520h is 1.85ohms. So maybe a combo of the coil and the long spark wire? Im going to swap the coils and see if that makes a difference on the 1991 (recently acquired)... 

 

Edited April 13, 2017 by Mastiffman

[Duramax7man7 505]

Well I've tried everything that I could tonight electrically.

Switching Wires, Found the lead for the condenser on the positive post instead of the negative, switched that back. Used the condenser from the good 520h, used the good 520h's wires, nothing... Pulled the wire on the front cylinder and the engine wouldn't even attempt to start. Do the opposite and the front cylinder fires up right away.

 So the compression test is next. I will have to get a tester. Any good suggestions of cheaper but effective ones?

 Seems like compression or Valve issue...?

[ericj 1,576]

is this the same motor that you had in the video ? do a compression test and let us know what you find. good luck

p.s. this is one of the times I hope I'm wrong

 

 

 

 

 

eric j 

[Duramax7man7 505]

Yes. The one from the video. I plan taking the heads off when I pull the motor so no big deal either way. Ive had my 1970 P219 apart and replace the rings, rod bearings, rods and put gasoline pistons back in it as it came with NG pistons. So if it needs rings, so be it. I havent replaced valve seats though. 

 

 

[Duramax7man7 505]

Here is what I found... The rear cylinder intake valve seat is very loose.  But it is fixable. I've seen a couple videos on "Pinging" the aluminum around the valve seat to compress the aluminum into the valve seat to tighten the block around it again. But I want to do this right.

 There are 10, 20 and 30 over valve seats online... I'm assuming I would have to take the entire stripped block to a machine shop to have them cut the valve port to the right size? or is there a safe way to do this at home without rebuilding the entire engine?

 

 

[ericj 1,576]

I have had several valve seats put back in, in onan's. I use a local NAPA  that has a machine shop. They order a new generic valve seat and machine it  to fit the block. usually by the time you notice that the valve seat is out the .10 oversize is too small to fit. I take the exhaust, intake and heads off, wiring flywheel, trigger and stater. drain the oil also. what they do then is machine the seat .005 bigger then the block. then you heat the block to swell it up and freeze the valve seat to shrink it. then they pretty much fall together and when cook have a tight fit. this is also why it is so important to not overheat the onan motors.

 

 

 

 

eric j

[WH nut 553]

On ‎4‎/‎13‎/‎2017 at 9:31 PM, Mastiffman said:

Found the lead for the condenser on the positive post instead of the negative

The condenser on the onans go on the positive side.

[Duramax7man7 505]

I will try the local NAPA and see if the can do this. It all makes sense though. The heat and cold and machining. Good to hear though and thanks for the ideas. 

 

 What I did today was pick up a compression test kit from HF. Figured I cant get a wrong reading with that. Got about 10psi on that rear cylinder. Ha ha. Front one was about 80psi. Pulled the flywheel, stator and Ignition module. Waterproofed the carb, crank breather and fuel pump vacuum line. With the rear wheels off and jacked up, I connected the rear hitch to the 520s inverted hitch with an old 1/2" drive extension and pulled the 1991 out into the driveway, wrap the steering tower and all electrical with a plastic bag and power washed the engine and tractor. Its ready to pull and get de carboned and hopefully fixed this week.

1 hour ago, WH nut said:

The condenser on the onans go on the positive side.

Interesting...The one my 1990 520h has been on the negative side this whole time. 

[ericj 1,576]

make sure the throttle is wide open when doing the compression test, it makes a big difference 

 

 

 

 

 

eric j

[Duramax7man7 505]

10 hours ago, WH nut said:

The condenser on the onans go on the positive side.

 

So what would happen if the Condenser was on the negative terminal long term? This is how it's been on 1990 since I got it. Ran it all winter like that with no issues. That I realized. ha ha.

 

 

5 hours ago, ericj said:

make sure the throttle is wide open when doing the compression test, it makes a big difference 

 

eric j

 

 

Eric, doing this only gained me about 3 psi...

[ericj 1,576]

4 hours ago, Mastiffman said:

 

Eric, doing this only gained me about 3 psi...

I guess sometimes it don't make a big difference, I have had it  go both ways, now the last one's I did it did make a difference, that's why I said something. now if I could just figure out what the difference is.

 

 

 

 

 

eric j

[Duramax7man7 505]

Ha ha. I hear ya. It was a good suggestion to get the most accurate measurement so thanks still! I put some engine restore in this afternoon and ran it for a bit and that alone took the 1990 520h compression from the 84psi to 90psi. So hopefully after a few hours of running that will make it optimal as it can be until I rebuild it. The 1991 with a valve issue will be taken to the machine shop. 

Edited April 16, 2017 by Mastiffman

[Duramax7man7 505]

Okay, so with as loose as the valve seat seems in this engine I'm wondering if I could just get a .002-.005" over valve seat, freeze the seat, then heat the block and press the new valve seat into the block that way instead of boring it out?

 

 What do you guys think?

 

EDIT: I think that I'll just take the old valve out, measure the OD and the ID of the current valve seat bore and then get the difference and make sure that I'll I have a tight fit with the new Valve seat. Is there a rule of thumb of the difference between these two measurements being that it's pressed in. Would it be .0005"-.001" over the ID of the bore, for the measurement of the OD of the valve seat? 

 

EDIT 2: I Spoke with a local recommended machine shop in Toledo, OH and the tech said that they'd rather not cut the bore on an aluminum bloc and would attempt the hot cold installation of an oversized valve seat. So I ordered a + .005 and +.010 valve seats and will see if that will allow me to press it in with the Hot Bore/ Frozen Valve Seat technic. I'm getting up to .008 difference when mic-ing them both. I will check the bore after heating it up to see how much it expands by and also check the new valve seat to see how much it shrinks and make a decision from there before attempting to install it. 

 Any tips on install if it goes down that path?

Edited April 18, 2017 by Mastiffman

[Duramax7man7 505]

Anyone every thought of using this stuff? Or experience using it?
Loctite sleeve retainer.

It can be used for clearances up to .007" and can withstand 400F
 

[aHorseofCourse 261]

I have used the Permatex bearing mount to hold the timing gear on my briggs crank.  It seems to have firmly mounted it.  I think the valve seat will reach temps far above 400degrees being in the combustion chamber tho.   If you get it pressed in and then stake it, it should stay.  A note on the freezing technique, the parts have to be very hot and very cold for it to work.  You only have a second and one shot at it before the temps even out.  It won't allow two parts that normally wouldn't fit, TO fit.  Basically if you don't think they could be pressed together with moderate force, it's not going to work.  I'm not sure what the difference in measurements should be for it to work, but it sounds like you are on the right track.

[Duramax7man7 505]

Thanks for the tips! Called a couple of shops today and tracked down one guy that will bore out and press in a mew seat. Will include the new seat for $80. Seams a little steep though. But at the moment Im battling a snapped off head bolt. Well whats left of it. I tried working it out to begin with but that rear cylinder must have cooked it because its rock hard and brittle. Tried three different juts welded in and non held. All broke off. Tried an unscrewing tap(?) And that broke off in the hole. Got that out with a tiny hole on the outside of the bolt hole and then tried a few bits. Fried them all at lowest drill press speed. Put in a cobalt bit and that did real well but the presses table began to drop. Tried tightening it and BOOM the bolt broke. Done for the night. Need a cool down. Smh. Beginning to think a new block might be better. Tried to take every precautions possible for this to go smoothly and its been nothing but a PIA. 

[Tuneup 1,416]

Just a note on the HF compression gauge. Bought one and it was horribly inaccurate. Almost pulled my Honda engine for a rebuild until I questioned the value (was reading a little over 100 psi on what I knew used to be 150). Garbage. If it's reading low according to your experience, get a second read from another gauge. My 2 cents...

[Duramax7man7 505]

I will rent one from.the auto parts store then. Thanks for the heads up. 

[aHorseofCourse 261]

Steel bolts in aluminum blocks never work out well when they sit.  I think you're right to get the shop to put that seat in.  They could prolly get that bolt out too.  It's all how much you want to spend.  Machining isn't cheap that's for sure.