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post #1 of 22 Old 12-29-2018, 10:02 AM Thread Starter
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1987 JD 420 compression issue

Hello all. This is my first post and I thank you in advance for any advice you can offer. My JD 420 (just over 1,200 hours) Started acting funny this past summer in regards to some loss of power when mowing through tall grass which was never an issue before. Additionally, it became increasingly difficult to get the tractor started this summer and by the end of summer it was taking several minutes to get the tractor started and then it would only run at about half power until the tractor warmed up for 5 minutes. Once the tractor did warm up it ran rather well and did a fine job cutting my 1.5 acres. At one point my negative battery cable melted resulting in the need for a new cable and battery. After the last cut for the summer I was unable to get the tractor started at all so I trailered it to a local JD mechanic. The mechanic did a compression test and told me I had almost 0 compression and one cylinder and about 40 in the other. He told me it is a valve issue and that the old Onan P220G engines are notorious for valve issues due to high heat. His advice was to re-power with a vanguard 23 hp due to the Onan being an L block engine. He told me a valve job would be so labor-intensive I wouldn’t save that much money and then quoted me an estimated cost of $2500 to replace the engine with the Vanguard.
I put almost $2000 in two new tires, all new filters, oils and plugs along with having the mower deck patched, needle scaled, sandblasted, primered and painted so I do intend to keep this tractor for a long time.
Based on all the reading and research I have been doing it does sound like valve issues are common with the engine I have. I have had no issues with burning oil, just the compression issue so I was wondering:
Could this possibly be a coil issue, or a head gasket issue, or could the valves just need cleaned up and adjusted? A friend of my family it is a quality auto mechanic and Crew chief for a nationally sponsored drag racing team and he is willing to help me out as a favor. Does anybody have any advice on where to start in the effort to get the engine running properly again. I would very much like to avoid re-powering at this time if at all possible.
Thanks for your help!
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post #2 of 22 Old 12-29-2018, 10:16 AM
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Re: 1987 JD 420 compression issue

If it were me, I'd be taking the heads off and having a look. That's a no-cost start for you.
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post #3 of 22 Old 12-29-2018, 10:52 AM
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Re: 1987 JD 420 compression issue

+1

I'd be buying whatever the favorite beverages your mechanic likes and asking him to bring equipment to do a leak down test when he arrives and then be prepared to pull heads. Maybe part of problem is you have no valve clearance.

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post #4 of 22 Old 12-29-2018, 11:14 AM
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Re: 1987 JD 420 compression issue

Well I would start with a compression test first followed by a leak down. Most likely with that amount hours, Do you have documentation on the last decarbon of the heads and valve adjustment? Are you the original owner? Onan are also known to wear the bore in an egg shape, that's why a compression test is needed, could be it's time for a bore and larger pistons/rings. The other thing that should be checked is the intake manifold. It is a 2 piece with a seal that can go bad.

Also check the oil pan, I found sludge to the bottom of the oil pickup and a nickle sized fuzz ball (dandelion or c/d wood). Which can cause heat due to low lubrication.

Russ

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Last edited by RussKish; 12-29-2018 at 11:20 AM.
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post #5 of 22 Old 12-29-2018, 11:43 AM Thread Starter
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Thank you guys very much for all this information! My dad bought this tractor with 90 hours roughly 15 years ago in gifted it to me three years ago. It has never been Decarboned and the valves have never been adjusted. My mechanic friend is out of town until the end of the week but I took the tractor to his garage and dropped it off. I will share all of this information with him. We already had a compression test done and it showed zero compression in one cylinder and 40 in the other. Have not had any issues burning oil but if we’re going to open it up we might as well take a look at the cylinders too. Thank you again!
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post #6 of 22 Old 12-29-2018, 12:12 PM Thread Starter
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does anybody have any suggestions on the best place to purchase parts for this engine in the event we do need a valve kit and or a piston and ring kit?
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post #7 of 22 Old 12-29-2018, 01:01 PM
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Re: 1987 JD 420 compression issue

Best place to get parts is either through Boomer (Google Boomers Onan) or through Onanparts.com.

Boomer can provide great advice over the phone.

The valves on the p series are frequently the issue. I wouldn't recommend redoing the valves at this point if that is indeed the issue... But the 40 and 0 has me a bit perplexed... Generally one one valve seat falls out not on both sides.... I'll be interested to hear what you find.

The true test of skill is when you can put something back together and it works... not the dusty half disassembled pile of junk in the corner....

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60's Power King, '64 JD 110, '65 JD 110, '65 JD 110, '66 JD 60, '66 JD 110, '66 JD 110, '66 JD 110H, '66 JD 110H, '67 JD 112, '67 JD 112, '69 JD 112, '69 JD 140H3, '69 JD 140H3, '70 JD 112, '72 140 H1, '77 JD 212, '79 JD 212, '80 JD 317, '80 JD 317, '80 JD 317, uk Cushman 898807, '83 JD 318, '84 420, '86 JD 318, '86 JD 420, '87 JD 430, '87 JD 318, '88 JD 318, '88 JD 318, '88 JD 855, '89 JD 318, '90 JD 318, '91 JD 430, uk Cab Cadet 2140, '92 JD 320, '93 JD 320, '96 JD 445, '98 JD 425, '99 JD 3215A, '00 Sabre 2046, '01 JD 455, and '01 LX277


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post #8 of 22 Old 12-29-2018, 05:13 PM Thread Starter
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Thanks for the info for Boomer. I will definitely provide an update as to what we find after getting the heads off. I so hope it only needs decarbed and perhaps valves adjusted. Thank you all again for the help and Happy New Year!
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post #9 of 22 Old 02-06-2019, 12:20 PM Thread Starter
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Finally got the heads off to get a look at things. My mechanic friend is coming tonight to look over everything in an effort to decide what is next. I have two pictures attached. I’m no expert but I see a good bit of carbon buildup around the valves and possibly a head gasket leak.
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post #10 of 22 Old 02-06-2019, 01:27 PM
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Re: 1987 JD 420 compression issue

Looking at the pics you shot, there is carbon blow by at the top and the center area, so check the head covers for flatness and cylinders top, but might be that head bolts were loose, also get a pic of the top of the cylinder/piston this is where a gap will develop (beginning of egging).

Russ

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post #11 of 22 Old 03-13-2019, 11:22 AM Thread Starter
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My mechanic friend quickly identified that the valve seat on the one intake valve is completely dislodged and moving up and down w/the valve. He also felt there was a head gasket leak. Any advice for pressing the seat back in?
Thanks!
Dave
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post #12 of 22 Old 03-13-2019, 12:23 PM
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Re: 1987 JD 420 compression issue

Shops normally heat the block (oven) and freeze the valve seat (liquid nitrogen). In your case, a heat lamp aim at the area for the valve seat and a freezer for the valve seat over night. Hopefully, the seat will go in. Then let it come up to room temp and check the fit, then they will cut the seat for fit with valve.

Russ

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post #13 of 22 Old 03-13-2019, 05:09 PM Thread Starter
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Thank you for the feedback!
Would a high heat Loctite be a good idea?
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post #14 of 22 Old 03-13-2019, 05:43 PM
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Re: 1987 JD 420 compression issue

None of the shops I know of use it. They prefer just a physical hold, normally if it is still loose after that it is time to go over size on the replacement seat and cut it down to proper spec/fit.

Russ

Grew up loving tractors and vehicles.
Proud Owner of: 318/420/430
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post #15 of 22 Old 03-13-2019, 05:52 PM Thread Starter
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thank you again very much!
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