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Thread: 1986 might max not firing on two cylinders.... jet valve adjusment or testing?

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    1986 might max not firing on two cylinders.... jet valve adjusment or testing?

    Hi, I have an 86 might max that isn't firing on two of the cylinders. There is spark, i'm wondering if the jet valves may be plugged or not adjusted correctly? any advice on how to troubleshoot? Also another post referred to a service manual in your "manuals" section, but I haven't been able to find the manuals section. Appreciate your help, thanks guys.

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    the manual section is only available to donators - it is a one time 10.00 donation that gets you edit, manual and selling privilege's on the site. For your firing issue, are you sure the plug wires are correct on the cap?
    Pennyman1
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    Living the D-50 lifestyle since 1980

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    Quote Originally Posted by pennyman1 View Post
    the manual section is only available to donators - it is a one time 10.00 donation that gets you edit, manual and selling privilege's on the site. For your firing issue, are you sure the plug wires are correct on the cap?
    I double checked the firing order, but looks good. I checked the compression just for kicks... The working cylinders were at 75, the non working came in at 50. Would that tell you guys anything? Thanks

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    Try the compression check wet too. But bother are pretty low pressures.
    This is the result I got on my G63B 2.0 its cyl 1234 in order.
    Dry 157 152 155 170
    Wet 170 170 165 167

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    Quote Originally Posted by pennyman1 View Post
    the manual section is only available to donators - it is a one time 10.00 donation that gets you edit, manual and selling privilege's on the site. For your firing issue, are you sure the plug wires are correct on the cap?
    Manuals are open to any member. Just selling and edit and larger mail is donators. And some other perks.

    Manuals are here.
    http://www.mightyram50.net/vbulletin...er-Source-Info
    Members come and members go, But the board keeps track of them.
    Find me on FaceBook
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    Or look on YouTube Click Here.

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    With compression that low, you won't get anywhere near an adequate combustion cycle. Check your timing belt to make sure it's set correctly. If all of the marks line up and the belt isn't stripped anywhere, I think the next step is taking the head off and inspecting the pistons and bores etc.
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    Quote Originally Posted by geezer101 View Post
    With compression that low, you won't get anywhere near an adequate combustion cycle. Check your timing belt to make sure it's set correctly. If all of the marks line up and the belt isn't stripped anywhere, I think the next step is taking the head off and inspecting the pistons and bores etc.


    Re-checked the compression, and you're right. Cylinders 2&3 are getting about 60-75, but 1-4 got up to 160. The truck will run on the two cylinders and actually doesn't sound as bad as i thought it would. (I disconnected spark plugs on 2 and 3 while it was running and no difference was made telling me that they for sure aren't firing). I put some oil in they cylinders and rechecked compression and it didn't make any difference, so would that rule rings out? Where should I start from here. Thanks again.

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    There aren't many options on this - the jet valves may be burnt out (not uncommon), possibly exhaust/intake valves (the head would still need to come off to perform a leak down test) or the rings could either be broken or frozen into the pistons with crap. At this point I don't think you have much to lose by trying an engine flush. No guarantees but it may free up the rings and slowly restore some (or maybe most...?) of your compression if it works. I've written up a guide on how I do it and so far I've had positive feedback from other members who have tried it out -

    http://www.mightyram50.net/vbulletin...ll=1#post49592
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    Adjacent cylinders low on compression is usually a head gasket in my experience. But certainly burned/hanging valves are a possibility.
    My compression tester has multiple spark plug thread adapters, all with a short hose with a male quick connect plug. This lets me pressurize the cylinder with a standard compressed air hose. With the cylinder at BDC with the valves closed, the source of the air leak will lead you to the problem. Coming out of an adjacent cylinder spark plug hole, the radiator fill, or between the block and head, head gasket. Coming out of the exhaust or intake, burned valves. Coming out of the oil fill cap, burned/broken rings. (there will always be some coming out of the oil fill cap, as every engine has some amount of blow by)
    Last edited by Giovanni89; 12-03-2019 at 06:04 AM. Reason: word salad

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    Good ideas Giovanni and geezer. I'll pressurize the cylinder first and see where that gets me. Appreciate the input.

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    Quote Originally Posted by Giovanni89 View Post
    Adjacent cylinders low on compression is usually a head gasket in my experience. But certainly burned/hanging valves are a possibility...
    +1 on that. I was thinking the same thing that the gasket between cylinders was bypassing after I made my initial post. It fits the symptoms.
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    pressurized the cylinder and had lots of air coming into and through the other bad cylinder, so like you guys thought, sounds like a head gasket. I'll start pulling the head and report back with what I see. Thanks

  13. #13

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    Well, at least it isn't roasted/bent valves. A head gasket is a straight forward deal. Hopefully the head is solid and there aren't any other rude surprises waiting for you...
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    edit,
    Last edited by claych; 12-05-2019 at 12:13 PM. Reason: wrong info

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    welp, sure seems to be the problem. hopefully getting it back together monday. Thanks for your help.

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    What does the head surface look like especially where the gasket failed?

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    check the head carefully for any cracks, particularly around the valve seats. Jet valve heads are notorious for cracking.

  18. #18

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    Well, i didnt think to check before I put it together haha. Got it running though. Next time I'll be sure to. Thanks for your help

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