Johnson 5hp TD20 ignition timing facts & figures?

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  • legendre


    Replies: 389
    Topics: 8
    #14395

    Alright, it seems that both of my TDs have the same timing geometry.. that is, they both fire significantly ATDC when at lower speeds. Full speed looks to possibly have some amount of advance BTDC but it’s not been measured. Someone apparently ‘borrowed’ my good degree wheel.. so all I can do is approximate.

    And I’ll have to admit.. at the moment, I really don’t know what might be the trouble with this one. So here’s a thought, let’s try to rule-out the head gasket without throwing $20+ at it.

    If the gasket is bad, it’s going to be having an effect on compression, among other things. What if I pull the head & gasket, goop it up with some decent sealer, and then re-install. Once it’s dry, I can take some compression readings and compare – if the readings come up, that tells me something, eh?

    Re: Use of sealer on head gaskets – I generally would never suggest it, but in this case it will never have to hold more than about 80-100 psi so it should be fine (I don’t plan to run it that way). Sound like a reasonable diagnostic?

    BTW – Do you or anyone else know of any sources for reasonably-priced head gaskets for these models? It’s not too intense.. really seems to be plain rubber / fiber about 1/8" thick or so. Not a composite design, no seals ’round the holes, just die-struck from plain sheet material.


    aquasonic

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 634
    Topics: 42
    #14421

    I read that you fabricated a replacement for the long vertical linkage rod. Are you sure that the linkage is all assembled correctly. I had one TD-20 with two different linkage rod issues. The short rod had broken, and the PO brazed it back together in the wrong orientation by 180 degrees. The long rod had been disassembled and put back together with the spring on the wrong side of the stop. Having said that, you mentioned that you had a good runner as an example to go by, so that is a great help.

    Going back to your original post, you mentioned that the compression is at about 65 PSI. That reading is on the low side and that motor may never idle well. I have a small collection of these TD-20’s, and the lowest compression of the really good runners is in the 72-73 PSI range cold and dry. When all is right with these motors, they will be able to idle just ‘two clicks’ up from all the way off with the ignition advance lever.


    legendre


    Replies: 389
    Topics: 8
    #14483

    Howdy aquasonic,

    I sure can’t find any issue with the new carb linkage – the main carb’s butterfly valve appears to be opening & closing just as it ought to.

    And yes, the various springs & keepers in the linkage are exactly where they ought to be. My only changes are substituting proper hairpin (aka ‘R’) -type spring clips for the factory ‘bent up chunks of wire’ in the linkage – and no, none of the clips are snagging or hanging up on anything else.

    We can agree – the 65 psi compression readings aren’t exactly ideal, yet they’re not below the generally accepted figures for a motor of this type and age. And my ‘good’ motor is in about the same psi range as well.

    And just as you note, my ‘good’ motor will idle very reliably at around 2 or 3 clicks ‘up’ from the full low-speed setting. Something is hobbling the present motor, and I’ve still not nailed it down..


    frankr

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 6715
    Topics: 51
    #14493

    The last TN I owned had 60-ish compression, and would idle, but not like it should. We have to remember here, compression is only half the story, controlled mainly by piston rings & cylinder. Crankcase compression is the other half of the story, controlled mainly by piston skirts & fit as well as crankcase sealing.


    legendre


    Replies: 389
    Topics: 8
    #14537

    Hey Frank & All,

    Correct me if I’m wrong, but this model lacks any type of conventional crank seals – yes? It must accomplish sealing by virtue of the plain-bearing bottom end & crank – only 0.0005"-0.001" clearance in the journals – which, in theory, is fairly well-stopped by oil in those tight spaces.

    So unless the main bearings are loose (they are not) we need not worry about leaks between the two crankcases, or between the outer mains and atmosphere. So that leaves us with leaks to atmosphere between the joined case halves.. which I suppose might be a possibility. The POs did do some damage near the case parting lines (chisel / screwdriver) but I couldn’t detect any warping. For the record, I sealed the cases together with the Permatex "Ultra Gray" product, which is automotive OEM approved for gasket-less joints (Nissan / Honda / Subaru / Chrysler) and seems to be well up to the job – but I’ll take advice on that.

    I did find a promising lead two days ago, but chased it down and got nothing. However, I’m using a new diagnostic tool for the ignition, and it seems that I just might have some spark issues with the lower cylinder. Changed the wire & condenser for that one, to no effect. Next I’ll change the coil and take a very good second look at the points.

    And having spent time carefully listening to it run, I +swear+ this motor is dropping a cylinder when I try to take it below 1/2 throttle.. and that’s what’s killing the low-idle range. It starts to shake, sound and feel very rough, the exhaust note sounds off too..


    legendre


    Replies: 389
    Topics: 8
    #14550

    Went all the way, and swapped the complete magneto plate from the good-runner to the problem motor. It’s too late to fire it up (neighbors..) so will report-back when I’ve had a chance.

    If this doesn’t do it, then I’m going to swap the carbs.. and if +that+ doesn’t do it, then I’m pretty well flummoxed.

    (I did try the carb swap earlier, but it was prior to diagnosing & correcting the air leaks in the intake. So that data is invalid.)


    csw


    Replies: 13
    Topics: 6
    #181801

    I’m sorry to have to crash this post, but I have a TD20 that I need to know how to set the points. It’s not like a newer motor, this the first one that I’ve worked on. Thanks Scott


    lloyd

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 453
    Topics: 4
    #181803

    Compression on that motor should be at least 70psi cold and dry to idle down slow. 75 psi is ideal and about 77 psi is like new. Timing is fixed and not an issue, as noted above.


    csw


    Replies: 13
    Topics: 6
    #181809

    the compression is 70psi, what I’m trying to figure out, is does the crank need to be turned to set each set of points?


    frankr

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 6715
    Topics: 51
    #181811

    The crank is mostly round, except for the “flat” that causes the points to close when the push rod enters the flat. So, the points should be set to be open 0.020″ when the push rod is anywhere on the round portion of the shaft. If you have to rotate the shaft to get the push rod on the round portion, well sure.

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