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Info My GJ72A - The usual: compression, rings, shims, etc.

Discussion in 'Suzuki 250cc In-line 4's' started by Zippy, Jul 11, 2022.

  1. Zippy

    Zippy Active Member

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    Hi all, thought I'd start a thread to share the 'fun' I will soon be embarking on!

    In summary, the bike had been running fine... Then in recent months it became really unwilling to start. Then after a few weeks of getting to the point of almost not starting at all, on the last couple of outings the bike would run like **** after it had warmed up - pretty much unusable.


    Carb Clean
    I dived into the carbs, turns out there was a blocked pilot jet on the left carb of the pair. I managed to predict this after noticing that exhaust manifold pipes 1 & 2 didn't get warm on initial startup!

    Having cleaned out the carbs, things seemed promising. Still not instant starting, but much better than before and I thought any residual difficulty would be cured by fine tuning of the fuel screws, idle speed, etc.

    However, on the most recent ride, once things had warmed up, the rough running returned.

    I started reading around this most helpful forum and this brought to light the need for regular valve adjustment and the potential for low compression. And with the bike virtually unrideable, what did I have to lose?!


    Compression Testing
    So in goes the compression tester, which provided the following warm readings:

    cyl - psi
    1 - 130
    2 - 130
    3 - 140
    4 - 140

    Not looking good!


    After adding a teaspoon of oil (on a different day, so engine was cold):

    cyl - psi
    1 - 270
    2 - 250
    3 - 280
    4 - 240

    Quite an improvement!


    Measured again when cold 18 hours after adding the oil:

    cyl - psi
    1 - 190
    2 - 180
    3 - 180
    4 - 190

    So, I'm assuming it's worn rings which are affecting compression and making it still hard to start and run poorly. But I don't see how that would make the warm running take a nosedive, after previously being fairly consistent.


    Valve Clearances
    So I proceeded to measure the valve clearances, with pretty much everything either around or below the lower acceptable range.

    The bike has done around 30,000 km's and based on the struggle I had getting the valve cover off, I doubt it's had much maintenance over the years.

    I guess this is why some people stay away from older bikes!


    Next Steps
    Anyway, as of this weekend the engine has now been removed.
    I plan to take it a local garage for a leakdown test (as I don't have proper system and I couldn't be bothered messing around trying to make a tyre air compressor fit one of the cheap leak down tools by bodging the appropriate air fitting).

    I'm assuming this will confirm the worn rings and potentially highlight additional issues with the valves, etc.

    Therefore I can imagine it will be a case of new rings, hone cylinders, replace a number of shims, etc. etc!


    I suppose only after cracking the thing open, having a closer look and taking some more measurements, will I know for sure. :confused:

    Would be happy to receive any advice from others who've been here before and what things to do, or avoid doing, whilst going through this process!

    I'll update this thread as things progress.
     
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    Last edited: Jul 11, 2022
  2. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Those compression numbers are fine. A teaspoon of oil is a lot in a 250cc four cylinder. The combustion chamber is only about 5cc. Also compession tests on such a tiny engine need to consider the volume of the hoses to the meter etc.
    1/ Set the valve clearances.
    2/ Check the air cleaner element, fittings to carb mouth, all hoses and inlet manifold rubbers.
    3/ Spark plugs, leads and caps need to be perfect.
    4/ Carbs need to be perfect. A very rare thing in todays world of "Green" ethanol blend fuels that are the exact opposite of green.
     
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  3. Zippy

    Zippy Active Member

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    @maelstrom Interesting!

    I was comparing my compression readings with what the book says: 13kg/cm2 (roughly 185psi). So I thought warm readings without oil of around 130/140 psi suggested this was too low. Also when adding oil had a big increase on compression, I thought this demonstrated the rings were leaking when dry.

    However I take your point about the effect of the compression tester itself as part of the equation. This is what I'm using:
    gunson.jpeg

    Just measured the length between pressure relief valve and end of the fitting, which is 350mm with a rubber hose diameter of 12mm. Estimating a tube wall thickness of 50% = 6mm internal diameter. This gives an internal area of 28mm at a given cross section, multiplied by 350mm length = 9800mm, so around 10cc of extra volume, or 4% extra on the 250cc engine?

    Applying that to the 185psi expected compression reduces it to around 177psi, but I know I've made some serious assumptions here! :oops:

    If the combustion chamber (when making compression) is only 5cc, then an extra 10cc of hose when testing a single cylinder would be huge!... So now I'm wondering, if compression test figures for these engines are so sensitive to the testing device used, how does anyone ever get an accurate reading?


    Anyway, @maelstrom if you say that those readings when warm don't seem too bad to you, it's good to have the benefit of your experience.

    Might be a week or two until I get to do a leakdown test on the engine, but will be interested to see (or hear) the result.
     
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    Last edited: Jul 12, 2022
  4. Zippy

    Zippy Active Member

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    Also, thanks for your helpful pointers at #'s 1 to 4 above.

    What makes this more confusing is that most of these factors haven't changed between when the bike was running fine when hot, and when it started to run rough / virtually not run at all.

    I suppose that doesn't rule out an intermittent fault with any / all of the above though.

    :(
     
  5. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    So now I'm wondering, if compression test figures for these engines are so sensitive to the testing device used, how does anyone ever get an accurate reading?
    I would say they don't. They get a comparative reading. If they are familiar with their tool and the engines that they work on then they will know what is an acceptable result.

    When thinking about how things work it is always easy to take the extreme case. Imagine your hose connects to a sealed 44 gallon drum that has a compression gauge fitted. You can crank your engine over for 300 years and the gauge will never register anything.

    TCI units failing is becoming more common as they rot away after 30+ years.
     
  6. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Those compression figures look good - it's the differences between cylinders, which are within range

    As @maelstrom has suggested - check your electrical connections between TCI and plugs - all component connections, there may be resistors [removable/replaceable] which can be accessed by unscrewing the brass connectors where the plugs insert

    Check stator to rectifier connection(s) also

    Electrolytic capacitors [dry out over time] and trigger transistors in a TCI unit can be replaced easily - opening the unit may be the hard part, possibly made more difficult if there is polyurethane potting inside the plastic case

    As far as anything engine related, don't go thinking of anything complicated until you've done the valve clearances and got them back into spec with the correct shims - tight valve clearance symptoms are both difficulty starting and poor running when warmed up because the valve stems grow with the heat and the clearance shrinks still further
     
  7. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Remember the volume of the compression tester tube affects the compression ratio as it increases the fully compressed combustion chamber volume

    99% correct, that extra 10cc volume is on one cylinder, so 16% of 62.5cc chamber volume - just putting 185x.86 [the inverse of 1.16] yields 160 for a compression pressure figure, BUT that's incorrect as the swept volume doesn't change - it's bore X stroke dependent only - compression ratio is swept volume -> compressed volume

    Going the other direction, 185 PSI compression reading would mean ~12.5:1 compression ratio - I assume that is standard spec.

    I reckon that fittinq/tube might have a bit lower volume than 10cc, but if you take the ideal 62.5cc -> ~5cc fully compressed for the 12.5:1 compression ratio

    You've getting reading(s) of ~140 PSI, which is about 9.5x standard air pressure

    Working backwards, 62.5cc swept volume [this isn't altered by the compression tester] -> compressed combustion chamber volume [this is affected by the volume of the compression tester] which yields ~9.5 compression ratio, which would be ~6.6cc which is a 30% increase over the 5cc combustion chamber

    HUGE!

    Easy to see that even a very small volume in that fitting and tube of the compression tester is going to have an enormous influence on the readings...

     
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    Last edited: Jul 13, 2022
  8. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    You fellas have way too much time to spare working on all those formula's etc... suffice to say, on small capacity 4 cylinder bikes... DONT do a compression test for anything other than comparing 1 cylinder to the others... it doesnt tell you anything.
    A leakdown test done correctly will tell you everything.
    If the the bike isnt using oil the rings are most likely fine... majority of issues will be electrical (due to age), fueling (always going to be an issue) and valve clearances..
    It would be very rare to find a high performance small 4 that had really worn rings and all the other items listed above were fine.
    I think people assume that because these bikes have a high Rev limit that they will wear out rings etc before anything else... sure, if you are going to do a top end overhaul (head off for valves etc) then by all means hone and replace rings but unless there is damage to the bore I would expect most are very serviceable and NOT the root cause of the any reliability issues.
     
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  9. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    But I like the math, it's good grey matter exercise

    Agree on all of the rest, but until the valve clearances are in spec, me personally, I wouldn't remove another bolt towards disassembly
     
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  10. Zippy

    Zippy Active Member

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    Thanks all for your input, this really is the 'home of the quarter litre'.

    Things I'm taking from this discussion so far are:

    • The compression readings I'm getting from using my gauge on my engine are not necessarily comparable with the manual specs
    • Others with more experience aren't immediately thinking the rings are shot from the readings I've obtained
    • Comparative compression results is the main benefit of a what I've done so far and those results are positive
    • Tight valve clearances could be the biggest problem (which is in line with the measurements I've taken so far)
    • A leakdown test should help locate any compression issue (if this is what the problem is)
    • The problem could easily be electrical in nature (further enquiry to be undertaken after leakdown results obtained)
    • Never get into a discussion down the pub about maths or physics with @ruckusman !
    :drinks:
     
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  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    :prankster::prankster::dance2:
    AWW c'mon, I'm not that bad, at least I show my working notes as opposed to just blurting out the answers
     
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  12. Zippy

    Zippy Active Member

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    Update - With the engine out of the bike, I tested each cylinder with a 'differential cylinder pressure tester'.

    At 90psi, the leak of each cylinder was:

    cyl - % location
    1 - 12% exhaust
    2 - ~2% not noticeable
    3 - 14% timing chain area
    4 - 14% inlet

    Not as bad as I was expecting. I did this test mainly to try and identify whether the next steps could be top-end only, or whether it would be rings all round. So far I'm only suspecting that piston rings could be an issue on cylinder #3.

    Given the above measurements, I believe this says there at least a couple of things to address with the valves / head, given that performing the leakdown test when cold at TDC means that valve clearances don't come in to play.

    Next steps will be to re-measure the valve clearances so that I know what shims might be needed, then remove the head and hopefully what I observe will confirm the leakage numbers and apparent locations of the leaks.


    Also to highlight - everyone's comments on compression testing were shown to be true - the warm and cold compression readings really didn't show the difference between cylinder #2 (which is good on leakage) and the rest of them.

    ;)
     
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  13. Zippy

    Zippy Active Member

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    Apologies for the lack of update in recent months!


    With the engine out, I re-measured the valve clearances:

    clearances.jpg

    The range of the tables is the min and max for each, as per the book... So the valves clearances are all nicely within tolerance!


    However, that suggested to me that something must be at fault deeper within the engine.

    So out came the cams:

    cams.jpg


    no cams.jpg


    Don't forget to stop and admire Mr Suzuki's machining work all those years ago!

    tappets.jpg


    However, some not very promising signs:

    water tube.jpg


    Which were continued overleaf:

    inside view.jpg





    So I poured some petrol in the combustion chambers to see if fluid would leak passed the valves in the same way as the air leak-down test had shown:

    leak test.jpg


    The result didn't make sense, because the first to empty as #2, which performed the best with the air leak test!

    leak test result.jpg




    Anyway, I gave things a bit of a clean in preparation for pulling the valves.

    cleanup.jpg

    Carb cleaner and a plastic bristle brush did ok, but clearly this grime needs soaking or something more industrial.


    But even this level of cleaning did reveal something interesting:

    A strand of carbon around an inlet valve on cylinder #4:

    inlet.jpg


    And a piece of something trapped between the exhaust valve and seat on cylinder #1:

    exhaust2.jpg


    These things directly correlate to the results of the air leak-down test results taken previously, so maybe the cause of the reduction in compression on those two cylinders!
     
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