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Help Suzuki Across Low Compression.

Discussion in 'Suzuki 250cc In-line 4's' started by AndrewStockham, Apr 24, 2019.

  1. AndrewStockham

    AndrewStockham Member

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    Hey Guys,
    First time poster.
    20180930_155054.jpg I have a 95 Across that i have been restoring and I'm scratching my head at the moment. It has 50PSI compression across all four cylinders, hot, cold, wet or dry the compression stays the same.

    It starts but obviously has no power and revs hang for a little bit before returning to idle
    Below is all the things i have changed/checked:
    • New timing chain
    • Valve clearances good (valves appear to seal with fuel not leaking past as a test)
    • lapped intake and exhaust valves also de-carboned exhaust ports
    • new standard sized piston rings
    • honed bores (measured by professional and within in limits)
    • pistons inspected within limits (measured by professional)
    • new head gasket
    • news o-rings on intake manifolds.
    The timing is spot on in relation to the service manual. I have vacuum/carby balancing gauges connect it and it indicates late valve timing. My current thinking is what if i advanced the inlet cam by one tooth and see if that improves compression? how close is the valves to pistons? Is there a a high risk of the two coming into contact with just a single tooth of difference? do i just do it to the inlet alone? advance both cams at the same time? someone also suggested to see if the head is warped, would this make all cylinders be at the 50psi mark?

    Can someone please help me look for something i haven't done yet? Do i push it out side and set it on fire or take it to a mechanic and have them sort it out?
     
  2. kiffsta

    kiffsta Administrator Staff Member Dirty Wheel Club Contributing Member

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    I’d be checking your compression gauges , at 50psi across all 4 pots , it would be very hard to start if not at all.

    How are you doing your compression test, are the carbs off or the throttle held fully open ? Is your comp gauge a screw in job or push in ?

    The across manual is fairly comprehensive in setting the timing , are you removing the cct and setting the exhaust cam first then the intake? Do you have the correct cams in their positions ? Should be 16 pins between the marks on the cams

    Great looking bike.

    Chris
     
    Last edited: Apr 24, 2019
  3. kiffsta

    kiffsta Administrator Staff Member Dirty Wheel Club Contributing Member

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    I had a thought, the first time I did an Across, I mucked up the timing mark , look at the attached images carefully, TDC is not on the T mark

    gsx250f_tdc.PNG gsx250f_tdc_2.PNG
     
  4. Andych

    Andych Well-Known Member Premium Member

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  5. AndrewStockham

    AndrewStockham Member

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    This is the mark I have been aligning everything to.
    Compression tool is a new one bought off ebay, I have been screwing it in. It is difficult to start, needs start ya bastard most if the time. I have done it with carbies off, another time with them on and open throttle, warm, cold, wet and dry. I am certain I have all the timing good.

    15560998974275928351399532980182.jpg
     
  6. kiffsta

    kiffsta Administrator Staff Member Dirty Wheel Club Contributing Member

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    That is the correct mark , when you tighten the cams , can you see the match marks running straight against the top of the engine ?

    157AB086-FE11-40B4-BB0A-BF98F5C9F2FB.jpeg

    Compression test should be done with carbs off or at least at wide open throttle , do you have access to another set of Comp gauges ? I’d also follow @my67xr advice and do a leak down test , easiest way to determine where the issue lies
     
  7. AndrewStockham

    AndrewStockham Member

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    Yes timing marks are all spot on and accurate to the manual, I'd take pics but will need to drain coolant again and remove head cover. The tool I got off ebay says it is a compression and leak down tool. http://www.ebay.com.au/itm/263595464760.
    Sadly I don't have a compressor at home to use air
    I believe the cams are the original ones, is there a way to tell?
    I will take the head cover again over the weekend. Thanks for your help so far guys, definitely look at a way to do the leak-down test.
     
  8. Andych

    Andych Well-Known Member Premium Member

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    That is just a Compression Tester as far as I can see. With a leak down tester you don’t crank the engine over. You set the cylinder to test at TDC and pressurize with the tester and compressed air. If it holds pressure constant all is good but if not you check for air leaking from the crankcase ( rings ) or inlet / exhaust ( valves ) or air in the coolant ( head gasket )
    You can use a little portable compressor for pumping up tyres at a pinch.
    It is very unusual to get exactly the same reading on each cylinder.


    Sent from my iPhone using Tapatalk Pro
     
  9. AndrewStockham

    AndrewStockham Member

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    Follow up post.

    I Macgyver'd the tool i have and a bicycle tire compressor to pressurise one of the cylinders (with the cam lobes definitely away from the valves) and i could hear air coming from around the rocker cover area (rocker cover was removed). Couldn't pinpoint it as the air escaped too fast once the pump was turned off (the leak is that large). tried to reinstall spark plugs to see if it was bleeding across to other cylinders, snapped one off in the head, cracked the shits, got the threaded part of the spark plug out and walked inside and started looking for a replacement motor. Last week i purchased and picked up a known running motor from a wrecker for $200. I'm in the process of cleaning it up, painting it gloss black and installing it.

    I will more than likely continue to troubleshoot the problem motor, but for now i just want to get the bike on the road.
    I'm convinced the head is either warped or there is serious cracks somewhere, ill try torquing down the head nuts a little further in case they have stretched somewhat see if that makes a little difference. maybe install the old head gasket as well as the new one just for shits and giggles. Once i figure it out, i will give an update to what i found the problem to be.

    Thanks for the help offered here.
     
  10. AndrewStockham

    AndrewStockham Member

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    Bike is registered and back on the road.
    So my low compression issue was the tool its self, although it says it is for motorcycles, i would not use it on a small capacity engine. 60cc purely just hasn't enough volume to compress the cylinder, the 30cm of non-braided hose and drive this gauge. I ruled it out when i checked the the second motor (a known runner) and it too showed 50psi in all cylinders. This makes 8 cylinders showing the same thing, concluding the tool isn't to be relied on.

    My main issue ended up being that i missed the emulsion tubes within the carburettors, i didn't realise they came out, so once i saw on a diagram that they do, I punched them out with some timber dowel and a hammer, i found all 4 were corroded and blocked for about the lower 1/3 section, #4 being the worst. Essentially i was running on 2-3 cylinders. i discovered this by managing to get the bike started and soon after feeling the exhaust headers, 1&2 were hot and even, 3 was warm and 4 was cold. It wasn't getting fuel.

    So i didn't need to purchase the second motor, but if i didn't, i would not have followed the path of troubleshooting that i did.
    Catch 22 situation.

    Just thought i would share my findings and close out this issue.

    Thanks again guys
     
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