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  1. #1

    What is the symptoms of a bad reed valve?

    Ok I have been a 4 stroke guy all my life. Even though I do marine repair for a living I have never torn down a two stroke motor. I have always been curious how a bad reed valve reacts to a two stroke engine? Does it pop and fart out the carb like a burnt intake valve on a 4 stroke? Give me some examples./ I have been really lucky so far and never incounterd one and am not sure if I would know if an engine had a bad reed or not.


    I have not gotten into rebuilding engines because of the time frame in tearing one down and putting it back together. The time you spend on this you could be doing drives , lower gear cases, tune ups and such,


    Thanks Roger
    Last edited by BAD ASS BOAT; 05-05-2013 at 02:23 PM.

  2. #2
    Generally speaking its very easy to tell a reed is bad on a carb motor...the bad reed will cause a puff/spit from the carb...it's supposed to be a one way valve into the crankcase,not out.All reeds have a little reflux,but if one carb is spitting noticeably more than the rest check the reeds.its not so easy to diagnose if it's an Efi motor,but you'll develop a feel for it,Chris

  3. #3
    Join Date
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    Additionally, it may be hard to start or down on power. A compression check may not show a "mechanical" issue but it's there because the piston doesn't suck in the fuel and therefore doesn't run on the affected cylinder. I've seen chipp'd reeds as well as the complete thing gone. If the problem isn't fuel related, ie: not too much nor too little and isn't ignition related, ie: spark and timing at the correct time.... then it has to be some kind of "mechanical" problem. Placing fuel down the venturi should result in the same running charateristics as other cylinders, as would blocking off the venturi with your palm, as an example. You would expect to feel the same amount of suction... One other thing, outboards manufacturers always had us do compression checks. But technicly that is "secondary compression" ( the top of the highest port in the cylinder to TDC gives you the psi readings). Snowmobile mechanics have always pressurized the crankcase to determine the ability to hold pressure, that's actually primary compression.

  4. #4

    Reply to answers on bad reed valves

    That is interesting. I thought it might blow back out the carb similar to a bad intake valve on a four stroke. How do you pressurize a crank case on a snowmobile? You would have to have some kind of block off plates for the carbs. If you lose part of a reed it seems like it would get stuck in your crank case or go into the cylinder possibly.

  5. #5
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    Well, it may. The reeds are the "block-off" plates. The issues with stainless reeds is that they are durable and last for a long time. That's the idea anyways. For a standard recreational engine anyways! But prolonged high revs can/will/may make them weak. And if the engine injests pieces of stainless steel, those pieces may get chew'd up in the crankcase or damage piston rings on their way out of the cylinder. Or stuck in the pistom dome..... So, the go-fast guys would rather eat plastic reeds than steel. Hopefully, the resulting damage is less. And, lighter, more responsive reeds may give better throttle response. I'd like to go away from the stainless ones I run in my engine only because of the potential damages rather than the performance "improvements" I MAY get! One day I may but then again, I don't run prolong's high rpms...

  6. #6
    Quote Originally Posted by King Dad View Post
    Well, it may. The reeds are the "block-off" plates. The issues with stainless reeds is that they are durable and last for a long time. That's the idea anyways. For a standard recreational engine anyways! But prolonged high revs can/will/may make them weak. And if the engine injests pieces of stainless steel, those pieces may get chew'd up in the crankcase or damage piston rings on their way out of the cylinder. Or stuck in the pistom dome..... So, the go-fast guys would rather eat plastic reeds than steel. Hopefully, the resulting damage is less. And, lighter, more responsive reeds may give better throttle response. I'd like to go away from the stainless ones I run in my engine only because of the potential damages rather than the performance "improvements" I MAY get! One day I may but then again, I don't run prolong's high rpms...
    Plastic Reeds well I will be to to hell! I did not know there was plastic reeds. Boy wouldn't that make a nice noise no1 cylinder munching down on a stainless reed valve. YUM YUM! That would suck!

  7. #7
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    hard starting,backfiring,blowing crank seals out,damaging recirc valves&hoses.while most of these symptoms have more todo with older/smaller engines,any reed damage can give these type problems and others.-th

  8. #8
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    ...fyi, BAB, CCM is the "TheReedGuy" of the 2-Stroke Universe... ...Hi Chris...;-)
    >> IF YOU'RE GONNA RUN WITH THE PACK...YOU CAN'T PISS LIKE A PUPPY !!!<<

  9. #9
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    My discription of primary compression is not really accurate. The internet shows many better descriptions of two stroke theory than what I could ever attempt to explain. One that does come to mind is a book by Gordon Jennings. Tho it covers more about motorcycles and stuff, ( it can be interesting reading IF ya like math and all that crap!!! "Two stroke Handbook", I think it's called. I guess I'd call primary compression a lesson in theory and not-to-be-confused-with actual pressure checking the primary side of an engine. Again, I've known snowmobile mechanics (with way too much time on their hands, I guess) who would do ANYTHING if they thought it would help them win a race. Who am I to say they were too into their sport! All that math, when I was in school, we called it, ARITHMETIC.!!

  10. #10
    Quote Originally Posted by mn808gade View Post
    hard starting,backfiring,blowing crank seals out,damaging recirc valves&hoses.while most of these symptoms have more todo with older/smaller engines,any reed damage can give these type problems and others.-th
    This pretty much covers what can happen. Ok here is another question guys what is the purpose of the recirculation hoses. I have hooked plenty of them up but what is the purpose of them? I guess the primary problem you will see with bad reeds is the puking out of fuel from the carb.

    I just got done with a 5 month restoration of an old Mark 58 and notice one of the nipples on the recirc hose has a check valve in it. What is the purpose of that?

  11. #11
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    to recirculate the unburned fuel to keep it from being dumped "overboard",to lube upper bearings by the use of oneway valves,sometimes just dumped back into charge flow of another cylinder.one or more recirc hoses broken on a merc V6 will make it darn near impossible to make idle(just an open air leak). sometimes hard to find-under plates/behind starter/under coilpacks/behind powerpacks or switchboxes.

    same for lots other motors.-th
    Last edited by mn808gade; 05-07-2013 at 12:45 AM. Reason: addition

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