Help with diagnosis PLEASE

1992 Mercury 200 Carbs not EFI.

I have posted some questions about rough idle and some stalling at low RPMs in the past and followed the suggestions on this board. Motor runs great once the RPM's get over 1500 or so, but at idle it has trouble. So far is have tried disconnecting the idle stabilizer, advice from a previous post, replaced plugs, decarbed engine, cleaned all three carbs replaced and readjusted, verified timing and link and sync.

Another symptom is there is a lot less suction from the two lower carbs. With the air cover off I can place my hand over any of the top four carbs and it almost chokes out the engine. Placing my hand over either of the lower two has little affect, I feel very little suction. Is this a reed valve problem?

As an aside, all the aformentioned procedures have helped to some degree or another, however I feel the problem is in the lower two cylinders.

Your thoughts?

Re: Help with diagnosis PLEASE

If the compression and all adjustments are good I'd guess reeds or crank case sealing. You could try swapping the reed valves to eliminate that. If the reeds are bad you should get some noticeable pulsing back out of the carbs, but not always obvious.

Re: Help with diagnosis PLEASE

Try cleaning the recirculation valves as these if defective will cause fuel to puddle in intake an foul plugs at idle.

Re: Help with diagnosis PLEASE

donzi gt230 said:

If the compression and all adjustments are good I'd guess reeds or crank case sealing. You could try swapping the reed valves to eliminate that. If the reeds are bad you should get some noticeable pulsing back out of the carbs, but not always obvious.

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I had a local mechanic check the crank case seal by removing the lower unit. He said thats the way to check. He told me the seal was fine. More in more it sounds to me like the reed valves.

Do you know if they are easy to replace on this engine or am I going to have to rip half the motor apart?

Re: Help with diagnosis PLEASE

Faztbullet said:

Try cleaning the recirculation valves as these if defective will cause fuel to puddle in intake an foul plugs at idle.

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I have heard this before, but when I look in my manual, nothing is mentioned about the recirculation valves. Where would I find them, and where would I get instructions on cleaning, testing and or replacing them?

Re: Help with diagnosis PLEASE

JohnnySCV said:

I have heard this before, but when I look in my manual, nothing is mentioned about the recirculation valves. Where would I find them, and where would I get instructions on cleaning, testing and or replacing them?

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The bleed lines (little rubber hoses coming from the reed plate behind the carbs, are on little nipple thingies. Those are the bleed check valves.

Bad bleed lines or bleed check valves can mess up idle, but I doubt it would make much difference in the induction pulse.

If you haven't done a compression check yet, do that next. If compression is low, there is no "tune" item that you can do that will help.

Bad reeds won't affect compression.

To get to the reeds, take the carbs off, then take off the reed plate, flip it over on the bench, and have your way with them. Gotta be one of the easiest engines to service reeds on.

BTW, the manual has inspection procedure and specifications right in it.
You should have known that was coming.

hope it helps
John

Re: Help with diagnosis PLEASE

j_martin said:

The bleed lines (little rubber hoses coming from the reed plate behind the carbs, are on little nipple thingies. Those are the bleed check valves.

Bad bleed lines or bleed check valves can mess up idle, but I doubt it would make much difference in the induction pulse.

If you haven't done a compression check yet, do that next. If compression is low, there is no "tune" item that you can do that will help.

Bad reeds won't affect compression.

To get to the reeds, take the carbs off, then take off the reed plate, flip it over on the bench, and have your way with them. Gotta be one of the easiest engines to service reeds on.

BTW, the manual has inspection procedure and specifications right in it.
You should have known that was coming.

hope it helps
John

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Thank you! I forgot to mention that I did do a compression check. Highest cylinder was 114 lowest was 110, so compression is ok. I think I will take a look at the reed valves. Will I have to tune the carbs if I use aftermarket reeds?

Re: Help with diagnosis PLEASE

JohnnySCV said:

Thank you! I forgot to mention that I did do a compression check. Highest cylinder was 114 lowest was 110, so compression is ok. I think I will take a look at the reed valves. Will I have to tune the carbs if I use aftermarket reeds?

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You should do a link n sync.

How 'bout a little history. Has this engine ever been right for you, or did you buy the problems? Is it bone stock, or a frankenmerc?

John

Re: Help with diagnosis PLEASE

Engine is completely stock, and when I bought it the engine was tough to start and had a rough idle. From an astetic standpoint the engine looks great, it has had a link n sync done, once before I cleaned and dissaembled carbs and once after. I just don't know of anything besides the reeds that would cause a loss of suction through the lower two carbs (other than a crank case seal) which has been ruled out. I figure that being a 1992, and judging from carbs not having the appearance of being cleaned, I would think the reeds are probably original. Is it wrong as a part of the maintenance to replace them after 18 years, simply as a safegaurd that one won't get sucked through the engine?

Re: Help with diagnosis PLEASE

JohnnySCV said:

Engine is completely stock, and when I bought it the engine was tough to start and had a rough idle. From an astetic standpoint the engine looks great, it has had a link n sync done, once before I cleaned and dissaembled carbs and once after. I just don't know of anything besides the reeds that would cause a loss of suction through the lower two carbs (other than a crank case seal) which has been ruled out. I figure that being a 1992, and judging from carbs not having the appearance of being cleaned, I would think the reeds are probably original. Is it wrong as a part of the maintenance to replace them after 18 years, simply as a safegaurd that one won't get sucked through the engine?

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I would consider it normal maintenance to pull the reed plate and inspect the reeds when you go through the carbs.

It's a dozen bolts and one gasket deeper into the engine.

Mine are 22 years old and appear to be perfect. I plan to replace them anyway next winter. I'll put in a set of fiberglass reeds, not for performance, but just because if one fails they digest easier.