No power from bottom cylinder

[FredM]

Hi all,<br /><br />I have an '81 model merc 18hp two cylinder single carb. SN 59118844<br />Cruising along nicely and suddenly dropped revs.<br />Top cylinder runs fine. Have good spark, changed plugs, compression good - 145psi on both.<br /><br />With the motor running, I ground the lower HT lead - no change. Coils checked out ok. tested with test plug<br /><br />If I ground the upper HT lead the engine dies.<br /><br />Being a single carb and the top cylinder running fine, I don't think the carb is the problem. I am thinking maybe a broken or stuck open reed or blocked exhaust port.<br /><br />Am I on the right track? Just want to get some good advice before pulling things apart.<br /><br />Anything else I should check for?<br /><br />Cheers - FM

 

[FredM]

Re: No power from bottom cylinder

^

 

[NEECAPRS]

Re: No power from bottom cylinder

Greeetings Fred: Are you certain you have spark at the lower cylinder?

 

[OBJ]

Re: No power from bottom cylinder

Fred, rig a spark tester that can be set to 3/8 to 7/16 gap and test your spark from the high tension leads. Sometimes it may appear to be a good spark by grounding the plug but it's not.

 

[FredM]

Re: No power from bottom cylinder

Thanks for the reply Neecaprs and OutboardJim. I sure have spark at the lower cyl. It was even jumping 1/2" from the lead to the plug when I pulled it with the motor running.<br /><br />I changed to a spare plug and after running for a short time on muffs (still not running on the bottom cyl) removed it and from memory it was drier than normal and had a very thin deposit between the electrodes on the path the spark would normally take. Cleaned it put it back and still no joy. Might be wrong but seems more fuel related.<br /><br />Is it rare for reeds to be a problem?<br /><br />Cheers - FredM

 

[NEECAPRS]

Re: No power from bottom cylinder

Fred: This is a tough one--it's too simple! Has this motor been dormant for a while so that reeds could be gummed up? Frankly, that's really hard to imagine. Not sure you can get those compression readings with no induction source. If you remove the bottom plug and ground out the wire, then start it BRIEFLY on the muffs, does fuel vapor shoot out the plug hole. If not, maybe reeds stuck shut--puzzling because most reeds sit slightly off their seats at correct settings. If no fuel vapor, suggest carb cleaner with motor tipped back (down) about 45 degrees. There are some kinds of gum and varnish that only methanol will cut. So, that is a backup, but, not to be left in the tank or in the motor for long periods--attacks neoprene! You can spray or drip it in the motor; running will clear it. You can significantly help open this path back up with use of a vacuum sweeper discharge into the carb. Don't use more pressure than that. Turn the motor over to open both sets of ports.<br />Let us know what this approach or any other does for the patient!

 

[jim dozier]

Re: No power from bottom cylinder

You might check your fuel pump diaphragms. A leaking diaphragm can inject excess fuel into the intake port of the cylinder its attached to. Only problem with this theory is that you bottom plug (you said) was dry, usually they would be wet from an overrich condition. Anyway its easy to check and inexpensive to rebuild.

 

[FredM]

Re: No power from bottom cylinder

Thanks NEECAPRS and jimd, I'll give your suggestions a go (unfortunately won't be until Saturday) I'll report back then.<br /><br />By the way, how likely is a reed failure (broken not just gummed up) in this type of motor. Being from an auto backgroud I have always felt (wrongly or rightly) that the reeds would be a weak point given that they flex back and forth zillions of times during their life. I would be interested in your opinions.<br /><br />Thanks very much for your help

 

[Joey O]

Re: No power from bottom cylinder

In most broken/stuck reed situations you almost always will have gas spitting out from the carb throat. These type of problems are real hard to diagnose without seeing. I would'nt rule out a bad lower end cap seal.

 

[NEECAPRS]

Re: No power from bottom cylinder

Fred, Jim, Joey: This thing's tough! If a reed was broken it does seem like there would be some spitting back out the carb at low speeds. Anyway, at higher speeds wouldn't the cylinder produce a little bit of power so it sure didn't look dead?<br />Lower seal leak: Would that let excess air or exhaust in from mid section so lower wouldn't even fire?<br />Is there any way that both cylinders could be getting spark at the same time? Particularily when right time for top cylinder, but, totally wrong for bottom one?

 

[FredM]

Re: No power from bottom cylinder

Hi fellas,<br /><br />Here's what I'll do. <br /><br />I know I've got good strong spark to both pots so will hook up the timing light to confirm sparking at the right time for each cylinder.<br /><br />If that's ok I'll start her on muffs and check carb for spitting. [I've also read a neat trick - if you aim the timing light (connected to the opposing cylinder) at the carb, the light will flash at the same time any spit comes back through the carb. Dull conditions are better and are supposed to highlight the mist]. If it's spitting then pull the carb and check reeds. If not check for wet or dry plug again. Also check the two bleed tubes.<br /><br />I've ordered a rebuild kit for the carb so I'll give it an overhaul since I don't know when it was last done.<br /><br />If not reeds or carb I'll check for a blocked exhaust.<br /><br />Don't know how to diagnose a failed crank seal other than visual or by elimination or maybe low pressure applied to the crankcase with all ports closed and listen for leaks? Wouldn't you at least get some fire out of that cylinder if it was a leaking crank seal?<br /><br />Will let you know the result.<br /><br />Cheers for now

 

[FredM]

Re: No power from bottom cylinder

Reporting Back - Problem found<br /><br />Chunk (about 1/4" piece) missing from one of the 6 reeds feeding the lower cylinder.<br /><br />Here's how I found it without taking it apart first: Connected a flexible hose to the lower spark plug hole when piston at around BDC (this is when the intake port is uncovered and there is a path to the combustion chamber). Blew through hose and heard air coming from the throat of the carb. Tried the same on the top - could hear the reeds shut and block the flow when I blew harder and leak slightly when I blew gently.<br /><br />I employed the same method to check if the exhaust was blocked. - it was ok<br /><br />I couldn't see any mist out of the carb when the motor was running, but now I think back I could hear dull pulses. There was some power from the lower piston but not enough for it to idle on its own but the top cylinder would. Lower cylinder would run on its own but only at higher throttle setting.<br /><br />Have to order some reeds and gaskets now.<br /><br />The bit of reed must have passed though or got burnt up. Still have good compression (150 psi) on the lower cylinder. Can't take the head off to inspect because it's integral with the block.<br /><br />For anyone else out there the 18 hp is the same setup as the 18, 20 ('86 on)and 25 blue bands.<br /><br />Thanks for all your help NEECAPRS, OutboardJim, Jim and Joey.<br /><br />Cheers - Fred

 

[NEECAPRS]

Re: No power from bottom cylinder

Fred, Good to hear you found it! And good that compression is still up. <br />If those are steel reeds, still interesting to know where the 1/4" piece is. They're spring steel sheet, hard enough to scrape the cylinder wall. Just a suggestion, not to obsess about it, but can you get a look into the cylinder through either the exhaust ports or through a removed cap over the inlet ports? Either or both of these may require the powerhead come up out of the lower bowl. Then you'd have to decide if you want to bother. That piece may be stuck in the piston or somewhere. It's harder for them to get out the exhaust ports when the ports are up on the side of the cylinder.<br />Good Luck will continue, hopefully!

 

[FredM]

Re: No power from bottom cylinder

Hi NEECAPRS,<br /><br />There are 2 inlet covers that I can remove. I'll have a look. The bit that came off the reed is from one corner and probably closer to 3/16" tapering to nothing over about a 3/8" distance - bit like a long sided triangle. I would like to know if its stuck somewhere.<br /><br />Fred M