Merc 1500 not firing Cyl # 3

[Boatin Bob]

Hi Guys....I've got a 1976 Merc 150 hp inline 6. Didn't seem to have the same power as last year for pulling a skier so we pulled each plug wire off one at a time to see if all 6 were firing. # 3 was dead, swapped plugs with # 4, no change, grounded plug with wire on and lots of spark from plug, so we did a comp test, all 6 cyls in the range of 115 to 125. The plug was bone dry and yet # 4 (same carb) works fine. The manual shows a reed block that goes around the crank so I pulled the carb off and could see a few of the reeds for # 3 and # 4. As there are several reeds shouldn't I still be getting some fuel, what else can stop fuel from getting to the cylinder? Also if it is reeds does this mean the engine has to be dismantled? Thanks in advance for any ideas, suggestions.

 

[what200]

Re: Merc 1500 not firing Cyl # 3

If you have a broken or chipped reed (only takes one) the intake charge will be at least partially blown back out. The powerhead will have to be pulled and the case halves separated to get at the reed block.<br />One thing you could try is while you have the carb off and the ignition grounded, spray some wd40 in front of the reeds while cranking to detect any backflow of the intake charge

 

[12Footer]

Re: Merc 1500 not firing Cyl # 3

Switch the two switchboxes around..They are identical. If the dead plug moves from #3 to #6, replace that switchbox.<br />There is a long-running debate locally about switching them out as a "matched-pair", but I have never done that...Too danged 'spensive!

 

[what200]

Re: Merc 1500 not firing Cyl # 3

He said the plug was dry so that would point to a fuel issue. The only thing about it is that broken reeds isn't that common.<br />Boat Bob maybe you can clarify. Did you mean that the #3 plug was completely dry but the #4 plug was wet?<br /><br />In any case that is a distributor motor so the switchbox is unlikey to be your culprit. On the #3 plug wire- did it jump 3/8" with a deep blue color or was it orange in color?

 

[Boatin Bob]

Re: Merc 1500 not firing Cyl # 3

what200.....I'm not sure about the length or color of the spark. The # 3 plug was completely dry while the other 5 were wet with like an oily residue. I attached the plug wire to each spark plug (one at a time) and grounded the threads against the block. The spark coming out of # 3 looked just like the other 5. Yes # 4 was wet and when running when I pulled the wire off # 4 the rpm's definitely changed. I guess it's beginning to sound like reeds. I think there is 8 or 10? of them around the crank but you can only see maybe 4 of them with the carb off. Sounds like I'm into a teardown.....thanks for the feedback!