1975 merc 50 got lazy

[boatmanwayne]

I am going nuts trying to figure why this 1975 merc 500 with lighting energizer ignition won't open up past 3200 rpms. I have gone thru the carbs, they are clean & set perfect. Have inspected distributor, and replaced driver coils, have replaced ignition coil (all new parts), set timing low&high, checked resistance of all dist.wiring, Tested on the water, will accelerate to 3200 then slight steady miss, squirted premix into both carbs, boggs down, all plugs are slightly oily/wet when pulled, Any ideas anyone? also compression is great.

 

[petryshyn]

Re: 1975 merc 50 got lazy

Have you inspected the distributor cap and rotor carefully, inside and out? Hair line cracks or carbon traces can cause this. Try spraying the cap and wires with water while idling. Does this cause misfire? <br />Is it possible that the fuel pump diaphram is leaking, causing one cylinder to run rich? Have you rebuilt the pumps?

 

[12Footer]

Re: 1975 merc 50 got lazy

In addition to Schematic's advice, it just may be running rich, due to the mixture screw setting. You stated you cleaned the carbs, and the old adjustments may have been compensating for a goo problem before the rebuild...Now, they may be allowing too much fuel.<br />Get your first mate to operate the boat at a fast idle,with the cover off, and try leaning-out 1/4turn at a time,every 10seconds,until the engine starts to stumble. Then, run them back out rich,1/4turn,until it again runs rougher.<br />Now, split the difference,in the smooth range.<br />Be carefull when screwing these mixture screws all the way in.They are very easy to maul at rthe needle seat,and will never run right after that,until the seat and needle are restored.

 

[boatmanwayne]

Re: 1975 merc 50 got lazy

Thanks Schematic, I have checked & doublechecked,even ran at night to pin this on the distributor. No luck. Also, to answer 12footers thought I've leaned it out exactly this way with no better results. Please keep your thinking caps on, We'll pin it down sooner or anchor.

 

[petryshyn]

Re: 1975 merc 50 got lazy

have you checked the timing?<br />and how about the fuel pump?

 

[boatmanwayne]

Re: 1975 merc 50 got lazy

Thanks Schematic, but Iv'e already set timing and checked fuel pressure. Lo and behold, I think I found the problem today. Pulled the powerhead, found exhaust estension tube to be choked down to about 5/8" diameter withe some real hard carbon, Exit should be approx 2 1/4 to 2 1/2 inches. this will no doubt let the engine breath better and get the RPMs back. Thanks to all who responded. This is a great site. Will visit more often.

 

[petryshyn]

Re: 1975 merc 50 got lazy

Right on!!<br /><br />Be sure to use good quality oil, adjust those carbs and minimize low speed operation (don't want this reoccuring)

 

[boatmanwayne]

Re: 1975 merc 50 got lazy

Oh-Oh new problem. Took boat out for test, when accelerating,engine revs to about 3500-3700 rpms, then its just like the ignition switch is being turned on and off, throttle back a little and it runs fine. I checked my timing at cranking speeds from idle to wot using timing light and remote starter, When throttle is at approx 2/3 rd the timing jumps from 35 TDC down to 15 degrees, throttle back it jumps back 35, what is going on