1974 Merc 500 WOT problem

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brandonvarnell

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Jun 16, 2007
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Hi, I am having problems with my engine. It runs great on the ears. When I put it in the water, it will idle great and rev up just fine, but when the prop is engaged and it is under a load it will not get up to full throttle. The RPMs increase until about half throttle and then no more after that. It doesn't bog down, it simply doesn't rev any higher. It has no problem trolling through the water at low speeds. I have replaced the fuel lines, gas tank, rebuilt the carbs and changed the spark plugs. At this point I dont know what else it could be. Any advice will be appreciated.
 
jddenham

jddenham

Petty Officer 1st Class
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Aug 8, 2006
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393
Re: 1974 Merc 500 WOT problem

Sounds like you lost a cylinder or two. Check for spark on all four then let us know what you find.
 
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brandonvarnell

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Jun 16, 2007
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Re: 1974 Merc 500 WOT problem

I just tested the cylinders and all 4 were between 110 and 115. Any more ideas?
 
jddenham

jddenham

Petty Officer 1st Class
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Aug 8, 2006
Messages
393
Re: 1974 Merc 500 WOT problem

That is a good sign that all cylinders are within 10% of each other.

Now that you've trested compression on all cylinders, test them for spark.

No spark on one or two could be caused by bad coil(s) or switchbox.

Do you have a distributor or one coil per cylinder?
 
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brandonvarnell

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Jun 16, 2007
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Re: 1974 Merc 500 WOT problem

I have a thunderbolt - distributor lightning energizer ignition - pointless (alternator driver ignition) system. I have a strong spark at each cylinder. Any other ideas?
 
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brandonvarnell

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Jun 16, 2007
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Re: 1974 Merc 500 WOT problem

I removed the distributor cap and the bearings seemed ok. Tested the coils and here are the results:

Red wire to blue wire = 467 ohms
red to white = 457 ohms
blue to white = 10 ohms
red/blue/white to ground on distributor case = all had not continuity

My manual says red to white and red to blue should be 400 ohms. Could this be a problem?

Thanks for your help.
 
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tmcalavy

Rear Admiral
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Aug 29, 2001
Messages
4,005
Re: 1974 Merc 500 WOT problem

Did you do a linc-n-sych to synchronize the timing with the butterflies opening on the carbs? May be the problem, especially if it was a running ape before the carb removal and cleaning. I learned this with my 73 Merc 500...cost me about $50 to have the L-n-S done but then she ran beautiful.
 
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