mercury (73) blueband 115hp low rpms only

lithgow

Recruit
Joined
Jan 29, 2002
Messages
2
I have an old mercury (approx 73) blueband 115hp and am having trouble getting it to run correctly. I have measured the compression at 140psi on all cyclinders, cleaned carbies (single throat), rebuilt fuel pumps and set timing as per manual 0-2 degrees before tdc primary pickup and max 23 degrees BTDC. However, there were two choices the other being 5-7 degrees BTDC and max 23 BTDC. Symptoms are max revs of 3500rpm (not at full throttle) and dies (looses rpm) if use full throttle. Fuel line size is ok. I think the engine is getting too advanced at too low a rpm????? Is 140psi ok?<br />If I reset the primary pickup to 5-7 degrees do you think this will help? Should I aim for 4800-5500 rpm max.? Should I try and set the max angle BTDC by trial and error for highest rpm?<br />The engine was rebuilt before I got it and has a serial number beginning with 800****** which is not listed in my manual but the electrics and distributor suggest its about a 1973. It has two timing marks (original I measued with dial guage represents TDC--am using this one, the second hand made mark is 1-2 degrees btdc). The decal on the ingition support suggests 23 degrees is the max BTDC to use!! Does anyone have a graph of rpm vrs advance?<br />Any help would be much appreciated
 

lightbuilder

Cadet
Joined
Jan 7, 2002
Messages
9
Re: mercury (73) blueband 115hp low rpms only

Does after it reaches 3500 rpm lose power, if you run it at idle again for a few minutes then run it again back up to 3500 rpm and it dies back down again? check your valve on fuel tank, float level on carbs & fuel pump. As far as the comp. goes its only a problem if it varies from cylinder to cylinder 140psi on one and 110psi on the next. Just some thoughts. :)
 

lithgow

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Joined
Jan 29, 2002
Messages
2
Re: mercury (73) blueband 115hp low rpms only

T0 lightbuilder <br />Dont need to go back to idle for it to keep going just back off a bit.If I keep the revs below 3400 it runs fine, only when i open wide the throttle does it loose revs.
 

lightbuilder

Cadet
Joined
Jan 7, 2002
Messages
9
Re: mercury (73) blueband 115hp low rpms only

Have you ever had it running properly. Did you clean (replace) the main fuel jet when you rebuilt the carbs? Do you have the right prop on the boat? ( pitch, dia. ect)
 

12Footer

Fleet Admiral
Joined
Mar 25, 2001
Messages
8,217
Re: mercury (73) blueband 115hp low rpms only

Allmost surely a fuel delivery problem.<br />Run it up to half throttle,and kill the ignition. Check the plugs.Find the dry plugs, and rebuild that carb.<br />Run the idle mixture screw in until it bottoms out (DO NOT TIGHTEN), while counting the turns. This way,you can put it back where it was before the problem,and shouldn't have to worry about carb re-syncing.<br />Reeds can cause this symptom too, but less likely than fuel delivery from the pump or carb.
 

skier

Petty Officer 1st Class
Joined
Jan 31, 2002
Messages
336
Re: mercury (73) blueband 115hp low rpms only

I had this exact same problem. I had it out for the day and I stopped to drop soemone off and head back to the trailer. When I got out of the 5mph zone, 3500 rpm's was all she would do. Infortunatly I also had an idiot for a machanic and it took him all summer and still never fixed it. I sure hope that you have better luck than I did. :(
 
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