mercury (73) blueband 115hp low rpms only

lithgow · #1 January 30th, 2002, 01:49 PM

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?

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?

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?

Any help would be much appreciated

lightbuilder · #2 January 30th, 2002, 02:39 PM

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 · #3 January 30th, 2002, 02:51 PM

T0 lightbuilder

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.

Hawkeye1 · #4 January 30th, 2002, 05:29 PM

Try posting on this board below. They may be someone there that can help.

http://www.wmi.org/www/boating/boatboard/index.html

http://bbs.cartserver.com/bbs/a/1655/index.cgi

lightbuilder · #5 January 31st, 2002, 02:49 PM

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 · #6 January 31st, 2002, 03:04 PM

Allmost surely a fuel delivery problem.

Run it up to half throttle,and kill the ignition. Check the plugs.Find the dry plugs, and rebuild that carb.

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.

Reeds can cause this symptom too, but less likely than fuel delivery from the pump or carb.

skier · #7 February 1st, 2002, 11:04 AM

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.