Merc 850 with jet pump not getting full rpm

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I have not checked gap jump but when I did link and sync the other day, the spark plugs were making a bright blue arc. Also i pulled the wires while running the other day and the spark was jumping more than an inch to the wires, and I got shocked ! I have a few spare coils I could try.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Went down and bought a gap tester. All 4 wires jump a 7/16 gap at cranking speed.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

Have you momentarily bumped the choke/enrichener while at WOT to see if it pickks up any RPM?

Also try squeezing the primer to assist the fuel pump.

If either makes any improvement then you can be sure it is fuel limited.

7/16 mminimum spark should be good.

Air inlets to the motor clear, not restricted by old loose sound deadening foam? Bird or mouse nests?

When everything else checks out the only thing left is an exhaust restriction, carbon build up in the ports or mid.

Drop the lower unit and look up into the exhaust bell. Remove the exhaust side cover and clean the ports.

Have you tried using Sea Foam and decarboning this motor?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I pulled the jet pump and inspected the exhaust bell it is clean as a whistle, however i can not see above it. I have not pulled the inspection cover on block. I have tried squeezing the bulb, remains firm, no rpm change. I have not bumped the choke.
Just recently rebuilt carbs, no air obstructions. I did a seafoam treatment, to my surprise, it did not smoke alot.
Thanks for sticking with me CharlieB and others.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Just pulled lower inspection cover, no carbon blockage, very little on piston tops, looks good to me. I read in a manual ( for a 71 ) that if plugs are oily, that could be a weak ignition. What is the role of the switchbox on this motor ? Could the coil fire properly at cranking speed but fail at or limit higher rpms? I have read that these ignitions either work or they dont, so I dont beleive it is in ignition.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

Depends on the ignition system. Automotive style ignitions loose voltage with RPM as the saturation time reduces with RPM. Most outboard systems are mag driven and increase voltage. Late model switchbox system relys on the stator to produce voltage, a weakening high speed winding in the stator can limit upper RPM. But earlier distributor system relys on an amplifier, to extend dwell time for increased spark, generally if this system works at all, it is fine.

CDIElectronics has literally written the book on outboard ignition troubleshooting, thier guide is available for free download thru links on their support pages.

You mentioned seeing a one inch arc when pulling plug wires, that's pretty good proof the ignition is hot.

Try bumping the enrichener to see if it makes any positive improvement. If no improvement and the pistons domes are well blackened, indicating a normally fat mixture, OEM likes them that way, fewer melt downs. Lower the float settings an eigth inch to slightly lean the main mixture, test run to check
WOT RPM then inspect piston dome color again. Paper bag brown is a great color for peak power but even the slightest fueling restriction will melt a piston so I don't recommend going near that lean unless you are already to spend the money for a total rebuild.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

Which motor is this?

Have you checked the reeds to see if one or more are not quite sealing?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

How do I check reeds? No I havent checked them. I looked at cdi website, nothing seems to fit my situation. While looking at motor and inspecting exhaust ports, I noticed white crust or calcium around water jacket, Next time I dip motor, I will check for jacket leak (Have not noticed it earlier). This is the cdi battery ignition motor, not ADI, does not have high speed coil on stator. Stator has 2 wires. Motor serial #3495920

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Bump

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

Factory parts view

http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=63271

Reeds blocks are around the crank.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I want to revive this thread. Hope that is not against the rules. A year has passed, I have not ran the boat for a year due to a bit of restoration work I performed on the boat. Took it out for a dip and still only get 4100 rpm on chevron mid-grade gas with mercury oil.
Last post was about reed valves possibly limiting rpm. How could I verify that ? Any other thoughts on limited wot rpms ? Thanks