Merc 850 with jet pump not getting full rpm

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

You increased pump clearance from 010 to 025 with NO increase in WOT RPM?

Restart from square one as something is limiting power. Verify WOT opens the throttle butterflies in the carbs, compression is within acceptable #'s, minimum 7/16 inch spark.

Once the basics are confirmed there are only a couple of things that limit power, fuel flow and a weak ignition.

A restriction in WOT fuel flow is easily checked, a weak ignition at RPM most test stator voltage at speed.

DVA the ignition at speed

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I will have to buy a dva, to check ignition, butterflies are fully open. I am using johnyrude fitting on tank end of hose & tank fyi, but cant see that being a problem.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Here is a link to a video of the boat on open water. Did not know my friend was going to video it, but you get a feel for the boat on the water. Boat is definitely over loaded on this run but still reached 4150 rpm.
http://www.youtube.com/watch?v=UWAI3CMPr-Y

 

[chum1]

Re: Merc 850 with jet pump not getting full rpm

here is a vid of a properly running 850, knock on wood

your tach might be off as my engine is slightly higher, but not much

http://www.youtube.com/watch?v=dKy7s8_ZOiM

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

What should the stator voltage be at wot ?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Bump

 

[bktheking]

Re: Merc 850 with jet pump not getting full rpm

Depending on what stator is on there , according to CDI a bad rectifier can limit RPM to 4000, they say disconnect and retest.

As far as stator testing- red and red white disconnected should ohm out to rx1 56-76, mine ohmed out to 150, the replacement CDI was 50.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I will try both, I did replace rectifier with 25 amp bridge rectifier from radshack, as so many on this forum have. I pulled jet pump and checked exhaust tube for carbon buildup. It was clear, also I hand turned the shaft, feels free, with impeller resistance. so I dont think it's the pump bearing. Thanks for the reply bktheking

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

bktheking,
My motor is a 73 with a distributor, stator wires are yellow, I think you are talking about a newer version with red/white wires ? I ohmed the stator staticly, my ohm meter only has 2 settings rx10 and rx1k. On the rx10 setting stator reading was .1 ohms. Not sure what that translates to but I am thinking 1.0 ohms? My darn manual has walked off hopefully it will come home.

 

[bktheking]

Re: Merc 850 with jet pump not getting full rpm

Ah well then it's an older system before the 4 coil, strange part is they have the same test from 73-77.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Tach is working indicating rectifier is working, ignition is same as 74' 75' merc 850. Are you saying to disconnect rectifier and try wot ? I found one post that said only function of stator on this ignition is charging battery.

 

[bktheking]

Re: Merc 850 with jet pump not getting full rpm

The stator on a 74-75 is for charging, in 76 they changed to run the switchbox. There are tests for no spark and intermittent but not "will not go past 4000 rpm" type tests. Have you verified timing, it's not off a tooth or something silly like that.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I have moved the timing belt one tooth each way, and it will not come into time either way, only where it is currently set. I read a post somewhere that this motor should be timed at 23 deg btdc. (I have searched relentlessly, on other sites as well). Thought about trying it there just to see if that will get me the rpm I'm looking for. Shadowrider

 

[bktheking]

Re: Merc 850 with jet pump not getting full rpm

23 is safe for today's fuel.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

I am using johnyrude fitting on tank end of hose & tank

Somehow J/E fuel pumps manage to pull fuel through a smaller line than mercs use for the same HP.

Rig a test tank without that fitting and test run to find out if there is a noticable difference. I this motor is indeed fuel limited you run the risk of a lean top carb, raising combustion temps and eventually melting a piston.

Can you look thru the spark plug hole at the pistons and see what color each is, all should be the same and black, anything lighter indicates leaner mixtures.

 

[Mi duckdown]

Re: Merc 850 with jet pump not getting full rpm

Shadow.
I am using johnyrude fitting on tank end of hose & tank fyi, but cant see that being a problem.

Could Be.

I forget the proper hose inner diameter, for that motor. Don't have my manuals handy.
What Charlie posted.
Could be as simple as that. Cheap test.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I will try to rig up a test and report back. In the mean time I just went back through link and sync, everthing is good there. Also I pulled plugs and they all have a drop of black oil on them. All look the same.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Just back from river, used all merc fittings, no noticeable rpm gain. Motor seemed to run rougher, could not find sweet spot idling, adjusting carbs as when completing link n sink.
Motor smoked more than normal. I noticed when motor fitting was disconnected that if I pumped bulb, the fitting would leak past check ball, wondering if maybe it was sucking air. Boats started coming into ramp, so I had to leave dissatisfied. Bout ready to send this to river bottom.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I found another motor fitting with a larger id, it's plastic but is .275 id and does not leak when disconnected. Hopefully my patience will return.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

Have you done a spark test using an adjustable gap tester?

A 7/16 inch is the spark a hot blue arc or orange?

A weakening ignition amplifier or coil will limit upper RPM power yet run perfectly fine otherwise.

The spark test or occilascope(spelling?) will prove the ignition.