Merc 850 with jet pump not getting full rpm

[shadowrider]

I have touched on this in the past but have new infomation and felt best to start new thread. I have a 73' Merc 850 with a jet pump. I recently added a tach to verify motor performance. I have been through link and sync proceedure and everthing seems hunky dory. Motor idles around 650 rpm, therefore I assume tach is right. At wot, I am getting 4150 rpm. Unlike a prop lower unit, a jet pump is not effected by load, and can develope full rpm at a stand still ( boat on trailer in water). This is a fact stated on the manufacturers website. I called the manufacture and talked to a technichian, he verified that the pump I had was the right one and I was using the proper 7 3/16 liner/impeller. I asked about changing to a smaller impeller and he said it would only gain 200-300 rpm and would not advise it. He could not shed any light on why I am not getting more rpm. He stated that I should be getting around 5300 wot, at least. Motor has 140 psi on all 4 cyls, runs great other than the low top end rpm. I can not find a great deal of info on jet pumps on here. I am stumped, Hope you guys can help me figure this out. Shadowrider

 

[canoemang]

Re: Merc 850 with jet pump not getting full rpm

Aslong as you have the pump mounted correctly i would lean more torwards the motor.

Prior to the link n sync did you clean your carb's and make sure everything is correct? Verify spark?

I have a merc 86 merc 115 jetted and WOT i get 5600 RPM out of the gate.

(motor is new to me just traded in my pos 84 merc115)

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Just rebuilt carbs, was wondering if bearing could be suspect and not allowing full rpm, however it idles great. Pump mounting doesnt matter motor should reach 5500 regardless of mounting. I will double check spark, has been strong in the past, does not miss a lick.

 

[canoemang]

Re: Merc 850 with jet pump not getting full rpm

It does matter but im sure youve been running jet's long enough you have it mounted correctly, just throwing out ideas.

Since youve added the tach have you ever hit the proper rpm's?

you got the tach set correctly for the amount of cylinders?

Does it GPS out the same as it did prior to the tach?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I disagree with you on mounting, a jet pump will reach full rpm at any mounting height. My motor is mounted properly, the motor has a 12 pole stator which is 6p on the tach dial, which is set. Motor hits 4150 rpm on open water or on trailer at ramp (in water of course).

 

[Mi duckdown]

Re: Merc 850 with jet pump not getting full rpm

Got pics of you jet setup.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Not sure why you want to see it but here it is:

 

[Mi duckdown]

Re: Merc 850 with jet pump not getting full rpm

How long have you been running this set up?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

about 2 years, Just recently added a tach, to be sure of rpms.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Anybody ?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Feel like I am posting to myself, anyhow I just got back from river and checked tach against a dwell tach and it was pretty close, so I can rule out false tach reading. I did spray seafoam in the carbs and will let sit overnite. Will fire it up tomorrow and see if that does anything.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

I did a seafoam treatment, and changed the fuel pump. No change in WOT. Is it possible that the timing set at 21 deg, is limiting the rpm ? Grasping for straws now.

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

If I understand correctly, you installed a pump 2 years ago and never got more than 4150 RPM or you bought the boat this way?

Either way, if you verified compression, spark, the carbs are clean, and the link and synch is done, the only thing left is pump adjustment.

I am assuming that mounting height is correct, the front edge of the pumpp inlet is even of within a half inch above the bottom of the hull. From your pictures it looks like you could come up an inch to gain a few MPH, but this will not cause a low RPM issue.

The pump rotor can be adjusted in or out on the shaft, there are shim washers on the shaft above and below the rotor. swapping the shim(s) from inside to outside moves the rotor closer to the case and increases load lowering RPM, where moving shims from the outside to the inside will increase rotor clearance to the case.

I don't remember the link, but San Leandro Pumps builds most of these pumps and has a service manual that describes the clearance specs for most model pumps. Either way, final adjustment must be done to each pump to 'tune' the pump to the motor.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Thanks for the reply CharlieB. Yes, I bought the boat with jet pump installed. It had a tach that was not working, so I installed a new tach, seemed to me that the boat was underpowered, or that motor was not turning up. Will I gain more rpm by increasing the timing from 21 to23 ? I am using a non-inductive timing light-does that matter ?
As far as the shims go, last time I installed impeller, clearence was closer to .010", where spec is .030". Been told as long as it clears, less than .030 is O.K. I have been on the San Leandro website and even talked to tech, and he is stumped too, and dont recall anything about tuning the pump to the motor. Would like to know more about that.

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Bump

 

[CharlieB]

Re: Merc 850 with jet pump not getting full rpm

The tighter the tolerance the more load, more water pumped. less 'slippage' between the rotor and case. Period.

That same pump is used on a number of HP motors by varying the rotor to case clearance.

10 thou is too tight and the additional load may be the reason for low RPM.

Shim it back out to .030 and retest to see how much RPM you gain, then readjust the shims as needed to dial in the RPM, 5600 is an ideal #

Timing is power, HOWEVER, fuel quality is critical as preignition, detonation will kill a set of pistons in less than a minute.

Thus we have been retarding timing 2 degrees on most all older motors to prevent problems.

You may elect to run a higher grade of fuel and reset timing to original specs but NEVER run a lower grade fuel with original or advanced timing unless you are ready for a total rebuilt or possibly a new engine.

 

[Mi duckdown]

Re: Merc 850 with jet pump not getting full rpm

Shadow. you realize that your transom design, semi v, Is not the best set up.
jet drives like flat bottoms.
And .030 is the best.
Are you sucking air Going with a wave?

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Thanks for the replys guys. I will definitly shim back to .030 to see if that helps. Boat seems to handle pretty well, and no I never suck air unless I try a hard power turn, which I have no need for. CharlieB, you say retarding 2 degrees, my original timing specified is 27 deg. timed down to 21 deg. per Clams Canino's link n sync article. So I am retarding 6 degs, not 2. Is that too much ? Thanks again, Shadow

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

Just got back from the river did a wide open throttle run, pump shimmed back out about .025" I have one shim left to take out but after moving 3 shims to other side of impeller I had no change in rpm. Very disappointed. Any more suggestions or ideas ? Thanks. shadow

 

[shadowrider]

Re: Merc 850 with jet pump not getting full rpm

bump