1995 90hp Johnson - Max RPM only 4100

[hinksj]

I pulled the gas line from the engine and let it burn the gas from the carb. Let it stand for 5 months. Started it 5 months later ... NEW gas .. RPM only reached 4100 when it used to get 5500.<br /><br />Engine uses a gas / oil mix.<br /><br />Pulled carbs and cleaned them with carb cleaner and compressed air. Visually inspected all jets and ait holes.<br /><br />Inspected Fuel Pump - ok.<br /><br />Check Ignition ... all 4 cyl's spark ... checked voltage going to coils - all at 200v per spec.<br /><br />Still engine only get 4100 rpm.<br /><br />While running wide open .. pressing the key in to choke the engine (this activates a selenoid which dumps gas directly into the carb throats) ... RPM increases to 4800.<br /><br />I have taken the carbs apart twice looking for something. No Linkage changes ... no prop changes.<br /><br />Suggestions.

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

Joe, by engine uses gas oil mix do you mean that vro is defeated and you mix at 50:1?Also to assure there is absolutely no obstructions in fixed orifices I usually remove them and use torch tip cleaners to assure no gummy deposits remain.Sounds like fuel restriction somewhere if motor seems to pick-up when bumping primer solenoid.Does primer bulb stay firm during low rpm running?Did you check fuel tank pick-up?Fuel filter?

 

[hinksj]

Re: 1995 90hp Johnson - Max RPM only 4100

ob ... engine does NOT utilize a VRO system ... gas is manually mixed 50:1.<br /><br />I agree it sounds like fuel obstruction somewhere ... I have taken the carbs apart twice. I pushed non-metalic lines thru all jet and openings .... I was very carefull Not to enlarge any ports. I have NOT checked the In-Boat fuel tank filter ... I don't know if I have one. The bulb does go somewhat soft while running. I dismissed potential problems up-stream of the fuel pump because it has enough gas at the carb inputs to handle the extra coke load from the chock solenoid.<br /><br />What does a soft bulb suggest?

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

A soft bulb would suggest air leak at tank fitting,restriction in tank pick-up.defective bulb,or tank not venting properly.

 

[hinksj]

Re: 1995 90hp Johnson - Max RPM only 4100

ob ... I installed a see-thru gas filter between the fuel pump and the gas inlet on the engine.<br /><br />Early on I noticed air bubbles from a bad gas line connection to the engine. All is fixed and no air. <br /><br />I would think if I still had an air leak I would see it.<br /><br />If the line from engine to gas tank was restricted, could I get enough pressure/flow to handle my coke test (stated earlier) but not enough to properly enable the engine to run at full speed?

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

You could be getting enough flow through to choke solenoid yet still not have volume necessary to sustain carb demand at high rpms.How exactly did you dtermine that the fuel pump was operating fine.Is this engine using the vro pump as conventional pump or has it been changed out when vro oiling system was defeated?Still need to insure fuel tank is vented during running operation.

 

[hinksj]

Re: 1995 90hp Johnson - Max RPM only 4100

ob ... the engine Never had vdo .... it came out of the box as a hand mix.<br /><br />I concluded that the fuel pump was ok because it supplied the required fuel when called upon by the choke solenoid when activated at WOT, increasing the rpm from 4100 to 4800.<br /><br />As a side note .... when this engine was started, I did not know that several mud-dabbers had made nests inside the engine compartment. I suspect that the air intake picked up some of the free sand and could have injested it. The engine operates great at low rpm so I concluded that no damage was done and that if it ran fine at low rpm it would not contribute to low HI rpm.

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

Even with that rpm change without looking I would still not rule out fuel pump.And,it is imperative that the fuel tank is venting during operation as if not it will not allow free flow of fuel mix to pump.Also,I believe your fuel pump is crankcase pulse driven air motor design.Inspect pulse hose for damage or kinks that could impair pump operation.A compression test would not be out of the question also since you post that carbs were run dry before storing.

 

[mellowyellow]

Re: 1995 90hp Johnson - Max RPM only 4100

just a guess, but perhaps fuel pump diaphrams<br />are weak? similar problem, no holes or tears,<br />just weak from age.<br />at least they're cheap...

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

I was just thinking Joe about when you dis connected fuel line and ran motor until it stopped.If the fuel line was disconnected from tank the pump diaphragm could have been damgaged since the Quick connect valve would have been closed .

 

[hinksj]

Re: 1995 90hp Johnson - Max RPM only 4100

I will replace the fuel pump diaphram and check to see if there is an in-tank fuel filter.<br /><br />Thanks for all your help.

 

[mellowyellow]

Re: 1995 90hp Johnson - Max RPM only 4100

the pump rebuild kit comes with a couple new<br />gaskets too (I think) it did on my merc.

 

[93evinrude]

Re: 1995 90hp Johnson - Max RPM only 4100

This is a guess but I will ask the question to others with more expertise.Is it possible that the fuel pump is slightly weak and in conjunction with the anti syphon valve cannot get full fuel flow when asked for.---Or is it possible that the anti syphon valve is sticking or gummed up not allowing a full flow of fuel.<br />If possible,perhaps he could use a portable tank and see if it improves rpm's.

 

[ob]

Re: 1995 90hp Johnson - Max RPM only 4100

All that is possible 93Evinrude,Might not be bad idea to try portable tank and rule out existing .Just need to verify that VENT is open.