1990 50hp 2k rpm @WOT

[FishinMO]

I have a 1990 50esdjd outboard on a 16' flat bottom that is nothing but trouble. The problem: motor idles well on hose/ in water, jumps out of the hole just fine gets up on plane and holds it. however it will not go over 2300 rpm. @ 2300 rpm it runs fine, does not stutter or shake. primer bulb is hard. if i turn choke on it dies.
What I have done. cleaned all three carbs (compressed air/carb cleaner), replaced all carb gaskets. replaced fuel pump. replaced all fuel hoses. cleaned out fuel tank. checked ign coils and pulsar coils. checked throttle movement. checked thermo-switch/buzzer. checked compression (116-115-112). checked spark in driveway on hose.
The oil injection system was removed by PO but i have all parts. is there something i should do with the 3 prong plug on harness side?

please help, i dont know what else to do. And yes i do have a manual i have gone through everything.

 

[mphelle8vld]

Re: 1990 50hp 2k rpm @WOT

If you have a 1648 with a 50/35hp that jumps out of the hole and gets on plane quickly, you are definately turning more than 2300rpm. How are you checking your rpm? I have a similar setup with an 89 50/35hp that would only plow along at 8-10mph at 2300rpm. It needs all of WOT (5200rpm) to "jump" out of the hole and around 2800rpm to stay on plane. From your information it sounds like you need a new tach.

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

Ok well enough i may need a new tach. but that is just what i am going on because my buddy with a 40/28 blows my doors off and his boat is heavier than my own. out of thew hole i have him but top end i cant even keep up. that is why i am thinking it is reading correctly.

 

[mphelle8vld]

Re: 1990 50hp 2k rpm @WOT

There is not alot of difference in power between the Yam 50/35 and the 40/28 since they have the same displacement. I've had both on my 1648, holeshot was about the same, top speed was 3-4mph difference. There are so many variables with your setup that will affect your speed. Impeller adjustment, load, load distribution, trim angle, dents in your hull, it's possible that if you swapped motors with your friend, you might still have the same speed issue. See if you can verify your rpm, mine has a WOT of 5200 and a top speed of 30mph with a 1648 riveted jon. If you are around 5000rpm and your impeller clearance is set properly thats about all it's got, could be time to look at the setup.

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

Ok well i guess i will get a new tach and try that out, to make sure or rpms. and i'll try to gps it to figure out MPH's. but just curious. if i am still not getting around 5k rpms what do you think is next? My impeller clearance is excellent and the leading edge is great with no nicks or dents. also the boat is a riveted 76', with a few dents in bottom. but not big enough to make a difference i thought.
I appreciate all your input.

 

[mphelle8vld]

Re: 1990 50hp 2k rpm @WOT

The RPM issue will be really important to troubleshoot the problem since my boat won't get on plane much less jump out of the water at 2300rpm.

Has your motor ever performed properly and then something happened, or has it been this way since you purchased?

Just to give you more info on loading and how it affects speed with a jet, if I add a 250lb friend with his gear, my top speed drops from 30 to 25mph.

Something else that's easy to check, I once purchased a 40/28 tiller and noticed that with the throttle handle all the way open, the throttle butterflies were only opening about 3/4. With your boat on the trailer, rotate your remote control all the way forward and then remove your carb intake cover to verify that the butterflies are completely opening.

If you're anywhere near St Louis send me a PM, I just might take a roadtrip.

 

[99yam40]

Re: 1990 50hp 2k rpm @WOT

If your tach reads well at idle then it maybe OK

Start the basics, compression & spark on all cylinders.
Check timing with a inductive timing light to make sure it is set properly and moves to the proper timing at WOT. IT will also show if a cylinder is loosing spark when the problem occurs by hooking up to the different plug wires.

The CDI unit on my C40TLRX controls the timing and is set at 7 ATDC at idle and 28 BTDC at WOT, not sure about your motor.
Check to make sure alarm system is working, high temperature will cause timing to retard and limit RPM.

Test fuel supply system to make sure it is not getting air in it and the pump is working properly. There is a good thread on that in the frequently asked questions part of forum.
You may still have a carb problem. I have found a piece of trash in one of the needle seats restricting fuel flow to 1 carb causing RPM problems before.
I have also found the jets had been drilled out in my motor causing a overly rich condition at high RPM and it would drop a cylinder. ( you have to love working with used motors)
Be careful, if you are running a cylinder lean it will eat a piston

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

well, I got the boat out on the water yesterday and it seems my tach was reading good at/below 2500rpm but above that it was incorrect. running yesterday with a new tach it came up to 5300rpm and ran good for about 1/4 mile then surge a little and drop to 4000rpm. i believe i have a air leak on inlet side of fuel pump. so i changed filters a couple hoses and some clamps. hopefully it has been fixed. i have checked carbs and they are VERY clean. no thrash at all. other than that it is running really well, idle all the way to full throttle.

 

[mphelle8vld]

Re: 1990 50hp 2k rpm @WOT

Excellent that your motor is operating close to where it should be, keep troubleshooting the fuel problem, don't want to be running lean on any cylinders.

Hopefully the fact that it drops to 4000rpm is the reason your top speed is off, once that's addressed, you should be leaving the 40/28 behind. Keep us up to date.

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

I'm going to take it out in the morning, so we'll see if it's running correctly!

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

so, took the boat out this morning. had a close eye on the tach the entire time. i put the boat in the water and fired up first shot. pulled it to shore, parked the truck. back to boat and fired up the same, i ran two miles with no issues, did 5400 rpm all the way! did great. fished for about 20 min. fired back up fine, started to go and traveled about 1/4 mile at 5400rpm and started surging again! then rpm dropped to a steady 4100 and stayed there with no issues, ran like that for about one mile. stopped, pulled plugs, they looked very normal with just a little oil on 'em like usual. oh yeah, this is all brand new fuel system. fuel pump/carb clean/fuel lines/fuel filter/EVERY fitting has a screw type hose clamp. the only thing that is not stock--i took out the fuel connection at the cowl to eliminate one possibility. I have run out of ideas!!!!!!!!

almost forgot i had a friend in the boat to help try pumping the ball while underway and it was rock hard. try the choke and it wants to die.

 

[mphelle8vld]

Re: 1990 50hp 2k rpm @WOT

Sounds like you're getting close, if your fuel filter was low/empty again, try swapping fuel tanks with your friend, might have an air leak or debris in the fuel pickup. (Make sure he is also using Pre-mix)

If the filter is full I would check the timing, tough to do since you need to see a change while it's malfunctioning. With the motor off, and the controls at WOT, is there any play in the linkage or control arm on the cdi?

 

[FishinMO]

Re: 1990 50hp 2k rpm @WOT

good idea about the fuel pickup, i didn't think that there might be an air leak in that! the top is good but i should def pull it out and inspect.
Also, i have checked throttle play throughout the entire range and it is very solid. (I replaced the carb sync rod and the cdi timing control rod. both are adjusted to specs and function great.
its hard to figure out with solid lines. maybe i could run clear pvc lines for a day and be able to see air bubbles??? that would def tell me where it isif it is in the lines and not the tank.