increase high RPM on 1971 100hp 'rude

[dstom]

Greetings!

I have had very good luck with advice here in the past.

My good old Evinrude ('71 100 hp) has been giving me lots of headaches this last summer. I thought I had them all fixed, and the other day took it to the lake for a last ride of the season. Everything was perfect, except one item- WOT. Every thing was fine at 4500 RPM, [steady tach, steady sound, steady speed] but at full power, it only climbed to 4900, the tach was bouncing and the motor sounded rough like an intermittent miss. [it had been running nicely at 5500 a few years ago before a slew of problems and issues]

I do not think it is fuel, as it occurred on two tanks, both with fresh gas. However, it could be in the carbs. Can I get and replace just the high speed orifices? Would running seafoam clean the carbs?

I don't think it is the spark plugs as they were replace in mid summer. If I pull the plugs, what should I look for?

I do not think it is the prop because that would affect RPM but would it also cause the engine to run rough?

Any suggestions/ recommendations/ possibilities are appreciated.

Thanks for all your help!


PS- I also had hooked up the digital mutimeter to check voltage on the run. It seemed to constantly grow, and after 20 minutes was at 17.5V. I was just reading a thread here where it was recommended to install a regulator. Where would I get such a item and where is it mounted?

 

[Faztbullet]

Re: increase high RPM on 1971 100hp 'rude

Check your rectifier and air gap on sensor

 

[Solittle]

Re: increase high RPM on 1971 100hp 'rude

Your post is very confusing. You indicate that everything was perfect except WOT rpm yet you also state that it is running rough. You state it was running at 5500 rpm before problems & issues but don't tell us what the problems & issues were. You follow that with some guesses on plugs, seafoam, & prop.

Start by telling us what troubleshooting steps you have done so far - not guesses. Do a compression test & report results. Do a spark test & report results. This will give the guys here something meaningful to go by.

 

[dstom]

Re: increase high RPM on 1971 100hp 'rude

This summer has been a series of headaches.

First trip- about a mile out, at 4K RPM, heard a loud clanking noise. Immediately dropped off plane and turned off engine. Starter would spin, but engine would not. Took off cover, and found loose flywheel nut. Result- towed to shore (by marine patrol), Tightened to 105# in driveway, and it started right up.

Second trip- Was out about 1.5 miles, doing fine at 4K, put a little more power and it started acting like it was starving for gas. checked bulb pressure and it seemed ok and tank was 1/2 full, so headed for home. On way home, it died a couple of times, but started right up after a minute rest. Managed to limp to dock. In the driveway, I noticed gas leaking (or more precisely bubbling) where the hose connects to the motor. Replaced fittings and installed a new bulb (all were at least 20 yrs old).

Third trip- basically the same as trip 2, but I had switched to a different tank. Suspected it was bad fuel as it was from a metal tank. On this trip, it would not restart, so I got a tow home. At home, transferred fuel to tank #1 but poured it through a coffee filter to catch any solid secondments. None caught. Also, cleaned tank, tank pickup screen and fuel pump screen, and installed new plugs (the old ones looked fine visually).

Fourth trip- I had met some recent HS grads who volunteered to be a chase boat (they had a 12 ft. skiff with a 20HP motor). Did not try to get above 4.5K rpm as it was running smoothly (and circles around the chase boat). They wanted to go wakeboarding, so we pulled to the dock for gear. etc. Tried to start, and it sounded like a dead battery (a very slow ruur-ump ruuuuur- uump). Removed truck battery and jumped. Same result. Tried it at home and could only hear the solenoid click. Took off the cover and tried again. Noticed a slight puff of smoke from the starter area. Found that a nut on the bottom of the long bolts that hold the top and bottom plates to the starter canister had come off, causing the starter to twist slightly when trying to turn the flywheel. Put on new nuts (with lock washers). Started right up on the hose.

The score for the summer at that point was 2 tows, 2 dead at dock, and 0 successful trips.

I have not done any tests after the most recent trip and it might be a few weeks before I get to it (something about grand kids being born). Besides compression and spark test, what other troubleshooting steps would you recommend?

 

[dstom]

Re: increase high RPM on 1971 100hp 'rude

Completed compression test and checked spark plugs. Results are:

Compression test:

Cyl Hand wrench
1 90 97
2 95 95
3 95 96
4 96 98

It was about 40* when I did the compression check. All checks were done using the starter for 2 seconds, with all plugs removed, and the throttle in the idle position. "Hand" means the tester was just screwed in had tight. "Wrench" means when I added about an extra 1/4 turn with an open end wrench.

Spark test

The spark plus all appeared good. The electrodes were all a dusty red color, and there were no oil deposits. The gaps were all at .028. For a tester, I made an instrument following Joe Reeves design-- a Board with 4 nails and a center post for ground. There was spark from all leads. #4 was a significantly stronger pulse. 2 &3 were the same, and #1 was weakest. This could be attributed to "crudeness" of the home made tester. When I test again, I will switch the leads to check if the weakness on #1 is due to the tester or it is consistently weak.

What tests do you recommend to diagnose the spark on #1?

Thanks!

Tom

PS- this is really a 1972 100 hp "starflight" model

 

[JSMoore]

Re: increase high RPM on 1971 100hp 'rude

Second trip- Was out about 1.5 miles, doing fine at 4K, put a little more power and it started acting like it was starving for gas. checked bulb pressure and it seemed ok and tank was 1/2 full, so headed for home. On way home, it died a couple of times, but started right up after a minute rest. Managed to limp to dock. In the driveway, I noticed gas leaking (or more precisely bubbling) where the hose connects to the motor. Replaced fittings and installed a new bulb (all were at least 20 yrs old).

Greetings, dstom......
You may have more than one thing going on here. But, one thing caught my eye in the quote above. And, this is just an idea for you to check on.

You never said what kind of gas you use in the boat. But..... if you've been running regular gas station, ethanol blended gas, I would suggest you do a carb tear down and cleaning. You indicated your motor was a 70's model, and that your fuel line/bulb was probably over 20 years old. The rubber compounds used in the fuel lines, gaskets & O-rings back then would not always stand up to ethanol. It would begin to deteriorate and "sluff" off pieces that could become stuck in your carb jets. If you've got some rubber junk stuck in your high-speed jets, that could easily explain why you're not getting full WOT RPM.

Again..... just an idea. But, perhaps, one that may be worth checking out!
Cheers!
JSMoore

 

[JSMoore]

Re: increase high RPM on 1971 100hp 'rude

Sorry...... Somehow got a double post.