1978 Evinrude 115 hp low RPMs at full throttle

[Stevethegreat]

I have a 1978 glassmaster super bassmate with a 1978 evinrude 115hp outboard. It's a 16ft tri-hull, with a 19 pitch aluminum 13 1/4 prop. The motor takes a few tries to fire up, but when it does it can hit 7,000+ rpms easily in idle. Whenever I have it at full throttle, however, it can only hit 3,300 RPMs, which translates to about 25mph in my boat. The spark plugs are all good, all cylinders are firing, I've sprayed the carb with carb cleaner and I've run a few bottles of seafoam through the engine. Everything I've read says the 19 pitch prop is the right size for this motor, and it's a pretty light boat. I need some advice about how to get the motor firing at higher rpms at full throttle. I'm new to outboard repair, so any advice would be appreciated. Thanks.

 

[emdsapmgr]

That engine will run 7000 at idle with only 2 or 3 cyls firing, so that's not a valid test. I would not recommend continued testing at 7000 rpm's when the engine is at idle and not under load. A quick way to put a connecting rod thru the side of the block. The true test is what will the engine run when under load (in gear) when it's in the lake-as you have done. 3300 rpm's is way too low. A couple of things to check: 1. do a compression check on each cylinder. That engine, when new, should have around 125 lbs compression on each cylinder. If the engine is down on compression, that can be a contributing factor to poor performance. 2. To validate the fuel pump, do a simple test. Have a friend go boating with you. When at WOT, have the friend constantly squeeze the fuel hose bulb. This will force extra fuel into the carbs. See what happens as long as you constantly prime the fuel hose bulb. Any change? 3. The ignition system could have a problem when running at WOT that is not seen at idle. Remove the cowling. When the engine is running WOT- put an inductive timing light on each plugwire. and check the flashes from the gun. They will show weak firing, intermittents, non-firing. These can be different from when you check it at idle. 4. Have you checked both carbs to see that the butterflys are level when at WOT? Remove the airbox cover to check this. 5. That engine runs the best when propped to run 5500 rpms at top end. The 19 pitch prop should work on that 115. If you drop down to a 17 pitch prop, you will only gain 300 rpm's. Not really a prop issue here. 5. Check the setting on the tach. Most marine tachs have a variable screw on the back to adjust to many types of marine engines. Check to see that yours is set to the number 6.

 

[Stevethegreat]

That engine will run 7000 at idle with only 2 or 3 cyls firing, so that's not a valid test. I would not recommend continued testing at 7000 rpm's when the engine is at idle and not under load. A quick way to put a connecting rod thru the side of the block. The true test is what will the engine run when under load (in gear) when it's in the lake-as you have done. 3300 rpm's is way too low. A couple of things to check: 1. do a compression check on each cylinder. That engine, when new, should have around 125 lbs compression on each cylinder. If the engine is down on compression, that can be a contributing factor to poor performance. 2. To validate the fuel pump, do a simple test. Have a friend go boating with you. When at WOT, have the friend constantly squeeze the fuel hose bulb. This will force extra fuel into the carbs. See what happens as long as you constantly prime the fuel hose bulb. Any change? 3. The ignition system could have a problem when running at WOT that is not seen at idle. Remove the cowling. When the engine is running WOT- put an inductive timing light on each plugwire. and check the flashes from the gun. They will show weak firing, intermittents, non-firing. These can be different from when you check it at idle. 4. Have you checked both carbs to see that the butterflys are level when at WOT? Remove the airbox cover to check this. 5. That engine runs the best when propped to run 5500 rpms at top end. The 19 pitch prop should work on that 115. If you drop down to a 17 pitch prop, you will only gain 300 rpm's. Not really a prop issue here. 5. Check the setting on the tach. Most marine tachs have a variable screw on the back to adjust to many types of marine engines. Check to see that yours is set to the number 6.

emdsapmgr,
thank you for your quick response. I should add that when I first bought the boat over the summer, it ran at 3400 rpm at WOT with a 15 pitch steel prop. After using it nearly every day, by the end of August it would hit 4300 RPM at WOT and occasionally jump to 4800 RPM, but would immediately fall back to 4300RPM. A month later I switched to the 19 pitch aluminum, which is what my manual recommends, and I'm at 3300. The butterflies are halfway at wot, and when I pump the bulb, the engine loses power. My next step will definitely be to check the compression, followed by the ignition system. Thank you again.

 

[Stevethegreat]

Also the tach was not working when I purchased the boat. I'm currently using a penny tach wound around a spark plug.

 

[emdsapmgr]

Thanks for the update on the prop. Based on your testing with the 15, it confirms the prop is probably not the primary problem here. Jumping of the rpm's may indicate an ignition problem. Weak ignition components tend to fail once the engine heats up to normal operating temps. May run just fine when cold, then start to crap out when it reaches normal running temps. Also based on your observation, the carbs may not be opening fully. You may need to do a "sync and link" on the engine. That's a service manual procedure where the carb butterfly movement is sync'd with the advance of the ignition spark (timer base.) The carb butterflys need to be fully open (parallel to the throat of the carb) when the throttle is in the WOT position and the timer base is against the max timer stop. Have not used a tiny tach, so am unsure how it determines the rpm count on between a 2 or 3 or 4 cyl engine.

 

[racerone]

How is the " tiny tach " set / adjusted between 2 and 4 stroke engines. ??

 

[Boat Doctor1]

From experiences- with the boat in the water (lake) - & intake coner removed-allowing you to see the carbs & when they're opening--- take a squirt bottle of -fuel mix-- get boat @ max rpm in gear & you or someone = squirt that fuel mix into each carb - (even =each side)- =whats the result?? if engine momentarily runs better - u have a fuel problem - high speed jets corroded or stopped up!! I Have seen this so many times!! These jets are in the bottom sides (both sides) of the float bowls!! Hidden behind what looks like fuel drains for float bowls!! Takes a good screwdriver to remove them ( omc makes a special tool for this -BEST!!) as jets can be damaged by an old worn out S D!! careful!! On each side of the float bowls -there is a 7/16 head small bolt -thats what looks like the drains for float bowls!! jets are inside that area! Jets are brass & will corrode (green usually & ) stop up!! again do this only with another person driving the boat & be careful!! fuel & rotating flywheels are dangerous!!

 

[boobie]

Do a cylinder drop test on it to see which cylinders aren't firing.

 

[Stevethegreat]

I've tested the cylinders at idle, they're all firing. I was planning on replacing the carb float and jets next week, but I may do that test first, Boat Doctor1, thanks for the tip. Also sorry for the confusion about the carb butterflies, emdsapmgr, I said "halfway" but I meant "parallel to the throat of the carb" at WOT. The tinytach/pennytach has a setting you can change whenever you get it that determines how many cylinders to count. Thank you all for the help, I'm going to determine compression and test the ignition system, as well as the carb.

 

[emdsapmgr]

Sounds like your carb butterflys are setup correctly-that's good. So, back to racerones' question. How does the engine know whether it's a 4 stroke or 2 stroke? That engine fires once every rpm. If it's counting for a 4 stroke the rpms will be off.