2002 75hp loss of power above 3/4 throttle

Ran it today with 11p prop. Ran consistently 4800 - 4900 rpm. Took back in and put on 13p prop. Ran consistently 4800 rpm. Two different pitch props and tops out at same rpm with what seems like a noticable surge. If I didn't know better I would swear it is bouncing off a rev limiter.

Been awhile but would like to revisit this if anyone is game.
Have tried several things on and off through late summer and now fall.
I find this quote from TexasMark interesting "Timing at idle is just a few degrees and at WOT on that engine is 18 to 22 BTDC depending on year model per my manual. Combustion comes from fuel and as rpms increase timing is increased to continue the torque curve (opinion). Since the engine is spinning faster the spark comes earlier to entice the cylinders to fire earlier hence increase rpms. Yea verily you continue to bring in more fuel as this is a venturi fuel system and upping the rpms ups the air flow through the carb and ups the fuel sucked out of the bowl, but the venturi is just so big and once the butterflies are wide open more air is harder to come by. They used to call this linkage system "back drag" and was an economy measure for cruising."
I find this interesting because it is exactly opposite of how my son's system is. With his the timing maxs out fairly quickly. Probably before throttle plates are 3/4 open. And anything extra comes from opening butterflies further.

Here is stuff we tried recently.
Replaced the plug wires because they had been poked and probed so many times I got shocked when hooking up a timing light.
The motor still runs up to around 4900 but no longer falls on its face when opening throttle all the way. Basically at 3/4 throttle when you open it all the way up there is an audible change in intake volume but nothing else happens. With cowling off it will run about 100 or so rpm higher but still doesn't run as it should.
Unhooked fuel pump and used a gravity feed container to supply fuel to carbs. Motor ran exactly the same so that pretty much eliminates fuel pump as problem. But we did verify that motor has excess fuel consumption, which we have suspected all along. It took exactly 6 minutes to run 1 gallon of fuel, which would equate to 10 gals per hr at WOT. Seems high based on suggested 10% of horsepower that everyone throws around.
Next I finally got some of the gapped plugs suggested earlier. Although I must admit I am not sure how to read plugs, what I found seems odd. After several minutes of WOT operation and immediate shutdown by turning off key, plug #1 looks brand new, as if not even being used. Plugs #2 and #3 on the insulator cones, one side of cone was milk chocolate and the other side of cone was like dark chocolate close to black. Seems 2 and 3 running pretty rich but not sure why 1 looks clean. It has spark and seems to be getting fuel since it has not melted down yet and still has comparable compression as 2 and 3. Since 1 seems to be not being used I want to try to run at WOT while spraying starting fluid or something into top carb to see if it makes any difference. Maybe 1 is getting just enough fuel to not melt down but not enough to make power. Would this be possible? Also thinking about rejetting since 2 and 3 seem to be really rich.

Any more suggestions from someone more knowledgeable than I? Thanks in advance.

daddy o said:

Been awhile but would like to revisit this if anyone is game.
Have tried several things on and off through late summer and now fall.
I find this quote from TexasMark interesting "Timing at idle is just a few degrees and at WOT on that engine is 18 to 22 BTDC depending on year model per my manual. Combustion comes from fuel and as rpms increase timing is increased to continue the torque curve (opinion). Since the engine is spinning faster the spark comes earlier to entice the cylinders to fire earlier hence increase rpms. Yea verily you continue to bring in more fuel as this is a venturi fuel system and upping the rpms ups the air flow through the carb and ups the fuel sucked out of the bowl, but the venturi is just so big and once the butterflies are wide open more air is harder to come by. They used to call this linkage system "back drag" and was an economy measure for cruising."
I find this interesting because it is exactly opposite of how my son's system is. With his the timing maxs out fairly quickly. Probably before throttle plates are 3/4 open. And anything extra comes from opening butterflies further.

Here is stuff we tried recently.
Replaced the plug wires because they had been poked and probed so many times I got shocked when hooking up a timing light.
The motor still runs up to around 4900 but no longer falls on its face when opening throttle all the way. Basically at 3/4 throttle when you open it all the way up there is an audible change in intake volume but nothing else happens. With cowling off it will run about 100 or so rpm higher but still doesn't run as it should.
Unhooked fuel pump and used a gravity feed container to supply fuel to carbs. Motor ran exactly the same so that pretty much eliminates fuel pump as problem. But we did verify that motor has excess fuel consumption, which we have suspected all along. It took exactly 6 minutes to run 1 gallon of fuel, which would equate to 10 gals per hr at WOT. Seems high based on suggested 10% of horsepower that everyone throws around.
Next I finally got some of the gapped plugs suggested earlier. Although I must admit I am not sure how to read plugs, what I found seems odd. After several minutes of WOT operation and immediate shutdown by turning off key, plug #1 looks brand new, as if not even being used. Plugs #2 and #3 on the insulator cones, one side of cone was milk chocolate and the other side of cone was like dark chocolate close to black. Seems 2 and 3 running pretty rich but not sure why 1 looks clean. It has spark and seems to be getting fuel since it has not melted down yet and still has comparable compression as 2 and 3. Since 1 seems to be not being used I want to try to run at WOT while spraying starting fluid or something into top carb to see if it makes any difference. Maybe 1 is getting just enough fuel to not melt down but not enough to make power. Would this be possible? Also thinking about rejetting since 2 and 3 seem to be really rich.

Any more suggestions from someone more knowledgeable than I? Thanks in advance.

Click to expand...

I know this is an older post but was wondering if this problem was ever resolved since I have an identical issue.

Thanks.

Sumyunguyz,
We never really fixed this 100%. Not sure if you read all the posts word for word, but the last best thing we did was replace spark plug wires and replaced the gapless plugs with the regular plugs someone recommended earlier in this post. The noticeable surge is gone, I think it was the plug wires leaking through to ground, because I got shocked a couple times when hooking up a clamp on timing light. It still will not run up to full throttle. For instance my son just destroyed his 13p prop the other day. So we put his spare 11p on it. It is also hot and muggy out, which is when the problems mostly rear its ugly head. When getting up on plane and then running it out it runs good up to about 3/4 throttle and won't go over about 4800 - 4900 rpms. Which is about exactly where it was running recently with the 13p. At 3/4 throttle if you go ahead to WOT it makes a noticeably louder audible sound when throttle plates open all the way but rpm does exactly nothing. So we usually just run boat at 3/4 throttle and accept that is the best it will run. In winter the throttle operates the same way but on good days, cold and dry, it will run in neighborhood of 5200 - 5400 rpms. The 200 - 300 rpm difference to me is normal due to the difference in cool and non humid weather. But what is not normal is the extra 1/4 throttle doing absolutely nothing whether it is hot or cold out. To me it really seems like the throttle plates open, making louder sound from extra air but it just seems like the motor can't make use of the extra air. It does the same thing with cowl on or off so I don't think it is an air intake restriction. I am leaning towards some kind of exhaust restriction. But without pulling powerhead and probably some extensive disassembly, I don't know of any other way to determine this. It is frustrating because I know there is more power in motor. Since it is my son's boat he has decided he doesn't want to spend the time and money to pull apart and at very least re-gasket everything and possibly find nothing wrong. You can reply back as to anything you have tried or found that has helped you or may be of help to us. Maybe somehow between us we can figure something out. Also one other thing we did early on that seemed to help a bit was decarbonizing engine. That is one of the earliest things we did that actually seemed to make a difference. It made the motor feel like it had more power out of the hole.

Thanks for the response daddy o and sorry to hear your still having problems. I too feel your pain with my 2000 75 hp Merc with the identical issue your having. Mine is on an 18 ft. Tracker pontoon. When I first got the motor about 3 years ago it had been left unused for a couple of years with ethanol fuel in it. Put new hoses on boat and motor side, cleaned and overhauled carbs spotlessly. Then I put a new fuel pump kit in it. Cranked it up but it seemed like a miss or surge started around 4000 RPM and never would get over 4200 (WOT). I too de-carboned the engine (1 can of Seafoam in a gallon of non-ethanol fuel). I seemed like it took the miss out of it slightly and the RPM's got up to 4900 however the fuel use was very high. Then the summer set in and it fell on it's face again. Took it to a Merc shop and they had it over 3-4 months. After their 3 tries of lake testing they just said they didn't know what was wrong with it and gave up. Even the Merc support guy came by their shop and said he was stumped.
Well in the last couple of years just like you it's got new CDM's, plug wires, trigger, stator, Racor filter, reed valves inspected, all motor side wiring inspected \cleaned, went from a 13p prop to a 11p, new gas tank and bulb. The carbs have been pulled and cleaned 3 times because I keep thinking it's a fuel delivery problem. Yes, the timing has been set and carbs sync'd. Compression at 112# on all cylinders.
I feel the same frustration as you on this little booger. My mind keeps telling me it's a fuel delivery problem. The last 1/4" throttle I can see and hear the throttle plates opening and lots more air coming in but no more power. If I hadn't had cleaned the carbs so many times before I would swear it's a high speed jet becoming lodged with debris but I know it's not. Good fuel pressure on discharge side of pump, no air bubbles in fuel lines, spraying a little fuel in the carb throat using the choke only bogs the engine down.

I haven't tried the regular plugs yet since I can't seem to find a conversion chart from gapless to regular. If you remember what regular spark plugs your using Ill give it a try. Thanks

Thanks for the info. I'll get a set and try it.

Just trying to do a little brainstorming here. Curious if you think a stuck open thermostat could cause this condition by making it run to rich, i.e. excessive fuel use. Seems like that's the only thing I haven't looked at yet.

A digital infrared thermometer would come in handy. Harbor Freight might have them. Friend found one in North Carolina for 20 bucks.

sumyunguyz said:

Thanks for the info. I'll get a set and try it.

Just trying to do a little brainstorming here. Curious if you think a stuck open thermostat could cause this condition by making it run to rich, i.e. excessive fuel use. Seems like that's the only thing I haven't looked at yet.

Click to expand...

I believe it probably could. I know that the motor we have supposedly had the thermostat changed because Mercury tech support told them to change it. I may revisit that aspect also. I have an infrared thermometer. Anyone know what block temp should be on a warmed up 75.

After 10 minutes at idle my IR gun read 116 deg. on #1 &#2 cylinders and 118 on #3. Inlet water temperature (lake) is 95 deg. Pulled the thermostat and it was fully closed and is opening at 120 deg. The thermostat appears to check out fine as far as I can tell using a high precision digital thermometer. My understanding is that the thermostat is primarily used to control temperature\water flow at low speed (<1500 RPM) and the poppet is to control temperature\water flow at high speed. Not really sure how to test the poppet valve but it, the spring and diaphragm seem to be in good condition as well. Still having some problems locating regular spark plugs for this motor but the auto parts store said I should have them tomorrow and will report back after a test run. Thanks.

When I got the plugs Autozone and Advance Auto both had to order. O'rielly had them in stock.

Thanks for the info! You were right. Read your post while I was in town and picked a set up at O'Rielly's. The length of these from the seat to tip of electrode is longer by around 1/8". Any issue with top piston clearance?

sumyunguyz,you are spot on the operation of cooling components function,If poppet valve did not function properly, engine would more than likely overheat. Check clearance of plugs carefully, I would turn engine over by hand slowly to ensure there no mechanical issues in piston to plug clearance.Reconfirm specs in manual.

sumyunguyz said:

Thanks for the info! You were right. Read your post while I was in town and picked a set up at O'Rielly's. The length of these from the seat to tip of electrode is longer by around 1/8". Any issue with top piston clearance?

Click to expand...

Was worried about that as well. Once installed we turned over by hand and did not feel any resistance. Bumped with starter and then removed plugs to be certain electrodes showed no sign of being bent, which they would be if the piston bumped them. You should be good, but absolutely check them yourself. For peace of mind if nothing else.

Just for a little piece of mind I checked it with a mic depth gauge and rolled it over by hand. there was plenty of clearance. Ran the heck out of it for 20 minutes jut like you and several others suggested. Here's the pics but I don't know what to make of it. Any suggestions?

I honestly don't know what to make of it but is interesting they look almost exactly like mine looked except that cylinders 1 and 3 would be switched. With #1 side electrode being lighter than the other 2 it would indicate that plug is running hotter which would seem to indicate leaner condition but with center porcelain being black it would seem to not be lean. Then with #3 porcelain being almost white would indicate lean but the black electrode indicates cooler running. I am so confused. Would be interesting to hear from someone that actually has lots of experience reading plugs.

As a side note, yesterday I pulled the plugs from our motor. They have been in since late last summer and since this is a duck boat it was ran a lot last hunting season. It also has not been in the water for almost a week since my son destroyed prop and we pulled boat out. #1 plug porcelain was wet and looked like it had been covered with a thick carbon but recently started flaking off as there were pieces of it missing and you could see white porcelain underneath in spots. #2 plug was wet and black. #3 was light black sooty and dry. Have no idea what this means but going to try to do another compression check and put some new plugs in it.

I'll be keeping an eye on this thread as well, similar symptoms with a merc blueband 50 4cyl 1980 outboard. only pushing an 18' crestliner deep V at 18-20mph GPS. Currently underpropped to test (and was sent a 10 1/4 x 10 pitch instead of 10 7/8 x 11 pitch that I ordered from ebay seller) I'd say I can give it up to 7/8 throttle and the last bit just gets it surging/bogging sound. I noticed that the first cold throttle-up would get right up to 5400 (mines rated 5500) but as the day got warmer and the more I ran it, i could only get 4700-4800 out of it. idles great all day long. hole shot good. nothing wrong with how it runs other than top end. new stator, switchbox, plugs, wires and other wiring where it needed it, new floats/needles/carb kits/gaskets/primer/line/etc. also new exhaust baffles, side plate, powerhead base/gaskets due to corrosion/taking on water . all good now. adjusted floats. linked and synced. THE EXHAUST DOWNTUBE WAS 75% CLOGGED when I changed the parts with the corroded/broken water passages. Before I discovered all that I couldnt push it past 1/2 throttle. Still not quite right tho. Same symptoms, bottom plug is wettest, top plug is dryest/hottest looking. middle 2 look fuel-washed but seems to be running strong on all 4.
the last thing ive done with it is clean out some fuzzies and dead bugs from the air intake passages, never thought about where the air enters the cowling. don't know if it helped until I take it out next weekend. Boat was last winters project and Ive had it out about 6 times now.
One additional problem with mine is a fuel weep from the carbs, and occasionally if I prime it too much when hooking up the fuel line, it will flood pretty bad and I have to unhook the line, run it, and hook it back up. New needles, and even have the floats set a touch low to try to stop the flooding.

My question is, on the tillitson/mercarbs that I have, is there supposed to be any seal or O-ring on the low speed needles? they were really finicky and touchy when tuning. would almost seem to work opposite of logic.

https://goo.gl/photos/gqzfaHWfKA5YkqMS6

here is a video. you can hear it start surging after running at wot for bit.
especially at 1:06 - is that detonation? prop ventilation? running lean or rich?

My only advice to this OP is to pull your lower unit off, get a good flashlight and shine it up the exhaust tube.. this is how bad mine was when I bought it. would have never known (but had crossed my mind when it bogged past 1/2 throttle)
https://goo.gl/photos/do9wn7bCbgTSzbC38

I don't know much about the carbs on that 50hp since mine uses Walbro WME's. Since I pulled the spark plugs and posted the pics, I've overhauled and pretty much sterilized the carbs, timed and sync'd them. I did find the #3 carb float level was just slightly (+/- 1/8") out of spec. I also replaced the Sierra primer bulb with a Quicksilver bulb. After a test run the regular plugs look consistent between each other but it still won't make 5000 RPM. Since I only have around 100 hours on this motor I didn't think of the exhaust tube being restricted. That back pressure would definitely cause a problem so I'll take a good look at it next time I have it off. With all the dirt daubers down here in Texas I wouldn't put it past them little boogers to have a big nest up in there. Thanks again for the recommendations, I would have never thought of it.

Just looking at your video I do see a couple of things I would recommend. Don't know if it will help but "best practices" might be an easy fix. Move your primer bulb within 12"-18" of your motor and position it so it is vertical (arrow up toward motor). Recommend using OEM Quicksilver bulb since the check valves appear to be slightly larger therefore reducing any restrictions.... Make sure that at WOT and carbs sync'd that all the butterflies are exactly the same and wide open.

You mentioned you had a crab dripping issue, have it been solved or still occurring? If it's still occurring I would check the reed valves.

Hopefully I'll have the carb dripping issue solved next week, last week I took it out and had nothing but fuel pouring out. The floats are both shot, seams leaked and were full of fuel. they are the typical aftermarket plastic floats. I hate these carbs, they have a shallow, wide float bowl, in my opinion terrible for an outboard with power trim.

Ive heard about bad reeds causing the carbs to "spit" fuel. I dont necessarily see them spitting, they're just always wet with fuel. Trying new floats and new needle valves (again... this time not Sierra's).
Checking the reeds would require a complete teardown of the powerhead on these old inlines, they are encased around the damn crank, not behind the carbs. if they are getting bad then ive gotta draw the line. already have $400 tied up in the motor on a $900 boat

I shortened up the fuel line between the primer and motor. based on how fast the thing flooded (I ran and revved the motor several times to clear the flood, with fuel line disconnected, and instantly flooded and died each time i re-connected fuel line while idling) then I'm absolutely sure its getting a good flow from the tank and thru the pumps. The floats were not closing and it instantly flooded each time within seconds

curiosities of mine.. Should I pop the welch plug on the side of the carbs? I really dont wanna spend $70x2 for the rebuild kits with the welch plug. Is there any site that sells any size plug id need if I got the micrometer out?
And the main stem in the middle of the carb- looks like its almost too long and partially blocks the orifice on the main jet that threads into the side of the stem.. if you can follow what I'm saying.. that may give me a lean/surging condition at sustained WOT ??

Why does this site log me out so fast??

this is the stem/jet situation..

I am having the same problems with my 2002 40 hp merc. I haven't checked exhaust. I changed to synthetic oil and it all started b4 i used a full gallon of oil.So i'm wondering if engine is getting to much oil or .Maybe reason for dark spark plugs after very little use.maybe high rpm is to much oil to fire correctly.I'm going to drain synthetic oil and go back to regular 2 cycle.Nothing i do or replace seems to fix problem..PLEASE let me know if u guys find fix...

Update on on my 2002 40 hp 2 stroke merc missing and losing power from 3/4 to wot. Tonight i took it out and ran it till it was acting up.ran good up to 4000 RPM than just started missing and wouldnt open up. I disconnected ground wire that attaches to plate that regulator and rev limiter are mounted on and engine ran great and did not miss anymore. was a black wire attached to top part of plate. So now i have it narrowed down and a least have something to work with.

Update. I took off bracket tonight and ground wire i disconnected was the ground going to rev limiter. So i ordered new rev limiter.the engine ran great as soon as this wire was removed from ground

I put new rev limiter on my 02 40 hp merc. I took it out ran full throttle , trolled for a while and ran full throttle again and it ran great...No more missing and losing power from 3/4 to full throttle.

Glad you persevered and found the problem,happy and safe boating this year.

I had this same problem with my 2 stroke 40 Yamaha. It acted the exact same way and it took me a while to figure out the problem. There were two things I did, one of which fixed it. I took coils off and cleaned them to make sure s good connection was being achieved. Secondly I adjusted the timing on my motor. Very little was done to the timing, but one of those the things got my motor running correctly at full rpms