mercury outboard help

[Marinescout]

I have a 204. Suntracker 20 ft pontoon with a 75 mercury same year two stroke. I cannot get over 4100 rpms or 14 mph per gps. With just me. I use to run 5200 rpms and 18 mph. Still too slow in my opinion. My motor will go over 5200 rpm as I accidentally reved it whil in neutral the other day. It also seems to want to go better in reverse than forward when backing off islands and such. I have had it in the mercury mechanics at my marina three times and they say it's fine but cannot get it over 4200. It runs great just not over 4200 at wot. It has had carbs rebuilt and engine gone through by two different marinas with everything being changed but the prop and hub and I mean everything including bypassing fuel filter and using auxiliary tank to the cost of over 2000.00 worth of repairs I can increase rpms to 4400 if I tilt it up but then it ventilated and revs real high. I'm frustrated with it and I'm almost about to replace with my 1985 115 mercury with 150 jets so I can use this thing. Please help is it just my prop? Because this boat use to not do this but I have been running same stainless 3 blade prop for three years. I think it's a 17 p but will have to check on that one

 

[Marinescout]

2004 model.

 

[racerone]

Mpst motor will rev over 5000 RPM in nuetral that means nothing to me !----------Have you checked for water in pontoons ??----Has the motor been run with a test prop ?

 

[Marinescout]

Water was in toons but has been let out and cracked weld fixed no water now. And no have not tried a test prop. I wouldn't know where to find a test prop around my area. Marinas here are not friendly with releasing props out. What do y'all think would be a good pitch for cruising.

 

[Yegboats]

If you used to get good performance with the current prop you have I would double check the pitch and use that as a reference. Did these " marinas" do a lake test or was it in test tank?

 

[Marinescout]

They said lake test I keep it in wet slip year round and they take it out for cleaning and took it out to do the maintained and put it back in. I can only go by what they say as the boat is in the slip before and after they work on it and I never see it at there shop. They said 4200 was good and it ran fine with them. But I want it to be at 5200 and faster as it will barely pull my kids on a tube. And the gas miles is sucking now.

 

[Marinescout]

They seem to be getting tired of my complaining and look at me like it's me but I've been running boats a long time and I know when it's not right. My marine corp attitude came out last month when I got a 482.00 bill and still had the same problem. And they told me to take it somewhere else. I am taking it out the water tomorrow as the winter just hit here in sc and will take it to another marina the third one so far but this guy is in my Masonic lodge so hopefully he will be squared away.

 

[ondarvr]

Several things can result in reduced RPM and speed and still allow the motor to appear to run well, and if they aren't doing enough to satisfy you the work may need to be done by you.

Did they give you a list of things they did, and a report showing you the compression numbers?
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A 115 with 150 jets would be a terrible running motor (if the jets are different), the jets do not determine HP, other differences in the motor require the jets to be different.

 

[Chris1956]

Gee, take a compression test and a spark test. If you have a bad cylinder, RPM will be reduced like you are seeing.

 

[racerone]

Step # 1 is to determine whether motor is putting out the proper power.----That is done with a test prop, not the same as trying another prop.---Or a more expensive tool called a dynomometer.----If motor is putting out proper power then there is something wrong with the boat !-------Start at home with a compression test !----Note a cylinder sleeve that has rotated will still show good compression but motor will not perform !

 

[Marinescout]

Thanks I'm getting it out of the water today and will check compression on it. I did get a long list of repairs and checks they did on the invoice but I don't see a compression test on here but all three coil packs appear to have been replaced.

 

[Marinescout]

What should the compression be on my engine and how do you check for the cylinder sleeve rotation? Also the 115 was rebuilt before I received it on my bass boat but I know it pushes my 16 ft bass boat crazy fast clocked at 62 on gps with two people. The guy I bought it from said it had 150 jets so that's what I was going with all I know is it runs great. Where my 75 isn't getting it done just yet.

 

[ondarvr]

Somehow people think changing jets does something, and, well, it will, it will make it run worse. It's one of those difficult to get rid of misunderstandings.

The compression numbers aren't as important as how close the cylinders are to each other, gauges can read differently, so unless you know your gauge it’s hard to give a number.

Make sure the butterflies in the carbs are opening all the way, and check to see if the timing is advancing all the way to the stop.

 

[Marinescout]

It appears the butterfly's are not completely open. Have not done a compression check yet but will be in the morning. Is there away to adjust the linkage to get them to open all the way? What is the link and sync I hear about and how do I do that. I am going to order a service manual but haven't yet. Any help on that would be great.

 

[ondarvr]

Make sure the cable has enough travel to pull them all the way open, sometimes it gets a little sloppy with use or fiddling.

 

[Marinescout]

Butterfly's and linkage are good they open all the way up now. Having to spray starting fluid in carb to get it cranked initially but after that it does it on it's own until it sits for hours then have to put sf in it to crank. Bulb gets hard and stays hard

 

[rjcamel2355]

Compression test: If all are within 15% or so, then you're probably okay (* as long as it's not a super low reading IE less than 90#'s or so)
Spark test: Make sure each plug is firing, and firing well.

If it used to run at 5200 RPMS, and the RPMS have dropped I am betting you have a weak cylinder, or weak spark. Unless you've hit something with your prop and it is beat up.

+1 on the "150 jets in a 115" That'll drown that motor and make it run like crap.

 

[Marinescout]

Ok so I found a lot out today. I had my son crank the engine over and still no start. He did it again and I pushed down on the enricher button on top of it. It was a little black button and I seen fuel pull from the top hose which comes from fuel pump area to the bottom hose which goes to the carb and it fired right up. So I'm assuming the silenoid is bad. Next I found when he pushed the throttle all the way forward the small arm that pulls via a metal rod and pushes down on the middle carb opening all three carbs up was slipping in the plastic bushing and it was not allowing the butterfly's to consitantly stay all the way open. It even popes out a few times so the little 2.00 bushing could be playing a roll so it will be replaced. I did a compression check with a cheap hf gauge that was about 5 bucks and it read about 80 on all three cylinders. The compression was exactly the same on all three. I'm going to get my brother in laws blue point gauge and check again. Does that seem low to anyone? I'm going to check it again before I spend any money on it at all. Also I tilted the engine up and has came out the bottom carb but was only what was left in the bowl because after a few seconds I pumped the bulb very hard and none came out but I ran it a few min then did it again and the same thing happens. Some fuel ran out then it stopped. It only does it when motor is tilted all the way up. And finally I noticed the thin metal ring on my prop but was loose and the big nut seemed to be also. My prop has about 1/8 inch play in all directions which I didn't think was good. So what do y'all think about all that

 

[Chris1956]

Compression gauges are often inaccurate, but usually reliable. That means it is likely your compression is even, which is most important. A different compression gauge will likely show different compression numbers, and I will bet that an accurate compression gauge will show all 3 are 90+ PSI.

 

[Chris1956]

Also, prop nut should be tight. Prop should not have any in-out play on propshaft. Carbs may dump fuel when motor is tilted. This is normal.