Cruising RPMs on a 76' 85 Johnson

[smitty_bb]

Any body have any suggestions on cruising RPMs for a '76 85hp Johnson? I run about 5000 at WOT, is it safe to run that for very long? Thanks for any help

 

[HighTrim]

Re: Cruising RPMs on a 76' 85 Johnson

that might be even a tad low, I believe your WOT operating range is 5200 to 5600 if im not mistaken. However this depends on your prop, weight of load, etc... As long as you dont have an overcharging problem or overheating, you should be ok.

 

[Texasmark]

Re: Cruising RPMs on a 76' 85 Johnson

Go to the Boston Whaler boat www site. Take any boat there and look at the performance sheet for that boat. All say the same thing as do most other planing hulls: Best economy is just out of the hole, where the bow comes down and the boat wants to pick up speed. Cut your throttle to maintain that just out of the hole speed (usually around 20 mph) and you're there.

Mark

 

[smitty_bb]

Re: Cruising RPMs on a 76' 85 Johnson

Thanks for the replys

 

[emdsapmgr]

Re: Cruising RPMs on a 76' 85 Johnson

Your engine was rated at 85 hp at 5000. It's normal operating range was between 4500 and 5500. It is normally recommended that you prop the engine to run in the upper end of the recommended range. 5500.

 

[Texasmark]

Re: Cruising RPMs on a 76' 85 Johnson

Gotta jump back in so there's no misunderstanding. Engine mfgrs and others associated with the industry, rate their engines at WOT. That means that you have firewalled the throttle, given the boat time to accelerate to max speed and trimmed for best mph.

The rating is an average of the engines built in a series, and even though the mfgr says it's ok to run +/- 500 rpms, the rated hp is produced at 5000 in your case. Obviously most engines will put out more than that to allow for mfging. tolerances.

Folks on here, that have been with the industry for years, like to prop up, for your normal loaded rig, to the upper rpm limit in 2 and 4 cycle engines. Apparently this has produced the best of what you want out of your rig....including reliability and they have learned that over the years.

Once you have setup your rig to WOT = 5500 in your case, you can run anywhere you want to on the rpm curve, but as I said, the most efficient, as proven in the Whaler curves, is just planing out from a hole shot. The worst place to be is just under planing speed where the wake is huge.....course if you're wakeboarding, that's where the board rider wants it.....but you have to suffer the consequences.

My 2c,

Mark

 

[smitty_bb]

Re: Cruising RPMs on a 76' 85 Johnson

I'm running this 85hp on a 16' aluminum center console with a 13 3/8 x17 prop. I can't get more than about 5200 rpms at wot which puts me at about 36 mph and I've been cruising at 4500 at 30 mph. should I change my prop to get higher wot rpms

 

[Texasmark]

Re: Cruising RPMs on a 76' 85 Johnson

I wouldn't as you aren't that far off. But I do stupid things like look at another prop when I have 4 already. You could increase an inch and pick up a little performance but the hole shot would be slower.

Mark

 

[ezeke]

Re: Cruising RPMs on a 76' 85 Johnson

Try moving the thrust rod out one hole and see if the top RPM changes. 5500 to 5800 RPM would be better for your engine. You should get your highest reading with the anti-ventilation plate parallel to the water and the boat on plane.

 

[smitty_bb]

Re: Cruising RPMs on a 76' 85 Johnson

I have to keep the trim all the way in to keep the bow from hopping. The boat does seem to preform better with it out one hole, but it starts hopping.

 

[ezeke]

Re: Cruising RPMs on a 76' 85 Johnson

You can buy shims that will set the motor out from the transom in a smaller increment than the thrust rod holes if you want to try them. Cook Manufacturing makes them in 2 degree increments; others are available up to 5 degress. Google "transom wedges".

 

[smitty_bb]

Re: Cruising RPMs on a 76' 85 Johnson

Thanks, I'll try to get some. Is the hopping caused by being trimed out to far,or could my prop be the problem, it has alot of cupping?

 

[Texasmark]

Re: Cruising RPMs on a 76' 85 Johnson

I assume you mean porposing when you say bow hopping. The bow rythmethically oscillates up and down.

Best speed under normal circumstances is just below where it starts.

If you have enough hp, you can blast through that region where, for the setup, the boat will stabilize.

Take a boat with trim. You can have the trim out at say 25 mph and punch it the boat will pick up speed and quickly go thru the oscillation area and continue on into "overdrive" if you will, if you have the power to do that. Guesstimate would be a boat capable of 40 or so mph, course the hull and setup have a lot to do with it.

Or you can have the trim in, go to max speed with the trim in and then slowly trim out which usually prevents you from porposing as you pick up speed.

Additionally, if trimmed out, cutting the throttle will subject the boat to porposing as you slow down unless you tuck it in a little as you go to compensate for the slower speed.

Putting your engine out one notch says that you are pretty close to your optimum speed with this setup and I agree that a 2 degree incriment shim could help you to get in just at the start of the porposing......but without trim it's hard to hold that under varying water and load conditions.

My 2c,

Mark

 

[ezeke]

Re: Cruising RPMs on a 76' 85 Johnson

First, I was assuming that the motor does not have power tilt and trim, because most of that model and year came without them.

Secondly, You said "I'm running this 85hp on a 16' aluminum center console with a 13 3/8 x17 prop. I can't get more than about 5200 rpms at wot which puts me at about 36 mph and I've been cruising at 4500 at 30 mph."

Your motor started life with a gear ratio of 13:26. A propeller with 17" pitch at 5200 RPM would have a theoretical maximum speed of +/-41.85 MPH if I am doing the math correctly.

I was thinking that you might get a little more top end with a slight change and it would probably give your top RPM a boost as well. It might be worth the $25.00 a set for the CMC 2 degree wedges.

There are other changes that could be beneficial, but this is fairly inexpensive and takes very little time to set up.

 

[speedcat2000]

Re: Cruising RPMs on a 76' 85 Johnson

I have a 1975 85 Johnson, probably the same motor as the subject of this thread. The boat is a 1974 15' Reinell tri hull. My prop is totally damaged, but still moves the boat. It seems to have plenty of power but not a lot of speed. I think the motor may be larger than what the boat needs, but that doesn't bother me as long as I can get it to operate within recommended ranges.

The prop is majorly dinged up, so I believe I need a replacement prop.

One reason I want to replace the prop is that I'm afraid of doing damage to the motor if it's operating out of the recommended range.

The other reason is that I notice after a certain point in opening up the throttle, towards the high end, there is relatively little improvement. RPMs go up, but the speed does not improve much at all. I imagine this is because the prop is missing chunks on all three blades, and a fairly huge part of one of the blades. I avoid this high end because I'm just chewing up gas and not getting extra speed out of it.

I will get an RPM guage so I can test the RPMs at WOT. Meanwhile, I understand the recommended props for this motor run from 12 3/4 to 14 in diameter, and from 11 to 23 in pitch. In this thread, the 13.5 diameter appears to be recommended, and I am fine with this concept. The current prop is also 13.5" in diameter.

The boat is quite old and I have no need to go real fast, in fact it will bounce like hell if I go that fast, I would not be surprised at all to catch serious air. I slow down for wakes to avoid catching air off them. The hull is in bad enough shape that I don't want to bounce around or catch air that much. Therefore I'm thinking of going with the middle to lower range for pitch, perhaps a 15. I'd like to retain all that power to tow people around, and just because it's cool to get up on a plane so fast, & so reliably.

Thoughts on prop selection here, especially on pitch?

Thanks

 

[ezeke]

Re: Cruising RPMs on a 76' 85 Johnson

The most important factor is that you not lug the engine; that requires that the prop be able to take you to the top half of your full throttle operating range with your average load.

It's not about whether you want to go fast or not, but whether you are overpropped or underpropped. If you go too low in pitch, you can run the engine faster than it is designed to run. If you go too high in pitch, you will lug the engine and accelerate the wear, with the risk of excess carbon, broken rings, etc.

Bent or broken propellers are unbalanced and cause wear on the whole drivetrain, but especially on the propshaft bearings and seals.

 

[speedcat2000]

Re: Cruising RPMs on a 76' 85 Johnson

Thanks ezeke. My motor really shudders at low rpms, but appears much smoother at high rpms. I'm not sure if this is prop related or what.

I decarbed yesterday using just Deep Creep, but the weirdest thing happened. I was able to shut the motor down by spraying into the top carb, but no matter how much I sprayed into the lower unit it would not affect the running of the motor at all. I did this twice on the lake. Waited at least 15 mins each time, then ran the motor for a while at cruising speed to burn off the carbon. There was some good smoke, but not for a very long period. More the second time. I didn't really notice any performance gain. The motor still shudders quite a bit at low rpms.

I have not re-tested the compression or replaced the plugs yet. Prior to this the compression was running between 100 and 110 for each cylinder. I have not done spark tests because the crappy spark tester from Shucks was a POS and broke on me. I might go to NAPA to pick up a real one today if they're open.

Any further advice on getting rid of that shudder?

Thanks, and Happy Fourth!

 

[ezeke]

Re: Cruising RPMs on a 76' 85 Johnson

You should be able to get an idea of what is going on by inspecting your spark plugs - they should look the same.

Spraying SeaFoam Deep Creep into the carburetors should have slowed the engine equally on either carburetor, so it would seem that you may have two cylinders that are not firing. To avoid serious damage to the engine, you should make sure that all cylinders are working properly; check compression, fuel, then spark.

 

[speedcat2000]

Re: Cruising RPMs on a 76' 85 Johnson

Thanks again ezeke. I apologize for hijacking this thread. Please let me know if I should start another thread and migrate this over there.

Did some more testing today. I'm not entirely sure about the state of the plugs, because I was focused on compression and spark test, but I thought that the lower 2 plugs (#3 and #4) were kinda dry, and the upper ones a bit less so. None of them were wet, per se, at least not at the point of gap. But the gap looked especially dry on some of the plugs, I think it might have been 3 & 4. But this is not for sure... I apologize but just don't remember exactly and unfortunately did not keep track of what plug came from where.

Meanwhile, it looks like the decarb improved my compression a bit. Here are the differences:

cyl PreDecarb PostDecarb
1 105 115
2 110 115
3 100 100
4 110 113

Again this points to a possibility that the lower 2 cylinders are not firing, as spraying Deep Creep in the lower carb would not change the running of the motor at all, while spraying in the upper carb was able to kill the motor.

Spark test, with known-good spark plug and setting the spark tester at a 7/16" gap:
1 Good, strong blue spark
2 Has spark, but it's pretty weak
3 Blue/white spark, did not appear as strong as #1
4 No spark. Works weakly with #2 spark plug wire attached.

I have not tested with a smaller spark tester gap.

Perhaps I need a new coil for #4? It looks like there are a few minor cracks in the coil. The plastic on the coils is light green in color (original?). #3 has epoxy on it as if it were repaired in the past, so does one coil on the other side of the motor.

I have no idea how to check the fuel situation, aside from checking that fuel is getting to the filter, which it is. I am pretty sure it is getting to the lower carb unit as I can see fuel leaking here and there if I flood it. But of course this leaking could be coming from the top unit and leaking over the bottom unit. Or, it could mean the bottom unit is not properly consuming the fuel. I get lost quickly when it comes to carbs.

Just to refresh for any new readers here: as of yesterday I can start this motor and run it fine at WOT or cruising speed. The problem I'm seeing is the motor shudders at lower RPMs. FYI the prop has big chunks out of it (and not evenly), so I suppose that could add to the shuddering at low RPM.

Thanks again for direction on this.

 

[speedcat2000]

Re: Cruising RPMs on a 76' 85 Johnson

I looked again and there is no difference in the appearance of the spark plugs. So scratch that. thanks