Help please, Merc 260's 2bbl vs 4 bbl

nass-t-habit

New Member
Aug 30, 2010
21
Lake Michigan
Boat Info
87 270 Sportfish
Rigged for scuba diving
Engines
twin Merc 260's
I need serious help, from somebody who knows Merc 260/5.7 carb engines inside and out, or at least has knowledge and experience working with them. Knowledge of these as twins, in the Amberjack configuration would be even better.

I've got a 87 270 Sportfish (Amberjack flybridge) with twin "260's" in it. I picked it up like 10th or 12 hand, both motors have obviously been replaced at least once.

The starboard motor is a 4bbl carb with v-belts. The port motor is what appears to be newer, 2bbl carb with the serpentine belt.

The starboard motor was running with perceivably far less power. The throttle had to be much, much further ahead to match the RPM's. It also built RMP's much slower than the 2bbl port motor. I found it had one cylinder with absolutely no compression, so I replaced it with a known good motor of the exact same configuration. But I still have exactly the same issue. It has far less power. And after installing Flowscans Saturday, I found out the 4bbl motor is burning TWICE as much fuel! But still providing less performance.

The 2bbl motor is burning 9 GPH, the 4bbl motor is burning 16! That's at 3400 RPM, with a heavy load, tabs down, drives tucked, in 2-3 foot seas, 22 mph on the GPS

The 4bbl motor has good and equal compression and leakdown on all cyliders. New plugs gapped properly. All 8 plugs fire equally (visually) when pulled out. If you look down the carb with the hammer down, you can see all 4 jets working, equally side to side, more from the back 2. It's not a throttle cable problem, I get full linkage motion with the full stroke of the throttle lever. The drives are matched. the props are indentical. It burns no oil. No leaks, no has gas fumes anywhere, no sheen on the water. It starts instantly, idles perfectly and runs great - unless you compare it the other motor. It's timed dead nuts on, the timing advance works within spec. New fuel in both tanks, new filters, problem is persistent no matter which motor(s) pull from which tank.

The tachs are on, the engine synch works. When the synch centers and the tachs are equal (one set anyway) you can feel that it's right. The ride smooths out, the vibration disappears, and the boat picks up speed slightly. I even did that "blind", when it felt right, the synch was centered.

WTF is going on here?

The ONLY thing re-used from the old motor is the distributor (new cap and rotor though), and the y-pipe. I replaced one flapper in the y-pipe, the other one is still rattling.

I will take any and all input I can get on this issue. However, what I'd really like to know is:
1. Can these 2 motors really be that different in performance?
2. Can the 4bbl really burn nearly twice as much fuel as the 4bbl.
3. Does it sound like there's something fundamentaly wrong with the 4bbl motor? (it does to me)
4. Any clues as to what the problem might be?
5. What SHOULD this boat be consuming for fuel? Assuming properly running 260 Mercs with Alpha drives, swinging 17x13.5 AL props. ( I'm guessing on the diameter, the pitch is right)
6. IF that 2bbl serpentine belt motor really is that much better of a motor, anyone have any clue as to what it might be as far as year/model? All serial numbers are gone.

I have no access to anyone local who knows what they're talking about. Any help will be greatly appreciated.

Thanks for taking the time to read this insanely long post, let alone respond.

And while I may be an idiot in some regards, I was Merc hack for 2 years about 20 years ago. I've owned 4 Searays since 1986, all with V8's that I've run long and hard, and never had enough money to hire anyone else to fix anything that broke. So I do have some reasonable hands-on experience and knowledge of the older ones.
 
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I think the first thing I would look at would be to see if the gear box ratios are the same. The 2 bbl motor is only 220HP and less torque and would have a lower ratio box. The legs could have been swapped out at some time. Did the 2bbl motor go in with its matching gear box ???

It sounds like the 4bbl motor is carrying most of the load and is why the fuel flow is up.

Solving this one could be interesting !
 
It sounds like the torque bands are different in the motors. This is accomplished by a few thing including but not limited to cam shaft duration, timing, carb setting, head porting size, and a few others. If they stuck an auto motor along side a marine motor you will see a difference. The blocks may be the same but the internals are different.

If you are using more fuel with less power than the 2BBl motor than this may be your problem. Your not making power on the same curve as the 2bbl motor. I have seen where a minor adjustment in carburation can make a huge difference in power band

BTW. If you are interested in selling your 4BBL and manifold off the old engine please let me know.
 
Chris, thanks for the input. I have no knowledge of what was swapped when. I did not put the drive on when I swapped the motor, I re-used the drive that was on there. At first I thought the drives deing geared differently was the issue. But then I found the dead cylinder and switched my attention to the motor. I have not pulled the drives apart and counted the teeth, or checked part numbers. But the boat feeling right with the RPM's matched should indicate indentical gearing yes?

Maybe I'll put it on the trailer and swap the drives one for the other this weekend. They're both RH rotation, as long as the port one isn't stuck on there, it should be a relatively quick thing.

Is there any other way to non-intrusively check the ratio? Everything else I read says to do it right, you've got the pull the drives apart. That's the ONLY thing I don't have the proper tools or patience to do.
 
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You've verified that the electrical timing is correct, how about the mechanical timing? Are the valves correctly timed to the position of the piston? Could the timing chain be one tooth off from being stretched or improperly installed? Could the problem be with the old distributor? Can you swap the distributors between motors and see if the problem stays or moves?

The 2-bbl motor with a serpentine belt could be rated at 250hp; most were that I have seen except for the earliest years (96 (maybe) and 97). 98 and up were rated at 250hp. It actually boils down to the heads. The Vortec heads with different ports and combustion chambers are more efficient than the older non-Vortec motors.

If you have a second person to help, the gear ratio can be checked by having one person watch the propellor for one full turn while the other turns the crankshaft with a socket & breaker bar and counts the crankshaft revolutions. That'll give you your ratio.

Larry
 
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Thanks Larry. No mechanicals on the electrical, Thunerbolt IV ignition. I have not swapped distribiutors - yet. Valves appear to be on, based on compression and leakdown readings, timing, etc. I've had t chain problems in the past, getting an idle is almost impossible.


Thanks for the confirmation on the newer motor. With the reseach I've done, that's what it appeared, but most people in the forums seem to claim they never made anything but the 230 with a 2bbl. Even though the Merc books from way back list 260s with 2bbls. I have not checked to see if it's got Vortex heads, I just found out I could look into that and have not been back to the boat since.

I tried checking the drives that way, but busted 2 perfectly good sockets on the crank, one Snap-on and one Craftsman. Too lazy to pull all 16 plugs first. Keep hoping there's an easier answer.
 
I agree that the boat will "feel" right when the engine RPM's are the same, but the 4bbl will be carrying more load, thus the higher fuel flow.

I wouldn't be messing around with the motors just yet, from what you say, they seem to be OK.

To find the gear box ratios, they are indicated by a letter stamped on the decal on the upper housing, port side. These decals usually go missing, but the letter is also stamped on the yoke of the universal joint that enters the upper housing.

The letters are:-

B = 1.98:1
C = 1.65:1
D = 1.84:1
F = 1.50:1
M = 1.50:1 Magnum

All I would do with the motors, would be to replace the 2bbl with a 4 bbl, later, after the gear boxes are sorted.

BTW. The letters aren't a sure thing, at times gear sets get changed and no one bothers to change the letter on the yoke. While the drives are off the boat, it is easy enough to turn them by hand to check the ratio.
 
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Could the 2 bbl engine be a Vortek and the 4 bbl engine be an older head configuration? Is it possible that the 2 bbl is producing more HP than the 4 bbl because of this? I'm thinking that the older 4 bbl is a 260 HP while the 2 bbl could be a 300 HP+/- Vortek.
All this is just me guessing but you may want to check it out.
 
Thanks Scorpio,

It could be, I need to get back down to the boat and check that out. Guess I didn't figure I could be lucky enough to get something BETTER than what I thought. And I never realized there was that much of a difference. Do you know what your 5.7's are burning in GPH?
 
Is the starboard motor a 5.0L with the 260 flame arrestor cover? Just a thought.
 
Alibuma, very little chance of that. Both the replacement and old motors acting the same? This particular motor that's giving me grief does have the serial numbers on it. I will get that checked out, just haven't gotten around to it. Oddly enough, I was afraid the 2bbl motor was going to turn out to be a 5L, not the other way around.
 
OK, so I found out that the 2bbl port side motor is indeed a vortec engine. That could explain the perceived power difference. Now I want to figure out if it could explain the double fuel consumption on the 4 bbl non-vortec motor.
 
I would think that the Vortek heads flow more efficeintly than the old heads and allow for a more efficient production of power for a given amount of fuel. The 4bbl Q-jet is a monster, if you open the secondaries, they are at least twice the size of the primaries. So a less efficeint head combined with a giant carb, IMO will account for the high fuel burn.
 
Assuming for the moment that both motors are in good tune.

Then the motor that is burning more fuel must be doing more work. Simple physics.

If the motors are doing the same RPM's and one is doing more work and thus burning more fuel there must be a miss match between the motors and the props. As the props are the same, the difference must be in the gear boxes.

Imagine an identical setup with two motors and 2 identical gear boxes but you change one prop, one being finer than the other. The coarse prop will carry more load at the same engine RPM and that motor will burn more fuel..

Nothing said above has changed my mind that there is a gear box miss match.

But then, I have been wrong before. :smt101
 
Chirs, Scorpio,

I agree with you both, that's what's driving me friggin' crazy!

The quadrabog is a pig, no doubt, but 2x !? I don't think so.

I need to swap the drives. I do have a spare drive in the garage, there's no visable stampings on the yokes.

Unfortunately I was tied up all weekend, and president what's-his-name was speaking in Milwaukee today so the harbor/river was shut down.

I hate tear into too much during the week. I run scuba diving charters with the boat part time, and whenever I've got one coming up within a few days, I hesitate to start anything. $500 evenings are hard to turn down. And as long as it gets there and back, the customers are none the wiser. It'd sure be nice to save $100 on the fuel each trip though.

Scorpio, do you know your exact fuel consumption numbers by chance, in GPH? I'd love to know just how far out of whack my totals are.

Chris, part of the reason I think you may be right is that when the stb motor is WOT at nearly 4K, that port motor still has a long way to go, and will easily if I start to open it up. But I'm still getting 30mph on the GPS, which is what I've been told by several credible sources is all I should ever get out of it. But that's with 260's too.

Ugh! I'll try to get the drives swapped.

Thanks, I really aprpreciate everyone's input
 
Just a quick note, my 383 burns 50L /hr or 13.2 US gallons / hour at 25 knots.

That is 2L / nm.

There is a 4bbl Weber on my 383. The boat weighs in at just on 4.5 tons loaded.

At WOT she'll burn 90L/hr or 24 US gallons / hour.

I have a fuel management computer on board and it has been calibrated.
 
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Keep in mind that your 4 barrel is still a 2 barrel until the secondaries open. If you are cruising and the secondaries haven't opened, you effectively have 2 barrels on both engines. This will completely take the carb difference out of the equation if the barrel sizes are the same and the jets are the same.
The issue has to be the ratio of the drives, the intake/valves, or both.
 
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When my fuel flow meters were working two years ago, I THINK I was burning something like 11-12 gph per engine at 3200 rpm just short of the secondaries opening. I have new meters installed but haven't had the opportunity to test tem out at speed yet.
I agree that the outdrive ratios may be mismatched, check that out too.
 
Thanks guys. Sounds like maybe my 25 GPH total burn at 3400 rpm with an incredibly heavy load really isn't that unreasonable then. Obviously there's something wrong that needs fixed, which will lead to better efficiency. But going on that "simple physics" philosophy that somebody brought up earlier, sounds like my fuel consumption=rpm x weight x speed x time isn't insanely high. If I get the drive ratios straightened out, theoretically the consumptions should come closer together, and the average will be in line with what you're burning.
 

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