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1995 7.4 L compression readings

C

canada randy

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I have a (purchased 2010) 1995 Maxum 2700scr 7.4 L 454 carb Bravo 2 . Did compression test all 102-108 psi warm/dry. Leakdown test 6-8 % on all. Oem manual says 150psi . Later bulletin from merc says 100 psi minimum ok. Thought compression seemed low? What are your thoughts? Installed new plugs ,wires, cap, rotor, fluids & filters, cleaned & new bottom paint. Never looked at carb yet. Weighed boat 7700 lbs + 110 gal fuel + 40 gal water + me & beer = 9000+lbs + passengers. Full throttle can only get 3950 rpm @ 33 mph. Cruise 3800 rpm @ around 30 mph 7/8 throttle. Factory recomended 21" aluminum prop. I can throttle back a little if not too many people and still stay on plane.A litte hard to get on plane with 6 passengers. Not sure I like having to use and run so close to full throttle. Is motor weak? Carb tune? Time to rebuild? Repower? Would like more power. Friend says don't fix it if it ain't broke & just enjoy what you got.What do you think of compression? Thank you
 
Are you sure it is a Bravo II.......?

There are three basic bravo's, I II III

I is good for most aplications up 500hp or so andused on many single motor bigblock boats

II is a very large bravo outdrive, maybe 20% larger than a Bravo I and uses a very large diameter prop. It is designed for larger boats with slightly under powered engines (meaning less costly so less motor in a larger boat)

III is a dual prop design

WHat I will say is a 27 ft Formula with a 454 would typically go 60 mph in my opinion.

The SCR is that a large cuddy cabin style boat?

Finally I would say that your motor has some hours of use, compression under 110 psi for any cyclinder on almost any engine is in my opinion worn out.

Maybe that is why there is no hour meter on the boat, Maybe it was removed (if it was ever there to begin with) to hide the total hours.

If you like the boat use it. If you want it back to original condition, purchase a rebiult long block.

Also make sure your getting full timing advance per manual.

I am sure you should be at wide open throttle closer to 4400-4800 rpms.

of course the issue of low rpms is of concern and it may just be the way it is propped. If it is a Bravo II the prop diameter should be close to 18 inches.(not sure just guessing from memory but i know they are very large) but it is signifiacntly larger than a standard bravo I or Alpha I prop they are ~ 14 or so inches in diameter.

If it is a bravo II and bravo II 21 pitch prop then a 17 or even a 15 would get you up to the higher rpms and would get you up on plane quicker. Speed is what it is due to boat design and wieght and other factors.

That is my opinion and questions.
 
Bravo 2= 2:1 gear ratio. 17 3/4 " diameter x 21 " pitch quicksilver prop. Boat came new with said drive & prop. 2700 scr is (29' o/a w/platform & pulpit) express cruiser (v berth & mid berth) 7.4 rated @ 300 hp @ prop I believe. Thanks
 
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I don't know about the engine and drive components but as far as getting on plane is concerned you could try a nice hydrofoil. Really helped my boat get on plane easier and faster, plus added a little bit on the top end.
 
Not the reccomended answer here. that is a nonreccommended fix for underpowered boats. it causes serious steering issues at high speeds. I am not even sure you could install one on a Bravo II...
 
just a thought... I was having a problem similar to him and it work. I put them on all of my boats, and man what a difference they make. From getting on plane in 10 seconds to getting there in 4.
 
Yes they do work and they work better on smaller boats, under 24 ft typically 19-22 ft.

They are dangerous because they can and do cause extremely hard sterring at high speed especially with boats that do not have power steering.

They take the place of a properly powered and propped boat.

Many boat manufactures in keeping pricing down very low and affordable choose to, as an example: put a 4 cylinder like a 470 in a 21 ft small cudy.

even on a good day this combo will have difficulty getting up on plane....

to avoid this they developed proppellers that shift. Change pitch based on rpms/spring combinations.

they start off at say a 13 pitch and when say 3800 rpms are reached they mechanically shift to a 19 pitch which is what the boat should be running..

either way I do not believe it is a good choice on a 27 foot "barge" of 9000 lbs.............a boat of that size should be running TWO big blocks and would probably use 1/2 the fuel for the same speed......again price vs function.......Most likely this boat was not designed for high speed and just for cruising so it was an acceptable choice when biult.

I stick to changing the pitch to a 17 pitch to get the better out of the whole performance and get the max rpms closer to 4,400.

but the reality is the compression is very low and with an added 40 to 50 psi this may be the ultimate true fix.

~40psi X 8 cylinders = 320 psi of additional compression.........more power/torque per cylinder per gallon of fuel used...........

the more power the less effort to gain speed and get on plane........
 
I've heard about handling issues with hydrofoils. I've considered it. The power option in this boat would have been twin V6 4.3 L. I actually have quicksilver torque-shift 2 speed prop for my 1988 peterborough 19 foot speed boat w/ 5.7 L 260 hp alpha one. Nothing beats more hp/torque. I wish I could hook a dyno up to my prop shaft to see what i'm putting now.
 
17p prop. would bring you up to recommended wot if there are no other problems besides the low compression number.

However.... First look at this list, see if anything else fits.

List of possible causes of low WOT

List of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

Above copied nd pasted from another website Don s is the author.
 
kghost said:
Yes they do work and they work better on smaller boats, under 24 ft typically 19-22 ft.

They are dangerous because they can and do cause extremely hard sterring at high speed especially with boats that do not have power steering.

They take the place of a properly powered and propped boat.

Many boat manufactures in keeping pricing down very low and affordable choose to, as an example: put a 4 cylinder like a 470 in a 21 ft small cudy.

even on a good day this combo will have difficulty getting up on plane....

to avoid this they developed proppellers that shift. Change pitch based on rpms/spring combinations.

they start off at say a 13 pitch and when say 3800 rpms are reached they mechanically shift to a 19 pitch which is what the boat should be running..

either way I do not believe it is a good choice on a 27 foot "barge" of 9000 lbs.............a boat of that size should be running TWO big blocks and would probably use 1/2 the fuel for the same speed......again price vs function.......Most likely this boat was not designed for high speed and just for cruising so it was an acceptable choice when biult.

I stick to changing the pitch to a 17 pitch to get the better out of the whole performance and get the max rpms closer to 4,400.

but the reality is the compression is very low and with an added 40 to 50 psi this may be the ultimate true fix.

~40psi X 8 cylinders = 320 psi of additional compression.........more power/torque per cylinder per gallon of fuel used...........

the more power the less effort to gain speed and get on plane........
Click to expand...

What?! Shift pitch as they pickup speed!! That is too cool! Thanks, for the heads up. I guess if I go any over 20' i'll try it w/o one, seems to work very well on my 19' deep v @ 2000lb.
 
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