7.4l 454 biii wot????

[dpvandy01]

I tried the search function with no luck and I'm trying to avoid a trip across town for my manuals. Does anyone have the WOT info for this set up handy? Reason for the inquiry is the last ride of the season had me about 1000 rpm higher than "normal". Normally cruised at 3600 rpm giving me about 28 mph... with a clean bottom at the end of the season I was at 4600 rpm for 28 mph. The engine sounded the same as normal, not over revving just the tach seemed off. I had Sea Ray check it during winterization with no definitive diagnosis. Any thoughts?

 

[No Name Sea Ray]

For the 1997 7.4L 300 & 330HP WOT is 4600.

Some of this info is on SeaRay.com

 

[320Bob]

Did you measure your speed with GPS or your speedometer? My old Mariah had the same engine, drive and length as your boat and at 3,600 RPM the speed was around 28 using GPS. The speedometer didn't work probably due to the sensor on the drive being clogged. Also when is the last time your had your engine alignment checked. That engine with a Bravo III drive is prone to coupler wear. I know first hand about this because I had to pay CSR $1,500 to replace it when I traded in my Mariah.

 

[Lazy Daze]

Do you know if they checked the selector switch on the back of the tach? It can be set for 4cyl, 6cyl or 8cyl. "Exercising" the switch back and forth usually does the trick.

 

[Loneranger]

I am not sure how much work you do on your own boat but if you do a decent amount the mercruiser scan tool is a great investment and will give you an accurate RPM reading

http://www.rinda.com/marine/marine.htm

I have the same set up by the way and have had no problems with coupler. Fingers crossed....

John

 

[Woody]

Did you measure your speed with GPS or your speedometer? My old Mariah had the same engine, drive and length as your boat and at 3,600 RPM the speed was around 28 using GPS. The speedometer didn't work probably due to the sensor on the drive being clogged. Also when is the last time your had your engine alignment checked. That engine with a Bravo III drive is prone to coupler wear. I know first hand about this because I had to pay CSR $1,500 to replace it when I traded in my Mariah.

Why would the Bravo 3 drive be "prone" to coupler wear?

 

[320Bob]

Why would the Bravo 3 drive be "prone" to coupler wear?

I got this from Chicago Sea Ray who told me they have replaced between 30-50 couplers this year including the one on my old Mariah. First the 7.4L is the old Chevy big block with a lot of torque which puts more stress on the coupler and I believe makes it more prone to go out of alignment. Second factory coupler is made out of aluminum while the Bravo III shaft into the coupler is steel. Steel is harder than aluminum.

They pulled the BIII drive to expose the coupler and showed me the alignment tool from Mercury they use to check engine alignment. It should slide in with no binding but on my boat it took a lot of effort to shove it in. When removed, I could see scoring in the grease on one side indicating the engine was not aligned. Then they shined a flashlight in the coupler opening and I could see that the splines had been worn to a point instead of being flat indicating wear caused by misalignment. Since the coupler is bolted to the engine fly wheel, the whole engine has to be pulled to replace it which is why it cost me $1,500. I bought the boat used from Spring Brook Marina with 323 hours on it. I put another 16 on it which was not enough to wear the coupler so the engine was out of alignment long before I got the boat. The Mercury manual for that engine and drive recommends the engine alignment be checked every 100 hours which obviously Spring Brook did not do before selling me the boat which was theirs not a brokerage boat.:smt013

 

[dpvandy01]

I got this from Chicago Sea Ray who told me they have replaced between 30-50 couplers this year including the one on my old Mariah. First the 7.4L is the old Chevy big block with a lot of torque which puts more stress on the coupler and I believe makes it more prone to go out of alignment. Second factory coupler is made out of aluminum while the Bravo III shaft into the coupler is steel. Steel is harder than aluminum.

They pulled the BIII drive to expose the coupler and showed me the alignment tool from Mercury they use to check engine alignment. It should slide in with no binding but on my boat it took a lot of effort to shove it in. When removed, I could see scoring in the grease on one side indicating the engine was not aligned. Then they shined a flashlight in the coupler opening and I could see that the splines had been worn to a point instead of being flat indicating wear caused by misalignment. Since the coupler is bolted to the engine fly wheel, the whole engine has to be pulled to replace it which is why it cost me $1,500. I bought the boat used from Spring Brook Marina with 323 hours on it. I put another 16 on it which was not enough to wear the coupler so the engine was out of alignment long before I got the boat. The Mercury manual for that engine and drive recommends the engine alignment be checked every 100 hours which obviously Spring Brook did not do before selling me the boat which was theirs not a brokerage boat.:smt013

Good information but would that cause my boat to indicate higher RPM's with reduced speed? As i said the "harmonics" sounded to be normal for the indicated speed, but the tachometer was indicating 1000 RPM's more than speed and sound would have suggested. As for previous questions, the speed is dash speedo read, not GPS BUT...... despite my inherent distrust for the pito-pushed speedometer I am leaning towards trusting it in this rare instance

 

[320Bob]

dpvandy01,
I am pretty sure the coupler was not causing your tachometer problem but I thought you might still have it in the shop so an alignment check could be done if you haven't had one in the last 100 hours.

 

[Loneranger]

Yeah I guess we missed giving you an answer assuming the tach is correct. If bottom is clean then it would have to be slip of the propellar or a significant change in weight. I am at a loss to explain it mechanically but I'm not that good at some mechanicals.

John

 

[dpvandy01]

Bottom clean.. check!, prop slip? not sure that's an issue with a BIII I suppose I'll vex over it for the next 176 days till the spring launch and perhaps take Sea Ray up on the in-water test... we'll see but keep the theories coming. I'll never scoff at advice and supposition.

 

[Carpedmman]

I tried the search function with no luck and I'm trying to avoid a trip across town for my manuals. Does anyone have the WOT info for this set up handy? Reason for the inquiry is the last ride of the season had me about 1000 rpm higher than "normal". Normally cruised at 3600 rpm giving me about 28 mph... with a clean bottom at the end of the season I was at 4600 rpm for 28 mph. The engine sounded the same as normal, not over revving just the tach seemed off. I had Sea Ray check it during winterization with no definitive diagnosis. Any thoughts?

dpvandy- I have twin 454's (carburated) and when cruising at a comfortable speed of 32 mph my engines are turning 3100rpms. When I speed up to 3400rpms there is an obvious change in sound and speed is around 37-38mph. If I push the throttles to the hilt and bring those wonderful 454's to screaming WOT of 4600rpm there is and OBVIOUS change in sound and speed increases to 48mph!

Yes, I do agree with the power output coupler as I had to replace that once on my port engine as I wacked mine out of alignment and didn't realize it until I heard the grinding noise....Check your alignment folks after ever hitting a submerged object. FYI.
My guess is that your problem may be coming from the speed input sensor on the hull. If you have GPS and can verify the speed, I would do it this way come Spring. Good Luck!

 

[beerman]

I never trust my tachs. I have the twin 5.0 in mine and they never read the same. I go off the speed by GPS and some days my rpms have been 1000rpms different between the 2 motors. You have run your 280 enough that I would think you could tell by throttle position and engine sound if the rpms were close. Just my .02.

 

[Lazy Daze]

Do you know if they checked the selector switch on the back of the tach? It can be set for 4cyl, 6cyl or 8cyl. "Exercising" the switch back and forth usually does the trick.

???

 

[beerman]

Was that for me Dennis?

 

[Lazy Daze]

Was that for me Dennis?

No, not at all. We were just posting at the same time. In fact my original thought was similar to what I think your line of thinking is. I was just curious if the OP had had a chance to ask his dealer about the selector switch. He stated the engine sounded like normal - meaning it sounded like 3600 even though the tach showed 4600.

Seems like it's worthwhile to start with the simple stuff.

 

[dpvandy01]

I never trust my tachs. I have the twin 5.0 in mine and they never read the same. I go off the speed by GPS and some days my rpms have been 1000rpms different between the 2 motors. You have run your 280 enough that I would think you could tell by throttle position and engine sound if the rpms were close. Just my .02.

You are correct, as I said, the harmonics and corresponding speed are consistent with the past 2 years of boating with this boat but I was just curious if there may be something else afoot. And speed + sound still get second guessed when I look at the Tach and see it at 5600 rpm! even though all of my senses and experience tell me it isn't so I still can't help but pucker just a bit! That's all

 

[dpvandy01]

Do you know if they checked the selector switch on the back of the tach? It can be set for 4cyl, 6cyl or 8cyl. "Exercising" the switch back and forth usually does the trick.

I haven't spoken to them yet about this, I can get at it on my next visit though if they have not... I HATE HAVING MY BOAT SO FAR FROM HOME!!!!!!!!! this is a first for me since it is usulally around the corner from my home (5 miles or so).. this year however it's about 35 miles away:smt021

 

[BonBini]

As far as tach being off: I don't know what ignition system you have but my old 230 had the thunderbolt. The coil wire would sometimes leak stray voltage over to the neg coil terminal and cause my tach to feather up some. I globed some grease there to confirm my findings. Never got around to replacing the coil and wire before trading up.

You problem doesn’t sound like any thing too serious to me, probably just some instrumentation. To me something like this can be very annoying and I would be getting it straightened out too.

BTW: being that new of a boat and probably fuel injected, does your engine have a rev limiter? ...Ron

 

[mjfrank]

Sorry to drag up an old post, just curious if a cause was ever found.

I just had a similar experience this weekend with our new-to-us '98 280BR with a 7.4l BIII. Cruising along, turning about 3100 rpms, trying to dodge 2x4s and small logs floating in the lake, I look down and suddenly the tach is reading 4100 with no speed change. Throttle back to 3100 and the speed dropped, so I used the Fonzi method, and banged on the face of the tach, which instantly dropped 1000 rpms to a more appropriate reading. Of course, while I'm expertly troubleshooting my tach problem :grin:, I missed seeing one of those logs in the water, which hit the lower unit and clogged up the speedo tube. So now I have a working tach and a non-functional speedo. :smt013 Oh well, I guess I should feel lucky nothing more serious happened from the impact.

I'll probably try the moving the switch method the next time I'm on it, to hopefully prevent it in the future.

--Mike