7.4 only turns 3800 rpms

I was asking Larry if he found his issue with the engine back then.
I'm having same rpm issues and have not figured it out so was curious what his issue was.
 
Right. But as I stated, there are many reasons for that "same issue". Which means IF he found the problem and IF he is still around... his reason may not be your reason.

We're happy to help you - but would need more info. It's OK if you don't want the help, too. Just offering.
 
Ya I understand that but I was following his post and he had tried many of things I have tested with similar results so I was curious if he had found anything.

I have a single engine 7.4 mpi in a 1998 sundancer 270. It will only pull 3800-3900 RPM's and max speed around 28 mph. I bought the boat from someone I knew without a inspection for cheap but have been fighting this since. The following things I have replaced or tried so far.

plugs and wires
fuel filter/water seperator
distributor with new module
checked timing
had injectors tested and flow tested
replaced tps sensor
replaced map sensor
checked fuel pressure, was around 70 at idle and 42 at WOT
Dry compression test 140 on a couple cylinders and rest 150 and 155
Dropped prop from 24 to 22 and didn't gain rpm's
I've checked wot on throttle body

The engine pulls to 3/4 throttle and then just stops pulling any harder. The engine runs smooth and never breaks up or anything like that. Have checked for codes with code reader and doesn't show anything.

thoughts?
 
Good - that is helpful.

PSI is funky.

Check for good spark at each plug.

Drive "trim" use?
 
Knot L8te said:
Ya I understand that but I was following his post and he had tried many of things I have tested with similar results so I was curious if he had found anything.

I have a single engine 7.4 mpi in a 1998 sundancer 270. It will only pull 3800-3900 RPM's and max speed around 28 mph. I bought the boat from someone I knew without a inspection for cheap but have been fighting this since. The following things I have replaced or tried so far.

plugs and wires
fuel filter/water seperator
distributor with new module
checked timing
had injectors tested and flow tested
replaced tps sensor
replaced map sensor
checked fuel pressure, was around 70 at idle and 42 at WOT
Dry compression test 140 on a couple cylinders and rest 150 and 155
Dropped prop from 24 to 22 and didn't gain rpm's
I've checked wot on throttle body

The engine pulls to 3/4 throttle and then just stops pulling any harder. The engine runs smooth and never breaks up or anything like that. Have checked for codes with code reader and doesn't show anything.

thoughts?
Click to expand...
Fuel pressure should go up at WOT, not down.
 
I don't think it ever went over 5000 rpms. Should have limiter
 
Knot L8te said:
Ya I understand that but I was following his post and he had tried many of things I have tested with similar results so I was curious if he had found anything.

I have a single engine 7.4 mpi in a 1998 sundancer 270. It will only pull 3800-3900 RPM's and max speed around 28 mph. I bought the boat from someone I knew without a inspection for cheap but have been fighting this since. The following things I have replaced or tried so far.

plugs and wires
fuel filter/water seperator
distributor with new module
checked timing
had injectors tested and flow tested
replaced tps sensor
replaced map sensor
checked fuel pressure, was around 70 at idle and 42 at WOT
Dry compression test 140 on a couple cylinders and rest 150 and 155
Dropped prop from 24 to 22 and didn't gain rpm's
I've checked wot on throttle body

The engine pulls to 3/4 throttle and then just stops pulling any harder. The engine runs smooth and never breaks up or anything like that. Have checked for codes with code reader and doesn't show anything.

thoughts?
Click to expand...

Well....for starters you are trying to move 8,000 pounds of boat with a single 350 HP engine.

Exactly how fast do you think it should go?

As to the rpms....the first thing I would look at is the bottom condition and what prop you have on the drive. It sounds like the engine is giving everything it has. How many hours are on the engine?
 
PlayDate said:
Well....for starters you are trying to move 8,000 pounds of boat with a single 350 HP engine.

Exactly how fast do you think it should go?

As to the rpms....the first thing I would look at is the bottom condition and what prop you have on the drive. It sounds like the engine is giving everything it has. How many hours are on the engine?
Click to expand...
After talking with others with the same boat and engine combo they were seeing upper 30's to low 40 mph. I,m just going off with what they are telling me. This is at first splash and clean bottom. Bravo 3 W/22 pitch prop.

techmitch what set up do you have? Looking at your picture you may have a 270 possibly.​

 
Knot L8te said:
At 3900 I doubt I'm even its power band
Click to expand...
Don't know how you are arriving at this conclusion. Rated at 4200-4600, that is within 10% of balls out.
The power band of your setup is neither that narrow, nor that high.

I have the same setup you do, and as you just surmised, I can get about 38 mph (with stock 22" pitch props).
Historical data in the plotter, including before my purchase, indicates that it has never seen 40. This is a big, heavy boat given its being a single screw. Even with no GPH meter I can hear it sucking fuel at such a rate that I make little effort to run much that way. (Have different boats for hauling ass)

I run around 3200 to maintain that same 28 mph.
Note that is not much above planing speed which can barely be maintained at 25 mph.

If your speed is GPS, it sounds to me like maybe your tach is to be held suspect.
If your speed is not GPS, check it against one, maybe that is suspect.
In any case, 28 mph does NOT jive with 3900 rpms, unless the bottom was REALLY FOULED.

The change in RPM vs speed with the prop change should have been telling. Did you literally see no difference?

For what its worth, my impression thus far is that 24" would be too much prop, at least for mine.
 
techmitch said:
Yup, electric fuel pump with pressure regulator mounted on the MPI fuel rail.
Click to expand...
OK, had to break out my service manual, the regulator senses manifold vacuum and increases pressure based on load. Now it makes sense. Yet another thing learned here. Thanks.
 
Knot L8te said:
After talking with others with the same boat and engine combo they were seeing upper 30's to low 40 mph. I,m just going off with what they are telling me. This is at first splash and clean bottom. Bravo 3 W/22 pitch prop.

techmitch what set up do you have? Looking at your picture you may have a 270 possibly.​

Click to expand...
Twin 4.3L's and Gen II alpha's as listed in my user info. on the left.
 
Big blocks make tourqe. Tourqe will turn whatever pitch props you have . A stock big block is not a high RPM motor unless it's full roller, aluminum heads ,massive camshaft all that stuff a small block will RPM a lot easier and faster than a big block will I'm very surprised to see that that motor ever went to 5000 rpm . We would only take street big blocks up to about 6800 rpm maximum where we could spin a small block 7500 8 grand no problem.
 
You sure your check isn't switched onto V6 or that the tack is working properly if you can get behind the helm and play with the switch move it back and forth a few times I had to do that to my Sea riay tach to get mine accurate that was a tip from scoflaw and it worked great
 
Scott215 said:
Big blocks make tourqe. Tourqe will turn whatever pitch props you have . A stock big block is not a high RPM motor unless it's full roller, aluminum heads ,massive camshaft all that stuff a small block will RPM a lot easier and faster than a big block will I'm very surprised to see that that motor ever went to 5000 rpm . We would only take street big blocks up to about 6800 rpm maximum where we could spin a small block 7500 8 grand no problem.
Click to expand...
That was someone elses post the MPI is rated 4200-4600. Checked tach RPM's with the timing light and they are correct.
 
Yendor said:
Don't know how you are arriving at this conclusion. Rated at 4200-4600, that is within 10% of balls out.
The power band of your setup is neither that narrow, nor that high.

I have the same setup you do, and as you just surmised, I can get about 38 mph (with stock 22" pitch props).
Historical data in the plotter, including before my purchase, indicates that it has never seen 40. This is a big, heavy boat given its being a single screw. Even with no GPH meter I can hear it sucking fuel at such a rate that I make little effort to run much that way. (Have different boats for hauling ass)

I run around 3200 to maintain that same 28 mph.
Note that is not much above planing speed which can barely be maintained at 25 mph.

If your speed is GPS, it sounds to me like maybe your tach is to be held suspect.
If your speed is not GPS, check it against one, maybe that is suspect.
In any case, 28 mph does NOT jive with 3900 rpms, unless the bottom was REALLY FOULED.

The change in RPM vs speed with the prop change should have been telling. Did you literally see no difference?

For what its worth, my impression thus far is that 24" would be too much prop, at least for mine.
Click to expand...
The MPH is off of GPS and RPM has been checked with timing light. I agree the boat has to be around 23+ mph to stay on plain. If I have a 3-4 adults I can get on plane fine but any more then that I have to move most everyone towards the bow. It just seems like a little over a half throttle the boat just stops pulling. To me it felt like timing but after rechecking that and also trying to power tune it while under WOT it is at its max on timing advance. When I dropped two pitch on the prop I didn't see any change in rpms or MPH. Just lost for ideas.
 
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