Throttle movement but no increase in engine rpms

[jststrd1]

5.7 Vortec 4 barrel Rochester alpha one gen 2. 2000 2352 Trophy

I was out last week and noticed that I get to a point with my throttle where I can move it but RPMs do not climb then they will climb normally.It is about 2 inches of movement. It feels like it is between the the time the secondaries should engage. No bogging down. It is just a dead spot. Then it climbs normally except it caps out at 34 to 3500 rpms

I also changed my prop from an 18x14.5 4 blade aluminium to an 18 x 15.25 4 blade stainless. My speed picked up handles so much better but RPMs max at at 34 to 3500. Are they related and would the prop cause me that much of a drop in RPMs?

 

[Scott Danforth]

secondaries on a rochester open by air flow from the engine. there is no mechanical connection to the rochester secondaries.

if you do have a rochester, your boat is 1990 or older or someone put an old rochester on the motor or the motor has been replaced. Rochester quit building carbs in 1989. your 2000 should have a mercarb

post the motor serial number.

topping out at 3500 RPM's means there is an issue.

you are either way over-propped or your motor has an issue. that motor should be spinning about 4600-4800 RPM
if you are over-propped, you will soon kill your motor by lugging it.

start with a compression test. post the numbers. all cylinders should be between 140 and 150 psi

verify the induction system

 

[alldodge]

changed my prop from an 18x14.5 4 blade aluminium to an 18 x 15.25 4 blade stainless. My speed picked up handles so much better but RPMs max at at 34 to 3500. Are they related and would the prop cause me that much of a drop in RPMs?

Blades are thinner so less drag but as Scott mentioned I think your motor has lost some HP

 

[Scott06]

5.7 Vortec 4 barrel Rochester alpha one gen 2. 2000 2352 Trophy

I was out last week and noticed that I get to a point with my throttle where I can move it but RPMs do not climb then they will climb normally.It is about 2 inches of movement. It feels like it is between the the time the secondaries should engage. No bogging down. It is just a dead spot. Then it climbs normally except it caps out at 34 to 3500 rpms

I also changed my prop from an 18x14.5 4 blade aluminium to an 18 x 15.25 4 blade stainless. My speed picked up handles so much better but RPMs max at at 34 to 3500. Are they related and would the prop cause me that much of a drop in RPMs?

Did it ever run at a proper WOT RPM or did this happen concurrent with the prop swap ?

In addition to the compression check would verify you are getting full spark advance

 

[cyclops222]

The secondary airflow sensor and mover are too draggy at the cross over point ?

 

[jststrd1]

Blades are thinner so less drag but as Scott mentioned I think your motor has lost some HP

secondaries on a rochester open by air flow from the engine. there is no mechanical connection to the rochester secondaries.

if you do have a rochester, your boat is 1990 or older or someone put an old rochester on the motor or the motor has been replaced. Rochester quit building carbs in 1989. your 2000 should have a mercarb

post the motor serial number.

topping out at 3500 RPM's means there is an issue.

you are either way over-propped or your motor has an issue. that motor should be spinning about 4600-4800 RPM
if you are over-propped, you will soon kill your motor by lugging it.

start with a compression test. post the numbers. all cylinders should be between 140 and 150 psi

verify the induction system

2 weeks prior had it out with aluminum prop 4300 RPMs. It runs great starts right up even after sitting. I am going to switch back the prop and see what happens. Will need to pull boat to do it.

 

[jimmbo]

The secondary airflow sensor and mover are too draggy at the cross over point ?

Airflow Sensor? The Air Valve over the Secondaries, is opened by the Pressure Differential that is between above and below it. It is dampened by Spring Tension.

 

[jimmbo]

secondaries on a rochester open by air flow from the engine. there is no mechanical connection to the rochester secondaries.

On a Q-jet, the Secondary Throttle Shaft/Plates are moved/opened by the Throttle Linkage. An Air Valve above it is what allows Airflow.
When the Choke is not fully opened, the Secondary Shaft is Locked and will not rotate, and a Spring on the Secondary Shaft, allows the Throttle Linkage to still make full travel

 

[Scott06]

2 weeks prior had it out with aluminum prop 4300 RPMs. It runs great starts right up even after sitting. I am going to switch back the prop and see what happens. Will need to pull boat to do it.

4300 is still low for that engine should be 4800 -5k at WOT.

If you go in shallow water lift the drive up would think you can swap props in the water , done it many times. or do you have an extended swim deck covering the drive ? Just gotta be careful the thrust washer doesnt stick to the prop and drop off in the water

 

[alldodge]

2 weeks prior had it out with aluminum prop 4300 RPMs. It runs great starts right up even after sitting. I am going to switch back the prop and see what happens. Will need to pull boat to do it.

This doesn't add up, 4 blade aluminum to 4 blade stainless and same pitch.
Maybe if the aluminum was damaged

 

[jimmbo]

This doesn't add up, 4 blade aluminum to 4 blade stainless and same pitch.
Maybe if the aluminum was damaged

The Al was 14 1/2 and the SS was 15 1/4
Don't know if the AL was cupped, or, actually anything about it.
If the AL was already at or below the low end of the Rpm Range, then more Pitch will really drag it down, more than the usual 200rpm/in

 

[alldodge]

The Al was 14 1/2 and the SS was 15 1/4
Don't know if the AL was cupped, or, actually anything about it.
If the AL was already at or below the low end of the Rpm Range, then more Pitch will really drag it down, more than the usual 200rpm/in

That's the diameter, pitch is 18

 

[Scott Danforth]

Im thinking the pitch/diamter is backwards on the OP's statement.

aluminum is 14-1/2" dia, 18P
and the SS is 15-1/4" dia, 18p

the alpha drive wont allow larger than a 15-1/2"

generally 4-blade props are even pitches, so going to assume the 18 is pitch

 

[jimmbo]

Here is what he wrote "18x14.5 4 blade aluminium to an 18 x 15.25 4 blade stainless"

Diameter is usually placed before Pitch

 

[alldodge]

Diameter is usually placed before Pitch

Not everyone knows or even does that, also never heard of anything other than solid numbers for pitch

 

[jimmbo]

Not everyone knows or even does that, also never heard of anything other than solid numbers for pitch

Less than full Inches by OMC for years.

 

[Scott06]

This doesn't add up, 4 blade aluminum to 4 blade stainless and same pitch.
Maybe if the aluminum was damaged

Either damaged or the aluminum prop didn’t have as much cup or bite

I have found that the aluminum 4 blades do not have the grip that a good SS 4 blade does. I get just about the same WOT RPM with a 23" Spitfire Al 4 blade as I do a 21" Revolution 4 , lots of slip with the two 4 blade AL props I have tried.

Still think the OP is down on power or drive may have been replaced with wrong ratio ...

 

[cyclops222]

Check the butterfly shaft for GROOVES cut into it. Or wear grooves by the shaft into the support pieces.

 

[Scott06]

Not everyone knows or even does that, also never heard of anything other than solid numbers for pitch

the revolution 4 xp is available in some 1/2" pitch increments , I believe also the ventera has some 1/2" increments . These are also getting to not be everyday props, I think out of their racing line

 

[alldodge]

I stand corrected, old medium and small outboards used less than inch pitch and currently high end Rev4 and Enertia Eco XP using special alloys

Question, other than a Bravo 2 who has ever put a 18 inch diameter prop on a Alpha or Bravo drive?