Everything done but low rpm at WOT. Distributor spring?

Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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There is no direct correlation between carb size and pitch

Your motor is an air pump. Just like your lungs are. Now put a snorkel in your mouth and try to run ....you won't get vary far breathing thru the snorkel. You get the point

Somewhere in the past a 600CFM carb was installed vs what was on the motor prior. And you now can't get on plane

Because the carb is now your restriction to get the motor to spin fast enough to do the work needed, you may have to drop pitch to get your boat on plane

Install a vacuum gauge in the intake and go for a run. If at WOT you have any significant vacuum, the carb is too small. A properly sized carb will get you to about 0 - 0.5" of vacuum at WOT and get you to full RPM

If it was me, and the motor is healthy.....I would put an 850 annular discharge carb on it
 
Scott Danforth

Scott Danforth

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VancouverBoat said:
I want to redefine my answer on the free rev. It seems like it takes time to get up to the higher revs and I hesitate to run it for too long at high revs in neutral. My previous boat would get the high revs almost instantly. This one takes like 5 secs. Is that an issue?
Click to expand...
In neutral, a motor should instantly throttle up.

Are you seeing fuel spraying from the accelerator nozzles?
 
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VancouverBoat

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Scott Danforth said:
In neutral, a motor should instantly throttle up.

Are you seeing fuel spraying from the accelerator nozzles?
Click to expand...
Yes, thank you. I found that he lower helm cable was sticking and slowing it down. All good now with pretty responsive free rev in neutral. Will try to check vaccum at WOT
 
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VancouverBoat

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Scott Danforth said:
There is no direct correlation between carb size and pitch

Your motor is an air pump. Just like your lungs are. Now put a snorkel in your mouth and try to run ....you won't get vary far breathing thru the snorkel. You get the point

Somewhere in the past a 600CFM carb was installed vs what was on the motor prior. And you now can't get on plane

Because the carb is now your restriction to get the motor to spin fast enough to do the work needed, you may have to drop pitch to get your boat on plane

Install a vacuum gauge in the intake and go for a run. If at WOT you have any significant vacuum, the carb is too small. A properly sized carb will get you to about 0 - 0.5" of vacuum at WOT and get you to full RPM

If it was me, and the motor is healthy.....I would put an 850 annular discharge carb on it
Click to expand...
I get 0 vacuum at WOT...
 
jimmbo

jimmbo

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Means no Restriction in the Airflow, which means the Carb is not too small.
 
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VancouverBoat

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jimmbo said:
Means no Restriction in the Airflow, which means the Carb is not too small.
Click to expand...
sure, but I have checked everything in the list:
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.

That just leaves Carb and prop pitch. Prop is good condition and was the size sold with engine/boat so..... I guess a new carb...?
 
jimmbo

jimmbo

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I'm not rereading everything, but is the Spark advancing and to the proper Max Advance? Has been any changes to the Engine that you are aware of, such as Camshaft, Heads, Pistons? Has the Timing Chain and Sprockets been changed? Harmonic Balancer?
How long have you had this boat and motor, and when did it start acting this way?

Edit: If the carb was too small, it would have better bottom end torque than a bigger carb, but it would be short of Breath at High Rpm, and you would have be seeing Manifold Vacuum at WOT. Since few boats get on Plane at 6000+ rpm, a bigger carb is not going to help
 
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VancouverBoat

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jimmbo said:
I'm not rereading everything, but is the Spark advancing and to the proper Max Advance? Has been any changes to the Engine that you are aware of, such as Camshaft, Heads, Pistons? Has the Timing Chain and Sprockets been changed? Harmonic Balancer?
How long have you had this boat and motor, and when did it start acting this way?

Edit: If the carb was too small, it would have better bottom end torque than a bigger carb, but it would be short of Breath at High Rpm, and you would have be seeing Manifold Vacuum at WOT. Since few boats get on Plane at 6000+ rpm, a bigger carb is not going to help
Click to expand...
I have had it for 2 summers and it has never achieved a plane. Had compression checked at 150 all cylinders. Since then, top end redone with new heads, plugs, wires, distributor. Rings/pistons were apparently ok. Timing chain and sprockets, camshaft were not changed. vacuum 14 at idle
 
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froggy1150

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Here is a dumb question...
Have you weighed the boat. Is it possible you got water under deck and have a heavy boat???
 
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VancouverBoat

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froggy1150 said:
Here is a dumb question...
Have you weighed the boat. Is it possible you got water under deck and have a heavy boat???
Click to expand...
Not a dumb question. I did not weight it but it is a pretty heavy boat to start with. It is almost 30 ft with command bridge and wide beam. Hull/deck are in pretty good condition - I don't think it is water saturated. The factory weight of the boat is about 7000 lbs. dry and unloaded.
 
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Bnewton530

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I am having the same problem with my Merc 350 attached to an OMC drive. When you get to 1900rpm cruise, have you tried to ram the throttle down to see if it will jump up onto plane?

Mine will reach plane rpm if I smash the throttle.
 
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VancouverBoat

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Bnewton530 said:
I am having the same problem with my Merc 350 attached to an OMC drive. When you get to 1900rpm cruise, have you tried to ram the throttle down to see if it will jump up onto plane?

Mine will reach plane rpm if I smash the throttle.
Click to expand...
Do you mean just push handle forward fast instead of gradually? I don't have any more travel left with my throttle handle all the way forward. I also removed throttle cable to see if the carb valve was being restricted by the cable.
 
jimmbo

jimmbo

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Bnewton530 said:
That’s the only way I can get into plane. If I go gradual I can only get 2500 rpm. I have half of the throttle travel left at that point and if I mash it I get onto plane
Click to expand...
Deleted, as I thought I was responding to the OP
 
Last edited:
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VancouverBoat

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jimmbo said:
That is an interesting little tidbit, wish you had mentioned it much earlier. I have never heard of that kind of Behavior, as I can get on plane with far less than Mashing it. However my boat is only about 19ft, but I also only have a 5.7. Yours is in the 30ft range, which in my books is a lot for just one Big Block.
When you 'Mash it' and it gets up on Plane, what Rpm is it getting up to at WOT?
Click to expand...
Jimmbo, that is Bnewton530 who said that (not me the original poster with this issue). Does not matter if I "mash" it or go gradual - no plane. I just bought a 4 blade prop online that has 2 inches less pitch. Will try that and report.
 
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