Can't get above 4000 rpm's?

[smithrock]

Took the boat out today, and after changing from a 14x21p prop (WOT 4000rpm's 41mph gps verified) I put a newly refurbished alum 13x19p prop and still at WOT 4000 rpm's 36 mph gps verified, I trimmed all the way up just to see if my tach would go higher and it did so I'm pretty sure my tach is working correctly. So what's my problem? I should definetly get higher than 4000 rpm's at WOT especially with the 19p on there. I need to be in the 4400-4800 range

 

[180shabah]

Re: Can't get above 4000 rpm's?

Start with the basics...

Fuel filters, Timing and all the other tune up stuff.

 

[smithrock]

Re: Can't get above 4000 rpm's?

Start with the basics...

Fuel filters, Timing and all the other tune up stuff.

Filters are new, plugs are in good condition, cap and rotor are new. I do want to replace my wires though. Just seems strange that with either prop I max out at exactly 4000 rpm's ?????

 

[180shabah]

Re: Can't get above 4000 rpm's?

Still need to verify initial timing is correct.
Is it advancing fully?
Is the throttle open all the way?
Is compression good?
Coil?
Is the fuel itself bad????

WHAT ENGINE are we talking bout????

 

[smithrock]

Re: Can't get above 4000 rpm's?

'85 Chaparral, Mercruiser V6 4.3L Alpha 1 (could possibly be a 3.8L though)

 

[wire2]

Re: Can't get above 4000 rpm's?

Most electric gauges utilise something called a D'Arsonval movement, in which a very small armature pivots on ruby or other bearings in a magnetic field. The clearance between the coil and magnet pole pieces is only a few thou. All it takes is a tiny piece of debris in there to hang it up and cause it to not read correctly. Can you borrow a shop tach from a mechanic to verify yours?
If the above scenario is in effect, your engine may be revving higher than the reading.

 

[smithrock]

Re: Can't get above 4000 rpm's?

Most electric gauges utilise something called a D'Arsonval movement, in which a very small armature pivots on ruby or other bearings in a magnetic field. The clearance between the coil and magnet pole pieces is only a few thou. All it takes is a tiny piece of debris in there to hang it up and cause it to not read correctly. Can you borrow a shop tach from a mechanic to verify yours?
If the above scenario is in effect, your engine may be revving higher than the reading.

Okay but then why did the tach read higher when I was trimmed all the way up?

 

[smithrock]

Re: Can't get above 4000 rpm's?

I'm still trying to figure this out, but I've come up with under load with whatever prop I have on, I can't get higher than 4000 rpm's. When it's not under load I can reach higher than 4000. I've tried working with the trim but it doesn't change anything as far as rpm's

 

[wire2]

Re: Can't get above 4000 rpm's?

A piece of debris won't be in there solid, the pressure of the needle wanting to swing higher may move it around some, but not to the correct indication.
Magnetic bits, (a tiny steel sliver) are the worst culprits.

 

[bruceb58]

Re: Can't get above 4000 rpm's?

An easy test to see if the needle gets stuck it to rev it up at idle.

I would check you max distributor advance at full RPM. You will either need to have an advance timing light or decals for the balancer to read that high an advance number.

 

[smithrock]

Re: Can't get above 4000 rpm's?

An easy test to see if the needle gets stuck it to rev it up at idle.

I would check you max distributor advance at full RPM. You will either need to have an advance timing light or decals for the balancer to read that high an advance number.

Okay, tested it with the muffs on and the tach seems to be working fine, had the rpm's up to 5000.
So what's next, check the timing? If that checks out--then what? It only gets to 4000 under load.

 

[180shabah]

Re: Can't get above 4000 rpm's?

lets try again

Still need to verify initial timing is correct.
Is it advancing fully?
Is the throttle open all the way?
Is compression good?
Coil?
Is the fuel itself bad????

 

[ParallaxBill]

Re: Can't get above 4000 rpm's?

"Okay, tested it with the muffs on and the tach seems to be working fine, had the rpm's up to 5000."

Never a good idea to run that much rpm's on the muffs even for a second or two. 1,500 should be max on muffs. It's bad on the impeller etc.

 

[ziggy]

Re: Can't get above 4000 rpm's?

is the bottom of the boat clean?


from one of dons' previous posts......

List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

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

__________________
Don S.

 

[95yj]

Re: Can't get above 4000 rpm's?

It's bad to free rev any motor over a couple thousand R's. Won't show you anything because motor reacts totally differently under load and all it will do is throw parts around your engine compartment.

*Marine growth on hull and outdrive

Still like the salutation: "if a dirty bottom slows you down what do think it does to your boat "

 

[smithrock]

Re: Can't get above 4000 rpm's?

It's bad to free rev any motor over a couple thousand R's. Won't show you anything because motor reacts totally differently under load and all it will do is throw parts around your engine compartment.



Still like the salutation: "if a dirty bottom slows you down what do think it does to your boat "

Who said I free rev'd it? And yes I know it reacts differently under load. That is what my thread is about! And yes it will show me something, it showed my tach does work!

 

[95yj]

Re: Can't get above 4000 rpm's?

"Okay, tested it with the muffs on . . .

"With the muffs on" implies a boat sitting in your yard connected to a garden hose; hence no water to place a load on the prop and no load on the motor. No load means free reving in my book.

Sorry to misunderstand your post.