HELP, Max 1200 RPM in forward, too slow

[mwh2319]

HELP, Max 1200 RPM in forward, too slow

My Atomic 4 starts rights up and in neutral will go up to 4000 RPM easily. But when I engage the transmission in forward it goes to a maximum of 1200 RPM.

I will admit the bottom of my boat, and probably my propeller too, are dirty. Which accounts for part of my boats slow speed of 1 MPH in forward. The hull speed is 6 MPH. Water temperature and oil pressure are fine.

But why will my Atomic 4 not get up to a recommended cruising RPM of 1800 RPM in forward gear?

Any thoughts would be appreciated.

Thanks,
mwh2319

[capnward]

If that was happening to me, I would take the carburetor apart and clean it, making sure no jets are clogged, including the small vents in the throat. At the same time I would replace the inline filter. You can drain the water separator to see if it is catching any water in the fuel. If you have a brown goo in the carb, that is caused by ethanol. Or maybe something is wrapped around your prop or shaft. a fuel pressure gauge would tell you if there is a fuel restriction before the carb. 1 mph is slow for 1200 rpm. When the bottom of my boat was incredibly fouled, I could still get 3 knots.

[Surcouf]

1 knot? that is reallllly slow.

Many different potential reasons...
- obvious one is of course a very dirty prop.... anyway you can anchor in clean water and remove barnacles?
- while in neutral at low speed, remove pulg wire one by one, to check that all cylinders are firing.
- after that it gets more complicated: fuel (pressure gage ok before carb? carb cleaned? no water in your fuel?), air (no problem on suction?), exhaust gases (unlikely, but is your exhaust pipe rubber? how old? Mine was crumbled inside, and "closed 75%", but still able to get 5 knots out of it), spark (spark plugs? spark plug wire test?), and compression (but to get only 1 knot, you would need at least 2 cylinders not running properly... A4 easily push most boats at 5 knots on 3 cylinders...)

The more educated Afourins will likely have more idea...

[tenders]

Invoking Occam's Razor, absolutely positively clean the hull and prop before removing a fastener from anywhere on the engine. Significant drag from a barnacled prop present exactly the symptoms you're describing.

[Easy Rider]

I totally agree with Tenders. Do the basic stuff first. Make sure the prop and hull are clean - including the very bottom of the keel. Even a few barnacles on a prop will cause what you are describing.

[Al Schober]

1 MPH??? While cleaning your prop, take a swipe at the knotmeter paddle. My knotmeter transmitter was removable from inside the hull - I just had to keep my bare foot over the hole while I cleaned it. Yes, it would grow barnacles too!

[Bayou Sailor]

I have had a similar issue. If your original prop was replaced with a larger or if it went from a 2 blade to a 3 blade it will reduce your max RPM somewhat. That combined with a dirty prop can cause extreme RPM loss.

[mwh2319]

Thanks everybody, very helpfull.

[Mo]

Last week I looked at a guys boat. He thought it wasn't running as good and not making the speed it did last year. First I looked at his firing order and checked his plugs for fire...all good there. Then looked at his distributer cap and the two hold down screws were rusty...not touched in years. We took it for a ride on the water and I got in the lazerette, loosened the 1/2 bolt on the dizy and rotated slightly... got her rpm back...knew then that he needed to get that cap off, replace it, and lubricate the centrifugal advance. With a little fooling around he sorted that out and it worked fine. I set the timing for him under-way a day later.

If the bottom is clean, prop is clean, engine seems to run fine in neutral generally I look for a sticking valve...Once you have cleared the prop and barnacles on the hull as culprits, in your particular case I'd check that the engine is handing out the power. Check compression, check fire, check timing...the small stuff. Then look at the weird stuff like a stuffing box too tight.