RPM Drops In Forward

[mhinnc]

The last time out the RPMs were 'normal', we could get 3000 RPM in forward. A couple of months went by before we could take the boat out again. No changes to motor/drive train during that time.

Next time out, 800 or 900 RPM was best we could do in forward, so we turned around, back to the slip. Asking around, the collective knowledge all pointed to crusted up shaft/prop. Two months ago it was cleaned.

This is the first time back at the boat. Same issue, mostly. Now I can get 1100 RPM in forward sitting in the slip, no more. In reverse, easy 2500 RPM, in neutral, 3000+. Any ideas? Thanks.

[joe_db]

So the prop got cleaned and the issue was fixed, or this is the first try?
Maybe it is fouled again?

[Al Schober]

I'm suspicious of the '3000 RPM in forward'. I know of no one with a propeller who gets to 3000 RPM in gear. I suspect some bogus RPM numbers.
Tell us about your tachometer. Is this a mechanical thing, electrical, or a handheld LED unit reading a reflective strip?

[mhinnc]

I'm suspicious of the '3000 RPM in forward'. I know of no one with a propeller who gets to 3000 RPM in gear. I suspect some bogus RPM numbers.
Tell us about your tachometer. Is this a mechanical thing, electrical, or a handheld LED unit reading a reflective strip?

What numbers would you think not to be 'suspect'? What should be the higher range in forward? 3k is best I remember from October. May have been less. I've limited experience with the A4.

The precise tach numbers wouldn't be the issue, anyway, at all, would they? The loss of RPMs is the issue. I would think this tachometer, accurate and precise or not, would be helpful to indicate a reference point as to whether the issue is worse or better as I attempt to find the solution. Any helpful advice to help solve the issue at hand is appreciated.

At this point, cylinder #1's lack of compression which I've just discovered, would point to a possible cause for RPMs to drop in forward, from all I've seen. Care to add? Thanks.

[msmith10]

I have a direct drive A4. I cruise at 1600. Can hit maybe 2200 wide open under load.
Is yours a direct drive or does it have a reduction gear? What's your boat? Solid or folding/feathering prop?
No compression cylinder 1? That's a new issue. I don't see that causing the kind of RPM drop you're seeing but I'm with Al. Unless you're geared down and have a small prop you're unlikely to hit 3000 RPM in gear. You're going to have to sort this. First thing to check with low compression is stuck valves. Cylinder 4 (closest to reversing gear) is more common to foul valves due to water intrusion. Check to see if the valves are opening/closing as the crank turns. You can look in the spark plug hole or remove the valve cover plate on the side. If valves are stuck, squirt Marvel Mystery Oil into cylinder as close to valves as possible. After soaking a while, you can usually free them up with an allen wrench placed on top of the valve thru the spark plug hole and some judicious tapping. Once free, exercise the valve and keep squirting MMO until they move easily.
Does the shaft turn easily by hand? You can get binding at the stuffing box, but that's usually apparent as the box will get quite hot.

[ndutton]

Do we know if this is a direct drive or reduction gear engine? Did I miss something?

[Dave Neptune]

Good info = good diagnostics

You state that the compression is "low" on cylinder #1 (flywheel end). What are the numbers for your compression test? Also did you do a dry test then a wet test? Do you also know what the numbers were before experiencing the loss of performance? NOTE when doing a compression test all 4 plugs should be removed, the choke OFF and the throttle set to full for accurate numbers.

Does the engine run as smooth as it did before or does it seem to be missing a bit? Do double check that the plug wires are in "order" a common mistake.

As per Joe's question, did the prop cleaning get your RPM's back after cleaning? A dirty prop can really reduce performance as can a loss of a cylinder with no compression.

How long since the engine had a tune including timing and a check of the centrifugal advance function?

Dave Neptune

[joe_db]

We seem to have 3 different issues going on, which always makes things harder to diagnose.
1 - The tach. No direct drive A4 is getting to 3,000 RPM. If it is, either the prop fell off or the clutch is slipping. Is this a direct drive engine? I think I might maybe get to 2500-2600 on a good day. Something is wrong here, maybe the tach.
2 - The prop. It was fouled. That will reduce boat speed to 0 if it is really bad. Is it fouled now? I know no one likes diving in January, but it needs to be checked.
3 - Compression. What are the numbers across all the cylinders? When was this done? Are you SURE the plug wires are back on correctly? If you switch the right (wrong?) 2, the engine runs smoothly with about no power.

[mhinnc]

The 'thumb test' proposed by Don Moyer was the method of finding the low (to no) compression in #1.
Thanks for the suggestions.