Still lacking power

[Bayou Sailor]

Hey guys. First of all Merry Christmas and Happy New Year! Well, we finally had some great weather down here so I have taken Margarita out a few times. She cranks right up. There's no more exhaust smell. She runs smooth, until you try to give her a lot of throttle. She is great up to about 1200-1400 rpm and then nothing. She doesn't clank and die like before, she just doesn't rev up. I got her back to the slip and tried it with same results. She revs up fine in neutral, but in gear she won't get past about 1200-1400. Now, I have recently rebuilt the carb, reset timing, new points, plugs, wires, dist cap, rotor, condenser, lubed the cent advance weights, rebuilt the exhaust, new alternator. My first thought is fuel. The tank is 50 years old. I plan on replacing the fuel/water and polishing filter today. Am I heading in the right direction? My only other thought is that the transmission is a V-drive. I have not changed the oil in it, but the oil level in it is a good bit over the "full" mark. Could that have anything to do with it? Any help would be appreciated!!

[joe_db]

First thing I would do is hook up a vacuum gauge. You might just have a fouled prop.

[Bayou Sailor]

Funny you should say that Joe. My buddy was sailing with me and mentioned that it's possible that a PO changed out the prop trying to increase speed at some time. There is a prop (possibly the original) in the bilge.

[joe_db]

Barnacles on the prop or a really over-pitched prop would show up about the same. This photo is after a bottom cleaning, but being badly over-pitched will cause the same problems as barnacles.

[edwardc]

What prop do you have?

My 13000 lb Pearson, with a 2:1 V-drive and 15x8 3-blade prop cruises at 5-6 kts at 2000-2400 rpm. With a clean prop & bottom, in flat calm conditions it gets all the way up to 3100 RPM at WOT.

You definitely should be seeing more than 1200-1400.

Air or fuel restriction, or exhaust restriction are the other two things that come to my mind, with exhaust being the more likely.

[Bayou Sailor]

Thanks Ed. Just replaced the exhaust so I'm thinking prop or fuel. I don't know what prop is on it now, but I need to pull out the one in the bilge and compare.

[Dave Neptune]

Bayou, do you have any info on what it did RPM wise before this episode as far as RPM's?

You replaced the exhaust, I assume it was the pipe portion or did you replace the hose too? A collapse exhaust hose will do the same and note they can collapse inside and look fine from the outside.

With a reduction unit like yours it would take a good deal of barnacles to cause such a drop even if over-propped.

The vacuum gage will tell you how hard the engine is breathing and the fuel pressure will let you know the carb has what it needs to perform properly. Both are worth the investment .

The oil level would make no difference other than not lubricating as well due to the age of the oil. It's not the trans.

Conditions that led to the drop in performance would be good info too, sort of a time line.

Dave Neptune

[Surcouf]

fuel is easy to rule out: rig a temporary fuel line from a clean tank located outside.

I have experienced experienced:
- the collapsed exhaust pipe so really something to check.
- a very fouled shaft at the end of a long season, I recovered several hundreds rpm after cleaning

But with the wrong prop, you could be limited in rpm even clean...

[Mo]

Two things come to mind for me:
1. Compression. Wondering if the valves are seating properly.

I worked on one about 10 years back and replaced a carb, changed a prop that didn't help a bit. I'd done the thumb compression test and it popped my thumb off. After a number of days fooling with this thing, I brought a compression gauge and it was down on 2 cylinders. Had to remove head, lubricate and reseat the valves. It was fine after that. It's the biggest mistake I made ever working on an A4 and to this day I don't rely on a thumb test.

2. Reduction gear? Just wondering if there is a reduction gear on the end of that reversing gear. Once saw an engine that had no power with a reduction gear. The problem was his prop...too big. They take a smaller prop.

OK, forgot you were a V-drive. Need to check the prop size there as well.

[tenders]

And are you certain the plug wires are run off the cap in the correct 1-2-4-3 order? Easy to get those mixed up, and the results can be very perplexing.

[ndutton]

All of the tests mentioned will return good information in chasing down the power issue but if the problem persists, an often overlooked test is exhaust backpressure. MMI offers a kit.

If the engine can't breathe - and that includes exhaling - no way will it develop the proper power.