Request for exhaust problem info

I have a 1971 Cal 29 with an atomic 4 gas engine. I may have a back pressure problem which is keeping the engine limited to about 1200 rpm under load. In neutral I do not have any problem with getting the maximum of 3000 rpm out of it.
What would be your recommendation for replacement of the exhaust system from manifold to transom?
Thanks,

Mel Root

[Don Moyer]

Mel,

Restrictions within the exhaust system serious enough to limit your RPM to 1200 under load would also usually affect your performance in neutral. Depending on the size prop on your boat (and whether or not it's clean) 1200 RPM could be quite normal.

If you do decide to check your exhaust system for a restriction, I'd start by removing the hot section between the back of the manifold and the water lift muffler. Most restrictions occur where the engine cooling water splashes into the hot section, creating a build-up of mineral deposit.

Don

[photoflash]

The prop should not be the problem. I recently changed from a MW 13 x 12 - 2 blade to a new Campbell 10 x 6 - 3 blade in an attempt to solve the problem (assuming I was overproped as there was less than 1/2" clearance between the prop and the hull.) The net gain was only abut 250 rpm and about .3 knots of speed under power.
The Campbell people suggested that it might be a back pressure problem.
Mel
aka photoflash

[Don Moyer]

The 13 X 12 two-bladed prop was definitely too much prop for a direct drive Atomic 4. In fact, that prop would be more appropriate for a 2 to 1 gear reduction model.

It would be interesting to know what your compression values are, and what boat speed you're able to attain.

Don

[photoflash]

Top speed with the 13x12-2 was a little under 4 knots at a top end of about 1000 rpm. With the 10 x 6-3 it "improved" to about 4.25 knots at a top end of about 1200 rpm. If possible, I'll do a compression check this weekend.
Thanks,
Mel

[photoflash]

Well, compression was good. The lowest cylinder (#2) was a little over 90, which, according to my spec sheet on the motor is adequate. The others were 100 or better. BUT, After a little tuning, new plugs, points & condenser, I was able to get 2000 rpm and about 6 kts of boat speed. The only problem now is that after running for about 20 minutes or so, the temp gauge starts climbing. I caught it at 240 and immediately throttled back to about 1500, which droped it to about 180. I am planning on changing the impeller this weekend to see if that is the problem. I hope it is that simple.

[hobbit91]

Sounds like you need to check the entire cooling system to check for proper flow. How much water is coming out of the exhaust pipe? When i had overheating problems i had to check every part of the cooling system including the riser and the rest of the exhaust system. Check that each part has water flowing through it. Check the through hull as well. Pull off each hose one at a time, start the engine for a second or two and check for flow, reconnect and do the next one. If water is moving through the engine its time to check the riser and the exhaust system. It take very little to block the water and the engine does not slow down if water is not moving through it. Aside from the possible engine damage, running hot with no water flow can seriously damage exhaust hoses. Usually from the inside.