Huntin’ for an idle

I’m trying to get the idle dialed in on my "79 Southwind. Cold start with the choke seems fine, and off-idle acceleration and performance at speed seems fine. Coming down from higher speeds however, the idle hangs at about 1200 RPM in neutral and slowly will drop to about 1000 (takes about 2 minutes). Disconnecting the throttle cable does not affect this. When hot, the motor struggles to maintain a 650 RPM in gear and seems to load up if idled over time. My prop is a 13 X 13. Motor is a PCM 351 with 970hrs and appears to be original. I did a tune up when I picked the boat up last spring , new points & condenser, cap, rotor, plugs and wires. I gapped the points at .019, dwell came in at 31 and timing is at 8 (checked this spring). The plugs look good and my compression test had one low of 105 and a high of 130. The carb appeared filthy so I had a very good carb guy rebuild it and it ran and idled perfectly on his test engine. I tested around the intake manifold & carb with starting fluid but there doesn’t seem to be a detectable leak. Intake vacuum is at 17 and it does jump about ½ lb at idle. I have tried swapping in a variety of PCV valves with no effect. As you can tell, I am open to suggestions. It’s open season.

Seeing as this is an original application, try looking at the mechanical advance and verify that it's operating as smooth as it should. If it's a little sticky, that could cause this type of problem I suppose. The distributor could just be tired too and need some TLC. There could be too much slop in it after that many hours. We've seen that on this form before. However, it does seem that the cause/effect was the carb rebuild.

Mine does that too. It hangs up on idle after some pulling skiers. I was thinking distributer mines pretty worn and things are loose on the inside. I just dont wanna chalk up the 350 bucks that they want at skidim. Im going to check for vaccume leaks this weekend. Mine also has had a carb rebuild at the start of this summer by a well known shop.
its weird about the hanging tho because it wond hang up if you let it run at rmp with no load but it does it when its under load. It should advance under any situation.

I also have had this same issue with my boat. After reading a Demon carb book, talking to Holley a few months back, and speaking to Don Gould who is currently adjusting the timing on my DUI/HEI distributor the following are some of the things that could lead to hanging RPM.

1. Right after I complete a high speed run and throw it in neutral the RPM's hang about 200-300 RPM's high. If I immediately turn off the engine and immediately re-start it I am right back at my desired idle of 850 RPM. Don has told me if you turn if off and restart it immediately the engine and the RPM's drop that is usually the mechanical advance springs/weights not releasing after high RPM but when you turn the engine off the weights/springs release and the RPM goes back to normal.

2. Holley said after a high speed run if the RPM hangs that means fuel is slowly leaking into the venturi somewhere like maybe the primary metering block. They stated it only takes a small amount of fuel to kick the RPM's up a few hundred RPM which sounds plausible.

3. Quite a few people have had the Perfect Pass hang up on the engine cover shroud so check there if you run PP.

4. Old, worn out, out of adjustment with too much slop/play throttle cables can result in the cable not consistently returning to the fully closed throttle position every time.

5. I am reading a Demon carb tuning book right now and it talks about (in this authors opinion) that 75% of all carb idle issues are directly related to throttle cable issues. They also mention that it is very critical to always have the carb return spring level with the throttle cable and not pointing at a downward angle which COULD result in an uneven return pull. The throttle cable itself where it attaches to the carb should be as level as possible with good support so it will not flex under wide open somehow causing the carb not to return to it's normal idle positon. They recommend one of these billet throttle support brackets that attaches to the carb because they are strong and provide good support and even return springs on the front of the carb. They make them for Morse cable applications. Here is a pic...

6. Holley and Barry Grant both state that if a carb is either too lean or too rich and out of whack on the idle circuit it can cause a hanging RPM condition off idle.

7. My question is could the vacum secondary somehow not fully release the secondaries after a high speed run leaving the seconday butterflies slightly cracked open pulling fuel beacuse most Holley/Barry Grant style 4 barrel carbs have a secondary idle circuit so fuel does not go bad sitting in the carb if you don't open up the secondaries for a while and could get a little fuel there.

Anyone else?

Thanks!

My 77 mustang hangs like that and it's all in the throttle cable end.I can bring it down by hand. I'm going to try a return spring to take up the slop. Farmer

OK, Back from the bay with exciting news- She runs just as smooth as my 1988 Nautique now that I swapped the carb and spacer from my Nautique onto it. Smooth idle, no problem with hot starts and good off the line- I'm a very happy man.

The swap went pretty smoothly, although the Nautique has a taller spacer by about 1/4" so I had to temporarily back the studs on the intake manifold of the Southwind out a touch. The throttle linkage post had to be moved to the upper hole on the plate but no big deal.

My suspicions now turn to the spacer plate from the Southwind. I haven't had a chance to examine it closely but I found fuel in the cavity between the manifold and the spacer- I'm pretty sure that is where the vapors from the PCV are to be routed- not fuel. Perhaps something happens after it absorbs all that heat from the intake manifold, causing some fuel seepage/vacuum leak. The symptoms only appear after it is thoroughly warmed up. My next step is to put the Southwind carb back on with the Nautique spacer and see what happens.     

As was once said (or written...) "All the king's horsemen and all the king's men.. couldn't get it to idle when warm once again."

Long story short, I learned a lot about my boat's engine and some about the carb. I'm good and real fast at puttin' 'em in and takin' em out now. I have re-checked and re-adjusted my cables, idle mixture, compression, re-torqued my manifold, timing, adjusted my points and verified them with a dwell meter... and still looked like an intox when trying to dock the thing.

So I went with a new carb from S/E Correct Craft and as Woody told me "You bolt it in, dial in your idle RPMs and you are done." I just got back from the bay and it runs great, cold and warm.

Thanks to everyone for the guidance and advice but for the hours put in, the new one is working for me despite the cost.   Now I can enjoy the tail end of the summer and into the fall.