RPM's hanging after a pull?

Anybody else have this problem?

I recently set timing on my 89 Sunsport PCM 5.8L. I set it to 12 degrees base. It runs much better now (was in the neighborhood of 20 degrees before), but I am encountering a new problem since adjustment. After pulling someone on a tube or board, the RPM's hang at ~2000rpm and will not drop to idle speed. Obviously, the boat won't come out of gear at that speed. If I turn the boat off and fire it back up, it runs at 700rpm idle speed.

What could be causing the rpm's to hang out up there? Engine has a Holley 650cfm carb on it, but the fact that it returns to normal after shutting down leads me to suspect something electrical? Any ideas are welcomed, thanks!

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Originally Posted by Knully

Anybody else have this problem?

I recently set timing on my 89 Sunsport PCM 5.8L. I set it to 12 degrees base. It runs much better now (was in the neighborhood of 20 degrees before), but I am encountering a new problem since adjustment. After pulling someone on a tube or board, the RPM's hang at ~2000rpm and will not drop to idle speed. Obviously, the boat won't come out of gear at that speed. If I turn the boat off and fire it back up, it runs at 700rpm idle speed.

What could be causing the rpm's to hang out up there? Engine has a Holley 650cfm carb on it, but the fact that it returns to normal after shutting down leads me to suspect something electrical? Any ideas are welcomed, thanks!

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12 is too much , 10 BTDC and adjust idle from there , or your throttle linkage is hanging up ..

20* was too much, I'm surprised it started well enough without kicking back, killing the starter, or detonating itself to death if you ran it there any length of time. Maybe you're running 91-93 octane and that kept it safe.
8*-12* is a good range for initial, and total around 30*-32*. Up to 16* is good for those inefficient heads with 36* total if it doesn't detonate, but I don't recommend that much timing in a towboat as you can be too close with detonation in an application where it's hard to hear. 10* initial and 32* total is pretty safe.

On the hanging revs, check your throttle lingage. May need to be adjusted to allow enough slack at idle to release fully consistently. Also set your idle mixture and idle stop screw after any base timing change. Check timing when it hangs, could be your mechanical advance sticking. Finally it could be an intermittent vacuum leak. I had one on mine that took forever to find, ended up being a leak to under the intake so only found the damaged gasket after removing the intake.

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Originally Posted by cadunkle

Check timing when it hangs, could be your mechanical advance sticking.

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I had this happen on an old boat. It had so many hours that the guides that the weight ran in had a notch worn into it at the fully advanced position! It would stick, i thought the whole distributor slipped, so I'd adjust it back, then it would spring back at random (and wouldn't advance further) so generally ran horrible...

Thanks everyone for the replies. I had already determined that the throttle linkage was not the cause but forgot to mention that.

This carbeurated engine is new to me - I have never owned something without EFI so trying to learn as I go. I will be sure to how-to setting the idle mixture. I have a rotor and cap to try out, what you are saying makes sense to me Korey. If timing advance is controlled mechanically, what is the function of the vacuum hose run to the front of the carb?

Thanks fellas, here is a picture of my weapon. Bought it in July, haven't had it out much. Life, weather and work get in the way. On the rare nice day, my brother has had his SAN 230 out so the Supra hasn't got much love.

http://i377.photobucket.com/albums/o...17968B630D.jpg


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Quote:

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Originally Posted by Knully

Thanks everyone for the replies. I had already determined that the throttle linkage was not the cause but forgot to mention that.

This carbeurated engine is new to me - I have never owned something without EFI so trying to learn as I go. I will be sure to how-to setting the idle mixture. I have a rotor and cap to try out, what you are saying makes sense to me Korey. If timing advance is controlled mechanically, what is the function of the vacuum hose run to the front of the carb?

Thanks fellas, here is a picture of my weapon. Bought it in July, haven't had it out much. Life, weather and work get in the way. On the rare nice day, my brother has had his SAN 230 out so the Supra hasn't got much love.

http://i377.photobucket.com/albums/o...17968B630D.jpg


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Then you have the wrong carb , should be a a Holley 4160 w/out vacuum advance port . Take a pic of what you got . There should a blow by hose out the back and then a over flow for the fuel pump . As well the carb overflows should have 90degree elbows pointing into the boosters


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It's possible I'm just confused. All I remember seeing on the carb is Holley and 650 cfm. I'll grab a picture later on today, it's stored at a friends place at the moment.


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http://i377.photobucket.com/albums/o...4E14F8AD93.jpg

Looks like you are right. What I thought was a vacuum line runs back down to the fuel pump. I need to take some time and read/tinker until I understand carbs a little better. It's all greek to me at this point.

Now, I had a chance to do a little tinkering tonight finally. My total timing is only 25 degrees, stops advancing about 3000rpm. Thinking the distributor would be the culprit here?


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What is your base timing set at ? And what does the boat idle at ?

I set mine to 600-750 idle rpm

8-10 degrees BTDC

And that should set about 26 -27 total timing of I remember correctly . Too much advance is no good , then you develop spark knock , just set it right when it starts with one key bump sometimes it's better to adjust by ear also .


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I have idle set at 700-750 now, timing was set at 600 RPM when I set timing.Thanks Lively, I did set base timing more by ear which is how I ended up at 12 degrees base. It just seemed to idle smoother there than 10 or 11 degrees. Timing seemed to bounce around when trying to set at 10 as well. My concern comes from me reading that I should be 30-34 degrees total timing, and I only have 25-26.

We do run 91 octane, and the boat ran great last time out aside from the RPM's not dropping. Had good, smooth power. Looks like it might be a winter project now, frost on my grass today. Boat is hopefully being pulled apart this winter to replace stringers.


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If you're new to carbs, (even if you're not) this is a great place to start http://www.bob2000.com/carb.htm

You can learn an awful lot about your tuning with nothing but a vacuum gauge and and tachometer. I too set my timing by feel (used the vacuum gauge) at idle and it was too far advanced. I think I'm at 9* now and the boat gained about 3mph top end. I had to up the curb idle to get it to idle nicely but it does. Based on the fact that it will idle nicer with a more advanced timing it sounds like the weights in your distributor may be sticking.

Second that on the mechanical advance , clean the springs and pivots with contact cleaner and throw some lithium grease on the pivots . You can also rebuild them as well


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Thanks for the responses fellas. We ended up going through the throttle linkage and snugging it up /adjusting and throwing on a new cap and rotor. Seems to have solved the issue. I will replace the distributor over the winter here, as I should be pulling the motor and trans this week. Am I going to have to pull the prop and shaft out to lift the trans out with the engine? It looked that way to me. I am really not looking forward to tearing this boat apart, but I better start chipping away at it.


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One more item that usually goes overlooked is the PCV valve. While running the engine @ low RPMs under load they get worked really hard. Usually cost under 10 bucks.Could be an cheap easy fix .