How "steady" is your idle RPM?

[TomG]

Tom,

My idle is a little jumpy too, & I have the Indigo PCV system...please report your results of the motor crack test if you find something.

My motor behavior has been so weird I don't dare blame it on anything just yet, but I am still looking for answers, or other experiences similar to mine.

Shawn,

Where does yours idle at rpm-wise and where do you set the idle mixture?

My engine runs perfectly except for this higher than normal idle.

[hanleyclifford]

Tom, Try comparing vacuum at idle with and without the pcv. Then watch the tach fluctuations with the pcv installed and see if the vacuum gauge flutters with the cycling.

[Dave Neptune]

Try leaning?

Tom, take a look at the vac if you have a gage, and if it moves around a bit thet means something is varying. I doubt it could be the crankcase as in these beasties the crank is not sealed and the cap/fill is a breather.

Note about the idle adjustment. I have found that after installing the carb and adjusting that after running I can go a bit leaner with the adjustment. I tend to get it close and then each of the next few cold starts I give the needle a "tic of a turn lean", sometimes I can do this 2~3 tiems before it is lean enough to disturb idle quality then go right back to where I was. I find that PCV engines tend to be adjusted to the rich side when I work on them.

Try this, disconnect the PCV and adjust the idle as best and as lean as you can having a smooth idle then open the PCV and try starting without adjusting. If it is rough try just a tic to the rich at a time and wait a minute or so between tweaks. I like to load them up on the dock lines and then back to idle as I work the needle as lean as I can. I get to the point that it is a bit harder to start and at that point it will idle all bloomin' day. I have had to idle for over 4 hours once last season and the lil beastie sat at 700 for 3 of those hours after the batteries charged up, not even a hick-up and the next morning I did a plug read and they looked GREAT. My compression numbers are low (-80 85 90 90) and I do have a bit of blowby.
Perhaps the throttle lever is "floating" a bit and is actually the culprit, but I doubt if it is that it would be cyclic in nature.

Dave Neptune

[sastanley]

Shawn,

Where does yours idle at rpm-wise and where do you set the idle mixture?

My engine runs perfectly except for this higher than normal idle.

Tom, Mine seems to idle about 850-900. The fluctuation is not always there, but it seems to be more prevelant if I try to get the idle any lower than that.

And, quite honestly, I need to take better notes on when it is occuring (i.e., after a long run, when it is cold, etc.)

I also have a vac gauge, but the problem with it is I still have a soft hose, so my readings are a little erradic (sp?), esp. at idle.

I just took my carb apart and cleaned it, so I had to reset the idle screw..I am pretty sure I have it at 1-3/4 turns out...trying to hang with the "lean brothers"..

My PCV was the first thing I did to my engine to reduce the blow-by into the cabin..That was in 2008..may be time for a new valve. (HOLY COW, have I really had this boat for 5 seasons??!?!?!)

Anyway, not trying to hijack your thread, but no sense in having two about the same subject.

[edwardc]

... With the PCV valve out of the equation, she idles very nicely at 7-750 rpm with the idle needle out about a turn and a quarter. With the PCV valve in, I can't get her to idle smoothly and consistently below 900 rpm and the idle needle set in to about 1/2 to 3/4 turn out from full rich....

TomG,

This just sounds so familiar, I have to tell this story. Your mileage may vary.

I too have a PCV valve, and I used to have trouble getting a smooth idle. I would end up with the idle jet air bleed set down between 1/4 and 1/2 turn to get a good idle at 750 rpm. But this is too rich, and would foul the plugs quickly. Any attempt to run leaner would not idle smoothly below 900.

After I installed the oxygen sensor and the fuel:air ratio gauge, it showed I was off the scale on the rich side! So I started experimenting.

First I set the idle mix by the gauge so it was very close to the ideal 1:14.7 fuel:air ratio. This ended up with the screw around 1 1/4 turns, but wouldn't idle well below 900. Then, I advanced the distributor a little bit. The RPMs came up a little bit, but I was then able to reduce the throttle and idle a little below 900 and stay smooth. A few iterations of this and I had a smooth idle at 750 with the throttle closed and the fuel:air mix still good.

And as a bonus, the hesitation I used to get at 1100 RPM when throttling-up (the transfer point between idle and main jet) completely disappeared.

So, you might want to give it a try. Just mark your distributor position beforehand so you can come back to it if this doesn't work for you.

[Dave Neptune]

Stop the Dancing

Shawn, hook your vac gage up with about 3 feet minimum of 1/8 tubing and it will settle and be easier to read.

Tom, Edward also makes a very good point regarding the timing. I have been asuming that your timing was good and did not consider that senario!!

Dave Neptune

[joe_db]

My PVC valve will sometimes rattle at low speeds. I'll see what it does to the idle next time I run the engine.

EDIT - can it go in backwards? That would reverse the open-close logic.

[ndutton]

Need I say it? Oh, I need . . . I'm lovin' this.

Dave and Ed's posts offer great advice for reeling in the carburetor after the installation of the PCV system. Regarding Ed's experience, I seem to recall (perhaps incorrectly) advancing the timing also has the effect of increasing the blowby so in somewhat of a vicious circle the PCV handles elevated blowby but may need a little more timing advance to idle well which may increase the blowby which fortunately (?) the PCV system is there to manage. So in essence, re-tuning with the PCV creates more of what it's trying to manage.

All of this assumes there's a blowby issue in the first place.

[edwardc]

... I seem to recall (perhaps incorrectly) advancing the timing also has the effect of increasing the blowby so in somewhat of a vicious circle the PCV handles elevated blowby but may need a little more timing advance to idle well which may increase the blowby which fortunately (?) the PCV system is there to manage. So in essence, re-tuning with the PCV creates more of what it's trying to manage...

I wondered if anyone would pick up on that!

Seriously, though, I found that the amount of advance required was still short of the amount you get when "power timing" the distributor for max rpm.

[ndutton]

Yeah well, leave it to me.

To recap my position on PCV systems, I'm not against them. I suggest they exist to manage excessive blowby gasses but address the symptom rather than the cause. In other words they are a stopgap measure to buy time.

As this thread demonstrates a PCV system has the effect of a designed vacuum leak after the carburetor and therefore introduces unmetered air into the combustion chamber. That's why this re-tuning is necessary. I'm not saying it can't be done, Dave, Edward and others seem to be able to dial it in but truly, you're tuning around a variable vacuum leak, variable depending on the position of the valve.

[hanleyclifford]

Ed, I was wondering if you could post your advance numbers for us. Like your total degrees at cruise rpm and your initial setting. Thanks, Hanley

[hanleyclifford]

Yeah well, leave it to me.

To recap my position on PCV systems, I'm not against them. I suggest they exist to manage excessive blowby gasses but address the symptom rather than the cause. In other words they are a stopgap measure to buy time.

As this thread demonstrates a PCV system has the effect of a designed vacuum leak after the carburetor and therefore introduces unmetered air into the combustion chamber. That's why this re-tuning is necessary. I'm not saying it can't be done, Dave, Edward and others seem to be able to dial it in but truly, you're tuning around a variable vacuum leak, variable depending on the position of the valve.

This is why the pcv system must be suited to draw the absolute minimum volume required to eliminate cabin leaks. The system can be regulated with a "snubber" set up which can include a circuitous path to the manifold. This is one of the reasons why I have blocked off the oil breather location with a plug and in effect draw the air thru the open flywheel.

[ndutton]

An alternative is to deal with the root cause of the blowby, engine wear and loose tolerances. Even with a PCV system the cause isn't getting any better so it's only a matter of time. I admit the PCV system might buy you a LOT of time but still it's on a declining scale.

For those who have no objectionable blowby (Tom?? Me??), consider the side effects of a PCV in return for little or no benefit beyond the slash tube.

[hanleyclifford]

The problem with the slash tube is that the flow is totally unregulated. But Neil is right in that first prize is tight tolerances.

[thatch]

Hanley,
I understand your comment about the slash tube not being regulated, but I tend to dissagree, at least in the terminology dept. The term "passive" more accurately describes the slash tube since the carburator intake flow doesn't really know that it's there. It just uses the airflow into the flame arrestor to pull the blowby gasses into the combustion process. It is however "regulated" by engine rpm's and flows considerably more "dirty air" at higher RPM's than at idle. The PCV valve on the other hand directly affects the carburator's mixture rate so it can be termed as "active".
I have noticed a considerably more "civil" approach in this thread than in previous similar threads on this topic, which I feel is a step in the right direction in making the PCV valve function properly on these engines, whether or not it belongs there.
Tom