Engine idle

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Clackety Clack
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Engine idle

Post by Clackety Clack » Tue Dec 27, 2016 12:34 am

I have this backed out exactly 2.5 turns. What happened was I installed a new distributor. A stock 205t. I took it for a ride and the idle was a little lumpy. I foolishly changed the idle speed without double checking the timing, which was retarded by a few degrees. I reset the timing and turned the bypass (large) screw all the way in and backed it out exactly 2.5 turns. This is what it sounds like. I don't own a tachometer and am looking for some advice on any further adjustments, and by how much.

https://youtu.be/EVjTHZJckJ0

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Re: Engine idle

Post by Marc » Tue Dec 27, 2016 2:21 am

First let's address the timing. The 113 905 205T (Bosch 0 231 137 035 or 036) is a vacuum-advance only distributor for which the correct initial timing setting is 0° (TDC).....from there it'll pull 32-35° BTDC total when the ported-vacuum signal is sufficient to max it out.

The factory-recommended method is to use a strobe light and set the timing to TDC with the engine idling at 800-950 RPM (right in the middle at 875 is good) and the hose disconnected and plugged (actually it won't make much difference if you plug it or not, but officially you shouldn't have the carburetor sucking in air - as for the vacuum canister on the distributor, plugging that off makes absolutely no difference).

One of the common "failure modes" for this family of distributors is for the return spring to lose effectiveness with age, with the result that the timing may already be partway into the advance curve at idle. A quick check for that is to momentarily connect/disconnect the vacuum hose while watching the timing with your strobe light and listening to the idle speed - it should NOT change if your idle speed is 950 or less. I know, you don't have a tach - but 950 is starting to sound pretty busy, to the point where people in cars alongside you at a stoplight may stare.

If there's any doubt, simply set the timing to TDC "statically" (using a 12V test lamp with the engine off). That'll allow the total timing to be correct even if it's off at idle. Still not good and something that needs to be addressed, but it'll get you down the road OK.

It's difficult to tell for sure from the sound track of your video, but my impression is that the idle's a little high right now. Once everything's adjusted, give the throttle a rapid blip/let-off to make sure that the idle recovers without stumbling/dying; if that's a problem raise the speed a bit until it isn't ;)

Now the carb:

The terminology for the two screws on the carb may vary depending upon what book you're looking at. The smaller one is the idle mixture, or fuel volume screw, while the big one is the bypass screw and is how the idle speed's adjusted. DO NOT use the screw which contacts the choke cam to set the idle speed, it should at most only lightly contact the last step on the cam when the choke is fully off.

That 2½ turns-from-shut setting is for the small volume screw, and is just a starting point that's usually somewhere close enough to get the car running, as the carb ages and starts to draw in unmetered air along the throttle shaft it's commonplace to need to back that mixture screw out further to compensate somewhat. I've never encountered one that needed to be out more than 4½ turns from shut unless there was some other issue such as an induction leak downstream of the carb. If you want to run it in to fully-shut while counting turns to see where you're at now, be gentle - turning it in too firmly can score the end of the needle and make it unpredictable when making fine adjustments.

There really is no "typical" setting for the large bypass screw (too many factors affect where it'll end up once the idle speed is correct) but more often than not 2½ turns out from shut would be slightly excessive. The timing has a big effect on it, it's not uncommon for it to be nearly shut to hold the idle down with a 0 231 178 009 centrifugal distributor, or for that matter any distributor that has an initial setting close to 10°BTDC.

The mixture and bypass screw settings can interact so it's standard practice to need to work back & forth between them as you zero in on the final idle mixture & speed.

This procedure mentions the 34PICT-3 carb, but the adjustment steps are the same on your 30PICT-3.
http://www.vw-resource.com/34pict3.html#carb

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Re: Engine idle

Post by Clackety Clack » Tue Dec 27, 2016 7:06 am

That's a really long and great response. I bought the refurbished 205t from a respected builder, who sent it to another respected old vw mechanic to test on his sun machine. The distributor worked perfectly. I was instructed to install the distributor and set timing statically for the number one cylinder. I did so and the car seems to run a lot smoother than the Chinese 009. This carburetor was tuned over the summer when the accelerator pump diaghrafm was replaced. I touches that big screw and nothing more. So, from the sound of the motor, what rpm is it now, and how far would you say I need to screw it back in?

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Re: Engine idle

Post by Marc » Tue Dec 27, 2016 3:40 pm

Best estimation I can make from the video is ~1000 RPM. With the engine fully warmed up, check that the fast-idle screw just touches the cam on the choke shaft, then slowly turn the big bypass screw in to bring the speed down, but not so far as cause a "rolling" idle - too low and one or more cylinders may start to intermittently misfire; if you're reluctant to tweak the mixture any, let's just go with your presumption that it's spot-on for now. There's no way to accurately predict whether that'll take a ¼ turn, ½ turn, etc. on your particular car.

The design cut-in speed of your generator is about 750 crankshaft RPM; so IF everything's working as-new the charging system warning light should be out at a normal idle speed - it may be right at the point where it begins to flicker slightly (particularly with the headlamps on).

The oil pressure warning light should also be out; after a freeway run on a warm day you may see it flicker as you come to a stop at the end of the off-ramp - that's still within "normal" because at that moment the engine & oil are still hot but the cooling fan and oil pump are both turning slowly. That light comes on at around 2½ psi and it's never a welcome sign because you don't have any way of knowing how much it's below that, so if you see it you should speed up the engine slightly until it goes out. By the same token, if you're seeing the oil pressure light come on frequently at stops (assuming the level's OK, the oil is good and of the proper viscosity for the ambient temperature, and the engine's not worn out or overheating for some reason) you could simply adjust the idle speed upwards a hair and not lose any sleep over it.

If you don't own a tachometer, that's about all I can offer. Perhaps you'll have an opportunity to connect a friend's sometime just to cement in your mind what 875 RPM sounds like on your engine ;) But don't obsess over it, anything close is still acceptable...

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Re: Engine idle

Post by Piledriver » Tue Dec 27, 2016 6:31 pm

Most major auto parts chain stores will "rent" you a timing light, (buy/return w/full refund) or use one in the parking lot for free.
The good digital timing lights have a tach built in, almost like cheating.
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Re: Engine idle

Post by Clackety Clack » Fri Apr 21, 2017 2:06 pm

So I'm ready to take the bug out soon. Here is what is going on. I had a 009 on the car that was timed and the carb was properly tuned. In December I had an original 205t rebuilt and restored. I installed and timed it. The car ran fine with no problems. Much smoother. Now the only thing I noticed was the idle was much lower and the bypass screw was almost turned all the way in. I turned it out a bit and the idle came up nicely. Now that the spring is here, I've had all winter to think of this. Can you do what I did? Or do you need to Futz with the mixture screw as well. Like I said, I changed distributors, didn't touch the carb, ran it, noticed the idle was a bit low and just backed the bypass out a bit.

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Re: Engine idle

Post by Marc » Fri Apr 21, 2017 2:53 pm

IF you take out a 0 231 178 009 centrifugal-advance distributor that was set at ~10°BTDC initial (in order to achieve the desired ~30° total at 2500+ RPM) and install a vacuum-advance-only distributor that's set at 0° initial, the idle speed will be lower - just due to the later timing - so it's expected that you'd need to back out the bypass screw a little to bring it back up.

With the 009 it's typical that the idle speed can be a little hard to bring down; even with the bypass screw shut it's sometimes necessary to lean the mixture slightly, so it's possible that your mixture MAY be a bit leaner than normal, but if you aren't experiencing any stalling it should be close enough.
Should you want to experiment with it, turn the mixture (A.K.A. "volume) screw in gently until it stops, keeping track of how many turns that takes for reference, then back it out the same number of turns. When the engine's at normal operating temperature and the choke's off, try turning it in ¼ turn at a time; as soon as the engine starts to stumble back it out ¼ to ½ a turn which should return it to a smooth idle. There'll be some interaction between the mixture and the bypass screw settings so you may need to tweak them both.

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Re: Engine idle

Post by Clackety Clack » Fri Apr 21, 2017 3:06 pm

Thanks. I don't have any stalling issues, and I did buy a dwell tachometer around December but have not used it yet. I plan on checking my idle speed tonight. I have no experience adjusting these carbs, but is the correct way to turn the volume screw in, back it out 2.5 turns then turn it out until it reaches the highest rpm, then turn it back in until the idle dips 20-30 rpms? Then set the bypass to 800-950?

I'm thinking leave it if I have no running issues?

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Re: Engine idle

Post by Clackety Clack » Fri Apr 21, 2017 6:07 pm

So I went out and did this just now. Here is a video of the car running. I turned the volume screw all the way in gently and backed it out 2.5 turns. Connected the meter and turned it until I got the highest rpm. It only went up about 40 rpm or so. Then I backed it off 25 rpm and set the idle to about 925 or so. The volume screw is about 1.75 to 2 turns out from shut and the bypass is about two turns out from shut,is this typical? The volume screw was initially 2.5 turns out.
https://youtu.be/5CZ8NY4V7s8

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Re: Engine idle

Post by Piledriver » Fri Apr 21, 2017 6:29 pm

FWIW I agree with Marc and suggest do the adjustment the opposite way, running the fuel screw out for max RPM, adjust air screw, repeat both until desired idle RPM is reached, then as the last steps the fuel screw goes in just until RPM drops, then back out ~1/2-3/4 turn.

This is essentially the same as the "lean best idle" procedure on Webers, almost all carbs in general, or on CIS injection for that matter.
(using the idle air screw and CO adjustments, which more or less are equivalent function to the idle air and fuel volume screws)

Running the screw in after best idle speed insures you are leaner than ideal, which generally guarantees a rough idle or at least has no room for fuel/weather variation. A little richer is usually a better plan. Finding where RPMS drop is just to find the point to go richer from.
Last edited by Piledriver on Fri Apr 21, 2017 6:42 pm, edited 1 time in total.
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Re: Engine idle

Post by Clackety Clack » Fri Apr 21, 2017 6:40 pm

Doesn't it depend where you start with the mixture screw? If I started 2.5 turns out like vw resource says, I had to turn it in, in order for the idle to go up. So I effed it up?

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Re: Engine idle

Post by Piledriver » Fri Apr 21, 2017 6:47 pm

Presetting the screws just provides a sane starting point so it runs.
If the final settings are way off normal, you may need to look for a vacuum leak or such.
It doesn't sound like you have a problem, though.

The idea is to end up 1/2-3/4 turn richer than where the RPMs drop off, which should be around max RPM.

You have to start at the beginning, and understand the goal---
The goal is correct idle RPM, and a stable best lean idle fuel adjustment.
The former is nice, the latter is vital.
Finding the RPM dropoff point is the threshold, backing out the fuel adjustment 1/2-3/4 turn is the extra fuel for insurance.
But-- You start by finding the MAX RPM fuel adjustment for the required air volume (idle speed) setting.

On any modified engine, the "factory" idle RPM is kind of meaningless, and if you don't have 100% exact factory spec parts, it's modified.
I would do the best lean idle as described, and if the RPM ends up a little high due to base timing, that's just your new "normal".

Also note that the cooling fan puts out ~4 times the air at 1000 RPM it does at 500, where some folks seem to want to idle at just because they can.
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Re: Engine idle

Post by Clackety Clack » Sat Apr 22, 2017 7:43 am

I wound up putting the mixture back where it was before the distributor change and just used the bypass to get the idle to 900. It seems to be running really well.

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