Another Weber DCOE Post

[Inka’d02]

Howdy!

 

Another Weber DCOE question!

I have new motor with 9.5:1 and a Schrick 292, running 40mm DCOE's. I am having an issue with idle (of course). These were purchased in 2010 and were working fine on my old motor which has a stock cam.  When I let off the throttle it comes down to 1500 RPM and it then takes 20 to 30 seconds to slowly creep down to 1050 or so. It is REALLY annoying. I am running a cable set up pulling from the center. I have tried all different return springs from insanely stiff to no return spring, just the ones in the carbs. Throttle shafts move freely hot and cold. Synchronization is correct. Idle mixture screws are about 2.25 turns out. I feel like it must be a main to idle signal issue. Any ideas are appreciated.

 

Mains: 125

Idles 55f8

Emulsion: F11 

Air corrector: 180

 

Thanks!

 

 

[Chris_B]

I am not sure what "main to idle signal" is, because there is essentially no connection between the idle and main circuits other than possible overlap in the transition. This is a really common problem. You might want to try lowering your idle speed a bit. Also, carb to carb balance can have a big influence on this idle problem. At which RPMs did you balance  your carbs?

[Chris_B]

I should have explained. If your throttle plates are too far forward, they can draw fuel in from the first transition hole, which will cause the idle to not want to settle down. If you close your throttle plates at idle a bit, that might move your them enough to position them below the first transition hole sufficiently to resolve this problem. You really want your idle to be just a bit below 1K RPMs. 

Edited July 2, 2020 by Chris_B
Poor drafting

[2002Scoob]

What Chris has said^^

 

My first assumption was that you have sticky throttle shafts or linkage, which could cause them to not fully close, but then creep closed slowly... but you appear to have dismissed that. 

 

So like Chris, I'm gunna bet that your throttle plates are a bit too open at idle. You should check and confim this by pulling your progression caps, and shining a flashlight down the chokes to see where the edge of the throttle plate is sitting at idle (and take some photos!).

 

This will also help you visually asses if your mechanical sync might be a tad off. 

 

I've got a near identical motor setup, and it's happiest idling 920-1000.

[uai]

I‘d say you’ve got an ignition problem.

Get a ti distributor

[Preyupy]

Before you spend a ton of time and money playing with your carbs break out your timing light.  How much advance do you have at 800 rpm?  Now check it at every 200 rpm from there up to about 3000.  

 

WRITE IT DOWN

 

 Now let the throttle close and if it comes down to 1500 watch the timing change as it slowly goes back down to the 800 rpm idle. I’ll bet you have a tired distributor and the advance is not consistent down in the “idle” range. 2 deg of advance can make a 100-200 rpm change in idle speed without changing anything on the carburetors.  If the weights and springs in the distributor don’t reliably retard the timing as the engine gets down near idle you can’t set a reliable idle speed.

 

Just as an example if you have the throttle plates set at 1% open ( this is just theoretical) and the timing is at 8 deg BTDC you might be at 800 rpm.  If you advance the timing to 14 deg BTDC and leave the carburetors alone you could easily have the engine up to 1500rpm.  If you keep the timing at 14 deg you might need to go to 0.5% of throttle opening to get back down to 800 rpm. 
 

Now think about if your distributor advances 6 degrees between 800 and 1500 rpm how will you get it to go back to an 800 rpm idle?   You need to make sure the advance mechanism ALWAYS returns to a base setting by the time you are down to 1500-1700 rpm.  

[Hans]

Spanish or italian? The carbs that is.

[uai]

3 hours ago, Preyupy said:

Now think about if your distributor advances 6 degrees between 800 and 1500 rpm how will you get it to go back to an 800 rpm idle?   You need to make sure the advance mechanism ALWAYS returns to a base setting by the time you are down to 1500-1700 rpm.  

What he said.

But it might be necessary to recurve your distributor (no mechanical advance before ~1000 rpm)
Also take care that you're not advancing too early or too much.

On my distributor the spring preload is increased on both springs and both the stops of the weights are bent so it doesn't advance that much and the curve flattens earlier.
@'76mintgrün'02 has explained this in detail in his posts

Edited July 2, 2020 by uai

[Inka’d02]

4 hours ago, Preyupy said:

Before you spend a ton of time and money playing with your carbs break out your timing light.  How much advance do you have at 800 rpm?  Now check it at every 200 rpm from there up to about 3000.  

 

WRITE IT DOWN

 

 Now let the throttle close and if it comes down to 1500 watch the timing change as it slowly goes back down to the 800 rpm idle. I’ll bet you have a tired distributor and the advance is not consistent down in the “idle” range. 2 deg of advance can make a 100-200 rpm change in idle speed without changing anything on the carburetors.  If the weights and springs in the distributor don’t reliably retard the timing as the engine gets down near idle you can’t set a reliable idle speed.

 

Just as an example if you have the throttle plates set at 1% open ( this is just theoretical) and the timing is at 8 deg BTDC you might be at 800 rpm.  If you advance the timing to 14 deg BTDC and leave the carburetors alone you could easily have the engine up to 1500rpm.  If you keep the timing at 14 deg you might need to go to 0.5% of throttle opening to get back down to 800 rpm. 
 

Now think about if your distributor advances 6 degrees between 800 and 1500 rpm how will you get it to go back to an 800 rpm idle?   You need to make sure the advance mechanism ALWAYS returns to a base setting by the time you are down to 1500-1700 rpm.  

Thanks. I will look into this. For reference, I am using s NOS tii distributor which I switched to a Petronix. I will certainly check this. In your opinion what would you like to see for full advance? I have set to 33 degrees if I recall. Also, I have the idle set where it is because the engine rocks around at anything below 1000rpm. 

 

Thanks Again

 

[Inka’d02]

4 hours ago, Hans said:

Spanish or italian? The carbs that is.

Spanish.

5 hours ago, uai said:

I‘d say you’ve got an ignition problem.

Get a ti distributor

Has one

[uai]

1 hour ago, Inka’d02 said:

Spanish.

Has one

then I'd add a bit of preload to the spring of the first centrifugal weight in the distributor so it won't advance before 1000rpm (engine rpm)
(after having confirmed it's the underlying cause)
If you don't get a stable idle when setting the max advance at 33° (or better 32° - all depends on engine & fuel & compression etc etc)  then you have to limit your max advance so you can have a it bit earlier at idle.
But as stated @'76mintgrün'02 has written so much - not necessary to repeat.

 

this was my first attempt - I've changed more but haven't updated the curve since (engine rpm and degree - not distributor rpm & degree)
I had to flatten the curve between 2,5-3krpm as I still had pinging

Edited July 2, 2020 by uai

['76mintgrün'02]

1 hour ago, uai said:

then I'd add a bit of preload to the spring of the first centrifugal weight in the distributor so it won't advance before 1000rpm (engine rpm)
(after having confirmed it's the underlying cause)
If you don't get a stable idle when setting the max advance at 33° (or better 32° - all depends on engine & fuel & compression etc etc)  then you have to limit your max advance so you can have a it bit earlier at idle.
But as stated @'76mintgrün'02 has written so much - not necessary to repeat.

 

It sounds like you have a late style distributor uai.  Thanks for sharing your curve/process.

 

The Ti distributors were the earlier style, with two small symmetrical springs that rub alongside lobes on the center post and they stretch as it rotates.  The springs get a flat spot worn on the side that is up against the lobe, which makes them weaker over time.

 

There is an adjustment built into the early distributors that allows you to add tension to the springs by rotating the outer attachment pins, which may encourage it to return a bit better; while also slowing the start of the curve.  Unfortunately, you have to drill the pin and disassemble the distributor to get at the adjustment.  Fortunately, that is fun to do and not very difficult.

 

In both early and late distributors, you need to make sure the center post can move freely on the shaft.  Try grabbing the post and rotating it by hand and you'll feel the advance mechanism moving.  It should feel smooth and want to snap back to "zero".  I am guessing yours might be binding there.

 

You could try digging out the felt plug in the post and putting some oil down the center.  That is considered basic maintenance, but the instructions usually say to put a couple drips on top of the felt plug.  Pull the plug and put the oil underneath it, then put the plug back.

 

The early style distributors slide on a pad under the weights to prevent metal to metal contact with the plate below and the nubs on the bottom of the weights will eventually rub through it.  The pad is brittle and often breaks into pieces that can inhibit the weights' movement.  Those pads are available separately, (if you do decide to dig into it).

 

I am more than happy to type about this stuff, but you've probably already ordered a 123 by now.   

 

Tom

 

 

[uai]

2 hours ago, '76mintgrün'02 said:

 

It sounds like you have a late style distributor uai.  Thanks for sharing your curve/process.

I have both.

I use the modern to get the curve dialed in - as it's accessible for adjustments without disassembly.

Once it's finished on the dyno, I'll try to transfer the curve to the old style cast iron distributor.
I've attached a pic of my bending tools - as I know you like that kind of tools.

 

Edited July 2, 2020 by uai

[uai]

So red is about yours, and blue is for for engines with more compression and more cam.

So when you have it at 10° btdc at idle <1000 you'll end up with 32° at 3200 rpm.
As the 292 is still mild and I don't know the compression you should end up in between but do not have advance below 1000 rpm and a steep curve at the beginning

 

Edited July 2, 2020 by uai

[Chris_B]

8 hours ago, Inka’d02 said:

 I have set to 33 degrees if I recall. Also, I have the idle set where it is because the engine rocks around at anything below 1000rpm. 

 

 

 

The rocking can be a symptom of the carbs being out of sync in that RPM range.