1973 Dual Carb Bus - Hesitation At Low RPM

[mylopolis]

1973 dual solex, stock 1700

So I've gone and synced my carbs and she's running really well. Only major issue now is a lot of hesitation at low RPM. Tweaking the timing gets rid of it.

I've read and the common theory is the vacuum retard can isn't working, however when I pull the vacuum line from it the timing advances quite well.

Any thoughts? Anything I should measure (what should the vacuum on the line be, how far it should advance, etc?)

[Amskeptic]

1973 dual solex, stock 1700

So I've gone and synced my carbs and she's running really well. Only major issue now is a lot of hesitation at low RPM. Tweaking the timing gets rid of it.

I've read and the common theory is the vacuum retard can isn't working, however when I pull the vacuum line from it the timing advances quite well.

Any thoughts? Anything I should measure (what should the vacuum on the line be, how far it should advance, etc?)

I have no idea where you read The Common Theory, but let's not worry about it.

You *have determined* that the retard unit is placing your timing at 10ºATDC at an idle of 950-1,000 rpm, yes?

You *have determined* that the central idling circuit is hissing good and strong when you remove the air tube from its intake atop the left carb, yes?

Cuz, if so, you merely need to balance up your mixtures and the bus will run like fuel injection.

Remove vacuum retard hose from distributor at warm idle.
Remove wire from central idling circuit.

Pull 4mm hose from thermostatic vacuum valve in right air horn, the one with vacuum. Put finger over hose with vacuum signal. When you then crack a little air into the hose by rolling your fingertip away just a bit, the idle should go down just a bit. If idle goes down too much, dual carbs are too lean. Idle goes up? Too rich. Idle goes up a lot, the cause of your hesitation is here, rich dual carbs are drowning engine when accelerator pumps kick.

Now put wire back on central idling circuit, hose back on vacuum retard. Blip throttle to kick in the central idling circuit.
When you roll your finger off vacuum valve hose, idle should go up. If it goes down, the cause of your hesitation is right here, too lean central idling circuit.
Colin

[mylopolis]

Okay, I swear I got these right during my initial sync per your procedure.

However, double-checking now and the carbs were certainly rich.

I wonder if it's weather related or just beer-before-tuning fallout.

After correcting that, it's now less 'hesitant' and more 'gutless' at low RPM.

[Amskeptic]

Okay, I swear I got these right during my initial sync per your procedure.

However, double-checking now and the carbs were certainly rich.

I wonder if it's weather related or just beer-before-tuning fallout.

After correcting that, it's now less 'hesitant' and more 'gutless' at low RPM.

Hmmmmmmmmmm. Less hesitant and more gutless.

What is the idling timing (hoses on) AFTER you have adjusted the timing (hoses off) to 28* BTDC at 3,200 RPM?
Colin

[mylopolis]

I shall check. By "hoses off" do you mean the Vacuum Retard and the signal hose? or both Retard and Advance hoses?

[dhoch14]

both the vac advance and retard off. this will give your max centrifugal advance.

[Amskeptic]

both the vac advance and retard off. this will give your max centrifugal advance.

What we want to see is the range of your centrifugal advance.

If you have the centrifugal timing at 28* no fooling around, at 3,200-3,400 rpm, we want to see what the centrifugal timing is at idle. With the central idling circuit plugged in, the idle will be too fast and this will kick the advance. We need the idle timing under 1,000 rpm. Unplugging the central idling circuit should drop the rpms down around 6-800, and the timing at that rpm, I am interested to know. THEN plug the central idling circuit in, goose the throttle, stick the hoses back on and let us know what your idle timing is now.
Colin

[mylopolis]

Timing set to 28' at 3200rpm

Idle with hoses and central idle off = 820
Timing is 8' ATDC

Idle with hoses and central idle back on = 890
Timing is 12' BTDC

Runs much better at these settings, although hesitation/gutless has turned into "stuttering" at low RPM's.

Interesting thing I noticed: With everything hooked up, when I come off of high RPM my central idle isn't working. Blip the throttle and it comes back on.

Vacuum is bouncy around 12-15"

[mylopolis]

Another confusing observation:

I've adjusted the carb mixtures to very slightly lean, idle drops with reference hose off.

Then I went to adjust central idle. First confirming hissing, I can then adjust it to lean, with idle dropping with reference off. Then when I go to enrichen (is that a word?), it hits a point where the idle very slightly bursts up, before settling back on 890 again.

No matter how much I open the central idle mixture from then out does not seem to make a lick of difference.

My expectation: More rich, more RPM.. but alas it's stuck on the same behaviour for (at least) 4 full turns out.

[Amskeptic]

Another confusing observation:

I've adjusted the carb mixtures to very slightly lean, idle drops with reference hose off.

Then I went to adjust central idle. First confirming hissing, I can then adjust it to lean, with idle dropping with reference off. Then when I go to enrichen (is that a word?), it hits a point where the idle very slightly bursts up, before settling back on 890 again.

No matter how much I open the central idle mixture from then out does not seem to make a lick of difference.

My expectation: More rich, more RPM.. but alas it's stuck on the same behaviour for (at least) 4 full turns out.

If you cannot get a slight uptick in idle rpms while testing the central idle mixture, I have to ask, can you get idle speed to go up with big brass speed screw to let's say, 1,000 rpm? Sometimes the mixture of the central idling circuit cannot get a decent "pull" through the emulsion tube if the idle speed of the central idling circuit still doesn't have enough airflow.

If you cannot get the mixture slightly rich and the speed up to 1,000 with your central idling circuit screws, then you have to ask the dual carbs to help out a little.
a) increase throttle stop screws 1/4 turn each (use Uni-Syn to snchronize if you like)
b) increase the two dual carb mixtures from "slight rpm drop" at reference hose, to "remains just about the same".
c) now see if you can get a slight uptick in rpms when you test the central idling circuit mixture and see if you can get an idle speed of 950-1,000.
Lemmeeeeeeeee know.
Colin

[mylopolis]

If you cannot get a slight uptick in idle rpms while testing the central idle mixture, I have to ask, can you get idle speed to go up with big brass speed screw to let's say, 1,000 rpm? Sometimes the mixture of the central idling circuit cannot get a decent "pull" through the emulsion tube if the idle speed of the central idling circuit still doesn't have enough airflow.

If you cannot get the mixture slightly rich and the speed up to 1,000 with your central idling circuit screws, then you have to ask the dual carbs to help out a little.
a) increase throttle stop screws 1/4 turn each (use Uni-Syn to snchronize if you like)
b) increase the two dual carb mixtures from "slight rpm drop" at reference hose, to "remains just about the same".
c) now see if you can get a slight uptick in rpms when you test the central idling circuit mixture and see if you can get an idle speed of 950-1,000.
Lemmeeeeeeeee know.
Colin

Okay,

With the central idle tube off, I can get the RPM's to react a lot better with the air screw. Once I put the tube back on, they drop like a brick (650)

I ended up opening the air screw a half turn and, again, I see an uptick in the RPM's on the reference hose but it's short lived. Making that change certainly helped (although not completely eliminated) the flat spot at low RPM's.

I'm still very confused though, why does the central idle not "come on" when coming down from high RPM's? I constantly need to 'blip' the throttle when I slow down, to keep the central idle on. Otherwise idle RPM's drops to 500-600 and no sucking through the central idle intake. A quick blip, it comes back on, RPM's back to 950, and we're back in business.

In other news, I finally fixed my rear hatch. Just had to pop the panel off, cleaned the stuck latch with some brake cleaner and hit with lithium grease. Works like a charm!

[Amskeptic]

With the central idle tube off, I can get the RPM's to react a lot better with the air screw. Once I put the tube back on, they drop like a brick (650)

Rich. Very rich. Float levels? Dual carb mixture screws too rich?

I'm still very confused though, why does the central idle not "come on" when coming down from high RPM's? I constantly need to 'blip' the throttle when I slow down, to keep the central idle on. Otherwise idle RPM's drops to 500-600 and no sucking through the central idle intake. A quick blip, it comes back on, RPM's back to 950, and we're back in business.

Lousy electrical supply? intermittant power? Not secure in the carb? Plunger has insufficient preload? I use a little copper washer and do NOT use a rubber washer to seal it to the carb, because that plunger needs its grounding through there.

In other news, I finally fixed my rear hatch. Just had to pop the panel off, cleaned the stuck latch with some brake cleaner and hit with lithium grease. Works like a charm!

Well, right on. When I was there, it looked like you had a lot of work to get through, is it all under control and fun again?
Colin

[Amskeptic]

I do have electrical problems. My alternator can barely keep the battery charged and requires quite high revving to keep the headlights bright. You think that might be causing the idle plunger to close?

I thought even with the cutoff plunger closed, it would still suck air, no?

Regarding the rest, still a lot to do, but now it's a debate between going somewhere and fixing something :) Today was a trip up the hill to wine country with a nice afternoon nap in the back. Sure, I coulda tweaked carbs all day, or I could enjoy this beautiful sun with the family :)

Central Idling Solenoid closes off the airflow.
Further up, your timing numbers are giving me questions.
You mentioned that at 28* BTDC centrifugal, that your idle timing is 8* ATDC without hoses, and 12* BTDC with the hoses. This is bass ackwards. Typo?
Colin

[mylopolis]

I do have electrical problems. My alternator can barely keep the battery charged and requires quite high revving to keep the headlights bright. You think that might be causing the idle plunger to close?

I thought even with the cutoff plunger closed, it would still suck air, no?

Regarding the rest, still a lot to do, but now it's a debate between going somewhere and fixing something :) Today was a trip up the hill to wine country with a nice afternoon nap in the back. Sure, I coulda tweaked carbs all day, or I could enjoy this beautiful sun with the family :)

Central Idling Solenoid closes off the airflow.
Further up, your timing numbers are giving me questions.
You mentioned that at 28* BTDC centrifugal, that your idle timing is 8* ATDC without hoses, and 12* BTDC with the hoses. This is bass ackwards. Typo?
Colin

Er, yes, typo. I always confuse B and A ;)

28* BTDC centrifugal @ high RPM = 8* BTDC without hoses and 12* ATDC with hoses.

[Jaffa]

I went through a similar issue 2 years ago. Did all of Colins fantastic tuning procedures, but also rebushed the throttle spindles to elliminate air leaks.

Could you have leaking spindles causing a lean mixture coming off idle??

Craig

'74 1800 dual PDSIT