1973 Dual Carb Bus - Hesitation At Low RPM

[Amskeptic]

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

Central idling circuit can easily overcome throttle bush leaks. Unfortunately, myopolis has new and more ominous symptoms from deep within the engine.

How did the carb tuning procedure work for you?
Colin

[mylopolis]

Okay, kerthhhhhhhhhunking out of the way, we're back to carbs.

New observations: hesitation is worse when cold.

Tweaking timing has little effect.

Every time I come off high RPM's my central idle is "off". This often equates to dead-stalling at stop signs. Blip throttle (which is hard to do when braking) and it comes back.

[Amskeptic]

Okay, kerthhhhhhhhhunking out of the way, we're back to carbs.

New observations: hesitation is worse when cold.

Tweaking timing has little effect.

Every time I come off high RPM's my central idle is "off". This often equates to dead-stalling at stop signs. Blip throttle (which is hard to do when braking) and it comes back.

Timing will have little effect. Timing is not related to off-throttle behavior.
Worse when cold and no central idling when coming to a stop tells me that you need more central idling airflow and a richer mixture. You mentioned earlier that the central idling circuit behaved poorly when putting the intake hose back on, and that you could not get your idle speed up past 890. Seize the bull by the horns. You have enough foundation here to start considering tweaks. If you cannot get the idle speed up, increase timing from uour current 12* ATDC towards factory 10*ATDC. If you cannot get the central mixture to richen up, is the float level too low? If you take off the top of the left carb, will you find a cracked emulsion tube? Based on your adjustments thus far:

What is your current warm idling speed?
What is the mixture when on dual carbs only (no retard)?
What is the mixture when central idling is working (retard on)?
What is your current idle timing?
Colin

[mylopolis]

Will get the numbers soon, but made major progress increasing central air and mixture! Only a tiny flat spot left and a whole lot more power along the way.

I also believe you tagged it right and that electrical was an issue. I took off the central idle cutoff and sanded it's contact along with the carb-side contact. Torqued it back in and the central idle die-off seemed to disappear. Probable bad ground makes for bad central idle!

And to answer your previous question; Now that the panic of my nasty exhaust leak is over, it's back to being fun again :)

[mylopolis]

So, I apparently have a choice. Either a flat-spot of hesitation at low RPM's, or a very high idle.

Increasing the idle air dramatically helps the hesitation, but she just won't settle down at a stop sign.

Ideas?

[Amskeptic]

So, I apparently have a choice. Either a flat-spot of hesitation at low RPM's, or a very high idle.

Increasing the idle air dramatically helps the hesitation, but she just won't settle down at a stop sign.

Ideas?

With a genuine 10* AFTER (to the right of 0) TDC timing, you should have both plenty of idle air and a settled down idle speed. Is the left throttle plate stop properly adjusted? Actually both of them, the stop screws should be no more than about 1/4 turn past initial contact:

(#5 orange screws)

[mylopolis]

I'll take another look at throttle stop screws, but they were previously adjusted well.

I set the timing back to 8* After, which made a helluva difference in the 'noise' department.

[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

Central idling circuit can easily overcome throttle bush leaks. Unfortunately, myopolis has new and more ominous symptoms from deep within the engine.

How did the carb tuning procedure work for you?
Colin

Colin, your tuning procedure worked fine, just as you advised. I did have to alter the accelerator pump linkages though, way too much fuel being squirted in when coming off idle. As well as the throttle bushes I fitted new gaskets and set the carbs up as per the Bently and the Aussy Gregorys manuals. So I knew I had good carbs.

Craig

[mylopolis]

Word of advice for carb tuning:

Before following this procedure, ensure that the crappy tachometer your friend gave you ACTUALLY WORKS RIGHT.

Turns out mine was reading 1/2 the RPM's. No, it just wasn't one of those "double for 4cyl" things that I got wrong. It really was just dead-wrong. Piece of crap has been hosing me for months!

Got a new one at a garage sale over the weekend, kick up my idle air to bring it to (actually) 1000 for the tuning, and holy cow do these instructions make way more sense!

Unfortunately, now I gotta start the whole process over to 'perfect' it, but my flat spot is almost completely gone with the much higher (as intended) idle.

Yay!

[mylopolis]

For the record, a major improvement was gained (if not permanent resolution) by jetting the carbs up to 137.5's on the main jets!

The '73 stock had 127.5's, which is not what Mr. Bentley claims it should be?

I got the 137.5's out of a donor set from a '72

Must've changed with the Smog equipment (which I had removed).

Muuuuch better now :) And a thrill to drive again!