Idle adjustment DRLA 40s

Any chance that the linkage or throttle system components are expanding causing tension thereby increasing idle speed? Is there little to zero slack on the throttle cable where it attaches to the linkage?

What Robert said, leave some slack in the cable when the car is cold.

Also, loosen the little 8mm locknuts and CLOSE the idle air bypass screws.

Never thought about that. Great suggestions, I will check that throttle cable to see if it’s overly tight. When driving around at stoplights if I find the rpms a little high I kick the accelerator pedal rapidly and the rpms come down. Sometimes they go up again gradually but sometimes stay low. Will check on that. Thank you.

@Impala posted:

Never thought about that. Great suggestions, I will check that throttle cable to see if it’s overly tight. When driving around at stoplights if I find the rpms a little high I kick the accelerator pedal rapidly and the rpms come down. Sometimes they go up again gradually but sometimes stay low. Will check on that. Thank you.

Could be an indication there's something hanging up there. I forgot to mention to check where the throttle cable exits the Bowden tube and where it exits the shroud. Lots of places to get hung up. A dry graphite type lube may fix some of those issues.

Question: where is the initial timing set? The advice is always to set the timing for 30- 32 degrees total advance-- but the quality of distributors varies wildly, and some of them end up at 8 -10 degrees advance at idle if you set to 30 deg at 3000 RPM.

That may be enough to give you a too-fast idle.

I ran 8-9 degrees at idle with the Megajolt. No trouble with idle speed at all. If the distributor advance plate is hanging up, yeah that would do it.

I did have my cable set with ZERO play cold when I first built the car, and when the engine warmed up, so did the throttle housing. All the sudden it was idling at 1500-1800. I loosened the cable and it permanently fixed it.

Disconnect the cable and see what happens when it's hot, if the idle goes down. If you can't get it to idle low close the darn idle air bypasses. Either that or you have a vacuum leak. I'd rock the intake manifolds and see if they've worked loose.

Great suggestions. Originally timing was set at TDC at idle and it was still doing the same thing. We set it up to 7.5 BTDC as per Aircooled.net instructions on the SVDA; the advance is working fine as advertised when taken to higher rpms. The car seems to work a little better than before. I will loosen the accelerator cable a little bit and experiment with that too. Thank you all.

Your cam is significantly hotter than the one in his car, Danny.

My money would be on the vacuum leak or linkage-- but a mild cam, larger than average carbs, and a bunch of initial advance will bump up the idle when warm.

Can somebody please tell Impala here to close the damn idle air bypasses? It takes no time at all to do that, unless the screws are frozen, as they tend to be on 30 year old Dellortos. My money is on that, or an intake manifold leak.

I gave the accelerator cable some slack. Didn’t change much except take the accelerator pedal lower to the floor. I will check on those idle air bypasses. When I got these carburetors from Gabrielle in Italy he expressly directed not to touch those as they came calibrated with factory equipment that he has. I recall they even came with some paint on the screws. I will look into that.

“When I got these carburetors from Gabrielle in Italy he expressly directed not to touch those as they came calibrated with factory equipment that he has.”

That’s great, (he might even use a flow bench) but once you place them on the intake manifolds, everything has now changed to what your engine is doing, not his “factory equipment”.  Everything I’ve read and experienced with my Dells is start with the air bypass (bleed) screws all the way in and then adjust them out individually, as needed, as you sync all four throats.  

You’ve convinced me, guys. I will try that now. I have the Dellorto Book from CB Performance as backup too. I’ve also been talking with John Connoly from Aircooled.net about getting a crisper response on the pedal and solving a sporadic spitting issue through at least one of the carburetors on occasion when shifting from 1st to 2nd and even 3rd and he suggested installing an anti pulse valve between the carburetor and the vacuum advance of the distributor to smooth out the vacuum signal. Apart from that and looking at my engine specs and compression ratio (around 8.5:1) he suggested downsizing the idle jets from the 60’s I have now to 55’s.

@Impala

Roberto!    How'd you make out with the carb adjustments?

Thanks for asking, my friend. I’m ashamed to say that I haven’t touched the car since my last post. In fact I haven’t even gone to the place where I have it stored. I intend to go out on a drive today since it’s beautiful down here but I intend to get to it sometime.

We went through the spark plugs and they looked consistent and perfect; which tells me the jetting and mix are fine. We cleaned, re-gapped and re-installed them since they didn’t have that much mileage. We adjusted the timing, due to my mishap with the Pertronix module inside the distributor coming loose. Finally, we also installed a new anti-pulse valve on the distributor vacuum hose, which we sourced at Aircooled.net. I haven’t had a chance to drive it much since but for the short time that I drove it last for a gas station fill-up nearby it felt better and it idles a little lower. Not sure if it’s a placebo effect but as I drive it more I’ll report on the findings.