Racing Fuel Systems

Hello All,

Just curious to hear how you all get the best idle quality and what your steps you do to get it.

My complaint right now with my engine, unless I am idleing 1700-1800 rpms, when I pull it into gear, it will pull the motor down to about stall. It is a dominator on a 730 hp SBC 409.

IAB is .045, and the idle feed is .050.

What do you all typically do with timing at idle?

Thanks

corrections, it has a .055 ifr.

Currently the idle screws are out 1.75 turns.

Was thinking about drilling and tapping the IFR to a 6-32 or 8-32 so I can try a larger restrictor.

Any opinions?

You start tuning for idle by not touching the carb!

You get the idle ign timing sorted out first.

Which may be anywhere from 15 to 55*, depending on cam duration & other factors [ Yes, 55* at idle! ]

From D. Vizard, author of 30+ automotive books. Until I can find someone with more experience, I will take his word. Hint: this quote about idle timing is in a carb book.
" The optimum idle advance is typically 35-40* for a short cammed street engine & [ though not commonly realized ] as much as 50* for street/strip engine."

Use a locked dist, programable box or man connected vac adv to get the reqd timing.

Once this is achieved, square up the T slot in the t/bores [ bypass air holes may be needed in the t/blades to achieve this ]. Once this procedure is done, you may find you get a good idle without changing fuel or air jets.

So what is your method of determining how much idle timing is needed?

I have it locked at 34, but I have a 7530t, so rolling timing in at idle would be easy.

Trail and error, do you have a dyno curve for this engine? That could provide the WOT timing curve.
Vacuum advance can provide up to 20 degree extra at idle using manifold vacuum.
My msd 6al2 programmable uses a GM map sensor to give extra timing at idle and high vacuum operation.

I am good on WOT, I was just trying to improve idle quality and not have it drop 500 to keep it from dying when I pull it into gear.

I am going to try and throw some more timing at it at idle.

Locked at 34* sounds like going in the right direction.
Finding optimum idle timing is actually very easy.

Warm up engine, idling, put into gear. Loosen dist clamp & move dist for highest idle rpm. If you are starting from 34*, there will only be a slight improvement going higher than that, but worth doing. Once you have the highest rpm, toggle the dist adv/ret to make sure you have the highest rpm, the sweet spot. Now, check what the timing is...& that is what you need.

Make sure your timing DOES NOT CHANGE going from N into gear. This is the leading cause of problems. If timing does drop, it could be the centri weights started deploying in N but dropped back when going into gear. Probably the most common cause is people using stock VA units with low vacuum cams. Never gonna work!!
You need an adj VA unit, with the Allen key fully CW as a staring point. These can add up to 30* of timing for idle. My GTO idles with 48*

GTO Geoff

If you don't mind sharing , what are your cam specs including LSA?

Sol roller cam 236/232 @ 050. 106 LSA. Yes, less exh duration.

What fuel?