holley gurus

Hi i have a faily low run time 770 avenger carb, the carb is about 3 yrs old, but has low run time.. the problem is with the idle.. it sux and is very rough.. I am not familur with problem solving this carb.. what should i look for, so far the float level is ok, the choke is ok.. the fuel psi is about 3-4 lbs.

I can adjust the mix screws with little afect..

the plugs look blk..

is there anyone here i can send it to


here are the specs

SB 410 stroker

9.6 cr
ported 587 heads with 2.02 valved
comp cam XE274 230/236@488/491 lift with matching springs.
rpm airgap intake.

msd E-curve dist
hooker headers.
904 auto
355 gears
street driven

I had 770 that the plating exposed to gasoline was deteriorating and coming off in the metering block and float bowls getting into places that it did not belong causing a very poor idle.

Whats the base ignition timing?
Very crucial to idle performance

I would think a combo like that would like 18 degrees or more.

As stated above initial timing is critical to idle quality. I would try 14 to 16 initial with that combo. Its possible the power valve could be opening due to low idle vacuum increasing the initial timing would help this also.

i have tried my base all over the place, i can make it worse but not better..anywhere from 12 to 22

it could be a number of things. it could be a blown power valve. is it rich off idle as well? power valve opening at idle does not enrich the idle mixture. has the carb been modified or tinkered with? the idle feed restriction may be too large. do you see any fuel dribbling from the booster venturis at idle?

All good advice so far.

You say the carb has low hours on it. Has it always idled rich (we're assuming), or is this a new occurrence?

Be sure the idle air bleeds are not clogged. Look down into the top of the carb, into the venturis. On the primary side, you will see 4 little brass orfices that are pressed in. The outboards are the IABs. Run some very fine small diameter solid wire down each one, and follow it with a good blast of carb spray cleaner. Preferably, this will be done with the float bowls and metering blocks off so any dislodged crud will be washed out. Do the same proceedure on the secondary side. Blocked IABs will cause a rich idle.

While the metering blocks are off, take a large good quality file and carefully file the surface of the main body where it mates with the metering blocks. Rarely is this surface flat, and if it's bad enough, you can get internal leaks because the metering blocks are not seated tightly against the main body.

Put it back together using only genuine Holley blue reusable metering block gaskets. No cheap cork or (shudder) rubber gaskets.

If you have an electric fuel pump laying around, pump the carb full of gasoline and let it sit on the workbench for a day or two. If the power
valve(s) (I think your 770 has only a PV in the primary side) have a ruptured diaphragm(s) as suggested above, gasoline will leak out through the rupture and out onto the bench. If you don't have a pump, put the carb on the car, start the engine (to pump it full) the renove the carb and leave it on the bench for a while to observe for leaks. Find 4 long bolts and 8 nuts and put the carb "on stilts" for easy observation.

ETA: I missed this the first time:

"I can adjust the mix screws with little afect.."

Flip the carb over and look at the relationship between the throttle plates and the transfer slots at idle. The throttle plates should expose enough of the transfer slots at idle that all you see is a square. See photo. If more transfer slot is exposed, that could cause your problem. Shout back if this is the case.


Jim

kool thanks for the advice.. the carb was new about 3 yrs ago.. but has had the problem since i took it out of the box and installed it last year.. the holley warrenty is expired. i will check all of this out as soon as i can..