Idle tuning

[55 Tony]

Hi I just rebuilt a 17084266 and sorry to say I did not use Cliff's kit.  I just found this website and it looks like I hit the jackpot!  I did buy his book but I'm ADD and skipped through it a few times not sure what all I've grasped.

I had to turn up the idle to keep it running, I forget how many turns first on the cold idle then also 2.25 turns for the warm/hot idle.  I have a wideband  A/F gauge so that helps tremendously in tuning.  Anyway I've opened up the two idle mixture screws just about as far as they go and still it's very lean at idle and doesn't want to idle, (making me turn up the other 2 screws).

The main idle speed screw I had adjusted while it was apart and set the butterflies half way into the Oh can't think of the name right now, the idle *slots*?  To keep it running I had to turn the screw to open the butterflies 2.25 turns in.  Still at idle the A/F gauge reads 17 to pegged past 18 with the idle mixture screws probably close to falling out!

I took it out for a test drive using the same 72 jets and 51 rods it came with and I forget the secondary rod letters, that it came with but the mixture is GREAT throughout rpm ranges and WOT.  It's only the idle that is so lean.

[Frank400]

It would really help knowing what type of engine it's sitting on and also, what was done to the carb (modifications, if so) beside "rebuilding" it.

   Frank.

[55 Tony]

I made no modifications to the carb, just rebuilt it.  It is an 800cfm.

It is on a modified 454 10:1+ compression, a healthy roller cam, Edlebrock rpm manifold, 1.75" headers, timing 7

[55 Tony]

I don't know what is going on with my post but I had writen this?

Timing 7

[55 Tony]

OK this newsgroup must not like the degree symbol, it cuts off everything after it, I'll try a third time.

Timing is 7* static, 37* all out, plus vacuum advance ported to manifold.  Let's see if the third time is a charm.

[77cruiser]

What's the cam?
You really need more initial timing.
How much vacuum do you have at idle?

[55 Tony]

No to more initial timing advance.  At 8-9 degrees it's hard to start, kicks back and breaks starters.

Idle vac is 15 but the idle is a bit fast at 800.

Cam:
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=436&sb=0

Previous q-jet idled as low as 500 in gear when good and hot.  (17058212 I think?)

[77cruiser]

Your vac. adv. is hooked to manifold vac.?
Probably needs to have the idle circuit enlarged to provide more fuel.

[Frank400]

Yes, lots more idle fuel with a big roller cam. Most every other circuit will need attention as well, in my opinion.

   Also, if that is a healthy roller cam, 800 rpm at idle isn't that fast.

[55 Tony]

Yes vacuum advance is to the manifold.  I tested every port on the carb and none gave more vacuum than the manifold.  The one ported one was about equal but with more of a lag when accelerating.  Didn't all cars have vacuum advance to the manifold before smog laws came around?

As in my original post, all other circuits are working perfectly throughout their ranges.  No need for changes there, I am working on the idle only.

[Cliff Ruggles]

You will need to modify the idle system for that much camshaft. 

I would get into the book deeper to familiarize yourself with what will need to be done to the idle system.

Since you didn't use our kit, I'd also recommend installing the high flow N/S assembly, small float, HP accl pump and secondary cam/spring, at a minimum.....Cliff

[Cliff Ruggles]

"No to more initial timing advance.  At 8-9 degrees it's hard to start, kicks back and breaks starters."

I'd also check cranking pressure.  With that much cam in a 10 to 1 CR 454 it should not be "bucking" the starter on hot re-starts with only 8-9 degrees of initial timing.......

[55 Tony]

What is cranking pressure and how is it measured?

I do have high compression, 195 to 205 psi.

Also how does this effect idle mixture?  That's my only problem.

[55 Tony]

Edit:
Sorry, never heard of it called cranking pressure before, so the 195 to 205 is my cranking pressure.

The cam is advanced 2 degrees which is partially responsible for the high compression.

As for "bucking" the starter, that even happens on COLD starts if I advance over 7 degrees.  Could be my compression ratio is higher than I thought?  Don't ask why but deck height was never measured.  Only made sure the pistons didn't hit the valves.  My first rebuild.

[Cliff Ruggles]

Was the cam ICL set according to the cam card or did you just advance it 2 degrees from the "dot to dot" position?