Tuning Idle Pontiac 400 with 17058276, Comp xe262h

[PonchoVia]

17058276 Previously installed Cliff's kit, 73 jets, 44 rods, float level 17/64.

Engine Pontiac 400 0.040+, 96 heads, performer intake, comp 262 cam, HEI with centrifugal coming in at 1000, all in at 2900. I've been using manifold vacuum for advance. 2004r, 9 inch 3.25.

Had an issue with idling like crap but driving perfectly. Called Cliff to order a choke pull off, turns out I   didn't need it. Recently installed AFR/wideband found my idle problem was WAY too lean, mixture screws did next to nothing to correct.

Cliff is NOT A FAN of these cams and said has always had idle issues with them.

He suggested just to open idle tubes to .038, down tube restriction to .050, idle air to 1.0, and using the recipes in the book opened idle bypass to .070 and mixture screw holes to .10, cut 2 rounds from spring they seat.

CAR IDLED GREAT!! I   adjusted rpm, mixture screws with vacuum gauge and achieved 17" of vacuum from the cam that says it is good for 17.5".

THEN found the vacuum line to the vacuum ball for interior controls wasn't even hooked up!!

Corrected that leak, corrected for idle mixture, engine still runs better than before, but can barely achieve 16" of vacuum.

I   assume this means that I   need more idle air? Am I   just not supplying enough of what this engine/cam combination needs?

I'm thankful to have the idle I   have now, but the OCD in me is flaring up big time. I   am using these issues to try to understand the black art of carb control.

AND one more question...

I'm unclear how/where to measure the secondary flaps to determine optimum opening. I   see the spec is 1.271 (memory?) is fully open. from the back of the opening to the ...... upper face of the open secondary? I    can't make out where from the pic in the book.

[Cliff Ruggles]

A low compression 400 CID engine and 262 seat timing cam will not need a lot of cfm, so really no need to modify the stop for the air flaps on that carburetor.

Idle bypass air is used to lower the throttle angle to keep the main system off line at idle speed or no nozzle drip. 

Not sure why that engine combo would want a lot of timing at idle speed, might want to try ported vacuum to the advance instead of running a lot of timing at idle.   With some set-ups doing so will actually help idle quality and provide stable idle in and out of gear as well.

I really wouldn't worry about engine vacuum at idle speed as much as idle quality, control with the mixture screws, and RPM drop/idle stability when the trans is placed in gear.

I've tuned at least a half dozen low compression 400's with that cam in them and never once found that I had to use a lot of timing at idle speed to make them happy.  Matter of fact tuned one not that long ago that had 7K3 heads (functionally identical to your #96's) and it was fine with 14 degrees initial timing and the tach barely moved when the trans was placed in gear.

It had a near identical carb on it as well, and I used .038" idle tubes, .055" DCR's and .110 idle bypass air, all airbleeds main and idle were left at the stock sizes.........Cliff

[PonchoVia]

Thanks for your time. As usual, you were right. Car idles so much better.

[PonchoVia]

All the settings are good except....

Car starts up perfectly on the choke, idles at 1500, but once it warms up, it occasionally sticks on the last step of the fast idle cam. I   know its that because I   can adjust idle speed with the choke screw.

I   must have made a mistake installing the choke but damned if I   know what it is. Im not real clear on how all the choke components work together.

I   loosened the choke and disengaged it, I   can start the car relatively easily but no fast idle, and I   don't get it hanging on the cam, runs great.

I   had installed your electric choke 3 years ago, had no issues until I   went back into the carb recently.
I've been hooking the choke loop on the tang inside the housing and setting the 'rivet' on the second mark passed the center.

[Cliff Ruggles]

Make sure the choke element is heating up. 

Engine heat will eventually allow them to work some and open the choke a bit.

Dead cold turn the choke CCW until the flat just barely closes.  That will be the quickest release time.

Also lubricate all the parts involved with the choke and fast idle cam.  Also verify the fast idle cam isn't rubbing on anything and lifts freely clear across the range of motion. 

I've seen a few that needed some minor adjustments made to the part that is pressed on the center shaft to provide adequate clearance for the cam to move up and down......Cliff

[PonchoVia]

Thanks, will do.

[PonchoVia]

Carb idle tip in likes about 12.5 on my afr gauge.

But APT tip in took 5 rounds to begin being happy. Afr shows upper 13's to low 14's. i know You don't care for afr tuning but I'm a bit concerned about maybe being too lean.

Is 5 rounds out on apt too many?

[tayto]

Keep in mind if your cam has a lot of overlap your O2 could be lying to you. I do like the idea of knowing what my AFR is when the carb is setup right for future troubleshooting.

[Cliff Ruggles]

Tune for results not for a particular A/F ratio.  Don't forget about the distributor.  Vacuum advance is a BIG player for tuning in the "normal" driving range and allows much leaner A/F ratios to be burned, increasing engine efficiency, improving power, engine performance and increasing fuel economy.......Cliff

[PonchoVia]

I   had forgotten to mention that I   did take your advice and switched from manifold to ported vacuum for the distributor advance.

So I   think I'm ok.

So just to be clear, 5 rounds out on the APT is not a concern?

[Cliff Ruggles]

I usually end up around 4.5 to 5 turns up.  This is putting the metering rods tapered section between steps high in the jets.  Fine as long as the hanger arms are EXACTLY even, and you are reducing travel distance which speeds up enrichment ever so slightly........Cliff

[PonchoVia]

Would that not indicate a need for 1 size larger jet? And put the rods back down a bit?

[Cliff Ruggles]

No, the APT tunes light part throttle.  The jet size should be determined first as it controls heavy and full throttle fuel delivery......Cliff

[PonchoVia]

OK, lets move back to the choke.

I   think its close to being correct.

question is, there is nothing else other than gravity to move the fast idle cam down as fast as, and as far as the choke mechanism will allow?

Im pretty sure I   had the screw that holds the choke assy to the body of the carb too tight causing a bind in the mechanism.

[Cliff Ruggles]

That screw needs to be tight.  If you find that it is binding thing up you may have issues with the choke shaft parts.  I've had to "tweak" a few where the part is staked to the center shaft to create some distance for things to work like they are supposed to.......Cliff