evoluzione1

I did a search and no luck. I called Jegs and Summit, no luck. Need help.

I have a 400ci with flat top pistons and a 460, 480 cam with stock heads. It has a 600 CFM holly smaller then all recomendation and the plugs are black. Yes changed them out what seems to be every 2000 to 3000 miles. The fumes? are so bad that just standing by the car running makes you smell funny, like rich. I've taken the intake off 4 or 5 times now checking things. Reset the valve 3 times and switched to a elderbrock carb 3 different time. The elderbrock carb doesn't seem to be as bad but performace lacks.

The holly carb runs the best, et around 13.1. Performance doesn't seem to be a issue but the smell sucks and it does blow smoke when starting and at full throttle. Looks gray. Any help would be great been fighting it for 3 years and just ran out off ideas.

F-BIRD'88

probabily a combination of the carbs throttle plates being out of best position at idle and not enough initial timing at idle with the larger than stock cam.

Remove the carb and flip it over. Reset the throttle blade opening to just expose the idle transfer slots. Adjust both the primary and secondarys on a holley.
.030" transition slot exposed under the throttles.

reset the idle mix screws once warmed up. Adjust the idle speed using ignition timing. a big cam with a rough idle and low manifold vacuum will need lots of initial timing at idle.
This allow the carbs throttle blades to not need to be so far open at idle and correct the burn time of the fuel at idle with a big cam.
Replace the power valve with a 4.5" one. Will stay closed at idle ( lean) .

make sure you have a PCV hooked up and a valve cover vent. your carb is designed to use a functional PCV system. Without it the carb will be rich at idle.

Exactly what cam grind are u using ( specs)?

if you tell me about the cam I can be more specific on the timing set up.

In order to run increased idle timing but have the same total max timing at high rpm (32 to 36 deg) you have to modify the mechanical advance travel in the distributor.
(Limit it)

Most stock HEI's have 20-22 degrees of travel. you want to cut down the travel to about 10- 15 degrees. to allow more initial timing at idle. the larger the cam the more initial timing nessessary for a nice idle.

Once done you can fine tune the vacuum advance. usually also needs limiting to 10-15deg at high vacuum with the more aggressive mechanical timing.

very large long duration cams will want the igniton timing "locked out" 36deg at idle 36deg at rpm.

Once you sort out the ignition timing curve both the holley and edelbrock carbs will function like you want them to.

The extended duration and overlap of a big cam causes the fuel mix in the cylinder to be diluted with exhaust gas @ idle.
this slows the burn rate at idle allowing unburned fuel to pass into the exhaust. thats what you're smelling. thats what causes the cool lopey idle.
fireing the mix sooner ( increased timing at idle) allows all the fuel to burn before the exhaust valve opens.

Sonix

iTrader: (1)

damn f-bird, that's impressive...

just our of curiosity, as i'll be doing this myself soon, once you swap in a bigger cam, and go to tune the carb/timing from scratch, you can set the dist up the way you like, then set the throttle blades to be open the correct amount, then adjust the idle mixture screws, and base timing for a decent idle speed?

I figured if you set your idle speed, then your mixture, then adjust your timing to have a smooth idle, it'll be idling really high... (that's what happened to me when I tried manifold vacuum on my vacuum advance, super high idle...)
can you adjust your mixture screws to compensate???

evoluzione1

The cam specs are 224/234 dur and the lift is 465/488. I have a new MSD 8360, it a extra duty ready to run, if that helps. Stock ign. and stock coil.
Wow I just keep learning.

Sonix

iTrader: (1)

ooh! I just noticed you have the evo kit! I have that issue of kit car magazine where they unveiled it, pretty sexy looking!
Any pictures ? did you do the full kit, interior as well?

evoluzione1

Yea done it all and that is the car that runs rich. That smell doesn't make it fun to drive. Iwas just talking about it in the auto detailing forum.

F-BIRD'88

You'll want about 16 to 22 degrees initial timing at idle (700rpm) do like I said with the carb throttles and start from there. Start at 20deg initial. do not conect vacuum advacne at this time. Let the car warm up and do not touch the throttle ( idle speed) adjust the idle mix screws for best idle vacuum. check the timing at idle. More timing= more idle speed less = less idle speed. the throttles are set right within a 1/2 turn do that last. You may have an ignition problem. ground the engine to the firewall. Inspect the cap and rotor. Remove one spark plug wire at a time while its running and find the bad cylinder. ( manifold vacuum leak, bad plug wire, defective coil)
your MSD distributor has a fully adjustable mechanical advance system. So once you've decided on how much initial timing the motor likes at idle you can limit the mechanical travel within the distributor to get 34 36deg mechanical at high rpm max advance with the 16 to 22 deg at idle. read the distributor's instructions. you want the mechanical advance to advance from idle smoothly reaching max advance at 3000 to 3500rpm.

Sonix

iTrader: (1)

damn f-bird, thanks! I know this isn't for me, but I always wanted to know how to set the tune with a lumpier cam.... that's slick...
what did you mean by :the throttles are set right within a 1/2 turn do that last

as in, after timing is set, you can/should adjust the idle speed?

five7kid

iTrader: (14)

The distributor has vacuum advance, right? Are you using it? If not, why not?

Using manifold vacuum at idle makes a lot more sense than having that much initial advance. Starter kick-back is going to be a problem, and is completely avoidable with the system you already have.

I don't agree with the approach being set forth.

evoluzione1

I'll start working with all your suggestions tonight. This is the stuff I have never heard of or tried. This info is great.

Thanks for all the advice, I'll let everyone know how it all works out when I'm done. Thanks again.

F-BIRD'88

Yes: after baselining the carbs throttles aligned in the correct position in reference to the idle transfer slots and determining the best compromize for the initial timing and then carefully setting the idle mixture screws ( warm) for best idle quality ( not speed) you can adjust the throttle speed ( opening) a small amount. you can close the secondaries a small amount of the idle is too high or open them a little if its too low. Same on the primaries. But you must keep the throttle opening position in the sweet spot for the idle system to work right.
Yes:: the practical amount of initial timing you can use will be limited by the starters cranking strength, but the bigger the cam the less cranking cylinder pressure . allowing some more initial timing. If you don;t need 22degrees, don;t use 22degrees. But you will want and need more initial timing at idle for a nice clean stable idle at a nice speed on a motor with a larger than stock cam. it works.

This motor's initial timing requirements are not that big. The starter should work fine with a good bit of initial advance. ( unless it's pouched) The throttle responce will be much better too.

Note: the PCV system is designed in the the carbs idle system and is part the of the totoal idle airflow @ idle. If its not hooked up or hooked up improperly the carbs blades will have to be opened further to get enough airlfow. Use it to your advantage to keep the carbs throttle bores in the sweet spot ( amount of opening) at idle.

then vacuum advance is a separate issue. it also will need to be custom talored to suit your motor's non stock timing requirements. It's all in the details.

Edelbrock AFB's and GM Qjets do not normally use the secondaries at idle. (there is not sec idle circuit) They are closed at idle, but holleys do. On a holley it is essential to have the primary and secondary throttle blades both positioned correctly at idle or the idle and off idle afr will be out of wack.

This combined with the cammed up motor's need for more idle timing (slowed fuel burn) causes the "burning eyes" @ idle.

anesthes

iTrader: (12)

Thats why I like electronics. Can set cranking spark at 10*btdc, idle at 25 or whatever the heads/cam like, ramp out whatever to wherever, etc.

I think on my trickflow heads I idle around 25-26* btdc, cruise is around 40*btdc, and wot ramps up to 36* in by 3000rpm, than I take out 1* for each PSI of boost. VERY VERY streetable even with a big cam.


-- Joe

evoluzione1

Started tuning carb tonight and man what a difference in just the idle. This is the first time it seemed to ideal the same after every time I goosed it. I still need to put in the 4.5 power valve but all good so far. Still trying to figure out the dist. to but that idle plate thing was great.

evoluzione1

Put the 4.5 power valve in tonight and man that's the ticket. This is the first time in years I could stand around the car with out rubbing my eyes.

Thanks for all the help, you the man F-Bird88.

F-BIRD'88

Glad it worked out well for ya....