I have just built a 388ci with the following. Custom comp hyd. roller with 4/7 firing order swap, duration at.50 is 260/262 lift is 547, lobe seperation is 112, compression aprox 11.5 to 1, quench was .045. gapless 2nd rings, ported victor jr aluminum heads, rpm intake, mallory race HEI with vacume advance lockout 36 advance, and a carter 750 carb(factory jetting) with no choke. walboro 255l intank pump with malory return reg. Fuel preasure is set at 6psi. autolite 3923 plugs .035 gap. I assume my problem is with the jetting. It has no stumbles but while running the engine for aprox 2 minutes at 4000 rpm the collectors were glowing red. Any sugestions for jetting for this application?

 

it's too lean if the headers are glowing. You need to increase your primary jets. Not sure if that carb u have is vac or mech secondaries but if it is vacuum then the secondaries were not open unless the motor was under a load. just sitting in neutral at 4000RPM will not open the vacuum secondary. Now a mechanical may have been open by then, at least partially.

general rule of thumb. increase the secondaries by the same amount that you increase the primaries to get it running right.

once it runs ok, spend the money to take it to a dyno with a wideband O2 and tune it.(~50-100 bucks) This is better than reading plugs(which can get you close)

 

BTW - my combo in my sig likes 72 jets front and back. each car is different.
your cam is bigger and heads prolly flow better so you may need more.

Hodge

 

With that big a cam you're likely getting fuel pull through when you are just reving the motor to 4000 rpm in neutral with no load. The long overlap of the cam is allowing unburned fuel to be pulled right thru the motor ( overscaveging). If that is what is actually happening, no amount of carb jetting will change it. (the cam grind is wrong or the headersCollectors are wrong)

But first:

Try using vacuum advance. You need lots of advance when operating a motor at part throttle low rpm when the motor has a big cam in it. The air fuel mix is diluted by reverting exhaust gas which slows the burn rate.
The extra timing corrects the burn timing so the fuel does not burn in the exhaust (late burn) {glowing exhaust headers}

Lock out the mechanical advance mechanism and set the timing at 36deg BTC. @idle.. Add about 10 to 15 degrees additional vacuum advance at part throttle high vacuum (like reving to 4000 in neutral with little or no load) Vacuum advance does not affect WOT timing because there is little or no manifold vacuum at WOT to pull on the vacuum diaphram.

You should be employing vacuum advance for a street driven motor, that operates at part throttle ( cruising) A radical motor like yours needs vacuum advance even more so.
This is not a jetting problem. It is an ignition timing problem. Possibly made more sever by an over scavegening exhaust system.

 

Once you've corrected the ignition timing you can move on to the carbs idle circuit.
Set tyhe carb up for best idle at the speed you want on a fully warmed up engine. No remove the carb and flip it over and look at the throttle plates. The idle transfer slots should only be partialy exposed. Not more than .04-.050"
If they exposed more readjust the throttle opening ( idle speed screw) so the transfers slots is just exposed .030".

reinstall the carb. warm up and reset the idle mixture screws. Do not touch the idle speed screw.
is the idle too slow? You can increase the idle speed without monkeying with the throttle blades by hooking up a PCV valve system to the intake or carb base. This extra airflow will increase the idle speed. Be sure to vent the other valve cover. Hook up the PCV and again readjust the mixture screws. If the idle is just a little too slow or a little too high adjust the idle speed screw ( throttle ) just a bit to correct.
The idle will be much cleaner now. See if the headers glow at 4000rpm now. If they still glow at 4000 after correcting the timing and idle throttle plate position refernece then the idle feed restrictions are too lean( IDLE and off idle fuel mixture. This is not controled by the main jets.
You can test the idle and off idle AFR for excessive leanness by adding propane ( propane torch) down the carb while testing and observing the headers. if the AFR is infact too lean the extra propane will enrich the mix and the headers will stop glowing. If the propane trick works you can correct the IDle feed restrictions by opening them up .002" or .003" Small changes make a big difference.

I'll bet once you've corrected the timing issue and the throttle blade position at idle. the headers will be fine without touching the idle feed restrictions.

Some people drill extra airflow holes in the primary throttle plates to get the extra airflow( idle speed) on a big cammed motor. To permanent and unadjustable for me. The PCV valve trick works just as good ( you can even employ 2 PCV valves and adjust the auxillary airflow with a pepcock valve.) The PCV system will not affect WOT perfomance and will keep the oil cleaner.

Remember the carbs' throttle plates must be in the correct position at idle with the transfer slots just exposed or the idle and off idle will be out of wack.

 

Fbird you never cease to amaze me.
How do they treat you at Mensa anyway?


I'll have to keep that PCV valve trick in mind, an aquarium pepcock to add more airflow, as well as a fuel filter to act as an air filter eh? Is that because the PCV system has a choke point at a different point? (ie, not at the carb inlet?)