Running Rich at Idle

[bandit]

What is the concensus on how far open to have the secondary plates? I was getting very little response on idle mix screws so backed the secondary plate screw all the way out to close them more and now it is too rich at idle and can't lean it out enough(eyes are burning in garage at idle). Mixture screws respond a lot better now though. The transfer slot on the secondaries is perfectly square with the adjusting screw all the way out. It will not go any more to cover it completely. Some say this slot should be closed, others say to open it up a bit to get the perfect square. Mine was running decent but had little response on mix screws with it more of a rectangle. What is the deal with correctly setting these secondaries? Doing a balancing act between too rich and good idle mixture screw response and rough idle and very little mixture screw response. Also, I could get 14in vacuum with them opened slightly but maxing at 10in with them closed to the perfect square.

Should I go for 14in with secondaries open more and lose a lot of response on idle mx screws or leave secondaries closed(perfect square showing) and go with the 10in and figure out this rich issue? All this started with me trying to smooth out my idle. Tempted to just open them back up and deal with the rough idle.

 

[Hemirunner]

What carb??

 

[bandit]

Holley 750cfm with vacuum secondaries

 

[Meep-Meep]

If you have a high overlap cam you will have the burning eyes at idle. These cams don't idle well and hooked up to a gas analyzer they will peg the HC meter - raw fuel. High overlap would be a 108 lobe separation. Post your cam and we'll go from there.

 

[bandit]

Not sure what cam is in there. The previous owner raced it on 1/4 mile though so wouldn't doubt it is not stock.

 

[bandit]

If I drill 3/32 holes in the primaries to get more air, will I lose control of the mix screws?

 

[Meep-Meep]

Let's not drill any holes just yet. On a 750 vac secondary (3310 perhaps?) the secondaries should be lightly seated in the bores and that little stop screw should be holding them open just enough to keep the throttle blades from sticking. This means the fit of the blades should make almost a perfect seal by being free of raised material along the mating surfaces to the bore. When seated you should not hardly see any light around the edges. If the throttle shaft is really loose you will have inconsistent idle and that goes for the primary side as well.

Start by setting your timing to at least 10 deg BTDC and noting that your total advance is not beyond 38 degrees. If you have enough timing for that cam the primary idle speed screw should be about 1-1.5 turns from the primary blades lightly seated in the bores. Idle mix you can start with 2 turns out from lightly seated. If you need to crank your idle screw because the engine is idling too slow then you probably don't have enough initial advance. I have run up to 15 deg + on hot cammed big blocks and no problems. But remember to make the necessary adjustments in the distributor to keep your total below 38 deg - if you will keep the higher initial setting.

The goal is to balance the initial timing and throttle plate angle to keep the engine happy. Proper throttle plate position should uncover about one third to one half the transfer slot and if open much more than that the engine will likely run on.

 

[Hemirunner]

You're assuming he has vacuum advance with that 10 degree initial recommendation. If he doesn't, then crank up the initial advance or lock it out altogether. You're probably right on and the cam is most likely some "awesome purple stripe cam" that someone stuck in there. Bandit, where are you in Illinois?

 

[Meep-Meep]

You're assuming he has vacuum advance with that 10 degree initial recommendation. If he doesn't, then crank up the initial advance or lock it out altogether. You're probably right on and the cam is most likely some "awesome purple stripe cam" that someone stuck in there. Bandit, where are you in Illinois?

Good point about the vac advance. I was assuming it was out of the picture at idle. The 10 deg is a conservative start and will likely need a lot more so he should sneak up on it. One sign that the throttle is open too much is having a vac signal on the ported spark port. And contrary to the popular article that has gone around I use the ported port for vac advance.

In my experience the .484" cam has been a disaster for a stock engine, however, the .528" mechanical is a very streetable grind due to the 112 lobe separation.