Holley fuel idle trim setting

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70Dart340V8

70Dart340V8

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So I just started tuning my fuel idle trim on my 4 barrel holley. I hooked up a vaccume gauge and started 2 1/2 turns out on each side. I adjusted inward leaning out the mix and the vaccume increased. I kept going until I was at a half turn on each side. With that I'm pulling about 13 inches of mecury and it seems to run ok. That seems like a really lean mixture though....I'm new to carb tuning so maybe I'm wrong. It's a 1970 340 by the way. Any advice?
 
If it runs correct there run it. There are several factors that determine how far out you adjust the idle mixture screws but generally 1/2 turn to 1 turn is about right. 2-1/2 turns would be really rich in most cases.

BTW: if it's an automatic I suggest adjusting the idle mixture screw in gear. Adjusting for highest idle vacuum in neutral will generally result in a slightly lean mixture in drive
 
Ok I just thought it was weird it being so far off from Holley starting point. I will definitly fine tune it in drive. Did you have any problems with the idle going up high in park? In drive it's right around 800-1000 RPMs but in park it jumps to like 1500. It's tricky getting the idle because when I take the air cleaner off its right around 1000 but as soon as I put it back on it jumps to 1400-1500
 
What does it do when you tighten the screws all the way in?
 
If it continues to run with the air screws tightened up, it is pulling fuel from somewhere. My first guess would be the secondaries are cracked open a bit too far, but I am kinda like Tracy. If it runs good where it is and it's not idling too fast, run it.
 
Have you checked your timing first? Your symptoms sound like it may want more timing. Reset carburetor after the timing. Others may be able to give you a good idea where the timing should be, based on your engine combo.
 
Is this a stock cam and trans and torque converter? What Holley model? Not much info to go on....
 
Ignition timing at idle?

Make sure the linkage on your carb is not hitting the intake anywhere. Check the pass side/secondary linkage.

Which carb are you using? 1850, 3310, some other model?

Sounds like possibly a bad vacuum leak.
 
It has a 727 torqueflite that is stock (to my knowledge) it is cammed but I don't know anything about the cam. The pervious owner had a friend build the engine and doesn't know any of the specifics. I timed it by ear for now. I'm going to pick up a timing light with an advance from a buddy later today so I can get the timing locked down. I have a Holley 6708-1. It's an odd ball carb. I didn't see very much about them online aside from its basically an old 4150 with extra vaccume ports. Possibly a factory carb off something. I didn't think about the clearance issue. I'll make sure to take a look at that.
 
I am guessing the transfer slots are partially exposed requiring you to lean out the idle fuel circuit to compensate. You may try adjusting the secondary stop screw to increase idle air bypass so you can back off the primary adjustment and get the transfer slot covered so your idle circuit can work as designed.
 
mguner said:
I am guessing the transfer slots are partially exposed requiring you to lean out the idle fuel circuit to compensate. You may try adjusting the secondary stop screw to increase idle air bypass so you can back off the primary adjustment and get the transfer slot covered so your idle circuit can work as designed.
Click to expand...

X2 I do the secondary trick on every holly I rebuild not matter what,worth a try.
 
mguner said:
I am guessing the transfer slots are partially exposed requiring you to lean out the idle fuel circuit to compensate. You may try adjusting the secondary stop screw to increase idle air bypass so you can back off the primary adjustment and get the transfer slot covered so your idle circuit can work as designed.
Click to expand...

So many people never learn this,& fight primary transfer situations.... Cracked is correct, timing curve set up correct,makes initial carb tuning so much easier.
 
70Dart340V8 said:
I'm also going to pick up a can of starting fluid to check for any vaccume leaks
Click to expand...

Good idea because that gaining 500 rpm just by installing the air breather suggests something drastic is wrong. As mentioned by others also check all the linkages and choke and such for something interfering with them. Also check the throttle shafts for wear. If the shafts are loose it can suck air just like a vacuum leak.

BTW: my Holley "bible" lists your 6708 as being a universal 650 double pumper. Sounds like an old version of the 4777.
 
I found the air cleaner problem was a clearance issue. It just barley touches, but just enough to make the idle jump up. I'm about to set the timing. Any sugestions? I heard 12 BTDC is a pretty comfortable area
 
Find the sweet spot where the engine likes it but will still crank easy when warm. Then adjust the advance curve accordingly. All this provided it doesn't ping under acceleration.
 
Just a side note on the timing issue. With todays crappy fuel the old all in at 2500 timing advance is not very practical or effective in many applications. In cars with a fairly tight stock converter you will most likely end up with the small factory advance spring and one of the super light replacement springs. This gives you a more gradual advance from a increased initial timing setting. You will still need to limit mechanical advance in the prescribed ways. Usually that is by shortening the outer end of the slots the advance weight pins move in.
 
The front half of the air cleaner was just barley pressing down on the air vent rod on top of the bowl. Just enough to make the rpms increase. Putting it on as a whole it's almost unnoticable. I had to just put the bottom piece on and watch everything very carefuly. I noticed when I slightly pressed down on the front a very small amount of fuel came out of the squirter. Good thinking on that everyone
 
70Dart340V8 said:
I found the air cleaner problem was a clearance issue. It just barley touches, but just enough to make the idle jump up. I'm about to set the timing. Any sugestions? I heard 12 BTDC is a pretty comfortable area
Click to expand...

Good deal on finding the idle problem. The cam and compression ratio are mostly what dictates the timing curve. If those are unknown's, which it sounds like yours are your best bet is to hook the vacuum gauge up and at an idle slowly advance the timing until you find the highest vacuum reading. If it's much cam at all it'll probably take at least 16 degrees timing at idle. Once you find what it likes at idle you have to curve the centrifugal advance so it doesn't advance past about 34 degrees max. If it's a factory distributor that requires welding up the advance slots. Here's a link to an article on Hot Rod that shows the whole deal

http://www.hotrod.com/how-to/engine/mopp-0301-mopar-electronic-ignition-system/

BTW: I have ran across a couple engines that didn't have a lot of cam yet liked a bunch of idle advance so don't be surprised if it likes something on the order of 20-24 degrees at idle for max idle vacuum. The only thing you have to watch out for there is too much idle advance may cause kicking back on the starter when it's hot so sometimes you have to limit it to a little less that optimal for it to start good. I generally find the springs Mgunner mentioned do a good job on regulating the timing curve
 
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