Carb Tuning / Vacuum Problems?

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danielb927

danielb927

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I don't think my carb is tuned right and my engine isn't running as well as it seems like it should. I got a new Holley 670 cfm Street Avenger carb this winter and I'm using it on the stock spreadbore manifold with an adapter. When I first got the carb, it worked pretty well right out of the box. However, I didn't have much of an exhaust on then, and when I got a full x-pipe system put on, it didn't want to stay running anymore. I played with the idle mixture screws and they seem to work best a turn and a half out of the stock location, which seems like a lot, but that's what I got when I followed the instructions I had for adjusting them.

Here's my problem now: the car has very little vacuum at idle in drive (fluctuates between 2 and 5) and to get it to idle at ~800 rpm in drive, it wants to idle at about 1300 in park. I also pulled the air cleaner today and noticed black soot on one of the secondaries in the carb. It seems obvious to me that something isn't set right. First of all, the timing varies around at idle in drive but I know it's close to 12 degrees initial, and needs to be around 14. Other than that, I'm not sure. There is a pretty loud hissing noise that sounds like it could be a vacuum leak, I only really hear it at idle in drive (below 1000 rpm or so), but it's loud enough that I can hear it in the car.

Here's a video I took today. I start the car and have it in park, then I put it in drive and you can see how low and varying the vacuum is. I also got a little video of the black soot-like stuff in the carb. It has a Comp Cams 280H cam in it, so I know the vacuum won't be great at idle, but 2-5 seems way low even for that (around 700-800 rpm in the video, I think).

Anyone got ideas? Do I have a vacuum leak somewhere, carb need better tuning, other problem, all the above?

[ame="http://www.youtube.com/watch?v=byjEbGuzMwA"]YouTube- Duster Vacuum Gauge[/ame]
 
Do you have the right pushrods?It sounds like your valves are not adjusted properly,do a compression test,also take a small diameter vacuum line about 3 feet long and hold it up to your ear and snoop around the carb base and intake faces see if you hear a hiss.
 
Firing order, double/triple check it.

Plug off ALL the vacuum ports at the carb. Eliminates any secondary leaks

Check float level and baseline the idle mix screws at 1.5 turns out.

Set the timing at about 16-18* initial.

If it idles above 1000 rpm, it may be bleeding mechanical timing in and that gets pulled out when you drop it in gear. You time it for 12* at you 1300 rpm and it drops to 7-8 when in gear at 800 rpm. Your engine should idle EASILY at 850 rpm, imho.
 
horsehead said:
First thing I would check is the firing order.
Click to expand...

Done, it's right, triple checked. I know it ran to Kentucky and back with 1 or 2 loose plug wires though, and ran worse then. I think I'm gonna get new plug wires, one of them has a little scarring from sitting on the manifold (#5) and another doesn't "click" when I put it on the spark plug (#7). Any suggestions or are they all the same?

bigdemo1 said:
Do you have the right pushrods?It sounds like your valves are not adjusted properly,do a compression test,also take a small diameter vacuum line about 3 feet long and hold it up to your ear and snoop around the carb base and intake faces see if you hear a hiss.
Click to expand...

Don't know anything about the rods, I'll pull the valve covers sometime and check I guess. Will try with the hose.
 
Well I guess it all matters on what you want to spend. A good 8mm set would be tops but you can probably get by with a cheaper set. Just be careful routing the wires so they dont hit anything hot or where they will rub.Buy some wire looms to help ya out.Back in the old days I would go out at night,pop the hood,fire it up,and watch the light show coming from my old plug wires.That would be replacement time. Also if the wires werent fitting tightly is there any change they came off and somehow got switched?
 
horsehead said:
Well I guess it all matters on what you want to spend. A good 8mm set would be tops but you can probably get by with a cheaper set. Just be careful routing the wires so they dont hit anything hot or where they will rub.Buy some wire looms to help ya out.Back in the old days I would go out at night,pop the hood,fire it up,and watch the light show coming from my old plug wires.That would be replacement time. Also if the wires werent fitting tightly is there any change they came off and somehow got switched?
Click to expand...

I got a 35 dollar set of 8mm wires. Will be installing them, setting the timing to 16 initial, and doing a compression test tomorrow. What's the proper compression test procedure? Engine at operating temp., all plugs out, crank the engine over 5 times per cylinder and test each cylinder 3 times?
 
I'm with you guys something doesn't sound right at all.
 
Listened to your video again with headphones on and very first thing I would do is pull valve covers and inspect rockers/pushrods/lifters and lobes.When you ran it on your trip did performance drop as you drove?or did this happen all of a sudden?
 
Just so everyone knows: some of the noise is probably the fact that the driver's side manifold is leaking at the back of the head and also where it connects to the headpipe. Still doesn't sound good though.

bigdemo: It's hard to say but it seemed like it may have gotten a little worse as the trip went along. I definitely had a loose plug wire then and it wasn't running as well as it is now. Actually, aside from the bad idle, I feel like it's running about the best it's been since I got it once it warms up. It's never really run great, though.
 
could you have an arcing wire(s)?? it sounds like a massive miss...

try out some new wires, any new wires, IMO that will take care of most of it... then fiddle with the timing if you have to. (and seal up the manifold)

you'll be happy you did!!!:cheers:
 
Verify the plug wires in relation to tdc by removing the #1 spark plug ,And distributor cap.plug the hole with a finger,slowly turn the engine over in the direction of rotation until you feel air pushing your finger out. This will put you close to tdc.Now continue to turn until your timing marks align.This is tdc on the compression stroke.Check that the rotor is indeed pointing to #1 wire on the distributor cap.If its not correct this by reinstalling plug wires in their proper order starting with #1. The rotor should be pointing to #1 and towards the front of the engine.
 
I heard this trick about vacuum leaks. Find a way to drop, drops of gas around the manifold. If the idle stabilizes then you know there is a leak there. Of course you will want to be as safe as possible to avoid fires. Have a fire extinguisher handy and try it with the engine cold. It does mess up when cold? I know it sounds crazy.
 
I got the new plug wires on and they fit super tight. Part of the problem may have been that it's so dang hard to get the back two drivers side wires on with the hi-po manifold. I'm still not sure my new ones are on there all the way, but I'll fire it up and see. May have screwed up the plugs with all this stuff too so I'll look at them when I do the compression test later.

horsehead said:
Verify the plug wires in relation to tdc by removing the #1 spark plug ,And distributor cap.plug the hole with a finger,slowly turn the engine over in the direction of rotation until you feel air pushing your finger out. This will put you close to tdc.Now continue to turn until your timing marks align.This is tdc on the compression stroke.Check that the rotor is indeed pointing to #1 wire on the distributor cap.If its not correct this by reinstalling plug wires in their proper order starting with #1. The rotor should be pointing to #1 and towards the front of the engine.
Click to expand...

Done minus checking that it was compression stroke, but it has to be or the thing would be sparking 180 degrees wrong and there's no way it would run. It wasn't pointing right at 1, more like 1/3 of the way between 1 and the next wire ( 8 ), which I figured was for the advance.
 
Sounds like it jumped timing, verify all timing marks valves and distributor at TDC.
 
advancing/retarding the cam will move the slot off center from the terminal, let alone moving housing to time it.

what out for mopar distributors and their poor cap rotor phasing/alignment.

at high rpm the miss alignment/poor phase will cause miss fire to the next cylinder, like 7-5 dod on mine yrs ago=scuffed piston skirt.
 
Distributor with cap off, timing mark at TDC:

p1020474d.jpg


Cap on (#1 wire at front):

p1020475.jpg


Looks okay to me, it seems like it might be a little advanced of where it should be but I can't say for sure since I never noted where it was before, so I have nothing to compare to.
 
Purple Haze said:
Sounds like it jumped timing, verify all timing marks valves and distributor at TDC.
Click to expand...

I haven't unclamped the distributor hold-down since before the trip. I'll check the timing today, I figure if the timing jumped, then it'll be different from what I had it at before, if it's the same, then I can rule that out?
 
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