340 idle problems

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cudaman51

cudaman51

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stock 340 455 268 cam had 670 streat avenger on wont idle sent it back still nfg. had intake machiened to match ports. fuel presh 6psi.put 650 double pump on runs fine 10 inchs vacume at low idle had budies 750 with big jets runs fine.I paid real money for that avenger id like to use it. any tips would be appreciated.
 
10 inches of vacuum is way low for that camshaft, IMO. I have a similar cam in a 340 and it pulls an 14-15" at 750-800 rpm.

Make sure you have enough initial advance, no vacuum leaks.
 
You need more initial. I am running 15 on mine. I had it at 20 with a holley 3310
and swapped in a 670 for better throttle response as I am running a ***** of
a cam that hates to idle at all. The 670 is an awesome carb. Keep in mind it
is wet flow tested also so it is closer to a 750 than you think. I'd work with
the carb. Mine needed a slight tweaking out of the box, and I need to jet it
up a little. Right now I am running 15btdc initial, have the secondarys adjusted to be slightly open to get more air at idle. Putting in a wideband
af gauge today so I can really get it dialed in!

Don't give up on this carb, it's a nice piece of hardware!
 
It might something simple that's giving you a vacuum leak. I'll usually cup my hand over the top of the carb top to see if it's "looking" to be more rich. I also spray starting fluid in little shots around the manifold while it's running to see if that speeds up the idle.......some might say that isn't safe but in small shots, I don't see any huge danger. I just don't go hog wild spraying it non-stop.

I've had vacuum leaks from using a carb base gasket that didn't exactly fit the top of the manifold.
 
when i choke the air horn of the idle does not go up.when i take the cap of the small manifold vac port to put the gauge on the idle jumps up 3or400 rpm, this is with the double pumper.i acidentaly had the timeing up to 18 initial and it ran grate.14inchs vac nice low smooth idle. put it back to 10 then 12inches. if i put the avenger on it would shut of below 1500.I have bin messing with this for a long time and am running out off pationce.I had the manifold machiened put sealer on the gaskets steel plate under carb I am wondering if their could be a problem with the carb.I know the vac is a little low
 
cudaman51 said:
i acidentaly had the timeing up to 18 initial and it ran grate.14inchs vac nice low smooth idle. put it back to 10 then 12inches.
Click to expand...

That right there should tell you something!

If you are timing it for total with no regard for what your initial timing is going to be.... YOU'RE MAKING A BIG MISTAKE, imho.

You need to get the idle/initial timing and then the carb settings. Timing first, carb second. Likely going to have to bust into the distributor and limit mechanical advance.

Too bad the mods won't put a "how to set my initial timing" in the archives so we can just link to it. Let the vacuum gauge be your guide. Max vacuum at a constant idle, as you dial in timing your engine should pick up RPM, then back off 1". Lock it down and take a timing reading. That's a great spot to start.

Try to not put silicone sealer anywhere near a carb or it's gaskets. Most silicone is gas soluble. Very bad.
 
I need to find a shop that can do my disty work near Minneapolis. I don't
want to give up my points disty with pertronix ignitor, but when I get my
car running great it's at about 17 before, that puts my total at about 42 which
as I found out makes me go ping, knock, ping. Not so good for motor life. If
anyone has a link on how to limit the advance I'd much appreciate it.
 
The part that blows mi mind is why do i have do go so far away from stock parameters with such a close to stock moter. Its even a mopar cam.I fully understand every thing you said to do and can do it if thats what it takes.It will be interesting to see where the initial ends up.Thanks I will give it a shot.
 
Why so far from stock parameters... Because the stock parameters were for a multitude of reasons other than max performance.

IMHO, when you build this stuff, don't pay one bit of attention to stock settings for timing, idle, carb adjustments. They are pretty much worthless if you want the car to run properly.

Let the car tell you what it wants with the tools you have available. That's the best way to go about any tuning adventure.
 
well i gues i must be naive i thought they were all the same.I learned something.I played with it today. cant go over 12.5inches @15d without ping.runs grate.Still have to do dist.hows 36 total sound with10.5cr j heads
 
Hey don't forget that sometimes the harmonic balancer outer ring will shift out of time with the inner one, giving an inaccurate timing reading.
 
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