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Old 04-14-2013, 04:41 PM
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Demon carb in chevelle not smooth

Hey guys been checking out the forum and it has tons of great info. I've been looking for info regarding my Chevy 327 having issues with running smoothly and have yet to find a solution.
The 327 is in a 69 chevelle I recently purchased and I am having issues with it running smoothly when I push the throttle. It does not typically happen under full throttle conditions but more of a light throttle. And this sputtering only happens when I'm driving. Not when car is sitting and I rev the motor, it actually sounds perfect when it is just sitting and I rev it by grabbing the throttle at the carburetor. When I say it's sputtering I mean its just not clean and crisp acceleration. And when I get to a higher rpm it seems to run a lot smoother. I really think it has something to do with the carburetor. Any suggestions on what I could do?
I have checked all the spark plugs and they look in good condition (milky chocolate color)
I adjusted the timing at about 35 this was done with a timing light and revving the motor to approx 2000rpm. The gas is priemum and there is an inline filter so hopefully no bad gas issues.

Any help is appreciated

Edelbrock performer intake
Demon 650 carb
Msd distributor magnetic pickup and vacuum advance
Msd 6al box
Mild cam
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Old 04-14-2013, 04:44 PM
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Also checked the direct manifold vacuum and it is about 10hg at idle
Fuel pressure is approx 7-8psi with mech pump
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Old 04-14-2013, 04:59 PM
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What is this. 10 is bad, or could be half of what is normal. Fuel psi is not bad. Even 8 psi wont unseat any of my carbs needles, but you never know.

Why only at 2000 rpm?
Did you go past to see if the curve keeps advancing?
Where is the vacuum advance hooked?

Get at least 14btdc mechanical for idle + more vacuum advance from the pod later. Make sure it comes in by 3000 and stops providing mechanical advance at 36.

Check every plug for spark.
Look for leaks, bad wires, odd sounds...
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Old 04-14-2013, 06:09 PM
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Define mild cam.... what cam is it.
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Old 04-15-2013, 12:17 AM
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I will doulble check the vacuum tomorrow jus to make sure but yes should be around 15-20 right?

It's a comp cam hyd 231 duration with 110 separation
Also has afr heads 180cc
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Old 04-15-2013, 12:22 AM
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Spinn

I thought that the 2000rpm was standard. But yes you make a good point because I didn't check to see if it advanced any more beyond that rpm. I will do that tomorrow as well. All cylinders hav spark but it does run maybe like a cylinder is down. Not sure. Thank u
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Old 04-15-2013, 05:33 AM
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Probably lean at transition like my Demon's were. You've got some work ahead of you so I hope you are good with carbs. P.S. start by cleaning the metal shavings out of it!
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Old 04-15-2013, 09:21 AM
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Quote:
Originally Posted by Zz4bronco View Post
Spinn

I thought that the 2000rpm was standard. But yes you make a good point because I didn't check to see if it advanced any more beyond that rpm. I will do that tomorrow as well. All cylinders hav spark but it does run maybe like a cylinder is down. Not sure. Thank u
The curve starts after idle around 1100rpm for most people. You do not want to be advancing at idle, but stable. The curve adds more until the springs stop it advancing, or the advance plate runs into a stop bushing/screw. Tuning for power at a common rpm, the total should not be more than 36. Make sure it does not keep going up. During wide open throttle a curve endine around 3ooo rpm is a good place to start. Then you can dial it quicker for better performance. Any sooner than 2000 and you might look into locking out the curve and using a starter retard.

Low vacuum is a indicator of a timing fault, distributor or valve.

Some guy said something about metal shavings, did you pm him something about that? were you drilling? or did he just come up with that on his own?
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Old 04-15-2013, 10:56 AM
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Quote:
Originally Posted by spinn View Post
Some guy said something about metal shavings, did you pm him something about that? were you drilling? or did he just come up with that on his own?
Apparently BG carbs were notorious for being shipped full of metal shavings.
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Old 04-15-2013, 11:12 AM
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Im picturing a carb bowl full of lathe shaving.

I have a mighty 750dp , and had a vs 750 too. When I got them , you set them for baseline, open the bowls check the float height, etc. I never saw a bowl full of shavings. I never saw anybody I knew have the problem either.

Now that you mention it I remember people had a lot of complaints. Rich idle, high idle, lean circuit, get too hot, 4 corner this and that, etc. To me it was a nice double pumper.
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Old 04-15-2013, 11:16 AM
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Set the timing up first, then do the carb. Some info on what to look for regarding the timing: Ignition advance. The page was originally on the GM HEI but the timing info still applies. BTW, info on vacuum advance can be found on the Ignition advance page as well. Work w/the vacuum advance after the rest is dialed in. On that same page there's a link to another page "Timing tabs and damper TDC lines SBC" that'll show you how to tell if the damper line and timing tab are accurate. If you don't have a dial back timing light or a degreed damper, there's also a link "How to make a timing tape" showing how.

The idle vacuum should be higher. Be sure there's NO vacuum leaks. The intake gaskets are sometimes the problem and they can be hard to diagnose if the leak is on the lifter valley side. A little help on this can be seen here. The vacuum line to the transmission modulator is often overlooked, so be sure it's OK. One way to be sure there's no vacuum leaks from the vacuum accessories is to disconnect and plug the vacuum ports on the carb one at a time, looking to see if one changes the idle vacuum. You may hear the change in the idle by ear. With the vacuum advance, check it to see that it holds a vacuum then reconnect it. Depending on whether it's connected to manifold or ported vacuum, the idle speed may change when it's disconnected/reconnected. The power brake vacuum line can usually be left in place, if the booster's leaking the brakes will usually give it away and often there'll be a sound that gives the booster away. Be sure the PCV system's hoses are all good and the valve is good, too.

One thing that often happens is the carb's primary throttle blades are opened too far to get the engine to idle. That causes the transfer slots to be over exposed, and in turn that causes transition problems. So the carb should be baselined to have the transfer slots form a square as viewed w/the carb upside down (see drawing below). Some info on this is immediately above where the Ignition advance link above takes you.



Once the transfer slots are set right, you can still adjust the idle speed using the carb's curb idle adjustment screw (thus changing the transfer slot), but just don't go too far. If it looks like you're going to need to open the primaries too far, you need to add initial timing to increase the idle speed, w/o opening the throttle blades. That keeps the transition slot exposure correct. Remember- when you add initial timing, you need to remove mechanical timing to keep the TOTAL timing from being too high. On your MSD distributor this is done by changing the bushings.

Spinn, not speaking for Blown Camaro but there were problems w/machining chips, etc, being left inside some Demon carbs early on. Later this problem was mostly taken care of.
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Old 04-15-2013, 05:48 PM
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Quote:
Originally Posted by malc View Post
Apparently BG carbs were notorious for being shipped full of metal shavings.
Of the two demon carbs I purchased .... One of them had a bunch of metal shavings in it.

So its worth checking for it
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Old 04-15-2013, 06:55 PM
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No metal shavings. Carb has been mounted a few years

Thanks for the help everyone so far

Anyways I checked the vacuum again and here is what I have. The timed vacuum for spark advance is at 0hg at idle then peaks out at 15hg once I rev motor. The direct manifold vacuum is 5hg at idle and then peaks out at 20hg while increasing throttle.

Also I checked timing again and I have my gun set at 35 on dial and at approx 2000rpm I hit zero on the indicator on h balancer. Once I lift throttle to approx 3000rpm the indicator is no longer increasing and is at about the 21 mark.

What do u guys think? Sorry I'm not an expert at this stuff but I think I can get around ok. Thank u for the help again
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Old 04-15-2013, 07:03 PM
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The timing goes down as rpm rises with the vacuum pod unhooked?

You sure it wasn't 21 at 2000 and 35 at 3000.

Hard to understand what your saying it isn't read that way.

YOu could be 21 past 35 degrees, like 56..

Set gun to zero , shine at balancer /engine at 3000rpm..what does it say?

Said "Lift the throttle" to 3000. So as you release throttle the rpms rise? Oh no.
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Old 04-15-2013, 07:03 PM
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Cobalt327 thanks for the info on the carb. I will check that once I figure out if I have the timing set correctly. Btw carbs are like jet engines to me so I will do my best to keep up.
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