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Old 06-20-2011, 12:30 PM
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Demon carb tuning

Took the Monte for a drive finally this weekend and came across some issues with this damn Demon carb.

sbc400. Bottom end stock, alum heads, headers, demon carb, and not sure on the cam off the top of my head (which I know you guys are going to yell at me for that )

Originally, I set the idle screw to where there is just a little square showing under the butterfly. The screws on the metering block were tightened up until they bottomed out, then backed out 1.5 turns as said so in the video and instructions.

Car idles great. But when it gets put in gear, she wants to die. I have a vacuum gauge on the 2nd vac port (the non-dizzy one) and no matter how I change the metering screws, it's not changing the vacuum at all. Only way I can get it to sound decent in gear, is turning the idle screw up, but then idle in park is too high.

I know these carbs take sometime to dial in, but I really did not have any luck this weekend with adjustments. Seems almost like it wasn't doing anything at all when changing the metering. Did die a couple times.

Any help is apprecriated.

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Old 06-20-2011, 05:32 PM
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Not knowing the cam size is problem one, if it`s a fairly large cam it needs a stall torque converter to idle correctly. Other thoughts is a vacuum leak or lack of enough spark timing. What is the timing set at? Is Vac advance being used? Setting it how the instructions says so is just to get it running, not it`s permanent set. 1.5 turns out is likely too lean. The idle should be set with a vacuum gauge. Turn one idle adjustment at a time to get the highest idle you can. When both sides are set even at the highest vacuum it`s set.
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Old 06-20-2011, 07:47 PM
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If your running a lopie cam with low vacuum and a stock stall torque converter, there's your problem. You need a torque converter that's loose enough that it doesn't drag the idle speed down when in gear.
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Old 06-20-2011, 08:12 PM
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Quote:
Originally Posted by sqzbox
If your running a lopie cam with low vacuum and a stock stall torque converter, there's your problem. You need a torque converter that's loose enough that it doesn't drag the idle speed down when in gear.
I still have all the boxes at the garage. I didn't buy the parts, the previous owner did, and according to him had bought it based on his goals. Everything was supposed to work together right.

The torque converter is not stock though. I want to say its a 2500 or 3500 rpm converter.

Timing is about 8btdc at idle, but still working on that. Thinking it should be a little more. Vac advance is hooked up. With the vac gauge hooked up, I believe it was about -11 vac and in gear was -6 vac or so. Don't take my word on that though. I'm useless when it comes to this lol. I need to get the exact readings this week and get back to you guys.
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Old 06-20-2011, 08:25 PM
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I have run into this issue before (the cause was a lean idle), and I also ran into the issue of the vacuum guage not really reacting to the idle mixture screw adjustments.

Just put the idle mixture screws at 2.5 or so turns out each and start her up and put it in gear, see how it acts.
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Old 06-20-2011, 09:18 PM
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You don't say what size or type of Demon it is. You must know your cam's duration. Most demons won't even begin to like life on an engine without at least 12* advance timing. Go online and dig up one of their carb sizing charts and you'll see what I mean.
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Old 06-20-2011, 10:36 PM
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Quote:
Originally Posted by cool rockin daddy
Go online and dig up one of their carb sizing charts and you'll see what I mean.
http://barrygrant.com/demon/default.aspx?page=5
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Old 06-21-2011, 03:04 AM
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You need more initial advance for starters, my Speed Demon needs 18-24, I use 18.

Find and downoad your Demon manual here.
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Old 06-21-2011, 05:29 AM
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Does your carb have the idle eze feature? This is essentially a controlled vacuum leak and may be part of the problem if not adjusted properly.

I agree with others you need more initial timing.
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Old 06-22-2011, 07:37 AM
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I found the box for the cam, but no specs on it, so I'm going to look up the model and find that all out for you guys. Carb model, honestly not sure and couldn't find the numbers on the carb.

But I think I may have another problem causing the issues above. When it was running, I took the timing light to the car, and noticed that on the harmonic dampner, the timing numbers go from 0-65 (left to right), and when the car was running, was reading about 60btdc on the timing mark @ idle. Obviously, that's way off... but wouldn't it not even come close to running or starting? I tried turning the distrib clockwise and tried to start and it just backfired drastically. Turned it back to where it was, and she fires right up. Still bad dies in gear and exhaust is very smelly.

Should I pull the dizzy and set it back a gear or 2? Originally when I put everything together, I had the dizzy 180* off and it was backfiring badly and such. So flipped it around and fired right up. But is seems like 60* wouldn't even make it come close to starting. Do you think this is my problem?
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Old 06-22-2011, 08:52 AM
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Found Cam

And just for reference, I found the cam.

Erson Cam E110822
Intake/Exhaust Duration
282.0 / 290.0

Says to use a 3000rpm converter. That's what I'm running.
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Old 06-22-2011, 04:36 PM
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Quote:
Originally Posted by david-b
I found the box for the cam, but no specs on it, so I'm going to look up the model and find that all out for you guys. Carb model, honestly not sure and couldn't find the numbers on the carb.

But I think I may have another problem causing the issues above. When it was running, I took the timing light to the car, and noticed that on the harmonic dampner, the timing numbers go from 0-65 (left to right), and when the car was running, was reading about 60btdc on the timing mark @ idle. Obviously, that's way off... but wouldn't it not even come close to running or starting? I tried turning the distrib clockwise and tried to start and it just backfired drastically. Turned it back to where it was, and she fires right up. Still bad dies in gear and exhaust is very smelly.

Should I pull the dizzy and set it back a gear or 2? Originally when I put everything together, I had the dizzy 180* off and it was backfiring badly and such. So flipped it around and fired right up. But is seems like 60* wouldn't even make it come close to starting. Do you think this is my problem?
If you have any doubt you need to verify the rotor is pointing at the number 1 plug wire with piston at top dead center before going any further. If not you need to reinstall the dizzy. When you are able to confirm correct dizzy install and your timing is still off then it is possible your balancer has slipped give you a false reading. Must get timing right before attempting to tune the carb.
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Old 06-22-2011, 04:54 PM
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Is your carb a vac secondary or double pumper.

Here's your cam:

I'm looking at a different cam for my 406, right now I'm looking at an Erson E110821.

It's a solid flat tappet with specs:

Duration @ .050:
246 / 246

Lift:
.510 / .510

108* LSA

Better figure out what Demon you have. if you look at the selection charts linked, you're either going to want 14-16* or 16-20* depending on size on carb. That cam has you in the wacky wonderland of straddling the recommended settings.

Last edited by cool rockin daddy; 06-22-2011 at 05:04 PM.
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Old 06-23-2011, 08:00 AM
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Carb is a double pumper.

The dampner fits on the key on the crank, so not sure how it can slip. It only fit on the one way. There were 2 bolts on the back of the dampner, holding on what I'm assuming is the weight, but those were tight and wouldn't be able to slide or anything like that. Plus, it wouldn't even fire up or run if it was that off, right? This thing fires right up when starting, and if timing was that bad, it wouldn't start.

Sadly I'm out of town this weekend so can't work on it at all, but really need to figure this out. Thanks to everyone so far.
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Old 06-23-2011, 08:26 AM
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The outside ring of the damper is fixed to the internal ring with elastomer,
over time it degrades and the outer ring slips.
This moves the timing mark off.
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