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  #1 (permalink)  
Old 09-12-2010, 02:29 PM
redride's Avatar
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tuning Demon Carb

Ok, I built a 502 and put on a Barry Grant rebuilt Mighty Demon 850. I had rich carb issues from the beginning and ended up getting gas in the oil which washed out the bearings. I have now pulled the motor, which only had 350 miles on it and after replacing the main bearings, rings and polishing the crank I got it back together. I took the motor to a local dyno shop that ran the motor with my carb and it put out 540 horse but the owner told me that something was way wrong with the carb and after a couple other pulls he pulled off my carb and bolted on a new Mighty Demon 825 that was on another motor he had just built and knew the carb was good. With this carb my motor went from 540 horse to 624 horse power. I went home and ordered a new Mighty Demon 825. The tuner at the dyno shop told me to pull out the power valve and put a block off in it and then go up 10 jet sizes front and back.

With the motor back in the car I bolted on the new carb which was jetted 86/86 which is 10 up from where it is from the factory. The car is running rich (smoke coming out of the pipes at idle and acceleration) so I pulled the carb back off and jetted the front to 84, turned the carb over and set the butterflies so they had about .020 of the transfer slot showing, turned the Idle-Eze screw all the way in then back out 1 1/2 all as the instructions said.

Installed the carb and started the car but it would not idle at all. No matter how much I turn the Idle-Eze screw it does nothing. I had to turn the primary idle screw in about two turns to get the car up to idle at 950 which now opens the butterflies way up the transfer slot. When I attempt to adjust the carb by use of the air fuel screws the car runs best when I bottom out a screw, I know this is not right. What gives, what should I try next???? I only have about 10 miles on this fresh rebuild and I am not about to ruin another motor.

Timing is set at 34 total which is as high as I can run it and still get it to start when hot.
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Old 09-13-2010, 06:29 AM
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I spent yesterday tuning my 750 vac sec Speed Demon on my radical SBC. I was having problems with it running rough and rich at idle....black smoke, eyes burning. The first thing I did before I screwed things up too much was to go on the Demon web site and into the 'Trouble Shooting' page. Their suggestions for running rich were as follows:

1. Increase initial timing (mine was Ok so I left it alone)
2. Go in on the mixture screws (mine were 'out' 1.25 turns on all 4 corners & I ended up with them nearly all the way in...a big improvement here)
3. Readjust butterfly positions (I adjusted them open slightly)
4. Lower float level (I adjusted the float in the primary bowl below the middle of the sight glass...this was a huge improvement). The floats on the secondary side were also adjusted below the middle of the sight glass.
5. Decrease fuel pressure (no change slightly over 6 psi)

One thing I did was to use a vacuum gauge to obtain the highest reading while making my adjustments. Maybe I was lucky but the car runs better now than it ever has. Oh yeah, one more thing .... make sure your engine is up to operating temperature before any changes are made. Good luck.

ZAPPER
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Old 09-13-2010, 06:34 AM
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Using an aggressive amount of initial timing can't be stressed enough, when the camshaft specs get 'out there'. Some radical cams can use all of the total timing that is required at WOT- even at idle.
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Old 09-13-2010, 06:40 AM
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Quote:
Originally Posted by cobalt327
Using an aggressive amount of initial timing can't be stressed enough, when the camshaft specs get 'out there'. Some radical cams can use all of the total timing that is required at WOT- even at idle.
Hi Cobalt 327;

Exactly...the timing in my engine is nearly 'all in' at idle....

ZAPPER
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Old 09-13-2010, 06:51 AM
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Quote:
Originally Posted by ZAPPER68
Hi Cobalt 327;

Exactly...the timing in my engine is nearly 'all in' at idle....

ZAPPER
What all you running?
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Old 09-13-2010, 06:58 AM
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Quote:
Originally Posted by cobalt327
What all you running?
36*

ZAPPER
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Old 09-13-2010, 07:54 AM
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lol I meant what are you running for an engine combination.
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Old 09-13-2010, 10:01 AM
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LOL good one....SBC built by American Speed out of Moline Il. It's a .030 over 350, steel crank, H beam rods, Edelbrock RPM heads & intake...agressive Crane offroad flat tappet solid cam. It dynoed @ 456 HP...I have since changed the Edelbrock intake for an original Chevy # 3917610, installed the original valve covers and painted the heads orange.

I wanted the engine to look and sound the same as the 302 (fresh in storage) without having to worry about grenading the original. I drive the snot out of the 355 and love every minute of it. I also installed a Tremec 5 speed to make it more friendly on the street. The low 1st gear and .68 o/drive is perfect with the 3.73:1 diff.

ZAPPER
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Old 09-13-2010, 10:31 AM
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Old 09-13-2010, 10:39 AM
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Wow hey cool that you have a 302

355 sounds like it runs good too!

I just wanted to throw in that if you say had a 283 cheap from craigslist or something, and you tuned that baby to get the most of it, then whenever you wanted to put that 302 in there then you would know exactly what parts to put on, you use the same parts as the 283. The 302 would prolly not run very good with the same parts as make a 350 run good, but the parts that made the 283 sing will be beastly on the 302 for sure. It is just protecting your investment and trying to only spend money on parts that you will get alot of use from. That is what I am learning, I am just trying to understand, but I'm having fun doing it.
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Old 09-13-2010, 07:05 PM
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My initial timing is 21 and total is 35. I can not go any higher on the timing or the car will not want to turn over when hot. The vacuum at idle (950) is 8 inches. As far as the motor goes it is a 454 bored 100 over and then stroked to 505. Comp Big Mutha Thumper hyd roller cam with AFR 305 heads and air gap intake.
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Old 09-13-2010, 07:51 PM
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Quote:
Originally Posted by redride
Ok, I built a 502 and put on a Barry Grant rebuilt Mighty Demon 850. I had rich carb issues from the beginning and ended up getting gas in the oil which washed out the bearings. I have now pulled the motor, which only had 350 miles on it and after replacing the main bearings, rings and polishing the crank I got it back together. I took the motor to a local dyno shop that ran the motor with my carb and it put out 540 horse but the owner told me that something was way wrong with the carb and after a couple other pulls he pulled off my carb and bolted on a new Mighty Demon 825 that was on another motor he had just built and knew the carb was good. With this carb my motor went from 540 horse to 624 horse power. I went home and ordered a new Mighty Demon 825. The tuner at the dyno shop told me to pull out the power valve and put a block off in it and then go up 10 jet sizes front and back.

With the motor back in the car I bolted on the new carb which was jetted 86/86 which is 10 up from where it is from the factory. The car is running rich (smoke coming out of the pipes at idle and acceleration) so I pulled the carb back off and jetted the front to 84, turned the carb over and set the butterflies so they had about .020 of the transfer slot showing, turned the Idle-Eze screw all the way in then back out 1 1/2 all as the instructions said.

Installed the carb and started the car but it would not idle at all. No matter how much I turn the Idle-Eze screw it does nothing. I had to turn the primary idle screw in about two turns to get the car up to idle at 950 which now opens the butterflies way up the transfer slot. When I attempt to adjust the carb by use of the air fuel screws the car runs best when I bottom out a screw, I know this is not right. What gives, what should I try next???? I only have about 10 miles on this fresh rebuild and I am not about to ruin another motor.

Timing is set at 34 total which is as high as I can run it and still get it to start when hot.
I didn't even see your post in this thread, sorry I sort of hijacked it talking about the other carb/engine.

The 'usual' rule of thumb is to go up about 8 numbers maximum from 'stock' when blocking off the PV. I cannot imagine you needed to go up 10 numbers on the secondary side too, though- unless you were jetted way too lean to begin with. That's the problem w/not starting w/the factory settings as a baseline. The primary side should be able to use a PV after you get it closer to tuned right, depending on how much vacuum you have at idle, that is.

What are the specs on the engine?

There are a couple options- first thing I would do, is to baseline the carb back to the way it was delivered and tune it from there.

I would question the need for 10 sizes larger on the primary side, especially w/the carb already running rich!

The secondary side comes w/the PV blocked off, w/85 pri./93 sec. jetting if down leg booster. If it's annular booster, 80/88. Both use a #65 primary PV. There are also changeable air bleeds- these need to be looked at to determine what- if anything- has been changed w/them.

Another idea is to use their troubleshooting suggestions.

Another is to call BG and talk to the techies there. BG says: "Due to continual R & D, specifications may periodically change. Contact Technical Support (706)864-8544 before making any calibration changes." I would strongly suggest you take them up on this offer!

Yet another is to send BG the carb back for them to flow it to see if there's a problem w/it, if you cannot get a good answer from them.

I do not use BG carbs, but according to their web site, either of the 850's should work- as should the 825, AFA your engine size goes.

But I would strongly suggest you talk to them personally- they deal w/their own products as a matter of course and are who should know what to do if anyone should.

Last edited by cobalt327; 09-13-2010 at 07:57 PM.
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Old 09-13-2010, 08:34 PM
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What distributor are you using Red ride?
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Old 09-13-2010, 09:11 PM
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Big Mutha Thumper cam. How many more threads are we gonna see where nobody can tune their engines with these cams in them. Change the cam.
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Old 09-14-2010, 07:41 AM
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What about the air bleed sizes? Bigger to lean it out?

Since you had good results with someone elses carb it has to be all carb related issues, no?
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