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Timing?

Since we are asking all the pertinent information, I’ll need your SS number as well. You can send that in a pm, lol!!

What is the springs set in the distributor, light or heavy?
Ya know, I don't remember changing them, and I think the factory (firecore) setup is one light and one heavy?
 
LOL!! Advance it 5* and see if she doesn't act better.....
Yes it acts better up to about 34 or so...that was my concern. Why so much this time. Build-wise, the compression ratio came up a little but the cam timing events aren't much different.
Just for grins, throw a different coil on it.
Thanks I didn't try that..I have a spare in the trunk too.
 
Which model Demon carb?

Are you saying you adjusted the mixture screws with the engine warm, idling in neutral @800, while watching for highest idle speed(highest vacuum), and you ended up at 7/8 of a turn?

Imo, it’s idling lower than I’d want it to.
Hot street motor, I’m usually at least 1000 in neutral.

How much rpm does it take to reach full advance?

FWIW, your readings seem okay to me.
Cranking pressure, vacuum...... right where I’d expect.
These things are always happier with as much initial timing as the fuel will tolerate.
If you can lock out the distributor, set it at 35*, and not have issues with pinging or hard starting...... just run it like that.
Though, my guess is you’ll likely need some curve in the distributor.

As a starting point I’d set it at 38* total timing, and if the initial ends up at 22-25, that’s a good place to start.
You’ll want to have full advance before whatever your normal cruise rpm is.

After setting the total timing at 38*, idling at 1000 in neutral........ engine warm, re-set the mixture screws.
I’d be pretty surprised if they ended up at only 7/8 turn.

If it still pops a little when you blip it, then it’s time to start trying bigger squirters.

If it still not satisfactory......... add 2* initial at a time.
Each time, re-set idle to 1000, then readjust mixture screws.
Whatever the initial timing is at where you feel your happy with it........ you’ll have to modify/adjust the advance in the distributor so you have the initial timing that gave you satisfactory response and idle quality, but only 35-38* total timing.

There’s nothing about that combo I see that seems like it wouldn’t work perfectly fine with 25* initial/35-38* total timing.

I know that carb worked fine on the old combo, but before I spent a bunch of time on the ign....... I’d see if I could borrow a “known good” Holley of QFT with 4 corner idle and see it it acted any better.
My experience is the Demons can be pretty finicky.
 
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Which model Demon carb?

Are you saying to adjusted the mixture screws with the engine warm, idling in neutral @800, while watching for highest idle speed(highest vacuum), and you ended up at 7/8 of a turn?

Imo, it’s idling lower than I’d want it to.
Hot street motor, I’m usually at least 1000 in neutral.

How much rpm does it take to reach full advance?

FWIW, your readings seem okay to me.
Cranking pressure, vacuum...... right where I’d expect.
These things are always happier with as much initial timing as the fuel will tolerate.
If you can lock out the distributor, set it at 35*, and not have issues with pinging or hard starting...... just run it like that.
Though, my guess is you’ll likely need some curve in the distributor.

As a starting point I’d set it at 38* total timing, and if the initial ends up at 22-25, that’s a good place to start.
You’ll want to have full advance before whatever your normal cruise rpm is.

After setting the total timing at 38*, idling at 1000 in neutral........ engine warm, re-set the mixture screws.
I’d be pretty surprised if they ended up at only 7/8 turn.

If it still pops a little when you blip it, then it’s time to start trying bigger squirters.

If it still not satisfactory......... add 2* initial at a time.
Each time, re-set idle to 1000, then readjust mixture screws.
Whatever the initial timing is at where you feel your happy with it........ you’ll have to modify/adjust the advance in the distributor so you have the initial timing that gave you satisfactory response and idle quality, but only 35-38* total timing.

There’s nothing about that combo I see that seems like it wouldn’t work perfectly fine with 25* initial/35-38* total timing.

I know that carb worked fine on the old combo, but before I spent a bunch of time on the ign....... I’d see if I could borrow a “known good” Holley of QFT with 4 corner idle and see it it acted any better.
My experience is the Demons can be pretty finicky.
Thank you! I've had suspicions about this carb for a long time. I've been through it multiple times and have had some good performance with it, but it's always seemed to be inconsistent. (850 speed demon 2003 vintage by the way)..thanks again for the advice!
 
This thread made me curious, and a web search came up with this article. Here is a quote.

"Most engines will idle happily with an ignition advance of 15 - 32 degrees. This is a very wide range - some engines will certainly not be happy at 32 degrees and others won't be at 15 degrees! An overly high amount of ignition advance for a given engine will result in lumpiness at idle, excessive hydrocarbon emissions and sometimes exhaust popping, while too little advance will also cause lumpiness. If the engine runs closed loop fuel control at idle, too much idle timing advance can disrupt the oxygen sensor reading, causing the self-learning process to overly enrich the idle mixture. Setting the optimal ignition timing can therefore best be done by trial and error variations"

://www.autospeed.com/cms/article.html?&title=Getting-the-Ignition-Timing-Right&A=109132
 
What is the distributor curve or if stock what number?
 
What is the distributor curve or if stock what number?
I had it set for 14° advance, it was all in by 2500 rpm. That was setup at 22/36 total (without the vacuum can). Firecore unit.
 
Pretty fast it seems adjust it down (mech) a bit or put a heavier spring combo in it. Then add the vac advance that is prob 10 or so way high way to fast
 
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Pretty fast it seems adjust it down (mech) a bit or put a heavier spring combo in it. Then add the vac advance that is prob 10 or so way high way to fast

Thank you HH, that distributor setup was how I had it on the engine's previous life. The problem I'm having with the new build is that the engine is taking 30+ degrees idle timing to get happy(best vacuum/rpm). Runs shaky..ignition components are okay as far as I can tell and the distributor has good reluctor clearance, no shaft wobble or anything out of the ordinary (at least that I can detect).
 
So I did some experimentation and for driving safety set the mech. advance for 6° only, and cranked the initial to 30°. Idle speed up to 950 neutral/about 850 in gear. Vacuum came up to 13 in, from 10-11 and got much steadier. Double checked transfer slots before re-adjusting things and they were okay. 4-corner screws, still ended up at 7/8 turn out.
It does run better, things got better but upon slowing down to a stop or idling it still shakes to the point of clanking (header against torsion bar)....
One thing I'm noticing. AFRs are all over the place. 14+ at idle at one stoplight, 13 at the next, and so on and never settles in..cruising it fluctuates from low 12s to mid 13s even at a steady rpm..I haven't been able to swap a different carb on yet as Mr. Porter advised but I'm starting to lean that direction more as I drive it around.
As I drift into recollections...I had some of this issue on the previous build, though not nearly as intense. I was able to tune to a point it wasn't so noticeable (I also was not running an A/F meter, so...). This carb is the one thing that's remained constant (except the distributor) and although I've rebuilt and tuned and gone through it multiple times maybe there's something just not right with it. (There was lots of talk of issues over the years with these older BG carbs...)
 
Great you have an A/F meter. Do you happen to have a laser temp gun? Quick and easy to measure each header tube just out of the head when the engine is running rough, a discrepancy could indicate an intermittent ignition misfire on 1 cylinder, bad lead or plug for example. A misfire would show up as a lean condition on your A/F meter. Only takes a minute to do.I'm not saying it's not the carb, or some other issue though, you seem competent and knowledgeable.
 
Great you have an A/F meter. Do you happen to have a laser temp gun? Quick and easy to measure each header tube just out of the head when the engine is running rough, a discrepancy could indicate an intermittent ignition misfire on 1 cylinder, bad lead or plug for example. A misfire would show up as a lean condition on your A/F meter. Only takes a minute to do.I'm not saying it's not the carb, or some other issue though, you seem competent and knowledgeable.
Thank you, yep got the temp gun too, all pipes are hot and within about a 50-degree spread of each other.
(And like 150 degrees cooler than when I had Hooker headers on there...gotta love the TTIs!)
 
When was the last time you replaced the metering block gaskets?

Try shooting carb cleaner through the air bleeds.

Make sure the PV isn’t dried out and stiff.

Where do you have the floats set?
I’ve had the best luck with those carbs with the floats just above 1/2 way up the site glass.

Try running the mixture screws a little richer...... maybe 1/8-1/4 turn.

That thing should run perfect with the initial at 30* and the total at 36*.

If none of that sorts it out....... I’d be looking for a known good carb to borrow.

We were a BG dealer when the Demon line came out.
Those things were a huge PITA to get sorted out 90+% of the time.
I had a used stock Holley 850DP(4781)I took in trade for something, put a kit in it, and used it for a dyno carb.
Countless times we’d be fighting with some Demon on the dyno trying to sort out some issue........ swap to the Holley(basically to verify the problem was the Demon carb and not something else), and whatever problem we were having just magically disappeared.
Despite the fact that they had a decent dealer program for the BG line, where you could sell them for the same price as Summit and still make it worthwhile to sell carbs...... we stopped carrying them.
Many times they required several hours of dyno time to get them to run well enough to let the motor leave with one of them on it.
In the end........ they were more trouble than they were worth.
And........ that’s not even counting the ones that had metal shavings in them, or the ones that were put together with flat out defective parts, or the ones that got damaged during assembly........ all of which somehow made it past QC.
There were several times when it was pretty quiet on the other end of the phone after describing some of the things we found wrong with those carbs while trying to get them to work.
 
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One thing I'm noticing. AFRs are all over the place. 14+ at idle at one stoplight, 13 at the next, and so on and never settles in..cruising it fluctuates from low 12s to mid 13s even at a steady rpm..I haven't been able to swap a different carb on yet as Mr. Porter advised but I'm starting to lean that direction more as I drive it around.
As I drift into recollections...I had some of this issue on the previous build, though not nearly as intense. I was able to tune to a point it wasn't so noticeable (I also was not running an A/F meter, so...). This carb is the one thing that's remained constant (except the distributor) and although I've rebuilt and tuned and gone through it multiple times maybe there's something just not right with it. (There was lots of talk of issues over the years with these older BG carbs...)
I have had similar results with my 850 Demon carb. My combo has shifted around in terms of cams, timing, compression but the A/F numbers seem to hover in the range that you listed despite jetting or PV changes. I've tried different carburetors that have shown a different range of numbers, a Holley 870 Street Avenger and a 3310 model 750.
 
When was the last time you replaced the metering block gaskets?

Try shooting carb cleaner through the air bleeds.

Make sure the PV isn’t dried out and stiff.

Where do you have the floats set?
I’ve had the best luck with those carbs with the floats just above 1/2 way up the site glass.

Try running the mixture screws a little richer...... maybe 1/8-1/4 turn.

That thing should run perfect with the initial at 30* and the total at 36*.

If none of that sorts it out....... I’d be looking for a known good carb to borrow.

We were a BG dealer when the Demon line came out.
Those things were a huge PITA to get sorted out 90+% of the time.
I had a used stock Holley 850DP(4781)I took in trade for something, put a kit in it, and used it for a dyno carb.
Countless times we’d be fighting with some Demon on the dyno trying to sort out some issue........ swap to the Holley(basically to verify the problem was the Demon carb and not something else), and whatever problem we were having just magically disappeared.
Despite the fact that they had a decent dealer program for the BG line, where you could sell them for the same price as Summit and still make it worthwhile to sell carbs...... we stopped carrying them.
Many times they required several hours of dyno time to get them to run well enough to let the motor leave with one of them on it.
In the end........ they were more trouble than they were worth.
And........ that’s not even counting the ones that had metal shavings in them, or the ones that were put together with flat out defective parts, or the ones that got damaged during assembly........ all of which somehow made it past QC.
There were several times when it was pretty quiet on the other end of the phone after describing some of the things we found wrong with those carbs while trying to get them to work.
Ok thank you. The carb got a refresh before the break-in, so it's had thecbreak in time plus maybe 10 miles..the floats are at the lowest of the 3 lines on the sight glass. I'll bring them up to half and re-check things. Yeah the issues you describe were pretty common it seems..
 
I have had similar results with my 850 Demon carb. My combo has shifted around in terms of cams, timing, compression but the A/F numbers seem to hover in the range that you listed despite jetting or PV changes. I've tried different carburetors that have shown a different range of numbers, a Holley 870 Street Avenger and a 3310 model 750.
Thanks KD, I'm getting the feeling that the calibration of this carb could be part of the problem, or I just got a turd.. but it's not adjustable without drilling/tapping certain things. Admittedly I was fairly inexperienced when I bought it, and back then the internet wasn't the bounty of information it is today. I was relying on my chevy friends(lol) and articles in the mopar mags for parts selection and those carbs were the new hotness...we'll see how it works out! The engine does sound more powerful through the exhaust than it did before, so that's positive!
 
Awright...this is driving me nuts! Updating this thread. Swapped on a QF street-Q 750. Not much change. Still won't idle below 24-26 btdc and barely idles at that. Read about the rotor/pickup woes out there so I swapped the pickup wires. Fired it up and the timing read 50 degrees(it was at 30) so I backed it down...same crap. No good much below 30. Revs and gets smoother as it approaches 40...Surprisingly (to me) it runs pretty much the same with the wires switched. Why won't this thing run without so much idle timing! I mean 40 is too much....Drove it at 30 with a 6-degree advance curve and it lacks response. Put on some good wires and plugs again, no change. I adjusted some preload out of the valves, to make sure they weren't hanging open...no change. Vacuum still a jittery 9-10 in gear. Spare coil and ECU didn't change anything, shakes and rattles at stop lights. I'm going to try a different distributor but I have to wait for my buddy to get back in town, he has a good spare one..
 
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