I have a BG Speed Demon 850 on my zz502 motor. I called BG and they had me change a bunch of crap on my carb, now it won't run and it's backfiring, really really loudly, it's backfired twice, so I gave up.

I adjusted the primary and vacuum secondary butterflies (hopefully that is the correct term). The tech explained that inside the carb there is a little vertical line, I needed to adjust the front butterflies to where it looks like a square and the back ones to where I can't see any of that line at all. If you need a visual you can go to: http://www.barrygrant.com/pages/manuals.aspx and download the speed demon manual. The line I am referring to is on page 3. We also found that my Edelbrock rpm air gap intake wasn't wide enough for the carb and it was causing a vacuum leak, so he had me go buy an edelbrock 1/8 inch spacer so the vacuum ports on the bottom of the carb would be covererd, which I have done and installed. My timing withoug vacuum connected is at about 12 and when the car was running it would idle at 700-800 rpm's.

I also adjusted my mixture screws as well, they are out about 1 1/2 turns.

I adjusted the butterflies and the car seemed to run better, but I tried to start it today and it won't start at all. There was one time when tons of smoke/unburnt fuel came up through my air cleaner when the car died, then shortly after that when trying to start it again I heard the loudest backfire I've ever heard. The BG tech asked me about backfiring yesterday and mentioned that I would need a new power valve if I hear it backfire yesterday and at that point I had not, but since I've heard it twice today, should I get a new one? What size 5.5, 6.5, 7.5???

 

Backfiring through the carb is likely a timing issue,..has timing been reset recently?

 

Yes, BG had me adjust stuff and my timing was at 4 (yes 4) so I bumped it up to about 12. I think it was round 18 before.

 

i would look at the spark plugs if they are wet or black then clean or change them, if they are fouled that would cause the engine to backfire. most of the time we set all 4 idle mixture screws at 3/4 turns from bottom, if the engine still can idle with the mixture screws bottomed out most likely you killed the power valve, the carb comes with a 6.5 power valve and that should be fine.

if you can bump the initial timing up to 16 to 18 btdc it may run better but keep the total in the 36 degree range plus vacuum advance

 

I will readjust the timing and the idle screws to 3/4 and maybe replace the power valve anyway. I think I have a 4.5 in there now, what is the difference between 4.5, 5.5 or 6.5?

 

i think 6.5 was original, if the vacuum with the car in gear is below 7 inches then 4.5 is fine be carefull if you go below 4.5 because at wide open throttle if the vaccum goes above the power valve closing vaccum the motor could go lean

 

I'll just play it safe then and put a 6.5 back in it, if that's what it came with.

 

Also,..if you have the original harmonic and your timing isn't making sense, it could be the outer ring on the harmonic has slipped giving you retarded (pardon the pun) readings,..and that will cause backfiring through a carb.

Too rich will usually cause backfiring in the exhaust,..as unspent fuel is re-ignited by heat.

 

 

I was looking at the manual and it tells you to adjust both the primaries and secondaries the same. You should see a small square at the transfer slot. My guess is you are choking the motor. Allowing way more fuel than air. You will find your plugs are probably very wet. First I would open the secondaries up to spec .020 or a visual square. I am not sure on the demons but for the price they should have backfire protection for the power valve. I am betting that they do. 6.5 is a good starting point.

 

As for the harmonic balancer, I at one point thought it was wrong, so I manually cranked the engine to TDC of the compression stroke and it was right on 0 on the harmonic balancer. This was the correct way, right?

 

Ok, here is an update, I've put a new 6.5 power valve in, adjusted all the mixture to screws to 3/4 up through 1 1/2 and am still sitting at about 10 on the vacuum test gauge. After I got it running and idling at about 800-900 rpm's I took it out for a drive. It ran fine up until about 3000 rpm's and then the motor cuts out like crazy, I can shift and it will be really strong until about 3000 again. Any idea why it's doing it? I remember that this part was happening even before I started adjusting my carb.

My timing is at about 28 at idle without vacuum and 30 at idle with vacuum on a warm motor. This seems a little high, but if I take it down to 12 the car idles at about 500-600 which I've been told is too low. I tried to time it at 3000 rpm's but I think my light is crappy because it starts bouncing all over the place.

BIGGEST concern is the engine cutting out at 3000, any idea why? I put new plugs and wires in about 2 months ago.

 

28 inital timing (at idle) is way too much,..this would give you about 50 degrees total and cause the pre-ignition.

Reduce to 10 degrees and adjust your carb idle speed as needed.

Good luck.

 

The tech at barry grant told me that I should never adjust the screws on the carb, my idle should be adjusted by timing, but I don't understand that. There are is a fast idle screw underneath the electric choke and there is a nother screw on the drivers side of the carb, underneath the throttle linkage. I had used that one in the past to adjust idle, but the tech told me that is wrong and if I have messed with that screw, then I need to take the carb off and adjust the butterflies to show only a little bit of the transfer slot.

The other thing I forgot to mention was that I can turn the mixture screws in the rear of the carb completely in and the motor still runs fine, whereas before it would die. Can someone help explain all of this?

 

Ok, I think I've got it. I mainly keep posting as a guide for someone that might ever have the same problem. I forgot all about Lars's BG carb paper, so I opened it and followed it to the letter and what do you know, the car runs a lot better. I wish I would have remembered this paper several days ago, anyway, I took the carb off, readjusted the butterflies so they both show a little square of the tranfer slot, I then started with all 4 idle screws turned exactly 1 turn out from just barely seating. After opening the choke the car seemed to start just fine, it idled a little high, right around 1200 but Lars expected that it would. I then dropped the timing to 16, then to 12 and backed the idle screws out 1 1/2 turn and it seemed to idle at 1000 and run just fine.

Hope this info helps someone else and a BIG THANKS to everyone who helped me out.

 

Unfortunately, you've received some really bad advice.

With as much timing as you have in her, I don't know how the starter managed to turn it over,..usually this much timing causes 'kick-back',..caused by the plugs firing too early in the compression stroke.

You don't set your idle speed with timing. The idle adjustment screw near the throttle linkage is for this purpose.

Set your timing at 10 degrees,..you'll likely have to up the idle speed as you retard the timing. You retard the timing by loosening the distributor hold-down clamp then rotating the distributor clock-wise (looking down).

After your timing is 10 degrees at idle, you're in the ballpark. The ZZ-502 should idle around 850 rpm with a manual shift, or about 650 in gear if automatic.

Then once you have proper idle speed and timing at 10* take her for a spin,..if she still pings or rattles, you'll need to retard her a couple of degrees,..if not, try advancing the timing to 12 degrees at idle. Each time you adjust the timing you'll likeky have to adjust the idle speed.

Then when you have a pretty good setting, adjust your idle mixture screws, Screw them out three full turns. Screw them a quarter of a turn each (take turns) until the engine's rpm's start to drop, then back out a quarter turn.

Good luck!

 

sorry to be so late!

-backfire can occur also when you try to start engine when your carb is too lean

-timing must be set with vaccum advance disconected!

- On some crate engine don't connect vaccum advance! my 385 Fast Burn musn't run with! ( sallee chevrolet advice) but fot a ZZ502 ?

ZZ502:
HP 502 @ 5,200 RPM
TQ 567 @ 4,200 rpm
Spark Timing: 36° total at 5000 RPM, 8° BTDC @ 800 RPM

 

I recently had similar trouble with my Mighty Demon 750. It needed some serious tuning before it ran right.

There was a big backfire if I'd rev the engine up quick in neutral. And a really bad bog off idle when I got off the clutch in first gear. I had several problems:

First, my engine made 6-7 inches of vacuum and I had a 6.5 inch power valve. So basically it would open way too soon, which made then engine run way rich. I replaced it with a 4.5 power valve to fix the problem.

Second, my squirters were too big. Installed were 31s, I replaced them with 28s and after I did this and the power valve, I had no more off idle bog and no more backfiring.

Finally, it was running pretty lean, so I upped the jet sizes 2 sizes on the primary side and three sizes on the secondary side. Runs great now.

Also, I had to take off the base of the carb to crack the idle eze open. It runs better with the idle eze all the way open. And my idle mixture screws are 1 turn out, I set them where the engine produced the most vacuum.

And the thing with setting the idle, they want you to set the butterflys so that you can see 20/1000s of the transfer slots on each side. So that the lines look like squares. Then you're supposed to set your base timing till the car idles at around 800. My engine seems to want about 16 degrees base timing. I had to change out the bushing in my distributor to limit my max advance to around 34-36. If I didn't I would have had close to 40 degrees max timing at WOT. I recommend doing this only if you have a good distributor. The engine idles fine with the trasfer slots in this position and my MSD dist at 16 base timing. But with my old HEI the engine would not idle at 16 base timing, it wanted more. If your engine wants more than 16, consider moving the trasfer slots until you can buy a better dist.

Good luck.

 

It is much better to adjust your idle with timming when using a BG. You set the butterflies in the correct position as BG calls for. Then adjust your timming to set idle.... this is correct.... You then change your distributor stops to set total timming. You do not adjust total timming with initial.

 

I'm with you until "you then change your distributor stops to set total timming. You do not adjust total timming with initial". What does that mean?