Hi Guys! I've chasing my tail with an issue on my 70GS. My gas mileage has went really really bad like 3.5MPG in town babying the throttle (never used secondaries) and also the performance is as bad as the Mpgs My car has a 4 speed muncie, HEI distributor, 625CFM Street demon, completely stock internals, heads valves seat and guides recently made and using holley fuel pressure regulator set at 3PSI Sometimes when driving I feel like the engine starts failing like if it were misfiring but actually is like it looses power, when that's happening no matter how much throttle I gave it the revs doesn't increase sometimes even backfires. I though I was running too lean so put a bigger main jet and nothing changed just that now my plugs are all black. If I adjust the idle screws the car feels a bit better but not as it should even the idle isn't completely smooth Already checked my timing mark and is stable so the chain should be in good shape. I'm running 18° initial BTDC with vacuum advance. Where else should I check?
How do you know you were running lean? If you don't have a wideband on it, you really have no way of knowing your air fuel ratio. 18 degrees initial?? Why so much? 10-12 initial and 32ish total plus the vacuum advance. You may need to limit the advance from the vacuum advance. I would recommend S-canning that Holley and having Ken or Mark build you a nice Q-Jet for it. You'll get plenty of performance and with the small primaries, better fuel economy Here is my car after tuning my Q-jet- 13.8 AFR cruising. You can see the tach is up around 2700 rpm. Starts good, runs good and has plenty of power
And why do you have a fuel pressure regulator? What kind of fuel pump? Your fuel pressure should be more around 6 psi for that Holley
According to Holley support 2 - 3.5 PSI is enought for this carb, more than that will cause the primaries to drop fuel (according to support tech) Running lean was my first guess due to the weird behavior, backfiring and poor performance. About the initial, I live at 5600ft over sea level, I've been told in the past that 1° extra is needed per each 1000 ft over sea level
The altitude would have an effect on your carburetor mixture due to less oxygen in the air but I have a hard time believing it would have an effect on timing unless someone can explain that one to me.
Heres how to be sure of the fact that you are too rich. Make sure that all parts of the carb are functioning as they should, make sure accelerator pumps are functioning, not blown. Make sure that everything works, period. Then, take 4 jet sizes out of both sides. If the problem goes away, tah dah Jason, lean mixtures burn slower, so with even less air per cubic foot, more timing needs to be added to account for the added elevation.
Sounds like you are way too rich. The power valve might be blown or incorrectly sized. At altitude, you need leaner jetting. And i think you misunderstood the Holley tech. No carburetor wants 2-3.5 psi. Ditch the regulator. A Q-jet is gonna be much better, get one built, and make sure you tell the builder you are at 5600 feet. Oh, and your initial timing tells us nothing. What is it advancing to with all the mechanical advance in, and the vacuum advance.
No I'm sure he told me that running 5PSI (that's what my pump makes) will cause fuel drop on primaries By the way this street demon doesn't has power valve, just accelerator pump. I'm going to take out the carb to see if something is out of place and also I'm going to check my total advance
I don't know, a 6-8 degree timing adjustment for 5600 feet seems a little drastic to me. I'd think more of a slight adjustment would be more in order.
As an FYI, a quick Google search says that a Holley Street Demon is similar to an old Carter Thermoquad. When I hear "Holley", I just think its a conventional Holley with side hung or cathedral style floats
OK, Street Demon, not a Holley, a Carter AFB clone. The primary enrichment is done with metering rods. Those are under the plates secured by a single screw on each side of the primaries. https://images.holley.com/583x/1900.jpg https://www.chevyhardcore.com/tech-...-the-most-out-of-the-street-demon-carburetor/
Carb is way too small.. Get back to us when you have a real carb on it. That motor wants 750-800 cfm. Not a chevy tuned holley either. 3 psi fuel pressure is not enough...you need 5-6 minimum
Sorry guys forgot to specify that I talked with Holley support just because they actually bought Demon so nowadays they're the manufacturer of these carbs. I still have the quadrajet that came with the car when I bought it, I think is the original carb of the car but is kind of weird because it has the earlier 70s style but with mid 78 serial number (like this). The problem with that carb is that somebody destroyed the idle restriction tubes so the carb is junk until I change those Anyway getting back to what I have now. I checked my advance and with 18° initial and vacuum unplugged I have like 41° at about 2850 - 2950RPM. Follow your advise guys and decreased the initial to 15° and now have 38° total without vacuum. Also checked the carb floats level and didn't noticed anything serious, one of the floats was a bit high so corrected it but the issue is still there. Idle isn't smooth as it used to be and I feel like the performance isn't even closer to what it should. Checked for vacuum and the best I can get is almost 19" but before the heads job it was like 21", to be honest I never checked the vacuum after the heads job until today that I'm figuring out this issue so I'm not sure if the vacuum was 19" since the valve job or if just lowered now with this issue
