acerulz

Stock 350 vortec with dual plane intake and holley 650 and coil in cap distributor, took it to the "local" dyno guy (3 hrs drive away from me one way) and we had my car running perfectly. Drove home that night and ran the best it ever ran. The next morning I drove it and all it would do is backfire. Now going to the dyno the car was running crazy lean, so I figured that maybe it was just the plugs over heated so I replaced them using delco iriduims. The old ones were really carboned up so I figured i was on the right path. Now, not even 50 miles later the car is backfiring out the pipes, has no pickup and when I put my foot into it it will sometimes cough out the intake. Plugs are once again carboned up badly. Does this from startup to operating temperature. Timing is at 4 deg and did not jump, plugs, wires cap and rotor are new, coil tests ok. We have a 5.5 power valve in it, 70 jets primary and secondary, fuel at 5.5 psi. I'm strapped for cash right now and am trying to get my car back on the road as cheaply as possible. What can I look for that would do this overnight like someone hit a switch? I unfortunately don't have the cash right now for an air fuel gauge, the plugs look like its running rich but there is no smoke out the pipes or any big smell either. Sorry for the long winded story, just trying to cover all the bases lol.

acerulz

Anyone?

zed-028

a free test

put it on ramps, crawl under the car and disconnect the exhaust before the cat

take it for a loud drive and see if your performance issues disappear.

acerulz

I got that part taken care of already. No emissions testing where i am so no cats.

naf

iTrader: (7)

Holley tuning sticky? That's where I'd start.

pancherj

If it truly happened over night, then something went wrong and it is unlikely that it is the carb. It could be the coil or the module in the HEI. The symptoms you are describing sound timing related.

acerulz

Ran great when I shut it off. 12 hrs later its backfiring out the pipes. Literally overnight lol

PaulyC

Check your PV and make sure its not damaged, make sure the fuel level is correct, check the tuning sticky, the carbon is telling you a story.
I agree the ignition can cause this problem but they won't carbon up plugs, the two could be unrelated. You can have the module tested to confirm. Ill also bet the 70 mains are a fuzz too big for a stock 350, it will run and run well but if you slap a wideband on it I'm guessing you'll see a 10.5-11.5 and that will carbon up some plugs.

acerulz

Ya I dropped it to 67s in the front from a 70. Didnt seem to really like that tho but it's hard to tell cuz its running like crap right now. When I had the car at the dyno it was running just about optimum air fuel. Very next day it ran like crap. Checked out the PV and it seems to be working fine.

sofakingdom

iTrader: (1)

I'd start by rebuilding the carb. Put it back to its STOCK configuration, as a baseline. Record carefully what all the settings from the dyno were, but for the time being, put it back to what it came as. Go here for details, according to your carb list #. http://documents.holley.com/techlibr...al_listing.pdf

A 5.5 PV is usually just one part of Holley's BAD factory calibration. What the PV does is, when vacuum is high (low load), it is closed; but then as load increases, and vacuum drops as a result, when the vac drops to the number of the valve, it opens; and richens the mixture around 15 - 20%.

In a properly working carb, you want it closed at all times of low to moderate load, including idle (both in and out of gear), cruising, LIGHT acceleration, and of course coasting; but the instant your driver demands power (opens the throttle), it should open. The jets should be chosen to provide a lean mixture at cruise, typically something close to 14.7 but usually a bit less than that; then the PV should fatten it up to something in the high 12s.

A jet choice that provides good cruise mixture, but then a low-# PV, typically produces the classic "Holley bog" or flat spot; where you're kind of driving along, and you give it a little throttle, and the engine starts to stumble, so you give it more, it stumbles more, you give it more, it keeps getting worse, you give it more, and ALL OF A SUDDEN, it snaps out of it and starts to go. What just happened was, as you opened the throttle, the mixture stayed lean while needing power enrichment, but the vac was still above the PV threshold; then when you finally opened the throttle far enough to lower the vacuum to the PV operating point, it opened, correct power mixture was achieved, and the bog disappeared.

See the Holley Tuning sticky at the top of this forum for instructions on how to set it up.

Make sure your ignition timing is in the ballpark, and that you have NEW spark plugs (not "I think they're still good": NEW only) before starting to work on the carb!!! A typical timing curve for a "hot street" small block will be about 34 - 36° "total", consisting of around 16 - 18° "static" (dist body setting) plus 16 - 18° of "centrifugal" advance (springs & weights) with the advance starting just off idle (maybe 1200 RPM) and "all in" around 2800 or so, then about 12 - 15° of vacuum advance on top of that, active when vac is at cruising levels (typically 15 - 18" or so). Most factory distributors CANNOT produce a curve like that: they have WAAAAAAAYYYYY too much centrifugal advance, sometimes as much as 28°, meaning that the "static" has to be set to well below 10° in order to keep the "total" (full power) at 34 - 36°, which VIRTUALLY ALL engines need approximately that same amount ± just a few degrees, because it is mostly a property of FUEL and not ENGINE. The low "static" # that a stock dist forces on the engine produces lazy throttle response at low RPMs and high engine temps at idle, and attempting to calibrate a carb around that for crisp throttle response and peppy off-idle performance can be quite frustrating.

The Tuning sticky may or may not produce a calibration that gives "max power", but should be close; and will reward your driver the other 99.999% of the car's operating lifetime that it spends doing something other than that. I repeat, dyno tuning for "max power" is NOT a good idea for a street car, the way that it is for a race-only one.

A "stock 350 Vortec", as in, EXACTLY how it came out of the truck except with a different carb & dist on top, will want a shade less "total" advance than most other SBCs, maybe 32 - 33°; the same 16ish at idle and all that; and since idle and cruise vacuum is VERY high, will want a VERY high # PV. Probably a 12.5. Hard to say what jets, since that's a function of what carb you have, and its air bleeds and all that other stuff.

Go through the Holley Tuning sticky IN ORDER, start to finish, starting at the beginning and following through to the end, IN ORDER, skipping no steps and adding no others, IN ORDER, start to finish. Did I mention, you MUST do it IN ORDER: ign timing, fuel level, idle transition slot, jets looking for a "lean roll", PV, secondaries. DO NOT just "assume" the early steps are right, and jump to something right in the middle.

https://www.thirdgen.org/forums/carb...ey-tuning.html

Go do that and let us know how it turns out.

acerulz

Finally got back onto my car. After retracing my steps, and finally putting another distributor into it with new pickup coil (couldn't find a pickup coil for my aftermarket distributor) half my problems went away. It would only pop out the tailpipes above 2000rpm. Put a new cap and rotor on it and presto! Runs pretty good. Just in the process of dialing in my jets, but the car feels like it's got more power now versus when it came off the dyno. Thanks to you all for all your help and suggestions. You thirdgen folks never cease to amaze me with all the help! Time to go enjoy my car!