Engine tuning help

[DruBlazer]

Hey guys,

I am having some trouble with my acceleration. It seems as though the engine is not advancing well from ~ 0 - 3000 RPM. To the point where if I'm at a stop light and stomp on it, it'll die. Above 3000 it seems to work fine. It's a 71 with a 350. It has the stock HEI setup with a Edelbrock 600 performer on a conversion plate on the stock manifold. It also has the stock fuel pump. I've taken the distributer off and the vacuum advance seems to be working as well as the mechanical advance. The weights were free and the springs seemed good. It idles a little rough, but not bad, and the timing seems appropriate too.

Any Ideas....Help!

Thanks

Drew

[blazerparts]

perhaps dirty fuel filter,gas, or weak fuel pump?

[Paul72]

Perhaps the carb is too rich/lean. I would guess it's on the rich side, since you're getting good RPM's. Maybe a choke adjustment?

I'm not an Edelbrock guy, so I can't help you on the details...

Paul

[CHEVIBUBBA]

several things could be wrong.
sounds like the engine is flooding with fuel.a sudden lean condition will result in a backfire,(backfire=thru the carb. afterfire=thru the exhaust).
1: shooter may be too large.
it's the nozzle that points into both primaries from the front center.
2: pump stroke too quick.
this coupled with above will pump alot of fuel all at once into the primaries. try moving the accelerator pump link rod to the hole in the pump actuator lever that is closest to the end/furthest from the pivot point of the lever.this will cause a slower pump delivery.
3: metering rod spring too stiff. this will cause the metering needle to rise too soon flooding the engine
4: rod/jet calibration incorrect.hows the fuel mileage?
5: float level too high.this will also cause these symptoms.
you said the vacuum & mechanical advance seem right.how did you test them?
some more details/history may help in diagnosing the problem.
carb history, who tuned/installed it?
what's it off of or is it new?
this sounds like a fueling problem. i would concentrate my efforts in the carb.
& get rid of that damned adapter plate.
a good, used performer rpm intake should be available on ebay.it'll do wonders!
hope this helps.

[67shortie]

Yes, the adapter plate is not helping things here, but I imagine the plate is in there because it is on the stock spread bore iron manifold. The only things you can do to save grace here is to make sure your timing is set up correctly, including your vacuum advance, and your carb set up. I would place your vacuum line on the timed port, not full. Now this will not help full throttle, but it may help you on step in when you put your foot down. Also, try bumping your mechanical timing (turning the distributor) a few degrees and see if you get a ping or if it helps at all. Lastly, you could put lighter advance weights in, just to see if you could get that full timing in earlier.

The carb setup, CHEVYBUBBA pretty much covered that subject.

Also, was the engine doing this before? Or only after you installed the new carb?

[Paul72]

Check your plug wires also. I had a bad wire once, and I spent all day working on the carb trying to fix the same symptoms you are having.

[DruBlazer]

Hey thanks guys. This board is such a great resource.

As far as how it ran before, I have no idea. I only bought it a few months ago and have been cleaning up the POs problems ever since.(typical) Anyhow I tested the vacuum advance with a hand pump vacuum tester. I can't remember off the top of my head what the values were. I'm getting about 10-11 mpg. I drive it pretty light footed. I'm not getting any backfires out the carb or the exhaust.

Thanks again for the help.

[CHEVIBUBBA]

here is a link to the only book you will ever need for your carb;
"super tuning & modifying carter carburetors"isbn number:0931472113
http://sabooks.carshopinc.com/produc...ts_id/46844/11
read this & you'll be an authority on carter carbs!
i cannot recommend it enough.
considering you won't get a qualified tech to even look at your carb for less than $20 it's a steal.
you may find it cheaper at halfprice books.go in & browse
edelbrock has tuning info on their website for these carbs too.or they did.
these carbs also respond well to simple cleaning.just spraying the venturi's & all with a good carb cleaner will clear up some performance degradations.
oh, in order to find out if it's lean or rich you need to read your plugs.
here are some excellent articles on plug reading:
http://www.dragstuff.com/techarticle...ark-plugs.html
http://www.dragstuff.com/techarticle...ead-plugs.html
http://www.strappe.com/plugs.html
http://www.centuryperformance.com/spark.asp

good luck.

[DruBlazer]

Thanks for the help chevibubba.

So the Carter book has good info that I could use for the Edelbrock carbs?

[CHEVIBUBBA]

yes. a few years back edelbrock bought the patents & tooling for the carter carbs.
with little or no refinments or mods, edelbrock started making/selling new ones.
edelbrock started selling a new carb back last year iirc.calling it the thunder avs.
this IS the same as the old carter avs carb,(air valve secondaries).
whats old is new again!
i believe they even kept the p.n. system for the rods & jets.
good move by edelbrock imo.
the afb is a wonderful carb.
i used to pull the air horn off almost everyday to make small changes.it's just so tuner friendly.
some say it's down on power vs. a holley but, all the #'s i've seen are within the margin of error for the dyno.so they are meaningless.
the holley design is just too sensitive.changes in as little as 10% relative humidity will sour the tune.

[cdowns]

with the hei distributor you should make sure that the PO ran a new wire to the dizzy cause if he used the original wires you'll only be getting 7-8 volts to power it
also inside the tank there's a fuel strainer sock that is not uncommon for it to have crap from 30+years clogging it

[67ChevyRedneck]

cdowns is right, check to make sure your ingnition wire has enough power. Also, take the cover off the coil and make sure the ground strap is in there. I forgot to put it in my truck and I couldn't figure out why it ran like crap for 2 months, it was shorting itself out, but like you said, it ran fine down the road, but wanted to choke/die under acceleration. The HEI module or the coil could be about to die. Your local advanced auto should check them for you for free. Then take the cap off itself and look for "lightning" marks, this will be an indicator of an electrical problem, misfire. Pull each plug wire out one at a time and start the truck to see if it runs different. Reinstall it and pull the next wire. Pull your spark plugs and check for wear/odd discolorations. I'd also check fuel filter. I think these things are fairly quick/easy to do. If none of this is it, I'd start messin with the carb. Could be dirty. You have one of those problems that could one of a hundred little things. good luck!

[DruBlazer]

Thanks again folks. I've give that stuff a try.

[curbyk]

Don't know if this would cause your problem but, where is the vacuum advance hooked up? There was a thread a while back talking about where it is supposed to be hooked up, I believe it is above the throttle plates. I'm sure some one will know if this is correct.

Good luck.