Steven6282

Ok, well some of this stuff is going to be hard to explain what exactly it's doing but hopefully someone can interpret my ramblings and I can get some feedback on this. I'm no mechanic, I can fix things and work on my car but this is where my inexperience shows as I have no idea what this thing is doing.

Let me start by saying the car ran fine on my quadrajet, it just had some issues with the carb seeping gas down making it hard to crank, and low end performance issues because the carb was to big for my engine (750 cfm on a mildly built up 350).

Now, I put on a holley street avenger 570 cfm carb (my best estimate using calculators and such said I needed a 550cfm, and this was the closest I could find to that): http://www.summitracing.com/parts/HLY-0-80570

Out of the box after installing it I ran it down the road and it seemed to run ok, had a slight lag in lower RPMs. I figured it needed to be tuned a little no biggy. I could feel when the secondaries open which added to the thought that it needed to be tuned since from my reading you shouldn't feel them open if it's tuned properly.

Saturday I decided to take it for a drive, again it seemed to be fine initially, and actually it didn't seem to have much lag in the lower RPMs that day. It was rainy the first day, so I thought maybe the weather and higher temp was helping it run a little better. Then, under one acceleration (not a hard acceleration, but not casual), in third I hear a thumping noise in the exhaust. Kind of like a backfire but not the loud bang a backfire gives. After that I start noticing a lot of stumbling / hesitating when going up hills, can feel the car jerking a little and such. I turn around to head back home with it, when starting to accelerate after turning around the thing just completely falls on it's face. It didn't stall, but it's like as if all cylinders stopped firing simultaneously for about 3 seconds... I don't think that is really what happened but that is the best way I can describe it lol.

I made it back home with it, but I really have no clue what to even look at with what it's doing. I knew how to tune the quadrajet some based on if it was bogging or whatever, but with what this thing is doing I'm lost. The carb is the only thing that was changed.

One of my friends suggested that it sounded like timing issues, but I'm not sure how changing the carb could effect the timing? My distributor is a mechanical and vacuum advance distributor, however I don't run the vacuum advance on it because the mechanical advances it as far as my engine spec recommends on it's own. I do plan to check the timing again to verify though.

Anyone have any ideas what could be going on?

pancherj

There are some things that definitely changed going from the quandrajet to the Holley. For one thing, the holley is vacuum operated secondaries that open at some set vacuum signal. The quadrajet is closer to a "secondaries on demand" kind of opening. Also, the quadrajet has much smallar primary barrels which give a good signal to the boosters and that can mask issues that the Holley wont like (like no vacuum advance).

No reason not to run vacuum advance. Hook it to manifold vacuum. You should have something like 42 degrees of advance at idle and light throttle. That may not be enough or it could be too much. You need to hook it up and see. After that, set the idle speed and adjust the idle enrichment. Next, adjust the vacuum secondaries to open as late as possible. It should run down the road at this point. From here, you can adjust the secondaries to open quicker, mess with jets, etc.

Steven6282

42 degrees of advance at idle?? seriously? lol

My engine specs from when I bought this engine almost 2 years ago are 17 degrees at idle (750 rpm) and a TOTAL of 38 degrees at 4500 rpm.

Last time I checked it when I replaced my intake I was running at about 17 when idling and advancing to around 44 @ 4500, with no vacuum advance on it. If I hooked up the vacuum advance it was doing like 23 at idle, and over 55 @ 4500

five7kid

iTrader: (14)

That's mechanical advance. Vacuum advance is supposed to be on top of that.

pancherj

This is what we call a teachable moment...

17 advance at idle (as low an idle as you can turn it so you are 100% sure the mechanical advance isn't kicking in) is pretty good. With a stock HEI, it may overadvance at higher speeds. You might have to limit the total mechanical to achieve 34 to 36 at something around 3000RPM.

The vacuum advance is on top of the mechanical, but only at idle and low throttle opening. Once you start to open the throttle, the engine vacuum drops and the vacuum advance goes away and you operate on purely mechanical.

If you have anything other than a stock camshaft, your engine will want the vacuum advance and it will want at least 42 degrees at idle. I run 52 degrees on mine!

bigal55

iTrader: (1)

Old school stuff! Most small blocks that aren't overly modified like about 38 to 40 degrees total advance. As stated above, the vacuum advance works under high vacuum conditions (idle, light load and coast conditions). The mechanical advance should not be "in" at idle, but should steadily add timing as rpm increases to about 2500 rpm by which time it should be all "in". With the vacuum advance disconnected (line plugged) and your idle set around 650 for a manual trans and around 750 for an auto trans in park, your "base" timing should be around 6 to 8 degrees advance for a manual trans and around 10 to 12 for an auto. When your motor is running about 2500 to 3000 rpm (light load conditions) your timing will be the total of all 3 parameters, ie. 8 degrees base timing + 20 degrees mechanical advance + 10 to 12 degrees vacuum advance for a total of about 38 to 40 degrees total advance. These are of course general guidelines. The more mods you make to your motor the more these parameters will have to be tweaked, but you should never run beyond 40 or 42 degrees total advance unless you plan on rebuilding your motor often. By the way, if I remember correctly, the quadrajet is a vacuum secondary carb not mechanical secondary. My guess about your carb problem would be jetting. I owned a 300 hp 350 Nova years ago and that car came from the factory with a quadra jet that was sized around 750 cfm (I don't recall the exact size). The 570 cfm Holley carb is modestly jetted and I believe it is designed to be primarily used in dual carb setups. When sizing a carb for a given cubic inch displacement, a larger vacuum secondary carb is recommended because the motor (vacuum signal to the secondary dashpot) will determine the amount of fuel it needs and recieves (this is the basic design reasoning for a vacuum secondary carb). When running a big cam, the vacuum signal to the carb is altered and a mechanical secondary carb is recommended. So, after saying that, I would recommend you look at your plugs to see if you have done any damage to your motor already (hopefully not). Get your timing set correctly and possibly your distributor curve setup to achieve optimum advance. As for the carb, I would either exchange it for a 700 to 750 cfm range vacuum carb or you are going to have to modify the 570 to suit your engines needs. Hope this helps and good luck!

Steven6282

Ok, I don't understand.

According to the company that built my motor the engine specs I was given are for total advance. Not mechanical only and then vacuum is allowed to go over that. I was told that I should run the engine as close to 38 degrees total advance (regardless to vacuum or not) as possible.

As for the carb sizes, every single calculator I use says I need between a 520 and 560 cfm carb. Even if I put in the specs of my engine as a race tune on some of the sites the highest I've seen it suggest is 560 cfm. The problem with the quadrajet is no matter what I did, low rpm always bogged down, exactly what everything I read says will happen if the carb is to big.

I would question the 570cfm being to small, because when it was running good it had no problems and strong power up to 5500 rpm. It never stuttered like it was lacking gas. The problems didn't start until later as I described above. They were showing under cruising speeds only when going up a hill, and moderate acceleration not hard acceleration.

Why would I go against all the stuff I've read, and what all the calcs tell me, and put a bigger cfm carb on the engine? I've still got my quadrajet, I could just throw it back on there, but then I'm still going to have the low end bogginess, and the gas seeping down making it take 30+ seconds of turning it over to get the gas back up to the carb and crank it up.

The specs to my engine are to the left, but just to be sure, it's a mildly built up 350. .440" Cam, some ported heads, nothing fancy, but it's enough to hit 315 rwhp on a dyno.

JaBoT

For the timing your confused, You want to have total mechanical advance at around 36 degrees and usually all in by no later than 3000 rpm. Depending on what distributor you have an how it is set up depends on how much timing you will have at idle.
Then there is vacuum advance which increases timing on cruise to provide better fuel milage and a smoother running engine among other things. This has absolutely no bearing on the total timing of 36 degrees at 3000 rpm. You can also hook the vacuum advance can to full manifold vacuum so you can increase the timing at idle to smooth it out on bigger cams. And once again this has no bearing on total timing. Once you go hard on the gas the vacuum drops and the vacuum advance basically shuts off.
Forget those carb calcs, they suck.
I would go with a holley style 600 or 650 double pumper. In the end the engine is only going to suck in as much air as it wants and no more.
Keep in mind though. Any carb you bolt on is going to need some tuning.

bestracing

When you get into the upper RPM's the manifold vacuum will be at or low enough not to matter so your total timing would be just mechanical and you would set there. Cruising doesn't demand much from the motor so vacuum advance will be fully in and give you the best economy and power for that application.

A 570 is really more geared more towards a low reving 305 motor out of the box than a mild 350. I've had great results with a Holley 650 on a 350 with mods like you have. (9:1 compression 350, stock 1973 2.02/1.60 GM heads, long tube headers, Crane fireball cam) The 750 was too much and I didn't feel like messing around with a Carter carb to get it tuned in at the time.

bigal55

iTrader: (1)

Above you state the motor pulls good to 5500 rpm. The 570 carb will limit your upper rpm power just as the long tube small diameter runners on a tpi system do, it will only pass so much air. It will run fine in the lower to mid rpm range provided the carb is tuned correctly for your setup, ie. jetting, power valve sized correctly, etc., etc. If you are set on this carb, don't expect any high rpm power, but it can be tuned to run right. I would suggest you go out and buy a good book on Holley carbs and read what is needed to make this carb work correctly for your setup. Every carb and every engine is different and will require it's own tune so to speak. No one here is arguing the fact that the 570 carb is wrong, it's just that there are better alternatives which may prove easier and less costly to tune to your setup. Good luck and keep at it, you'll soon be motoring happily down the road with a lot more knowledge and the satisfaction that you did it yourself.

check out this post on timing. https://www.thirdgen.org/forums/carb...m-vs-full.html

bestracing

Well said, that is more of what I really wanted to post when saying the the 650 would have been a better choice.

To the OP: Also check out the sticky thread at the top of this section and read the thread on how to set up your Holley by "sofakingdom". A lot of good info on where to start and how to set up your carb.

bygddy

Which "ported" heads are you using, what's the part number on your cam, lift and dur @.50 pls.
The 570 is not even in the ballpark of "right" Carb....perfect for a stock 305, not even close for a motor that hit 300+ whp.

bygddy

As for timing, set your initial at *16, assuming a stock HEI this will give you right around *36 total advance. Then hook your vaccum advance to full manifold like everyone is telling you, stop worrying about what your engine builder says because unfortunately they tend to talk to everyone like were all builders and understand the lingo. That isn't always the case, you will likely want to buy an adjustable pod, they are cheap and easy to install but you can look at that after you get it up and runnimg.
Sell the 570, go buy a 650DP, its not too much, I actually prefer a 750 on any reasonably strong 350, but they are more sensitive to timing and tune so stick with a good 650, the HR650 from quick fuel is an excellent piece.