I'm gonna repost the first section of this from CF, and follow that up with the new info from today.

Posted 9-14

Having some trouble with my new 383 engine. It's in a C4, but since I carbed it I figure this is the better area to go for tuning advice. Engine is a relatively mild 383. 9.8:1 compression, ported stock 462 iron heads, cam is .465/.488 with 1.5's and 224/234 @ .050. Distributor is a vacuum advance HEI MSD from Summit (just a basic distributor, no MSD box or anything like that). Base timing is at 10*.

So here's the deal. I had a Barry Grant carb on this engine and had nothing but problems with it, so I changed to the Holley StreetAvenger 670. Vac secondaries, with the heavier springs in the package (the yellows instead of the blacks). Got the engine barely broken in at this point, maybe 500 miles. It fires right up no problem. Choke kicks off and idle settles in at around 850. Seems just a tad rich at idle right now, but I'm not done fiddling with that yet. The engine produces good vacuum and all that at idle. But the preformance between idle and ~2000 rpms is terrible.

When leaving a stop the engine is very sluggish to respond at lower rpms, and you really have to feather the clutch and be quick with the gas pedal to keep it from stalling. As it accelerated from off-idle to 2000 rpms the engine stutters and bucks a bit, almost like it's too lean, but it doesn't backfire. At around 2000 rpms the engine clears up and runs great from there to 6000.

Once above 2000 rpms it cruises fine too, with very light throttle the engine maintains 2000 no problem and runs nice and smooth. From idle-2000 you gotta give it a lot of gas and it still bogs terribly. If you try to cruise at 1400-1800 rpms is when the stuttering is worse. It gets a bit better under heavy throttle at low rpms, but it's still pretty rough until 2000 rpms.

So, I'm wondering where to head first here. I'm thinking either the distributor isn't advancing to where it needs to be until the higher revs, or the carb isn't opening the secondaries early enough. Like I said, it feels lean based on the stuttering, but it doesn't backfire. Fuel pressure is a rock-solid 6 pounds because it's an electric pump with a return-style mallory regulator and a gauge. Anyone got a suggestion? The car is down at the moment to have the trans rebuilt, so I can't hit the road again for a little while. Anyone had a similar problem? The Barry Grant had a similar problem of very poor performance down low, so I'm thinking it might be the distributor curve? Possibly the vac advance mechanism? It is adjustable, but I'm not sure how to set it statically since it's based mroe on engine load. I guess I could unhook it and see if the car behaves the same way?

And the new info from today:

I had the trans rebuilt, and finally got back to this engine problem this weekend. Here's what I found.

For starters, we had the distributor hooked up to ported vacuum (wrong). I have now tuned the timing following the various procedures here and in the Lars papers. The base timing is 14, it's all-in mechanically to 36 by 2500 rpms. With the vac canister hooked up (to manifold vac this time) it's idling at 34 and a total timing max of 50. All seems fine on the distributor end of things.

So, I took her out for another road test and the results were not impressive. At first it seemed like I had the exact same problem. Took me numerous tries to get it out of the very gently sloped driveway. Car revs just fine when sitting still, but the first sign of a load and it falls flat on it's face. Anyway, got it to the road and cruising seemed ok. A little bucking under light throttle, strong performance under heavy throttle. But then it changed.

I haven't had a good long road test for a while (cause of the trans problem). I guess I didn't realize it, but this seems to be more of an intermittent problem for some reason. Case in point, once in a while I'll come to a stop and leave it just fine. 1000 rpms, light throttle, no bog, no hesitation. Next time I might have to play a freakin symphony on the cutch/throttle to keep it running long enough to leave a stop.

Also, I hit a good sized hill nearby and lugged it down to check for knocking after raising the timing so much. No knocking, but it did act really weird. For the first 2/3 or so the enigne ran great. 1300 rpms in 3rd gear, really lugging it down. Then it just fell flat again. No change in pedal pressure or anything else, engine just flat out lost it. But I pushed the throttle hard to the 3/4 mark or better and it picked back up again.

With that information, I'm now wondering if it could be a fuel supply problem? Perhaps I'm draining the primary bowl dry and when I open it up I draw enough from the secondaries to keep from stalling? I know the accel pump is working, I can see it squirt and it seems centered on the cam fine. I replaced the stock pump 2 or 3 years ago and ran it as a fuel-injected car fine for several months on this pump. Now I'm running the stock in-tank pump through a Mallory 4307 bypass regulator. Rock-solid 6psi atleast when sitting still, no matter how hard I mash the pedal. I can't understand how I might be running the bowls dry with that setup though.

Also, I've had two different carbs on here with almost the same problem. I originally had a BG 650 mech secondaries carb which I removed for this similar issue (I guess it wasn't the carb after all, but BG has had so many quality issues I thought it was). Makes it seem unlikely to me that it's the carb, but I guess it's still a possibility.

Anyone got an idea? A test I can perform? I may try to rig up a way to see the FP gauge from inside the car, but it'll be tricky since the reg is mounted to the front of the engine. What else might it be at this point?

Lean it out, add more advance under 2k.

What is the vac look like when it is stumbling??

I think your pumpshot might be a hair too much.. I have not tuned a carb in over 10 years, but carb, efi, same thing. sounds like you have too much acceleration enrichment.

When I tune EFI cars, I usually get it to the point were it pops on an accel, then add fuel in.

-- Joe

Not sure what the vac looks like when it's stumbling. I'd have to get a longer piece of vac line and watch it I suppose. Is there something I should/should not see?

I'm not sure about the pump shot, though. I can't even rev it to a steady 2500 and ease the clutch out. It'll hold at 2500 no problem while not moving, but as soon as I start to ease the clutch it'll fall flat immediately and the only way to catch it is treadling the gas (more pumpshot, I suppose). This one's really got me stumped (as if I wasn't having enough problems with this car these days).

The other weird part is the fact that it's somewhat intermittent. Occasionally I can ease from a stop at 900 or 1000 rpms no sweat, barely any throttle. Other times I can't get it to leave at all without lots of pumping the gas and feathering and bucking and stumbling. I still think it's a fuel issue at heart, but damn if I can figure out how.

Forgot to add something, which I also find really weird. I can not get the car to stop running rich. At idle you can smell that it's rich, and the exhaust smokes (not oil smoke). I lean it out in steps and it doesn't stop. Eventually I get to the point that it's too lean to idle and it stalls, but it never clears up the idle. And it's not just at idle, either. My buddy followed me on my test drive yesterday (in case I got stranded) and he said it smoked almost the whole time. The only time it didn't smoke was when the engine was really working hard, like accelerating up a hill. Then it cleared up.

The plugs are all a very healthy tan color, none of them have fouled during my tuning sessions. Yet the car continues to be too rich. Can't find anything in the Holley manual about troubleshooting this. Any thoughts on that too?

With the gas today its a little harder to judge by appearance of plugs. Im assuming carb is running on its idle circuit meaning throttle blades arent open too much at idle?

Stock air bleed sizes? what size Power valve? Checked it to see if it is good?

It sure does act like a powervalve problem, and the thought has occurred to me. StreetAvengers are supposed to come with a standard 6.5" PV, which would be just fine for my 15" of vac at idle. Now, I suppose it's possible it came with a defective PV? But these carbs are supposedly wet-flowed and calibrated at the factory. Surely they would have noticed a defective PV?

I am reading this so I can refresh and reinforce my really crappy knowledge about how carbs work and how to tune them.

A few suggestions.

Replace the power valve. If the engine has ever backfired through the carb the power valve is probably toast. This a common Holley problem.

Check/set the float levels.

With as much cam as you have you may have the the base idle screw adjusted down/in to far which will cause the carb primary to engage the main metering system at or just above idle. The trick with cammed engines is to adjust the stop screw for the secondary butterflys so the throttle plates for the secondaries are cracked open a bit allowing more air into the engine at idle. This will allow you to back off the throttle stop screw & get the carb back on the primary idle circuit. Reset the idle/air adjustments.

Manifold vacuum to the distributor vacuum advance is a bad thing. Reason: As soon as you apply throttle the engine vacuum drops causing a drop in timing which aggravates an off idle stalling condition. Attach the vacuum advance to a ported vacuum source.

Churchkey, the SA has a check-ball style PV blowout protector, and I've done the float levels a couple times now. Yes, the idle transfer slots were way too far exposed, but I still couldn't get a decent idle even with the transfers at .020" and opening up the secondary bleeds. Fortunately I had a eureka moment this weekend and I was not just having normal issues with this carb. I took the bowls off finally to make sure the PV was the correct 6.5 that it should have come with and not an 8.5 for some reason, cause that's what it was acting like. You know what I found?

[IMG]http://i86.photobucket.com/albums/k113/JLeatherman_CF/Vette/CIMG1532.jpg[/IMG]

That's right, that is my PV with [B]TWO[/B] gaskets on it from the factory! That's why it ran a bit rich through the whole range, but not rich enough to be a blown PV. It was leaking fuel between the gaskets. I pulled both gaskets off, cleaned it up, and put a new gasket on it. Reassembled the carb at baseline settings again and you know what? The car runs so much better. I still have the usual Street Avenger stumble to correct, but it's not doing all the stuff it was before. Just a bit of hesitation off the line. Atleast now it idles, stops, and doesn't smoke me out of my garage.

wow!!!! whatta find!!!

WOW!! That was a pretty good catch!!

Quote:

I don't agree with that. The manifold vacuum can be used to advance your timing to get a steady idle, and the mechanical advance in the distributor should take over at speed.

With ported vacuum, you'll need a ton of base timing and a very slow distributor curve to avoid too much advance at high speeds and heavy throttle.

Quote:

Eh?

Ported vac, high base advance especially if it is cammed. Most performance motors like around 24-25 degrees at idle. You want the mechanical advance to ramp out to around 36 degrees by 2500,3000 RPM, however on a cruise when your vac is very high (like 40kpa) you want the ported vac source to keep your advance in the 40s for lean cruise but you don't want it to effect your idle advance.


-- Joe

Good find Leatherman.

Suggest working with or changing the accelerator pump cam to dial out the off idle stumble. If memory serves, there's an assortment of 5 cams in a Holly track pack.

Finicky carbs may require a squiter nozzle change & or a larger volume accelerator pump to get past the off idle flat spot.

I guess either strategy can work. I've asked and researched the same question before. I wanted to minimize the advance during heavy throttle and low rpm to avoid pinging, but my cam isn't too crazy either.

It would be interesting to compare the total timing curves of the two options. I guess itd be a 3d curve.

Ditto on experimenting with cams on the acc pump. Once fine tuned youll love it, should be as snappy as anything else out there.

Even with PV blowout protection those can still blow, had that happen. Checked everything except the PV lol

with 15 inches of vacuum would not a 7.5 powervalve be better? my rule of thumb was, 1/2 of idle vacuum.

Also, does your distributor have vacuum advance? I assume, can you adjust your vacuum advance, by inserting a 3/32 hex wrench in the vacuum tube fitting? Its been about 10 years till 2 weeks ago since I tuned a carb, however I always did the power tune method. I use to advance it till ping than retard it. I use to start with dizzy weights, and finished with the vacuum advance.

Just went through this pain in my plow truck, 10-1 355 sbc way over cammed 292 comp cam. took 2 weeks after work to get it right, included 3 carbs, and finally rebuilding the originally carb. I feel your pain. I did find 10* initial was the sweet spot also.

I don't know if they make a 7.5 PV. They seem to go in 2" increments. 2.5, 4.5, 6.5, and 8.5. 8.5 is too big, obviously, so 6.5 it is.

The vac can is adjustable, but I need to pick up a kit to set different stops on the mech adv weights. The car seems sluggish to start without the vac advance and only the ~16 initial advance. I'd like to have more base timing, like 20 or 22, for startup but I can't do that without limiting the mech advance weights to like 12-14 and limiting the vac can to another 14 or so. Otherwise my cruise timing will be way too high. Been outta town this week, but I'm picking up some more gaskets, jets, and distributor stuff this weekend.

BTW, at idle this engine doesn't seem too happy until atleast 28 degrees (actually, right now it's running 34 thanks to the vac can and manifold vacuum, and it seems plenty happy).

I'd have thought ported would lower my advance at cruise? At cruise I'm barely on the pedal, so manifold vac is high and the can adds 16* or so. With a ported source wouldn't my ported vac be very low with my light pedal pressure on cruising? Car runs close to 52 degrees at cruise right now, and it hasn't pinged yet. I'd like to back it off a bit though. It's got hyper pistons and they really don't like pinging. Plus it's on a 30/70 CAM2/premium mix for tuning and I wanna get it to straight up premium.

ported vacuum source on a holley is designed for a "lean cruise" if you can understand that. Meaning, weak at idle, strong at cruise, and weak at WOT. This is so for fuel mileage along with performance. At hoghway speeds 45* + in timing is not that uncommon, with light load. i ALWAYS USE THIS PORT. Oh and by the way, they do make a 7.5" power valve.

Now I would highly recommend a cheap Mr.Gasket distributor recurve kit. I used offset color springs, however I can not recall whitch colors I ended up with, along with what offset in stops.
. Also my base start with vacuum canister was as follows,however only really start playing with this after you get WOT in the pall bark, I completely turned counter-clockwise, than went 4 full turns in. I DID THIS ADJUSTEMENT PRIOR TO PLAYING WITH SPRINGS, THIS WAS THE FIRST ADJUSTMENT ON dIZZY I DID DO. I tried there, THAN SCREWED WITH THE WEIGHTS.

And after 2 weeks of tuning, after countless times removing cap, and other trial and error, I may off went like approximately and additional 1.5 turns on the vacuum advance. this is were I am at, again this was for my snowplow truck. with that engine, th400, 4.56 gears and 36" tall tires. Call it what you will with that cam, I expected a dog till 3000 rpns with torque, however, after tuning I am now pleased.

Interesting. So, maybe if I went with 24* initial, with 12-14 total from the mech advance by 2500 (I'd obviously need stops and maybe springs) I could then run ported and have better startups. How do you set the max advance from the vac can then? Since it's only coming in at cruise, and obviously without a dyno I can't be standing next to the car under cruise.

How does the ported do weak at idle, strong at cruise, and weak at WOT? Is it related to the air velocity above the throttle blades then?

I'm still confused why manifold vac isn't as good though. The manifold vac is high at cruise and idle, low at WOT or other heavy loads. Seems to make sense to me that I'd want to add timing at idle and cruise, and take it away under load. Both manifold and ported sources mean more timing at cruise and less under heavy load. The only difference between manifold and ported is the idle. Manifold adds timing at idle and ported doesn't, right? So couldn't you tune to either one, depending on your distributor stops and initial advance?

i am tossed between 10* and 12* initial. If your going to 24* initial AT IDLE SPEED, vacuum advance unplugged and carb ported vacuum port plugged, and your engine restarts normally, something is wrong, your balancer timing orientation is off!

No, I meant maybe I should. Right now the car's set to just around 16* initial. That's idle, vac can unplugged, choke off, all ports blocked, etc. But the car is really really unhappy at 16*. Pretty poor idle, hard starting, etc. With the vac canister hooked up to manifold it's around 34* at idle (18* from the can, 16* base, no mech), and the car loves it. Idle smooths right out, exhaust clears up, etc. This engine loves advance.

I'm wondering if that's part of my off-idle stumble problem. When I go to leave a stop, the first time the engine lugs down the vacuum drops and the timing drops with it. But the engine's still only at around 900-1000 rpms so the mech advance hasn't kicked anything in yet. That means my timing goes to base, 16*, when I leave a stop. The car isn't even happy idling at 16*, so it'd make sense that dropping the timing to 16 when I'm trying to load it would give it a major miss.

I figure a possible solution might be more base timing, 22-24 (about the point the idle first smooths out) so that the timing can't possibly drop below that when I pull out. Limit the mechanical to a total of 12-14 more (36* by 2500 rpms), and then run ported manifold limited to another 6-10 or so. That'd be a minimum of 22-24 at any given time, and a max of 42-46 under light cruise. It's just a question of whether I can get away with that much initial on pump-gas. In the current form I haven't been able to get so much as one ping outta it, but that's because the timing is falling off a ton under load.


OR, if I kept the manifold source and gave it more base and less vac. advance it might accomplish almost the same thing. Let's say I give it 20* base timing, and 12-14 more from the vac cannister hooked to manifold (32-34 at idle). That'd keep my nice smooth idle, but still wouldn't let the timing drop below 20* when I leave a stop. I'd have to limit the weights to an additional 16*. That'd be a minimum of 20*, 34* at idle, and still a max of 48-50 under light cruise. I already know the car can stand 52 under light cruise cause that's what it runs now.

I'm back to the same conclusion, that ported and manifold do basically the same thing if you set your initial and mech limits properly. Or am I just being crazy?

I disagree, they can't do the same thing, and I believe manifold vac gives you more options. I'm a noob at this but hear me out.

Using ported vac requires lots of base timing for a clean idle. This forces you to reduce the mechanical advance of the distributor to prevent too much timing at higher rpms.

Manifold vac gives you clean idle, allows lower base timing, so you can use more of the mechanical advance curve. It also increases timing during cruise.

Check this out:



Ported vac equals manifold vac but turns off at throttle under ~20%.

When my truck was setup on ported vac I had to increase base timing and increase the idle screw a bunch to get it to idle properly. And it pinged under acceleration at lower rpms. Also, the increased idle speed was actually causing the carb to start running on the primary jets which meant the adjusting the idle mixture screws didn't do hardly anything.

IMO ported vac is good for low performance cars with a puny cam that have to pass a smog test.

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24* total at idle.

A majority of people who think their car sounds awesome and "big cammed" really just have too little advance at idle.

Any cam over 220 degrees or so should have around 24 degrees at idle. Even a stock L98 idles at 20 degrees.


-- Joe

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CC, thanks for the graph. One thing, though. I'd like to know where it came from and if there was any qualifying information. It looks like it's a graph of the engine under no load. Load reduces manifold vacuum. I'd be interested to know if it affects ported vacuum at the same rate.

The purported advantage of manifold vacuum, being able to run less base timing and make up for it with the vac can, is what I feel might be hindering my car. I'm concerned that load at take-off is reducing my low-speed timing too far. That's why I'm thinking about running more base-timing on purpose, to limit how far retarded the car can be at take-off. Yes, I would have to reduce the mechanical advance, but after 2500 rpms it doesn't matter anyway because the car would be at 36 no matter where my base timing was. As the graph shows, after a certain point it doesn't matter which port you're timing from.

Anesthes, I'm actually kinda shocked at how low the timing dips on that graph under heavy acceleration. 24* total timing at 4800 rpms, even under 90% load, seems excessively retarded. Any car with a performance HEI dizzy is running a min of 36 at anything over 2500, even under hard load like that. It's that kind of retarding at low rpm loads that I'm looking to eliminate, assuming I don't get a knocking.

I agree with CC, 24 * with advance included, I was not figuring in the mechanical advance. I was doing the old school Method, hell I can throw the advance light on, I already know @ 2000rpm 's I am getting almost a full 38* degrees TOTAL TIMING. Tomorrow, I will throw the advance light on it, report my findings. Pretty bad over the weekend, I smoked a mustang with my plowtruck, up to 60 miles an hour, of course he would of had me beyond that, but I was in 4 wheel drive, and he could not man up to it 93 mustang being beat once, so I did it 3 times. anyways, I can tell you though I had a horrible loss of torque before I used the power tune method. My engine did not like anything beyond 16* initial NONADVANCED TIMING. sounded like garbage, and was not running right. Trust me you will know when you go too much advance, the problem is, carbs dont have a knock sensor to retard the timing, even when the inaudible knock does happen. I always error on the side of safety here.

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It's a driving test, so the WOT is under load.

http://www.gofastforless.com/ignition/advance.htm

"You're probably thinking, "Sure there is no manifold vacuum at WOT but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart."

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Your forgetting, this is a STOCK timing table, meaning their is a PE (power enrichment - i.e, wot) advance adder. I believe by 4800rpm it has added a total of 7 degrees to the 24, so 31 degrees total. I wanted to post a STOCK bin, not one of my molested tunes. I usually disable PE timing, along with a ton of other spark related tables and run off the main table.

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Yep, though be careful. Some heads don't like 36 degrees of advance. Remember, your advancing the timing to get complete combustion of gasoline at at RPM. Complete combustion makes power. Too much advance and you'll be building too much cylinder pressure before TDC, to little and it will turn the headers red and run like fuzz.



-- Joe