Started from scratch with the Edelbrock, advice needed...

Relevant specs:

Edelbrock 1406 (600cfm) on a 350ci/290hp motor... Cam: ".450'' intake and .460'' exhaust lift", and "Flat Tappet Duration @ 0.050'': 222° I, 222°"

1986 K5 Blazer, emissions stuff not connected. Vacuum to brakes, PCV, and vacuum advance (ported).

13" Hg of vacuum, 8 degrees BTDC at idle/850rpm.

I've been playing with changing the metering rods and jets, and I'm making progress with it. I should have it running pretty good soon. I'm getting good results with that but need another couple sets of metering rods to try out.

Anyway, it STILL does not idle to my liking. I am having the damndest time with it.

Initial timing was 16 degrees BTDC (really). While idling, I brought it down to about 8 BTDC, leaned out the mix a 1/4 turn on both screws, and reset idle speed with the adjustment screw to 850rpm.

I thought, maybe closing the idle screws in to 1.25 turns (from the recommended 1.5 turns) combined with increasing the idle speed would help. It has a little, but beyond that it gets really irratic. Vacuum continues to drop.

If I set the idle for max vacuum, it just gets richer and richer, and the timing gets more and more advanced to compensate. I suppose if max vacuum at idle were the goal, then I could achieve that easily, I have before. But it SHOULD NOT burn your eyes out because it's running so damn rich at idle.

I later took the idle mix screws in to 1 full turn out...a 1/2 turn less than recommended as the starting point. I hit the gas once, and it backfired through the carb. At this point I'm gonna have to replace my air filter, as it's the foam type and now it's showing signs of burn damage on the inside (it's singed and shriveled up a bit).

WTF am I doing wrong here? I just want it to idle at the right mix. Or is this carb just FUBAR? Was a 600cfm Edelbrock too much for my motor? I know the stock carb on the old 305 was a 795cfm carb, but a Quadrajet is a WAY different beast, so I don't know if cfm's would be meaningful here.

Thanks,

~jp

Haven't checked valves yet. I've read that backfiring through the carb can be the result of a lean condition. I should've made that more clear--it backfired once. One singular pop and that was after I'd changed the step-up springs (which made it so rich it wouldn't run) then changed them back. It's possible that there were residual something-or-anothers from when it'd been run incredibly rich a few minutes earlier. Not sure. It popped once, then proceeded to run as it had before the spring change. Hasn't backfired since.

The vacuum is stable, yes. I don't suspect the valve train. I could check it. I think I have a large, cone-shaped attachment that came with the gauge. I believe it's probably for sticking into a spark plug hole and testing compression or something like that.

I've had valve issues in another vehicle before. Granted, it was a

4cyl Toyota Camry, but still, I think it would probably cause running problems at speeds other than idle.

Either way, I just want to defeat this carb, LOL. It's won every battle, but I'm gonna win the war.

As a side note. I took it out to the store to get beer and smokes, and put some gas in it. Here's what I noticed:

1) off-idle is OK...just ok, but no real surging or weirdness.

2) beyond that, with vacuum reading between 6" and 7" Hg, it runs rough, like it's lean, it surges. Not a problem, I just need another set of metering rods to take it once stage richer. But I noticed that it "hisses" at me, quite noticeably, through the intake. When I push it beyond that, and hit 5" of vacuum, it smooths out and runs better, but still hisses. At 4" of vacuum, hissing ceases, it hits "power" mode in the carb and the truck takes off, as it should at that point seeing that I have the 4" step-up springs in there. Maybe the hissing is normal when the carb is in cruise mode, maybe it's just because it's running lean...I don't know.

3) When I nailed it going up a hit, I hear a lot of noise from the intake. That may be normal for the carb I have and the air cleaner I'm using (foam element, triangular shaped Edelbrock with the "screen" on top). But it still doesn't have the power I'd expect it to. Granted, it's a K5 Blazer, not a Dodge Neon, and will need a considerable amount of power to fly up the hill. But still, as far as I can tell, this relatively new 290hp/350ci motor doesn't have much more go-power than the 16yr old 305ci motor with a blown head gasket did--even after swapping from 3.08 gears in the diff to 3.73's. The off-idle response if I nail it though, is impressive :-)

~jp

David Johns> You know your vacuum so I assume you have a vacuum gauge. Does it show a

you mention hissing allot.... does it have a CAT converter on it? plugged cats can make a hissing noise under load. Kinda sounds like an air leak really. I guess it kinda is an air leak now that I think of it :)

Dumb question, but whose fuel are you using?

I'm right outside Atlanta, GA. I have wanted to pick up an O2 sensor as well, for this specific application. Not to mention that someday I plan to install a TBI setup and it'd be necessary.

~jp

David Johns> Alright, nice steady needle rules out valve problems. Check float levels and

It's definitely from the intake, not the exhaust. I'm starting to think it's probably somewhat normal for my current setup.

~jp

ShoeSalesman wrote:

Not sure who supplies the gas for Quicktrip (QT) but that's the main gas station I go to. Occasionally Chevron. My choice in gas has everything to do with location and whether I'd have to just turn right or cut across a street to get it :-)

Either way, it's never had anything but 93 Octane in the entire time the engine has been in the truck.

~jp

Bill wrote:

A couple od things here. 220+ degrees at .050 is a bit of cam. If you ar running stock compression, it will have lower idle vacum but 13 is too low for your setup.. ALso, 8 BTDC is not really enough. It should be around 12 or 14 BTDC initial and a total of 38 to 40 degrees at

3800 RPM or so. If you are retarding the spark to maintain 87 octane abilty, you are realy kinda shooting yourself in the foot here.

----------------- The SnoMan

Ok... now this is the kind of info I've needed for 2 years. I knew there had to be some way of figuring a starting point based on the engine specs.

What's the maximum limit on advance at idle? Should I follow the old rule of advancing it till it's hard to start then backing it off? I only backed it off because I was trying to lean it out and didn't want to induce knocking.

I agree, 13" of vacuum is low. I'll advance the initial timing. That will probably raise the vacuum some at idle. In your estimation, what should I be shooting for here? 15"...17"? More???

And this raises another question I've had for a while...the vacuum is what pulls the gas and air through the carb and into the manifold, so would lower than normal vacuum keep the fuel from properly atomizing into the airstream, and keep it from burning properly? Logic tells me that the two are related.

The Edelbrock manual states:

1) "If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of adjustability and erratic idles."

2) "Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold vacuum."

Why would I want to connect it to manifold vacuum? Seems it would advance a lot at idle, then retard as the throttle opened...

Concerning the cam... I wasn't as well informed when I bought the engine. I could've had a 260hp motor for $300 less--minus a warranty on anything newer than a 1985 truck. Mine's an '86. So for $300 more, I could get a warranty or the 290hp motor (with a warranty). The 290hp motor is the SAME engine but with the hotter cam.

In retrospect it may have been a better engine for something smaller and lighter, like a mild Vette or anything but a heavy truck. Granted, it's not a terrible choice. But I need low-end torque more than I need top end speed, and the cam may need to be swapped out at some point.

~jp

SnoMan wrote:

Here's my engine:

It says 34 degrees of total advance on 87 octane fuel. 8.5:1 compression.

I suppose it would be good to know how much total advance my mechanical and vacuum advances are providing to know what the total max advance is.

I'm also guessing that number can be higher than the listed 34 degrees given that I run 93 octane...

~jp

SnoMan wrote:

So a less restrictive Cat? I could see that a restrictive exhaust system could be a problem. After all, the engine can only suck in as much as it can pump back out at a given time... the complete exhaust system is stock for the truck with a 305ci motor.

So the low vacuum could be a cause of the flat spot...by not sucking in enough gas?

I'll play with the timing in a bit.

So ya think Chevy put together a bad combo of parts or is it just a bad combo for my truck?

~jp

Shep wrote:

Yeah... I figured as much. As a future mod, do you think a more appropriate cam would be a good idea? What's the deal on the RV cams. I know there can be a bit of variation between the different manufacturers, but what would you consider to be an ideal cam profile for a truck like mine?

Also, while I'm asking, I'd like to know more about cams in general. I understand what they do, but would like a more in depth explanation of how the profile of the lobes will affect things like compression, vacuum, peak power, etc... Got any good links about that kind of stuff?

A new exhaust setup wouldn't be too awfully much. I was wanting to get new headers/exhaust manifolds that are pre-drilled for O2 sensors (for the future TBI and/or a fuel mixture gauge) as well as a cooler sounding muffler. I suppose it wouldn't be too hard to throw a newer, better flowing cat in there as well. What would you suggest for pipe diameters?

Thanks,

~jp

Shep wrote:

Jon,

Where these guys have been saying a looser converter, they are not speaking of a freer flowing catalytic converter, but rather a "looser" torque converter that allows more RPM before "hooking up" takes place.

Ah...gotcha. Here I was looking in the LMC catalog for better flowing exhaust components.

Still, I do want to replace the manifolds with ones ready for O2 sensors, as well as a tougher sounding muffler. I will probably replace the cat while I'm at it.

But I don't think a looser torque converter is a good solution. Sure, it'd have more power on takeoff, but I like where it takes off. Seems more like a band-aid than a real solution.

I'll be playing with timing and whatnot, trying to get vacuum up over the next week. But I'll also be looking into a better cam for my application.

Thanks,

~jp

George wrote:

JP...

I'm no expert but you can find a number of sources from your local book store to searches with goggle.

In my view the first concept you need to think about is that all the components of the engine work as a team.... you can't just change one and expect magic results. Plus add the transmission and the rear end (and front end for 4*4) gearing to the mix.

The other part is how wild do you want to go and how much money do you want to spend, is this thing a daily driver, will it be fuel injected, carbureted... does it have to pass emissions testing.

I'm catching this on the tale end but if you run fuel injection or your truck is currently fuel injected you are very limited in the type cam you can run.

on a truck...in general you want to create torque and horsepower in the lower rpms....where you can use that power... Think about the use of your truck....are you just going to hit mud holes wide-ass-open or generally slower moving type stuff?

I think for a daily driver "whatever the brand" the closer to stock the better. Minor mods to exhaust, maybe a small cam "up-grade", cooling up-grade, gear upgrade, decent set of off-road lights, maybe a winch / bumper, set of tires for what you want to do...

in my view you want a stump puller for 4*4 stuff, you have to compromise here and there if you have to drive the thing daily.

You generally have to look at these things as a package deal, the cam matches the heads, which matches the fuel setup , with is compatible with the gearing on the truck and tire size. You get one of these out of whack and its not going to run like you want it to. Conservative, minor modis to a stock truck is the way to go, especially if you are new to this stuff, or have to pay someone else to do it.

A good idea would be to check out what some of the other guys are running in your area, maybe see what works for them.

Just my thoughts......

----------- Elbert snipped-for-privacy@me.com

Thousands of pages could be written and have been written on proper cam selection. You can find probably 100 books on cam selection and maybe as many computer programs. Everybody can make a suggestion but cam selection depends on many factors, among them are static compression, valve size, rocker ratio, TQ stall speed, gear ratios and where you want your engine to make power. Any of your major cam manufacturers and large suppliers can take this information and suggest a cam. As far as RV cam's that is just a broad term used for cams designed for RV use that provide low end TQ. There are probably hundreds of grinds that could be considered "RV" cams.