Technical Timing with Dual Quads - HELP!!!!

Hey guys, I'm having a hell of a time getting my engine to run right with two fours. This is my first foray into multiple carburetors, so I could use some help getting this thing dialed in.

First off, specs on the set-up.
-GM 350/290hp crate engine. Brand new engine.
-Compression is low, 8:1. I did a compression test as a confirmation and had an even 130psi in each hole. That seems about right for that compression ratio.
-Camshaft lift is .450" intake / .460" exhaust. Camshaft duration (@.050") is 222 degrees intake and exhaust. Lobe centerline is 114 degrees. As per the manual, normal engine manifold vacuum for the should be 10-12" Hg at idle (650-750 rpm)
-Carbs are new Edelbrock 500 cfm AVS2 series. Vacuum secondaries. Electric choke on the primary rear, manual on the front. They are jetted for a dual quad application.
-Intake is an Edelbrock C26, low rise, dual plane intake with a front oil fill tube.
-Linkage is progressive, front carb comes in at about 30% throttle.
-Distributor is a new Pertronix "stock look" unit with he IgnitorIII internals
-Trans is a 700R4 with a stock converter if it makes a difference.

This issue I'm having is two-fold;
1) It's taking a lot of initial advance to get to idle. Something like 25 degrees of initial timing. If I back the timing out of it, it doesn't want to idle, will stumble in gear, and fall on its face, if not stall entirely.
2) If I keep the initial timing in it so it idles in gear smoothly, and starts nicely, it detonates on the top end and under added load.

I can't seem to get it to both idle smoothly with good vacuum, and not detonate under load. Fixing one issue creates another.

Engine manifold vacuum is about 14 Hg at idle with more advanced timing, a little less with less timing, but then stumbles.

Anybody have any ideas?

 

Did the engine come with the balancer? If not did you verify tdc? There are a couple of different 350 balancers.

 

Disconnect 1 carb completely including fuel line . Use the carb gasket as a pattern & cut out a blocking gasket , put the carb on to hold it in place see if you can get to run with 1 carb , try a different distributor ,again , see if it will run. Check cam installation, TDC , Simplify & conquer .

 

2 carbs are a bit much BUT it can be done . What distributor and have you degreed it . Change weights , limit amout of advance , change springs . Probably eliminate the vacuum advance or get an adjustable vac unit . Double check balancer TDC, . These gm engines came with 390-410 lift cams , did you change it out
What do your plugs look like ?

 

Balancer is a **** street dampener. 8". The engine did not come with a balancer. I (very scientifically) checked TDC with a screwdriver in the #1 plug hole as I rolled the motor so the timing mark was on 0. Seemed accurate to me.

Here's specifically what I used.
https://www.jegs.com/i/****/942/SCA8002/10002/-1#

 

@sidewayzz69 to the white phone. Steve was running the 500 edelbrock carbs and finally gave up on getting them to idle. He switched to Holley and never had another problem. But as lumpy said, I've seen it multiple times on my junk that I had the wrong balancer as far as the timing mark in relation to the keyway.

 

I did not change the cam, I used what the engine came with.

My rear carb is primary.

Plugs looked like ****, which was likely as a result of running way too advanced on the initial break in, a couple cylinders definitely got pretty hot. I cleaned them up and sent them back on their way, but the problem still persists, though a little better with the plugs cleaned.

 

What about the distributor advances ? Have you played with them ?
I just looked at the balancer , IIRC the tdc is offset from the ketway and about 25-30 deg on the picture of your balancer . Of course it matters where your timing mark is . The pictured balancer should have the later model timing mark which was run on motors with long water pumps and the mark was behind the water pump almost straight up , say 12:00 position .
So it begs the question, where is your timing pointer ?
AFBs have a vac enrichment that can be adjusted with springs , again IIRC. I use holleys and Q-jets mostly

 

Is it running rich? Like burning your eyes? Every edelbrock I've ever bought had the floats set too high.

 

I had that exact issue an oddly enough it was fuel pressure related.
I had everything setup for the old engine with a pair of Holleys, dropped in new engine with a pair of Carters and never backed the fuel pressure regulator down.
This kinda drove me crazy, like you as soon as I set the timing it wouldn't idle or restart without twisting the **** out of the distributor, reset timing to 34 all in and again no idle and wouldn't restart without advancing the timing.
When I reset the fuel pressure from 7.5 down to 5.5 damn thing started and ran like normal with correct timing, weirdest thing ever.

 

Yeah Joe, along with what Jimmy is saying, take your 1/4" nutdriver and loosen the retaining screw on the metering rod cover at idle, twist it around and look at the metering rod, it's should be pulled all the way to the bottom, not bouncing.

 

Okay, the thing that jumps out is that you say you have the timing set at 25 degrees initial. I don't think a starter would turn it over at 25 degrees initial timing. How did you achieve that? What rpm was the engine running? Something is wrong with that statement.

 

This engine should have been sold with a OEM damper installed, with the correct timing tab, unless it was a base long block.

If this is a one-piece rear main seal engine, with cenerbolt heads, then you may not have the correct damper and tab on it.


Without the use of a piston stop, you may not have an accurate ***essment of TDC.

Did this engine have a timing tab on it when you got it?

 

Eddy's only like 5 to 5.5 psi pressure............

 

Long story short:

If you have earlier-style components on a later engine, where the timing tab would be at the 12-o'clock position, then your actual reading of the timing is 30-degrees off.

25-degrees of advance in that scenario would be 5-degrees ATDC.

 

are these real 500s or did someone convert 600s?
were they electric chokes with thermostats removed?

 

You can test tab/damper theory, almost for free.

Clean the damper on the surface, ahead clockwise (viewed from the front). Cut a piece of tape that is 1-3/4" long. Apply it with the left edge right up to the groove in the damper, and the other edge heading clockwise (again, viewed from the front). This will simulate another timing mark 30º off from the current one.

Fire it up, and see where your timing is, based on the edge of the tape. If it is about -5º-ish, then you just have the wrong timing tab.

If this is the case, and you are cheap, make a straight, clean scratch at the edge of that tape. Use that as your new timing mark.

 

OR! Hook up your vacuum gauge to manifold vacuum and adjust the timing until maximum vacuum is attained.

Ben

 

Are you using Edelbrock 7525 intake? If not what intake are you using? Some Edelbrock intakes require a metal gasket 2732 or you will have vacuum leaks around both carburetors.

 

I looked at the balancer he used as the picture shows its not correct IF He has the timing mark on the side . Balancer has 0 at the keyway .

 

He has a C26 intake.

 

What he has is a 10º damper, meaning that the timing mark is 10º before the keyway.

Later engines, with the timing tab at the 10-o'clock position need a 40º damper, being one that has the timing mark 40º before the keyway.

 

Fellas, can't thank you all enough for the comments and brainstorming. Admittedly, I had not considered that the balancer could be incorrect for the application. I'll take some pictures of the timing tab that's on it, but it's not the one at 12 o'clock, it's sitting at around 2 o'clock on the driver side.

 

You have a zero-balance application, meaning your damper, in any case, has a zero balance.

That means that it is not "wrong". It may just be that the mark on it does not match the desired configuration.

The "fix" here would be to get a piston stop ($10), and a stick-on timing tape ($5).

Find true TDC, and stick the tap on there, putting zero on the tape where zero on the tab is.

Easy peasy, and it sure would explain why you can idle at 25º BTDC!

 

I wish all of my issues could be solved for $15 haha

 

It could be just $10, even!

You don't really need the timing tape!

 

Yep, a piston stop , a piece of making tape and a sharpie will do the trick.

 

So your saying two carbs screwed up your timing?

 

It might be that nobody involved here, including him, actually knows accurately what his timing is.

That needs to be ***essed and corrected, if need be, first.

 

(Hey guys I'm having a hell of a time getting my engine to run with 2-fours.) He didn't say I have a timing problem on my engine with 2-fours. You all are saying it's a timing problem not him. Lippy