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Old 06-12-2012, 04:09 PM
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Eddy Carb hesitation off idle.

Hey everyone. Just thought I would get some ideas on what my next step should be in tuning my carb. Its an edelbrock AVS 650cfm carb on top of a 10:1 vortec headed 355. cam is a howards 180325-08

I have an issue with a hesitation if I jump on the gas. It was really bad at first but I adjusted the accelerator pump rod to the setting closest to the body of the carb and it made it better. Also I took the base timing to about 22" and am running 10-12ish* of mechanical advance. Which also helped but its still hesitating when I jump on it. I have the fuel set at 5.5psi not sure if I should bump it up some? I also read that there is a spring under the metering rods that lift it off the seat to let more fuel in. Should I run a stronger spring to allow fuel to come in sooner?

Can anyone suggest some good reading material for carb/generic engine tuning theories. I found one online awhile back that seemed really good but can't remember for the life of me what happened to the bookmark.
Edit: Found it http://www.4secondsflat.com/Demon_Tuning_Guide.html... any good? anyone read it? anyone recommend it?

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Old 06-12-2012, 07:06 PM
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Edelbrock Owner’s Manual covering Performer and Thunder series carbs- HERE. With the info from it and from Edelbrock Performer Series Carburetor- Complete Guide to Set-Up, Tuning, and Performance Jetting, you can go about adjusting it for better performance.

Be sure the accelerator pump is discharging the instant the throttle is moved. Changing the step up springs on the metering rods won't help the symptoms you describe.

If you're using a vacuum advance hooked to manifold vacuum, you might need to use ported vacuum. See if it likes that, if it is sluggish you might need to add some initial (and take away some mechanical to keep the total where it should be), if the engine acts like it wants more initial when the vacuum advance is switched to ported.

You may be able to adjust the vacuum advance can (if it's adjustable) to work w/manifold vacuum, this is a trial and retrial type of deal. More http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Vacuum_advance]here[/url]. The page is about the GM HEI but the advance info is universal.
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Old 06-12-2012, 08:13 PM
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That 108LSA cam will want a good bit of initial base timing.
24 to 26deg 34 to 36 @max advance.

Try a slightly richer primary cruise calibration.
(smaller lean step on the metering rod, or just a larger pri main jet.)
a richer primary helps throttle response.
Make sure the pcv is connected to the back of the carb. not to the front.
no vacuum leaks. Brake booster etc

May need a larger accelerator shooter. ( drill out +.003" or edelbrock
service part shooter kit)

A afr gauge helps you dial it in.

carb spacers sometimes create throttle response issues.

trans torque converter stall speed is critical.
a stock converter won;t work very well.
3000 stall minimum....3500 stall is not too much for that cam.
When does it bog-hesitate? rolling into the throttle from cruise,,, or when you rug it hard?
rug it from idle.....
rug it from steady cruise.....
rug it from hiway cruise.....
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Old 06-12-2012, 08:20 PM
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Awesome thanks! I'll read through that tomorrow when I have some down time at work. Any specific reason why my car will stall if I turn hard? Usually when I turn right. I have to ride the gas to keep it from stalling.
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Old 06-12-2012, 08:23 PM
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Quote:
Originally Posted by F-BIRD'88
That 108LSA cam will want a good bit of initial base timing.
24 to 26deg 34 to 36 @max advance.

Try a slightly richer primary cruise calibration.
(smaller lean step on the metering rod, or just a larger pri main jet.)
a richer primary helps throttle response.
Make sure the pcv is connected to the back of the carb. not to the front.
no vacuum leaks. Brake booster etc

May need a larger accelerator shooter. ( drill out +.003" or edelbrock
service part shooter kit)

A afr gauge helps you dial it in.

carb spacers sometimes create throttle response issues.

trans torque converter stall speed is critical.
a stock converter won;t work very well.
3000 stall minimum....3500 stall is not too much for that cam.
When does it bog-hesitate? rolling into the throttle from cruise,,, or when you rug it hard?
rug it from idle.....
rug it from steady cruise.....
rug it from hiway cruise.....
Pcv is hooked to the front brake booster is in the back. Why does it matter which one? I have a 2800-3000 stall converter. It hesitate when punching it from a dead stop. Its fine rolling into out.
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Old 06-12-2012, 09:15 PM
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Quote:
Originally Posted by zildjian4life218
Pcv is hooked to the front brake booster is in the back. Why does it matter which one? I have a 2800-3000 stall converter. It hesitate when punching it from a dead stop. Its fine rolling into out.
The PCV is fine where you have it. The hesitation is likely either the vacuum advance falling out (if connected to manifold vacuum and supplying more than 10 degrees of vacuum advance) or the accelerator pump circuit is not supplying enough pump shot, or the pump shot duration is insufficient. Or a combination of these.

Different size shooters are available or drill yours. If you drill too big- which you may do if searching for the "perfect" size- solder them shut and re drill to the correct size as determined by testing. Then if you want, you might be able to buy a new shooter of the correct size or get one as close to the right size and drill it. This is because sometimes getting a good spray pattern from a soldered over and re drilled shooter is hard.

And again- you have to get a pump shot the instant the throttle linkage moves. You can buy an accelerator pump piston assembly separately, same page as above.
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Old 06-13-2012, 07:04 AM
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Quote:
Originally Posted by cobalt327
The PCV is fine where you have it. The hesitation is likely either the vacuum advance falling out (if connected to manifold vacuum and supplying more than 10 degrees of vacuum advance) or the accelerator pump circuit is not supplying enough pump shot, or the pump shot duration is insufficient. Or a combination of these.

Different size shooters are available or drill yours. If you drill too big- which you may do if searching for the "perfect" size- solder them shut and re drill to the correct size as determined by testing. Then if you want, you might be able to buy a new shooter of the correct size or get one as close to the right size and drill it. This is because sometimes getting a good spray pattern from a soldered over and re drilled shooter is hard.

And again- you have to get a pump shot the instant the throttle linkage moves. You can buy an accelerator pump piston assembly separately, same page as above.
I do have the vacuum advance can going to full manifold vacuum. I heard so much about how full manifold is the way to go but I guess in my case it will help with the idle transition by switching to the ported. I wanna start looking at getting an AFR gauge. It would make tuning so much easier.
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Old 06-13-2012, 07:53 AM
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Quote:
Originally Posted by zildjian4life218
I do have the vacuum advance can going to full manifold vacuum. I heard so much about how full manifold is the way to go but I guess in my case it will help with the idle transition by switching to the ported. I wanna start looking at getting an AFR gauge. It would make tuning so much easier.
My take on what vacuum source to use, is to use what works. On most engines, using manifold vacuum- as long as the rest of the advance curve is tailored to it- will be fine, and the engine will run best that way. But there are cases (possibly like your case) where ported will work better. Now, that's not to say you can't set your engine up to use manifold vacuum. But the way things are NOW, using ported may be the better choice. Obviously the only way to know for sure is to try ported to see what the result is.

Be prepared to raise the idle speed w/the curb idle adjustment if the idle drops too much when the vacuum is switched to ported. But if the butterflies need to be opened too far to get the engine to idle, that puts the transition slots too exposed and the off idle response will suffer because of it. That usually happens w/a big cam.

So for now, I will suggest you try ported just to see if the problem is the vacuum advance dropping out. If it IS, the fix can be addressed. If it isn't, you have at least eliminated one thing from the possibilities, and you can look into other things, like the accelerator pump shooter or other carb problems, etc.
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Old 06-13-2012, 08:03 AM
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Quote:
Originally Posted by cobalt327
So for now, I will suggest you try ported just to see if the problem is the vacuum advance dropping out. If it IS, the fix can be addressed. If it isn't, you have at least eliminated one thing from the possibilities, and you can look into other things, like the accelerator pump shooter or other carb problems, etc.
So by switching to the ported vacuum I will be ruling out or confirming that the problem is either fuel or ignition related right?
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Old 06-13-2012, 08:29 AM
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Quote:
Originally Posted by zildjian4life218
So by switching to the ported vacuum I will be ruling out or confirming that the problem is either fuel or ignition related right?
It will tell you the vacuum advance falling off when you accelerate (and the vacuum drops) is not the cause. Everything else will still be on the table. But you have to start somewhere, at least this is quick and easy.
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Old 06-13-2012, 08:56 AM
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Quote:
Originally Posted by cobalt327
It will tell you the vacuum advance falling off when you accelerate (and the vacuum drops) is not the cause. Everything else will still be on the table. But you have to start somewhere, at least this is quick and easy.
Alright Cool! Thanks for the help. I will do that friday when Im back home and see if it makes a difference. Any input on why my car is stalling when I turn?
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Old 06-13-2012, 09:30 AM
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Quote:
Originally Posted by zildjian4life218
Alright Cool! Thanks for the help. I will do that friday when Im back home and see if it makes a difference. Any input on why my car is stalling when I turn?
A too high fuel level can cause it. Check the float level seting and be sure the float hasn't taken on fuel and is heavier, causing the fuel level to be too high.

I suppose a too low level could cause it, but the level would need to be very low, being as how the jets are on the floor of the float bowl.
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Old 06-13-2012, 07:07 PM
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Quote:
Originally Posted by cobalt327
A too high fuel level can cause it. Check the float level seting and be sure the float hasn't taken on fuel and is heavier, causing the fuel level to be too high.

I suppose a too low level could cause it, but the level would need to be very low, being as how the jets are on the floor of the float bowl.
Edelbrocks have both a float level and a drop setting and adjustment is described in the manual. Both have to be 'on' target.

You also need to check the accelerator pump as some of the earlier versions didn't have an alcohol resistant cup and will.
start to deteriorate. Also, these carbs are sensitive to fuel pressure with 5 to 5.5 psig max. And any dirt - that will really screw things up.

Your vacuum advance need to be hooked up to the correct port for your particular ignition system. The RHS as seen from sitting in the driver's seat is ported and the LHS is manifold vacuum. Your PB in the back is fine along with the PCV in the front
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Old 06-13-2012, 08:35 PM
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To get the launch throttle response dialed in.
do your tuning with no vacuum advance at all.

get the initial base timing, centrifical curve and the total max mech timing
correct first. Then work in the amount and rate of vacuum advance.
The edelbrock carbs usually work best with a healthy dose of inital base timing.

24-26deg base at idle. 34-36 at max mechanical .
Limit the centrifical curve to around 10deg to allow this.
start there.

|Again the carb can be sensitive to where the PCV hose is connected to.
(front or back of the carb.) I recomend the back of the carb for the PCV.

Once you get this all tuned in you can get a fully adjustaBLE vacuum advance
and use ported vacuum as a signal source.

Stalling on hard turns.
The float setting is a starting point. You can fine tune from that as required.
and play with the fuel pressure a bit too. 5 to 6psi.
usually its cause the float is running slightly too high. or the fuel pressure
is a bit too high.
The fuel inlet needle-seat size also plays a role.
a a large needle seat assembly (.110"+) acts with more force on the float than a smaller one does. (='s less fuel control on turns)
there is also a off road spring leaded needle seat assembly for increase fuel
control.

You can also convert the carb to dual fuel inlets ( right and left) and block the internal connecting fuel channel between the right and left bowls.
If your carb is the electric choke version
start your tuning using the default jetting of the manual choke version.
The calibration is a bit richer and will give better throttle response.
You can tune it from there.
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Old 06-14-2012, 04:59 AM
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Based on your original question, I'm not sure when you are feeling the carb drop out. If its during mid-throttle to full throttle you need to check the step-up springs in the Edelbrock.

These springs work against vacuum to transition the rods from cruise to power. In cruise mode vacuum pulls the rods down against the springs and the carb runs relatively lean. When vacuum drops during acceleration the springs push the rods up into power mode. If the spring pressure vs. vacuum is off, the transition to power mode is too slow. Edelbrock sells a set of springs that are rated in 3" to 8" of vacuum. A higher rated spring transitions to power mode sooner, which can help part-throttle bog.

Bruce
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