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Old 07-12-2019, 05:36 AM
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Avs2 1905 tuning ideas

Hi guys!

I've been running an AVS 1905 on my 350 smallblock for 1000 miles now. Its runs great, loads better than my old qjet, but I have the feeling it's not perfect.

I'll start with my engine specs :

350 Chevy smallblock
416 heads with 1.94" valves
lunati barebones camshaft (280/290)
Avs 2 Carb,with factory jetting on an edelbrock performer intake
true dual exhaust, block hugger headers.
Custom advance curve distributor, gives 36 total advance at 3000 rpm.

I have 2 issues, although they probably are related. My biggest issue is that the underhood temps are HOT! Engine is perfect at 180 degrees, but under the hood it feels so so hot. The exhaust itself is very warm as wel, the exhaust tips even feel very hot.

The second one is that the car stumbles when you just touch the throttle after braking or taking a turn. ( imagine driving to a roundabout, braking, letting the car coast around the roundabout and when you then touch the throttle it stumbles for a second)

I included some pics of the #2 sparkplug, looks a tad lean to me, wich would make sense with the hot exhaust and stumbling.
Happy to hear your thoughts!
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Old 07-12-2019, 08:09 AM
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Stumble like that with an Edelbrock carb, first places to examine are step-up springs need to release sooner, or accelerator pump needs adjustment or moving to bigger shot hole on the linkage.

Getting off the gas coming up to a turn or stop creates high vacuum, this "dries up" the manifold and then it is too lean when you get back on the gas.

If you adjusted the idle mix screws for highest vacuum at idle this will typically be too lean on performance motors, turn them back out richer 1/16-1/4 turn and see it that fixes the problem.

What's it in??....a small engine bay will tend to hold heat in.
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Old 07-14-2019, 04:25 AM
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Its in a C3 corvette
I adjusted the idle screws a bit to get the highest vac reading ( previously used a CO% tester)

This is 1/8 richer than the highest idle. I dont like the fact that its jumping around that much though.

The stumble is almost 100% gone now

https://www.youtube.com/watch?v=6kmt...ature=youtu.be
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Old 07-14-2019, 06:52 AM
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Worry less about the gauge and give the engine what it wants.....
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Old 07-14-2019, 07:17 AM
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Quote:
Originally Posted by predator carb guru View Post
Worry less about the gauge and give the engine what it wants.....
Exactly ^^^^
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Old 07-14-2019, 07:23 AM
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X3
And your motor wants more timing too.
More initial, less mechanical, less vac advance.
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Old 07-14-2019, 04:34 PM
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A cam getting that long in duration is going to want a lot of advance. Probably getting into 18-22 degrees static with 14-18 degrees in the mechanical.

With big cams you shouldn't tune to highest vacuum idle that's for grocery getters. When you start cracking 200 maybe 210 degrees at oh-fifty you have to tune into what the engine wants more than what the book or WEB trolls talk endlessly about.

Bogie
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Old 08-13-2019, 02:38 AM
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Quote:
Originally Posted by RWENUTS View Post
X3
And your motor wants more timing too.
More initial, less mechanical, less vac advance.
Would it be worth getting a different distributor with adjustable VAC etc?

Im at about 18 static atm, 36 @ 3K rpm, stock vacuum hooked to the manifold vac.

Thanks!
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Old 08-13-2019, 07:06 AM
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18 static but at what rpm is that checked at?
And with your numbers we need to assume you have 18 degrees mechanical.
What did you do to the mechanical timing to adjust it?
All in at 3K!
What’s your timing at 3500?
You don’t need a new dizzy with an adjustable pot.
You need to limit the one you’ve got to give only 10 degrees.
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Old 08-13-2019, 07:43 AM
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Quote:
Originally Posted by RWENUTS View Post
18 static but at what rpm is that checked at?
And with your numbers we need to assume you have 18 degrees mechanical.
What did you do to the mechanical timing to adjust it?
All in at 3K!
What’s your timing at 3500?
You don’t need a new dizzy with an adjustable pot.
You need to limit the one you’ve got to give only 10 degrees.
At idle, so about 800 rpm in P

Adjusted the weights and springs, doesnt advance once past 3k rpm
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Old 08-13-2019, 07:54 AM
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Adjusting the weights/springs only changes how fast or slow the mechanical timing comes in.
What method did you use to limit it to 18 degrees?
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Old 08-13-2019, 01:56 PM
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Those numbers are all over the place!

My 1962 Chevrolet checks at the following
350 CI
10.3:1 Comp. Ratio
L-79 GM Camshaft w/ 222 deg. Duration, intake and exhaust
1967 heads, 3890462, aka double hump
PerTronix Stock Look distributor w/ adjustable vacuum advance

Timing:
12 deg. initial advance at 600 RPM idle w/o vac. advance
24 deg. Intermediate advance at 900 RPM idle w/ vac. advance
36 deg. total advance in by 2,400 RPM

I must be doing something wrong.
Please advise.
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Old 08-14-2019, 12:56 AM
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Modified advance stops
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Old 08-14-2019, 05:21 PM
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The way I set my own daily driver up which is pretty typical of many street builds I've done in recent years using the 350 with L31 or better heads and a cam around the range of timing and lift as the Comp XE268H is 18 to 22 degrees static, 12 degrees max of vacuum attached to the timed port, and 18 to 14 degrees in the centrifugal. It can be a little reluctant to crank when the engine and day are hot so I have a manual switch on the ignition so it can be turned off till the starter gets a few turns on the crank before the ignition is turned on.



In my case this is with a 2800-3000 stall converter ahead of a built 700R4, a 3.08 posi rear and 26 inch tall tires, the vehicle weighs 2800 pounds even with me in it and 5 gallons of gas.



Bogie
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