Carb help!

-
4404spd

4404spd

Well-Known Member
Joined
Nov 23, 2010
Messages
1,612
Reaction score
80
Location
Towanda PA.
Im working on my sons 1970 340 Dart and about to pull my hair out.

First off its a 340 thats 30 over 10:2:1 compression J heads cut out to 202 valves, Mopar Performance cam 280/280 @50 238/238 centerline 110 and lift is 474/474, Wieand 8007 dual plane intake and Eldelbrock 1406 Carb.

The problem is the idle. It won't idle down. I found out the ported side of the carb is pulling vacuum at a idle. If I pull the vacuum advance off it will idle down. This problem is the same with 3 1406 Eddy carbs i tried. I tried a square flange spacer and its the same. I put the weakest metering springs in and the largest rods 1407#. The cam is not radical but with the centerline its only pulling about 9" of vacuum. Is this intake junk or what...
 
The intake is an excellent one.

The distributors vacuum advance on a Edelbrock carb should connect to the left port as you look at it. Where is the distributor set at? (You did set the distributor up without the vacuum advance connected right?)

Regardless of how you jetted it or what springs you have in the carb, it has nothing to do with the idle speed. Even more so evident since it issue down once you pull the vacuum hose.

This leaves me to believe the springs in the distributor are very light and not stock.
 
rumblefish360 said:
The intake is an excellent one.

The distributors vacuum advance on a Edelbrock carb should connect to the left port as you look at it. Where is the distributor set at? (You did set the distributor up without the vacuum advance connected right?)

Regardless of how you jetted it or what springs you have in the carb, it has nothing to do with the idle speed. Even more so evident since it issue down once you pull the vacuum hose.

This leaves me to believe the springs in the distributor are very light and not stock.
Click to expand...
Yes, im using the port side...The left side vacuum. I have two stock distributors and both act the same. The springs do make a difference with low vacuum in the carb. They pull off quicker.
 
I got this off another site.



Not 100% sure, thought only difference between Manifold and Ported
is with Ported there is no vacuum signal at idle.
Otherwise Ported vacuum is pretty much about the same as Manifold Vacuum.
Ported vacuum signal, since it's picked up in the sidewall of the primary carb venturi,
may rise at a little bit quicker rate than Manifold vacuum, but eventually they'll equalize.

If there is any vacuum signal at all on a Ported Vacuum port at idle,
then the primary butterflies are too far open at idle
and the secondary butterflies should be cracked open a little bit more
so the primaries can be closed down a little bit at idle.
This prevents any vacuum signal at all on the Ported Vacuum port, while at idle.

If at idle the primary 'flies are opened enough to get a signal on the Ported Vac port,
then most likely the primary Transfer Slots are also over exsposed at idle
causing idle mixture, main circuit activation, venturi fuel dripping, and off idle stumble problems.

Understanding is Vac Adv on Manifold vacuum is a way to get an engine with
a large cam with a LOT of overlap to idle down well and give "useable" idle vacuum levels.

A lean mixture burns slower than a ricj mixture.
This is what Vac Adv is "meant" to do, advance the timing during "lean" conditins,
like when cruising at steady state speeds when the 'flies are closed down,
engine "choked off", the power valve closed, and running on a lean mixture.
As soon as the 'flies are cracked open, vacuum drops, power valve opens,
main boosters come online, mixture goes "rich", ~12:1 for power,
and less advance is needed to light the fire at the proper point
to maximize cylinder pressure at the optimum piston in bore rotational location.
 
IMO, 10 isn't close. Double it for a mid 230's cam in a 340. Will need 18-22ish.

I know 340-8bbl has a cam/compression in similar range and he's running more than 20 initial.

Give dist a twist CCW and see if the idle comes around. If the engine picks up RPM when you twist CCW, it wants the timing at idle. Going to have to shorten up your mechanical to get total number right.
 
The throttle plates on the primarys were not closing enough letting the port side on the carb to have vacuum at a idle. I adjusted the secondarys to stay open slightly to add air so the primary's could close and stop vacuum to the port. Mission acomplished. I could get it to idle fine before but I had vacuum at a idle and thats not suppose to happen.
 
I recurved and welded up a distributor. I have 16 initial timing with 34 total advance. It seems to work good at the moment I have to fix a antifreeze leak on the thermostat housing before I can road test it.
 
You must log in or register to reply here.
-

Back
Top