rwatson5651

iTrader: (5)

I have an SA with the 36 DCDs. I love these carbs. I have been tracking this car some lately and generally speaking am very pleased with them. I have none of the starvation issues when cornering that I have heard about from other carbs. However, I have discovered that they do have an issue with fuel starvation under very hard braking. When I brake very hard from very high speeds and then try to blip the throttle to downshift it bogs pretty hard, and under if I slow down to a near stop the engine dies completely. So, I have been investigating. I am convinced it is because the fuel inlets are at the back of the fuel bowls, here is a pic:

rwatson5651

iTrader: (5)

So I have decided to try to make extensions to extend the inlets more toward the front to see if I can fix it. I measured the inner diameter of the inlet, .151 in. Then I found some copper tubing at McMaster-Carr that had an OD of .159 in., then made some extensions, here is a pic:



My question to the carburetor gurus is do you think this will fix it? Or is there some other possible reason for the apparent fuel starvation other than the location of the inlets that I am too dumb to consider?

t_g_farrell

iTrader: (1)

Whats the orientation of the carbs when installed? Do you have an engine bay picture with them
on the car. It will help me understand where these are in relation to whats happening.

Remember that your brake booster could be a vacuum leak and could cause similar symptoms. I would try disconnecting
the booster and do some runs to see if it persists.

rwatson5651

iTrader: (5)

The fuel inlets (circled in red) are toward the rear when the carbs are installed. I have already replaced the booster with no affect on the problem.

t_g_farrell

iTrader: (1)

I wonder if you can test this by putting the car at an angle such that it reproduces the issue?

So are you seeing it lean out on the AFR guage when it does this? You have one right? I can't
remember.

rwatson5651

iTrader: (5)

Yes, it goes lean. AFR gauge confirms this and also I can diminish the problem by pulling the choke **** activating the starting circuit, thus making the mixture richer. This greatly reduces the problem, but does not fix it entirely. I did a preliminary test this morning by temporarily installing the extensions and trying them out. Didn't fix the problem, but they may not have stayed in place, since it was just a friction fit at this point. I will have to take the tops back off the carbs to see if they stayed in place. If not I will try again with something to hold them in place and seal them to the inlets. If they did then its not the key to the problem and I will be scratching my head to figure out why the mixture would go lean if the inlets are still submerged.
Do you think it is possible that the decal gs are affecting the fuel that is inside the circuit of the carb? I had dismissed this because the circuits run right to left rather than front to back, in my mind minimizing any affect the decal gs would have on the fuel inside the circuit.

t_g_farrell

iTrader: (1)

Maybe its prematurely closing the needle on slosh which cause starvation. I read something about that recently related to
webers but not this one.

So how does t he choke circuit get gas or does it just limit air coming in?

rwatson5651

iTrader: (5)

The "choke" circuit does not limit air, it is an independent circuit with its own jet and air jet that introduces additional air and fuel at a much richer amount of fuel so that it enriches (is that a word?) the overall mixture AND raises the idle speed. This is why Weber calls it a starter circuit rather than a choke. (To them a choke is a venture) .

It is hard to be sure as the diagrams do not show it well, but I believe it gets fuel from the accelerator pump circuit. The fuel inlet for this circuit in the front center of the bowls and can be seen in the pics above.

Also I did additional testing today and to be sure I could eliminate the booster as part of the problem, I disconnected the booster from the manifold vacuum and the problem remained, so tonight I will remove the carb tops to see if the extensions stayed in placed. Maybe they did not stay in place long enough for my test this morning to be valid.

rwatson5651

iTrader: (5)

Well, no luck. The extensions are still in place, so apparently the problem was not fuel starvation.

I'm stumped.

Why would the mixture go lean under deceleration if the fuel inlets are still submerged?

j9fd3s

iTrader: (3)

which way does the passage between the emulsion tube well, and the venturi point?

rwatson5651

iTrader: (5)

It is kind of hard to explain so I'm looking for a pic I can post. The emulsion wells are located behind the bowls when mounted in the car, so I am realizing that the fuel level in the wells would drop when the fuel sloshes forward, but I do not believe that would affect the mixture under deceleration because the main circuit is not active when the throttle is closed. However the fuel level in the idle passageways would also tend to lower under deceleration since they are also behind the bowls, possibly causing the idle circuit to have less authority, leading to the lean condition, maybe??

Maybe a different combination of idle jet / mixture screw setting would make a difference.

Maybe a higher idle speed would keep the circuit flowing even when the level drops . I could raise the idle speed while at the track and then re-adjust for normal operation. Just thinking out loud ............

rwatson5651

iTrader: (5)

In the pic at the top of the page the emulsion wells are the two at the back next to the throats. In this pic the one on the left is missing and the e-tube is in the one on the right with the air correction jet missing. The idle circuit passageways are in the part of the body between the bowls and the ventures and run right and left.

Qingdao

iTrader: (3)

Stupid thought but are there any off road versions of this carb? I thought about getting an off road holley one time because the off road version is made to be operated at extreme angles. Off road carbs have baffles in the bowls to help with awkward angles.


Maybe you could adapt some of the offroad carb's attributes to your carb. or get some baffles from a kit or something.

t_g_farrell

iTrader: (1)

Robert just stop using your giant brakes so much