Dying under braking with Weber 32/36

[Boogiefab]

It's prolly not a vacuum issue. If your Weber is a 32/36 DGEV mounted with the fuel float bowl towards the rear, the stumbling/stalling on medium/hard braking is a known issue we just learn to live with and mitigate with gentler braking more in advance of an expected stop whenever possible.

Under braking, the fuel in the bowl still has forward momentum, so with the bowl at the rear, braking causes a slight surge of fuel into the intake resulting in a rich-condition stumble/stall. Sometimes raising the idle a bit can mitigate this, but that may also cause a bit of run-on "dieseling" after you shut off the engine.

It's possible to mount the DGEV with the bowl in front, but this complicates throttle cable routing, as the cable would then need to approach the carb from the intake side, rather than from the exhaust side as stock. Some have handled this by simply flipping over the cable bracket/guide on the firewall, and then bending the guide tube up a bit to help the cable housing clear the brake booster/cylinder, but perhaps that might not even be necessary with your smaller booster.

The mirror-image DFEV carb eliminates those problems, as it can be mounted with the bowl in front and receive the cable approaching from the exhaust side as stock. However, DFEVs are harder to find and tend to cost a fair bit more, so the better availability and cheaper price of a DGEV is worth the compromises for many (including myself). Having the bowl in front causes a slight lean condition on braking and going steeply downhill, and a slight rich condition on hard acceleration and going steeply uphill, which is all preferable to the inverse which occurs when the bowl is at the rear.

As for "vacuum beast", I think that reputation is because one of the vac-advance barbs on the DGEV actually provides "manifold vacuum" (taking vac from below the throttle butterfly), so hooking up the ignition advance to that causes strong advance at idle, diminishing up to partial throttle where it starts to taper off more steeply. The other vac-advance barb just above that one (usually capped with a small screw on new DGEVs) provides "ported vacuum" (taking vac from just above the closed throttle butterfly), so hooking up the advance to that one only causes advance just-off idle and up to partial throttle. Originally, all vac-advance used manifold vacuum, but then as emissions regs tightened, they invented ported vacuum as a way to disable advance at idle, thereby raising combustion temps at idle to help burn hydrocarbons more thoroughly in that state.

Sub,
What you described is EXACTLY how it acts while braking to a stop. I'm amazed there is no work around for it. My fuel pressure is 4.5 psi and I checked the float. Thank you for the explanation.

I may try raising the idle speed and watch for dieseling. My warm idle is about 750 rpm, right now.

I'm trying to understand the vacuum advance you described. I'll have to verify if I'm taking vacuum from below or above the butterflies, if I can.

[SubGothius]

I'm trying to understand the vacuum advance you described. I'll have to verify if I'm taking vacuum from below or above the butterflies, if I can.

Looking at the side of the DGEV where the choke element/linkage is mounted, there's two vac barbs. One is slightly above and to the left of the other, which is normally plugged with a small screw; I'm not certain but think this one provides ported vac, whereas the other, lower one provides manifold vac. I amended this diagram to show what I mean:

[TintedRam50]

I would turn the carb around so the fuel bowl is forward facing. Modifying linkage isnt too bad. Ive had that problem years ago with other carbs due to gas splashing into carb. If you take off the filter and look down into the carb you may see where there are two little vent holes going directly into fuel bowl where fuel can also come though if you stop hard enough, regardless of float level settings (if i remember correctly). at least on the 38/38 weber. Personally, i would lower pressure to around 3psi also.

[Boogiefab]

I would turn the carb around so the fuel bowl is forward facing. Modifying linkage isnt too bad. Ive had that problem years ago with other carbs due to gas splashing into carb. If you take off the filter and look down into the carb you may see where there are two little vent holes going directly into fuel bowl where fuel can also come though if you stop hard enough, regardless of float level settings (if i remember correctly). at least on the 38/38 weber. Personally, i would lower pressure to around 3psi also.

You read my mind. I spun the carb last night. I flipped the cable 180 at the firewall and it worked perfectly. I can't believe I never read about this problem when I was researching the Weber swap.

[Boogiefab]

Looking at the side of the DGEV where the choke element/linkage is mounted, there's two vac barbs. One is slightly above and to the left of the other, which is normally plugged with a small screw; I'm not certain but think this one provides ported vac, whereas the other, lower one provides manifold vac. I amended this diagram to show what I mean:

I spun the carb and it stops perfectly now. I was also able to tweak the linkage, so the secondary comes in way more smoothly, too. It revved up a lot better, for some reason. I didn't realize until this morning that I forgot to hook the vacuum advance when I test drove it. Not sure what that means.