Dying under braking with Weber 32/36

[Boogiefab]

Greetings, all. I've been experiencing an issue under medium/hard braking. I have a Redline Weber 32/36 and am running a smaller Summit Racing brake booster.

The Weber runs great, brakes feel fine, under light braking, in gear. Bled brakes multiple times, confident there is no air in lines.

Vacuum line to booster is at the stock location on the intake manifold, by the #4 cylinder.

The reason for the booster, is my truck is on air suspension, with a body drop. I needed a smaller diameter booster, for wheel tub clearance. I've had a single diaphragm booster and a dual diaphragm booster, both of which had the same dying issue.

21" of vacuum at idle, 10-15" of vacuum at medium/heavy throttle load. 21-25" vacuum at light throttle while cruising.

Vacuum gauge bounces somewhere around 10-15" of vacuum when experiencing the dying issue. Depending on how careful I brake, the truck will sometimes recover on it's own. I usually heel/toe, to keep it from dying.

Questions
1. Has anyone ever experienced this?

2. I've heard the Weber carb described as "a vacuum beast" what does that mean?

3. I have one vacuum source available on my stock manifold, under the carb. Would this equalize the vacuum and help the dying issue?

4. Will a vacuum reservoir between the factory port on the manifold and the booster help my issue?

[SubGothius]

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.

[Fingers]

not related to your stumbling issue, but I see a lot of people using an electric booster for their brakes. They can be sourced from a Tesla or way cheaper from a Honda. Same units, different price.

[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:

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27923&stc=1

[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:

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27923&stc=1
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.

[TintedRam50]

Good to hear. Yes i did the same on firewall setup, flipped the metal housing coming out off firewall. But, the cable housing rubs the reservoir on the brake master. Not a huge problem with a zip tie to hold cable steady to slow down rubbing.

[Boogiefab]

Good to hear. Yes i did the same on firewall setup, flipped the metal housing coming out off firewall. But, the cable housing rubs the reservoir on the brake master. Not a huge problem with a zip tie to hold cable steady to slow down rubbing.

I still have to pretty up the cable but it worked better than I'd expected. I also need to redo my return spring but it's a vast improvement.

[SubGothius]

Great news, maybe I'll give another go at flipping my DGEV around. BTW, my return spring setup:

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27944&stc=1

[Boogiefab]

Great news, maybe I'll give another go at flipping my DGEV around. BTW, my return spring setup:

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27944&stc=1
That's a much better return spring setup than I had in mind. Thanks for the help!

[SubGothius]

BTW, the anchor brackets for the cable housing and spring on the left, and the curved bellcrank for the cable at the right, came with the Weber linkage kit for a Mazda pickup, Redline Weber part no. 99007.675.

[TintedRam50]

I need that L bracket. I used the golish colored one that didn't line up for anything and made it work somewhat. It does pull fully open though (throttle).

[SubGothius]

Which L bracket? Note the bracket for the cable housing is separate from the adjacent one anchoring the return spring. Looks like the Mazda pickup linkage kit now provides the latter as a straight piece with large (bolt-down) and small (spring anchor) holes, rather than having that dogleg bend in the middle, but should work about the same either way. Or you could probably just make your own with a strip of sheet steel in a bench vise to form the bends.

[TintedRam50]

The bracket holding threaded end of cable jacket. The gold bracket you see here in my pic came with kit years ago, still dont know what its for.

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27945&stc=1

[TintedRam50]

Also, what is the best way to upload pics here? So far I have learned to resize pics to post but just shows link to then view.

[TintedRam50]

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27946&stc=1

[SubGothius]

Aha, that "boomerang" bracket in your first photo is meant to go on top of the valve cover, to anchor the cable housing there if it was coming at the carb from the exhaust side as stock. It won't be used with the cable coming from the intake side as you've got it. The linkage kit for a Mazda pickup (Redline Weber part no. 99007.675) will include the brackets and curved bellcrank I'm using:


https://www.carburetion.com/Products/ProductDetails.aspx?Part=99007.675
https://www.piercemanifolds.com/MAZDA_LINKAGE_B2000_B2200_99007_675_p/99007.675.htm



Also, what is the best way to upload pics here? So far I have learned to resize pics to post but just shows link to then view.

See here for tips about uploading and inserting pics:

http://www.mightyram50.net/vbulletin/showthread.php/89-How-To-Pictures-and-Attachments-Tips-and-tricks

[TintedRam50]

I appreciate your help. May be ordering bracket soon, as a rebuild kit is suppose to be here tomorrow for the 38. After looking at my own pics I may put a spacer in to raise carb and rotate the crank of shaft upside down. (pulling underneath from opposite but stock direction, putting cable back in stock position over valve cover). Its going to take alot of spacing to raise carb though if possible.
Im going to double check soon.

[SubGothius]

You can use the Trans-Dapt 2107 adapter to mount your carb an inch or two higher; that's what I did. It's made for a Datsun but fits our manifold bolt pattern perfectly, gives a bit more low-end torque for slightly less high-end peak power, but these engines aren't real high-revvers anyway, and torque is preferable for haulin' duty.

Note if your manifold has a coolant port in the carb mounting flange for the stock Mikuni, the Trans-Dapt won't fully cover that port like the Redline adapter plate does, so you'd need to tap that port with threads to accept a grub/set screw (M6 or M8, I forget which) as a plug.

[Law Dog]

Redline sold a thicker spacer before that fit but you needed longer studs to put it on.

[TintedRam50]

Got the rebuild kit in. The redline weber kit, one of the gaskets look like crap. Looks like partial creases and rips on the paper mounting gasket were there from manufacturing. I guess im going to have to add some sealer incase. just sucks to spend money on authentic parts only to get generic cheap looking parts. Bought off ebay, may send back but dont really want to lose time waiting to ship back and wait for replacement. Not so much sellers fault, you can tell the factory didnt do well making it. Makes me miss the Holley 5210.
I will look into the recommended suggestions on spacers.

[SubGothius]

Funny you mention the Holley 5200 series, as those are just licensed (and variously emissions-altered, depending on the 52*0 variant) copies of the Weber DF-series, which is basically a mirror-image of the DG-series.

Redline Weber seems to be the official US importer/distributor/agent for genuine Weber carbs and parts, but they're not Weber themselves, and all the other Redline-branded stuff (kits, linkages, accessories, etc.) is their own product. If you'd purchased from them or one of their sub-distributors (Pierce Manifolds or Carbs Unlimited), I might contact them about that dodgy gasket, see if they'd send a replacement for it.

Might be worth sending an email with a photo anyway since it's their brand, worst that can happen is they say No, and if you don't ask, the answer's already No anyway.

[TintedRam50]

http://www.mightyram50.net/vbulletin/attachment.php?attachmentid=27954&stc=1

[SubGothius]

Looks like that base gasket mighta been slightly bent over a shelf edge or something like that, and since it's fairly thick and rigid, it didn't bend or crease but instead kinda roughly cracked/delaminated.

I'd dress any carb base gasket with a light surface coat of Permatex #85420 anyway, which should press together and seal up that rough streak nicely enough to do its job just fine.

Don't dress that other paper gasket tho', since you may need to be able to pull off the carb top now and then to swap main jets for tuning, or service the needle valve or float, and that doesn't really need to be a fully airtight seal anyway since it's above the throttle butterflies.