I've been all over this engine trying to find a vacuum leak. It's a carter bbd, the second one I've put on. The first one did not have a proper ported vacuum port, so I bought a second one. I believe they are both 318 models. I'm 99.9% sure I'm using the proper flange gasket, which has an oval hole with notches on each side. I'm diagnosing it as a vacuum leak because when I back the curb idle all the way out, and make sure the throttle plates are as closed as they can be, it will still idle, and pretty high too. It also seems to kick in only after it warms up. I thought I had it beat when I put some gasoline resistant pipe sealant between the gasket and the intake, and put it back on and tightened the bolts down. I didn't tighten real tight, as I didn't want to squish the gasket or create a problem. When I tested it the first time it died when I backed the curb idle off. Then today, when it's nice and hot out, it started and idled around 600-700 rpm, which is where I had set it before. Then, after a bit of moving the car back and forth and warming it up, it seemed to suddenly start idling at around 1000 rpm. Then I went to check it and indeed when I backed off the curb idle, it was still idling at 1000 rpm.

I've pinched off the vacuum advance, tried pinching off the brake booster line, and sprayed all around, but nothing really produces much of a change. When I spray around the intake to head area, it very very slightly raises in rpm for just second, then returns to normal. I have to really spray right at the intake to head gasket area though. I'm sure the linkage isn't holding the throttle open or anything like that. I've had that problem before so I've made sure that wasn't it. It's killing me! I have a 12 hour trip home to make soon and although I have a couple weekends of work on the suspension to do, this damn leak is what's probably going to make me have to have it towed for tons of money! Any ideas?

The gasket looks like this



and the bottom of the carb looks like this



It doesn't have the holes drilled in the choke plates, and it does have a proper ported vacuum port.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

Aluminum or cast iron intake?

Definitely cast.



http://www.semisynthetic.net/kill-9/gal ... C_1260.jpg



That's not the carb that I have on it now, but the original that had no ported vacuum.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

The place where I bought the kit from with the gasket shows the same kit for a 77 volare slant six 225 BBD model and a 76 318 BBD model, even though the 76 (the one I had on previously) does not have the notches on the base plate, and the BBD 65-79 parts manual calls for a different base flange gasket and a different zip kit.

For example, 1 77 volare/plymouth/aspen/etc slant 6 225 calls for 1a-403 flange gasket (carbs 8087s kit 902-325)

A 77 318 calls for the same gaskets and kit

77 318 1a-403 flange gasket (carbs 8133s/8134s/8135s zip kit 902-325)

and the 76

76 318 1a-316 flange gasket (carbs 8069s kit 902-311)

Now they could have just included all the gaskets needed for either in the kit, but there is only one thick base gasket, with notches like the one shown, and the other two base gaskets are too thin, but also have the notches.

This is what led me to believe the base gasket could have notches whether the base did or not, despite what the BBD manual says. But now I'm questioning everything.

http://u225.torque.net/cars/SL6/docs/BBD_Manuals.pdf

It says that both the super six and 318 BBD in 77 got the notches on the throttle plate. Mine has notches, so mine must be a 77 or up. Through these notches, intake manifold vacuum is applied to both the vacuum kick diaphragm and the inlet air door diaphragm.

If by vacuum kick diaphragm they mean the choke diaphragm, that appears to be working correctly. The inlet air door diaphragm outlet is blocked off with a nipple right now I believe.

Now they go on to say that this change (adding the notches) required a new flange gasket. They say that using a 77 and up gasket will block off the intake manifold vacuum on a 76 or earlier BBD (presumaby because without notches it will then have no way to provide this vacuum).

OK that's fine, I have notches, but the question is, what would happen if I used the older style gasket with notches (the only one provided in the kits) with the 77 and up body with notches?

They then go on to show an even more confusing picture where they show super six gasket and 318 gasket, where the super six has no notches and the 318 does, despite having just said that they both have notches in the new configuration.

The manual then says I should use the gasket with no notches if I have notches. Great, but since I can't find one, would this cause a leak? It seems like it would definitely cause problems with manifold vacuum reaching the vacuum kick diaphragm and the heater door diaphragm, but the choke diaphragm seems to be pulling.

I guess if it comes down to it, does anyone know where to buy this elusive gasket with no notches?

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

Everything I'm reading and all the kits are showing that it really doesn't matter if there are notches in the base gasket. So I guess the assumption from this is that my vacuum leak isn't from this.

If I put my hand over the carb slowly, and not fully, it will slightly raise in rpms. If I put my hand over it much more than that, it will die. If I had a massive intake manifold leak I would think I would be smelling it, have bad idle quality, and some smoke. I don't see any of this. If I didn't know any better I would think it was idling great, just a little high, until you back off the curb idle and realize it's stuck at 1k rpm.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

There's a hole inside the throat of the carb, towards the back, right at the top of the main body just where the top and main part meet. Put my finger over it and all the sucking and hissing stopped. Now I just need to figure out what that is, why it's sucking air, and how to stop it.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

It seems to be the idle bypass assist. My old carb didn't have this. So, more research I guess.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

Is your only complaint a high idle?

What is you base timing?

Have you verified that the outer ring of the vibration dampener has not slipped?

Have you verified the accuracy of the TDC mark on the damper?

Try plugging the hole with a bit of RTV sealant.

The only complaint is too high an idle. Everything else is set properly. I can't get it any lower than 1k. When I plugged the bypass hole with my finger it dropped to the proper idle, or close to it. I'll try some black rtv like you said. I thought for a while about some jbweld but then I thought that if that fails it's going to send pieces of it into my valves. So I would rather shove some rtv in there and have it fail and have to redo it then have pieces of jb weld in my valves.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34

Your damper may indicagte the proper timing, but have you verified that the damper itself is accurately indicating TDC?

How far out are the idle mixture screws?? Never seen any mention of that. If they're too far out it will idle very high. I usually close the throttle all the way, turn the curb-idle screw in until it just touches (used as a stop), then adjust the mixture screws in/out until it idles as low as it can while still being smooth.

The hole your talking about plugging is the idle air bleed if Im not mistaken (im assuming its the very tiny little hole in top)...

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"if it aint broke, fix it till it is"
78 Plymouth Volare Super Six wagon
89 Volkswagen Golf GTI 16v
92 Chevrolet K1500 5.7
98 Ford Escort ZX2 zetech

They are about 1.5 out at the moment. They have been in and out to troubleshoot this though. The timing has been verified as well, using all the valve events and everything. I had a lot of other problems before this with timing and starting that caused be to go over every single inch of this thing. This damn bypass hole is the last in a long long story of learning every inch of this slant.

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1970 Plymouth Duster, 198 Slant Six, Super Six setup w/ EI conversion

http://www.semisynthetic.net/kill-9/gal ... p?album=34