carburetor tuning questions

[a-marshal]

I've got an older set of PDSI carbs, the ones in the CB performance kit and I've been chasing down and intermittent lean condition. After dealing with a few issues, and screwing a few other things up, I've rebuilt the carbs. I've got a fuel pressure regulator installed and that fuel level in the float bowls is 14mm. In the driveway everything seems great, I've got a smooth idle at 800 RPM and the snail gauge shows 6.5 for both carbs, they stay balanced as I run the RPMs up.

Under load it is a different story, on (attempted) acceleration it pops and shudders and will not really pick up speed, backing out the volume screw seemed to help a fair bit but I am now over 4 full turns out from bottom and it is still not drivable. At this point I think that I want a larger idle jet but since this is all new to me I thought I should ask.


Current set up
28mm venturi - Idle Jet = 50 - Main Jet = 140 - Air Correction Jet = 130 (Altitude 328 feet )
009 distributor timed @ 28* BTDC at 3000 RPM no hoses ( trying to keep the temps down) Valves were set before I started down this rabbit hole...


I've read everything I can find on the subject of jetting but if there are any sources that you can recommend I would appreciate it.
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-Ob. vehicle info....
1973 Westy w/ '74 1.8 liter / PDSI (type3?) carbs / 009 distributor / pertronix

[SGKent]

don't assume it is lean unless you measure it with an A/F meter. Make sure the timing advances smoothly also and that the accelerator pumps work right. Transition holes must be above the throttle plate at idle. Fuel must be reasonably fresh and not 2 years old.

Bentley shows typical for PDSIT carbs (different models have different combinations). I would start with the appropriate factory settings and change from there.

26 mm Venturi
130 to 137.5 mains
140 to 175 air correction jets

See page 30 of Fuel in Bentley
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[a-marshal]

Thanks SGKent.. I know the fuel is good and the timing was advancing fine before I started this process. I'm having trouble finding Jets for these carbs, I've got mains that cover the range of stock sizes but both ACN and CBP only carry Air Correction Jets up to 130 for these things.. (the AC jets originally in the carbs were marked as 115 for what ever that is worth (127.5 mains & 52.5 Idle cutoffs))

I tried a pair of 55 Idle jets, it was better but still just barely drivable.

I'll try and get back to as close to stock as I can, I'll check the accelerator pumps again .. and I'll take a look at the transition holes while I'm at it.

As a side question, the idle cutoffs haven't had the sealing washers for the 11 years I ran them... what should I have noticed about that?
_________________
-Ob. vehicle info....
1973 Westy w/ '74 1.8 liter / PDSI (type3?) carbs / 009 distributor / pertronix

[SGKent]

I don't know because I am not familar with those carbs. It is just that I have rebuilt a lot of single and dual carbs in my life and the problems like that can be hard to trace. You may blame the carbs but it can be something else.

If the timing advances too fast or far the engine can backfire. Same for too retarded (I am talking a lot not just a few degrees).

If there is a problem with too much cam or too big of valves it can cause an engine to gasp.

If the plug wires are shorting to one another, or the cap has carbon tracks in it that can cause cylinders to cross over under load. That is why one always starts with a check of the ignition system.

Anything that makes the mixture too lean causes the fuel molecules to be too far apart. They burn slower and extend out to the next cycle which then causes backfiring. There are even two kinds of backfiring - one which comes back up thru the carbs and the other is the ignition of unburned fuel from a misfire happening in the exhaust when the exhaust from another cylinder ignites it. One has to learn all these things to get the carbs dialed in.

That said, gasoline burns in a mixture from somewhere around 12:1 to maybe 16:1. 14.7:1 is equal air and fuel. So when an engine backfires and does the kinds of things you describe, it has to be way off somewhere to do what you describe.

Tell me - are all 4 plugs the same general color?
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[a-marshal]

[xyzzy]

If they're factory PDSITs, then 140 mains are way too big. My stock 73 came with 127.5 left and 130 right -- and they should have 55 idle jet solenoid cut off valves.

I have a 2L in mine, and 135 mains are right.

The problems you're describing sound to me like you have something unrelated to the carbs causing your problem. Air leak? Timing checked? Distributor problem?

Have you performed Colin's tuning procedure? Are you able to get through that accordingly?
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1973 VW Westfalia 2L hydraulic, dual Solex PDSIT
1987 VW Vanagon Westfalia

[busdaddy]

[a-marshal]

[busdaddy]

[a-marshal]

[a-marshal]

[Nitramrebrab72]

As regards to jet sizes go onto ratwell they have a chart and depending on slight variations similar carbs aren't the same.

Those carbs are renound at leaking air through spindles after high mileage, it's not the amount of air that is sucked in through them but more the loss of depression (negative pressure) which reduces substantially the amount of atomized fuel sucked through the venturis.

To do a really proper job and to get those carbs running as like they did from factory, you will need a new pair of spindles, 4 slitted spindle bushes and 4 weber spindle seals. You will need to get the carb casting machined to accommodate the bushes and seals.

[a-marshal]

I'm back close to where I was when I started, it is drivable. I need to do some fine adjustments but that can wait til it isn't tipping down rain.

Hopefully the float valve was the last of the problems and I'm done messing with the carbs..
_________________
-Ob. vehicle info....
1973 Westy w/ '74 1.8 liter / PDSI (type3?) carbs / 009 distributor / pertronix

[a-marshal]

Just trying to make sure I understand things correctly ..

Everything is back together and working. I've got one tiny issue left, when running up the engine at idle or gently cruising my CHTemps are nice and even left to right, but if I start to jack-rabbit around through stop-and-go traffic or just through town with lots of lights the temp on Cylinder 3 starts to slowly edge ahead of Cylinder 1.

I believe this means that the accelerator pump on the right side is providing slightly more squirt than the left. (I've run the 10 squirt test a number of times and if there is a difference it is within the overall margin of error)

So the question is, if I want to adjust this (and pretending I have an adjusting nut rather than a cotter pin and fixed stop) and I want to decrease the squirt I would loosen the "adjusting nut" to move the accelerator pump lever lever out on the connecting rod .. am I understanding this correctly?
_________________
-Ob. vehicle info....
1973 Westy w/ '74 1.8 liter / PDSI (type3?) carbs / 009 distributor / pertronix