Engine tuning - 40HP carb / distributor matching

[germansupplyscott]

on a '63 ghia with original engine we are trying to get proper match between the 28PICT and distributor. the distributor is the ZV/PAU4R5 with the large vacuum can and large cap. the carb is a 28PICT but there seem to be different varieties of 28PICT, some have a tiny vacuum orifice and others have a larger orifice. images from classified section:

issue with the engine is the vacuum advance was 'all in' at idle so not possible to get any advance off idle into acceleration phase. the car is not with me now but i think the carb had the larger vacuum orifice. i tested vacuum with a gauge and carb was pulling more vacuum at idle than was required by the canister to start pulling, so at idle the canister was fully advanced.
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SL

[danielsan]

Try sending a pm to andy / "glutamodo" http://www.thesamba.com/vw/forum/profile.php?mode=viewprofile&u=26332 ...and to let him know about this thread -- he knows stock carbs better than anyone else.

[tasb]

I know that the later 40 hp aluminum bodied distributors have the same restriction at the canister nipple. The earlier cast iron 40 hp distributors do not have this restriction. Which goes with which, would indeed best be left to Andy.
_________________
Roads Scholar &
1957 Kombi low mileage 36 hp governor equipped M 178 Slow Drag Winner 2014, 2015, 2018

1965 hardtop Deluxe Microbus owned since 1990 M 620 factory 12 v 1500cc

1961 (October)Single Cab- Road Trip Workhorse

[germansupplyscott]

i've seen the restriction in the canisters. the distributor for this engine has been rebuilt by glenn ring using the canister we had, which has no restriction. the distributor is cast iron.
_________________
SL

[glutamodo]

The restriction in the vacuum pipe was introduced along with the extension pipe into the venturi, or at least that's how it looks here. VW temporarily used a small number (only 840, in May 1962) of a variant distributor which was only to be used with that modified 28PICT carb. However the Service note doesn't say anything about the 4R5 having issues. I'll include the note below. (Tasb, have you ever seen any of those 840 ZV PAU 4R7's?)

The restriction in the vacuum nipple won't actually reduce the amount of vacuum reaching the vacuum canister. It will only just alter the time it takes for that vacuum to reach it. If you're like idling at a constant speed, the vacuum will just build up and be the same.

Now, there have been reports from people about the vacuum advance kicking in too strong at idle on 28PICT carbs. The only reason I can think this would be would be something like the EMPI 30PICT-1 carburetors from several years ago where they placed the lower vacuum drilling slightly off from where it probably should have been in relation to the throttle butterfly. ( http://www.thesamba.com/vw/forum/viewtopic.php?t=278191 ) A freshly rebushed carburetor will see a much stronger vacuum signal as well.

I would make sure the upper advance drilling or fitting in the venturi is clear, as the vacuum signal is a balance of what comes in at the throttle butterfly and middle/upper end of the venturi.

-Andy

[germansupplyscott]

i am having a bit of difficulty being sure i am interpreting this chart correctly:

set timing to 10°BTDC at idle, hose off.

at idle, with hose on, timing should be 10-16° BTDC

all in, timing should be 14-22° BTDC

what doesn't make a lot of sense to me is that if i am reading the table correctly you could have total advance from of only 4° (End Degrees 14-22). obviously the engine will not pull very well or run at higher speeds very well with only 14° total advance. ideally you'd want to have a smooth advance off idle from 10° to 22°. but how do you control this with the vacuum only distributor? spring tension alone? and how do you ensure the carburetor is pulling vacuum in the right amount at the right rpm?

again reading the chart the carb should pull 16-25mmHg at idle, 35mmHg midrange and top out at 43mmHg. there's no rpm's given for the vacuum figures. the book i am using has lots of graph for timing curves but doesn't seem to have one for this distributor.
_________________
SL

[glutamodo]

VW always specified 15-21 or 14-22 degree range for maximum vacuum advance on the 07015 vacuum canister for the 4R5 distributor - but all the ones I've ever run were more like 20 or 21 degrees. Double the numbers on the left, for engine degrees:

[germansupplyscott]

Ok that graph is unequivocal, thanks.

is there any way to control or modulate the amount of vacuum the carb is pulling?
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SL

[glutamodo]

The vacuum signal is a balance of the vacuum at the throttle butterfly and the drilling or fitting in the middle/upper of the venturi. To change it, you'd have to modify the sizes of those openings. I actually did that in that other thread I mentioned above, to get an EMPI 30PICT-1 to deliver a better vacuum signal. Here's that link again:

http://www.thesamba.com/vw/forum/viewtopic.php?t=278191

[tasb]

No Andy, I have not come across an R 7 yet. I would love too though!
_________________
Roads Scholar &
1957 Kombi low mileage 36 hp governor equipped M 178 Slow Drag Winner 2014, 2015, 2018

1965 hardtop Deluxe Microbus owned since 1990 M 620 factory 12 v 1500cc

1961 (October)Single Cab- Road Trip Workhorse

[germansupplyscott]