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Discussion Starter #1 (Edited)
Hey guys,

I've got a problem with my timing where I'm getting maximum vaccum advance at idle.

I'ts a rebuilt dual port 1300 engine. Currently has a 31Pict-3 carb & a geunine Bosch 034 SVDA distributor. It's been running great but having issues with it advancing properly without it hesitating and getting flat spots.

I've set the mechanical advance (with the vacuum line plugged) at 3500rpm + at 30deg BTDC - this results in a 7.5deg BTDC at idle.

However when I go to attach the vaccum line the idle jumps to about 14deg BTDC?? The Vacuum line is attached to the left side of the carb and not the other port facing the rear (for retard line).

I've sprayed carb cleaner in all the suspect areas checking for air leaks when running but the idle speed doesn't change. The Carb has all been rebuilt as well and tuned to spec.

Funny enough, I ran the car just today just on the mechanical advance alone, with the vacuum line plugged and it ran much better with the flat spots practically gone.

Is my carburettor just junk or is there something else i'm missing?

Any help would be great.

Thanks.
 

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Does your distributor have a return spring on the vac advance base plate ? (and has it become disconnected ?)
 

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Yes, there is a spring, it's inside the vac can and it should return the plate when the vacuum signal falls - to test suck very hard on the pipe going to the vac can - you should be able to see the plate rotate to full advance. Hold the vac and the plate should stay put, release the vac and the plate should snap back (spring inside the can doing the work)

However, assuming you have set the throttle arm stop screw correctly (see Moby's pic below) I think your fault must be with the gasket between the carb & manifold - here's why.



You have full vacuum at idle, with the throttle plate closed. The only place that full vacuum pressure exists at idle is below the throttle plate.

The vacuum advance port drilling goes from the brass tube, down to the base of the carb where it travels through a transfer slot in the base and up another drilling that comes out inside the carb right on the edge of the throttle butterfly.

Therefore, if the gasket is not making a proper seal across the full base of the carb, maximum vacuum will leak through to the transfer slot and appear at your distributor. You may well not have any obvious leak externally on the gasket, but I think you probably have an internal leak to the transfer slot in the carb base.

This could be caused by -
Warped carb body ( carb base flange not fully flat)
Warped manifold (mating flange not fully flat)
Damaged or incorrectly sized gasket.
Hole in the manifold larger than the hole in the base of the carb - such as a 1300 carb mated to a 1600 manifold.
or a combination of any of the above.

Warped flanges occur when the bolts are over tightened.

If you suspect this the best way to correct it is to get a piece of float glass (preferably a thick piece) and some wet & dry sandpaper about 120 grit.
Scribble over the mating face with a permanent marker pen then rub across with the sandpaper on the sheet of glass. Continue till you have removed all the marker pen, then repeat a couple of times till you're happy it's flat.

Obviously for the carb you will have to remove the studs first, and take precautions to stop abrasive and dust entering the manifold if you do it on the car. (stuff a big greased rag down the hole first, pull it out when finished, then wipe all the grease from the sides with clean paper towels, or better still do it off the car and clean properly before refitting.

Dave.
 

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Discussion Starter #6
Hi Dave,

Thanks for taking the time to reply. With the vacuum can test you mentioned, the plate is holding it's position and then returning to it's fixed position after the suction is lost.

Regarding your answer to the manifold gasket, you could be correct and I will check it out first thing tomorrow morning. I'll do as you suggested and make sure both the manifold and base of carb are completely flat first before installing a new gasket.

Would you reccomend I use a thicker gasket for better sealing and should it meet the edges of the carburettor hole so it's flush?

I was getting some advice from thesamba.com and they suggested that my carburettor (german solex 31pict-3) won't run well with a SVDA as the vacuum signal is too weak and therfore I should mate it with a DVDA. But then I'm getting a strong vaccum signal, it's just that it's all maxed out at idle. Any thoughts on that? I assumed that over here in the UK, on the AB coded engines, we never used the DVDA distributors. Yet, I do have a timing mark for it (5degATDC) on the pulley and the extra retard vacuum port on carb.

Many thanks.
 

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Hi Dave,

Thanks for taking the time to reply. With the vacuum can test you mentioned, the plate is holding it's position and then returning to it's fixed position after the suction is lost.

Regarding your answer to the manifold gasket, you could be correct and I will check it out first thing tomorrow morning. I'll do as you suggested and make sure both the manifold and base of carb are completely flat first before installing a new gasket.

Would you reccomend I use a thicker gasket for better sealing and should it meet the edges of the carburettor hole so it's flush?

I was getting some advice from thesamba.com and they suggested that my carburettor (german solex 31pict-3) won't run well with a SVDA as the vacuum signal is too weak and therfore I should mate it with a DVDA. But then I'm getting a strong vaccum signal, it's just that it's all maxed out at idle. Any thoughts on that? I assumed that over here in the UK, on the AB coded engines, we never used the DVDA distributors. Yet, I do have a timing mark for it (5degATDC) on the pulley and the extra retard vacuum port on carb.

Many thanks.
I would be wary of thicker than standard gaskets, as they make it easier to distort the flanges assuming they are not distorted already. If both faces are truly flat then a paper thin gasket should be fine. If you look at the base of the carb when you remove it you will see the slots for the vacuum advance, the gasket needs to seal these off from both the outside and the inside when the carb is bolted down. The hole in the centre of the gasket should be exactly the same size as the bore of the carb and should line up with it. Those slots in the base of the carb should be sealed by the base gasket just as well as the rest of the carb.

According to this site - http://vw.zenseeker.net/Ignition-Distributor.htm - look up 043 SVDA - your svda should be fine with a 31 pict-3. DVDA's not that common over here either - I wouldn't bother myself.

Dave.
 

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Discussion Starter #9
Thanks, I have read that info before on vw-resource but will do so again, it's a great site. In fact, it's usually my first port of call when I've got trouble!

Also, regarding the vacuum port on the left hand side of carb... I've seen two different sized diameters (see pics below). Which one is correct? I'm assuming the smaller one? Or does it not matter? I can only guess that larger one is for vacuum only distributors?

Pictures are taken from sources on thesamba. (TheSamba.com :: Performance/Engines/Transmissions - View topic - Solex 30 pict-3)



 

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Those pics are of 2 different carbs, the top one is a 34 PICT-3 for a 1600 twinport engine with a small vacuum can on a SVDA distributor.
The bottom carb is a 30 PICT-3, I have one of those on my 1500 single port engine with a Single vacuum only distributor which has a larger vacuum can.
 

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Discussion Starter #11
Ah thought so, thanks. I've got the SVDA type port.

So, I think I might have found the issue thanks to Mr.Whippy. The old gasket was really worn and wasn't completly covering the vacuum port at the base, allowing air to travel up the chamber. I cut out a new one and made sure both the base of the carb and inlet manifold were perfectly flat and smooth.

I'll start it up tomorrow and see if it's made a difference. Have added pics below.

New Gasket:



Old Gasket:





Open port not covered:



Old gasket fitting poorly:

 

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Yes, that gasket looks pretty suspect. Let us know how it goes.
With your svda at tickover without vac connected it should come out around 7 - 8 BTDC, if you then connect the vac it should stay the same, or only change very slightly. If you then rev it up it should go to about 40 BTDC maximum (that's mechanical plus vacuum.) Actual RPM figures are irrelevant, the only critical thing is that you don't go more than 32 BTDC at maximum mechanical advance.

Dave.
 

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Discussion Starter #13 (Edited)
Thanks, Dave.

So started it up this morning ( test carried out on the driveway) and looks like my issue is sorted. I've re-checked the mechanical advance (vacuum port blocked off) and I'm getting 30deg BTDC at around 3500rpm (it doesn't advance more than that with increased rpm) as you say, the rpm is irrelevant. I guess I could to 32deg BTDC but will see how it performs under load.

Once at idle, it comes in at 7.5deg BTDC exactly. Then I hooked up the vacuum line, the advance barely shifted (maybe one degrees). But I'm happy with that compared with what it was doing before (the full vacuum advance).

Thanks again for this great resource.

Hope to get to a few shows this year! Is bugjam still on??

Just as a side note, I got a bit creative today and put together a sticker for the '69-71 air cleaner. They don't reproduce them anymore but just using Microsoft paint and some glossy printing paper, I thought it finished off the engine bay nicely.

Take care and stay safe everyone.



 

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Good news you have it sorted !

Sticker looks good - Is it self adhesive ?

Dave.
 
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