Intake Manifold

Regarding the three-piece dual port intake manifold -- Why a three-piece manifold? Gives the veedubber one more thing to watch for, to make sure air isn't getting sucked into the manifold around those rubber expansion joints.

The ‘71 did get the 34PICT/3 carby and three piece manifold. My theory is this-- (re the one-piece/three-piece switch)-- the cylinders actually 'walk' front/ back, and up/down a few thousandths when the engine is running (the heads are clearanced to allow for this), and also the iron cylinders and aluminum heads expand at different rates with heat. This makes the engine change shape fractionally with heat. The one-piece manifold, with it's soft copper gasket at the heads, is probably at the limits of it's capacity to stretch with this engine movement on the 1500 (it runs colder than the engine remember, so doesn't expand as much), and with the greater heat of the 1600, they needed to alter the design.

I also wonder if they were not anticipating an extended stroke for another increase to say 1700. An extended stroke would have required a wider engine, and the three piece manifold could cope with this OK.

On p. 1-17 (of the Bentley manual) there is an exploded view of the cylinder head. Two heads are shown: the 1971 and later head and the 1970 head. The major difference appears to be that the 1971 head is double port, whereas previous the head was single port. Is this correct? And as I look at the 1971 head it shows two studs that the intake manifold attaches to. It is at this point that there is a compressible copper gasket, right? If the nuts are not sufficiently tight this would be a very likely place for air inleakage. I don't find a torque specification for these nuts in the Technical Data section; do you know what torque they should be tightened to?

In 1970 the US got the first 1600cc engine, but that was with the single port heads, same as my 1500cc engine. I think the valve sizes may be a bit larger on the twin port heads too.

I'm fairly sure they are paper or fiber gaskets on the twin port. The RMMW catalogue has a photo of a twin carby setup with the gaskets on the right hand side (black -- 'double ring' shape).

Can't remember the torque setting -- but it wasn't very much (small studs).

The '71 manifold was also designed for the 34 PICT carburetor. The 34 PICT came with the first 1600tp in the ‘71 model year. ’71-‘73 had the dual vacuum distributor with the 34, then 74+ got the single-vacuum dual-advance (SVDA) distributor and still kept the 34 PICT carburetor.

There IS the possibility that the ‘71/’72 manifolds (with generator) were a little different to the ‘73 (with alternator -- larger diameter), but I haven't heard of any difference (which doesn't mean much as I have less knowledge after ‘70 than before).

I've been having a frustrating time myself getting the old manifold out and putting the "new" one back in. Can you give any advice?

On my '73 SB I have to strip pretty much everything off of the engine (carburetor, alternator, etc.) to get the intake manifold out. And to do that, of course, it's just easiest to remove the fan housing.

I found that by removing the left tp "corner" manifold piece and lifting the fan assembly with generator attached I could finangle the new manifold centre section in. Since my Bug doesn't yet have cooling flaps back in (hard to find here -- almost all VW's have had them removed and junked!) I could lift the fan without worrying about the thermostat linkage.

I'm trying to visualize the system and what may have changed. The two right cylinders are running, so you know you're getting petrol into the system. And you know that the two plugs on the left are firing, which means that somehow between the carburetor and the cylinders the fuel mixture on that side is outside the 1.4% - 7.6% flammable range for petrol. There's no way it could be high, so excess air in the system on that side is the only logical conclusion.

I even tried pulling back the rubber boot on the left side of the manifold to see if I could spray WD40 in there for a test, but the right cylinders wouldn't fire with the left boot off. It was this which finally made me think it MUST be the left tp manifold piece, since that was the furthest away from the right cylinders which were obviously getting a "close enough" mixture to run.

Finally I just sat and looked, then tried moving the left rubber boot to try the WD40 trick at the left cylinders, which settled the thought about where the leak had to be, even though I couldn't see it and WD40 near the head/manifold joint didn't make any effect (couldn't get the straw right down there, and the leak must have been over too wide an area).

(Later) It turned out to be the removed dual port manifold piece which had not snugged down properly. When it's in the car, the bolts for that piece are almost impossible to reach (socket with 1foot extension bar for the front nut and an open-end spanner for the rear bolt, turning it 1/12th turn at a time and flipping the spanner over each 1/12th (the heads are angled 1/6th turn). So it had not bedded down and I couldn't see that from above it. After loosening and a good wiggle I got an extra 2-3 turns on the nuts and it's fine now. Idles smooth, lots of power.