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Carburettor Conundrum

Simeon

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blog-0811490001452063364.jpgHaving spent some time sorting out my ignition, I was pretty happy with how my car is running. The idle is a lot more stable than it was before but I can still detect a slight miss fire occasionally. Coincidentally to finishing the ignition I noted that I had a problem with the water choke on my 32/36. This was normally as regular as clockwork but for some reason had stopped engaging.

I took the air cleaner off to watch the operation of the choke and while I was there I looked down into the primary choke. When the throttle was opened a fine mist of fuel was present as the engines vacuum pulls the fuel from the auxiliary venturi in the primary along with the output of the pump jet (this was a nice healthy squirt when seen with the engine off). When the throttle was returned to idle, the visible mist disappeared as the engine is now running from the idle jets but periodically a big drip of petrol would fall from the auxiliary venturi and drop on the back of the closed throttle plate. Looking at the carb in general there was some seepage from the top cover so, along with the choke issue, I figured this would be a good time to pull it for a rebuild.

I had previously rebuilt the carb about 3-4 years ago and put in new jets to match CD's jet prescription. At the time I noted that the base flange was a little warped but I didn't do anything to correct it as I wasn't convinced I was going to keep the carb then. I do have a pair of Solex PHH carbs, manifolds and most of the linkage necessary to fit. I had been holding off, awaiting a rebuild of my engine but now I am cooling on the idea of the side drafts.

Most of my driving is around town, with pretty low speed limits common (60km/hr - approx 40mph). At this speed, I find that the car in its current state of tune seems to sit at this speed in 4th somewhere between idle and the throttle barely open - just at transition. This makes it a little bit annoying to drive - I often find myself rolling along in 3rd gear just to get the revs up and get off the idle circuit. If I rarely use anything other than the single 32mm choke then why would I move to 4 x 34mm chokes? I might reconsider that depending upon how far I go with an engine rebuild. I will have 9.5:1 compression ratio pistons (I have NOS that I picked up off eBay for a steal!) and the engine is already fitted with a 292 cam following a head rebuild a few years back. I will stick with the 32/36 for now and take a view on it when I have the new engine fitted.

My carb is actually a Spanish 'Bressel' licensed copy of the 32/36 which dates it as pretty old since this was made at the factory in Spain that Weber now use to make carbs. The carb is simple to pull, though you need some patience and long fingers to get at the nuts past the various obstructing hoses. Once the carb is off, it is straight forward to strip - removing the top cover first. I checked the float level first and this seemed OK. I had been confused slightly as to whether I had previously set it at 41mm (the factory figure) or 40mm (as quoted by CD) - I appeared to have picked the factory figure as this was pretty much bang on 41mm. Subsequently I worked out that the 40mm CD quotes includes an allowance for the gasket. I needed to slightly tweak the height of each float so that they are perfectly level (there was a hairs difference between the two) but I will need to look elsewhere for any internal flooding issues. I swapped the needle valve even though the one in place had no visible wear. I was already committed to using parts from the rebuild kit, so I may as well do the whole thing while i am there.

The auxiliary venturis were slightly loose in their slots and could be jiggled slightly. From what I have read, this is likely to be the cause of my problems. I can't remember how loose they were last time I rebuilt it but I don't remember reading any special advice beyond making sure I got them the right way around and checking that the feed tube in the centre had its slot facing the bottom as they have been known to rotate. The bottom of their housing slots had some 'bright' wear visible, so it looks like they have been vibrating in their slots for a while.

The body of the carburettor was sanded flat using #80 paper held on a sheet of glass. This took a while to get all of the warp out (I wonder if the carb had been overheated in the past, I think the warp contributes to the loose venturis). Note: I actually did this before disassembling completely as, depending upon how effective it was, I would have junked the carb if it didn't work. Having got the base flange flat, it makes the body a better base for the rest of the rebuild.

I took the rest of the carb apart, including the throttle shafts. I bought replacement bushes as there was also a little play in the bushes and these are another source of vacuum leaks. The shafts had the linkage removed from the outside and the screws were carefully undone. Since they had been previously peened with a chisel to prevent loosening, I was able to back the screws out carefully to allow their reuse. Other advice is to grind the projecting ends of the screws with a point in a die grinder but this seemed too risky if I could get them out without destroying them. Once the shafts and bushes were removed the body and the top were cleaned with carb cleaner, scotchbrite and a small wire brush. All of the passages were flushed with carb cleaner and then blown out with compressed air.

When reassembling, I first needed to address the loose auxiliary venturis. There seems to be a number of techniques to resolve, including staking with a centre punch and even gluing with epoxy. I chose to add shims to the blank ends to push the open ends up tight against the corresponding passage in the carb body. I cut the shims from a soft drink can as the aluminium is thin enough that multiple layers can be used to dial in the thickness exactly. I followed this up with a single touch with a centre punch to stake everything in place. The venturis needed to be carefully aligned and pushed into place, definitely no wobble so they should be much better.

The problem with the choke seemed to be related to the choke diaphragm and its actuator rod. For some reason the whole rod in mine had been replaced with a similar but different part which was incomplete. Someone had previously ground a slot into the rod, within which the operating lever that controls the stepped cam and butterflies runs. This had subsequently worn further and instead of pulling the choke butterflies open with engine vacuum the operating lever had wedged on the rod, binding the whole linkage up. I did a quick examination of the local Australian Weber parts specialist (http://www.weberperformance.com.au) who don't stock the choke diaphragm / rod assembly. I had to order one from the States. Since this would leave my car stranded for a week or so while I await delivery from Pierce Manifolds, I decided to have a go at a repair.

Part of the issue is that the rod on this diaphragm is much thinner than the correct part so this allows the rod to be displaced away from the choke operating lever as it is unsupported within the housing. To minimise this, I added a small section of rubber hose at the end that the rod can run in and it would keep centred. I rotated the rod, relative to the diaphragm, so that there was more steel facing the front where it meets the operating lever. I then filed a new groove into the rod, ensuring that there is a nice hard edge for the operating lever to act against when the diaphragm pulls the rod forwards. The small vacuum passage on the body was also blocked, so I cleared this out as well. There are two styles of choke pull down diaphragm for different 32/36, the one with the thin rod which, when complete, has a couple of washers and a spring retained by an 'e' clip which act upon the choke linkage and the solid rod style where the linkage runs in a machined slot. I ordered a solid rod diaphragm from Pierce Manifolds before removing the choke body from the carb in situ to swap the rod. This time I didn't touch the coolant pipes, I just removed the three clamping screws and pulled the water choke and spring body clear of the choke body.

I struggled to find a lot of information regarding adjustment for the choke pull down via the adjustment screw at the end of the diaphragm. Similar style chokes on different carbs are adjusted based upon setting a given size opening in the choke plates when the vacuum is strongest at idle. After fishing around on several old Ford websites I found out that the opening should be 7mm and this is measured by levering the diaphragm backwards up against the adjustment screw. This is done after refitting the choke body to the carb but not the water choke / spring body to allow access to the diaphragm rod. Set the choke by opening the throttle and manually shutting the choke plates (no choke spring fitted to close them for you). You then lever with a screw driver the diaphragm rod towards the back of the car and up to the stop. The choke plates will pop open to give a 7mm gap. In my case, it was a few millimetres wider so I removed the diaphram end screw and screwed the adjustment screw in a few turns before checking again. Refit the water choke / spring and adjust the rotation of the choke plates and then check the fast idle screw setting and your done.

When the car is first started, the choke plates are tightly shut. Once the engine fires and develops a vacuum, this pulls the diaphragm and opens the plates to the 7mm position to prevent the engine from being over choked.

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7mm drill in position to measure plate opening when diaphragm is levered towards the rear of the car and up against the adjustment screw.

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The original 'thin rod' diaphragm

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Comparison with new 'thick rod' diaphragm

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Auxiliary venturi shimmed with Pepsi can and staked with a single centre punch 'dot'

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'Bressol' licensed from Weber



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