Converting an L-Jet Beetle to carburetion

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Converting an L-Jet Beetle to carburetion

Post by Marc » Sun Jan 01, 2017 4:29 pm

There are several ways to go about converting a `75-up Beetle from L-Jetronic fuel injection to carburetion. It requires some forethought to settle on which route to follow, based upon what you have to work with and where your priorities lie. First, if you're starting with an L-Jet engine you should be aware of the differences between it and a carb'ed 1600DP. The engine case is cast from AS21 alloy, which is arguably better than AS41 - it was chosen for its higher strength at high temperatures. There is no provision for a mechanical fuel pump, so you'll be committed to using an electric one. The L-Jet cylinder heads are similar to later DP carb'ed ones, but have "1300-sized" 33x30mm valves with 9mm exhaust stems rather than 35.5x32mm/8mm. These won't cause a significant reduction in power on a mild engine, but would be a liability if your plans include increasing the displacement. You'll need an alternator pedestal and strap from a carb'ed engine since on the L-Jet engines those are integrated with the intake manifold. The thermostat opening temperature of the L-Jet engine is higher; if you retain a functional t'stat system (as you should) you'll need a "carb'ed" t'stat. Lastly, there are a few small differences in the length of certain m8 studs in the case & heads - replacing them will result in a cleaner-looking finished product, but if you're leery of breaking them you may want to just use spacers under the nuts.

The L-Jet pump now in the car puts out far more pressure than any carb can handle - it could be regulated down but not simply, best to go with an aftermarket pump intended for carb use. The existing fuel pump safety circuit won't work anymore, so you'll need to provide another means of switching the pump off should the engine quit with nobody present/conscious to turn off the key. The front-mounted stock pump pushes unused fuel back around to the tank, so there're two fuel pipes through the tunnel; all fuel lines are larger than those used in carb'ed cars/engines so some adaptation will be needed.

What exhaust system and heat exchangers do you want to use? The heavier and more complex F.I. setup does offer improved heater output since all four cylinders' exhaust is routed through the heat exchangers, and there'll be no issue with rear apron clearance. The rear sheetmetal on the F.I. engine, besides being slightly wider, has the holes for the freshair hoses and the warmair pickup hose to the aircleaner relocated compared to the carb'ed piece - routing the hoses with the wrong combination of parts can be challenging. Staying with the F.I. heat/exhaust setup would seem the best for most applications were it not for it having no provision for an intake manifold heatriser pipe to connect. Unless you only drive on warm sunny days you do NOT want to live with a single-carb'ed dualport with no manifold heat. But with dual carbs preheat isn't needed - a pair of small 1-bbls suits a mild engine well.

If you opt for a "carb'ed-style" exhaust system the heat exchanger control valve linkage needs modification and the RH cable will need to be extended. The small levers at the valves can be bent 180° so they'll be pull-to-open (`68-`71 Bus heat exchangers or the Dansk aftermarket "universal" boxes won't need that). The `75-up rear apron won't clear the tailpipes of a stock carb'ed muffler; this won't be an issue with an exhaust header, or with many aftermarket muffler setups...but keep in mind that 99% of those, although they have connection points for the intake manifold heatriser pipe, do not provide near the net flow that a stock muffler does so there could still be some driveability issues with a single carb on a cool/damp day. The "sled" tins between the engine case and the heat exchangers are narrower on the carb'ed setup so you'll need those too.

The L-Jet engine's fan shroud can be used on a carb'ed engine. The throttle cable guide tube through the shroud will need to relocated to align with a carburetor (or most dual-carb linkage setups). Figure this out in advance, there are subtle differences in the internal vane arrangement that complicate moving the tube - and it needs to clear the oil cooler. The tube is longer than that used on carb'ed engines. You'll need the correct combination of guide tube, Bowden tube, and cable lengths - simplest is to use 1974 parts for all. The L-jet shroud has an integral "Hoover Bit" (that little L-shaped piece of tin that wraps around the base of the cooler on carb'ed engines with a doghouse cooler) and an added venturi ring at the cooling fan inlet which improves its efficiency. The mounting point for the ignition coil is different, which may be a problem with some dual-carb linkage designs.

The ledges on either side of the engine compartment are scalloped out to provide clearance for the fuel injectors and their hoses when removing/installing the L-jet engine. The overcylinder sheetmetal is therefore wider. The rear sheetmetal is also slightly wider at the ends; you can get away with using a "carb'ed" piece here, the resultant small gaps in the corners won't allow a lot of hot air to recirculate into the engine compartment - but something must be done about the overcylinder tin. If you don't have the correct pieces, extension plates can be built out of sheetmetal. They can be designed to sandwich in at the front & rear tin attaching screws, or can be riveted/brazed/welded onto the cylinder covers.

Should you be using an early, externally-regulated alternator or a generator, there are insufficient wires in the L-Jet car's harness to support installation of a regulator under the back seat so you'll need to mount it to the firewall or shroud and build a harness for it. Other wires to the engine may need to be extended as well.

Up front, remove the L-Jet electric pump and connect the tank outlet directly to the feed line going into the tunnel. The return line going back to the tank needs to be capped, better to do that right at the tank rather than at the rear so that line isn't "wet" and a potential source of a leak. Or you can replace the tank with a carb'ed one and peddle yours. The larger outlet pipe and nut will fit, or you can simply use an inline filter below the tank to adapt the fuel line size up. The fuel supply pipe coming out at the rear will require another adapter to go back down to the carb'ed fuel line size; again an inline filter could be used but it's not the most convenient place for one (plus it's too close to exhaust heat for my liking).

If you're using an L-Jet engine case with no mechanical fuel pump, install a low-pressure electric one. Best place for this is up front since electric pumps are better at pushing than at pulling. There's a good spot on the passenger side, across from the master cylinder - route the lines so that they have ample clearance to the steering linkage at all points of travel. Don't overlook providing a safety circuit for the pump; one simple way is to power it via a simple relay in the trunk that's controlled by the charging-system warning light wire. Locate it where it's accessible to be bypassed in an emergency - like, if you should run the carb(s) dry or the alternator dies - or provide a push-button or spring-return toggle switch under the dash to accomplish that.

The L-Jet harness is independent of the car's main harness so it can all be removed, along with the ECM and double relay behind the back seat. This'll leave a rather large hole in the firewall to be plugged off - an oiltight hole seal made for electrical panels (Rittal, Weigmann, et al) does a sanitary job, if a bit pricey. The series resistors for the injectors reside behind the RH engine compartment tarboard; remove with care so you don't break off the leads at the resistors. Most of these parts are no longer available new and there's a market for them, along with goodies like the oil filler assembly and the intake manifold-to-head spacer blocks.

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Re: Converting an L-Jet Beetle to carburetion

Post by bug » Tue May 15, 2018 12:15 pm

I have a 1975 VW SB convertible which has undergone a body off restoration after being stripped of everything except the wiring harness. Prior to this the fuel injection was not working and the car not running. I have purchased a re-built engine and am wondering if I should replace my wiring harness with a harness for a 1974 SB. The 1974 Super Beetles used carburetion and I would like to convert from fuel injection to carburetion. What is your take on this?

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