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Carb restoration: What we do, and how we do it

Carb restoration: What we do, and how we do it

Before and after

The SU carburetors are an amazingly rugged carburetors. At 30 or more years old they are probably due for some major attention. Rich-running carburetors cause carbon build up around the piston rings and valves, it also dilutes your engine oil causing accelerated engine wear. Lean-running carburetors can result in burnt valves or even pistons. Worn carburetors are difficult if not impossible to tune causing poor performance and wasted fuel – at $4.00 – $4.50+ a gallon every drop needs to used to its fullest.

All our restorations include:

    • Four-step cleaning process to restore color and appearance of all castings and parts
    • SU dashpots available as highly polished or original ‘dull shine’
    • Steel linkage replated and polished
    • All SU carburetors include new throttle shafts and bushings. Strombergs with brass shafts receive new shafts and bushings. Water choke Strombergs receive shafts/bushings as needed.

This thing could use some work!

Every restoration pays critical attention to the metering of your carburetor. This starts with the replacement of the metering needle and jet, using the recommended needle for your application (or any needle of your choice if you have special tuning). Most rebuilders offer this much, but more is required. The SU carburetor is highly vacuum dependent for its operation. Vacuum leaks at the throttle shafts are a common problem on these carburetors. These leaks allow unmetered air into the mixture and prevent the piston from raising properly due to the lack of vacuum in the system. Either of these alone are enough to cause a critical lean-running condition, combined they can quickly lead to major problems. To correct this carburetor bodies are bored and fitted with new bushings and throttle shafts. HD8 sets use teflon bushings which do not require machining. Strombergs with steel shafts receive new shafts only as required as they tend to not wear, new throttle shaft seals and bushings are installed. With the vacuum leaks solved it is important to make sure the piston rises and falls according to factory specifications. A piston drop test is conducted and compared to factory specs. Pistons which drop too slowly tend to run rich, whereas pistons that drop too quickly will run lean. Fixing slow drop is a matter of light sanding to increase clearance, fast dropping pistons have required replacement in the past. Most often overlooked are the piston springs, most carburetors I receive have mismatched springs in them. The proper calibrated springs are essential to the correct metering of fuel. Every set of carburetors receive a new set of matched springs. Bugeye Sprite H1 and TD H2 carbs do not use springs.

As good as new!

The float bowls are often taken for granted, but with the HS carburetors there are a couple of concerns. The bowl must be held at the proper angle otherwise the fuel level will fluctuate causing either rich or lean run conditions. Alignment is maintained with a nylon or rubber adapter and a ‘lug’ cast into the body of the carburetor. Very often the lug is broken off, I repair the broken lugs and replace the nylon adapters in all HS restorations. It is important to have adaptors in proper condition to prevent fuel foaming.

Floats are cleaned and then precisely weighed. Weight must conform to new specifications. The floats are then submerged in a ‘test fluid’ and held under pressure. The float is then weighed again; any increase of weight indicates a leak in the float. Replacement of floats as required is included in the standard restoration price. The HIF series of carburetors does have a history of bad floats. I keep new HIF floats on hand and have a good supply of used floats for H, HD, and HS series of carbs. New needles and seats are installed and the float level is set. Once assembled, test fluid is pumped into the carburetor at fuel-pump presure. This ensures that the needle and seat are functioning properly and that there are no leaks in the fuel path.

All part of the job.

The proper operation of your carburetors is the primary objective, the appearance is important also. Your carburetors are completely disassembled and all parts cleaned. I use state-of-the-art cleaning equipment with does not damage your parts. The connecting and choke linkages are replated in bright zinc for years of corrosion protection. The dashpots are lathe sanded to remove any minor nicks and scratches, then polished to a high shine. Everything is reassembled using new stainless steel screws for the dashpots and float bowl covers (HS series). The result is a set of carburetors that perform and look as good or better than the new carburetors offered at more than twice the price. As a special bonus I will clean your heat shield and manifold at no additional cost.

My price for this extensive restoration is very reasonable, especially if you consider the cost of doing it yourself. These are the currently advertised prices for the parts you would need:

Item Moss Part No. Qty Price each Total
Carburetor Kit 375-358 1 $154.95 $154.95
Throttle shaft 372-540 2 $19.95 $39.90
Throttle bushings 372-545 4 $2.95 $11.80
Piston Springs 370-520 2 $4.45 $8.90

Not included in the above is cost of machining the bodies for bushings, easily $100.00 — IF you can find someone qualified and willing to do it.

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