7 - Carb Attack
Breathing new life into the old Linkert M74B has been one of the most enjoyable jobs I have done.
I wanted to go up
a size in Linkert terms from what would have been stock for
I could just use a later Chief Linkert 1-1/4" three bolt model, and the stock 1-1/8" (28.5mm) venturi, or maybe scratch it out to 1-5/32" (29.5mm), but that would be making a bigger venturi in a small carb.
That would cause a delay to the operation of the main jet circuit, as the diffence in cross-section area between carb throat and venturi would be smaller. In other words, the air would not speed up as much as with a larger difference in area, and suction on the jet would be less, so the throttle would need to be open farther for the same air speed increase through the venturi and suction on the jet. Hence, the carb would "come on the jet" later.
So this is why I needed to go up to a 1-1/2" (38mm) four bolter. The M74B with a smaller than the stock 1-5/16" (33.3mm) venturi will allow the carb to come on the main jet earlier, and the main jet is far better equipped to supply a richer mixture of methanol than the low speed jetting circuit. You need to flow a lot more fuel through the jets on methanol than with a petrol/gasoline engine.
I also wanted to try out some of the Old Time speed tune hot tricks that were performed on these old carbs. and would be in keeping with the spirit of the build of this bike.
Moen had what could only be described as a derelict M74B that he was willing to donate to the cause. As I intended to perform unholy surgery on this poor old relic, it would be exactly what I needed.
I boiled the whole thing in a pan of piss-thin Gunk engine cleaner - that pulled out an amazing amount of crap - then let it soak in carb cleaner over night. This at least made the battle scarred body useable, and that was as good a starting point as I could have hoped for to make me a Race Carb.
The front flange was cut off, the choke discarded, and a 15 degree taper brass air intake bellmouth soft soldered into the mouth of the carb, so it was the same size as the 1-1/2" carb throat - with this then blended into the Linkert body to make a smooth air pasage interupted only by the venturi, main nozzle and slim-line butterfly.
The low speed needle has been retained on a fixed adapter instead of the needle valve lever.
I have made a new venturi out of brass with an 1-1/4" ID. The stock M74B 1-5/16" ID venturi gives a 71% butterfly to venturi cross-section area ratio (i.e. the area of the venturi is 71% of the area of the carb throat). With the 1-1/4" ID venturi, I now have a 64% ratio. This ratio change will advance the main jet into operation much quicker, as the air flow will be speeded up more in the smaller venturi and, thus, suck harder on the main jet.
The butterfly is soft-soldered into its shaft so that the fixing screws can be discarded and the shaft/disk made slim.
The fuel bowl was in a sad state and could not be used. This was replaced with an M88 side feed bowl, which has a larger fuel capacity (another tune-up tip!). And, of course, the old favourite "Quick Fix", an IPE supplied P/N 102447F foam float.
I can not recommend this product high enough. I have used these floats on my mates' Schebler and Linkert carbs as first point of call when the guys I know can't get "this S*^T heap F^%+ing carb to run!". On the back of these foam floats I now have a name as "the bloke to see" if your old Linkert won't play!
Last job on the carb was to make the linkage for the butterfly valve with a spring return, and a fixing for the low speed needle.
Earlier Installments of the DiXiE Story:
Part 1 - Presentation
Part 2 - End Float Mystery
Part 3 - DiXiE Oils Up!
Part 4 - Pumphouse Preview
Part 5 - Fit to Roll
Part 6 - It went BANG!
from Victory Library is a good place to start if you want to look into
making your Linkert carb perform better.
More Chouts and Vintage Sprinting:
as a race requirement I had to fit a steering damper, and I'm trying to
cut the weight off! There is no way I will fix up a modern hydraulic one,
now that would be sacrilege. The only way to go is the Deluxe Andre,
well you would wouldn't you!
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