|View single post by Mark Rosenbaum|
|Posted: 03-19-2005 01:28 am||
|In the Stromberg carburetor, the amount of fuel delivered to the engine is determined by the main jet, which is of fixed diameter, and the fuel mixture needle, which is tapered. The amount of fuel delivered depends on how far the needle penetrates the jet -- or, to be precise, on the area of the annulus (ring) formed between needle and jet at any specific penetration.
I recently acquired a copy of the Burlen Fuel Systems booklet titled 'C.D. Carburetter Needle Profile Charts', which lists the characteristics of about 275 different needles used in various Stromberg carbs. As I discovered upon perusing this document, the profile, or taper, of a mixture needle is determined by its diameter at a number of different points. By manufacturer's convention, a needle is measured every 0.125" down from the top shoulder, resulting in 13 measurements.
For the carbs used on the JH, two different needle profiles, code-named B1CM and B1DK, have been used. It's been accepted wisdom that the B1CM needle provides a richer fuel / air mixture than the latter. As will become apparent below, this is not quite correct.
The table below shows each needle's diameter in mils (0.001") for each of the 13 steps. The third line of the table shows the result of a moderately complex calculation. For each step, I calculated the size of the annulus for the B1DK and for the B1CM, determined the ratio of the two areas, and then multiplied that ratio by 1000 for formatting purposes. This number is a reliable indicator of the relative amount of fuel delivered by each needle under the same conditions.
Step =01= =02= =03= =04= =05= =06= =07= =08= =09= =10= =11= =12= =13=
B1DK 97.4 93.6 92.0 89.6 86.4 82.8 79.6 76.6 74.3 72.7 72.1 71.4 71.4
B1CM 97.8 94.4 92.5 89.7 86.5 82.9 79.7 76.7 74.3 72.7 72.0 71.2 71.2
Ratio 1179 1138 1064 1009 1007 1005 1004 1004 1000 1000 997 994 994
So what does this all mean? Well, first of all, if the ratio figure is over 1000, the B1DK needle provides a richer mixture than the B1CM needle, and conversely. Second, by observation of the carburetor during operation, one can say that Steps 1, 2, and 3 correspond to its state under low-speed low-load conditions such as idle; Steps 4 through 7 correspond to part-throttle conditions such as cruising; and Steps 8 and above correspond to a high-power-output state.
From this, it's apparent that the B1DK needle actually provides a much richer fuel / air mix than the B1CM at idle, is slightly richer under cruise conditions, and is slightly leaner only under full-throttle conditions.
Admittedly, none of this will matter much to most JH owners, but I can see two areas where some might care. First, for cars that fail emissions tests because of excess hydrocarbons at idle, a change from a B1DK to B1CM needle for the test might offer significant benefits. Second, owners of racing cars with Strombergs and an exhaust gas sensor can use a spreadsheet configured for the appropriate calculations to quickly determine what alternate needles might be worth trying on the track.
In closing, I'll comment that this technique should be applicable not only to JHs but to any other car using Strombergs, and I see no reason why it would not also work on cars with SUs.