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Zenith Stromberg Carburetor Fuel Mixture Needles  Rating:  Rating
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 Posted: 03-19-2005 02:28 am
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Mark Rosenbaum
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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

Annulus

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.

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 Posted: 03-20-2005 08:56 pm
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j23mau
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Mark, Is there any VIN specifics that would tell us when each needle were used?  Much like the rope seal, just curious if it would make a difference if you where to s/o for different carbs.  My dad has about 4 different sets of carbs sitting around. 

Would it also make a difference if one carb was a B1DK or B1CM?

Jeremy

Last edited on 03-20-2005 09:00 pm by j23mau

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 Posted: 03-21-2005 01:35 am
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Mark Rosenbaum
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According to the shop manual, the B1CM needles were used until engine 4802.  I should have mentioned that but to be frank I just didn't think about it.  Of course there's no way to know what a PO may have done, so any car could have needles of either type.

Both carbs need to have the same type of needle.  If not, I'd expect that it would be impossible to ever get the carbs properly balanced.  And, since one of the sets of carbs I bought via eBay last year, supposedly from a running JH, had two different needle types, this problem may be a lot more common than people would expect.

Fortunately, the mixture needle code number can easily be checked without disturbing the carburetor adjustments in any way.  Remove the carb dome, remove the long spring, remove the vacuum valve and diaphragm, pull gently down on the shaft of the mixture needle, and read the marking on the very top of the needle.  Assembly is the reverse of the above, and once done, it's a good idea to top off the damper oil.

Finally, do ensure that the tab on the rim of the diaphragm fits into the recess for it on the carb body, and that the alignment mark on the dome faces toward the carb inlet -- from what I've seen over the past few years, about 1 in 20 Strombergs is reassembled incorrectly.

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 Posted: 03-21-2005 02:46 am
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Ron Earp
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Great info, I appreciate it being posted.

Strombergs are basically very large motorcycle carbs, at least, like most CV motorcyle carbs you find.  As such you can do a lot with them as Mark has indicated.  Other tuning methods are more "crude" but can yield excellent results.  You can shim the needles up a but using very small spring steel washers under the needle seat and between the piston which will raise the needle a bit, and enrichen the mixture.  Go in small steps such as 0.005" to 0.020"  at a time, a little goes a long way.  You can also taper the needles yourself using abrasive methods, but it is quite imprecise.  These methods are best used on race cars where you don't care about transistions in throttle position too much since you'll get poor uniformity.

In the end a pair well prepped Stromberg is plently large for the 2.0L JH motor and will provide excellent performance.  I think, from what I can tell from comments about them, is that they are misunderstood and folks just give up on them without ever understanding how to get them to perform.

R

Last edited on 03-21-2005 02:48 am by Ron Earp

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