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What is defferent between BS34SS carbs

JTGS850GL

JTGS850GL

Forum Sage
Past Site Supporter
A thread on another topic has got me curious and concerned.

Besides jetting (Air, Main, Pilot and Needle), what is different between the BS34SS carbs on an 8V GS1000/GS1100 and a 16V GS1000/GS1100 engine? I was told by another forum member that there are differences and can accept that to be true, but I can not get any definitive answer as to what the differences are and what, if any, jetting changes would be needed to make any one type adapt to which motor assuming a stock configuration. Pilot circuit changes? Main circuit changes? Needle circuit changes?

The only identifier I was told was that the 16V engines used carbs that had 4 holes on the air filter side while the 8V carbs had 2 holes. Both carb types have two holes. One for the pilot air and one for the main air jet but the other two holes look like they're blocked off. On other Mikuni carbs I've seen these two blank holes look like cast bosses for mounting studs that are not drilled and tapped out in our application.

The reason for this question is that I have several sets of BS34SS carbs of both varieties and always assumed that, other then jets, they were interchangeable. Now I've been told that they are not, so I would like to know what the differences are in order to better understand what changes would have to be made to adjust them to the application needed.

The carbs that came on my 1980 GS1000 were 4 hole carbs and the ones I just picked up from a bone stock GS1100G had 4 holes as well but the ones that came on my Frankenstein 1000GL had 2 hole carbs. I have two other sets that have 2 holes as well as another getting ready for rebuild that has 4 holes. History on those is not know, so I really could use some clarification on this.

Any help would be greatly appreciated.
 
Ok, found the answer to my question. With the help from Chef and then going through several factory service manuals, I found what should be the definitive answer.

According to Bill the only difference he was aware of between an 8V and 16V carb is the Main Air jet size. I verified this by going through all the FSMs and indeed the 8V used a 1.7mm Main Air jet while the 16V version of the carb used a 1.2mm jet. Since the jet is not removable it distinguishes one from the other. You can drill the 1.2mm air jet out to the 1.7mm specification thus allowing the use of a 16V carb on an 8V engine. Now I'm sure that you could vary the other jets to compensate for the air jet that's in there. This is more of an academic exercise then a tuning discussion.

I created a spread sheet that broke down each carb by ID# and subsequent specifications for all GS1000 and 1100 derivatives that utilized the BS34SS carbs. After that I inspected each of my carb sets and found something even more amazing. There is NO correlation between the number of holes on the air filter side of the carb and the application it was intended for.

As an example I had a carb set that had the number 49400 imprinted on it's body. That ID# crossed referenced to a GS1000G 8V engine. Another set I had was ID# 49220 which crossed to a GS1100E 16V engine. I measured the ID of the Main Air jets and found them to match the specifications in the FSM. The part that was most amazing to me was that BOTH sets of carbs (8V and 16V) had 4 openings on the air filter side. One opening for the Pilot Air jet, one for the Main Air jet and then two cast bosses that were dead ended. The other identifier that I found reliable (so far) was that ALL 8V carbs had a notch cut out of the bottom bar that ganged the carbs together to clear the cam chain tensioner while the 16V versions did not.

My conclusion is, that the only definitive method for determining what the "original" application for a carb was, is to use the Mikuni carb ID# and determine which year/model used that version. I have the spread sheet and will expand on it to include all GS models from the GS400 to the GS1150 that I can obtain FSMs for. I'll try to find a way to post that sheet once completed so that everyone will have a good reference to use when looking up carb specs.

Much thanks to Bill (Chef1366) for helping me with this. Truly a good guy who knows his carbs.
 
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