Needles are in pairs ?

[ST1100Y]

And all four of them are the same part number in the above listing, which is what I wrote.

Smaller mains hence less lateral movement?

 

[Andrew Shadow]

Hi Martin:
I have become confused and no longer understand what you are trying to convey or if we are still discussing the same issue.
The discussion between us was whether or not the US/CM models have different carburetor needles for the LHS carburetors verses the RHS carburetors.

... ergo some unique shape for LHS and RHS...

To this previous post, I responded that on the US/CM models the parts list states that the needles for both LHS carburetors and both RHS carburetors all have the same part number. Because they are all the same P/N, all four carburetors use the same needle. If they are all the same needle, they do not have unique shapes on the LHS verses the RHS, on these models.

Smaller mains hence less lateral movement?

You very well may be correct about that. I have no idea about the mains and I did not look them up to compare. I can also assure you that you have more experience with that and you would certainly know better than I do. I have interpreted mains to mean main jets?
If so, I don't see the relevance of whether the mains are smaller or bigger however. The size of the mains does not change the fact that this parts list states that the needles on the LHS carburetors have the same P/N as the needles on the RHS carburetors, which is what I believed we were discussing.

 

[ST1100Y]

The discussion between us was whether or not the US/CM models have different carburetor needles for the LHS carburetors verses the RHS carburetors.

Got widely distracted as this issue is spread across several threads in here, plus I had 2 tabs with parts listings open as well...

To make it more confusing: AFAIK do US carbs have plastic pistons, whilst the EC spec ones are coated aluminum...
I'll talk with a mech these days, maybe he can add a few grains of thought into this...

 

[aSTerix]

Got widely distracted as this issue is spread across several threads in here, plus I had 2 tabs with parts listings open as well...

Two threads only !

This thread which is about "Why are the pairs of needles are different"

The other thread about "How to interpret a Honda fiche diagram" so that I can work out whether its Left-right pairs or Front-Left pairs

 

[ST1100Y]

OK, after debating and (over)thinking that at length, I conclude that this must be for temperature compensation (hence indeed front vs rear pair of carbs!)
As the rear carbs get warmer, the mixture there enriches while the forward ones remain unaffected, which would result in uneven output of the cylinders... not good...
Without actually seeing the needles I assume the forward ones being shorter or more progressive to even the mix for all 4 cylinders as good as possible.

 

[aSTerix]

Without actually seeing the needles I assume the forward ones being shorter or more progressive to even the mix for all 4 cylinders as good as possible.

I will try and find my micrometer and when I do I will take some measurements and also photograph the needles side by side

Unfortunately at the moment I have no idea which needle goes where !

 

[ST1100Y]

I will try and find my micrometer and when I do I will take some measurements and also photograph the needles side by side

Just check with the parts catalogue/fiche if the J1BP or the J1BQ go in front/rear... or left/right

Because then again I checked with the NRP site, only to find that they offer kits for the right and for the left carburetor, making my thesis nil and void...

Search - ST1100

nrp-carbs.co.uk

 

[aSTerix]

Just check with the parts catalogue/fiche if the J1BP or the J1BQ go in front/rear... or left/right

and thats were it all started ! it isn't clear where they go

 

[aSTerix]

Because then again I checked with the NRP site, only to find that they offer kits for the right and for the left carburetor, making my thesis nil and void...

Search - ST1100

nrp-carbs.co.uk

overhaul kits for left vs right makes sense because of the way the float chambers are handed , it doesn't necessarily void your thoughts on front vs rear

I can think of many reasons why there might be a difference in air-fuel ratio front to rear but as of yet I can't think of even one for a left-right difference

 

[ST1100Y]

overhaul kits for left vs right makes sense because of the way the float chambers are handed , it doesn't necessarily void your thoughts on front vs rear

I recently did the carbs of my '94 with OEM spares:
- 4 identical idle jets
- 4 identical main jets
- 4 identical valve seats
- 4 identical valve needles
- 4 identical bowl gaskets...

I did not dig deeper into them, as I only had to fix clogged jets after a longer downtime, leading to crappy tickover/idle...

 

[aSTerix]

Its going to be a long time before I get the carb block back together, by then I am hoping I will have received a definitive answer from Honda Japan

 

[aSTerix]

I have found out the term for this is "staggered jetting" , apparently it is commony used on V fours to make the rear cylinders run cooler (richer mix)

It is also used to correct for differing intake lengths or differing cooling on the exhaust headers.

I am not aware of any difference on St1100 air intakes, if we consider the cooling and exhaust headers then the difference if any would be front to rear

 

[aSTerix]

From my research, it is ONLY the US & CM models that have same needles for all 4 carbs, every other bike (at least ST1100AY) has two pairs of needles that have different tapers between the pairs

I still maintain that the reason for the difference is "Staggered Jetting" and it is to make the rear cyilinders run slightly cooler at mid throttle. To prove this I now need to work out why the US/CM emission control system negates the need for the Staggered Jetting

For the life of me (and I have thought very hard about this) I cannot see any justification for Staggered Jetting Left to Right.

I'm getting close to putting money on this !!

[I also maintain that it is possible to interpret the parts diagram either way ie front vs rear or left vs right ]

On the plus side, I've gone from knowing very little about carburettor theory to knowing a bit more !!! At least I got to the point I could understand the charts in the Honda Common Service Manual when it talks about what influences fuel mix at different throttle settings

 

[Andrew Shadow]

[I also maintain that it is possible to interpret the parts diagram either way ie front vs rear or left vs right ]

Below is why I see this diagram as distinguishing left from right as opposed to FWD from Aft.

Honda defined the orientation of the motorcycle in this diagram via the arrow and the term FR.
I added the red line to extend this orientation through the illustration. It delineates left from right using the centre-line of the engine.
The red numbers 1 & 3 are the carburetors of cylinders 1 & 3 on the right-hand side of the engine.
The red numbers 2 & 4 are the carburetors of cylinders 2 & 4 on the left-hand side of the engine.

Following this logic;
Item # 20 is the needle of the carburetor for cylinder # 3 on the right-hand side of the engine.
Because it is not listed separately, and because the quantity is listed as 2, this indicates that carburetor # 1 also requires item # 20.

Item # 19 is the needle of the carburetor for cylinder # 2 on the left-hand side of the engine.
Because it is not listed separately, and because the quantity is listed as 2, this indicates that carburetor # 4 also requires item # 19.

 

[aSTerix]

Below is why I see this diagram as distinguishing left from right as opposed to FWD from Aft.

Honda defined the orientation of the motorcycle in this diagram via the arrow and the term FR.
I added the red line to extend this orientation through the illustration. It delineates left from right using the centre-line of the engine.
The red numbers 1 & 3 are the carburetors of cylinders 1 & 3 on the right-hand side of the engine.
The red numbers 2 & 4 are the carburetors of cylinders 2 & 4 on the left-hand side of the engine.

Following this logic;
Item # 20 is the needle of the carburetor for cylinder # 3 on the right-hand side of the engine.
Because it is not listed separately, and because the quantity is listed as 2, this indicates that carburetor # 1 also requires item # 20.

Item # 19 is the needle of the carburetor for cylinder # 2 on the left-hand side of the engine.
Because it is not listed separately, and because the quantity is listed as 2, this indicates that carburetor # 4 also requires item # 19.

Your assessment of the diagram is valid, however, if you flip the line so it separates front from rear it also is valid

Here is what I did, I asked myself how would I draw the diagram if it were a front rear split, my initial decision is to explode carb 1 and put needle 19 there, however, its just easier to rotate the CAD and change a label so thats what I would have done i.e. exactly how its shown !!

 

[Andrew Shadow]

Your assessment of the diagram is valid, however, if you flip the line so it separates front from rear it also is valid

If I underdstood you correctly, I am not sure how how that could be accomplished without also changing where the FR. indicator is and this is not an option that is available. The longitudinal centre-line of the engine can not be rotated without also rotating where the front of the engine is.

The location of the front of the engine I don't see as an issue for debate. Honda has defined it in this diagram by the FR. descriptor. They also defined which direction the front of the engine is facing by including the arrow in the bottom left of this diagram. This indicates not only where the front of the motorcycle is, but also in which direction it is facing.

By defining where the front of the engine and motorcycle are, and by defining its directional attitude, they have de-facto defined where the longitudinal centre-line of the engine is as well. This is because all of these items are in a fixed relationship to each other. None of them can be arbitrarily rotated without rotating the others the same amount in the same direction.

Anyway, that is my take on it.

 

[aSTerix]

If I underdstood you correctly, I am not sure how how that could be accomplished without also changing where the FR. indicator is and this is not an option that is available. The longitudinal centre-line of the engine can not be rotated without also rotating where the front of the engine is.

The location of the front of the engine I don't see as an issue for debate. Honda has defined it in this diagram by the FR. descriptor. They also defined which direction the front of the engine is facing by including the arrow in the bottom left of this diagram. This indicates not only where the front of the motorcycle is, but also in which direction it is facing.

By defining where the front of the engine and motorcycle are, and by defining its directional attitude, they have de-facto defined where the longitudinal centre-line of the engine is as well. This is because all of these items are in a fixed relationship to each other. None of them can be arbitrarily rotated without rotating the others the same amount in the same direction.

Anyway, that is my take on it.

Yes I see your point, it will be interesting to come back to this when I get an official answer from Honda (probably in a few months time)

 

[aSTerix]

On an aside but related, what possible reason is there for having different needle tapers (Jet Staggering) between left carbs versus right carbs ? Assuming its to affect mixture at mid throttle then what could be the difference between left and right side of the engine that would necessitate this

I am going to pull the top of my second bike now I think and have a look at the needles, this bike hasn't been dismantled since i got it from the police auction so will have been exactly as the dealer left it , might take a while because its now 40C in the basement where I work on the bikes so I tend to overheat and need to take frequent breaks !

 

[aSTerix]

Okay, here is the result:

Carbs 1&2 both have J1bP needle , no washer shim

Carbs 3&4 both have J1bQ needle, no washer shim

I have no idea what the needle decode is but generally it gets thinner (ie richer mix) as the letters go from A-Z .

I have no idea whether this arrangement is as manufactured but it is the arrangement that is on my untouched ex-police bike

At some point I will hopefully get an official answer from Honda and will update

 

[Andrew Shadow]

At some point I will hopefully get an official answer from Honda and will update

It will be curious to see what they respond. If they state that it is indeed a FWD/AFT split, as opposed to the LH/RH split that I have illustrated above, I hope that their answer also contains an explanation of how that marries with the existing directional indicators already contained in their diagram.

Unfortunately, your question will probably be addressed by a dealer support representative whose job it is to field questions from dealers. They are the equivalent of customer support representatives for the public but but are assigned to responding to questions from dealers. This person quite possibly has less proficiency at reading and understanding that diagram than you do. That has been my experience with Honda Canada the two times that I had a technical question at any rate. The person responding to the queries understood less than myself and, much to the frustration of the dealer who was asking these questions on my behalf, much less than him.
Unfortunately, the days of being able to speak to someone who is actually in a technical/engineering capacity in a large corporation are long gone.