Keihin CR carbs 31s or 33s?

[wyly]

I'm modifying my 84 GS1150ef, a vintage buddy has a set of unused CR31s still in their original box and offered to sell them to me for $800cdn(Sudco's price was $1140usd and I think that was dealer price).

My mechanic says 33s would be optimal but 31s would be an improvement over oem as well.
Has anyone here experience with 31s?
How much improvement will the 31s offer vs oem?
How much performance would I be giving up not going with the 33s?

I'm going with Hindle exhaust 4 into 2 into 1.

 

[Steve]

Others here know a LOT more than I do about different carbs, but they are going to ask you if there are any changes inside the engine, like larger pistons, cams, things like that. You mentioned a different exhaust, what about intake? Stock airbox or pods? If pods, which ones?

.

 

[blowerbike]

mikuni RS36mm are perfect for a stock 1150 with a pipe.
easy to tune and maybe less money.
i have did many like this...turns an 1150 into a near monster(seat of the pants) feeling anyways.

 

[blowerbike]

don't forget to add an HD backing plate to your basket or bad things will happen fast.

 

[wyly]

Others here know a LOT more than I do about different carbs, but they are going to ask you if there are any changes inside the engine, like larger pistons, cams, things like that. You mentioned a different exhaust, what about intake? Stock airbox or pods? If pods, which ones?

.

the head has been given a refresh so it's like new and stock, K&N pods probably.
This a long term project I plan to do modifications every winter and see how it evolves over a few years.
this winter coming up-Carbs/pods/exhaust/brake lines/ remove anti dive from forks maybe change from the oem fairing to a smaller 1/4 fairing. I already have the fairing and a complete headlight assemble from a 1150e.

A friend has the complete front end from a V Strom if it's suitable and I can make it fit.

 

[wyly]

mikuni RS36mm are perfect for a stock 1150 with a pipe.
easy to tune and maybe less money.
i have did many like this...turns an 1150 into a near monster(seat of the pants) feeling anyways.

I'm considering mikuni as option too I just haven't been able to find much info on them as yet. More power is ideal but I'm striving to keep the budget as low as possible.

The Keihin 31's are relatively inexpensive $600usd (about half the price that Sudco sells them) and available whenever I want them.

I have no idea what a "HD backing plate" is. I do have an excellent mechanic(world super bike and motogp tuner) who is guiding me but I'm also looking for guidance and experience of the GS forum familiar with everything GS

 

[Big T]

The HD backing plate is for the clutch hub

 

[GregT]

The 31's will be an improvement over stock - mainly in the midrange. The problem is that they aren't really street carbs - and are not as sophisticated as the RS's as far as idle and off idle adjustments are concerned. If you do use them, you'll finish up changing later IMO.

 

[steve murdoch]

the head has been given a refresh so it's like new and stock, K&N pods probably.
This a long term project I plan to do modifications every winter and see how it evolves over a few years.
this winter coming up-Carbs/pods/exhaust/brake lines/ remove anti dive from forks maybe change from the oem fairing to a smaller 1/4 fairing. I already have the fairing and a complete headlight assemble from a 1150e.

A friend has the complete front end from a V Strom if it's suitable and I can make it fit.

Gee whiz Robert....it is only mid August and your talking about winter already? haha.

 

[wyly]

The 31's will be an improvement over stock - mainly in the midrange. The problem is that they aren't really street carbs - and are not as sophisticated as the RS's as far as idle and off idle adjustments are concerned. If you do use them, you'll finish up changing later IMO.

Good to know that's why I'm asking for input, experience I don't have. Thanks

 

[wyly]

Hi Steve, there's only a month of riding weather left Steve and a lot of that will be cool and wet. It's not as tropical here as where you are

 

[wyly]

The HD backing plate is for the clutch hub

Do I really need that? I'm only going to have a power increase of 10-15hp and maybe 8 ftlb of torque.

 

[Nessism]

The HD backing plate is a nice addition even for a stock bike. For one thing it reduces noise from the clutch due to the springs rattling around, and for another it makes the clutch action smoother during engagement. I had a HD clutch in one of my GS1000's and it made a big difference.

 

[wyly]

mikuni RS36mm are perfect for a stock 1150 with a pipe.
easy to tune and maybe less money.
i have did many like this...turns an 1150 into a near monster(seat of the pants) feeling anyways.

what is day to riding like with a 36mm? It sounds a bit large for street use? I spoke with a buddy who had 33's and said they worked well.

Where is added power delivery with a 36mm? all through the rpm range, bottom end, middle, top end? My real world riding is mostly city riding with very occasional highway blasts.

 

[blowerbike]

trust me i have done many....day to day is fine/fun and fast...less MPG is the only downside.
if you want to blow your basket a part and through the cases just add a set of hot rod carbs and a pipe.
my brother blew a basket in a brand new 1150E with pipes and a jet kit.
FYI
tune and test night he was running low 11's....11 teens and 0's.
i removed my RS series from my 1150 and put on his bike....next pass was a 10.70 flat.
the RS36mm will be great on a stock 1150 daily rider and this size can run 8 second 1/4 mile passes on a drag bike....been there,done that.

 

[blowerbike]

depending on gearing...if staying stock gearing the power is in the mid and top end with these carbs.
these carbs let a person grow if you make changes in the future....
as with any mechanical carb....if you grab a handful at low revs you will get a slight bog...above 4 grand or so you will be wondering if the bike is going to flip over when you grab a handful.

 

[wyly]

depending on gearing...if staying stock gearing the power is in the mid and top end with these carbs.
these carbs let a person grow if you make changes in the future....
as with any mechanical carb....if you grab a handful at low revs you will get a slight bog...above 4 grand or so you will be wondering if the bike is going to flip over when you grab a handful.

you have valuable experience I don't have. a lot of good info and a lot to think about.

A friend thinks the 36s are too big he had 33s smooth-bores in his GS, my mechanic thinks they're big but should work fine. I worry about going large but I'm leaning toward 36. You, the supplier I found and my mechanic say 36 is fine.
The supplier says he can set me up with 36(or 34s if I'm concern with the size) jetted for the GS with pods or adapters for stock air box so that it's ready to go. He claims 25% more power which I find hard to believe, 10% is believable.

Less mpg is something I'm familiar with, my CBX, easy highway cruising 40mpg, misbehaving 20mpg. Regardless of the rpm on the cbx from idle up when I grab a handful it never bogs, instant response. Awesome carb technology on that machine it revs like a sewing machine.

 

[blowerbike]

i tried 34RS on an 1150 and wasn't as happy as i was with the 36mmRS carbs.
these carbs are easy to tune(at least at my altitude..1000' or less/more depending on conditions).
do not try and use an air box...use either APE or K&N offset ovals.
what is 3 tenths in the 1/4 mile equal in horsepower gain?
i wish some of the other GS 1100/1150 tuners would post so i could see there opinions.

 

[GregT]

you have valuable experience I don't have. a lot of good info and a lot to think about.

A friend thinks the 36s are too big he had 33s smooth-bores in his GS, my mechanic thinks they're big but should work fine. I worry about going large but I'm leaning toward 36. You, the supplier I found and my mechanic say 36 is fine.
The supplier says he can set me up with 36(or 34s if I'm concern with the size) jetted for the GS with pods or adapters for stock air box so that it's ready to go. He claims 25% more power which I find hard to believe, 10% is believable

RS36's are a generation better than your friend's 33 smoothbores. Much more tunable over a wider range. No comparison.

 

[wyly]

yeah no doubt there's been considerable improvement over 30 years