Keihin small body FCR 33 carbs

[jseng1]

I've been working on a small body FCR33 twin carb. As far as I know this is the 1st and only time this 33mm FCR size carb has been installed on a Norton. Height is 6" and length w/o adapters is 2" so there is plenty of frame clearance. Seems to have very good throttle response in the mid range with immediate kick in. I'm experimenting with limiting the accellerator pump so it only squirts the first instant when you need it instead of wasting fuel. I've only had it running for a few days and I just have one set of carbs for now but so far everything is looking good.

Setup on my test bike.

 

[acotrel]

I suggest the major advantage of a fuel injection system is its level of control of fuel delivery at quarter to three quarter's throttle. How is the amount of fuel the pump delivers controlled ? With my motor I use methanol fuel, and the jets deliver twice the normal amount as for petrol. So errors in jetting are about one-half as critical. The difference between slow and fast taper needles is still quite substantial. It is like riding a two stroke when the needles are slow taper - the throttle needs to be fed on. However less throttle with lean mixture gives more power than more throttle with rich mixture. The motor spins-up much faster with lean mixture.

 

[milfordite]

I would guess that for a street bike in factory tune, or a little hotter, the 33's would be a great choice with maybe better response than the 35's.

 

[jseng1]

I would guess that for a street bike in factory tune, or a little hotter, the 33's would be a great choice with maybe better response than the 35's.

Which is exactly why I went to all the trouble. And because the 33s are physically smaller and less bulky than the larger body 35s/37s. No Bog out when dumping on full throttle. Perfect idle. Linear and and smooth when you roll it on. Screams bloody murder at full blast.

Fortunately they ran great out of the box. I refined it with richer main jets (my head is ported with the raised intake an exhaust floors). I leaned out the idle jet a tad but I noticed it still missed a couple beats when shifting on the instant you opened the throttle. The accellerator pumps shoot out fuel like a squirt gun and were loading it up for a second until it cleared out. It's OK and most people would put up with it but I wanted it perfect. So I disconnected the accellerator pumps and the problem went away. It still jumped without hesitation when dumping on a handful of throttle - it fact I couldn't find a hesitation anywhere. The flat slides are ultra resporsive - so what the hell good are the accellerator pumps for other than wasting gas?

I've studied it and I may adopt the option of removing the accellerator linkage and replacing it with a simple button similar to the Amal tickler - turning the accellerator pump into a primer choke. This way you can push the button once or twice to squirt gas into the manifolds for cold starts. It needs more testing and street riding but I think it will work.

 

[Schwany]

I think that is 1/4" or a little more clearance to the tank and frame than one gets with the 35's. The additional clearance would be great for mounting to obtain more downdraft than one gets with the 35's.

As a person that has been using the FCR 35's for almost 5 years I think, I would miss the accelerator pump. The accelerator pump works exceptionally well on my engine and combination of supporting parts as long as I am in the right gear and not in 3rd doing 20mph going uphill when I turn the wick full on.

Setting the accel pump up as a tickler is a good idea. My engine would be very difficult to start without the accelerator pump. I might have to spend time retuning it with the accelerator pump disabled. I really don't want to waste the time finding out myself.

I have gotten 67mpg out of the 35's with the accelerator pump. Typical is more like 64 mpg. Easy to do on the freeway with a little bit of city riding thrown in. Doesn't seem like I'm wasting fuel with the accelerator pump working.

All that said and with perfect hindsight knowing what I know today... if 33's would have been in a Sudco kit for the Norton 750/850 when I bought the 35's I would have gotten the 33's. Much better for short shifting I would imagine.

 

[milfordite]

When I ran 36mm Dellorto's on my race bike, I disabled the accel. pumps per the rules, but found it ran just fine w/o them. Perhaps have an option to be able to disable the pumps if you decide to market them.

 

[Steve W.]

Wow! Those carbs look really nice. Very Interesting. Cheers

 

[jseng1]

Here's what I came up with for the "primer choke" - see photo. Seems to work great. Installs easily - just press into place. Push it once or twice in the morning and you're good. Its removable and you can easily go back to the original accellerator pump just by removing/reinstalling one allen bolt for the original linkage parts. I'm choosing to leave the new primer choke on my own bike because I find no use for the accellerator pump. With these carbs and w/o the accellerator pump you can give it instant full throttle from 2500RPM on up w/o hesitation. With the accellerator pump installed you get a momentary miss (flooding) when shifting and that's the reason I removed it. But its the riders choice. BTW, these smaller 33mm carbs don't bog out when giving it instant full throttle from 2500RPM on up (w/o accellerator pump.

 

[Schwany]

I was looking around a couple of days ago and found a size chart for Keihin FCRs and it indicates the 33mm and 35mm bodies are the same height. So for Commando owners the fit under the tank and in the frame issues would be the same. Benefit is the bore size. The website that had the size chart seems to have gone belly up today. I think I found it on keihin-fcr (dot) com.

 

[jseng1]

I just talked to Keihin-fcr. They compared the two and the FCR 33mm is 17mm/6.69" shorter than the FCR 35

 

[Schwany]

I just talked to Keihin-fcr. They compared the two and the FCR 33mm is 17mm/6.69" shorter than the FCR 35

That's a lot. They do have a different top cover.

Size chart needs to be updated. I got on his site and couldn't find that darn chart. I should have copied it, but forgot I'd have to prove it. I swear I didn't make it up.

 

[jseng1]

That's just the mid body measurement difference. If the 35 top cover and the bowl thicker then the measurement is greater. Won't know exactly until someone measures a complete 35.

The 33 is 6-1/8" from bottom to top cap.

 

[Schwany]

Definitely much smaller. I must have been sober when I looked at that chart.

I eyeballed the measurement with the 35's installed on my bike and sent you my crude rough numbers in a PM. They really need to be on the bench to get an accurate number. My desire for obtaining higher internet cred is less than zero though, so I'm leaving them on the bike. Maybe one of the members that have a 35mm FCR set sitting around on a bench could help out.

I wonder if that 33 is a new design or if the chart had a copy and paste error in it.

 

[jseng1]

I just learned that FCR carburetors have two different “body” sizes. Small from 28mm – 33mm and large 35mm – 41mm.

 

[Matchless]

I just learned that FCR carburetors have two different “body” sizes. Small from 28mm – 33mm and large 35mm – 41mm.

Same as CRs. Small body up to 33mm, large body 35mm +. Needles are also in two lengths to suit.

 

[Schwany]

Found the bad diagrams on Amazon that showed the 33mm and 35mm FCRs being the same height. I knew I didn't hallucinate it. Really bad source though. Somebody selling fake FCRs.

Fake FCRs

If you click on the second image in the image lineup on the left of the product page it shows a size diagram. If you do that for the 33mm and 35mm models, the height sizes are the same.

Hope I didn't throw the earth of its axis with the bad info.

 

[robs ss]

Here's what I came up with for the "primer choke" - see photo. Seems to work great. Installs easily - just press into place. Push it once or twice in the morning and you're good. Its removable and you can easily go back to the original accellerator pump just by removing/reinstalling one allen bolt for the original linkage parts. I'm choosing to leave the new primer choke on my own bike because I find no use for the accellerator pump. With these carbs and w/o the accellerator pump you can give it instant full throttle from 2500RPM on up w/o hesitation. With the accellerator pump installed you get a momentary miss (flooding) when shifting and that's the reason I removed it. But its the riders choice. BTW, these smaller 33mm carbs don't bog out when giving it instant full throttle from 2500RPM on up (w/o accellerator pump.

What silicone tube did you use there Jim?
I have a similar manual plunger on my FCR 35's. The Keihin bellows are hopeless (& expensive!) and are useless in no time!
Cheers
Rob

 

[jseng1]

My first silicone tube was 1/4" OD from Mcmaster Carr but its too stiff with1/16" wall thickness. So I'm switching to 1/32" wall if found on Ebay

Silicone Tube 1/8" ID x 3/16" OD 3.3ft Rubber Tubing Air Hose Water Pipe | eBay

Specifications:Size: 3mmx5mm(I.D.xO.D.)Length: 1m/3.3ftFeatures:The thickness of silicone tubing = ( OD - ID ) / 2.The tubing can withstand temperatures from - 60 °C to 200 °C(- 76°F to 392°F).Applications:Transferring liquid and gas, such as liquid handling, pump, drain, water&air hose for...

www.ebay.com

 

[robs ss]

My first silicone tube was 1/4" OD from Mcmaster Carr but its too stiff with1/16" wall thickness. So I'm switching to 1/32" wall if found on Ebay

Silicone Tube 1/8" ID x 3/16" OD 3.3ft Rubber Tubing Air Hose Water Pipe | eBay

Specifications:Size: 3mmx5mm(I.D.xO.D.)Length: 1m/3.3ftFeatures:The thickness of silicone tubing = ( OD - ID ) / 2.The tubing can withstand temperatures from - 60 °C to 200 °C(- 76°F to 392°F).Applications:Transferring liquid and gas, such as liquid handling, pump, drain, water&air hose for...

www.ebay.com

Thanks for the reply Jim.
Your plunger, with the brass knob. Is that something you made yourself or something readily available?

It's getting a bit cooler here now so I need to use mine, at start-up, more regularly.

My version is more complicated than yours - just a handle on the linkage. Keihin's garbage rubber boot is its weak link!

Cheers
Rob

 

[Café au Lait]

Seems to be an available option

Primer choke for eliminating 33 & 35mm FCR accellerator pump

33mm FCR carbs run better without the accellerator pump because the pump causes momentary flooding when shifting normally through the gears. You can replace it with a “primer choke” similar to the…

jsmotorsport.com

You’d probably be able to knock one up for yourself for pennies