FCR 35 tuning

I have been mucking about trying to learn/tune my FCR 35s which came with my bike from cNw nearly five years ago. They were way to rich then.
I had just posted this info on a PM to three other FCR 35 users but thought others may be interested.

I have just made my latest adjustment - good results - turned the slow air screw out from 1.5 to 1.75 turns out. 65miles today - mixed riding - all good.
The last adjustment I had to reverse - had taken the slow fuel screw out to 2.0 turns out - now back to 1.75 turns out.
So... My current set-up is:

Main Fuel Jet - 145 (thanks @KiwiShane !) - down from Matt's 152
Main Air Jet - 200 (as fitted by Matt)
Slow Fuel Jet - don't know (as fitted by Matt) - current screw adjustment doesn't indicate change is needed
Slow Fuel Screw - 1.75 turns out - was 1.0 turn out from Matt
Slow Air Screw - 1.75 turns out (will try 2.0 sometime soon) was 0.75 turns out from Matt. Very happy that Matt fitted screws rather than jets!!
Needle - EMR (as fitted by Matt) (can't remember clip position - will let you know when I next look)
Pumper disabled - now have made a manual handle (see below) so I can give a starting squirt in winter. I think it needed it last winter!

Bike runs very well with crisp acceleration in all gears/revs - just the rare pop/fart instead of rev blip on downchange.
Plugs (Denso VW22) are pretty good after 1000 miles.

I'm still learning how these things work - so different to Mikunis & Amals
Cheers

BTW - my bike is 850, Fullauto head, standard compression, light pistons/long rods, 22 tooth front.
Cheers

Interesting. Mine, after much experimentation:
140 main jets (came with 152s)

Needle clip in #2 slot (from top; there are seven slots; the needle clips came in #5 slot I believe)

Stock pilot jets

Slow air screw out two turns (were originally out one turn)

Slow fuel screw out 5/6 turn (Right side was originally about 2/3 turn out. Left was originally 1.5 turns out)




Curious why you disabled the pumper?

Rob, forgive my ignorance mate but I can’t quite grasp how deleting the pumper makes the carbs better. I understanding about being over rich starting it on a hot day but surely the pump ain’t just there for starting. If you find no performance benefit for having it could that not point to there being another issue?

My understanding is that the AP is for acceleration, heck it’s a Japanese racing carb so why would they care about start ups? Webbers of the 1960s like those fitted to Ferrari V12s had APs and I gather the car’s performance was crippled if the AP was not operating correctly.

@8.52

Also of confusion is the colour of those plugs. I know colour is less of a thing with modern fuels but I assume you’re running E0 anyway. My (out of date?) recollection was that the ceramic should be a light tan and that a plug running pure white is an indication of a lean mix which has a tell of popping on the the overrun. Another tell is lean equals hot IIRC? FWIW my plugs are a nice mid tan and directly after a run I can hold my hand on the timing case for as long as I like unlike my modern lean burning FI Kawasaki which scolds to the touch.

Anyway, like I said, what do I know but I am curious nonetheless.

BrianK said:

Curious why you disabled the pumper?

Click to expand...

I was uncomfortable with the volume of fuel that was squirted into the engine (have you seen it? Pull your carbs off, point them at something (not your eyes!) and whack the throttle open) and felt it unnecessary on engines such as ours. I initially pulled the lower actuating link off and experimented with throttle openings (fast and slow) at various revs and in various gears. Found no bad effects at all.

Do you really think an un-atomised stream of petrol is a good strategy. There's a reason carb makers go to so much effort with bore venturis, etc to get a good mixture ready for burning.

Stops wankers washing your bores by twisting the throttle on your parked bike too!

As for @Café au Lait comment that not having a pumper operating pointing to an underlying issue with my bike - I think quite the opposite. Having a dirty great petrol pistol squirting whenever you open the throttle is more likely to hide poor tuning.

I think the facts speak for themselves on my current set up:
Nice clean plugs
Good, smooth throttle response - 2% pop/fart instead of rev blip on down changes (yes - I've counted!)
Will pull to 7500rpm in top with 22 tooth front

Why would I want to change that?

Brian - what is bike set-up?

Hi Rob. Mine is pretty much stock, engine wise. '73 850, TriSpark ignition with 4-ohm dual coil. Have big K&N pod filters on the FCRs and Jim Schmidt's port sleeves on the intake ports (not his D-shaped ones; those and his exhaust counterparts are next....!)

My plugs look good - not as clean as yours to be sure but nice color.

I've gotten up to 60 mpg during sustained 40-50 MPH steady cruising so doesn't seem to be wasting a lot of fuel. No lack of power; more than I need (I didn't say "want").

I love the FCRs. Always looking for improvement however....

BrianK said:

Hi Rob. Mine is pretty much stock, engine wise. '73 850, TriSpark ignition with 4-ohm dual coil. Have big K&N pod filters on the FCRs and Jim Schmidt's port sleeves on the intake ports (not his D-shaped ones; those and his exhaust counterparts are next....!)

My plugs look good - not as clean as yours to be sure but nice color.

I've gotten up to 60 mpg during sustained 40-50 MPH steady cruising so doesn't seem to be wasting a lot of fuel. No lack of power; more than I need (I didn't say "want").

I love the FCRs. Always looking for improvement however....

Click to expand...

You're aware of potentially disastrous failures of the vacuum release plates?

I suggest the pump does the same thing as a quick taper needle in other carbs. It richens the mixture to compensate for loss of vacuum if you whack the throttle open. If you feed the throttle on in a controlled manner, you do not need a richer condition and you usually get better performance anyway. When you lower the needle one notch in any well-tuned motor, the motor should cough when you ride the bike - that is slightly too lean. If the mixture is correct and the ignition system is OK, your motor should never miss. If the mixture is too rich, the motor will be slower, but safe. Plug readings are usually done to check main jets for high speed running. Around town with a road bike, you probably do not use the main jets for metering fuel. A four-stroke motor should never detonate.
Anyone who road races, probably never whacks the throttle open. It is probably a dirt-bike thing.

BrianK said:

Hi Rob. Mine is pretty much stock, engine wise. '73 850, TriSpark ignition with 4-ohm dual coil. Have big K&N pod filters on the FCRs and Jim Schmidt's port sleeves on the intake ports (not his D-shaped ones; those and his exhaust counterparts are next....!)

My plugs look good - not as clean as yours to be sure but nice color.

I've gotten up to 60 mpg during sustained 40-50 MPH steady cruising so doesn't seem to be wasting a lot of fuel. No lack of power; more than I need (I didn't say "want").

I love the FCRs. Always looking for improvement however....

Click to expand...

I guess there are too many variables to compare mpg figures. These bikes, at least the Norton parts are hardly at modern tolerances. Differing jetting, fuel, riding style, even altitude. FWIW once warmed thoroughly I ride mine in a spirited fashion shall we say and get 53-59mpg. When I rode it at a constant 56mph and topped up the tank it was 67mpg. These figures with the AP of course.

@robbss yes I’ve had the carbs out when I had that intermittent fast idle issue, (thankfully long gone) and they’re like a spitting cobra! Obviously by design. I profess no knowledge on the subject but as long as the bike rides well, shows no obvious signs of either too lean or too rich a mixture and get these mpg figures I sure ain’t gonna mess. I’m glad you’ve made the improvements that suit your bike though that is always the end game. As for the ‘Throttle Wankers’, I use a combination cable tie either around the throttle and brake lever or through my lid when placed there; it’s neatly stored in the left cover with my disc lock wedged in a foam holder I knocked up.

robs ss said:

You're aware of potentially disastrous failures of the vacuum release plates?

Click to expand...

No! I've never heard of this. Please fill me in. Tx - B

BrianK said:

No! I've never heard of this. Please fill me in. Tx - B

Click to expand...

About 4 months ago the Australian Keihin distributor told me of a case where the FCR vacuum release plate seal (#22 on the diagram) failed on a customer's bike culminating in the slide being sucked into the engine with catastrophic results. I did a bit of "forum hunting" and it appears that on some of the big singles erosion of the vacuum seal leads to chattering of the vacuum plate, cracking it so that pieces get sucked into the engine, holding valves open with expensive noises ensuing.
Also happened on the "gixxer" forum - Suzuki GSX I presume.
So, after buying a new set of plates and seals https://www.ebay.com.au/itm/256405830563, I pulled mine apart and found some minor damage to the bottoms of both plates - in the centre, where they come down on the complex shapes surrounding where the needle projects - photo below.
No much and I can see no marking on the carb bodies. I can feel a bit of curling with my thumbnail on the plates.
So I fitted seals to the new plates and will keep the old set as emergency spares. I will also be careful not to "snap" close the throttles as I think this must be the cause
Anyway, something to look out for during routine maintenance.

Every engine is different as are riding styles.

I run 35mm FCRs with the accel pump connected and none of the lean tuning mentioned in this thread. Without the accel pump I'd have a heck of a time starting the engine when cold. My Norton 750 engine sits straight up so less downdraft angle into the intake tracts than a Commando. I get 63 MPG keeping up with traffic on the HWY 70-80mph and passing cars in the mountains. I use more throttle in the mountains.

My plugs will run white if I do a couple miles at 80mph. Otherwise riding around town and in stop and go traffic they are on the brown side.

I have squirted the accel pump with the FCR carburetors off the bike. A couple of squirts is no worse that flooding the bowls with the ticklers on Amals. You can dial in the squirt timing and length a little. It is a time-consuming process though.

BTW I did try 142 mains and the lean tuning suggestions when I first installed the FCRs, and unfortunately the results were too lean.

It is always what works which counts. The only carburetor setting I ever take note of is needle jet size. With a four stroke, it is critical for performance. If it is wrong in a two-stroke carburetor, the motor might not even start. Everything else is usually adjustment. In a normal carburetor, the taper on the needle compensates for loss of vacuum at low revs, when the bike is accelerating. There is probably a formalised way to set the pump in a pumper carb. I would set it low and increase the setting until the motor performed well when the bike is accelerating. Even when using petrol, you should not get detonation.

robs ss said:

I was uncomfortable with the volume of fuel that was squirted into the engine (have you seen it? Pull your carbs off, point them at something (not your eyes!) and whack the throttle open) and felt it unnecessary on engines such as ours. I initially pulled the lower actuating link off and experimented with throttle openings (fast and slow) at various revs and in various gears. Found no bad effects at all.

Do you really think an un-atomised stream of petrol is a good strategy. There's a reason carb makers go to so much effort with bore venturis, etc to get a good mixture ready for burning.

Stops wankers washing your bores by twisting the throttle on your parked bike too!

As for @Café au Lait comment that not having a pumper operating pointing to an underlying issue with my bike - I think quite the opposite. Having a dirty great petrol pistol squirting whenever you open the throttle is more likely to hide poor tuning.

I think the facts speak for themselves on my current set up:
Nice clean plugs
Good, smooth throttle response - 2% pop/fart instead of rev blip on down changes (yes - I've counted!)
Will pull to 7500rpm in top with 22 tooth front

Why would I want to change that?

Brian - what is bike set-up?

Click to expand...

Not to mention a big difference in fuel consumtion.

I was surprised when fitting slower taper needles into my MK2 Amals made an improvement to throttle response. It was not what I expected. Too much or too little fuel, both have adverse effects. I rode motorcycles on public roads for about 10 years, and never even tried raising or lowering the needles in the carbs, let alone using a different taper needle. I did not ride Japanese motorcycles. I suggest that is where much of their speed comes from. My mate has an RZ350 Yamaha. He found a pair of new, old stock needle jets on the internet - just lucky. They are too complex to re-manufacture. A lot of guys do not recognise that when we get an old motorcycle, the first thing to replace is the needle jets.

FCR carburetors on my Norton 750 get me milage in the 60's. I think I got 67 mpg out of them on one trip. Typical is low 60's. In town they consume a bunch of fuel though. Too many on/off/on throttle events that use the accel pump in town.

FCR accelerator pump timing and duration can be adjusted to some degree. It's not the best setup for adjustment though. Fortunately, it is very close as delivered with a straight up engine. The slot shown below is the accelerator pump adjuster slot and can be widened to delay the pump shot or closed up to start the pump shot sooner. It's really easy to screw things up, but not difficult to get it back to as delivered if the width of the gap is checked before getting started. Keihin documentation says, "In normal applications, the accelerator pump should not require any adjustment"



I get why people lean the FCRs out and disable the accel pump. If I start my bike and run the engine in the garage blipping the throttle the plugs end up very dark from the added accelerator pump fuel. It has to be ridden to clean the plugs up.

I think I am going to try the 145 mains again. I've had no complaints with my 140s, but maybe I can get the motor to run a little cooler at speed. Besides, good excuse to fettle....