Fuel flow limitations of the concentric carb

[worntorn]

I don't think it does run right out of fuel, fuel is still flowing in at a good rate, but slow enough to limit rpm.
For example, with my Vincent Rapide, with one fuel tap open it will sit right on 100 MPH or thereabouts. It runs smooth, but will not pull higher. Open the second tap and it pulls up to 115 MPH.

The Harley xr750  flatrackers ran into this fuel flow problem at the tap. The Pingel high flow fuel tap was developed to get around the problem.



I need to get the Carb changed and give it a try, however nothing but heavy rain in the forecast for the next few days.

Glen

[muskrat]

G'day Richard.
It usually gets to the point where the needle/seat can't deliver as much as the motor wants. The fuel level drops slowly (still being fed fuel) causing lean burn. Eventually the fuel level gets that low the needle/seat is acting as the "main jet" and the motor keeps running (somewhat). If your stupid enough to still be holding the throttle wide open you deserve the holed piston.
I ran two 932's on my A7SS on methanol, getting about 12 MPG. With 640 main jets she was still pulling at WOT at the end of Eastern Creek straight. Only problem was on a cool day they would freeze wide open . I'd have to hit the kill button for a few seconds for the slides to drop.
Cheers

[Rocket Racer]

bsa group triples must have both taps open to run properly

[BSA_54A10]

Confused about fuel flow vs. horsepower. Wouldn't sucking all available fuel out of the bowl just kill the engine or lead to so much coughing that you had to let off the throttle? When accelerating full out with an engine built for speed (say 50 HP, for example), wouldn't you be using the full 50 HP right up until you ran out of fuel, at which point you'd drop to zero HP?

Richard L.

No you are not draining the bowl dry.
What they are expousing is that the supply of fuel at WFO is not enough to keep the float bowl completely full
So you get to an equlibrium point where the bowl is mainained at say 3/4 full.
This means that the venturi has to work harder to pull the fuel up from 3/4 the way down as distinct from pulling the fuel from the top of the float bowl.At WFO the venturi is at it wealest unless you have velosity stacks or bell mouths fitted, in fact this the reason why bell mouths were fitted in the first place.
What people seem to be missing is if the engine runs better with both taps on, it is not a problem with the carb, but rather with restrictions in the fuel delivery system or perhaps the 1/4" fuel line is just too small to flow the volume of fuel required to do better than 100 mph by gravity alone.

With the older plastic needles apparently they bounced around inside the carb which restricted the fuel supply but the use of the brass needles stopped this

[nimrod650]

regarding float bowls and starvation problems i remember and people on here know far more than me that people used to replace the huge float bowl on the amal gp carbs on dbd 34 goldie manx norton ect with a small matchbox type float ?

[Rocket Racer]

With my methanol A10 I tested the fuel line flow rates to make sure I had no such problems.

On the triples online forum there is a lot of discussion around the inconsistency of float level setting in concentrics which particularly affects triple carbed bikes if they differ between carbs  There is plenty of online literature about setting the float levels correctly on concentrics.
I had to correct my 600 series float levels which had variations

[RichardL]

No you are not draining the bowl dry.
What they are expousing is that the supply of fuel at WFO is not enough to keep the float bowl completely full
So you get to an equlibrium point where the bowl is mainained at say 3/4 full.
This means that the venturi has to work harder to pull the fuel up from 3/4 the way down as distinct from pulling the fuel from the top of the float bowl.At WFO the venturi is at it wealest unless you have velosity stacks or bell mouths fitted, in fact this the reason why bell mouths were fitted in the first place.

What is "WFO"? Something "fully open"?

 I was about to say this makes sense, and maybe it does, but there is still something (among a million other things) I don't understand. The main jet isn't changing length, so, if the bowl is full or if it is 1/4 full, it would seem that the vacuum head needed to draw the fuel up through the main and needle jets would be about the same. The only difference, it seems, would be  due to the weight of 3/4 bowl of fuel making extra pressure at 3/4 depth, but that seems insignificant.   

Richard L.

[BSA_54A10]

Well you are right & you are wrong.
WFO = wide flat out
And you do need to apply more vaccuum to raise the fuel thet extra 1/2 ".

However when the throttle is completely open the the venturi affect on the emulsion tume ( the brass bit that sticks out into the carb ) is at its lowest so pulling the fuel up the extra fraction of an inch does make a difference.

[worntorn]

What people seem to be missing is if the engine runs better with both taps on, it is not a problem with the carb, but rather with restrictions in the fuel delivery system or perhaps the 1/4" fuel line is just too small to flow the volume of fuel required to do better than 100 mph by gravity alone.

The one tap two tap example was taken from another bike ( Vincent Rapide) to explain that slightly insufficient fuel flow doesn't cause a complete cutout, which was Richard's question. Perhaps that example muddied the waters.

The real question is from the first post- and that is, are standard single MK1 Concentric carbs only capable of delivering 30-35 HP due to their limited fuel flow design?
This was the result found by Jim Comstock , a very knowledgeable fellow who approaches things in a scientific manner.

Still raining here so I haven't had the Super Rocket out, but the Monobloc is on there and ready to go.

[worntorn]

I finally got around to testing with the monobloc fitted. With the Concentric fitted,  a run up the local test hill gave 65 MPH at the top after hitting the bottom of the hill at 60 MPH in third, then WFO up the hill, GPS speedo.
With the Monobloc fitted the speed at top was 67 MPH. So only a small improvement and even that may be due to replacing the round paper element of the Concentric to a K&N clamp-on with the Monobloc.

Glen

[Rocket Racer]

Glen,
 If you wanted to consider the performance of the a65 thunderbolt which had a single concentric, contemporary road tests of the day reflected these bikes having performance effectively identical to the twin carbed models at around 108mph and no doubt only a fraction lower due to compression and cam differences. (typically around 2 mph lower)
Whether these performances typically attributes to factory HP claims of 45/55 actually equate to 35 at the rear wheel I have no idea. But the factories did not fit these concentric carburetors in the sixties to compromise their performance.
I would only expect a twin carbed period pommie engine to outperform an equivelent single carb engine at revs most A10's never reach.

[jachenbach]

Ah, the wonders of Google. Been wondering what the heck a Pommie is. Finally looked it up.

[worntorn]

Glen,
 If you wanted to consider the performance of the a65 thunderbolt which had a single concentric, contemporary road tests of the day reflected these bikes having performance effectively identical to the twin carbed models at around 108mph and no doubt only a fraction lower due to compression and cam differences. (typically around 2 mph lower)
Whether these performances typically attributes to factory HP claims of 45/55 actually equate to 35 at the rear wheel I have no idea. But the factories did not fit these concentric carburetors in the sixties to compromise their performance.
I would only expect a twin carbed period pommie engine to outperform an equivelent single carb engine at revs most A10's never reach.

 That makes sense. Perhaps the 35 rwhp fuel flow limitation that Jim found on dyno  is not a limitation for a 650, whereas it is for a 750 or 850 Commando, the bikes he is interested in.
After testing the Super Rocket I fitted the GPS on the Norton 650ss and did the same speed test. The 650ss came over the top at 75 MPH . The SR and 650ss are about the same weight and have an almost identical overall ratio in third gear. They also have a similar HP rating with the ss at 49-52 depending on the source and the West Coast 63 SR (9 to one CR) at 46- 50 depending on the source.
The difference in actual power between the two bike on the road is great, will have to keep searching for a problem in the BSA. I expect it will always be a bit slower than the SS , but they should be close in performance.
The S R engine is as new, it tests very well on leakdown. It runs nicely, just not a lot of power coming forth.


Glen

[Rocket Racer]

Glen,
 I'm pleased you're moving on from blaming the humble concentric.
It does sound like your super rocket is not quite as super as it could potentially be  and I hope you can enjoy it while you ponder why that is, if that is important to you.
I would certainly expect it to effectively match any period 650 of the day in a comparable state of tune.
As someone who likes performance my road rocket is a match for any 650 on the track and has regularly embarrassed far bigger machines  .
Tim

[worntorn]

Hi Tim
The Carb change was worth a shot and it did pick up a tiny bit of thrust in the process, plus it's nice to have original type equipment on there.
The bike runs very well, idles nicely and is really smooth for a solidly mounted parallel twin.
I will slowly pick away at things to see if more power is hiding somewhere, but if not, she's a nice old bike as is.

Ignition is next thing to look at. That is, after I do one last check to make sure the brakes aren't dragging:-) 

Glen