Carbs - Not a problem with them YET!!

[MR.E]

now then

2 questions.
As i'm building the bike to race it i'm looking at the simpler ways to tune the bike before i go to extremes of milling flywheels, conrods etc.

The 1st thought is to increase top end by drilling the main jet by a mm or half, question is has anyone tried it??

2nd question is would the bings have any problem with a fuel pump been added to the circuit?
I'ts a 1980 honda XRV pump, and it more ensures the fuel is there all the time, i am thinking of a switch on the handlebar to flick the pump on whilst held at high revs

Thanks

Hope you're all well

[montmil]

Elliot- Bing sells main jets in a variety of sizes. You might save a few experimental bucks by buying rather than drilling. I knew fellas back in the cheap days that had to solder up and re-drill their main jets that they had "improved" upon.  

Fuel pump? Are you concerned that the Boxer's float bowls will run dry at full revs? Dat's passin' a lotta gas!

[MR.E]

Monty

Thanks for the reply.
I didn't know i could buy different jets, but then i haven't looked.
I'll do some digging on that tonight before i go drilling!!

As for the fuel pump, the last r65, at about 5000 revs in 4th started to die off, almost like it needed putting on reserve.
The thought is the fuel pump will keep a steady flow to the carb, and remove any doubt of this been the problem.

Thanks

[steve hawkins]

Surely for a race bike you would be going done the Mikuni route?

Rev. light

[Barry]

Some very rough guidance on jet size increments.  One jet size increase on main jets is considered to be 5 units i.e increasing from 130 to 135 and that increase will flow 9 -10% more fuel. That 9-10% is the approximate part as it's not quite as simple as working out the increase in cross sectional area as bigger jets flow slightly more fuel per unit area than small ones.  

[Luca]

Have you considered dual fuel taps instead of a pump?

The stock carbs should be able to sustain high rpms.  I've got no problems running my street bike at 6k down the highway for 15 minute stretches.  Sounds like your old r65 might have had a bad diaphragm or obstructed fuel tap.

Once you get the bike up and running, you can do the roll-off test to help you home in on the right jet size.  I believe this description of the test is from Mikuni USA:

"ROLL-OFF:

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.

If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.

If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one."

You can also carefully play with the ignition advance for a bit more power.

[georgesgiralt]

Hello !
Once valve lash is correct and ignition set properly, you'll clean and check both carbs (diaphragms, state and correctness of jets and needles) yo can follow this :
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html#1.%20Top%20end%20%28full%20throttle%20/%207.5k%20to%20redline
and tune perfectly your engine to give you as much power at it can ! You may also tune for ethanol added gasoline (in Europe we are going to have 10 % ethanol in our fuel... everywhere.... )
But it is just me ....

[montmil]

...The stock carbs should be able to sustain high rpms.  I've got no problems running my street bike at 6k down the highway for 15 minute stretches.  Sounds like your old r65 might have had a bad diaphragm or obstructed fuel tap.

Agree. It's not a lack of fuel nor even fuel head pressure that's causing your drop in power at a lower than normal rpm. Something, or things, ain't right within the carbs.

Are you running the Bing spec'd main jets in your carbs?
Are the diaphragms lifting the slides all the way up?
Date of last full carb rebuilds?

[MR.E]

The last bike had a carb rebuild, new diaphragms etc.
The problem seemed to be at roughly 100mph the bike would rapidly lose power, it always seemed like the bike needed to be put on to reserve.
so as i'm building this bike i wanted to see if i could avoid the situation, i think as the bike needs finishing and i can't test any improvements yet, i should slow down and get the thing rolling as stock first.........but when will i do that!!

Thanks for the help

[Luca]

Maybe on a racetrack at WOT and high revs the carbs do dry up?
Does anybody know if it is possible that the air in the venturi is going supersonic?  That would ruin atomization.

Again though, two fuel taps should give you a lot more fuel without the weight or electrical (read "horsepower") drain of an electric pump.

Another way you could tweak the mixture is to raise the fuel level in the bowls.

[Bob_W]

Is this an R 65? Don't expect it to have a lot of power left after reaching a true 100 mph.  BMW gives a max speed-rider prone- of a little over 100 as I recall.
My monoshock would hold around 90 for hours, but this was indicated not true mph and there was not a lot left for speeding up.
I agree with Georges, set everything to specs before making any "improvements".
Bob

[MR.E]

Yeah i've looked in to two taps, but i've cut the tank down and it'll be a bit of a squeeze as the underside profile has changed a bit!

I generally ran the old carbs with the needle lifted 1 notch, it was crap on tickover but from 3-4000 revs it was a nippy little bugger.
The fuel pump i've got here is smaller than one of the stock coils so it won't be a massive impairment if i run it and i'm only running 1 coil as ive opted for an electronic ignition-thingy set up

I'll drag up this thread in a few weeks when i've, optimistically, planned for the bike to be finished!!

All the best

[MR.E]

Is this an R 65? Don't expect it to have a lot of power left after reaching a true 100 mph.  BMW gives a max speed-rider prone- of a little over 100 as I recall.
My monoshock would hold around 90 for hours, but this was indicated not true mph and there was not a lot left for speeding up.
I agree with Georges, set everything to specs before making any "improvements".
Bob

Yeah i agree with you both, but i like to jump WELL ahead before i'm ready
Thats probably why it's still not finished!!
Plus i'm tempted to dyno it as stock, so i can see if all the planned improvements give me ANY gain.
Again jumping ahead of myself but afyer the carbs i'll be looking to a camshaft mod, more low end torque and higher valve lift.....tempted to do the work myself as i got to learn somehow!!

Thanks

[Luca]

Plus i'm tempted to dyno it as stock, so i can see if all the planned improvements give me ANY gain.

If you can pay for dyno time that's a great idea.  Be sure to mimic the dyno conditions as closely as possible to racing conditions, such as fuel type and engine warmth.  When comparing dyno graphs, it is best to make sure the air temp and humidity are close on both tests too.

I'm sure you know this, but don't neglect the bottom end when hopping up the top.  With more lift, heavier valve springs, and more horsepower you will need to make sure the conrods, pushrods, and tappets can take the load.