Grrrrrrrrrrrr

[Steve H]

Still having problems with the DR. Symptoms are
- With a steady throttle its fine at any speed
- Open the throttle in 5th at anywhere between 50mph and 80mph and it will stumble cough and be reluctant to accelerate.
- Any other gear is fine, I can nail it from 20mph in second up to 90+ and it is fine.
- Bike is largely standard
- CV carbs have been replaced with a tidy secondhand set, and certainly helped.
- Only happens on my way home, never in the morning when cold.
- Particularly bad on one stretch of windy road which is 25 - 30 miles into my journey home.
- I tend to ride in top and roll the throttle on and off, which exacerbates the problem (and no I dont want to change the way I ride)
- Lowering the needles makes no difference.

My feeling is its one of two things.
- The slides in the CV carbs are rising too quickly making the mixture too rich.
- The slides are sticking.

Gut feeling is the later, but how do I fix it. They are plastic slides in an aliminium housing,  and dont appear to be worn, is there anything I can use to lube them that wont just wash off.

Steve

[andy230]

hey steve

now this prolly aint an answer but if it were me, and I were spending a lot of time, and not inconsiderable money, I'd check all the hardware works as close to "as it should" ie not holes in the exhaust, or suspect carb rubbers) and get the thing on a dyno.  Probably 60 to 100 quid, but it may be worth it if it helps.

cheers for now matey

a

[Steve H]

Andy, cheers for that, it reminded me that Pat had mentioned a Dyno in Caldicot. Its now booked in.

[guest27]

How much for the dyno and what does it include - just thinking ahead.

R

[Ken]

Not wanting to complicate things but I think you will find on a CV carb that it is rich when the Piston and needle is low. As it cant draw air,  it draws fuel until it raises the piston so giving a rich mixture on acceleration.

Ken

Still having problems with the DR. Symptoms are
- With a steady throttle its fine at any speed
- Open the throttle in 5th at anywhere between 50mph and 80mph and it will stumble cough and be reluctant to accelerate.
- Any other gear is fine, I can nail it from 20mph in second up to 90+ and it is fine.
- Bike is largely standard
- CV carbs have been replaced with a tidy secondhand set, and certainly helped.
- Only happens on my way home, never in the morning when cold.
- Particularly bad on one stretch of windy road which is 25 - 30 miles into my journey home.
- I tend to ride in top and roll the throttle on and off, which exacerbates the problem (and no I dont want to change the way I ride)
- Lowering the needles makes no difference.

My feeling is its one of two things.
- The slides in the CV carbs are rising too quickly making the mixture too rich.
- The slides are sticking.

Gut feeling is the later, but how do I fix it. They are plastic slides in an aliminium housing,  and dont appear to be worn, is there anything I can use to lube them that wont just wash off.

Steve

[Steve H]

This is quite useful
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html

[peterj]

Good article in general, but who makes bike carbs with fuel mixture screws rather than air screws? Harley maybe? I'm pretty sure every bike I've ever had had air screws, hence the instructions in the last section need to be reversed for most folk's bikes.

On your slide lube question - I'd try some silicon spray on the basis that it's a pain to remove when you don't want it on something. Alternatively how about a spot of 2T at 100:1?

On the Dyno, it pays to ask when booking whether they can provide a fuel readout on the graph. If not, it can take an awful lot of runs to get it right. To track down this kind of fault, a proper load cell dyno is better than an inertia type as you can hold it at the troublesome revs and get an accurate readout.

If it proves to be an intractable thing to cure, how about raising or lowering the gearing to shift the stumble out of your favourite cruising speed?

[Steve H]

Yep I checked they can monitor fuel on the runs. He also mentioned its a an inertia dyno but its been modded to allow for holding a particular revs (dont know how this works). Interesting though the inertia aspect is more useful as its only when the throttle is opened quickly and in fifth that the problem occurs.
The article was interesting as it was emphasising the importance of float height, I have checked the height several times but not experimented with different heights.
I can overcome the problem to a large extent by changing gear before accelerating, but to me part of the reason for riding a single is it suits my 'lazy' riding style.

[ken]

Just be sure your plug lead is not breaking down when the bike is under load. If they are going to play up this is when it will happen More cylinder pressure makes the spark more difficult to jump the gap. try closing the gap or changing the lead, cap , coil just to see.
Most likely fuel but this is worth a try.

Ken

[Peterj]

The float height bit was definitely interesting. I've never tried it as a fine tuning tool, but I did have a 750/4 hole a piston because of a wrong setting on one carb, so it definitely can have a major effect.

Any chance of getting/posting a pic or two of the dyno mod? I've never heard of anyone doing that to an inertia type, and it certainly sounds like a way to get the best of both worlds.

[Steve H]

I thought I would measure the actual fuel height on the DR before pulling the carbs to check them again. I bought some clear pipe windscreen washer pipe and made up a short length to connect to each float bowl drain. Its not something Ive tried before but I was surprised to find the fuel level was above the gasket level on the float bowl. A quick 'google' would suggest that the fuel level should be just below the float bowl gasket. Anyone used this method before ?
Oddly adjusting the float height so it is now 2mm lower (2mm increase in height) seems to have little effect on the fuel height. I'm going to check the float valves tommorow.

[guest27]

Is the tube narrow enough that capillary action is drawing the fuel up above the true height?  What happens if you just dangle the end in a pot of fuel?  any lift should be deducted from the measured? Do the float bowls vent to air and is this clear and unobstructed etc - any rise - even slight - in pressure in the float bowl would push the fuel up the pipe that much further - hydraulic advantage. Not sure why either would occur but would be a bugger if they were...



R

[Steve H]

Well float height made sod all difference !. Odd it will take full throttle from 3000 revs in top and rev through to 90+ but open ii up at 4000-5000 and she stutters.

[Steve H]

Ok, Ive just got back from Roystons in Caldicot. Quite a small concern but struck me as being professional without all the bullshit which goes with a larger dealership. The guy was quite helpful and listened to me before running the bike on the Dyno. The run was quite extensive covering lost of roll-ons from different speeds and gears. Probabaly spent around 1/2hr on the Dyno.
And the conclusion was.
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A bit rich but nothing out of the ordinary. He then took the snorkel off the inlet to see if it made any difference, and was surprised to find it did quite a good job and put it back on. After another run he managed he reckoned it was getting quite hot and was starting to run rich and wasnt running quite right, but that there wasnt much that could be done.
Hes given me a couple of main jets to try which might help, but it looks like I might have to either put up with it or adjust my riding to compensate
The point made earlier in the thread about changing the gearing might help as it might move the problem away from the problem speed of 60-70mph, somthing that didnt occur to me when the suggestion was made.

Even though it wasnt sorted I would recommend  Roystons.

[guest27]

Know it is a rude question - but how much did 1/2 hour of dyno cost?

R