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Discussion Starter #1
I have scooterised the bike 3 years ago (170/52, stock needle, the air filter cover is not completely cutted, I have left 1/4 of its circumference uncut to prevent "HyperCharger Syndrome" ...).
Bike is running very well, no issues, but this summer I have experienced this symptom :
=> when riding and keep my right foot into normal riding position, as in the pic "A", all is OK, bike is running very good.
=> when I keep my right foot like in pic "B", bike is poor running. To compare both cases, if I keep the throttle into a constant position, keep the foot like in pic "A" and run with 100 km/h, then I remove my foot and keep it like in pic "B", the bike speed drop to about 75 km/h and feel that has no power.

IMO, by its position my foot is influencing the air flow coming into the filter and the effect is not enough fuel when I keep the foot like in pic "B".

It doesn't bother to me because, of course, always the foot is like in pic "A". My wife doesn't even notice that :) (she is riding the bike).

What do you think I can do to solve this issue ? I'm thinking at two solution, if the rout cause is not enough fuel :

1. raise a bit the needle using a shim
2. use an air filter cover with less cutted holes, so that the air will not go so easy into the filter, means more underpressure into the carb, which means more fuel.

Sorry for long story ..
 

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Sure, too much air make You too lean ;) Playing further with needle shims will not solve Your main problem by default. You should stabilize the airflow to prevent its dependency from Your position on bike, first. IMHO, changing needle shims will only mess up everything You have by now. After increasing the needle height, theoretically You should consider the jet sizes again. Just put back the original filter cover ;)

EDIT
Or start with covering Your filter little by little to find the right cover size limit.
 

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Discussion Starter #3
Sure, too much air make You too lean ;) Playing further with needle shims will not solve Your main problem by default. You should stabilize the airflow to prevent its dependency from Your position on bike, first. IMHO, changing needle shims will only mess up everything You have by now. After increasing the needle height, theoretically You should consider the jet sizes again. Just put back the original filter cover ;)
I cannot put back because the air filter housing is completely modified (irreversible) due to scooterising (is completely closed), need holes on air filter cover for the air to come in.

I already bought a stock air filter cover and thinking to make some holes on it, not too many for the beginning, then riding to see the results. If the mixture is too rich, I will make an additional hole ... and so on, until my foot will not have any influence on the afr and plug color will be OK. What do you think ?
 

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That's the best!
Or before You start, do an experiment with some kind of tin can of proper diameter size and try those holes on it ;)
Just to make sure, it helps as expected.
 

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Discussion Starter #5
Sure, too much air make You too lean ;) Playing further with needle shims will not solve Your main problem by default. You should stabilize the airflow to prevent its dependency from Your position on bike, first. IMHO, changing needle shims will only mess up everything You have by now. After increasing the needle height, theoretically You should consider the jet sizes again. Just put back the original filter cover ;)

EDIT
Or start with covering Your filter little by little to find the right cover size limit.
Yes, this is best to do.

Tanks for reply.

:good:
 

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<and the effect is not enough fuel when I keep the foot like in pic "B".>

The opposite, don't touch the carb, your bike is running great now, leave needle as is, instead of closing off cover hole less, you need more opening for more air
 

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Discussion Starter #7
<and the effect is not enough fuel when I keep the foot like in pic "B".>

The opposite, don't touch the carb, your bike is running great now, leave needle as is, instead of closing off cover hole less, you need more opening for more air
My judgement : as I have less power when foot like in pic "B", I assume the engine receive less fuel. To get more fuel, need more vacuum into the carb, to "suck" the additional fuel. If the air filter is open, or very permissive, I cannot get enough vacuum into the carb.

or ?
 

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Something like that. In other words, the engine sucks in the easiest available substance. In Your case Your leg works like an air spoiler. In one position it decreases the air access, but in another increases.
 

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Actually, there could be some truth in the idea of btom, too. I got what he means :)
If You have too much fuel, You also need a sufficient amount of air to burn it effectively. So, Your leg in picture A could work like an obstacle and prevent sufficient air amount to the engine. When Your leg is in the position B, air access is better.

To work out two these theories, I would:
1) Inspect the spark plugs in longer period of time. If they are getting black, You have too much fuel and not enough air (the idea of btom)
2) First try to partially cover Your air filter by 1/4. (If no result, go for 1/2.) If Your problem is solved, my theory is the right one.
In any case, don't touch the carb setup. Operate only with Your air filter.
(I still bet on mine.) :D
 

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1. Carburetors work on carb signal, not suck. That is what the carb "sees" on both sides of the carb.
2. If you run the engine on the dyno with no air intake system, one would notice many peaks and valleys in the torque curve.
3. What the factory does is design an intake that flattens the torque curve (among many other things).
This makes the carb easier to tune for the entire rpm range.
4. When an intake is modified, one runs the risk of upsetting the torque curve and re-introducing the hills and valleys. This makes carb tuning much more difficult.
5. What has happened in your case is that the intake is sitting at the raw edge of being waaay out and leg position is sending it over the edge.
6. Manufacturers design an "air box" into the system to ensure that the air is as stable and consistent as possible to avoid the very issue you are having.
7. Now, many say that they have modified with success, but it involves fattening up the carb to effectively "cover up" the design flaw of the modification.
8. On some Vulcans, 88 being one, the air box is so exactly engineered that modifications always result in driveability issues that are extremely hard to dial out.
9. Remember, air "quality" is as important as air quantity.
 

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Discussion Starter #11
1. Carburetors work on carb signal, not suck. That is what the carb "sees" on both sides of the carb.
2. If you run the engine on the dyno with no air intake system, one would notice many peaks and valleys in the torque curve.
3. What the factory does is design an intake that flattens the torque curve (among many other things).
This makes the carb easier to tune for the entire rpm range.
4. When an intake is modified, one runs the risk of upsetting the torque curve and re-introducing the hills and valleys. This makes carb tuning much more difficult.
5. What has happened in your case is that the intake is sitting at the raw edge of being waaay out and leg position is sending it over the edge.
6. Manufacturers design an "air box" into the system to ensure that the air is as stable and consistent as possible to avoid the very issue you are having.
7. Now, many say that they have modified with success, but it involves fattening up the carb to effectively "cover up" the design flaw of the modification.
8. On some Vulcans, 88 being one, the air box is so exactly engineered that modifications always result in driveability issues that are extremely hard to dial out.
9. Remember, air "quality" is as important as air quantity.
I understand and fully agree with your comments. I realy have doubts that the power has increased due to scooterising, could be in some cases, I don't know, but was not the idea in my case. I wanted an open intake .... just nice to have :) :).


1. what is "carb signal" ? what makes the fuel to come up through the jet, only venturi tube effect, or also an underpressure, in the camera above the jet ?.
4. I understand. I can feel those picks in TQ curve, when running through entire rpm range.
5. sorry I'm not an English native, so I do not understand 100% what do you mean. Could you please be more specific ?
7. I understand and fully agree with you. But is nice to have an open intake :) :).

Thanks
Marcel
 

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If I correctly understand sfair, the carb signal in this case in general is nothing more than opened or closed throttle grip. And being "out and thereafter over the edge" means the intake's dependency from air flow which is too unstable. (That's what I was already talking about previously. Correct me if I'm wrong.)

marcelem, why do You think that having an open intake is nice?
 

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no such thing as 'carb signal' on an 800
 

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Discussion Starter #14
If I correctly understand sfair, the carb signal in this case in general is nothing more than opened or closed throttle grip. And being "out and thereafter over the edge" means the intake's dependency from air flow which is too unstable. (That's what I was already talking about previously. Correct me if I'm wrong.)

marcelem, why do You think that having an open intake is nice?
I'm reffering only to appearance, it looks much better for me. And in general having something open, not covered.
 

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Carb signal includes:

1. Instantaneous vacuum at that time.
2. Intake reversion/vacuum due to valve timing, exhaust system, rpm, throttle position, etc. It is complicated and every engine specification has an influence. Change any one item and carb signal changes.
3. Conditions outside of the mouth of the carb including standing reversion pulses, air turbulence, etc.
4. Factory intakes/exhaust try to damp down all of the influencing conditions so that carb tuning covers all situations.
5. Now, one can also take advantage of, say, a power spike that occurs at a high engine rpm with the modification of exhaust/intake system, but it usually is at the expense of power some where else in the power band.
6. Motorcycles can be more difficult to dial in than cars because 1 or maybe 2 cylinders share a carb with little to no intake plenum, and this aggravates the situation.
 

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Let me give an example.
Carb signal has a major influence on carb slide position.

1. Remove air cleaner, start engine and bring up to 3000rpm and note slide position.
2. Then run down the road, bring up to 3000 rpm in high gear, start up a hill and note slide position.

You WILL see a difference. If you were to alter exhaust system, you again would see a difference in slide position.

So if you make a statement that 800's do not operate with carb signal, you better be able to back that up!
 

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That all is correct and true and nobody argues. The only problem is that people here (including me and seems like btom also) have never heard that those values and bunch of factors are called "carb signal" ;)
 
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