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 Mid-range problem

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AuteurMessage
COCONUT

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Nombre de messages : 6
Age : 32
Localisation : Toulouse France
Date d'inscription : 22/03/2017

MessageSujet: Mid-range problem   Jeu 20 Juil 2017 - 20:21

Hi all,

I just finished my race bike and had ridden it for 2 days at Magny-cours two weeks ago
After putting it on a dyno we founded that the engine run very well with lot of power and torque but i'm having a serious gap on the mid-range.

Engine is a tuned YPVS with a cheetah 68mm kit 421cc (+4 mm crank).
I'm assuming this engine has no valve, but i've seen much better curves with a similar setup.
This project took me 2 years, but development of the engine is not finish until i can sort out how to get more HP at mid-range

I can give you almost all parameters of the engine... just ask, there is so much
Here is the curve, it's now way better since i was 10 pts too rich on the main jets on the dyno, see the gap between 6000 and 7500 rpm :

[Vous devez être inscrit et connecté pour voir ce lien]

Going to test news spark plugs, new timing, intake spacers soon...may be also a scavenging problem due to an other parameter
I'm pretty sure i will find out what is wrong but i have seen a lot of very good advices here so any help is welcome, not the solution but a way to follow :)

Thanks
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JanBros



Nombre de messages : 228
Localisation : Belgique
Date d'inscription : 05/12/2011

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 1:01

typical dip at 2/3rd of rpm of max torgue because the pipe does more bad than good at that time. they all do that. Frits could tell you more on how to (partialy) solve that.
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 9:11

COCONUT a écrit:
Engine is a tuned YPVS with a cheetah 68mm kit 421cc (+4 mm crank). I'm assuming this engine has no valve
YPVS is an abbreviation of Yamaha Power Valve System, so there should be a power valve. Maybe the Cheetah kit doesn't have one, but how can you assume a thing like this? The presence or absence of the power valve should be clearly visible from the outside.
JanBros a écrit:
typical dip at 2/3rd of rpm of max torgue because the pipe does more bad than good at that time. they all do that.
Frits could tell you more on how to (partialy) solve that.
Quite correct Jan. The real solution would be the power valve, but you can lessen the problem by advancing the ignition timing only in the rmp area where the dip is and by enrichening the carburation in this area.
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COCONUT

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Nombre de messages : 6
Age : 32
Localisation : Toulouse France
Date d'inscription : 22/03/2017

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 10:25

Sorry for the misunderstanding, my English is bad sometimes

Off course YPVS is an abreviation, i mean my engine is a RDLC 350 31K, cylinders are not YPVS anymore off course because i changed them for the cheetah cub.

Yes Frits the real solution for such an engine is powervalve but since i have since much better curves, i'm sure i can improve this. I advanced the ignition timing a lot in this sector : 30° from 3000 to 7000 rpm and it is going down after 7000, to me it is already very high.

Does a very short and high ignition advance point between 6500 and 7000 could be a good idea ?
what do you mean by richening only in this area, carb needle setting or porting ?
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 15:01

COCONUT a écrit:
Sorry for the misunderstanding, my English is bad sometimes. I advanced the ignition timing a lot in this sector : 30° from 3000 to 7000 rpm and it is going down after 7000, to me it is already very high. Does a very short and high ignition advance point between 6500 and 7000 could be a good idea ? what do you mean by richening only in this area, carb needle setting or porting ?
There is nothing wrong with your English, Coconut. 30° ignition advance from 3000 to 7000 rpm should be fine. I would not advise more than 30° advance past 7000 rpm. What is the ignition doing at higher revs? If you can post an ignition curve, maybe I can be more helpful.
Enrichening the carburation should be done with carburettor settings, not with the porting. If you have a programmable aftermarket ignition, chances are that it has an option for a programmable power jet as well.
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COCONUT

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Nombre de messages : 6
Age : 32
Localisation : Toulouse France
Date d'inscription : 22/03/2017

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 15:30

That's exact Frits, i use an ignitech and here is the map i used when i have done this dyno curve :
As you can see it's falling after 7K (blue curve is the stock 31K RDLC mapping for comparison)
[Vous devez être inscrit et connecté pour voir ce lien]

Yes the ignitech does have a powerjet control but not my model, also, my Keihin carbs (Pwk 35) does not have powerjet too.
But it may be an option if we found that it is the solution.
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 15:49

The bad news is that both your power curve and your ignition curve look quite good, so we will have to find a solution to your problem elsewhere. You could experiment with a richer carb setting overall. That may spoil the maximum performance but it may give you a clue of what can be achieved with power jets.
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COCONUT

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Nombre de messages : 6
Age : 32
Localisation : Toulouse France
Date d'inscription : 22/03/2017

MessageSujet: Re: Mid-range problem   Ven 21 Juil 2017 - 17:57

Thank you Frits and Jan for your help, it's a bad news but still a good news that eliminate some issue.
I'll try to change the jetting as you say, it's going to be a pain since i was already 5 or 10 jet size too rich when i've done this curve
For curiosity about the engine, see here : [Vous devez être inscrit et connecté pour voir ce lien]

I will also try intake spacers, not sure it will solve it but performance is experimenting first !
It seems it may change a lot the bottom case pre-compression since this case it not designed for a 421cc engine at all

Also what about increasing the volumetric ration ? (actually 7:1)
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