421 cheetah help

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alex2218
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421 cheetah help

#1 Post by alex2218 » Sun Nov 13, 2016 10:03 am

Hi guys,

I did not found any presentation post so i do it here.
My name is Alex, 30 years old, riding some 2 stroke in France and specially the nasty ones :smt003

Actually i just finished a Cagiva mito race bike with a 421cc engine
This project has been pushed to the end with the help of all the friends from the french 2 stroke forum, thanks to them
The bike is running well and need some smalls tuning to get finished

Image Image

BUT

I have a problem with the engine : There is no enough power on the top
There is no problem with the exhausts, they run very hard on a friend race bike but he have a different tuning
Mine pulls hard from 4k with lot of torque but the top end is smooth, the engine is not violent at all and i can't pull the 6 with a 17x38 bracket (banshee gearbox)
Here is a video of the very first ride on track, jetting was too rich but as you can see its hard to beat a tuned 350 at 3.20min : https://www.youtube.com/watch?v=MNRHwjhHSyU" onclick="window.open(this.href);return false;

The only mod i made is to use Vertex blaster 200 pistons instead of the Wiseco ones (seized them due to a premix problem on the first test...)
I'm using the very good Wicked head with 22cc domes that i modified to match the vertex pistons because they have a slightly more domed top
So now i have a little more compression with different dome angle but i'm trying to find if my problem is here.
See the difference (vertex on left / Wiesco on right )

Image

This is why i'm asking for your help because i have seen this post : viewtopic.php?f=1&t=9390" onclick="window.open(this.href);return false;
We have some way to follow but it seems you know this engine kit well here and i would have your advice too

I can supply more informations if needed but here are the base lines :smt002
Thanks

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JanBros
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Re: 421 cheetah help

#2 Post by JanBros » Sun Nov 13, 2016 2:13 pm

alex2218 wrote: There is no problem with the exhausts, they run very hard on a friend race bike but he have a different tuning
different tuning means different timings I guess ?
if so, there's the reason.
if it runs, you can race it !

alex2218
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Re: 421 cheetah help

#3 Post by alex2218 » Sun Nov 13, 2016 2:41 pm

He started with these exhaust with practicaly the same setup before tuning the engine much more.
I'm sure it's not the problem, i trust him a lot
But you may be right and i'm going to try with a +4° stock map to see the difference instead of my actual map

I'm wondering if the air filters are restricting the engine because the foam seems very dense but not sure, i did not tried without filters, its too risky
They are UNI 6229AST, i'm waiting for an answer from them about this issue....

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Questo vecchio rz
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Re: 421 cheetah help

#4 Post by Questo vecchio rz » Sun Nov 13, 2016 5:57 pm

Im not qualified to speculate on your port timing and pipes.
But generally when one gets to this level of tuning, your port maps dictate your exhuast design. If your engine or clylinders are in any way diffrent than your friends engine design, then technically your pipe design should be diffrent. Although one would think it would still perform decently, and not. "fall flat" on top end.

You post is titled Cheetah 421. Are your cylinders CPI Chettah cylinders or Athena? The two are completly diffrent designs requiring diffrent setups.

Also you say Banshee bottom end, run your calculations compared to a stock YPVS gearset and see if that could be your limiting factor in top end speed. (Dont) go by 2nd hand information, Only go straight to the manuals and compare all your gearing for your calculations.Speak with (MK here) as he has 1st hand experience with Banshee gearboxs and primaries.

I doubt its your foam filters, UNI s breath quite well, even with added sand covers/sleeves/wraps they still flow quite well.

Also what are your carburetors ? & what size are they? And what is your reed choice and their condition? Many times if not usually carb related issue.

There might be a little lost to translstion, but if your advancing your timing to + 4 and doing so with even more added compression, Be careful! as many of these engines expire at sustained speed, they dont like heat. Even with cool head designs.

(Note*) CPI Chettahs may or may not have longevity issues on roadrace applications which lead to seizures. This is only info I've read from "some" I cannot verify that, do some reasearch.

Finally, dont be discouraged by not being able to accelerate at high speed with a tuned YPVS with "stock ported barrels" as there are quite a few perfectly tuned YPVS racebikes that could easily be a performance match or better than a "average" 421 or one thats not tuned to its potential. I'm specificlly referring to a roadrace bike & tune.
Last edited by Questo vecchio rz on Sun Nov 13, 2016 7:19 pm, edited 3 times in total.
Banshee (Baja) race bike,+ 2 A Arms,L.E.Ds, Toomey, +4 stroker IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura rep.
85 custom Tri-Z
RZ/YZR bike(project)
86 VFR750 RC24 Merkel replica (project)
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waltmil
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Re: 421 cheetah help

#5 Post by waltmil » Sun Nov 13, 2016 6:08 pm

I also hate to speculate but are those Uni filters pods? If so and you are not running in dusty conditions, pull the filters and see what happens. I would do that before messing with ignition timing.
Red/White US '84, Spec II pipes, Y-boot w/K&N, Fox Shock, Mikuni carbs?

alex2218
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Re: 421 cheetah help

#6 Post by alex2218 » Sun Nov 13, 2016 7:34 pm

Thank you for your messages, i'll try to answer :
But generally when one gets to this level of tuning, your port maps dictate your exhuast design. If your engine or clylinders are in any way diffrent than your friends engine design, then technically your pipe design should be diffrent. Although one would think it would still perform decently, and not. "fall flat" on top end.
Engine was using stock cylinders when these exhausts were designed, they are based on the Wobbly exhausts. When i modified the domes, i raised the cylinders from 0.012 to 0.032 with thicker base gasket so the exhaust duration is slightly higher than stock.
You post is titled Cheetah 421. Are your cylinders CPI Chettah cylinders or Athena? The two are completly diffrent designs requiring diffrent setups.
Yes, they are cheetah cub 68mm cylinders, juste grinded the ports a little to remove the cast iron nothing more.
Also you say Banshee bottom end, run your calculations compared to a stock YPVS gearset and see if that could be your limiting factor in top end speed. (Dont) go by 2nd hand information, Only go straight to the manuals and compare all your gearing for your calculations.Speak with (MK here) as he has 1st hand experience with Banshee gearboxs and primaries.
Yes, i'm experiencing a serious problem when passing the 5 and the 6 is terribly long ! When i have a very small bracket like 15x38 it is an accelerating monster, assuming i have a some other 2 stroke i can say it pull hard but not as expected. I would try with a 31k gearbox but the 6 still long.
I doubt its your foam filters, UNI s breath quite well, even with added sand covers/sleeves/wraps they still flow quite well.
I'm running ONLY on race track, there is no dust and filters are not oiled and clean
Filters i'm using are the UP-6229AST but i removed the RED foam (pre filter)
Also what are your carburetors ? & what size are they? And what is your reed choice and their condition? Many times if not usually carb related issue.

carbs are PWK 35 with V-Force 4, jetting still a bit rich according to the spark plug after a WOT run.
There might be a little lost to translstion, but if your advancing your timing to + 4 and doing so with even more added compression, Be careful! as many of these engines expire at sustained speed, they dont like heat. Even with cool head designs.
I use 22cc domes and 98 pump gas, the domes with Vertex pistons at the TDC volume of the head is 19.1cc with spark plug hole. Ignition is an ignitech with a custom map made on the dyno, also running on other quad and bike using cheetah dyno. I think this setup is really safe, also the engine does not heat so much, i have to tape the radia when it's cold !

I have to admit i'm really happy of the engine, it's pulling from 2k to 10k with no flat spot and strong but i'm really thinking there is missing HP on the top
I'm not a gearing specialist but maybe the gearbox ratio can't handle a so long bracket... but as i said, i should pull harder on the top end

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Questo vecchio rz
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Re: 421 cheetah help

#7 Post by Questo vecchio rz » Sun Nov 13, 2016 8:21 pm

I'm "thinking" that your gearing might be the issue.
I have a Banshee with a P/V Cheetah cylinders, and stock front & rear sprocket ratios, I'm thinking it only can hit around 90 mph maybe more, but thats about it. It gets there VERY fast, accelerates quite hard. It obviuoslly has no speedometer but I've ran alongside some custom trucks/sandrails that do, they have said thats about what they were doing.

A bone stock YPVS can achieve 115 mph or more.

Best do a search for a gearing calculator.
Banshee (Baja) race bike,+ 2 A Arms,L.E.Ds, Toomey, +4 stroker IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura rep.
85 custom Tri-Z
RZ/YZR bike(project)
86 VFR750 RC24 Merkel replica (project)
Royal Enfield Bullet 500

ghezzi
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Re: 421 cheetah help

#8 Post by ghezzi » Sun Nov 13, 2016 9:59 pm

Banshee gearing calculator - just input your primary & final drive, and wheel size.

http://www.everything2stroke.com/conten ... or-Banshee" onclick="window.open(this.href);return false;

You can also alter the individual ratios ......................

Or find the appropriate calculator for RZ350.
I'd rather ride a slow bike fast, than a fast bike slow.

alex2218
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Re: 421 cheetah help

#9 Post by alex2218 » Mon Nov 14, 2016 5:07 am

Already done all this calculation with the gearing commander : http://www.gearingcommander.com/" onclick="window.open(this.href);return false;

It's clear that the 1 2 and 3 are perfect on the banshee but 5 and 6 are way too long to pull
You can't compare with a banshee since the wheels are much much smaller

After all, my problem may be only a bad gearing with the banshee box that restrict the engine from 5 to 6

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Re: 421 cheetah help

#10 Post by MK » Mon Nov 14, 2016 3:28 pm

As already mentioned the rpm drop from 5'th to 6'th is by far too large for a roadrace application.
That's the reason why you feel a 'flat' top end.
For my kart track bike it's ideal as I can now use the 6'th. With the YPVS gearbox the difference was too small.
YPVS:
Image

Banshee gearbox:
Image

I'll be glad to swap your Banshee gearshafts with YPVS ones. It's a plug & play swap, but engine disassembly is required.

Secondly a stock ignition curve is crap for those cylinders. You need to dial it in on a dyno.
Raising the stock curve about 4 deg with the higher compression pistons may be lethal.
Btw. high compression and high advance kill your overrev, another possibility for your issues.
Bye
Martin

alex2218
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Re: 421 cheetah help

#11 Post by alex2218 » Mon Nov 14, 2016 5:13 pm

Thanks Martin, i have a similar graph on Excel, it show very well how the banshee gearbox is not adapted for race track use
I have a 31k gearbox in stock and going to fit it, not a problem to open the engine, i'm used to that lol

My compression is good and not too high, it's the same as my RS250 en RD350 track bikes wich are running very fine even on top end

I compared my custom ignition curve to others cheetah users and there is no much difference, only some few degreed depending on the setup

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Questo vecchio rz
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Re: 421 cheetah help

#12 Post by Questo vecchio rz » Tue Nov 15, 2016 4:15 pm

alex2218 wrote:Thanks Martin, i have a similar graph on Excel, it show very well how the banshee gearbox is not adapted for race track use
I have a 31k gearbox in stock and going to fit it, not a problem to open the engine, i'm used to that lol

My compression is good and not too high, it's the same as my RS250 en RD350 track bikes wich are running very fine even on top end

I compared my custom ignition curve to others cheetah users and there is no much difference, only some few degreed depending on the setup
Please post your impressions after you have sorted your new gearbox ratios. Also any photos of your trackbike would be welcomed, in addition to your suspension setup.
Banshee (Baja) race bike,+ 2 A Arms,L.E.Ds, Toomey, +4 stroker IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura rep.
85 custom Tri-Z
RZ/YZR bike(project)
86 VFR750 RC24 Merkel replica (project)
Royal Enfield Bullet 500

alex2218
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Re: 421 cheetah help

#13 Post by alex2218 » Tue Nov 15, 2016 6:36 pm

Photos + video is on the first post... ;)
Suspension is stock on the front and EMC shock on the rear

alex2218
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Re: 421 cheetah help

#14 Post by alex2218 » Sun Nov 20, 2016 3:44 pm

Problem probably in way to be solved, i tried to load the stock 1WT 350 map into my ignitech and added +5° everywhere and the engine pull really stronger on top.
I'm wondering if somebody tried the bdk map or any more advance than 5° on cheetah engine please ?

MK
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Re: 421 cheetah help

#15 Post by MK » Sun Nov 20, 2016 5:51 pm

Did you verify the base advance?
With a stock stator plate it should be around 18 deg.
I usually test it with a mark at measured 2mm BTDC which is 20 deg. Mark the rotor at that position.
Then program a curve with a constant 20 deg over all rpm and strobe the running engine.
If the mark is off, correct the base advance until it matches.

After that, program the curve you need. The stock 1WT curve is way off for your demand.
On the last pages of this pdf you can find a basic ignition guide: http://l3s9113.zeus09.de/RD350/Auspuffb ... glisch.pdf" onclick="window.open(this.href);return false;
Bye
Martin

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