tzr has a flat spot at 5000rpm, help!

[simon1221]

my tzr seems to have a flat spot at about 5000rpm, anything under that speed it is fine, but anything over is in the powerband, so thats fine, its just that from 4500rpm to about 5500rpm the engine sort of coughs and splutters through it and doesnt really generate much power, so then the only time i can really get to the powerband is if i am going downhill or downshift a gear to whack the revs up

it has a new spark plug and air filter sponge (standard),

however the previous guy said "er, it runs better on super unleaded rather than normal unleaded mate, so use that"

i just filled up with unleaded as i didnt think that it would make any difference as a higher octane fuel is unlikely to boost a tiney engine, especially one that wasnt designed for it

cheers for any replies!
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DNA50 (sorry) ->> TZR125l >> GPZ500S

[Kickstart]

Hi

I would agree that the petrol is not likely to make that much difference. Not due to the engine size (which is irrelevant on this) but as you have not mentioned that you are running a higher compression ratio.

Some stumbling before hitting the power band is quite usual. Might be worth playing with the needle jet and experimenting with using different amounts of throttle through that rev range.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[simon1221]

thankyou i will do that

cheers
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DNA50 (sorry) ->> TZR125l >> GPZ500S

[DM]

I have the same problem with my RX and it's more noticeable now since I dremeled the intakes and exhaust, shifting the powerband even higher (but allowing me to accelerate into the red ).

But there's a definite flat-spot in the 5,500 -> 6,500 range that drives me bonkers. I'll have power below and above that range, but within it you'd think the bike was in horrible condition. In lower gears I can usually give it full throttle and there's enough torque to carry it through, but usually I'm in the higher gears, give it a full-twist and it starts hesitating and/or choking. To combat it I generally just slip the clutch and power past easily, but I've been trying to take it easy on my plates and not create a racket everywhere that I go .

What I think it comes down to is the quality of carburator or how it's set-up. Someone told me that Delhortos, one of which I'm using, usually have a flat spot no matter how you tune them, and switching to a Mikuni Flatslide should solve the problem. Something I'll look into when I get back from university this summer.

[satans_BIG_helper]

I have a MK1 Mito 125..... and mine does the same thing around 5 - 5.5 K all i need to do is sort the needle on the carb but im lazy and cant be bothered as the bike rides fine and it only stumbles if i keep it at those revs

it will pull through no problems so i just gotta get round to sorting it out...or wait til i find a 34 mm carb to replace the 28 thats on it at the moment

[Kickstart]

Hi

Don't think Dellortos are bad, and they are certainly easy to get parts for with loads of different jets and needles available.

Any high performance engine will have a flat spot at half peak power. Basically everything is tuned to work well at a certain RPM, with the exhaust pressure waves bouncing back to such in the next load of mixture / suck out the next load of exhaust gas. However at half the peak revs the pressure waves do quite the opposite, shoving all the burnt exhaust gas back into the engine.

A power valve is an attempt to get round this problem. Does work to a large extent by changing the exhaust port timing but is unlikely to correct it totally.

With 4 stroke engines they often engineer in these flat spots worse than they need to be to make the noise tests easier.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[DM]

Well, I think the ignition might be at fault as it obviously isn't stock considering the mental way the power kicks in compared to a regular RXS. Even with the stock exhaust it exhibits the same behaviour, just not quite as aggressive.

One thing I haven't tried yet is adjusting the needle position. Might be something to experiment with tomorrow...if I'm recovered from tonight, of course .

[Kickstart]

Hi

The porting changes are far more likely to cause the flat spot than the ignition.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[DM]

Well, the bike had the same flat spot before I got ahold of it . When I rode it for about 6 months, the block was completely stock. When I got ahold of a new piston and had it rebored to an oversize, I noticed the piston windows were smaller than the intakes and didn't quite match up, so I enlarged the piston windows to match the cylinder intakes. Then I slightly raised the exhaust (maybe 1/2mm) and it was enough to shift the powerband up a bit and allow me to accelerate past redline, which I just wasn't able to do before. The flat spot has remained more or less the same.

Tonight I had a bit of time and adjusted the carb needle so it sat a bit higher. After assembling it I had problems getting it started. So I roll-started it in my small backyard, got it going and it revved up to something like 12,000 rpm, redline being 9,500 rpm. I let out the clutch slightly so it wouldn't cold-sieze or anything and the back wheel starting digging a hole in the yard. I finally managed to reach the keys and turn it off without losing total control and later found out that my throttle cable had started to unwind and the loose wire was sticking in the cable housing . Of course, I have to wait until Tuesday to get a replacement, which means no transportation in the meantime . I'll try and sort something out tomorrow...quite funny how many things I've temporarily fixed up with the intention of doing properly later and have forgotten about.