Tuning question, high speed drop in power?

[nzspokes]

Well as much power as my old 250 can give.

At 8 to 9k rpm in high gears I get a drop in power. The motor is pulling strong then it has a marked drop in power. It will pull strong through 10k in lower gears.

I suspect fuel starvation at speed. Could I use bigger float valves or drill mine out a bit?

I have been running Gull fuel but have read yesterday the Ethanol fuel can create a lean condition at high RPM in older motors. Needless to say I wont be running it again.

Any other ideas?

[Grumph]

What's the bike ?

I well remember the headscratching over a GSXR400 which would redline in every gear except 5th and 6th. Turned out to be a Jap home market speed limiter....

If yours will redline at all, in any gear, it's not fuel flow.

[nzspokes]

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What's the bike ?

I well remember the headscratching over a GSXR400 which would redline in every gear except 5th and 6th. Turned out to be a Jap home market speed limiter....

If yours will redline at all, in any gear, it's not fuel flow.

86 CBX250rs. No limiter as it happens in a 500 rpm range. It can be fuel flow as it will hit max rpm much quicker in a lower gear so not using as much fuel.

[slofox]

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I have been running Gull fuel but have read yesterday the Ethanol fuel can create a lean condition at high RPM in older motors. Needless to say I wont be running it again.

Any other ideas?

If you were running pure ethanol you would indeed need a bigger main jet(s). But at 10% (Gull blend) I would not expect a major difference.

The main deal about running Gull is whether or not the components of the fuel system are ethanol tolerant.

[nzspokes]

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If you were running pure ethanol you would indeed need a bigger main jet(s). But at 10% (Gull blend) I would not expect a major difference.

The main deal about running Gull is whether or not the components of the fuel system are ethanol tolerant.

They have been so far but will be trying other fuel from now on. What I read about this was from NZ.

[Fast Eddie]

if its only happening in a 500 rpm range that is very strange. Clean the carburettors first and reset the floats/jetting back to factory specs and drain all old fuel out and use fresh fuel that is factory recommended fuel. i.e. my fireblade tells me it wants BP premium unleaded. And then go from there. Check air filter/fuel filter is clean as well and spark plugs are clean.

Start basic. and eliminate all the easy stuff.

sound really wierd, I'm gonna think outloud now - that at high speeds there is alot more load on the engine/wind drag so in the last few bits of speed it has to give its starting to misfire a little or the fuel/air mix isn't right or not enough spark etc when under a heavy load of top speed/wind speed.

that depends how fast ur going tho, im picturing 150+ haha..

start with the basic and clean everything out reset everything back to factory specs

I wouldn't drill out or modify anything yet. The bike would have been running fine out of the factory with its original parts - so get it running how it should again without drilling anything or swapping any bits.

[bogan]

I know in other countries with high ethanol blends people put additive in the tanks, can't remember why, but it's performance related I think. Could be worth checking out as it might be a simple fix.

[nzspokes]

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if its only happening in a 500 rpm range that is very strange. Clean the carburettors first and reset the floats/jetting back to factory specs and drain all old fuel out and use fresh fuel that is factory recommended fuel. i.e. my fireblade tells me it wants BP premium unleaded. And then go from there. Check air filter/fuel filter is clean as well and spark plugs are clean.

Start basic. and eliminate all the easy stuff.

sound really wierd, I'm gonna think outloud now - that at high speeds there is alot more load on the engine/wind drag so in the last few bits of speed it has to give its starting to misfire a little or the fuel/air mix isn't right or not enough spark etc when under a heavy load of top speed/wind speed.

that depends how fast ur going tho, im picturing 150+ haha..

start with the basic and clean everything out reset everything back to factory specs

I wouldn't drill out or modify anything yet. The bike would have been running fine out of the factory with its original parts - so get it running how it should again without drilling anything or swapping any bits.

Carbs have just been done. But have just bought near new carbs that I will swap over with my jets in. Air filters are fine and its plug is new.

Will pull the filter out of the tank etc on the weekend to check that when im fitting my 5th turn throttle.

[nzspokes]

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I know in other countries with high ethanol blends people put additive in the tanks, can't remember why, but it's performance related I think. Could be worth checking out as it might be a simple fix.

Yeah, screws up mixture at high rpm aparently. Not worth blowing motor up for.

[Fast Eddie]

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Carbs have just been done. But have just bought near new carbs that I will swap over with my jets in. Air filters are fine and its plug is new.

Will pull the filter out of the tank etc on the weekend to check that when im fitting my 5th turn throttle.

I guess drain all fuel and replace with the same fuel we all use, Shell or Mobil or BP 95. shouldn't be any problems with those.

If the carbs have just been "done" then I wouldn't swap haha.. and near new carbs can still be dirty. especially if they have been off bike or sitting around for a while.

If the state of tune and condition of engine is good then yea eliminate the fuel, its an easy one to drain and try fresh before digging further.

hopefully just a fuel starve/block issue I had fuel tap and fuel tank outlet blocks before due to old petrol tank/rusty and sediment. That would starve it very slowly but surely. Not sure on the setup of your 250cbx though.

[sinfull]

I'd say try the near new carbs without changing out jets first !

Ethanol could and prob will with your age bike eat away at seals and the like so steer clear of it !

The higher octane fuels stop preignition in older bikes i believe, so shouldn't be the prob with what you have !

At 8 to 9k rpm in high gears I get a drop in power. The motor is pulling strong then it has a marked drop in power. It will pull strong through 10k in lower gears.

This is weird cause it wouldn't be a fueling prob if it pulls all the way in lower gears but not in top !
There wouldn't be something in or around the airbox that could flap and close off the air entry at high speed ?

[nzspokes]

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This is weird cause it wouldn't be a fueling prob if it pulls all the way in lower gears but not in top !
There wouldn't be something in or around the airbox that could flap and close off the air entry at high speed ?

In the lower gears you get through the rev range quicker so not as much fuel is used. in top it takes sometime to get to the higher revs so is using more fuel and could be emptying the float bowls. will check fuel delivery to the carbs first I think. Being a 250 it takes awhile to wind it out in 6th.

[nzspokes]

Oh, and its on pods so no flaps etc.

[sinfull]

Good on ya, ya just gave me an idea, seem to be having a similar problem in top on my HD, just been rebuilt 8ooo ago and started doing the same thing as yours in top on the last ride before my 3 month walk a couple ago ! Biggest change i made apart from High comp pistons was the jump in cam, won't kick in till 135/140 clicks in top and bogging down two up on an incline at say 120/130

Was gonna pull the S&S carb and rekit it (does need it), was going to rejet it smaller also, as it seems to be running a bit rich now and fuel consumption has gone through the roof since the rebuild and it should've gone down lol, but you've made me think to perhaps look at fuel delivery as well, seein i've never had the fuel tap out or even looked at it, or have an inline filter, Know the reserve is dodgy lol

[SS90]

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Oh, and its on pods so no flaps etc.

Can you just clarify that one for me?

By "Pods" do you mean those K&N style pod filters?

If so, what (if anything) did you do to the jetting to compensate for the air filter change?

Also, if you did fit pod filters, did this problem occur at the same time?

What do you mean by "no flaps etc"?