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]

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]

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]

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]

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]

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]

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]

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]

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"?

[nzspokes]

Im now convinced its a fuel delivery issue. Stripped the petcock valve and it was clean, but the holes in it are tiny for the fuel to flow through. So on long straights the bike slows as it leans out.

So can you get higher flow valves for the tank? I would presume racers have a fix for this?

[bogan]

I'd be very surprised if it was a design flaw like that, how tiny do you mean? Easy check would be to open it into a measury thing, and calculate the flow rate.

[nzspokes]

I'd be very surprised if it was a design flaw like that, how tiny do you mean? Easy check would be to open it into a measury thing, and calculate the flow rate.

So like how long it takes to fill 100cc you mean? That wont be to hard. How much shoule it be?

But it is fuel flow. as soon as I back of the throttle for 30secs and open it up again the bike pulls hard again.

[bogan]

So like how long it takes to fill 100cc you mean? That wont be to hard. How much shoule it be?

But it is fuel flow. as soon as I back of the throttle for 30secs and open it up again the bike pulls hard again.

yeh, no idea what the flow rate should be, but wouldn't be too hard to work out if it was in the ballpark.

I have a similar issue to sort out with a rich cruise after 30secs-ish, pin it, or cut throttle and it'll be good to go for another 30secs. Got a few ideas, but nothing that fits really well.

Incidentally, where does your float bowl vent come out? Honda made a mistake with my bike in that cross wind would create a pressure change in the bowl, and it'd start rich fouling, maybe your's goes lean at high speed for a similar reason.

[nzspokes]

yeh, no idea what the flow rate should be, but wouldn't be too hard to work out if it was in the ballpark.

I have a similar issue to sort out with a rich cruise after 30secs-ish, pin it, or cut throttle and it'll be good to go for another 30secs. Got a few ideas, but nothing that fits really well.

Incidentally, where does your float bowl vent come out? Honda made a mistake with my bike in that cross wind would create a pressure change in the bowl, and it'd start rich fouling, maybe your's goes lean at high speed for a similar reason.

It comes out and splits into two hoses, one goes up and one goes down.

It took about 20mins to drain the tank from half full on the weekend to check the valve if thats any indication.

[ducatilover]

I wouldn't be surprised if it's your float vents getting pressure or having pressure sucked out of them.
Route them somewhere where there is not going to be any turbulence or funny wind spots.
Had a similar problem on my CB400

[nzspokes]

I wouldn't be surprised if it's your float vents getting pressure or having pressure sucked out of them.
Route them somewhere where there is not going to be any turbulence or funny wind spots.
Had a similar problem on my CB400

Thats intresting. Where would be the best place to run them?

Theres a port out of the top of each carb and they would also vent out the fuel over flow pipes that drop down below the motor.

[ducatilover]

Thats intresting. Where would be the best place to run them?

Theres a port out of the top of each carb and they would also vent out the fuel over flow pipes that drop down below the motor.
Not sure with your bike mate.
You want to have them vented to atmospheric pressure, any more = rich, less = lean.
I'm not sure where you'd route them on your bike sorry mate.
Also, has it always done this? It may be a jetting compromise, but your bowl drain issue is also quite possible, is your tap vac operated? If so, maybe going to a manual tap would be ideal.

[nzspokes]

No its manual. Will try drilling the gallerys to help flow.

Will look at where the vents go to, but think one goes under the seat and the other just goes down by the motor.

[ducatilover]

Good luck with it man.

[tigertim20]

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?

after all the posts you made about newer bikes, why dont you just sell the old piece of shit and get a new fuckin hayabusa or a 2011 ZX10r?

[nzspokes]

after all the posts you made about newer bikes, why dont you just sell the old piece of shit and get a new fuckin hayabusa or a 2011 ZX10r?

Because that would be to easy. And I cant afford it.

[Phreak]

Guaranteed it's just some dirt in your carbs, mate. The main and/or pilot jets probably need a good clean out, and the carbs could probably do with a good balance and tune at the same time. A flatspot in the rev's range (will be way more noticable in higher gears) is almost always a sign of a bad state of tune. Alternatively, if you've changed air filter (increasing or decreasing airflow through the carbs) or exhaust (increasing or decreasing exhaust flow) in which case you'll need to retune your carbs to suit anyways, possibly even requiring different size jets and needles in your carbs... I discovered this when I overhauled mine during a prolonged stint off the road, and now I give my carbs a good going over at least once every couple of months. Remember, manufacturers spend big money developing motors and bikes, and I personally know that my fuel petcock chamber is tiny, even though my bike happily revs out to 19'000rpm regularly (on the track of course!)

How does the bike run at idle? If you blip the throttle (in neutral) does it react quickly and smoothly? Or does it bog down a little bit before gaining rpm? Or worse still, does it hunt for idle or take it's time finding a nice smooth idle after blipping? All these are signs that a carb tune is needed. I learnt to balance my carbs by sight pretty accurately, then tune them by ear. Not an easy task with a four cylinder, but not impossible. They used to do it everyday back in Grandad's time, the old school way of doing things works!

FYI: I use Shell 95 octane, have found most other companies gas tends to foul plugs and other shit that makes my bike run like crap. Maybe that's just me though.

[nzspokes]

Guaranteed it's just some dirt in your carbs, mate. The main and/or pilot jets probably need a good clean out, and the carbs could probably do with a good balance and tune at the same time. A flatspot in the rev's range (will be way more noticable in higher gears) is almost always a sign of a bad state of tune. Alternatively, if you've changed air filter (increasing or decreasing airflow through the carbs) or exhaust (increasing or decreasing exhaust flow) in which case you'll need to retune your carbs to suit anyways, possibly even requiring different size jets and needles in your carbs... I discovered this when I overhauled mine during a prolonged stint off the road, and now I give my carbs a good going over at least once every couple of months. Remember, manufacturers spend big money developing motors and bikes, and I personally know that my fuel petcock chamber is tiny, even though my bike happily revs out to 19'000rpm regularly (on the track of course!)

How does the bike run at idle? If you blip the throttle (in neutral) does it react quickly and smoothly? Or does it bog down a little bit before gaining rpm? Or worse still, does it hunt for idle or take it's time finding a nice smooth idle after blipping? All these are signs that a carb tune is needed. I learnt to balance my carbs by sight pretty accurately, then tune them by ear. Not an easy task with a four cylinder, but not impossible. They used to do it everyday back in Grandad's time, the old school way of doing things works!

FYI: I use Shell 95 octane, have found most other companies gas tends to foul plugs and other shit that makes my bike run like crap. Maybe that's just me though.


Howdy

The carbs are clean. This is a long term issue. Now my 2 carbs dont need balancing as its a progressive system. The second carb starts to open at 1/4 throttle. So if I WOT her it will almost stall due to low air speed over the needles. Now the bike is a single. Your bike if im correct has 4 carbs, which means you have 4 bowls holding fuel to a much smaller piston area so they dont need to flow as much per carb as it were. But in saying that your motor is a much more modern design producing 10 more hp.

I got a pair of carbs from a much lower K bike than mine and last night I stripped them down. Apart from being like new inside I could see that the fuel delivery to the secondary carb is very restricted. It runs through the primary carb to it.

All the plug chops Ive done have come out with the plug looking lean. So what my next step is to do is on the new carbs drill the fuel delivery circuits and do the same to the vent ports to match. I believe the primary carb jetting is good but the secondry is out. So im going to try lift the needle 1 notch (once ive worked out how to get the bugger out). If that doesnt help I will break out the jet drills untill she turns up to rich.

I measured fuel flow out of the tank last night and got 100mm in 19sec. That should be more than enough.

[ducatilover]

I'll donate you one to four Keihin CVK-D32's if you want mate, you pay postage and you'll no longer have gay as fuck tuning issues.

An offer you can't refuse, surely.

[Phreak]

Hmmm okay, I wasn't 100% familiar with your bike's mechanics, but I think you have hit the nail on the head - sounds like you need to lift the secondary needle up a notch or two (I raised mine using small washers, has the same effect). From the sounds of things you still have your original carbs handy? Have a look at those and see if the jetting / needles are any different...

Just remember, once you drill something out, you can't go back!

[geoffm]

Rather than drilling the jets, buy new ones. they are cheap enough, and drilled jets don't flow consistnatly, and the sizes are limited. Check the jet sizes from yours (with pods) vs the new ones - has the jetting been altered due to the pods? Might be worth trying a larger main and seeing if it makes a difference.

Pods also do funny things to engines - eg in Yam RDs, the reflection from the solid endcap is well known for causing flat spots, unless they are spaced out 25mm or so rather than just on the end of the carbs.

Must get the dyno controller fixed, then you could dyno it which would help find the prrblem.

[nzspokes]

Rather than drilling the jets, buy new ones. they are cheap enough, and drilled jets don't flow consistnatly, and the sizes are limited. Check the jet sizes from yours (with pods) vs the new ones - has the jetting been altered due to the pods? Might be worth trying a larger main and seeing if it makes a difference.

Pods also do funny things to engines - eg in Yam RDs, the reflection from the solid endcap is well known for causing flat spots, unless they are spaced out 25mm or so rather than just on the end of the carbs.

Must get the dyno controller fixed, then you could dyno it which would help find the prrblem.

Jets went up 2 sizes when the pods went on. I drill jets as I have the drills and not lots of spare jets.

[ducatilover]

Jets went up 2 sizes when the pods went on. I drill jets as I have the drills and not lots of spare jets.
I've never had any luck with drilled jets.
I reckon to CVK-32's would look grouse on it :yes:

[nzspokes]

Well today I drilled a jet and the world didnt end. Took the primary carb main jet from 112 to 118. Seems to have made a big difference.

Either its in my head or she has picked up power right through the range and spins up to top speed faster.

Thinking about lifting the needle 1 notch on the secondry or going to bigger jet.

[ducatilover]

Good stuff mate! :yes:

[nzspokes]

Just got to get my head around what lifting the needle will do. I know it makes it richer but not sure how it effects the whole range.

[ducatilover]

Makes it richer until the slide has lifted, so around mid way up the rev range, maybe a little lower in higher gears. It shouldn't have any effect on your top end.
If it's flat in the middle (like Keira Kinghtley) then raise the needle one clip

[nzspokes]

Makes it richer until the slide has lifted, so around mid way up the rev range, maybe a little lower in higher gears. It shouldn't have any effect on your top end.
If it's flat in the middle (like Keira Kinghtley) then raise the needle one clip

Cheers, good explanation. Its a bitch to do but have done it on the new carbs on the secondry. You have to pull the carbs to do it.

But jebus what a difference tonight. I seem to be able to stay in 6th more and hold higher speed. Very happy with it. Cant wait for a longer blast.

[ducatilover]

Good stuff! Remember to run it and thrash the fuck off it before re-tuning. You want to see how it performs at all temps. Good excuse to have that needle in the red lots too :Punk:

[nzspokes]

Well working on the time honored theory of if some is good then more is better ive go up in main size on the secondry carb. Seems to rev harder....

Cant ride it untill tonight after kid dutys....

[mossy1200]

I was looking at some info does your bike have direct pull primary and cv secondary card
If so you need a airbox rather than pods or your secondary carb wont want to open at top end.

Yamaha circumnavigated this difficulty first on its XT5 50 trail thumper by the simple solution of having two carburettors, one to feed each inlet valve, and linked so that one of 'em does the business at low and mid-range revs and the other comes in to join it when serious grip-twisting and 'go for it' situations occur. The Yam system uses a slide carb primary and constant velocity carb secondary set-up, but when Honda gratefully thieved the idea, it found that a pair of Throttle response is instant and smooth from tickover to the ten-and-a-half-grand red line


http://www.motorcyclespecs.co.za/model/Honda/honda_cbx250%2087.htm

[nzspokes]

I was looking at some info does your bike have direct pull primary and cv secondary card
If so you need a airbox rather than pods or your secondary carb wont want to open at top end.

Yamaha circumnavigated this difficulty first on its XT5 50 trail thumper by the simple solution of having two carburettors, one to feed each inlet valve, and linked so that one of 'em does the business at low and mid-range revs and the other comes in to join it when serious grip-twisting and 'go for it' situations occur. The Yam system uses a slide carb primary and constant velocity carb secondary set-up, but when Honda gratefully thieved the idea, it found that a pair of Throttle response is instant and smooth from tickover to the ten-and-a-half-grand red line


http://www.motorcyclespecs.co.za/model/Honda/honda_cbx250%2087.htm

The secondry carb is mechanically operated. So it starts to open at 1/4 throttle.

[ducatilover]

I would have smacked a CVK on there by now :pinch:

[nzspokes]

I would have smacked a CVK on there by now :pinch:

To be honest, I dont know what a CVK carb is. :Oops:

So the bike has beter top end and speed but seems a bit flat mid range. So Im gunna try either staying with this jet size and dropping the needle one notch or going back to the original jet and lifting the needle one notch.

[ducatilover]

To be honest, I dont know what a CVK carb is. :Oops:

So the bike has beter top end and speed but seems a bit flat mid range. So Im gunna try either staying with this jet size and dropping the needle one notch or going back to the original jet and lifting the needle one notch.

http://www.google.co.nz/search?um=1&hl=en&safe=off&rlz=1C1CHKB_en-GBNZ458NZ458&q=keihin%20cv%20carbs&gs_sm=e&gs_upl=39037l39037l0l41555l1l1l0l0l0l0l325l325l3-1l1l0&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1166&bih=784&ie=UTF-8&sa=N&tab=iw&ei=SawjT9vDFsqfiQe1u-jTBA


A (http://www.google.co.nz/search?um=1&hl=en&safe=off&rlz=1C1CHKB_en-GBNZ458NZ458&q=keihin%20cv%20carbs&gs_sm=e&gs_upl=39037l39037l0l41555l1l1l0l0l0l0l325l325l3-1l1l0&bav=on.2,or.r_gc.r_pw.,cf.osb&biw=1166&bih=784&ie=UTF-8&sa=N&tab=iw&ei=SawjT9vDFsqfiQe1u-jTBA)s I've said, I have a spare set of them, off a ZZR400, I would imagine one of them with the correct jetting will make your bike rather happy. They are vac. slides and reliable as all hell.
One of them has a smashed pilot screw holder, the other three are sweet.
Do a bit of research if you're interested and if you want 'em they're yours for the cost of postage, I'll throw in an intake rubber or two also and some jets.

[cowpoos]

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.

Simple....get a bigger motor!!!

[nzspokes]

Simple....get a bigger motor!!!

Yup, planning of chucking a CB1000 donk in there.