Tuning question, high speed drop in power?

[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.