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There are drain holes that let water out of the plug tunnels if any gets in. If the cam cover gasket gets disturbed when refitting, oil gets into the tunnels and weeps out of these holes giving the appearance it is leaking from the head.
 

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Discussion Starter #62
Well, that would be inconvenient. Guess I will find out when its reassembled next week and still runs like garbage.
 

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Hi Mikey,
I got stuck once with my bird, I serviced the bike, and forgot to connect back the suction pipe from the head to the petrol tank valve ( when I put the bike together, I thought it was an overflow pipe, it was the first time I serviced it myself, and what caught me out , was it started perfectly and ran about 10km before it started dying and wouldn't rev at all ), this thin pipe sucks open the petrol valve to allow flow, Sometimes the valve will let fuel through when not running but not enough. Maybe it lets through a bit more than usual, that's why it runs but runs lean. I'm thinking that because you've put everything back the way it was after making the modifications, Carbs won't suddenly go out of sinc, or float levels change if you have not done anything to them. So it must be something small. That could be why it runs better when you close the choke then it sucks harder to get fuel.

I had a Honda 750F2 before my BB, and I put K&N filters on, it hat absolutely NO power and was drinking fuel at about 3km per litre. I then put the old box back and it was perfect.
I have a 1998 BlackBird and has 74000km on the clock.
 

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Discussion Starter #64
That's a good point. I replaced that hose, even tested sucking the valve open myself and let it drain into a can. It flowed FAST. But that was me. Maybe the bike isn't producing enough vacuum.
 

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Discussion Starter #65
Rebuilt now, still runs the same. After letting it warm up it is stumbling and hiccups pretty bad. Even backfired.

This is with timing double checked and verified. All stock parts.

At this point I think something may have went wrong in the motor.
 

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Hmmmmm
Wondering if you've got a heat induced issue. Might want to get you a can if freeze spray. And hit both of the ckp's to see if the condition changes. You might want to pull a hot compression check. What did the heads look like. Specifically valve seats. I'd also put a vacuum gauge on the taps and see if anything shows there as well.
 

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Time for some 'swaptronics' Imo. Swap the pickup and see what happens. No change, try the ignition box. Carefully check all said wiring also. I'm wondering if there is a break in the pickup wiring where it goes through the grommet in cover.
 

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Discussion Starter #68
Alright. Well we did a lot today. I'm pretty sure something is mechanically wrong with the 10,000 mile motor.

We did an engine swap. Dropped both motors and put the 30k mile motor in. Runs way better. Dead spot is still there.

I'm going for the throttle position sensor next. It still ohms out fine but I don't trust it. I did the old "unplug it and see if it changes" trick and the problem was similar but worse. So maybe it's just partial, or messing up.

It acts the same with the petcock off and fuel flows fast through the hoses I've checked by sucking it open.

It's not the pickup.
It's not the engine, valves, timing.
It's not likely the carbs any more.
It's not a vacuum line.
It's not fuel flow.
It's not bad gas.
The stator charges at 13.8v

What exactly does the tps and ignition module do? Is there an advance function? If the tps has bad contacts and fails to read throttle when on the the throttle what exactly would it do?
 

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13.8 at idle?
The TPS signal is used to help determine ignition advance based on the throttle position. It gives a 2nd input to the equation along with RPM. SO that could be part of your issue. are you using the same carbs for both bikes?
 

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Discussion Starter #70
Same carbs. That other one has a crack.

13.8 is the max I see a little above 3k rpm if I remember correctly.

Really hope it's tps.
 

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Thats not good it should be closer to 14.6 to 14.7 2 5K. You may be running out of juice. Might explain why it stayed even with the engine swap.

I'd swap TPS sensors next then. Teh carbs your running are non Cali versions right?
 

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Ok
Also wanted to be sure air cut valve wasn't in them either. Something to rule out.
 

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I'm with you an leaning towards the TPS. Have you tried tapping into it and watching the signal as you rotate the throttle slowly. You'll want to see a steady increase without jumping.
 

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Discussion Starter #75
EDIT: We ohm'd it on the bench and rotated it, constant resistance increase and decrease as rotated.

Ordered a stator and rectifier for $70 as a package from caltric on ebay, they are always quick and have done good by me. Something will solve this mystery. I actually rode the bike yesterday a bit, just a handful of miles.

It pretty strange going WOT at 3k and it torques forward, reaches 4500 and its like it has to think, there is a moment of nothingness as it creeps to 6000 and then tried to rip your arms from their sockets. And if you shift to keep it in the power band you would never know.
 

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So do you have 2 separate TPS pots? Try switching them and see if you see a difference. If you haven't already.
 

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Darn it, why'd you go and do that. How you supposed to troubleshoot if you go breaking all the spares lol.
 

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  • This will not help unfortunately. I have a ‘97 carb’ BB. I’ve owned it since 2014. It has a problem that I never solved which is along the same lines as yours but I tried everything and still no joy. Well obviously not everything as it still has the problem:D
It will rev through fine from idle in neutral and on fast acceleration but when sitting at say around 4000 rpm and open the throttle full. You get to approximately 6000 rpm and it’s like you turned the ignition off/cut the fuel/flicked the kill switch the lot. Like hitting a wall and then bang, everything back on within less than a second with a kick up your backside and away you go again smooth as you like. This will happen most of the time in this scenario but not always.

I don’t give up easily but this has truly stumped me and still does after many hrs/days/weeks of head scratching, Spannering and part changing/swapping. I just know what to expect or pop it down a gear or 2 as a remedy. I’d love to know what it is so shall keep an eye on it to see how you get on.

They are a strange beast but what a lovely bike😊 it has 54,000 ish miles on and apart from that a great machine.
 
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