I'm struggling to get this engine running well.

It starts up great on about 1/2 choke, and after a few minutes I can start reducing it until its running on no choke.
It will rev up ok, but has a tendency to hang.
The main problem is the unstable idle. I get it set ok, but after a minute it will raise, fall and just be quite inconsistent. I'm constantly tweaking the idle screw.

So far I have;

Checked valve clearances twice (all within tolerance)
Cleaned carbs
Fitted a full new set of jets, floats etc to the carbs.
Set float level using the clear tube method
Set pilot screw to 2 & 1/4
Balanced the carbs
Made sure the carb rubbers are tight and not leaking (checked with carb cleaner while running).

I dont know what to try next..

Definitely not an air leak ?

I'm pretty sure it isn't.

If I squirt a bit of carb cleaner into the air box the revs pick up quite a lot (as expected).

But squirting it around the carb rubbers and anywhere else there could be a leak does nothing.

[Edited on 31/7/21 by nick9one1]

From experience, hanging revs are a sign it's too lean/weak. I would firstly increase mixture screws another half turn out and check if the idle speed has increased. I would also re balance the carbs, did you use a multi carb set up or a single in each carb mouth?

Thanks. I used a multi carb balancer. Its only a cheap one but I calibrated it by joining all the hoses with tees and a vacuum tool.

I'm now at 2 and 3/4 turns on the mixture screw..

This video illustrates the issue better than I can explain it.




https://youtu.be/2mJC_Uzh9YY


[Edited on 1/8/21 by nick9one1]

Do you have a return spring on the carbs to mimic the action of the bikes push pull set up? . Needed one on my fireblade car though the zx10r was fine without

Yes, the carbs have quite a strong return spring. After letting off the pedal the carbs quickly close and rest on the idle screw.

Check the carb diaphragms for splits or poor sealing.

I used to fill the sealing groove in the carb body with rubber lube to ensure a good seal.

Thanks. Already checked this. I completely disassembled the carbs and rebuilt. The sliders move freely and raise when you rev the engine.

[Edited on 1/8/21 by nick9one1]

Hanging revs is usually an air leak or weak mixture, I notice that you don’t have an air box fitted, this may make the mixture weaker.
I’ve had loads of problems recently with my YZF750R carbs, I stripped, ultrasonic cleaned and rebuilt my carbs twice (tried different float heights) and it was still running similar to yours. I used brand new inlet rubbers from EBay and these were letting air in, so I used the old ones and they were fine. I still had a problem so stripped them down again for the third time. This time I used an airline on everything, I could feel that the air wasn’t passing through the pilot jet (mixture) hole as well as it was on the other 3 so I used a piece of wire to clean the hole out until the air was flowing through equally. The only other thing I found to be wrong was two of my main jets were the wrong way around. I had 2x125 jets and 2x125.5 jets and the two larger jets had to be in the centre carbs.
I didn’t see what was blocking the pilot jet it was so minute, but these two things combined fixed the carbs.
It may be worth stripping and cleaning again, it was for me.

Airbox is fitted in the picture.

My R1 has a strong return spring on the throttles bodies but still suffered hang-up until I fitted an additional one. A friend has a blackbird and has an additional return spring on the throttle cable.

Thanks for the suggestions. I'm pretty sure it's not a sticky cable/spring issue, but next time I test it I'll disconnect the throttle cable.

I've tested rubbers for a leak using the carb cleaner trick - spraying around them while running. If it's leaking the revs pick up quite distinctly (it doesn't).

But it could still be blocked inside somewhere. I'll disassemble and clean it again. I've also got a wideband o2 sensor on order, this should indicate if a bank is running lean.

It's not that the cable sticks. In the bike the throttle has 2 cables. One pulls open and the other pulls shut. Without the shut cable, it seems that often the throttles don't shut properly. It's common on BECs.

Never got a decent explanation of why but the butterfly's seem to hang or flutter (seems a suitable term for a butterfly ) when shutting off quick. I suspect it's a product of the relatively high angle of them when closed in bike carbs/TBs as opposed to the near 90deg in car throttles.

Ah I see! Thanks

quote:

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Originally posted by Gord
Hanging revs is usually an air leak or weak mixture, I notice that you don’t have an air box fitted, this may make the mixture weaker.
I’ve had loads of problems recently with my YZF750R carbs, I stripped, ultrasonic cleaned and rebuilt my carbs twice (tried different float heights) and it was still running similar to yours. I used brand new inlet rubbers from EBay and these were letting air in, so I used the old ones and they were fine. I still had a problem so stripped them down again for the third time. This time I used an airline on everything, I could feel that the air wasn’t passing through the pilot jet (mixture) hole as well as it was on the other 3 so I used a piece of wire to clean the hole out until the air was flowing through equally. The only other thing I found to be wrong was two of my main jets were the wrong way around. I had 2x125 jets and 2x125.5 jets and the two larger jets had to be in the centre carbs.
I didn’t see what was blocking the pilot jet it was so minute, but these two things combined fixed the carbs.
It may be worth stripping and cleaning again, it was for me.

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