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Messages - Wenman

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1
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 02:21:42 PM »
Quote from: Trigger on July 14, 2020, 02:14:41 PM
Quote from: Wenman on July 14, 2020, 02:03:04 PM
I’m wondering if the return spring is incorrect?

Running out of ideas now

Does anyone have a spare throttle return spring I could purchase? DSS have them as discontinued??

Thanks

Alan

Sliver do the spring Link >>> https://www.davidsilverspares.co.uk/CB750K2-FOUR-1972/part_221951/

Thanks trigg, currently showing out of stock tho 😩

2
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 02:03:04 PM »
I’m wondering if the return spring is incorrect?

Running out of ideas now

Does anyone have a spare throttle return spring I could purchase? DSS have them as discontinued??

Thanks

Alan

3
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 01:55:34 PM »
Quote from: jeremypeachey on July 14, 2020, 01:43:09 PM
I’m clutching at straws here but could there be a little suction pulling the needles up in their holes? If that’s not nonsense, maybe allowing more air via the pilots (1/4 turn anti each) might help.


I understand your logic here, however if the lifters were closing fully and the revs hand that could be a cause.

The revs hanging is being caused by the lifters and subsequent sliders are not falling quick enough

4
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 01:23:07 PM »
Mine operate the same as that and they do slam shut when opened up anymore than 10%.

It’s only occurring when I apply a very small amount of throttle.

I posted a video of it earlier in the feed and you can see where the hang up is

If I open the throttle anymore than 10% the weight and force of the spring slams them shut.

If I apply a very tiny amount of throttle that closing momentum created by spring and the weight etc isn’t present so they are closing but very slowly?


5
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 01:01:29 PM »
Right so carbs off.. slides out all cleaned reassembled, slides move and fall freely off the bike, there is zero friction on the sliders, return spring added and they slam shut.

They still very slowly drop when I apply a very small amount of throttle.. this is def being caused by the engine running, maybe it’s characteristics??

Please if anyone has a CB750 can they try running it apply a small amount of throttle and watch how fast the sliders seat?

Maybe I’m just going mad!

Thanks you

6
CB750 / Re: Hanging idle please help
« on: July 14, 2020, 11:19:16 AM »
Okay so this morning I removed the cables from the carbs ran it and still playing up, then I also removed the pod filters to see if they causing the issue.

Still defective

It seems no matter what I do when I open the throttle slowly raising the lifters only about 5mm they will NOT seat back quickly.

I have done another video which shows the slides themselves very slowly returning to where they should be.

https://youtu.be/yKIJlmy-rHc

Please have a look and see what u think?

Is there anyway of raising the sliders so they don’t go down as far as they do when they slow down by screwing the idle screw in raising the idle but then somehow reduce the idle on the air/fuel screws.

I’m running out of ideas now 😩

7
CB750 / Re: Hanging idle please help
« on: July 13, 2020, 06:31:57 PM »
Quote from: jeremypeachey on July 13, 2020, 06:21:58 PM
Is it possible for the return spring to bind in its shroud (if it has one )?
[/quote

Great idea and on reading this I got very excited to think it might be something simple as that however as previously mentioned it dosent do the fault when engines off and if it was the shroud I’d expect it to do so both engine running and engine off??

Because it fails to do so when off I don’t believe it’s anything mechanical?

Could be wrong tho, have been many times before!

8
CB750 / Re: Hanging idle please help
« on: July 13, 2020, 02:28:45 PM »
I’m pretty confident the internals are clean and gum free, I stripped them down prior to rebuild and there was nothing untoward.. plus I’m a sucker for carb clean so they would have had a fair share of it.

Is there anything that may cause this effect when the engines running like vacuum?
Only it dosent repeat the fault when the engines off.. they seat well and quick. Only does it when the engines running?

Thanks for all your replies.. together we will sort it!!

Alan

9
CB750 / Re: Hanging idle please help
« on: July 13, 2020, 12:21:34 PM »
Quote from: jeremypeachey on July 13, 2020, 11:50:46 AM
Could it be slides gumming up? Mine did when I left it for a long time. I sprayed GT85 or similar via the air box which seemed to help and use Sta-Bil in the fuel.

Worth a try.. I just sprayed them with gt85 and it didn’t revolve interestingly it does NOT do it when the engines off!!!
So whatever is causing it is a result of what’s happening when the engine is running!!

Please help 😂

10
CB750 / Re: Hanging idle please help
« on: July 13, 2020, 11:40:24 AM »
https://youtu.be/wTdmuhMUE1Y

Okay so I think I’ve found the culprit.. you were correct in that the sliders aren’t seating properly.
If I blip the throttle the Return spring and Weight pulls them closed.

If however I slowly open the throttle to say 2500 revs and let go they struggle to fully seat and remain open about 2-3mm. They do slowly go down which can be seen in the last 3 throttle pulls in the video.

What could remedy this? I believe the cables are adequately routed with no sharp bends. Could it be a fault internally inside the carbs bent needle or faulty slider.

Any recommendations would be helpful thanks guys!

Alan

11
CB750 / Re: Hanging idle please help
« on: July 13, 2020, 10:18:34 AM »
Okay thanks for all the replies!

The bike has aftermarket clip ons with venhill Cables. I have a domino aftermarket throttle too.
The venhill cables were cut to length by myself. I have slight play at both the carb end and about 5 degrees at the throttle grip.

Routing is somewhat questionable however there are no sharp bends.

On opening and closing the throttle when looking into the open carb bodies the sliders slam shut as the return spring pulls them down.

If the sliders weren’t closing properly surely I would feel that in throttle grip as if one holds open they all must be open due to all being on the same lifter?

Thanks again I’ll try and get some pics!

12
CB750 / Re: Hanging idle please help
« on: July 12, 2020, 07:27:49 PM »
Thanks for those responses I have some 38’s that I can try so that will be my next move. I feel like it’s not a jetting problem tho!

The issue is I can be going 20-30 mph and when I change gear with the clutch the revs maintain whatever I’m revving say around 5-6000rpm, I also feel like I’m braking against a reving engine.. like having one foot on the brake and the other foot on the accelerator (car reference before someone asks why I’m using pedals lol)

Occasionally when coming to a stop the rpm also remains high when clutch is disengaged??

Alan

13
CB750 / Re: Hanging idle please help
« on: July 12, 2020, 01:20:09 PM »
Okay so went out for a ride today.. it seems a lot better but still getting the occasional hang up??

What is going on.. I’m at breaking point now.. feel like I’ve tried everything and not sure what direction to go.

The only thing I can think of is an air leak in the inlet rubbers but when I had the carbs off I inspected them all and they all looked good??

Don’t really want to start throwing money at it when I don’t know what the exact problem is?

Any more help would be appreciated if you guys/girls have any ideas??

Thanks

Alan

14
CB750 / Re: Hanging idle please help
« on: July 04, 2020, 05:54:31 PM »
Quote from: K2-K6 on July 03, 2020, 05:23:35 PM
Push cable usually has no adjuster,  so just fit and leave.  But that one shouldn't give you a problem unless something is misplaced on brackets.

As it has an affect when you push the top of slides it may be that there's not a parity of the slide home position from assembly. Are you familiar with Bryan's bench synchronisation method?  As that should remove an error like that if present.

Thank you so much!! And thanks to you Bryan as well..

So last night I whipped the carbs off and bench synced them using a red tipped welding electrode 1.6mm (1/16) adjusted all the floats again and went through the jets with a guitar string.
Popped them back on and it’s purring like a kitten.. idle drops back down responsively and it’s sounds great!!

I have the air/fuel mix screw out 1 turn and it seems bang on!

Not had chance to get her on the open road Yet but it’s a far cry from what I was dealing with!!

Once again this forum comes to the rescue! What would I do without you guys!
If you lived closer I’d get you a pint!!

I’ll keep you posted after the run out but needless to say I am very excited!!!

I’ll be clapping for you guys on Thursday night!!!

Cheers

Alan

15
CB750 / Re: Hanging idle please help
« on: July 03, 2020, 06:13:54 PM »
Quote from: K2-K6 on July 03, 2020, 06:04:13 PM
This thread is worth a read too as it looks specifically at accommodations for running without air filter box etc.

http://www.sohc.co.uk/index.php/topic,22031.0.html

It'll be interesting to hear how you get on with it.

Thank you.. I think this is def the best starting point! I’ll be sure to get back and let you know how I get on!

Alan

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