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Really High Idle CB250RS

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JamesKMX99
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PostPosted: 22:48 - 22 Jul 2017    Post subject: Really High Idle CB250RS Reply with quote

Hi,

So I just fitted my bike with a pod filter. I am doing a cafe racer build.

I turned the screw clockwise until it turned no more and started the bike up and the idle was ridiculously high, I do not have an exact RPM but it was not the same as with the air box on.

How do I lower the idle?

Thanks
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Stoker
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PostPosted: 23:20 - 22 Jul 2017    Post subject: Reply with quote

Turn it anti-clockwise?
Turning the idle speed screw clockwise (inwards) lifts the slide, allowing more air/fuel into the engine, thus raising its tickover speed.

Also, fitting an after-market filter such as a pod or cone type will affect the fuelling, you may need to change the jets and needle valve to match the different airflow characteristics
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RhynoCZ
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PostPosted: 23:33 - 22 Jul 2017    Post subject: Reply with quote

Never touch the idle speed adjuster when the engine is off and/or cold. Warm engines with carbs have higher idle speed than when cold.

Lean mixture also raises the idle speed. Other side effect of lean mixure is higher engine temperature (not good, especially with air cooled engines).
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Teflon-Mike
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PostPosted: 04:00 - 23 Jul 2017    Post subject: Re: Really High Idle CB250RS Reply with quote

JamesKMX99 wrote:
So I just fitted my bike with a pod filter. I am doing a cafe racer build.

Heathen fashion victim. 250RS was the most humble looking yet the pokiest of the Honda 250's, a device most suited to being kept standard as a 'sleeper' with which to humble more modern sheep-in-wolf's clothing 125 & 300 'spurts' bikes Laughing

JamesKMX99 wrote:
I turned the screw clockwise until it turned no more and started the bike up and the idle was ridiculously high, I do not have an exact RPM but it was not the same as with the air box on.


What screw you diddling? The idle speed 'slide-stop' or the primary air 'mixture' screw?

I'll ask again... are you sure?

If you are adjustng the primary air screw... Eeek... could be a nadger... take it all the way out and make sure it has the really thin needle on the end! They have a habbit, not needing adjustment of corroding the carb body, so they shear when messed with after... what? Thirty years?

Other wse, adjust idle speed o the slide-stop screw.. and always lift the slide with the twist-grip before adjusting the set=screw, and then letting the slideback down onto the stop and letting dle settle before repeating to make fine adjustment.

If you let the slide fall when you wind stop out slide can stick in the body and ot settle, and when you have done, the ide can end up well below what yu thought it was; if you try lift the idle screwing stop 'in' you can end up just jamming the slide in the body and denting the stop-set ramp rather than lifting the slide... so do that o the cable...

Make sure the cable has some slack in t on the top adjuster first too, then take out the free-play after setting the idle stop to suit.


Oh.. plus make sure motor warm; and don't assume you need to make micture rich to compensate for the pod; with air flow from the side having to turn 90 degrees in small volume insde teh filter, so close to carb mouth, than are often more restrictive, not more free-flowing than an air-box, and causing turlace in the carb mouth can screw the mixture either way over the jets, and rev range!

BUT more likely pushed onto the carb mouth 'cock' either the filter or the turbulant flow may obscure or mask the pilot air drilling front to back through the carb.. making sure the pilot air port has clear flow is probably as much 'set-up' as is needed TBH.
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iansoady
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PostPosted: 10:36 - 23 Jul 2017    Post subject: Reply with quote

RTFM comes to mind......
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Rogerborg
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PostPosted: 10:56 - 23 Jul 2017    Post subject: Re: Really High Idle CB250RS Reply with quote

JamesKMX99 wrote:
I turned the screw

Which one?

JamesKMX99 wrote:
clockwise until it turned no more

Why?

JamesKMX99 wrote:
started the bike up and the idle was ridiculously high

What did you expect to happen?

JamesKMX99 wrote:
How do I lower the idle?

Put it all back the way it was then step away from the bike.
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JamesKMX99
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PostPosted: 11:33 - 23 Jul 2017    Post subject: Reply with quote

Stoker wrote:
Turn it anti-clockwise?


I turned the whole screw anti clockwise, ran it for 1 minute and no change in idle. This is with choke off
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RhynoCZ
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PostPosted: 11:37 - 23 Jul 2017    Post subject: Reply with quote

JamesKMX99 wrote:
Stoker wrote:
Turn it anti-clockwise?


I turned the whole screw anti clockwise, ran it for 1 minute and no change in idle. This is with choke off


Is the throttle moving properly? Can you hear the carburetor click when you open the throttle and let go? Your throttle might be stuck open.

The other option would be a huge air leak in the rubber manifold between the carburetor and the head. BUT, you would still be able to get normal idle, (it would take some time for the carb to find it, but it would eventually).
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'95 Mercedes-Benz w202 C200 CGI; MZ 150 ETZ, '98 Mercedes-Benz w210 E200 Kompressor


Last edited by RhynoCZ on 11:40 - 23 Jul 2017; edited 1 time in total
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JamesKMX99
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PostPosted: 11:38 - 23 Jul 2017    Post subject: Re: Really High Idle CB250RS Reply with quote

Teflon-Mike wrote:
JamesKMX99 wrote:
So I just fitted my bike with a pod filter. I am doing a cafe racer build.

Heathen fashion victim. 250RS was the most humble looking yet the pokiest of the Honda 250's, a device most suited to being kept standard as a 'sleeper' with which to humble more modern sheep-in-wolf's clothing 125 & 300 'spurts' bikes Laughing

JamesKMX99 wrote:
I turned the screw clockwise until it turned no more and started the bike up and the idle was ridiculously high, I do not have an exact RPM but it was not the same as with the air box on.


What screw you diddling? The idle speed 'slide-stop' or the primary air 'mixture' screw?

I'll ask again... are you sure?

If you are adjustng the primary air screw... Eeek... could be a nadger... take it all the way out and make sure it has the really thin needle on the end! They have a habbit, not needing adjustment of corroding the carb body, so they shear when messed with after... what? Thirty years?

Other wse, adjust idle speed o the slide-stop screw.. and always lift the slide with the twist-grip before adjusting the set=screw, and then letting the slideback down onto the stop and letting dle settle before repeating to make fine adjustment.

If you let the slide fall when you wind stop out slide can stick in the body and ot settle, and when you have done, the ide can end up well below what yu thought it was; if you try lift the idle screwing stop 'in' you can end up just jamming the slide in the body and denting the stop-set ramp rather than lifting the slide... so do that o the cable...

Make sure the cable has some slack in t on the top adjuster first too, then take out the free-play after setting the idle stop to suit.


Oh.. plus make sure motor warm; and don't assume you need to make micture rich to compensate for the pod; with air flow from the side having to turn 90 degrees in small volume insde teh filter, so close to carb mouth, than are often more restrictive, not more free-flowing than an air-box, and causing turlace in the carb mouth can screw the mixture either way over the jets, and rev range!

BUT more likely pushed onto the carb mouth 'cock' either the filter or the turbulant flow may obscure or mask the pilot air drilling front to back through the carb.. making sure the pilot air port has clear flow is probably as much 'set-up' as is needed TBH.


https://i.imgur.com/MplpRYL.gif

The highlighted red it what im twisting. I have turned it all the way out and the idle is still very high
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JamesKMX99
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PostPosted: 11:44 - 23 Jul 2017    Post subject: Reply with quote

RhynoCZ wrote:

Is the throttle moving properly? Can you hear the carburetor click when you open the throttle and let go? Your throttle might be stuck open.

The other option would be a huge air leak in the rubber manifold between the carburetor and the head. BUT, you would still be able to get normal idle, (it would take some time for the carb to find it, but it would eventually).


Yes the throttle is moving properly, its the first thing I checked. There is no air leak also.
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Rogerborg
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PostPosted: 12:23 - 23 Jul 2017    Post subject: Reply with quote

It's a slide carb so the only reason for it to be revving hard is that the slide isn't free to move all the way down.

What I suspect has happened is that the slide has rotated so that the angled ramp part of it is no longer above the idle speed screw and the whole slide is sitting on top of a very slightly protruding part of the screw. If you can remove the idle screw completely it should drop down and confirm that (catch the spring).

Alternatively, you've managed to pull the needle clear of the main jet and it's stopping the slide moving all the way down.

At this point I'd take it apart and see what's going on. You should be able to sort it just be taking the cap off, withdrawing the slide and re-inserting it so that the ramped part of the slide is above the idle screw, and checking that it slides freely all the way down.

This is assuming that you haven't been messing around with the throttle cable as well. No chance that you've strained it or pinched it under the tank?
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike
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Teflon-Mike
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PostPosted: 13:27 - 23 Jul 2017    Post subject: Reply with quote

Rogerborg wrote:
It's a slide carb


NO! No it isn't!

I assumed it was too, but if its the same carb as that CMSL schematic... its a hybrid CCV.. 'mongrel' cam-assisted vaccum-lift slide ... eek!

I which case .... All bets off... these things are a law unto themselves.

I have on in the shed, that was on one of the Super-Dream motors, but I think originally from a CM.... the idiosyncrasy, and quirkiness and a pre-mullered butterfly is why it's in the shed! It too had a pod filter on it; almost imposible to set up anything like, I chucked it in the bits box in disgust and fitted a couple of plain PD slide carbs!

At a guess part 10 isn't adjusting the 'idle' but the initial initial lift ramp of the slide.. if pod is choking the flow at the carb-mouth, as they are want, then it's causing excess manifold vacuum at the carb entry, and if so, that could be lifting the slide more than it should hence lifting it of the assist ramp,screwing the syncronisatn of the mechanism....

Sub-Assy 6 look like the idle-air needle; the one with very fine brass needle wont to corrode and shear in the body... be effin careful if you touch that.

Sorry.... no helpful hints on this one... other than either ditch the pod, and or go to a 'proper' carb! Horrible mongrel hybrid cantankerous, quirky illogical over complex techno blunders these things!

ALCOHOL.. advice large doses! Wont help... b-u-t.. in the morning at least you'll KNOW why you have a head-ache Laughing
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'
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JamesKMX99
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PostPosted: 23:54 - 23 Jul 2017    Post subject: Reply with quote

Teflon-Mike wrote:


Sorry.... no helpful hints on this one... other than either ditch the pod, and or go to a 'proper' carb! Horrible mongrel hybrid cantankerous, quirky illogical over complex techno blunders these things!



I have a few ideas from this thread on things to try. If i have no success what carb would you recommend? Being new to building bikes ETC would it be a "plug and play" ordeal or would modifications to the engine need to be made?

Thanks
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JamesKMX99
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PostPosted: 23:54 - 23 Jul 2017    Post subject: Reply with quote

Also the diagram is the same carb on the bike Smile
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salem1987
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PostPosted: 07:28 - 24 Jul 2017    Post subject: Reply with quote

had the same bike with pod filter. Ran really well with.......

130 main jet
disable accelerator pump
mixture set to 3 turns out


This was perfect for my high mileage, slightly oil burning rattly engine. May be slightly different for yours.
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