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rotax 604 carb phm; anti surge washer and idle jet
Topic Started: Oct 15 2016, 09:06 AM (1,008 Views)
housewinnie
Knight
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Hi Again and so sorry for keeping on going on about carb probs but here goes again.

Have ordered new jets etc from Eurocarb and planned to strip, clean and replace in one final attempt to get her running well.

Ordered a new idle jet size 40 to replace the 40 which I have but its alot smaller in length and also my anti surge plate is cut away to accomaodate this jet, is this how it should be or is it something someone has done and could this account for my bike running rich? please see pics.

I am wondering whether iI should get a new anti surge plate with no cutaway?

Cheers so much again

Mark

[IMG][URL=http://s347.photobucket.com/user/housewinniehousewinnie/media/DSCF0188_zpsctrgwxkp.jpg.html]Posted Image[/URL][/IMG][IMG][URL=http://s347.photobucket.com/user/housewinniehousewinnie/media/DSCF0189_zps1v68inyc.jpg.html]Posted Image[/URL][/IMG]
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stevo604
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God
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the cut out is standard on the surge plate on the phm40 carb

if it were me I would get the right length of jet

I bought a rotax lately which had been sitting for 4 years or more a drove it home with fresh petrol and cut out twice on me
as if it was running out of petrol,checked float height and carb was clean .
it turned out to be a dud plug the whole time
Edited by stevo604, Oct 15 2016, 10:04 AM.
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Gonzo250
Member Avatar
Working-class moto psycho
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I had the same conundrum re: the shorter jet. Put it in anyway and all has been well. I wouldn't worry.
I drink too much, I swear too much, I ride too fast and I crash too often.

It is a universal impossibility for me to leave my tools alone and my bikes standard. 'It must go faster!!'
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housewinnie
Knight
[ *  * ]
Just out of curiosity the cut out anti surge washer is it important and was it something CCM customised themselves as the only ones I can find have no cut out?

Also would it make a difference if I replaced it with one without a cut out?

Cheers

Anyway just waiting for a new choke jet and needle as I thought I would replace these too as have replaced everything else, maybe should have bought a new carb nevermind.

I may have found the reason though why she has always run rich as the mixture screw was only half a turn out and the throttle idle screw was seized, felt like I was going to cross thread that one carefully going in and out but it winds in and out fully by hand now so perhaps things are looking up.

cheers again for the help

Edited by housewinnie, Oct 18 2016, 01:53 PM.
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vinny900
Marquess
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The big copped washer is an anti surge/ anti vapour lock device. Its job is to make sure that there is always a good pure supply of fuel around the main jet. Surges from acceleration ( although these bikes cannot outaccelerate gravity) and air bubbles or fuel vapour will affect the main jet less if one of these is fitted. I dont know much about these carbs but maybe the washer thing without the hole is used with the shorter pilot jet, and the longer pilot jet needs a washer with a cutout in it.

On the Dellorto carbs the mixture screw you are speaking about is just that. It controls the idle mixture. Further out equals more fuel mix ( already mixed with air) so 1/2 a turn out would mean a very lean idle mixture. Maybe the idle was set high with the idle screw and the mixture screw set in to reduce the high idle.
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vinny900
Marquess
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If you look at this link you will see what i mean about the idle jet and fully round antisurge device - note short idle jet

http://advrider.com/index.php?threads/rebuilding-a-dellorto-phm-sd-carburetor.635497/

I have not seen one of these carbs in the flesh but they are similar to the dell orto trial carbs, and usually have a drilling right at front of the engine side carb mouth which is the idle mixture feed, when throttle is fully closed. This is what is controlled by the mixture screw.
When idling the slide should be right down into its recess, so the little holes behind the slide are covered up. This is why you need to adjust the idle mixture with the screw, with the slide down as much as possible. If the slide is up a bit, then fuel will be drawn from the rearward holes, which combined with mixture coming in from the front idle hole, will make the idle too rich, so often the idle mixture screw is closed to compensate.

This may not be the case with your carb, but is generally true. I would start with the idle mixture screw out 1.5 turns, and slide down. Then turn throttle screw until (hopefully) you get a nice clean tickover at factory suggested revs
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housewinnie
Knight
[ *  * ]
Thank you for the reply, gradually little by little I am starting to understand how the carb works, just to clarify as I am still getting a little confused the mixture screw on my carb controls the air coming in so if it is wound right in that would mean no air and a rich mixture?

Cheers again

Mark
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Gonzo250
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Working-class moto psycho
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Wound right in, you wouldn't get any fuel at idle. The way I understand it, is it allows the air through to enable the fuel to be sucked up through the idle circuit, sort of like a spray gun. No air flow = no liquid flow.
I drink too much, I swear too much, I ride too fast and I crash too often.

It is a universal impossibility for me to leave my tools alone and my bikes standard. 'It must go faster!!'
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vinny900
Marquess
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Mixture screw on dellorto carbs ( the one nearest the engine) controls the amount of mixture ( already mixed with air) that can get out through venturi drilling between slide and engine.
On most other carbs the screw which is on the air filter side controls the amount of air that is allowed to mix with the fuel in the pilot ( idle and low revs) system.
So air screw - more out more air = leaner mix. Mixture screw - more out more mixture ( fuel and air) = richer
The normal rule is that if the control screw is on the engine side, it is a a mixture screw, and if it is on the filter side it is an air screw. Most 4 strokes have a mixture screw, most 2 strokes have an air screw.

I only found this out when playing with 2 stroke trails bikes, where the Dellorto PHBL carb is commonly used, which has a forward mixture screw, despite being used on a 2 stroke.

As mentioned before, the front drilling in the venturi supplies the fuel/air mix for tickover, as slide should be almost completely closed, blocking off other drilling(s), so this needs a separate control for the mixture.

When the slide is lifted the vacuum runs across the other drilling(s) ( should be just in front of the needle hole) and pulls mixture out of this hole. Sometimes there is more than one hole, or a bigger hole, as more mixture is required as revs increase, but needle has not lifted enough to provide more fuel through needle jet. On most carbs the mixture screw only controls the flow to the front hole, the rear holes being controlled by fixed pilot jet and fixed air jet ( you can buy different sizes of both)

As the slide moves up further the taper on the needle allows more fuel through the needle jet as revs rise, although some fuel is still drawn through the pilot holes. When the slide is fully up, the needle taper is out of the needle jet and the size of the main jet is controlling the amount of fuel at full throttle.
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housewinnie
Knight
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Clearer and clearer, its all starting to make sense.

Thank you all so much.
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vinny900
Marquess
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If you really want to understand how the carb works in principle then save and print out this little gem and read it instead of watching eastenders tonight

http://www.rotaxjetting.com/PDFs/Documents/The%20Old%20Dellorto%20Tuning%20Manual.pdf

Further down the pages it shows some diagrams of how the fuel and air go through the carbs at different throttle positions. You can probably work it out but the yellow stuff is fuel, blue stuff is air and green is atomised fuel/air mixture

You can see in these diagrams how the two different control screws work.


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Sportax Racing
Anything & Everything ROTAX
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The longer pilot jets were for off road bikes - mainly trials bike that go up or down steep hills for example. The jet sits in deeper fuel witch is less likely to be starved of fuel at a steep angle...

Either size will work perfectly on your CCM Rotax - providing it is of the correct size and I'd say 40 is lean from what we learnt on the Dyno..
www.sportaxracing.co.uk
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housewinnie
Knight
[ *  * ]
Excellent again and thank you for the help.

Will maybe start with a 45 then and go from there, would it make much difference if I use a surge washer without the cut out?

Cheers
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tommysdad
Earl
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I had similar problems, strip & rebuild with new jets 185 main, 40 idle ,mixture 1.5 turns out,replaced float needle valve (size 300) ,I found its all about setting the float height right, its fiddly but worth doing right.
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I'm a man with a fork in a world of soup
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