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Valve cutaway?

Tuning

 
 
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  #1  
Old 08-27-2009
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Default Valve cutaway?

So last week a friend came over and was checking out the RXL, which is in the middle of a carburator swap.

Now I've installed rack mounted flatslide carbs off an SRX700 Yamaha triple. I had to change the throttle cable mount to work with the polaris throttle cable and used a Yamaha fuel pump for the same machine, also had to modify the coolant rail to accept the carbs all the way into the carb boots, and modified the airbox(though I think I'm going to run K&N filters instead. I've fired it up and ran it in the garage quite a few times but never rode it.
Idle is shitty and have to play with throttle to keep it idling. Throttle response just feathing the throttle and jamming to wot was VERY fast compared to the old EFI system I took out. Revved to high rpms very quickly and I was very happy. I figured when I switch it all over it should run good, once I get it to idle correctly.

He took one look at it and asked me if the throttle valve cutaway was the same. I was kinda baffled and told him I didn't know. He said it won't run right if the cutaway is different. I've done some research and found very little about valve cutaway but found it does make a difference 1/8-1/4 throttle like jetting would.

I'm in the process of swapping everything over to a better tunnel/bulkhead and curious if it will work? I'd hate to get it all swapped over just to find that I have to reinstall everything for the EFI. I think it should work so I plan to keep on track with the conversion, but curious to see what everyone thinks. Am I going to run into a problem? Could this possibly be why it wouldn't idle on it's own?
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Last edited by Gotmud : 08-27-2009 at 08:01 AM.
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  #2  
Old 08-27-2009
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You can buy other sized cutaway slides.

Yes, that may be the reason it doesn't idle correctly.
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Old 08-27-2009
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who makes the carbs? they should make slides with different cutaways.....
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Old 08-27-2009
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I'll have to look into different slides. Is there a way to measure to find what the cutaway is? Not sure if the throttle bodies are different?
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  #5  
Old 08-27-2009
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Now the question is are you not getting enough air or are you getting too much air? To me it sounds like not enough air and your choking the motor and by feathering the throttle you're keeping her alive. Just my thoughts.
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Old 08-27-2009
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Quote:
Originally Posted by MX86 View Post
who makes the carbs? they should make slides with different cutaways.....
They are Mikuni TM33 rack mounted carbs.

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Originally Posted by Triple650indy View Post
Now the question is are you not getting enough air or are you getting too much air? To me it sounds like not enough air and your choking the motor and by feathering the throttle you're keeping her alive. Just my thoughts.
Exactly why I thought it wouldn't idle and I didn't mess with it at the time. It acted like it was starved for air at idle and I just assumed it was the air screws. If I either feathered throttle or opened choke a little bit it would idle, rough but idle. I didn't mess with any jets or adjustments when I did run it, I ran it set up for the SRX700 engine figuring they should be about the same for jetting.

I just found this and learned a little something. I'll have to wait until I get it back together to see if the valve will work or not for sure.http://www.iwt.com.au/mikunicarb.htm
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  #7  
Old 08-27-2009
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Unless I'm mistaken, just take the slide out and put it on a flat surface, and measure the top of the cutout to the surface.
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Old 08-27-2009
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That link I posted said it should be stamped in the valve.
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The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).
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"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

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Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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  #9  
Old 09-29-2009
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We cut our own slides as long as you need to go higher (i.e. 1.5 to 3.0) , you can scribe and "cut it away". You'll find it makes the most difference just off idle in the 5000-5500 rpm range.
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Old 09-29-2009
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Sweet I'll pay attention to it whenever I finally get it back together lol.
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"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

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Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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  #11  
Old 09-30-2009
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Check www.sudco.com out
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  #12  
Old 11-25-2009
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What benefit does flat side carbs have vs. round Mikunis? It seems to me the round carbs are hands down more reliable and simple to rebuild/clean. Everything I have worked on with the flat carbs has been a pain in the butt.
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  #13  
Old 11-25-2009
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Quote:
Originally Posted by VanDamage007 View Post
What benefit does flat side carbs have vs. round Mikunis? It seems to me the round carbs are hands down more reliable and simple to rebuild/clean. Everything I have worked on with the flat carbs has been a pain in the butt.
the tm and other rack mounted mikunis are more tunable than the round slides, better throttle response and flow more air than a same size round slide. they are pretty much better in every way just need to take time to learn them.
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  #14  
Old 11-28-2009
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Face it mud, the RXL is a piece. You will never have it working right, I am going to start charging you for blown belts when I have to pull it home again.
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  #15  
Old 11-29-2009
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just like stormy the legend of an rxl goes back into the day. many old rxl's and 650's set performance standards that made you love stormy so much.
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  #16  
Old 11-29-2009
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Quote:
Originally Posted by Dirty_Harry View Post
Face it mud, the RXL is a piece. You will never have it working right, I am going to start charging you for blown belts when I have to pull it home again.

WTF you tow me home 1 fuckin time and I get this shit? You just wait until I have to tow your ass, and that'll be this year.
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"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

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Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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