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  #1  
Old 21st October 2021
sml1226 sml1226 is offline
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Default HSR idle mixture screw

Hey guys. I have an HSR42 that I’ve never been able to get right. I stuck an O2 sensor on to check AFR here recently and it doesn’t seem to want to move any while turning the air screw. I’m not really sure if it’s always been like this or not but I can’t really tune the rest of the carb if the idle is never right.

The needle, needle jet, main, etc shouldn't be affecting me here, should they? I’ve had a 20 and a 22.5 in there so far and both seem to settle right around 13:1. I can get it to go richer, but even there, not a ton. I think it hit 11.5:1 before it stumbled.

So I guess the first question is, is there anything that would lead to the screw being largely useless? I can’t see anything blocking the air passages. The air jet is a blank on these. The choke plunger, as far as I can tell, is fully seated. An air leak somewhere would have the opposite issue.

I can try a 17.5 jet too, as I do have one, but I’d still expect the AFR to move even if the jet were too large.

Any other suggestions?
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Old 21st October 2021
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Never touched one, but maybe this will help.

https://r.search.yahoo.com/_ylt=AwrV...ZMtGKc5MNZa.Q-
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Old 21st October 2021
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Just a thought, the choke plunger on that if it’s the modified H.D. Cable can hang up and that would give you a richer mixture. The Mikuni cable, is a much better alternative and recommended.
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Old 21st October 2021
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Quote:
Originally Posted by sml1226 View Post
Hey guys. I have an HSR42 that I’ve never been able to get right.

Any other suggestions?
Are you turning the screw the right direction? On the HSR 42 it's an air screw. Turning it in richens the mixture, out is leaner. The usable range is normally between 1/2 and 2 1/2 turns out. The off idle transition is frequently best with a pilot jet that has a best idle with the screw 1 - 1 1/2 turns out.

The other thing that comes to mind is that like having the throttle blade open too far on the CV carbs, you can have the slide lifted too high on a slide type and have it pulling fuel thru the transfer ports.

Make sure you have enough ignition advance at idle, 12 - 15 degrees, that helps it idle with less slide opening.

Idle AFR is interesting but best idle is with the mixture screw setting that gives the highest manifold vacuum, or the lowest MAP. I would expect "best idle" AFR's to be in the lowish to mid 12's.
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Old 22nd October 2021
sml1226 sml1226 is offline
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Mikuni cable is on there (yes, Mikuni spring and plunger too). I never liked the Keihin cable on these carbs prior to the swap. As far as I can tell, that’s good.

And yeah, I know it’s an air screw. I’m not so much trying to tune it for a specific AFR as I am confused by the lack of change as I do turn the screw. But tuning it by MAP is something I hadn’t thought of and if I can ever get it working, I’ll be sure to run my monitor and look at that while setting it. As for the slide height at idle, that was honestly my big concern. The AFR wouldn’t do much with the screw if it’s already on the needle jet. The slide is pretty low as is, but I can’t say for certain on that one. I’m not entirely sure where it starts pulling from that circuit on these just by looking. It is as low as I can get it to idle, but I suppose if it’s that much too lean on the pilot, I could potentially have turned the adjuster in enough to lift it past the idle circuit height.

And I’d have to check on the ignition. I haven’t paid that much attention here.

Thanks for the suggestions though. I’ll see what my ignition map looks like if it needs any tweaking around idle and see if going richer on the pilot lets me lower that slide any more than it is now.
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Old 23rd October 2021
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i have a question, are you using the correct heated sensor?
a sensor that cannot maintain a set temp is a poor choice as the O2's will be off. also remember how the engine is fueled, it is not a steady state stream and each cyl can/will effect the other.
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Old 23rd October 2021
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I would assume it’s heating properly, but yes it’s a heated wideband sensor feeding a gauge designed for that particular sensor. I forget the model but it’ll run a short little “heat” cycle at the gauge before reading the ratios.

And absolutely. The idle is probably going to be the least reliable AF number I see, but I still expect movement of the number as I turn that screw. That was the biggest concern, and my only thought that made sense was that somehow I wasn’t running on the idle circuit at all. I haven’t had a chance to make changes and see if they help yet, but unless something’s screwy in the carb, that’s still where I’m leaning.
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Old 24th October 2021
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Quote:
Originally Posted by sml1226 View Post
...my only thought that made sense was that somehow I wasn’t running on the idle circuit at all. I haven’t had a chance to make changes and see if they help yet, but unless something’s screwy in the carb, that’s still where I’m leaning.
One way that happens is because the slide lifted too far to keep it idling because your idle mixture is way off or you've got too little ignition advance at idle.

This is where a vacuum or MAP gauge really helps. You adjust the air (mixture) screw to get the highest vacuum / lowest MAP. That normally raises your idle speed so you lower the slide to bring your idle speed back down and repeat the process as the lowered slide will change how the idle circuit / transfer ports pull fuel. When the transfer ports are pulling fuel the effect of the air screw is diminished.

It's kinduva a dog chasing its' tail process but that's how you get it right.

You also need to keep track of how many turns out the air screw is and change the pilot jet size so best idle is with the screw 1 - 1 1/2 turns out.

What cams are you running?
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