Mitch81special
XS650 Enthusiast
Maybe a dumb question but will the position of the fuel mix screw affect how well the bike cold starts?
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Fuel mix and starting
- Thread starter Mitch81special
- Start date
I would guess very little, if at all. The choke (enrichment circuit on our CV carbs) richens the mixture quiet a bit.
Mitch81special
XS650 Enthusiast
Thanks Jim that's what I figured. Iv been playing with my carbs for the past few weeks and I'm having trouble getting it dialed in. Turning the mix screw out makes it richer right? (Bs34's)
Correct.
Here's a rule of thumb...
If the screw is between the intake and the slide, it's a fuel/air mix screw. Opening up the screw adds more fuel.
If the screw is between the slide and the air filter, it's an air/fuel mix screw. Opening up the screw adds more air.
Here's a rule of thumb...
If the screw is between the intake and the slide, it's a fuel/air mix screw. Opening up the screw adds more fuel.
If the screw is between the slide and the air filter, it's an air/fuel mix screw. Opening up the screw adds more air.
Mitch81special
XS650 Enthusiast
Jim if you or anyone else would care to have a listen here's a couple videos. First one is 3 turns out and there's this slight burp/pop I can't figure out but it's only on the left side. Second video is at 4 turns out. That burp seems to go away but it's my understanding 4 turns out is way to much.
Well, the factory setting for the -34's is 3 to 3-1/2 turns, so I wouldn't call 4 turns too much.
My speakers suck, so I can't really tell much from your vids. If it sounds happy at 4 turns, let it be.
My speakers suck, so I can't really tell much from your vids. If it sounds happy at 4 turns, let it be.
Mitch81special
XS650 Enthusiast
If it sounds happy at 4 turns, let it be.
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This is the advice I needed. Thanks, I over think things
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This is the advice I needed. Thanks, I over think things
Just noticed in the vids you're using the K&N style pleated filters. They've been known to not play nice with our BS carbs. You might want to try some foam pod filters.
Mitch81special
XS650 Enthusiast
Just got some yesterday!
One other thing... it's best to rev the engine a time or two 'tween adjustments.... kinda let it clear its throat if you will, then see how it sounds.
Sounded pretty good to me even at 3-1/2. What’s your idle speed?
Mitch81special
XS650 Enthusiast
12-1400. It seems like I need to turn it up to get it started then turn it down as it warms up
To warm up my ‘76 I do turn the idle up for a few minutes then turn it back down to 1200 (approx) when at operating temperature. As Jim says, the enriched choke circuit provides quite a rich mixture and I too doubt thr mixture screw would have a marked effect on startup.
I see you also changed the mufflers? Have you re-jetted for these changes (air filters and exhaust)? Having to run the mix screws more open than the spec is an indication you may be too lean and need to change the pilot jets. On the BS34s, you can also fool with the air jet to tune the idle circuit. Decreasing it's size will richen the idle circuit a bit. It doesn't have as big an effect as a pilot jet size change so it's useful for fine tuning.
There was never a factory setting given for the BS34s because they were set and capped over from the factory, and this was all E.P.A. mandated. Once you uncap them, you would usually find them set around 2 - 2.5 turns out, not ideal but needed to pass emissions. Through experimentation, we found they run best at 3 to 3.5 turns out. But this is on a stock bike with stock jetting and no mods. Start re-jetting richer and the screws may not require as many turns out. My '83 with the typical "pipes and pods" mod likes about 2.5 turns out. This is with the pilots increased one size, air jets reduced one size, and the mains up 2 sizes.
Wow .the second video at 4 turns ( First video stumbles ) is pretty much perfect 8 on a scale to ten
It is a bit high revving but otherwise I have never in close to 40 years had it that good
If that is on a warm engine without choke I would set it down a bit at least for getting into first better
After some riding the plugs will most likely go black and the idling will be affected
getting better again after a long ride
After some short rides it is worse black again
A little worn carburetors and so --- I don't think it ever gets better than that.
It is a bit high revving but otherwise I have never in close to 40 years had it that good
If that is on a warm engine without choke I would set it down a bit at least for getting into first better
After some riding the plugs will most likely go black and the idling will be affected
getting better again after a long ride
After some short rides it is worse black again
A little worn carburetors and so --- I don't think it ever gets better than that.
Mitch81special
XS650 Enthusiast
I'm went up 1 size in the pilot and main. 45 and 135. I never thought about reducing the air jet, but that makes sense. I'll order a size down and see what that does.
Also, there's an o-ring on your mix screw. If it's old (original) and all hardened up, the screws may not function properly. The o-ring seals the screw into the carb body and if it's bad, you may be sucking some air in. That might explain it running better with the mix screws 4 turns open.
Last thing is the butterfly shaft seals. They work on vacuum, being sucked in tight and sealing by the air flow through the main bore. New, they are very soft but they harden up with age and don't work so well anymore. The faster you go (higher RPMs), the harder they get sucked in and the better they seal. That means at idle, if they're old and hard, they may not work very well.
If your carbs are U.S. spec with the fixed needle, you could also probably do with another size up on the mains. If they are Canadian spec carbs with the adjustable needles then the 135 may be fine. Also, if they are Canadian spec carbs, you may already have smaller air jets. The U.S. carbs got 135s but the Canadian ones may have 130s. If that is the case then I'd look at the rubber bits, they may be your issue.
Last thing is the butterfly shaft seals. They work on vacuum, being sucked in tight and sealing by the air flow through the main bore. New, they are very soft but they harden up with age and don't work so well anymore. The faster you go (higher RPMs), the harder they get sucked in and the better they seal. That means at idle, if they're old and hard, they may not work very well.
If your carbs are U.S. spec with the fixed needle, you could also probably do with another size up on the mains. If they are Canadian spec carbs with the adjustable needles then the 135 may be fine. Also, if they are Canadian spec carbs, you may already have smaller air jets. The U.S. carbs got 135s but the Canadian ones may have 130s. If that is the case then I'd look at the rubber bits, they may be your issue.
Mitch81special
XS650 Enthusiast
Thanks for the input 5twins. I did change the o rings on the mix screws. Also changed the outside shaft seals but I was a bit nervous removing the butterfly plates and I didn't do the inside ones.
I have the adjustable needles set to 3rd slot from the top. And iv got 135 air jets in currently. I think I'll try running it like this and see how it goes(maybe swap the air jets cuz that easy). On to the next task which I think is brakes and sprockets.
I have the adjustable needles set to 3rd slot from the top. And iv got 135 air jets in currently. I think I'll try running it like this and see how it goes(maybe swap the air jets cuz that easy). On to the next task which I think is brakes and sprockets.
