Fuel/air screw question

Beck Built Fab

Beck Built Fab

Custom XS650 chopper parts!
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So, last night I was trying to sync my carbs a bit better. I was using the dead cylinder method. While turning the fuel/air mixture screw, I noticed that nothing had changed as far as idle. Throttle response change (very sluggish and boggy until I get past 1/4ish throttle).

What could this be?? Is my pilot jet too big?

I have BS34 carbs. Pilot jet is 42.5. I don't know the size of my other jets.
 
I think with BS34 the peak is more of a plateau, very broad. I'd just set it at the recommended turns, if it works ok there

P.S. your jet is stock size. If I recall correctly, the recommended setting is 3 1/4 turns out, or 11/2 turns if you change to the next larger size jet (45).
 
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When using the dead cylinder method you need to slow the idle speed down as much as you can and still keep the bike running.
The pilot jet feeds an air fuel mix out of three tiny holes near the throttle plates. The mix screw feeds an air fuel mix out of a larger hole between the throttle plates and the engine.
To set the mix screws with the dead cylinder method you need to slow the idle speed down as low as you can. This blocks off the pilot jet feeds out of those tiny holes. This makes the effect of the mix screws more noticeable.
This will get the mix screws set right but won't sync the carbs.
To sync the carbs you adjust the cables(early carbs), linkage on the later linked carbs.
On the early carb once you set the mix screws you hook both plugs up and set the idle speed back to normal then adjust the two cables so they open the throttle plates at the same time.
On the BS34 carbs and earlier linked carbs the idle speed screw is on one carb. The other carb is linked to the first carb by a tab and a spring loaded adjustment screw. Syncing is just getting the second carb to open and close the same as the first carb. You do that with the adjuster screws. This is located between the carbs.
Eye balling this adjustment to get the throttle plates even is done on the bench. Getting the same amount of light around the plates while looking through the carbs or by using a feeler gauge of some sort. I use pieces of paper.
You have yours on the bike so using a manometer works the best. A manometer is easy to make. A 10 foot piece of clear 3/16 tubing and an easy to see liquid. Some use 2 stroke oil, ATF, motor oil or something of that sort.
A stick of some sort, A yard stick works. Find the middle of the tubing. Mount it to the stick so this center is at one end of the stick. Run the tubing to the other end of the stick. Use packing tape. zip ties, whatever is handy to attach the tubing to the stick. Hold the stick up so the loose ends of the tubing are up. I drilled a hole in my stick so I can hang the stick on a nail
Now you put enough liquid in the tubing so as it sets in the bottom of the loop of tubing on the stick, and comes up about a foot up in each side of the tubing.
Most bikes that used the BS34 carbs used carb holders that have vacuum ports on them. A small brass tube sticking out of the holder. One has a hose to the petcock, the other capped of.
Unhook the hose to the petcock, turn petcock to the PRI position. This lets you run the bike. Uncap the other port.
On your new manometer hook one end of the tubing to one of the vacuum ports. Hook the other end to the other port.
Now start the bike. If the sync is way off the fluid will get sucked up into one carb or the other. Hopefully this wont happen.
If it does you need to try a rough adjustment of the screw. Turn it half a turn toward to carb that sucked up the fluid.
Refill your manometer if needed. Try again. If it still sucks the fluid up the same came turn the screw the opposite way a full turn.
At some point of rough adjustment you will get it to run and not suck the fluid out. The fluid may bounce up and down in the tubing. When one side goes up the other goes down. Adjust the screw slowly till the fluid is the same on both sides. Once you get the fluid bouncing the same on both sides your done. Shut the bike down, remove the manometer hoses and reinstall the cap and hook the hose back up. Turn the petcock back to on.
There are fancier ways the make a manometer but this basic model is very accurate.
Leo
 
Ok. So when initially setting idle as low as I can, that is done with both cylinders working like normal, correct? The unplug one and ground the cap to the engine and then go from there?
 
DB90 said:
Ok. So when initially setting idle as low as I can, that is done with both cylinders working like normal, correct?
Click to expand...
No. If you set it as low as you can with both firing, it won't idle on one cyl. Pull one plug wire and then set the idle as low as it will go and still keep running.

Are you running an electronic ignition? If so, be sure to ground the disconnected lead. I just use a spare plug and some safety wire.

IMG_20190812_131902.jpg
 
Jim said:
No. If you set it as low as you can with both firing, it won't idle on one cyl. Pull one plug wire and then set the idle as low as it will go and still keep running.

Are you running an electronic ignition? If so, be sure to ground the disconnected lead. I just use a spare plug and some safety wire.

View attachment 147334
Click to expand...

Ok thank you for thay clarification. That might be where I was going wrong. But I still haven't noticed a single change in the sound of the engine when turning the mixture screw. I definitely should right?
 
The BS34s are highly E.P.A. mandated and stifled so adjusting the mix screws with the dead cylinder method doesn't show much of a change. I don't bother even trying to use it on them. As mentioned, just set the mix screws around the BS34 recommended setting (3-3.5 turns out). Set them both the same. You can test the setting and experiment with other settings close to it using small throttle blips. You want the revs to drop back down to idle quickly and not hang. Also, you want little to no popping out the exhausts. You can start the screws out at around the 3 turn mark and adjust each one in 1/4 turn at a time, and test with the throttle blips. When the symptoms I've mentioned start to appear, that's a sign you're getting too lean with the setting. Open the screws back up until the symptoms go away. If you need to open the screws more than about 4 turns, that's a sign your pilot jets are too small.
 
5twins said:
The BS34s are highly E.P.A. mandated and stifled so adjusting the mix screws with the dead cylinder method doesn't show much of a change. I don't bother even trying to use it on them. As mentioned, just set the mix screws around the BS34 recommended setting (3-3.5 turns out). Set them both the same. You can test the setting and experiment with other settings close to it using small throttle blips. You want the revs to drop back down to idle quickly and not hang. Also, you want little to no popping out the exhausts. You can start the screws out at around the 3 turn mark and adjust each one in 1/4 turn at a time, and test with the throttle blips. When the symptoms I've mentioned start to appear, that's a sign you're getting too lean with the setting. Open the screws back up until the symptoms go away. If you need to open the screws more than about 4 turns, that's a sign your pilot jets are too small.
Click to expand...


Ok. Thank you all for the info! I do have feeling that my pilot jets might be a tad big. It tends to run pretty rich no mater what lol.
 
5twins said:
If you're checking your plugs right after a start-up using the choke, that's why they're black. The choke does that to them.
Click to expand...

I haven't done that. They were brand new about 100 miles ago, maybe? I've checked em 2 maybe 3 times and they have been solid black each time.
 
There's supposed to be a rubber plug or cap over the top of the pilot jet. This allows it to only pull it's fuel supply from the main jet tower, not all the rest of the fuel sitting in the float bowl. Without it, I'm pretty sure you'll run too rich. Do you have those plugs in there?

1miKX9O.jpg


You should also check your main jet size. Stock is 132.5 but there's no telling what some P.O. has installed, especially if they did some mods like pipes and tried to re-jet for them without really knowing what they were doing. Some people seem to think they need huge jet size increases when they do mods and that's just not true. Yes, you sometimes need an increase but usually only by a size or few.
 
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5twins said:
There's supposed to be a rubber plug or cap over the top of the pilot jet. This allows it to only pull it's fuel supply from the main jet tower, not all the rest of the fuel sitting in the float bowl. Without it, I'm pretty sure you'll run too rich. Do you have those plugs in there?

1miKX9O.jpg


You should also check your main jet size. Stock is 132.5 but there's no telling what some P.O. has installed, especially if they did some mods like pipes and tried to re-jet for them without really knowing what they were doing. Some people seem to think they need huge jet size increases when they do mods and that's just not true. Yes, you sometimes need an increase but usually only by a size or few.
Click to expand...

I have those plugs but they are alittle bit dry and starting to crack.

My bike is running semi open pipes and the 2 layer Uni filters. I haven't checked on the main jet size yet. I'll do that sometime this week.

Where is a good place to get those rubber plugs? Also, where can I get replacement mixture screws? The heads on mine are on their way out.
 
You can usually find 4 packs of the rubber plugs on eBay for about $10. Yamaha gets a bit over $5 each for them. But, the mix screws are best bought from Yamaha. You get the complete mix screw assembly (screw, spring, washer, o-ring) for what I think is a very reasonable price, and it's O.E.M. original. Aftermarket replacements usually cost more .....

https://www.partzilla.com/product/yamaha/12R-14105-00-00

As I mentioned, your BS34s were set up very lean from the factory. With your mods, I think the bike would barely run if it wasn't re-jetted already. You really need to check that main jet size. Like I said, someone may have gone hog wild and put a huge one in there. You probably need to bump the pilots up one too. Your sluggish performance below 1/4 throttle seems to indicate that. Basically, I think you're too rich on top, on the mains, and too lean on the bottom, on the pilots.
 
Yes, they'll burn themselves clean but it takes a few miles of running. I sometimes need to clean mine if I've done multiple cold starts with the choke and only short rides.
 
5twins said:
You can usually find 4 packs of the rubber plugs on eBay for about $10. Yamaha gets a bit over $5 each for them. But, the mix screws are best bought from Yamaha. You get the complete mix screw assembly (screw, spring, washer, o-ring) for what I think is a very reasonable price, and it's O.E.M. original. Aftermarket replacements usually cost more .....

https://www.partzilla.com/product/yamaha/12R-14105-00-00

As I mentioned, your BS34s were set up very lean from the factory. With your mods, I think the bike would barely run if it wasn't re-jetted already. You really need to check that main jet size. Like I said, someone may have gone hog wild and put a huge one in there. You probably need to bump the pilots up one too. Your sluggish performance below 1/4 throttle seems to indicate that. Basically, I think you're too rich on top, on the mains, and too lean on the bottom, on the pilots.
Click to expand...

Is there anywhere that sella both the mixture screw and the pilot passage plug? I'd prefer to buy all from one place.

Mike's wants $10 to ship 2 rubber plugs.
 

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