Pilot jet questions

[Tim L]

Hi,

I have put a pair of linked carbs from a later model XS650 on my XS2 and she won't idle.
I cleaned them out pretty well and the only change was that I used the float bowls from an earlier pair of carbs as the pilot jet in one of the bowls was all chewed up.
I can get the bike to fire and run for a few seconds with the choke on but it won't idle and dies after a few seconds. It won't fire with the choke off.
I left the pilot jets from the older carbs in the bowls I used as they were the best I had.
I have tried with a variety of pilot air screw setting, a little different but not much.
I am going to drop the bowls off in the next few days and have a look.
Does anyone have any suggestions?

Many thanks.

Tim.

 

[Classic]

Idle jets varied by a large margin so swapping them between carbs can cause serious problems. Check out the carb section under TECH.

 

[jamesgs4]

you should use the angled carb holders from the later year carbs, otherwise carbs will be sittting at the wrong angle because the early carb holders were straight.

 

[Tim L]

Many thanks for your reply.
Do you know if the float bowls changed much between 76 and 79 model carbs?
If they were the same I will get some 79 pilot jets from the XS650 shop and try them.

 

[5twins]

'76-'79 carbs (and bowls) used the same type pilots, the VM22/210 style. '76-'77 ran a #25, '78-'79 used a #27.5. '75 and earlier carbs used the BS30/96 type pilots sized in the low to mid 40s. You have to use the correct pilot type (and size) with a bowl designed for it .....

The difference is in how the air is fed to the jet. If you're using early bowls ('75 or older) then you'll need the BS30/96 pilots (sized in the 40s) to go in them, the VM22/210 style will screw right in but won't flow correctly.

'76-'77 bowls are the same as '78-'79 ones as far as how the pilots get their air flow. The difference is in '78, the overflow tube inside the bowl was removed and so was the overflow nipple on the bowl bottom.

 

[Tim L]

So, is it okay to use 27.5 pilot jets in 1979 carbs with 1977 bowls?
I have got 25's in at the moment and my XS2 won't idle for longer than a few seconds, either with the choke on or off.

 

[5twins]

Yes, you want at least 27.5 VM22/210 type pilots in '78-'79 carbs. That was the stock size for that 2 year carb set. If you have mods (air filters and exhaust), you'll probably need 30s or 32.5s.

All the years are basically the same old 650 motor. Carbs, however, changed their jetting specs quite a few times through the model run. You must jet (or re-jet) a carb set based on it's year, not the motor's year. Here's a chart I drew up that might help you out .....

Your case is a little different in that you are using another year float bowl. But, since it's the VM22/210 type (just like the original for that carb set), you want to base your jetting on the jet size (27.5) that came in the carb set. You don't want the jet size that was originally in those bowls for the '76 carb set (#25).

If you were trying to use '75 or older bowls, you would need to try 42.5 or 45 BS30/96 type pilots to start and fine tune from there. Those were the 2 stock sizes for that bowl type but exactly what would work best when those bowls were put on a later carb set would need to be determined by experimenting.

 

[Tim L]

I have modified the air filters and exhaust so may go to 30 or 32.5 as you suggest.
Where can I get them from? I see some on XS650 shop but they have side drillings and my 25's don't, will they be okay?
First sunny day today in the UK, so I need to get on the road before the rainy season starts, that's April to February by the way!!!!!!!!!!!!!!!!!!!

 

[5twins]

You can use the jets with the side bleed holes. They're not required for your application but won't hurt anything if they're present. You may have trouble finding the 32.5 size. It's a relatively new size (along with #37.5) that Mikuni introduced just recently. You will probably need to source it from a dealer who sells genuine Mikuni jets. On the other hand, you may not need to go that big, the #30 may be fine. Try that 1st.

Also, make sure you start your mix screws out at the recommended setting of 2 1/4 turns out and fine tune from there. If you're running them now at like 1 1/2 turns out, that's too lean and could be contributing to your problems.

 

[Tim L]

Can I ask you another couple of questions.
I have put new 27.5 pilot jets in now, took a few days to get them. I have also set the mix screw to 2 1/4 turns out.
She runs for a little longer, maybe 3-5 seconds so I may be going in the right direction.
I am going to order some larger ones, not sure whether to go 30 or 32.5. What do you think?
One thing I don't understand. When she runs for the 3-5 seconds surely opening the throttle a little will bring the main jet in to play and I should get a longer run. Maybe I should go bigger on the main jet as well.
Should I also be looking at the needle positions?
Are you in the UK or US, or elsewhere?

 

[5twins]

With your mods and using '78-'79 carbs, you will want 30 or 32.5 pilots, about 140 mains, and you will probably need to lower (lean) your needles a step. Only testing will tell you for sure what works best. These bikes all respond differently to mods and jetting. Your '72 motor has a hotter cam and better porting than the later models so it may need even different jetting than I suggested. It could need slightly bigger mains (142.5 or 145).

Just make sure you do indeed have a '78-'79 carb set. If you look at the chart above, you'll see the jetting specs between them and the '76-'77 set are VERY different. The slide design and needle length is different as well as some body details. The jetting I've suggested WILL NOT work in a '76-'77 carb set, it would be way too rich. That carb set usually can't take mains larger than the mid 130s.

The slides on these CV carbs don't really start to lift until near 4K RPM. That means the main doesn't really kick in and have a large effect until then. You run mostly on the pilots up to that point. That's why the idle or pilot circuit takes so much tinkering with on these. It plays a big role in how they run. The mix screw is an integral part of that. Don't just set it at the spec and call it good. That spec is just a starting or jumping off point from which you tune it for best operation. On your carbs, it may need to be down near 2 or up near 2 3/4 turns out to run best. You have to tinker with it and find out.

 

[Tim L]

I found a Mikuni dealer with a full range of jets in the UK.
I will order a few sets to play with from him tomorrow, they are a lot cheaper than the XS shops.
He has 3 styles of pilot jet almost identical really, except one is 13.5 mm long and the others are 14 and one is a 3.8 mm slot and the others are 4mm.
One has no side holes, which is the same as in the bowls at the moment, the other two have holes with different size mixing chambers.
Should I go for the VM22 or the ones with no side holes?

 

[5twins]

Get the VM22/210 type. There is a difference in the location of the metering orifice between these and the BS30/96 type. The VM22/210 style has the orifice at the top by the screw slot, the BS30/96 has it at the bottom end. Even though the 2 types will physically interchange, they flow differently and must be used in a carb designed for them. Your carb uses the VM22/210 type. .....

That 3rd style with no bleed holes is probably the N151.067 type and has it's metering orifice at the bottom just like the BS30/96. They won't work in your carbs.

 

[Tim L]

Many thanks.
I really appreciate all your help and advice.

 

[Tim L]

If I were to go back to using the original carbs do you think they would also need re-jetting now I have Mikes pod filters and an open exhaust system. They did run okay for a while but started to play up a bit.
I changed the carbs because the originals are split and I thought linked ones from a later model would be easier to set up.
I was having a bit of a problem where one side started to run a lot cooler than the other, making me think that there wasn't enough fuel getting through to the engine on one side.
I have tried 3 sets of carbs now, my original XS2 carbs, the 79 38's and some 34's.

 

[5twins]

Yes, any set would need re-jetting when used on a modded bike. Your originals may not need to go as big on jet sizes because they were richer to begin with from the factory, but they would need some increase. I think you did right by switching to a newer linked set. The BS34s are actually one of the best stock carb sets, they just take more fooling with to get right because of all their emission restrictions.

 

[Tim L]

I have now opted for plan B.
I took my old, separate, carbs. Cleaned them out, went up one size on pilot and main and moved the needle circlip one position.
She runs like a dream, the grin factor is back!
I am going to stick with these carbs now but thanks for all the help and advice.
I should have done that in the first place really and not bothered with the linked carbs. I know have a pair of 38's and a pair of 34's and a mountain of jets and carb spares so keep an eye on e bay.

 

[Tim L]

My XS2 is running pretty well with the new jets but I can't get any adjustment of idle speed using the pilot screws. I am relying on the idle screws, which means I cant adjust the air/fuel mix on the pilot circuit. I am wondering if this may be causing my plug fouling problems.
I have followed the guidance in the mckay document but I can screw them fully in or 3 to 4turns out and nothing changes with the idle.
I was expecting it to fluctuate a little as the mix got richer or weaker.
Any ideas?

 

[grizld1]

Nonresponse to mixture screws and excessive richness can indicate a problem in fuel level control. Either float level is set too low (resulting in high fuel level), or floats have developed leaks, taken on fuel, and are not closing the float valve, or the float valve (needle and seat) is dirty or defective, or a combination of the above. It can also indicate that your pilot jet is so large that the adjustment screws can't lean out the mixture at any in-range setting.

 

[retiredgentleman]

My XS2 is running pretty well with the new jets but I can't get any adjustment of idle speed using the pilot screws. I am relying on the idle screws, which means I cant adjust the air/fuel mix on the pilot circuit. I am wondering if this may be causing my plug fouling problems.
I have followed the guidance in the mckay document but I can screw them fully in or 3 to 4turns out and nothing changes with the idle.
I was expecting it to fluctuate a little as the mix got richer or weaker.
Any ideas?

When doing the dead cylinder method, lower the engine rpm (with the idle speed screw) to about 300 or 400 rpm. By doing this you are closing the butterfly enough to cover up the 3 by-pass holes in the carb throat. With the 3 holes partially covered, the pilot screw is then able to change the air/fuel mixture. With the butterfly positioned for 1200 rpm, the pilot screw can't make any adjustment.