Is a 150 main to big?

[Michaelrupp1]

Is a 150 main causing my midrange break-up?
I am running a bs38 on my 1978 xs650. I have pod filters and open exhaust. From every post I found I should run a 150 main and 32.5 pilots. I've done all the tuning from the tensioner, valves, timing etc... Synced them with a manometer, adjusted floats...
It seems to idle fine and I can rev it fine but under load it stumbles and break up after 2500 rpm and can't really get full throttle. I have gone out maybe 4 turns or so with my mix screw, I know that's too far, I just don't know what that means. It seems to pop if I don't take it out that far. I know this means something.
Think I need a smaller main? Some people say I don't need anything bigger than a 140.

Just looking for some help... And yes I did read the carb guide, but some things get confusing.
Thanks in advance for your help

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[ANLAF]

Well, Michaelrupp1, I have a 1979, with 1976/77 BS38s. I run 27.5 pilots and 135 mains, pods and through pipes (no silencers/mufflers - perfect! The needle float is set to position #2 (second notch from the top (the blunt end).

Anlaf

 

[jd750ace]

Perhaps this is more straightforward than the carb guide for you. I won't opine, given that you seem to be a reader (a welcome trait!)

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html

 

[ANLAF]

Michae3lrupp1, have you cleaned your pods? Soak them in a little petrol and watch the blackness - that will affect running. Check your plugs for running rich - clean them. If you are running rich get that needle position raised.

Anlaf

 

[Michaelrupp1]

I have the pods off for now trying to eliminate them from the issue. They are clean, but I think the they are the cheap ones that everyone says airflow can be blocked by the shoulders of the pods.

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[650Skull]

Identify your carbs before re jetting. The other day a guy was trying to set up his carbs for his 73. After he posted a pic they were identified as 76/77 carbs.

Pics will help with any probs.

What is your bike??????? original, Chopper/bobber, cafe. All relevant, post your bike details in your post like ANLAF. helps us to help you.

And Welcome

 

[XSLeo]

I might recommend starting at the stock size and test. Try one size up, test, one size more up test, You get the idea. In the carb guide is a section on jetting. It tells how to test, the possible results and what they mean. Using these results tells you what you need to adjust and how much.
Follow the testing procedures and let the bike tell you what it needs not some friend or whom ever.
The procedure that jd750ace linked is a good one.
Leo

 

[Gene]

I'm running some cheap pod filters and open exhausts. My carbs are set up with #140 main jet (which is 3 sizes up from stock), and #45 pilot jet (1 size up from stock). It runs and starts fine with this set up.

 

[ANLAF]

Gene, what carbs are you running? 45 pilots doesn't sound one size up from stock on BS38s. How about putting some details in your signature section. Thanks Gene.

Anlaf

 

[ANLAF]

Michaelrupp1, oh! and welcome to the forum.

Anlaf

 

[650Skull]

78/79 carbs. Stock 135 mains a 27.5 Pilot jet

 

[ANLAF]

That's my set-up, too, 650Skull - works a treat.

Anlaf

 

[Michaelrupp1]

[/ATTACH][/ATTACH]

Well, Michaelrupp1, I have a 1979, with 1976/77 BS38s. I run 27.5 pilots and 135 mains, pods and through pipes (no silencers/mufflers - perfect! The needle float is set to position #2 (second notch from the top (the blunt end).

Anlaf

I'm going to try you carb set up today. I wanna ride!!!!
Thanks for the info
Keep it coming... Every little bit helps



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[5twins]

From your pic, it appears you have '78-'79 carbs. 135 mains and 27.5 pilots are stock for that carb set so will be too small for your mods. They work for ANLAF because his carb set is from '76-'77 and had 25 pilots and 122.5 mains as stock. You will want a 30 or 32.5 pilot and mains in the low to mid 140s. Yes, 150 is too big and probably the cause of your upper midrange break-up. You will get that break-up with any main jet size increase on your carb set unless you lean the needles a step.

 

[Michaelrupp1]

Ok, I just put the stock back in, but per your advice I am going to put a 140 main and a 30 pilot.
I only. Have a 140, 150, 155 main. Pilots I have 30, and 32.5. I have my needle set in the second from the top. Crossing my fingers. Should my fuel mix be around 2.5?

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[5twins]

Yes, 2.5 turns out on the mix screw is a good initial setting for that carb set. Fine tune from there. Larger pilots may like slightly less turns out, like 2.25 or 2. But that's not etched in stone. Your bike will tell you what it likes and needs by the way it runs.

 

[jd750ace]

I have 79 BS38s. I have a 30 pilot, 3rd groove on stock needle and 140 mains currently with K&N filters in the original airbox and stock exhaust, 1.75 turns on the mix screws.

I have not decided if I'm changing the exhaust or dropping a clip position or main jet size yet, but she's rich currently. Not poor running rich, but poor mileage rich, and she could stand some breathing room in the exhaust.

 

[5twins]

Lean your needles a step and your fuel mileage should improve. You spend most of your time riding "on the needles" so their setting has a big effect on mileage. I'm surprised you can run 2 sizes up on the mains without leaning the needles. You should be experiencing upper midrange break-up under heavy throttle applications. I'll bet if you run that test (full throttle from about 3.5K up to near redline in 3rd or 4th gear), you will see the break-up.

 

[jd750ace]

Probably so. Mileage was down to 41-42mpg. I was nervous at the 58 I got with the stock mains, and my plugs, while not ashy, were "barely" tan. Now they are browning a bit with a slight black halo, so I know I'm on the rich side. Not convinced the 140 is the ticket with the stock exhaust, but it's what I had in hand. Probably could have gone 4th groove on the 135s and had decent results. I'll settle the mains when I change the exhaust out. the stockers are in pretty rough shape (appearance) but don't leak except the crossover.

 

[5twins]

Because you ride on the needles so much, they will greatly effect plug coloring too. When I was experimenting with mine, I had the needles leaned a step and knew that was good but wanted to see what putting them back in the #3 slot would do. My near perfect smoke ring at the bottom of the porcelain grew to cover like 2/3 of it.

Leaning the needle is a double edged sword. It is many times needed with bigger mains to cure that upper midrange stumble they produce but at the same time, it also leans the rest of the midrange and the upper idle circuit. This can make the midrange rather flat and unresponsive and also create a flat spot just off idle. A larger pilot usually cures that flat spot but you're kinda stuck with the flatter midrange response. That's why I like to lean the midrange as little as possible, just enough to cure the stumbles at it's upper end. I shim the needles with a washer to achieve half step settings. Mine is currently set 1/2 step leaner than the #3 slot, or at the equivalent of a 2.5 slot. This was enough to cure the stumbles but keeps the rest of the midrange as rich as possible.