New XS Chop Carb Tuning: Advice Needed

[muskallunge]

1978 XS650 with pod filters, straight pipes and bs38 Mikuni's. Just replaced pilot jets from stock 27.5 to 30, main jets from 135 to 140, upped the clip on the jet needle to the 2nd slot from the top, have the mixture screw out 2.25-2.5 turns and had the entire carb sonic bath cleaned.

Still getting studdering at higher speeds or 3/4+ throttle.

The manifold intakes aren't the newest and need to be changed most likely but I've sprayed with carb cleaner and no change rpm's.

Have plugs on vacuum petcock barbs with spring wire clips to hold them on.

Get a hint of popping letting off the throttle occasionally.

Bigger mains or pilots or both?

A little bit of gas is leaking from left float bowl when not running but I've got new gaskets on the way.

Please advise. Thanks.

 

[Endee]

When you say the needles are on the second slot from the top, do you mean the blunt side or the pointy side? If the clip is closer to the blunt side that is probably too lean and why you are cutting out.

Closer to the blunt side leans the circuit, closer to the pointy side richens the circuit.

 

[muskallunge]

I meant blunt side. I put them there as suggested by another member on some other post about a similar bike. I moved the clip back to the 3rd slot down from the blunt side and it didn't fix anything. Still breaks up or fires poorly after 1/2 throttle to 3/4.

 

[Endee]

Roger.
Sorry, I'm not the most knowledgeable with these carbs or bikes. The conversations always start with checking the carb manuals in the tech sections though and weeding out the most likely culprit first.
I increased my pilot jet 1 size because I was really flat off of idle and the 1 size increase didn't help that much. Still had a pretty noticable flat spot off of idle, so I'm going to increase it 1 more size.
If it breaks up and stumbles do you think the 140 main jet is too rich? A lot of the more experienced guys here will say that you don't really know if you're jetted correctly until you go a size over and notice it start to break up. Then when you go back down to the original size and it performs better, you know you are jetted correctly.

I'm pretty familiar with the carb on my vstar 650. One of the tricks I used to check and see that I was dialed in was to pull the choke cable. It would richen up my main circuit slightly and I would stumble, telling me I was running rich. That is expected since the choke is pulled. If I was running lean pulling the choke would make the engine run a little better. The bs38 carbs are different though,and there is no halfway point like with a choke cable, so I don't know if that will be helpful

 

[5twins]

Carb jetting for mods is all about experimenting. We can recommend a size or range of sizes that have worked for others but you may need to try several of the sizes in the suggested range to find what works best for your bike. You were right to lean the needles a step. That is required on the '78-'79 carb set with pretty much any increase in main jet size. You will get break-up under heavy throttle in the upper midrange without the change.

You said you have pod filters but didn't say what kind. If they're the pleated K&N style, they may be the problem. They don't work right on CV carbs. If that's what you have, try a test run with them removed. If you're going to run pods, the straight foam UNI pods work best.

If you have the foam pods then it may be a case of your jetting not being quite right yet. I don't think your 140 mains are too big. If anything, they may be still too small. I run 140s in '78 carbs with pods and aftermarket mufflers and they're good. With straight pipes, you may need more. You should probably try a 142.5.

The pilot jet shouldn't have much effect up where you're having problems but sometimes a 32.5 size does work well in the '78-'79 carb set. For choosing/checking the pilot jet, you want to observe the low speed/low RPM operation. Check how the bike takes the initial throttle opening from idle and transitions into the lower midrange. If it breaks up or stumbles, you could be too rich. If there is a big flat or dead spot, you are too lean. Also, find a parking lot or deserted back road where you can cruise along in 1st or 2nd gear at low RPMs, say 2K or so, just above idle. If the pilot size is good, you should be able to do this smoothly. If the bike surges, you're lean. If it's breaking up and stumbling, you're rich.

 

[muskallunge]

The filters are indeed the pleated kind. I'll test without them on after work today. Seems to idle fine with the pilot jet sized upped to 30 instead of the stock 27.5

 

[5twins]

It may sit there and idle fine with the pilot size you have but the true tests are the ones I outlined - how does it respond to opening the throttle and how does it cruise around at low speed/RPMs.

 

[muskallunge]

I was only able to find a 30 and 35 pilot size when I ordered them from Mikesxs but will try and find one in 32.5. Would an adjustment in the mixture screw fix the surging/lean issue?

 

[peanut]

a mid to full throttle stumble break up might not necessarily be down to your carbs . Under heavy load the bikes ignition and electrical systems are put under considerable stress and your ignition advance requirements are different at that rpm range too.
Do you have a battery fitted or you running a capacitor ?
What ignition system are you using ...stock points and advance ? or fully electronic etc ?

Regarding setting the idle (pilot) jets the amount of turns out you have to set the idle mixture will give you an indication if you need to up the size . If the jet needs adjusting to near full in or out then up then increase or decrease the pilot jet accordingly .

I run straight through exhausts BS38s foam filters and have upped my pilots first to 30 then to 32.5 where they are now. My mains were upped to 140 and the engine pulls like a train without hesitation or stumble from 1000rpm to 6000+rpm .However I do have a fully electronic ignition and advance system (Boyer Bransden Micro Digital ) which is the mutts nuts.

Try this little test . With the engine warmed up on the stand . Blip the throttle and see if you are getting a hesitation or flat spot. If you are try the test again whilst simultaneously applying various small amounts of choke. The chokes supply an immediate independant extra supply of fuel and air to your idle mixture that doesn't need any jet change and can be used as a diagnostic
If the throttle pickup improves with a little choke it indicates that your idle mixture is lean for some reason. If the throttle bogs worse then the idle mixture is too rich but you'd probably know that by checking the spark plugs and by the rich smell and smoky exhaust.

A critical setting that effects throttle pickup is how well sealed your diaphrams are . Carry out the standard diaphram test . Your diaphrams should take around 15 to 20 seconds to fall . Any more or less then you have a sticking slide or a leaking diaphram /seal.

 

[5twins]

Yes, the 32.5 pilot jet can be difficult to find. It's a rather new size just introduced by Mikuni over the last few years. Not every place seems to carry it yet. They skip from 30 right to 35 which was the way it used to be. 2.5 size increments could be had up to 30 then they jumped in 5's. Mikuni introduced the 32.5 and a 37.5 so now they don't jump in 5's until they get to 40.

Mike's jets are repops. I did use them many years ago when they were like $2 each but now they cost pretty much the same as genuine Mikuni jets. I don't see the point in using them any more, especially with the quality control issues Mike's seems to have. With a genuine Mikuni jet, you can be assured of the sizing and the quality. That's all I use now.

 

[weaselbeak]

My BS38's are good with FOAM type pods. I've never found a pleated filter that dials in. They allow too much turbulence at the mouth of the carbs, foam settles it to dead air. Slip foam sleeves over your pleats if you can & see the difference.

 

[muskallunge]

So, got home from work yesterday and took the pleated filters off. Took it for a ride and let 'er rip. No breakup or stumbling. Just hauling ass. It was indeed the filters after all creating too much restriction at higher speeds. Will probably get a pair of velocity stacks. A pal has some on his KZ chop and that things flys. I'm sure I need some little adjustments to make her run perfect but she runs damn good now without the flow restriction. Thanks for all the advise. I learned a lot.

 

[peanut]

good to hear that it turned out to be something so simple and easily remedied. 5Twins had the solution straight off.

I have to say that the tiny increase in power that might be available by using velocity stacks will be far outweighed by all the grit and crud being sucked into your cylinders and scoring the bores and valves etc . Stick some Ramair foam filters on and protect your engine would be my advice

 

[Hitndahedfred]

I gotta agree with peanut on the stacks. And if ya foam the cones that too will affect jetting. I learned this with endee when he came to show me just how carbs work.
Mines rips with the k&n pods,, but stumbled bad with the stacks,,then worse when I double foamed the bell to keep crap out.

 

[muskallunge]

Some velocity stacks come with a small mesh filter to help reduce the amount of debris going in. I'm down for a foam filter but to be honest, don't like the look and stupid logo printed on the side of the Ramair filters. Too modern of a look for me and I'm trying to keep this bike 70's style.

 

[5twins]

Your BS38 carbs need a filter with a 2" I.D. mounting flange. The UNI pod most of us use is the UP4200. It has a 2" flange and is 4" long. It is the longest filter that will fit on a stock bike between the carb and battery box .....

You won't have a battery box to contend with on your chop, instead you will have the upper down tubes on the hardtail. UNI does make a stubby pod but I don't care for them. I think they're too small to flow enough, also the ends are capped which cuts flow as well. But, UNI does make pods with a 15° angled mount. That might work for you. You could angle them down and/or out to clear the frame rails. They are the dual layer version of the UP4200, with a red foam "sock" over the basic black foam filter. If you don't care for that, you can simply remove the "sock" and you'll be left with the plain black filter. The part number for those is UP4200AST.

http://unifilter.com/online-catalog/clamp-on-air-filters/

 

[muskallunge]

Thanks. These UNI filters will keep the vacuum port on the top end of the intake from getting blocked? I've heard that is another issue with the pleated pod filter. I have an electrical box below my seat that I'll have to contend with but I'm sure I can make it work

 

[muskallunge]

The $17.95 is for one correct?

 

[5twins]

Well yes, but that's for the straight version, which may not fit because of your hardtail's upper frame rails. I think they're going to get in the way of a long, straight filter. That's why I recommended the angled version, this one .....

http://www.ebay.com/itm/2-Stage-Angle-Pod-Filter-Uni-UP4200AST-/391102400157

With this one, you can angle it down or out a little, maybe a combination of both, to clear the frame rails.

If you do get any of these UNI pods, don't get/use the special foam filter oil they sell for them. These are a very fine foam and that oil is too thick and will clog them. Just use a 50-50 mix of gas and plain motor oil to oil them. I bought the cheapest bottle of straight 30wt. at Walmart (their in-house brand) to use especially for this.

 

[muskallunge]

Ordered a pair. Thanks for all the help fellas.