jetting question thats bugging me

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fyrman41

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Okay, so i have a 79 special with the bs38s pamco with E advancer and high output coil. Xs performance pods and commando megaphone mufflers with the removable allen bolts. Question is ive gone from stock 27.5 pilots to #35, and stock 135 mains to 142.5. Am i still due to go up more? As it seems ive jumped up in sizes quite a bit already. The bike does not stumble pulls clean up to redline in 5 gear. Maybe a tini tony flat spot just off idle where it doesnt really pull too much. Timing is great, valve adjustment is good (been checked twice). Cam chain is adjusted. Brand new cam chain, chain guide and tensioner. Problem is it seems to run a bit hot jugs measured 289 with my infrared thermometer yesterday after a 55mph ride in 75 degree weather. And the plugs which have been in for a while are white other than a small light tan spot on the intake side of the plug. I cant find any air leakes my floats have been set i have good fuel lines and clean carbs. It just seems quite a leap in jets for just exhaust and filter mods and still have a leaner burn.

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that flat spot off idle is most probably those XS pods. The pleated type filter elements cause too much turbulance in the air flow through the carbs. This disrupts the vacuum signal for the slides. This prevents proper slide lift.
The shorter and more tapered just make things worse, the longer straight ones may be better. They also often have too much lip on the inside where the lip blocks off the ports in the intake bell.
On your plug color that seems pretty close on the jetting. When they took the lead out, reading plugs just got a bit harder. They don't run the tan like they used to. White with a touch of color on the side is ok. Mine run like that. They have a black color come up about half way up to the bend on the ground. If you look deep down inside you will see a fire ring. I don't recall just how far up the insulator it should be.
With that exhaust and pod filters I have found one up on the pilots is enough, one or two up on the mains. That's on my bike at my location, your bike at your location may like something else better. I might try one up on the pilots and mains. Do each by it self. Do two at a time can confuse things.
Make one change, test, compare, better or worse.
Yes, the jugs can run a bit hot. The 289 is about usual. If you search "Oil Coolers", you will find things you can do to bring the temps down.
Leo
 
Thanks Leo! That definitely sums up my questions. Though the black line on my ground straps are just at the very bottom. The rest of the way up is a light gray.

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Your plugs sound about right. As Leo said, today's clean burning fuels don't color plugs as readily as the old leaded gas did. It's hard to get brown or tan, mostly you're dealing with shades of gray. Here's my plugs, gray ground strap, black at the top of the threaded portion, and you can't see it but there is a ring of color at the bottom of the porcelain. The upper part remains pretty clean .....

PlugColor.jpg


The mixture or smoke ring on the porcelain should extend up maybe 1/3 from the bottom .....

SmokeRing.jpg


I'm going to say your 35 pilots are too big. That's 3 sizes up from stock. Most only need 1 or 2 up in this carb set. I don't think you've tested your jetting adequately. These CV carbs are very forgiving and will mask over minor jetting glitches unless pushed hard. I outlined test for the pilots and midrange/mains in the carb guide. You made no mention of changing your needle settings. There's no way you could go 3 up on the mains with this carb set and not have to lean the needles. But if you didn't test the upper midrange to main transition area under full throttle, you wouldn't know this.
 
Thats right I forgot to say my needles are in the middle stock position. My pkugs look just like yours.It pulls smoothly through all the ranges yet im not sure if i have lost power compared to the number 30 pilots i had before which gave me crispy white plugs and some detonation...but the detonation could be from the hard dry black stuff I cleaned off the pistons and heads when i had it all apart last month. I have 32.5 pilots in my tool box i could try. The 35s were all the shop had for the next jet up when i went to buy some last fall.
 
Your plugs will eventually get color but it can take hundreds of miles, so do check them as miles accumulate. The ones in my pic looked good in the short term but when I changed them out at 3 or 4K, they were pretty black all over. I think I need to lean my needles a little more. I have '78 carbs and mods similar to yours, Commando exhaust and UNI pods. I'm running 30 pilots, 140 mains, and the needles are shimmed 1/2 step leaner than stock. I think I need to pull the shim and lean the needles a full step.

Something you have to realize and understand is that the three circuits in your carbs overlap. Changing one effects the one next to it slightly .....

Carb_Circuits.jpg


Increasing the mains can eventually make the upper midrange too rich. Leaning the needle to fix that can make the upper idle and transition from idle to midrange too lean. That requires a larger pilot to fix it. I may encounter this when I lean my needle more. I may need 32.5 pilots then.
 
Ill try backing down to my 32.5 pilots and see what they feel like. If I like them ill try a 140 main and see what they act like. Taking a ride she feels like she dont have quite as much low end torque as before. Top end seemed a tad boggy also like she wanted to go a bit faster but couldnt. Theyr not drastic changes, you had to pay attention to really notice them. Mid range seemed fine though.
 
Did you test the midrange to main transition under full throttle? That's the test required. If you get stumbling or break-up in the 4 to 5K range, that's an indication that the larger mains have made the upper midrange too rich and the needles need to be leaned to compensate. Like I said, you have to work these carbs hard to sort the jetting.
 
Yes I did, once by myself and once with someone on the back with me. No stumble or break up. Very slow to redline in 5th gear but it can be done. Dont know as i feel a surge when the full main opens up like the carb guide say you should.
 
You may not be getting full lift of the slides and full flow through the mains. These CV carbs work on engine demand. That's why it's easy to over-jet them. The slides will only lift as much as the engine can use. For this reason, the bike can seem to run OK with larger than needed mains. Yes, try the 140s and run the same test. They might work better.

I don't understand the lack of break-up in the upper midrange under full throttle. On this particular carb set, even one size up on the mains usually causes it. Yours don't seem to be acting like a '78-'79 set usually does.
 
Well, you say you set the floats but exactly how did you do it? These carbs are very picky about this setting. Did you measure off the gasket surface as you should or off the gasket itself? Did you check both sides of each float? Many times these floats are twisted on their bracket and one side sits higher or lower than the other. If you only check from one side you won't catch this and the fuel level may not be what you think it is.

Also, what spec did you use? It varies between some of the different year carb sets. The '78-'79 carb set spec is 24mm ± 1mm. I experimented on mine trying 23, 24, and 25mm. It ran best @ 24mm.
 
im going to recheck them after work tonight. I can't rember if i checked them on both sides or not. Just another reason to pull them out and check. I measured off of the metal surface of the gasket seat.
 
So last night i took my carbs out found out the floats were crooked and measured around 38mm. Must have messed something up with previous measurements. So i brought them into spec 24mm dropped the pilots down to 32.5 gave here a test run and she seems to pull harder. Starting to get minor sputtering at WOT around 4k and up. The shop had no 140 mains so im going to have to order some from mikes. Should i get 137.5 mains to try too, just in case?
 
I don't think you'll need the 137.5s. I started out with them and they were too small. I prefer using genuine Mikuni jets. The small round mains used in your carbs are very common and may be available at a local shop.

MainsCompared.png


Be sure to re-visit your mix screw settings with the new pilot jet size. Stock spec for your carb set is 2.25 turns out and you should still end up close to that. It's no wonder your jetting attempts seemed futile. Those floats were way off, very lean (higher measured height equals lower fuel level in carbs).
 
I was very meticulous at setting them this time kept checking and rechecking both before putting the bowls back on. What style of uni foam pods are you using. My new xs pleated pods do seem to bother the carbs.
 

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