Post rejetting question

[MikeC23]

Good morning all. First off I did read the carb guide! I feel like I should throw that our before my question. Ok. On to the question. I decided to rejet the carbs on my 79 special. They were stock BS38's. I had put on UNI foam air filters. I went up one on the mains (to 140), down one clip from stock on the needle, and one size up on the pilot (to 30). Is my assumption correct that I now need to adjust my air mix screws to be leaner than factory (2.25 turns)? OR should I set them to factory and balance with a manometer from there? Those mix screws are the only thing I'm having an issue finding info on.
Thanks!

 

[gggGary]

Do your synch if you can get it to hold an idle, then; generally factory idle mix is at the limit on lean. standard technique is adjust one idle screw in, then out counting turns between till idle stumbles lean, rich, set screw half way between (maybe bias slightly towards rich side), repeat for other side. this usually works great for BS38's, BS34's not so much. if you can't open to get a rich stumble that's a clue the pilot could go up one more.

 

[5twins]

The factory spec is just a suggested starting point. From there, as Gary said, you adjust in/out to find the best setting. It should be near that spec, maybe even at it, but it may not be exactly at it. A pilot jet change can sometimes change the "best running" setting a little. Usually, a larger jet would require slightly less turns out on the screw, but this isn't always so. You just have to play with the setting and find the best spot.

What you're looking to find is what we call the "sweet spot", a small range of adjustment maybe 1/2 turn wide where the engine runs best, smoothest and fastest idle speed. Going past it in or out with the screw will make the motor start to stumble or the idle speed start to fall off. I like to set mine towards the rich side of this "sweet spot" range, actually as rich as I can without the idle being adversely affected. I find this helps alleviate popping on decel.

 

[MikeC23]

Solid advice, guys. Thanks! I have a good bit of decel popping right now. I'm going to monkey with it tomorrow night.

 

[5twins]

I also meant to mention that at 140, you're 2 sizes up on your mains. Jet sizes change in 2.5 number increments. Stock main for your carb set was a 135 so one up would be a 137.5. If you have changed the exhaust along with adding the pods, the 140 should be fine. If the pods are your only mod, a 140 may be a bit much and maybe a 137.5 would be better. As with any jetting for mods, only experimentation will tell you what's best.

 

[MikeC23]

I just ordered a set of 137.5's to see how they work out. The foam UNIs are the only mod as I'm running stock exhaust. I'm kind of enjoying this rejetting stuff!

 

[Marlin72xs]

I just ordered a set of 137.5's to see how they work out. The foam UNIs are the only mod as I'm running stock exhaust. I'm kind of enjoying this rejetting stuff!

I'm not enjoying it. LOL. Well, actually I'm not enjoying it because I've been a "honey-do" lately and can't get to make any adjustments. It's like a week or 2 between testing and adjustments.
Actually, I made too many changes at once and now I don't know what to change back. (I do have some ideas). Let me ask you this.
Did you solve your tuning problems?
I have foam filters in the stock boxes and shorty (glass pack), megaphones. I had major backfiring, with only a minor hesitation on take-off. I went up 1 on the pilot jet and main jet and raised the needle to the stock #4 position for a 1972 carb. Oh, and before any changes, the plugs were a nice light beige color.
Now, the plugs are a dark brown, the idle sucks, but no backfiring. But, also, no flat spots above idle to about 6.5K. 6.5K is about as hi as I've gone since the rebuild.
Just got an air compressor, so they're coming off one more time to be cleaned thoroughly, and put the stock jets back in. I've got a sneaky suspicion that the needle rise back to stock #4 will stop the backfiring and a thorough cleaning will improve the idle. Should know by this weekend. Honey do this! Christmas lights! And make a new kitchen table for the party! It's like a JAP's favorite wine...

 

[5twins]

Raising the needle along with larger mains and pilots is probably what turned the plugs dark. Needle position has a major influence on plug color because it tunes the midrange mostly and that's where you spend most of your time running. You might try the larger jets but with the needles leaned a step to position #3. The popping (backfiring) on decel and the flat spot just off idle are lean indicators. They would seem to show a larger pilot is needed. The mix screw setting can also effect this to a certain extent. Don't just set them at your carb's called for spec and assume they're right or best. Start there but adjust them both ways (in and out) to find what is the best setting.

 

[Marlin72xs]

Just got back from a spin around town, but before i left i set the mix screw to 1/2 or a 1/4 in from factory. It idled, wow. But still has that slight bogg on take off. I think there are other problems. I put it on the side stand while i helped my wife with the groceries and found a puddle of gas on the driveway. I generally shut the gas off at all times, just in case, but this happens about once or twice a year and it hasn't happened since i changed the float needles and seats last June.
Thanks, 5twins. I'm gonna go back to #3 on the needle, it makes sense. I also start from the petcocks on down. I've always had filters, but there cud be a clog somewhere else. When going to reserve this evening, before I cud switch over, it went dead and it took what seemed like forever to kick in again. I was going about 50 and it slowed to almost 10. My life passed before me. I had visions of walkin it. I'm just glad I wasn't on a busy highway. Gotta finish my honey doos then get hot and heavy on this. Maybe I'll put it in the basement after Christmas and replace the cam seals too. Now both are leaking. I've ordered the thin ones from partzilla. Thanks again.
As a wise man once said, "It's gotta work, it's only made of metal, rubber and plastic!"

 

[peanut]

I I've got a sneaky suspicion that the needle rise back to stock #4 will stop the backfiring and a thorough cleaning will improve the idle. ...

its not the jet needles that effect your deceleration its your pilot jets and your idle mixture settings as 5T said .

Think about it ....... On deceleration your throttle is closed just where it would be at idle.! so its the idle circuit that is providing all your fuel and air .

if you are using foam or mesh filters I would not be surprised if you didn't need to go a bit higher than 30 on your pilot or idle jets.
My 79 needs 35 pilot jets and I tried 27.5 , 30, 32.5 and 35 . Any less than 32.5 and I also got popping on decel

I would suggest that if you need to screw your idle mixture screws out more than 2.5 turns then go up one on your pilot jets try 32.5 or even 35 . If you buy both then you can experiment.

Don't forget to allow at least 20 seconds or so for each 1/4 turn adjustment to effect the idle mixture and settle before further adjustment .

 

[5twins]

Sounds like you may have a slow leaking float needle and seat assembly (or maybe both of them). Running down the road, you're probably able to burn up the extra fuel before it overflows, but parked or at idle, not so much. The bowls slowly overfill. This could be the cause of your off-idle flat spot and bogging. If you used aftermarket replacements, that could be the problem. My dealer told me he won't use anything but genuine replacements now. He had too many bikes returned with leaking carbs when he was using the aftermarket float needle and seat assemblies. It was costing him time and money. Genuine replacements from Yamaha aren't that expensive .....

https://www.partzilla.com/product/y...?ref=02c5827ff8797617442d86865a0c2d98f844117f

 

[Marlin72xs]

That

its not the jet needles that effect your deceleration its your pilot jets and your idle mixture settings as 5T said .

Think about it ....... On deceleration your throttle is closed just where it would be at idle.! so its the idle circuit that is providing all your fuel and air .

if you are using foam or mesh filters I would not be surprised if you didn't need to go a bit higher than 30 on your pilot or idle jets.
My 79 needs 35 pilot jets and I tried 27.5 , 30, 32.5 and 35 . Any less than 32.5 and I also got popping on decel

I would suggest that if you need to screw your idle mixture screws out more than 2.5 turns then go up one on your pilot jets try 32.5 or even 35 . If you buy both then you can experiment.

Don't forget to allow at least 20 seconds or so for each 1/4 turn adjustment to effect the idle mixture and settle before further adjustment .

Well, I only assumed that the backfiring started when I changed the needle setting to 3 back in 1978. The original mufflers, muffled, no pun intended, the noise until they rattled themselves to pieces inside. The replacement muffs were modified slightly, and the backfiring appeared. I was young and didn't mind the noise.
At the end of the Vintage Yamaha Outing, Maxpete suggested messing with the carbs. I adjusted the mix screw out at 1/4 turn intervals. Some of the backfiring went away, but a minor bogg at take off appeared then and never went away. So, on the pilot I went from a stock 42.5 to a 45. On the main, I went from a stock 130 to a 132.5.
I can't concentrate, because I have a bunch of students around me acting like jerks. I'm a substitute teacher. Later.

 

[5twins]

Yes, leaning the needles a step could induce backfiring in the upper idle circuit. You have to remember, the 3 circuits in your carb overlap slightly. Changing one can and often does influence a portion of the one next to it. In the case of the midrange (needle setting), a change will also effect the lower portion of the main circuit and the upper portion of the idle circuit.

Most carbs come stock with the needles set in the middle #3 slot but your '72 set differs. Stock setting for that set was the #4 slot.

 

[peanut]

On to the question. I decided to rejet the carbs on my 79 special. They were stock BS38's.
!

hang on a minute ......

if you have stock BS38 carbs on your 79 bike why are you fitting 40 pilots ? ................ they are for BS34 carbs (ie 1980 onwards) a completely different set of jets , slides and needles to a BS38 .

Have I missed a carb change somewhere ?

Ah I see ...We are addressing marlins problem but it was originally Mikes thread . doh..
It sure is helpful when folk start their own thread about their own problem rather than confuse things . it doesn't take much to confuse me these days

 

[grizld1]

Secondary effects of tuning changes can be very strong in these carburetors, and that's especially true of interaction between jet needle position and pilot circuit components. Moving the needle a single notch will always require readjustment of the fuel screw and will very often force a compensating pilot jet change. Another area of strong interaction is between needle jet selection and main jet selection (not many NJ options available for OE carbs). Excuse me for being a pedant, but there is no "midrange circuit:" the jet needle and needle jet are main circuit components. Component-and-range charts can be deceptive. The main circuit has an effect even at idle (through the slide cutaway), and the pilot circuit never turns off, even at WOT.

One thing to watch for is wear and/or damage to the needle jet. Aggressive cleaning, corrosion, and wear from needle contact over the decades can cause serious trouble. The most obvious symptom is heavy fuel consumption.

 

[Marlin72xs]

Hi Peanut. I did but in with my problem, but I was actually interested in Mike23's problem and his solution. It could be just me, but it seems like many threads go off on tangents.
Sometimes the threads just end and I'm sitting here wondering if the problem was solved. They could be from 2010. I need to know, dammit! LOL.
Hey Mike 23, what gives? What was your remedy?
If anyone cares, (of course you do), it's been either raining or really cold or both and I've been working a lot. So, no tinkering from me, lately.
Grizld1, I have a lot of wear on one of my needles.

 

[peanut]

Hi Peanut. I did but in with my problem, but I was actually interested in Mike23's problem and his solution. It could be just me, but it seems like many threads go off on tangents.
Sometimes the threads just end and I'm sitting here wondering if the problem was solved. They could be from 2010. I need to know, dammit! LOL.

I agree Marlin it is very frustrating when posters ask for help and then the thread ends with no posted solution or outcome. I always try to find out the outcome in order to close threads when I can so it can inform future searchers that arrive at the thread

What makes things both confusing and frustrating is when posters ask a question about their particular bike or component problem right in the middle of another posters thread.
When responding to someones request for information or advice we invariably look to the first post of the thread to aquaint ourselves with the model year and components , modifications and all the other vital information needed to get a full picture of the issue so that we can respond.

If someone hyjacks another members thread,...then all the information, specifications and advice gets mixed up and eventually 3 pages on, nobody is clear which information and advice is associated with which bike owner or particular problem without re-reading the entire thread .

Your bike has BS34 carbs as stock and Mikes bike had BS38s Whilst they are both CV carbs that operate in the same manner , they are different carbs in terms of jetting and component specifications.

It is always better to request advice or information about issues with ones own bike by starting a separate dedicated thread so that there can be no confusion.

if you are not going to ride your bike or work on it due to the cold weather , why not remove the carbs and strip and clean them in the house ready for when the weather improves. Thats what i do every Winter. I hate working in a cold drafty shed or garage when I can work in the warm comfort of my sun room with coffee and cake on hand lol

 

[gggGary]

Stoking up the basement wood stove this morning to start in on another carb set.
Well there's some bench cleaning first.
Keeping threads on track, cat herding, same odds of success.

 

[grizld1]

Marlin, if the jet needle is worn, there's going to be wear in the jet as well. Replace them; there's no effective way to compensate for that condition.

 

[peanut]

Stoking up the basement wood stove this morning to start in on another carb set.
Well there's some bench cleaning first.
Keeping threads on track, cat herding, same odds of success.

are you going to give us some tips on cat herding techniques gary
heres an example of successful cat herding

I can't even get my cats out of bed this time of year