Selecting #35 or #38 Slow Jets

[BonS]

I was an early adopter of rejetting from 35's to 38's. I did it for a couple of reasons. The OEM idle circuit was jetted lean due to EPA regulations which is a very common problem. This lead to low speed performance that I simply didn't trust when lightly accelerating from a dead stop such as a traffic light or stop sign. Secondly, my bike sat too much and the 35's would easily clog and were hard to "drive clean" when totally plugged. To correct the lean idle issue I initially ran my fuel mixture screws out to enrichen the mixture but couldn't run them out far enough to find the sweet spot. I decided (like many here) not to live with it anymore and I put in the 38's. To finish the job I followed this procedure:

1) Turn in the mixture screws (gently) all the way in and then back out three full turns.
2) Warm up the engine.
3) Pick a cylinder and slowly turn the mixture screw in until the cylinder begins to stumble or die from
    being too lean. Note the turns.
4) Turn the mixture screw out 1/4-1/2 turns at a time, wait 30 seconds for the engine to stabilize.
5) Repeat #4 until the motor stumbles a bit from the mixture being too rich. Note the turns.
6) Now set the mixture screw half-way between the two settings for the correct idle mixture.
7) If the mixture screw is more than three-turns out or one-turn in then a different slow-jet should be used.
8) Repeat for each cylinder or for an average do all cylinders at once.

Using 38's I generally have my mixture screws set to 1-3/4 turns out. I have reliable performance at low throttle near idle and I've haven't ever had to take apart my carburetors from terminally clogged jets (knock on wood).

It's my opinion that those with excessive carbon build up haven't properly tuned their carburetors after a jet swap and this is what leads to problems. The 1 to 3 turn limit on the mixture screw ensures that the selected slow jet is appropriately selected for proper metering.

Finally, yes, the slo-jets do contribute to the total fuel mixture at all throttle settings but they are a bit player at WOT and are not altered to correct WOT issues.

Attic Rat, MoodyValk, and all the other experienced tuners, can you help out here? I'm sure that there are a lot of us that are using 38's and want to be doing things right. Is there more to this than I've outlined above? I'm here to teach, learn and do things right.

[Hook#3287]

 

Excellent post

exactly my feelings and concerns.


 

[Attic Rat]

BonS you have describe the proper way to adjust the carbs. When I was doing all of my research using a digital fuel air ratio meter on a stock bike. My findings where as follows. Setting the mixture screws at 2 1/4 turns out with the 35's slow jets was almost perfect. However using the 38 slow jets with only 1/2 turn out the fuel mixture showed  to be way too rich. There for you don't really have any adjustment with the 38's to get the proper fuel air ratio mixture. Now given all of that I like to run my performance bikes just a tiny bit rich which makes them start and run better. However not to rich that you give up your fuel mileage. I still use the 35 slow jets on my performance bikes and they run very well and get very good fuel mileage. oh by the way I don't have problems with the 35"s clogging up either. My personal bikes set for months at a time with out any problem. Too much work and not enough time to ride. I have one bike my #2 as I call it and I haven't  ridden it 500 miles in the past 5 years but she will start right up and run perfect. All I do is fill it completely full of real gas and put stabil in the fuel I have never ran any fuel treatment of any kind in any of my bikes. If I don't ride one much in a year I will suck all of the fuel out and replace it with fresh fuel and stabil.
The proof is when you pull the heads and look at the combustion chambers and ports. I also look at the back side of the intake & exhaust valves. I have never pulled a bike down that was running 38 slow jets that didn't have the intake & exhaust valves with a lot of build up. The combustion chambers are also all coked up along with the ports. They are usually so bad that I have to grind all of this out. I pulled one down last year that only had 6k miles on it with 38 slow jet and it had all of the above problems as the high mileage bikes that were running the 38's.
BonS I hope this gives you some insight as to what you are looking for.

Regards,

Bob

[Ricky-D]

BonS, You are incorrect in the assumption that the screw has such a large influence

on the low speed circuit. It does not.

The low speed circuit has four apertures with the screw affecting only one single aperture.

The remaining three are controlled by the jet size alone. So when you adjust the screw you are adjusting

within the boundaries of the 25% portion of the low speed fuel flow when you turn the screw.

***

[moodyvalk]

BonS, You are incorrect in the assumption that the screw has such a large influence

on the low speed circuit. It does not.

The low speed circuit has four apertures with the screw affecting only one single aperture.

The remaining three are controlled by the jet size alone. So when you adjust the screw you are adjusting

within the boundaries of the 25% portion of the low speed fuel flow when you turn the screw.

***

ricky d, thanks for your reply buy BonS only asked for experienced engine tuners....   joking of course 

[moodyvalk]

well honestly I can not comment on the 38 jets because the "Turbo Valkyrie" is the only Valkyrie to this date that I have played with and it is running the oem 35's.  But I can tell you this from my experience on all other bikes that I have tuned with is that I have never ran anything other than the oem slow jet.  In my opinion, there is more to it than just making more power at every level of the powerband.  I'm not going to argue that there is a very good chance that making a richer condition just off idle with a 38 slow jet may make more "power" but just off idle/cruising I want proper fueling aka complete combustion aka stoichiometric.

 Stoichiometric on a wideband o2 sensor gauge is 14.7:1 air to fuel ratio.  From my experience on the valkyrie, just off idle/cruising with the stock 35's the a/f ratio is very nice and no reason to adjust the jetting.  02 monitoring system don't lie....its either lean, rich, or somewhere inbetween...

sure we can install 38's and adjust the idle mixture screws so that at idle it is tuned in, but it is off idle and cruising situations that we need to be concerned with when running the 38's.

Like I said earlier, I have not ran 38's because there is absolutely no reason for me to with the 35's on the air fuel monitor are very new stoichiometric.  I don't need more power at cruising personally....

hope what I wrote made sense to everyone...

[moodyvalk]

and more food for thought,

the 35's at 6500 ft elevation in Colorado Springs where a bit on the rich side of stoichiometric, could you imagine 38's up in the high elevations....

[BonS]

Thanks for all the good info. I'm going to go back to 35's on a spare set of carburetors that I'm rebuilding and will strap them on this Spring. I've got my newly completed six-channel digital carburetor balancer that I'm itching to use in this process as well. I'm spending about $25/carb for jets, needles, o-rings and gaskets for both the carbs and fuel rails. I'll keep my Dan-Marc solenoid valve in place as its been trouble free and cheap insurance against the unthinkable.

[Attic Rat]

BonS I would also run the interstate carb springs which will open the carb piston just a little quicker

[BonS]

BonS I would also run the interstate carb springs which will open the carb piston just a little quicker

Yup, thanks, I have an Interstate and my spare carbs are Interstate as well.

[John Schmidt]

Geez, you guys make me feel guilty now. I went to 38's quite a few years ago, didn't have any negative effect on mileage but it did clear up a tendency the 35's had to clog if the bike sat for an extended period, we don't have "real" gas down here. With the 38's I opened up the mixture to two full turns, initially was closer to 1.5 and the plugs appeared to be running lean so went up just 1/4 turn. Still showed lean signs but not as bad and seemed to appear only after an extended run on the interstate at higher speed. Finally went to two full turns and it all looks good, and gives me all the performance I could ever want. With decel at lower speeds I do get some burbling out the exhaust, not sure if it's the pipes or the mixture...or a bit of both. For about the last ten years I've been running I/S carb springs, K&N filter sans the prefilter, 4 deg. T/W with an I/S ICM, and some aftermarket pipes with moderate backpressure. On the open road I can get in the upper 30's mpg if I behave....don't get that very often.    Since I added a batwing fairing and radiator pods, it has affected mileage some but I like it so not concerned. I decided to leave well enough alone and just ride the darn thing. 

[Donncat]

Hello Gents. I have read allot here about jets and adjustments but have not heard any talk about running rich. My 99 STD is so rich my clothing smells of it when I come in the house. I have an even idle and wonderful throttle response but around home and short runs to the DQ and back the fumes are tainting my enjoyment a bit. Just bought it but my 6th flat 6 so experienced with the engine. Where would you fine scientists suggest I begin? Have not touched any carb adjustments. Thanks.

[The emperor has no clothes]

Hello Gents. I have read allot here about jets and adjustments but have not heard any talk about running rich. My 99 STD is so rich my clothing smells of it when I come in the house. I have an even idle and wonderful throttle response but around home and short runs to the DQ and back the fumes are tainting my enjoyment a bit. Just bought it but my 6th flat 6 so experienced with the engine. Where would you fine scientists suggest I begin? Have not touched any carb adjustments. Thanks.

I'd start with a new air filter.

[FLAVALK]

Geez, you guys make me feel guilty now. I went to 38's quite a few years ago, didn't have any negative effect on mileage but it did clear up a tendency the 35's had to clog if the bike sat for an extended period, we don't have "real" gas down here. With the 38's I opened up the mixture to two full turns, initially was closer to 1.5 and the plugs appeared to be running lean so went up just 1/4 turn. Still showed lean signs but not as bad and seemed to appear only after an extended run on the interstate at higher speed. Finally went to two full turns and it all looks good, and gives me all the performance I could ever want. With decel at lower speeds I do get some burbling out the exhaust, not sure if it's the pipes or the mixture...or a bit of both. For about the last ten years I've been running I/S carb springs, K&N filter sans the prefilter, 4 deg. T/W with an I/S ICM, and some aftermarket pipes with moderate backpressure. On the open road I can get in the upper 30's mpg if I behave....don't get that very often.    Since I added a batwing fairing and radiator pods, it has affected mileage some but I like it so not concerned. I decided to leave well enough alone and just ride the darn thing. 

John, I did the same thing shortly after you did yours. Matter of fact, you gave me the lead on SUTCO (I think) slows. I have not  had a lick of problem and the old girl runs great. Mileage is consistently in the high 30's. I must have put mine in eight years or so ago and have never regretted it.

[quexpress]

I went to 38's quite a few years ago,
For about the last ten years I've been running I/S carb springs, K&N filter sans the prefilter, 4 deg. T/W with an I/S ICM, and some aftermarket pipes with moderate backpressure.
... I added a batwing fairing

Mine has been set-up exactly like yours for approximately 10 years also John. 

[moodyvalk]

several people have talked about the not having "real gas" available and so on...

so here is more food for thought,

ethanol enriched fuel acts or should be jetted, rather, like an alcohol engine.  for an example, roughly with an alcohol engine will burn 3 gallons of fuel vs. gasoline engine.  therefore, if your running fuel with 10% ethanol then the idea of getting away with 38's vs 35's would make more since in the a/f ratio world.  if a person was wanting to convert to e-85 then they would have to increase all there jetting to get proper air to fuel ratio's. 

another thing that I would like to mention is that the talk about 35's clogging up seems funny to me, the Valkyrie is not the only bike or engine that has ever ran 35's or small jets.  so jetting bigger to stop a clogging issue seems like a bandage to cover up another problem instead of correcting the original issue....

[DarkSideR]

BonS you have describe the proper way to adjust the carbs. When I was doing all of my research using a digital fuel air ratio meter on a stock bike. My findings where as follows. Setting the mixture screws at 2 1/4 turns out with the 35's slow jets was almost perfect. However using the 38 slow jets with only 1/2 turn out the fuel mixture showed  to be way too rich. There for you don't really have any adjustment with the 38's to get the proper fuel air ratio mixture. Now given all of that I like to run my performance bikes just a tiny bit rich which makes them start and run better. However not to rich that you give up your fuel mileage. I still use the 35 slow jets on my performance bikes and they run very well and get very good fuel mileage. oh by the way I don't have problems with the 35"s clogging up either. My personal bikes set for months at a time with out any problem. Too much work and not enough time to ride. I have one bike my #2 as I call it and I haven't  ridden it 500 miles in the past 5 years but she will start right up and run perfect. All I do is fill it completely full of real gas and put stabil in the fuel I have never ran any fuel treatment of any kind in any of my bikes. If I don't ride one much in a year I will suck all of the fuel out and replace it with fresh fuel and stabil.
The proof is when you pull the heads and look at the combustion chambers and ports. I also look at the back side of the intake & exhaust valves. I have never pulled a bike down that was running 38 slow jets that didn't have the intake & exhaust valves with a lot of build up. The combustion chambers are also all coked up along with the ports. They are usually so bad that I have to grind all of this out. I pulled one down last year that only had 6k miles on it with 38 slow jet and it had all of the above problems as the high mileage bikes that were running the 38's.
BonS I hope this gives you some insight as to what you are looking for.

Regards,

Bob

Attic Rat,

Do you have any data on fuel mixture screws running a stock air box, with 35's and Super Trapp exhaust (& headers) with 5 disks?

[Attic Rat]

My air fuel ratio meter shows that 2 1/4 turns out on the mixture screws. The bike will  run and start good there. Also it doesn't seem to change your fuel mileage. Josh this will also work very well where you live too.

[DarkSideR]

My air fuel ratio meter shows that 2 1/4 turns out on the mixture screws. The bike will  run and start good there. Also it doesn't seem to change your fuel mileage. Josh this will also work very well where you live too.

Thank you much.

[tz89]

...I've got my newly completed six-channel digital carburetor balancer that I'm itching to use in this process as well. ...

I am anxiously awaiting your results. Be sure to post some pictures.