Bonneville America Forums Forums Technical Forums Performance Mods Jetting??????

ok, where to begin. The reason I'm posting is to find out roughly what I should be running for jets, so I can get some to start tuning.

The details:
First, the bike is an 03 speedmaster 790. Most of the time the bike is at around 4,500 ft elevation. I have an aftermarket set of shorty (12") exhaust w/baffles. For now, the air box is in the bike but I plan on removing it soon (air filters on their way), so lets just say it's gone and K&N pods are installed. The AI has been removed. Near future plans include an exhaust heat wrap. The dealer, who was also supposed to fix an oil leak and didn't, claimed to have jetted the bike for my elevation.

The symptoms:
The bike pops and backfires (a lot) on deceleration, with the throttle closed. It does not backfire any other time. When I replaced the plugs two nights ago they looked to be running too lean. Occasionally, when cruising, the engine will bog (more like choke itself if I'm right about it running lean) for a second when I twist the throttle open to accelerate. I'm not sure but I think it may also be idling high when warm (nothing extreme, just slightly 1,500 rpm or so). One other thing that scares me is that the dealer claimed the bike was running so rich, according to whatever machine the hooked up to it, that it should not have been running. The engine also seems to have a slight miss (barely noticeable) when cruising around 4,500 rpm.

Please help me make sense of all of this.

oh yeah and what's this I hear about everyone running TBS needles? If I'm going to have the carbs apart I might as well mess with what I can right?

Try downloading the "Jetting Calculator" from Pat's site. It's a good place to start. I threw your info in it and based on stock Pilot Jets (42), the calculator comes up with 158 Main Jets at your altitude using "open" pipes. I'm guessing the 12" shorty's you mentioned could be considered fairly "open"

The aforementioned executable runs alone on your OS and is virus free.

Regards,

Tom

Thanks Tom,

I couldn't get the calculator to open (the Mac didn't recognize an application to open it with). You're right in assuming that they are fairly "open". There is a four or five inch metal baffle inside with some holes drilled in it and that's about it.

I'm betting you are running lean. Try turning your air mix screws out and if that does not lessen your backfiring, up the pilots to 45. If you are doing away with the airbox, you might want to start with 145-150 mains and see how that goes.

The TBS needles have more of a taper than the stock needles. Changing to them would be like upping your main jet size slightly so it's an easy way to richen the circuit.

You can drill out the vacuum hole in the slide to 7/64" which will help make the throttle response crisper.

Please only do these mods one at a time. If you do them all at once and have a problem, you will have no idea which one caused the problem.

I'd work the pilot/low speed circuit first and then progress to the mid/high speed.

Thanks for the tips! I'll let you know how I make out.

If I drill the whole in the slide will I need to adjust the jets again? or is that something I can do after getting the jetting right?

Drilling the hole in the slides only improves throttle response, it doesn't alter jetting requirements.

I've looked and didn't see an air adjustment screw, I'm sure that it's there but I'm feeling like an idiot for not being able to find it. Which way do I turn it when I do find it? Thanks guys.

dead center right in front of the carb bowl, it may be covered by a plug.

Mixture screw is circled in red, phot is borrowed from Pats carb site in the Tech Vault.

You know, at 4500 ft I'd be surprised if you'd didn't find 45 pilots to be a good solution to the idle and popping issue right off the bat. It's hard to know about the mains if you're going for K&N pods but surely you'll need to be somewhere in the 160 range at that altitude.

Turn the mix screw to the left to open it...you might as well run it all the way in (not too tight or you'll deform the seat) so you can count turns on the way back out. The counting bit is only good for a reference, you really adjust the screws for the best idle (and also run at 2K in 2nd) so each carb can vary and might be set slightly differently. Greybeard and The_Dog33 have both described this at length so I won't repeat it here...you can do a quick search and the reading will likely do you good anyway.

Once you get it pretty good then you need to think about getting the pair balanced for optimal operation.

I really think 160 is going to be too large at 4500' probably more like 145 to 150 , air gets thinner the higher the altitude so the less gas you will want in the engine. I found 160 to be too large at 1800' as have many others at or below my altitude ,so I think at 4500' they would be way too big. Maybe 155 tops. I do agree with the 45 pilots though and then adjust for best idle. Turn in until it runs rough then out until it runs rough then split the difference. I disconnect 1 plug wire and do the other cyl then switch.

Thanks again! I'll feel better about running around on it this weekend if I can get it a little more rich. I don't have time to sit and really mess with the jets, nor do I want to prior to putting on the pods. I should but my Father just road his Harley the whole way down here from Pennsylvania, so I have some riding to do this weekend! No time for work! Anyhow, thanks for the tips and I'll start searching a little more regularly before asking questions as I'm sure jetting with these bikes has been covered quite extensively on here.

The jets I was talking about were to go with the pods not stock air box.

Would this be the same for a 2005 with an 865, Bub's, and K&N pods? The stock pilots on it are 40 rather than 42.

yep still go with the 45, I suspect they may have gone with the 40 rather than 42 for emissions purposes not performance.Remember jetting is different for different exhaust and intake set ups as well as different elevations.

Ok, so I've finally looked in the tech vault and have a better idea of how these things work. I do, however, have one question: What is the reason/purpose of shimming the needle?

To provide more fuel from the needle when the main jet comes into play, which is around 1/2 throttle (approximately) or more.

Normally shim it if you have a flat spot in the mid range.

That's interesting. I just completed my partial removal of the airbox and installation of the K&N's, as well as upping the mains to 150's. I haven't ridden the bike much, but it has a flat spot when you open the throttle abruptly from the 1/4 throttle range. At this point I'm not sure if it's starving for air or getting too much fuel. I'm kinda thinking that it's still running lean down low because it's still backfiring and popping on deceleration. I couldn't up the pilots yet because none of the local shops had 45 pilots. I'll be ordering a set tomorrow. Am I heading in the right direction?

Sounds like you are, don't expect very fast throttle response the slides are vacuum operated and will only open as fast as the vacuum can build above the slide. I meant a power flat spot. Once you have the K&Ns on there stock pilot will be way lean.