Bonneville America Forums Forums Technical Forums Performance Mods Different jetting between the carbs

I just had a dyno-tune performed on my "new" bike after the break-in period following a 904 BB (it was a 790cc). This was at a Triumph dealer. I asked the service manager if this included balancing the carbs and she said "of course". After it was all said and done (the guy doing the runs seemed to know his stuff, and spent a long time analyzing and tweaking with the sniffer in each exhaust) I ended up with:

Left Carb:
- 155 Main, 42 pilot, 2 shims

Right Carb:
- 150 Main, 42 pilot, 1 shim

In the notes he remarked that airflow was different from left to right.

I had a chance to talk to him and asked about this difference and remarked that I thought that the carb balance screw would remedy this. He said not really and showed me the different charts he ran and this and that. I was short on time so I didn't get into it, plus I knew he was consulting the the other mechanic who I know is Triumph and Ducati factory trained and works on race bikes and such while he was making these decisions.

Has anyone else heard of this? I don't remember reading where anyone else had different jets on their carbs.

BTW, my bike is running awesome. I went from 56 hp and 43 ft/lbs torque running long TORS and airbox eliminated pre-big-bore, to 70 hp and 53 ft/lbs torque with Specialty Spare Long slash-cuts.

Here's my dyno-run chart showing the old run and new ones:
[image]http://i89.photobucket.com/albums/k214/airbuspete/TBA/dynorun.jpg[/image]
Here's a lame cell-phone video of one the runs - taken from outside the room looking through a window:
[image]http://s89.photobucket.com/albums/k214/airbuspete/TBA/?action=view&current=0428091230a.flv[/image]

I thought I knew how to embed the photo and video but I guess not...You can see them if you click my name and check out my photobucket.

Thanks Erwin!

Interesting post. Did you have any head work done that may have caused an airflow imbalance?

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Did you have any head work done that may have caused an airflow imbalance?

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Hmm, I hadn't thought about that, but yes, I did. While the barrels were out getting bored I asked if they could "clean up" the head. They spent about 3 shop hours (I sometimes wonder what that translates to in real hours) porting and polishing (probably not a true port and polish, more of a clean up). They said it would yield a bit more bottom end torque. I hope that's not what is causing the imbalance! The bike runs great, I just hope it stays that way!

I'm betting the intakes are hogged out unequally. That's what they make flow benches for. I wonder what the mis-match is gonna cost you performance-wise?

I wonder what an engine that no one has messed with would show. I never really thought about putting a sniffer in each exhaust. The sniffer went in the closest exhaust and I jetted by that.

I presume that Bill H. ran the dyno. My 865 did 52 lbs of torque (uncorrected)on that same dyno, on a cooler day.

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I presume that Bill H. ran the dyno. My 865 did 52 lbs of torque (uncorrected)on that same dyno, on a cooler day.

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You obviously know the dealership. Actually it was Glen(young guy)who did everything, the BB, headwork, dynos. I was kind of bummed when I found out that is wasn't Bill, but Glen seems like a good guy and I hope Bill was kind of watching over his shoulder. Since I was there waiting while they ran the dynos I did see Bill looking at the charts in between runs, so I think he had an input to the proper jetting and tuning decisions. Hopefully it wasn't, "geez Glen, you #@%&'ed up that headwork, now how are we going to fix it?!" If I hadn't asked, I don't think they would have even told me what the final jetting was.

That's a great torque number for you Ken. I'm happy with my gains though, 20% in both hp and tq.

My bad, the max. torque on my Feb 09 run (865, stock internals) was 51.10 uncorrected...

My carbs have been ballanced with the merc tube vacuum gauge, but I'd bet there's a difference because my left head pipe is yellower than the right. The only change is TOR long slash pipes and rejet the same on both carbs. The left pipe was yellow right at the head the day I rode it home and has gotten worse since then. I thought it was the AI. causing the problem but some kind of flow thing could be causing it. I guess I'll be heading for a dyno tune myself sometime in the near future.

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My 865 did 52 lbs of torque (uncorrected)on that same dyno, on a cooler day.

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My bad, the max. torque on my Feb 09 run (865, stock internals) was 51.10 uncorrected...

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That's OK Ken, what's .9 ft/lbs amongst friends!

If you have the stock airbox, it makes sense. The reason the idle was so lumpy on the carburetted Harleys was because the rear cylinder actually runs ahead of the front one, so the vacuum in the intake is a bit higher when the rear intake valve opens. So, when the manifold pressure is lowest (higher vacuum), which happens with the throttle closed, the rear cylinder gets less air.
Makes sense that the 90 degree offset in our cranks would have the same effect, only at higher speeds, because the common point in our intake is the airbox.
This also shows just how restrictive the air filter is. Bet it would be a different story if you got rid of the airbox.

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Bet it would be a different story if you got rid of the airbox.

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Ed, thanks for the info, I did get rid of the airbox. I have the freak kit with K&N pods. She's running great now, so I'll leave well enough alone for this riding season.

Pete