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Old 10-07-2013, 09:10 PM
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Why the difference in the jetting front vs back cylinder. Is it temperature related or velocity stack related. If it's the latter, why can't I just run two of the longer stacks and jet them the same. It would simplify the fine tuning and evidently eliminate plugging one lift hole (as per 8541 Hawks great setup baseline)

Last edited by CaryDG; 10-07-2013 at 09:13 PM.
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Old 10-07-2013, 10:12 PM
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Originally Posted by CaryDG View Post
Why the difference in the jetting front vs back cylinder. Is it temperature related or velocity stack related. If it's the latter, why can't I just run two of the longer stacks and jet them the same. It would simplify the fine tuning and evidently eliminate plugging one lift hole (as per 8541 Hawks great setup baseline)
It is the latter, but at the same time, when running two long stacks or two short stacks the air flow is still different.

The air box was designed to be run with a short and a long so changing the way the air flows in the cylinder will not give you the same air flow into each cylinder (confusing, I know) just a different pattern of flow to the the object cylinder.

There is a reason there is a short and a long stack.
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Old 10-08-2013, 03:49 PM
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What's confusing is that in other threads it is being stated that running two Long stacks eliminates the need to have a plugged lift hole in the front slide. Is this know or just surmised?
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Old 10-08-2013, 04:18 PM
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CaryDG, I am by no means an expert, nor am I well educated on the topic, but from what i gather the reason for plugging the hole is to compensate and smooth out the vacuum signal from the venturi effect of the stacks. The rear stack, either makes a larger or smaller vacuum thus not needing one of the holes plugged. If you read through all the pages of the Hawk carb set up, you will find your answer most likely. Hope this helps.
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Old 10-08-2013, 04:33 PM
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Whoa hold on... the rear is jetted richer at least partially for cooling. That is correct as far as I know. Front cylinder gets more cooling from the air so the rear gets a little more gas to compensate.

Second, you don't need to plug either hole. It just delays the pressure signal a touch so the mains don't open too soon (the way that HRC does). You will benefit more from clean and balanced carbs than trying to guestimate a velocity stack/slide plugging option. I only say this because swapping a velocity stack for the sole purpose of not pulling the carbs apart to plug a slide is not really the best way to tune them. If you're having problems, a v stack won't fix them, and if you're tuning properly, you'll have the carbs apart anyway... hope this is making sense. The stacks are doing more than just opening the slides, so you'd be changing a lot more than you'd be fixing by doing this particular mod.

Also, I'm not saying not to do it (or that I wouldn't try it), I'm just pointing out that there's more to it than that!
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Old 10-08-2013, 04:53 PM
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+1 on 7moore there, accept I thought the plugging of the hole directly slows down the point as to when the slide starts to open and switches to the needle circuit, keeping you on the pilot circuit a little longer. The slide opening later indirectly affects the mains.

I think there is a good possibility that only reason for the shorter stack is to give the bike a little more top end. It helped Honda with their marketing and top speed figures against their competitors. I run two standard long stacks for that little extra bottom end/midrange punch at the sacrifice of top end. If you want both then use Dr Honda billet stacks.

(:-})
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Old 10-08-2013, 05:11 PM
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I ran two long ones on mine up until I crashed it, I don't think the short stack really helps with top end much, but YMMV
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Old 10-08-2013, 07:10 PM
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7, I was simply asking for info. My previous V twin was a built to the hilt NT 650/727 Hawk. It had different port configuration/angles on the two heads, hence the different jetting on it. + the cooling aspect. I ended up going with two rear cylinders to get the best flow and power. The VTR seems identical front to rear other than the stacks. I pull my carbs a couple of times a day while playing with the jets. I've got them pretty sorted out just trying to make it better I've only had it a couple of weeks. So I'm doing the DM F4 shock and fork mods next. I'm in the process of hooking up a portable EGA so I can verify seat of the pants.
Thank you everyone for the input.

Last edited by CaryDG; 10-08-2013 at 07:11 PM. Reason: spelling
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Old 10-08-2013, 07:14 PM
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I'm putting a bung in each head pipe so I can get differential readings. I'd love to get it in the 13.6 - 14:1 range through out on both cylinders.
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Old 10-08-2013, 08:27 PM
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Hey wasn't trying to be overbearing, just wanted to give too much info rather than too little! Bung in each header and an EGA would be further than I've taken my jetting!
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Old 10-09-2013, 09:43 AM
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7, are those tapered or spherical "over bearings" and where do I install them? Ha, Ha! Seriously though, thanks for the input. I can always rely on this site to give me all the info I need.
I'll let you know what I find with the EGA.
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Old 10-09-2013, 10:08 AM
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You install them over the under bearings, right next to the counter-steering strut.
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Old 10-09-2013, 10:31 AM
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LOL you guys.

I'm putting a bung in each head pipe so I can get differential readings. I'd love to get it in the 13.6 - 14:1 range through out on both cylinders.
This sounds very interesting. Where exacly will you be putting the bungs. Do you mean in the can outlet so the exhaust will pulse back through and out the other can or are you literally going to stop the two cylinders mixing by controlling it at the splitter pipe or something.

Keep us informed as I would be interested to see how you get on with this. Have you though about maybe fitting a Lambda sensor in each pipe closer to the head.

(:-})
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Old 10-09-2013, 11:07 AM
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I think he means what you are saying... a bung to screw a sensor into each head pipe:



Not a muffler bung...
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Old 10-09-2013, 11:51 AM
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Argh yes now that makes sense, a threaded insert. I've not heard them called bungs before but then chips are crisps to me. LOL

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