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Slow throttle response

Old 12-13-2005, 08:39 PM
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Slow throttle response

Hey all,

I am trying to understand what is happening to my bike since I installed a new exhaust. I got a Two Brothers Racing Exhaust (hell yes) from Ebay. It was used but well taken care of. Out of sheer excitement, I went ahead and took off my stock cans and made a neat little metal christmas sled with them out in the yard. 8)

Feeling the urge to hear my canless motor roar, I cranked up the Hawk and gave it a few revvs. Damn. Sounded like a harley, except a bit more refined. I took it for a spin around the block without any pipes at all, and, other than a little backfiring, the bike felt/sounded like a beast.

My TBR pipes came in the mail, and I installed them no problem. However, it's crappy weather and I can't go riding, only garage work for now. As I'm cranking it up since the install, the throttle response is about a half-second delayed, and I'm trying to figure out why. Also, one of the pipes (clutch side) is running hotter than the other (throttle side). In fact, the throttle side can isn't even hot. I only ran the bike for maybe 5 minutes because I don't want to kill myself Carbon Monoxide-style.

My initial reaction was carbs. Because the revving is more uniform when I revv slowly, I'm thinking it's a vacuum issue. Somehow the air/fuel mixture isn't right, and perhaps it's because the slides can't keep up with the increased exhaust gas outflow, but at least both cylinders are firing. My question is: is it required to re-jet, and/or get a K&N airfilter when installing aftermarket exhausts like Two Brothers?

Has anyone had a similar issue? Is it even possible that riding the bike without pipes somehow messed up the compression ratio or valve timing?

Sorry for the extremely long post,

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Old 12-14-2005, 04:33 AM
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I know that TBR says it is not necessary to rejet, but I think it is the way to go, get a Factory jet kit and put that in, I noticed a big difference with mine, also I recommend a BMC air filter to go with that.
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Old 12-14-2005, 10:29 PM
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Re: Slow throttle response

I 2nd the rejetting idea.
I have the same system & rejetted.
I went dynojet but the factory pro is a little easier.

It will seem like you have a new fire-breathing monster VTR after you do it! It REALLY opens up above 6K so hang on tight the first time you crack the throttle wide open at 6K.

Here's a K&N rejet kit for your VTR on ebay for $30 + $10 shipping

My memory fails me but I think the K&N kit may be a relabeled Dynojet kit (but could be a factory pro...i forgot)
Anyway it's a heck of a deal for $40 shipped to your door.

The factory pro kit uses the stock slide springs and gives you new #50 idle jets ( plus main jets, needles, & clips)

The Dynojet kit replaces the stock slide springs & has you drill a small hole in the slide (to improve throttle response they say).
They give you a jig & drill bit to use for the drilling (no big deal - was easy)
It also gives you main jets, needles, & clips but NO new idle jets.

Most of the guys here say the factory #50 idle jets are too rich & go with #48 idle jets instead (stock is #45)

The word is the Dynojet kit is harder to get dialed in but has more potential
The factory pro kit is install & go.

When I installed my Dynojet I followed the instructions & it ran fine but could use a little fine tuning (I think the idle to 3000 rpm is a little lean)

I got a set of #50 that I am going to try with my dynojet kit.
Maybe I'll get time in January to work on my VTR

Personally I like dealing with the Factory Pro people more than the Dynojet people. Maybe cause they are friendlier or something. They'll spend more time on the phone with you. (I used a factory pro kit in my last bike)

Here is a link to how carbs work.
It's a different model of carb than on the VTR but gives you an idea how they work.
Note "Part II-Tuning and Options" has a graph that shows the over laping performance & relations of the idle jet, needle, clip, & main jet. kind of gives you a better understanding how it works.

Go download the VTR shop manual here:
The factory website has lots of good info. It is a "frames" web site so I can't provide a link directly to the tuning page. Click on the "Product Support" link top center
Greg Nemish has some tips here:
And of course we have some info here:
and here

Some people like the K&N filter & some don't. (loss of mid range HP & Torque with K&N but more HP at top end)
I left mine stock.
With the K&N you must jet it MUCH richer on the mains
(it's noted in the jet kit instructions)
Mine was 180/185 with stock filter & 190/195 with K&N for applications below 3000 feet altitude.

Hope this helps.
There are many guys here that can help with your questions....just ask!
(I asked... they helped, informed, & educated me)

Enjoy your S'hawk & the forum


....and you thought YOU were long winded (NOT)
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Old 12-15-2005, 07:35 AM
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Re: Slow throttle response


I guess I was a bit spoiled because my last VTR had a TBR system and the carbs were already jetted. Jet kits are pretty straight forward, and a fairly affordable upgrade (as opposed to new pistons). I am assuming that the bike's computer system is not going to be affected by riding without pipes, and I guess that can be ruled out. I appreciate the input.

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Old 12-18-2005, 06:51 AM
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FYI, the K&N is the Dynojet kit.
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Old 12-18-2005, 07:11 AM
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I got my Superhawk used with a K&N and MIG pipes, but the prev. owner doesn't know if the first owner rejetted. My question: What do I look for that will tell me if I have non-stock parts?
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Old 12-21-2005, 03:58 PM
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Originally Posted by Loco";p=&quot View Post
one of the pipes (clutch side) is running hotter than the other (throttle side). In fact, the throttle side can isn't even hot.
On the throttle side, where the header meets the slip on pipe, there is an obstruction/overlap inside the pipe, which can be viewed if you pull the slip on pipe.

Some people grind it out.
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