Carb and muffler advice

**limbimtimwim** · 21st September 2005, 21:06

So I did the thing that my brain said was bad. And other people who know better. Those people save your heckling and just accept that I am stupid, and try and save me from more doom. :-)
I have a carb kit here (main jets, needles, drill bit) and I have a muffler showing up in the next couple of days. So now I am thinking; what do I do now?

Now applying what I know from the computer bizzo, the golden rule is one problem at a time. This means when you are changing stuff in around in a complex system, you only do very small changes, then you retest. You will eventually end up with the result you want.

Which in this case is at best less than 2hp and deafness (Good) at worst a pistons with holes in them (Bad).

So my question is this, do I:

* Fit the carb kit first, and dial it in as best as I can then fit the muffler after that. I imagine this means I *may* have to take the carbs to bits again to change the needle positions and will have to fiddle the carbs again afterwards.

OR

* Fit the muffler first, then the kit and try and dial it in. But have I changed too many variables at that point? Do I risk having extreme trouble setting up the carbs because the reference point has changed too dramatically? But then it is likely that I do not have to take the thing to bits again after that, saving me effort. I feel less pulling things apart if you manage it is good, less chance of screwing things up by damaging something delicate in the process.

Obviously, I don't have access to a dyno or an exhaust gas machine, so I have to do it with my bum, eyes and ears to figure out what needs adjustment.

I've never done this before, so it's going to be quite a little adventure...

**Rumble** · 21st September 2005, 21:27

My advice:

Fit the can,

Fit the carb kit

DONT F with the carbs, unless you do know what you are doing (which you sound like you dont) or have someone with you who knows what they are doing (which isnt watching you play with the carbs).

Take it to a bike shop, pay the $70-150 odd and get it done by someone who does that sort of thing everyday. Maybe even ride it to the shop pay a bit more and get them to fit the carb kit.

My experiences with carbs (apart from my 2hp suzuki outboard) have ALWAYS gone bad (or not as good as one would hope)

**TLDV8** · 21st September 2005, 21:28

A VTR250 is a twin carb V-Twin ?...If it was me i would put the new muffler on first,it shouldn't make a drastic difference to the jetting..then you could start by checking the sync on the carbs,then checking the pilot jets and airscrews/idle speed.....if you have to wind the mixture screws in much more than 1 1/2 turns out a bigger pilot jet can be installed (it may be fine)......then you can move onto the needles ..one way is to measure the taper on the stock needle at set points with a vernier caliper to get an idea of its flow,that can be compared to the ones in the kit..you may be fine just raising the stock needle one click or so...same with the mains,maybe a size or 2 up,you will know if it is rich or lean by the way the engine reacts...it just takes a little patience...one of the bonus's of fuel injection and a PC2,just plug it in to the computer and change the map

**cowpoos** · 21st September 2005, 21:37

do a web search....which you should be good at...find the company that you brought the carb kit off....find the user manual...or request it to be email to you....I'm guessing it'll be a dynojet???? request common settings for your bike with aftermarket muffler?

They should be able to provide that info as they have to use a bike like yours to find improvements to develop a kit to sell....easy...!!!!

**TwoSeven** · 21st September 2005, 22:11

Fit the end can first. Then run the bike up to temperature and do a plug chope at 1/8th interval carb settings (hold the throttle constant for 7 secs before hitting the kill switch).

Assuming you can read the plugs accurately, this will tell you what circuits inside the carb need to be modified. If you cant do that, spend 60 bucks on a dyno run and get the chap to tell you what needs to be done.

Usually you buy the carb kit after you fit the end can, so you know what jets and stuff to order. If its a standard off the shelf jet kit, then it may have been designed for the orginal zorst, it may not work for the aftermarket model.

**FROSTY:Me** · 21st September 2005, 22:28

save ya money.Those lil VTR's are pretty darn good as they are.
Take the bike to a shop with a dyno and have em set it up for you.
I must say ive fitted a shit load of dynojet kits and for NORMAL road use they actually made matters worse.They do seem to be aimed at better mid to top end power. If ya gonna do anything fit the muffler first

**nudemetalz** · 22nd September 2005, 12:02

Let me know when you're going to do the carbs, I'd be interested to watch what happens as I"m considering getting a "more authoritive" muffler for my VT also and the engines are virtually identical.

Chris.

**limbimtimwim** · 22nd September 2005, 20:11

Originally Posted by Rumble

My advice:Fit the can, Fit the carb kit DONT F with the carbs, unless you do know what you are doing (which you sound like you dont)

Nope, I don't :-) . I'm not completely ignorant however, I used to pull apart ancient japanese cars for fun. Carbs are new to me though.

Take it to a bike shop, pay the $70-150 odd and get it done by someone who does that sort of thing everyday. Maybe even ride it to the shop pay a bit more and get them to fit the carb kit.

I forgot to mention my other motivation; I want to learn something. If I take it to a shop for all the stuff I can do without spending money on tools, why should I? If I mess it up, I mess it up and I learn not to try this again :-).

My experiences with carbs (apart from my 2hp suzuki outboard) have ALWAYS gone bad (or not as good as one would hope)

I appreciate your advice. I hoping not to do anything that I can't undo.

**limbimtimwim** · 22nd September 2005, 20:21

Originally Posted by FROSTY

save ya money.

It's to late for that mate :-)

I must say ive fitted a shit load of dynojet kits and for NORMAL road use they actually made matters worse.They do seem to be aimed at better mid to top end power. If ya gonna do anything fit the muffler first

Ah well, who needs low end power? ;-)

I had a look at it last night and one of the needles looks wrong, so it probably will be going back to California. So the muffler will come first anyway, so lots of noise and no real gain :-).

I can't put it on myself, the darn system needs cutting and welding.

**Rumble** · 22nd September 2005, 20:24

Originally Posted by limbimtimwim

Nope, I don't :-) . I'm not completely ignorant however, I used to pull apart ancient japanese cars for fun. Carbs are new to me though. I forgot to mention my other motivation; I want to learn something. If I take it to a shop for all the stuff I can do without spending money on tools, why should I? If I mess it up, I mess it up and I learn not to try this again :-). I appreciate your advice. I hoping not to do anything that I can't undo.

I wouldnt mind knowing how to do it, in fact I would also love to learn, just the dont call it 'tuning' for no reason... Its very hard to get the proper comprimise between starting, power and efficiency...

**willy_01** · 22nd September 2005, 20:53

first of all good on you, i have been really keen to do what you are planning to do but just gave up on the idea due the very small gaines it would give my bike, that and my carbs are VERY hard to access. Anyway my bike crapped out on me and i was forced to fix my self (no money but was keen to learn) any way after reading a LOT plus taking a look my self carbs dont seem too hard to play with (famous last words).

From what i gather the best thing to gauge change is your air/fuel ratio and throttle position. So if i was you i would really want to get my hands on something that measures this ratio, then test it before you put the kit in (get some sort of a yard stick) then put the kit in, balance your carbs then start adjusting jets (remember 14.7 to 1 is the most efficient ratio). My theory is any mod you do after the kit ie better air intake you will need to re adjust the AF ratio via the kit.

**limbimtimwim** · 22nd September 2005, 20:56

Originally Posted by TLDV8

A VTR250 is a twin carb V-Twin ?...If it was me i would put the new muffler on first,it shouldn't make a drastic difference to the jetting..then you could start by checking the sync on the carbs,then checking the pilot jets and airscrews/idle speed.....if you have to wind the mixture screws in much more than 1 1/2 turns out a bigger pilot jet can be installed (it may be fine)......then you can move onto the needles ..one way is to measure the taper on the stock needle at set points with a vernier caliper to get an idea of its flow,that can be compared to the ones in the kit..you may be fine just raising the stock needle one click or so...same with the mains,maybe a size or 2 up,you will know if it is rich or lean by the way the engine reacts...it just takes a little patience...

Cheers, thanks.

one of the bonus's of fuel injection and a PC2,just plug it in to the computer and change the map

Pah computers! Oh wait..

Originally Posted by Cowpoos

do a web search....which you should be good at...find the company that you brought the carb kit off....find the user manual...or request it to be email to you....I'm guessing it'll be a dynojet???? request common settings for your bike with aftermarket muffler? They should be able to provide that info as they have to use a bike like yours to find improvements to develop a kit to sell....easy...!!!!

It's not a dynojet, I tried to ask them questions, and I tried about 3 email addresses linked to on their site. Stupid buggers hadn't setup the corresponding mailbox, turds for brains. So they didn't get my money. Rung these guys ( www.factorypro.com ) he (Marc?) was very helpful (Talked a lot of crap though, so we had things in common) so I got his kit instead, and it has been fitted to a 98+ VTR. . They were willing to make up parts for it if what they sent was no good.

It does come with suggested settings, and a guide on 'If it does this, do this, if it does that turn this'. It did make sense, but I imagine in words it's hard to describe how lean feels.

Originally Posted by TwoSeven

Fit the end can first. Then run the bike up to temperature and do a plug chope at 1/8th interval carb settings (hold the throttle constant for 7 secs before hitting the kill switch).

That's a good idea, I can see how that would work. I will do that, thanks.

Assuming you can read the plugs accurately,

By the taste right? ;-).

this will tell you what circuits inside the carb need to be modified. If you cant do that, spend 60 bucks on a dyno run and get the chap to tell you what needs to be done.

Thanks, that will probably end up happening, yeah.

Thankyou everyone, especially TwoSeven, that's a clever idea.

**limbimtimwim** · 22nd September 2005, 21:04

Originally Posted by Rumble

I wouldnt mind knowing how to do it, in fact I would also love to learn, just the dont call it 'tuning' for no reason...

Buy a kit, a muffler, pull out the tools and do something stupid like me then :-).

Its very hard to get the proper comprimise between starting, power and efficiency...

I know, so even when I think I am done (Or I've fucked it up) I think I will still be taking it to a shop for a looking at. And well, don't care to much about the last one, though if I halved my mileage I'd be rather concerned.

**TwoSeven** · 22nd September 2005, 23:43

A little trick.

The technical name for the exhaust system up to the baffle/end-can is called an attenuator. To attenuate something means to reduce it. For exausts you're attenuating the wave form of the power pulse, on top of removing spent exhaust gases. That attenuation can be used to either scavange the engine to increase flow, or to increase back pressure which aids combustion (two different tuning techniques).

Thats another way of saying, when you mod the exhaust system, your moving the power/torque around.

Most aftermarket zorsts that I have looked at, usually chop something off the top, to replace the drop at 5k for the noise regs to give a smoother delivery - I suspect they do that because peak power often happens well before the redline, so there is some excess to play with. A good can, shouldnt give you a drop on your overall power/torque and a really good one might give you 4-5bhp increase (because the OEM ones are slightly detuned for noise purposes). What you can do is change the width of the power band, and/or its position up and down the rev range as well as the start and end positions of the rev range itself. A savvy race team may run a couple of different exhaust systems depending on the characteristics they want (along with a matched jetting profile). A tight twisty track may require the power band a little lower to match the low gearing, and a long fast track may want it up higher for example.

Its not until you start working with the jetting (and other bits) that you'll realise the gains in power, which is why many cans need to be jetted afterwards. If you have to rejet the bike just to bring the power back, its likely you have a can that may be set up to scavange (the Harris Uk one I have on my 600 does that, so I lose 1k rpm off the top, but get a wider power band - good for that crackle and flame on the over-run but crap for road use), whereas if the can offers more power before jetting, it may be set up differently

What I did with my old 250 was to run it up on the dyno for an all gear run, then use that result to calculate my gear shift up/down points (ie. what rpms I needed to shift each gear, and what gears needed to be short shifted). Since most of my riding was in third gear and I had a 520 conversion on the final drive, I then looked around for an end-can that suited the profile I wanted (although I had some bias on the looks side of things which took precident). I ended up with a nice can that gave me about 4bhp all up in power and a ft-lb in torque and smoothed everything out to a nice linear drive which suited me nicely. Problem was, it made me deaf when I used it so I sold it on after a while. I've never been a fan of bikes that just kick in with the power or have heaps of grunt but no usability. I could have gotten another 4bhp out of it by rejetting, but I couldnt see the point.

My current project is to design a 4-2 underseat exhaust for the old Cibby F2, so you get pipes coming up each side. Still doing the math at the moment (due to no dosh for materials).