Ducati 916/VTR trumpet Carb jetting

[Varastorm]

Hi,

I have been running the bike with the Ducati stack mod for a couple of hundred miles now & the plugs are looking a little rich. Dark tan electrode & the threaded base a little sooty also. Don’t get me wrong it runs great, no flat spots or nasty popping at tick over etc. It’s a clear night & day improvement over the standard velocity stacks.

I am after any advice on jetting for a standard engine with large bell mouth stacks & open exhausts.

I have read that these engines can tolerate a rich setup (my wallet can’t though) & think it will be okay running it as is (rich) but I am afraid of going too lean by using the methods below.

When I initially started the Varadero & the fitting of VTR carbs thread over on the XRV forum
( http://www.xrv.org.uk/forums/varadero/7 ... carbs.html ) I had trouble finding the correct carb set up, then I found the carb set up thread from 8541HAWK. I threw away the Dynojet & free flow filter & have never looked back. Excellent advice, thanks.

Having read up many threads discussing the various ways to go, some say bigger mains, some say smaller mains & the single lift hole modification also. I was thinking of going with a leaner 170 front & 175 rear (I have these jets), also blocking one hole in each slide, using the glued "o" ring in the hole method?? Or just keep the standard mains & go for the single hole slide’s?? HELP.

Any recommendations would be great, thanks in advance.

[Stephan]

With Dr Honda stacks, I went one size down, from 180\182 to 178\180. I have plugged front slide hole, but I will plug rear as well.

Use epoxy for plugging.

e: I join with question, what are the opinions on the same jets. I have same lenght stacks, and exhaust is made to have the same flow from both cylinders, so I think it would be beneficial to have same size of jets. Just to add 1/4 turn on rear pilot screw.

[Varastorm]

Hi,

Thanks for the reply Stephan,

Just to clarify, I am running Hawks setup at the moment which is :-

#48 pilot jets, one thick washer under front needle & thick & thin washer under the rear needle,
Pilot screw set at 2 1\4 turns in the front and 2 1\2 turns rear.
Standard paper filter, Ducati stack mod (equal length, 5mm taller than Std tall stacks), shortened snorkel (to get it to fit on the bike).
Open race Renegade silencers & link pipes on Std headers.

It was running great & still is, but I feel it can be better. If I need to, I can dig out the Dynojet kit & use it for the smaller mains if needed......But then I read this.......

A note from Dynojet

We are often asked for a "cross reference" sheet that compares our main jets to Mikuni or Keihin. The fact is you cannot directly interchange the jets for a given size. Many things affect fuel flow through a jet of the same orifice size. The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburettor venturie. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed. At a certain speed the two may flow equally, but a change in velocity changes the flow characteristics. The Dynojet main jet hole size is measured in millimetres. For example, a DJ142 has a 1.42mm hole.

Michael Cory
Dynojet Research & Development



If I was to dig out the Dynojet kit for the mains, would I be right in thinking I can use the corresponding sized jets to decrese the flow & ignore the main jet taper bit? Or does that interfere with the hole size (fuel flow) also?

Quick answer to the above, NO!

This is the recommended dynojet Mains when you install the kit:- DJ180 front DJ185 rear which are Mikuni 169 front 173 rear.

If you cross reference the standard Mikuni jets:- 175 front 178 rear Mikuni = DJ186 front DJ190 rear.

I feel the urge for a much needed drink.....

[Stephan]

If you need to go one size down on main jets, you are just to buy one main jet for few usd or pounds, no need to recycle dynojet kit :-) But it is probably worth to run dynojet needles, if you don´t do so.

[Phil_H]

Hi varastorm, I have been following your bike mods with earnest over on xrv.
Regarding the jets, as I posted on here just this morning, don't feck about - stick to genuine Keihin jets (if that is what's in there now). I had a right old runaround a few years ago with some 'replacement' Keihin jets from ebay. Don't even go there, cross referencing with other brands is unlikely to be spot on.

Allens performance offer great service, have genuine Kehin/Mikuni parts & are no more expensive than spurious/ebay items. Just make sure you know tge jet part No before phoning.
I paid £12 for 2 x 48 pilots posted.

Keep up the good work

Me, I think I'll stay with stock carbs, just trying to richen the bottom end & lean the top end without losing the middle, which isn't as easy as it sounds :/

[Varastorm]

Hi Phil,

Thanks for the comments,

Have a go with the carb mod, it's all reversible & if you don't like it you can sell the bits back on eBay. I doubt you will though, also you get a taste of what these guys on this site are raving about, proper v twin power.

Your right about me nicking the Dynojet mains though, with the jet sizing & venturis they incorporate into the main jets, there's no point in me complicating things anymore than they are.

I was thinking of using them to get it a bit closer & when happy, get some genuine Mikuni ones. Or to get some help from members with proven large bell mouth setups & use them. Like Hawks set up guide which is an excellent base setup & read.

I have decided to go down the single slide hole route on each slide. Just not too sure on the mains though.

BTW, has any one any comments about the Dynojet needles? Are they any good? Or are they only used because of the ease of height adjustment?

Regards.

[VTRDark]

BTW, has any one any comments about the Dynojet needles? Are they any good? Or are they only used because of the ease of height adjustment?

Yeh having adjustable needles is handy especially if you want to lean them off (can't do that shimming) if running rich mains and/or pilot circuits.

Are they any good....put it this way they are designed to work with the dynojet mains which as you found out are a completely different design and are measured in flowrate according to the tapers. So it would be like mixing dynojet mains with the Keihin needles. It just complicates things. That's not to say that it can't be done.

This is one of the reasons I prefer the Factory Pro kits as their mains are exactly the same as Keihin with regards to design and measurement scale. This makes it a little easier working out where things are at. They also have smaller increment jets in the scale which helps with the differences between front and rear carb.

(:-})

[Phil_H]

Varastrom, what is the benefit of blocking one of the slide holes?

[tony.mon]

Varastrom, what is the benefit of blocking one of the slide holes?

And it'll add weight

[Phil_H]

Varastrom, what is the benefit of blocking one of the slide holes?

And it'll add weight

Eh...
And the benefit is????

[Varastorm]

Varastrom, what is the benefit of blocking one of the slide holes?

Hi Phil,

Because I am using the Ducati modded stacks, this is the theory below.

This is from the 8541Hawk setup guide.

The HRC type (single hole) slide is also the cure for the Dr. Honda stacks which cause the over rich mid-range. The problem here is the stacks create too good of a vacuum signal to the slides causing them to open too quickly. So in this case you would plug one hole in each slide. This will eliminate the rich condition without causing a lean condition on the top end.

[Varastorm]

Right then,

I have been using the (Superhawk forum) 8541Hawk setup for a few years now & it was good too, but since fitting the trumpets it has been running rich (dark - very dark plugs).

So back to the drawing board & more reading required, then I read an update to the 8541Hawk carb set up on this Vtr1000.org forum. Which he states,

Ok what I have to add is for the AL stacks. The vacuum signal idea for the rich mids was dead wrong. What I now believe the issue to be airflow stall in that RPM range.
The plan to address this idea will be to try and increase the airbox volume. I believe mikstr has done it already so I need to chat with him a bit but when I have time & some extra $$ I plan on mod'ing an airbox to remove the bottle and carb slide filters (yes still have them but out of the box) to see if that will help out any.

Now that threw me, to plug or not to plug the slides? If so, one or both?

It was running:-

Front, Pilot was 1 1/3 out, needle washer was a 0.7mm washer, 2 open holes & 175 Main.

Rear, Pilot was 1 2/3 out, needle washer was Std + 0.7mm washer, 2 open holes & 178 Main.

It ran very well with no problems, only dark - very dark plugs, it required no choke to start & was doing 33 miles to the gallon

So equipped with the plug colour & zero choke required to start (if any was used it would stall, hot or cold) plus the poor MPG, I thought there must be room for improvement.

So equipped with bit's n pieces from various setup threads I have attempted this:-

Front, Pilot 2/3 turn out, no washer under the needle, 1 open hole & 175 main.

Rear, Pilot 1 turn out, needle washer 1 Std thin washer, 1 open hole & 178 Main.

No test run yet, but now starts like it used to with half choke, ticks over at 750rpm to 800rpm when warm. Blip the throttle to 6-7000 rpm & she comes straight back down with no attempt to stall like it would sometimes try to before.

I know its a bit radical, but so are the Ducati stacks, they tried to suck my hand in whilst simulating an air filter @ 7000rpm

Any comments welcome.

[VTRDark]

Front, Pilot 2/3 turn out, no washer under the needle, 1 open hole & 175 main.

Rear, Pilot 1 turn out, needle washer 1 Std thin washer, 1 open hole & 178 Main.

That's not far off what I have except I'm using the factory pro kit with a BMC filter so my mains are one up at 178 front and 180 rear.

(:-})

[8541Hawk]

I'm getting quoted in a context that I can't even understand what I am on about.....lol

If you have any questions on why I rum my carbs the way I do, feel free to ask away.... I'm not here as often as I was but I'll get to them.

For the record, right now I run 175\178 mains, 48 pilots Dr Honda stacks with a hole plugged in each slide and a flo-commander and a set of HRC needles

Now this is VTR stuff a Valerado will need smaller jets to make things work

[Varastorm]

I'm getting quoted in a context that I can't even understand what I am on about.....lol

If you have any questions on why I rum my carbs the way I do, feel free to ask away.... I'm not here as often as I was but I'll get to them.

For the record, right now I run 175\178 mains, 48 pilots Dr Honda stacks with a hole plugged in each slide and a flo-commander and a set of HRC needles

Now this is VTR stuff a Valerado will need smaller jets to make things work

Hi 8541Hawk,

sorry I didn't quote the whole post, it is quite big, but here goes:-

Carb set up

Postby 8541Hawk » Tue May 28, 2013 7:40 pm

Carb set up

This thread has been around for a while on the US forum with many good results.
Just bringing it over here in case someone hasn't seen it or are not happy with your current set up.

Another way to put this after reading more on this forum and Roger D.'s post, this is more of and based on the Bob H. carb set up as he is the one who got me started with it and gave me my first base line settings for my HRC jet kit.

I will also ad a bit of new stuff at the end as I've been running the Al stack for a while now and have changed my opinion on a couple of things.
So here it is though the pic is missing..... though it is not really necessary.

As I've been asked a couple of time and with all the issues I've heard about with carb set up I guess it's time to pass on what I have learned in the last 14yrs of owning a SH (yes I picked mine up in 97)

Note: The following set up works with a stock airbox and stock air filter. If you want to run a aftermarket filter or airbox mods, you are pretty much on your own trying to get it dialed in correctly.


I have arrived at this set up by taking the info I have received from Dan Kyle, Bob Hayashida and HRC. Then by a little experimentation and looking at how HRC did things, I have come up with what I feel is a very good base line set up. As with all tuning, this is just a starting point and you will need to fine tune for your bike.

First lets cover a couple of things. First is the old axiom that the bigger the CV carb, the harder it is to tune. As this bike has the largest CV carbs factory installed, it is no wonder you hear about all the problems getting them "right". I've read many posts where people state that "the bike runs great, except at..... insert low end, mid-range, etc here.
To me this is unacceptable.

Then there is how the aftermarket jet kits (except HRC) deal with the stock set up. They address the low end /mid-range lean condition by running larger mains and a different needle profile and in the case of DynoJet by getting the slides to open sooner & quicker. While these methods work well with most bikes, on a SH there is IMHO a better way.

The HRC kit comes at things a bit differently. Though it is really for a full race set up and if you want to run ram air or a real high flow filter, then their method of plugging the air bleeds and running small jets is the way to go. On a street set up, this really isn't all that good of an idea. I'll just leave it at that for now. It can be discussed later but really doesn't have a lot to do with what this is trying to accomplish.

The main thing a street rider can get from the HRC set up is a modification they have done on the slides and their recommendation to run #48 pilots. The slide mod is useful to widen the power band on a bike with stock velocity stack. It also can be used for racing where the set up is 2 short stacks. This same method is what is needled with stacks like the Dr. Honda units which go rich in the mid-range. Though I am getting ahead of myself, so I'll explain the mod and what it does in a bit.

So back to the stock set up or starting point. The problem with a stock bike is it is a bit lean on the bottom and mid-range and pretty close on the top end by what I have seen. The good part is the stock needles actually are very good. In fact they are some of the best stock needles I have ever seen. Which leave us with how to get the best performance from the bike.

So here we go. The first step is to install a set of #48 pilot jets (per Dan Kyle & HRC). You will here people say they have tried this and it didn't work. The reason for this is that they have done it along with installing a jet kit. As I stated earlier, aftermarket kits install larger mains and different needles. When you change both, you end up too rich with a stock motor. It's a one or the other with this bike. By upping the pilots you cure the low end leanness and also add a little to the top end as the pilot circuit doesn't turn off, so larger mains are not required.

Then you need to shim up the needles. While they have a very good profile, they are a little too long for performance. The question here is how much do you shim them. Around .040" is a good starting point but will need to be tuned to each bike. Also there is more to it than just shimming both needles .040". One of the set up tips from HRC is to use one more shim on the rear needle than you use on the front (this tip might be helpful to try on bikes with an aftermarket kit also). So what I do when setting up a set of carbs is remove the stock thin washer that is under the front needle and leave it in place on the rear. With this method you actually end up with the front needle approx. .030" higher than stock and the rear .040" higher than stock which seems to work well.

So You can stop at this point, put it all back together and start with the pilot screw set at 2 1\4 turns in the front and 2 1\2 turns rear as a start and the bike will work pretty good but it can be better.

This is where the HRC slide mod comes into play. What is this "secret" mod? Well it really is quite simple. We'll start with this hijacked pick once again

Carb set up-slide-jpg

The 2 holes are called lift holes. It's where the slide gets the pressure differential signal which causes the slide to open. That is why DynoJet has you drill an extra hole, to cause the slide to open sooner, but that is the wrong way to go IMHO.

Because of the different length velocity stacks what is really going on with the bike is the front cylinder slide opens too quickly causing the front cylinder to go rich in the low to mid-range. FactoryPro even recommended running an emulsion tube from the rear in the front to help cure this problem.

Now the HRC cure for this is to supply a slide with one lift hole. This will lean out the low & mid while not effecting the upper mid-range and top end. So the last step would be to use some epoxy and seal one of the lift holes in the front slide.

This will give you a very good baseline set up. Yes some tuning will be required but all it involves is adjusting the washer stack ( you might need to raise or lower the needles a small amount) and set the pilot jets per the service manual.

The HRC type slide is also the cure for the Dr. Honda stacks which cause the over rich mid-range. The problem here is the stacks create too good of a vacuum signal to the slides causing them to open to quickly. So in this case you would plug one hole in each slide. This will eliminate the rich condition without causing a lean condition on the top end.

Hope this is some help to you guys.

Ok what I have to add is for the AL stacks. The vacuum signal idea for the rich mids was dead wrong. What I now believe the issue to be airflow stall in that RPM range.
The plan to address this idea will be to try and increase the airbox volume. I believe mikstr has done it already so I need to chat with him a bit but when I have time & some extra $$ I plan on mod'ing an airbox to remove the bottle and carb slide filters (yes still have them but out of the box) to see if that will help out any.

Like many which have stated before me, this is an excellent standard baseline setup.

But with me the fitting the bigger Ducati stacks ( & it drinking fuel) it made me look for answers for my new mixture problem & so since every thing you had to say on the matter has worked so far, I looked up what you had to say on the bigger stacks you had fitted.

Then I read the

The vacuum signal idea for the rich mids was dead wrong.

Now that got me, my Guru was

dead wrong

Now that was a sad read

I really don't want to modify the Varadero/VTR air box even though the( http://www.vtr1000.org/phpBB3/viewtopic ... irbox+mods ) post with Tony.Mon's huge airbox mod is an excellent read.

Its an eye opener to feel the amount of air these things suck, I placed my hand over the intake whilst blipping the throttle It is unbelievable that the (small) airbox works at all. The air being forced down the intake snorkel must be close to supersonic at full chat.

As I mentioned last post, I have not tried the jetting on the road, it might be pinking & surging. But I thought its worth a stab, be prepared though, I might be asking for more jetting help some time soon.

Also, the Varadero is running 48mm VTR carbs & open exhausts & will hopefully be fitting VTR cams in the new year too.

All I need to do then is ask Lloydie if he is still lightning flywheels?

Then I should have the Varadero that Honda should of built.