Carb Tuning guide (setting up a Dynojet or Factory Pro kit)

[davebrannigan]

Very interesting. I wonder if they're really needed at all then? One thing for sure is that they get in the way of adjusting the pilot screws!

I've read of others relocating the coolant reservoir but not sure where they put it. That is also a piece of shocking design putting that where they did. Combined with the hoses it makes life very hard.

I guess I'm just bitter as I cannot get this bike running well. I suppose once it's set up the "problem" pretty much goes away and the design is fine. Just wish I could get to that point.

[VTRDark]

Not only de-icing but the heat also helps with the atomisation of fuel. Look how close they are to the float bowls so warming the fuel in there kind of like pre heating an oven. It's like a thermostat only in reverse. A thermostat remains closed at first to help the engine warm up quicker and then opens once the engine reaches sufficient temperature and starts to circulate the coolant around the system. The coolant hoses under the carbs are the reverse and open when cold and when there is sufficient heat in the engine they close.

[davebrannigan]

Can someone please enlighten me as to what a "Needle jet" is and where I might find one in my VTR carbs. I've been in touch with a local tuner who says my issue may be a worn "needle jet". I explained that I have already replaced the pilot jets and asked why, if he was talking about the main jet, it would have an effect on the low throttle opening but he's adamant that this "needle jet" could be the problem. When questioned did he mean the pilot jet he replied...,

"NO , there is only one in each carb, it is not a fuel jet you can change with a screw driver. It will not have a size on it. if a bike has good a good idle and smooth low speed cruise but stumbles just off the idle rpm , a worn needle jet is a very likely cause."

What is he talking about?

He's offered a dyno run with air fuel check for £60.

I'm out of my depth here.

Dave

[8541Hawk]

Unfortunately a worn set of needle jets means you need to source a new (to you) set of carbs.

The needle jets really are not replaceable in these carbs though places like FactoryPro have changed them out but it is not something most could do at home.

[VTRDark]

Its the female end of the needle. As has been said this is not normally a serviceable part as it's press fitted into the body but is the kind of thing a specialist carb refurb company possibly could do.

[davebrannigan]

So back to an earlier question, with the bike having done just 18K miles is it likely that these have worn to the extent that they are causing hesitation/stuttering, especially given that my issues are all around very slight throttle openings?

I guess getting it on a rolling road would tell me one way or another, though doing that is half ways to the cost of a replacement set of carbs on t'internet. I'm just concerned that I buy a set of used carbs and they are no better than the ones I have.

What a nightmare.

Dave

[8541Hawk]

Even without sub filters 18K is a little soon to have that much wear on the needle jets.

[VTRDark]

I agree. Sounds to me like the tuner is just trying to make a sale. Very low throttle opening is down to the pilot and more so the fine tuning of the pilot mixture. Have you tried pulling the choke out to richen the mix ever so slightly to see if that makes a difference. Buying used carbs is like a lucky dip. Funnily enough one of the things that can wear the soft brass needle jets more is fitting aftermarket needles. The OM needles are a very soft material compared to say a FP TI needle.

[davebrannigan]

Everything is OEM.

As said previously, if I so much as move the choke cable a fraction she cuts out as if it's already way to rich. Note sure if that's normal with these type of enrichening devices which are not really chokes per se. Id love someone to try this on theirs and see what theirs does.

With the 48 pilots I could wind the needles all the way in and it made absolutely no difference and with the 45's I've now now fitted she barely starts to sneeze. Touching the choke with either jet causes a stall.

Frustrating. With it like it is it's no fun to ride so I don't.

Cheers

dave

[8541Hawk]

Possible the enricheners are not seated properly in the carbs, if they are leaking then you can have all kinds of issues.

[VTRDark]

Float valve would be another area to look at. If that is not shutting off completely and allowing fuel to continue then its going to flood. The cone shaped rubber base does wear in these and are a serviceable item easily and cheaply changed. I would also check the floats themselves for any leak too while at it. I expect they are OK but it does no harm to check and rule it out. Unfortunately the float height is non adjustable in these carbs. You certainly have a rich problem because if you pull the choke out it should simply raise the revs but could possibly cut out if fully extended and the bike is at running temp.I don't understand why your dropping the needle to test the pilot jet size. It's the amount of turns on the fuel mixture screw which will determine the right size pilot jet but by the sounds of the problems your having I don't think the mixture screw will have any affect at all.

[8541Hawk]

Float valve would be another area to look at. If that is not shutting off completely and allowing fuel to continue then its going to flood. The cone shaped rubber base does wear in these and are a serviceable item easily and cheaply changed. I would also check the floats themselves for any leak too while at it. I expect they are OK but it does no harm to check and rule it out. Unfortunately the float height is non adjustable in these carbs. You certainly have a rich problem because if you pull the choke out it should simply raise the revs but could possibly cut out if fully extended and the bike is at running temp.I don't understand why your dropping the needle to test the pilot jet size. It's the amount of turns on the fuel mixture screw which will determine the right size pilot jet but by the sounds of the problems your having I don't think the mixture screw will have any affect at all.

about all I can add to this is that you might also want to make up a float drop gauge as I have seen the tab that the float valve rides in wear.
It was the root cause of the last set of "un-tunable" carbs I worked on and as stated the only fix is a new set of floats.

[VTRDark]

As a side note, like the the needle profiles interestingly that's another area of subtle difference between carbs. The air filled inside the floats is done in accordance with atmospheric pressure of the country, probably filled inside a hyperbaric or barometric chamber. No major concern unless living on top of a mountain or way below sea level.

[davebrannigan]

Personally I don't believe it would make any difference if there was a vacuum inside the floats or if they were full of a heavy gas like Argon. The level it floats at is determined by the amount of fuel it displaces and the weight of air in such a small volume is negligible up to a couple of hundred bar compared to the overall mass of the float assembly - I don't think:-)

I've proven the float valves are sealing off by holding the petcock open using a vacuum pump with the engine stopped. at what level I don't know, but they do seat.

When I first got the bike the two "choke slides" were solid with lacquer. I managed to free them up and as far as I can tell (using a Vernier to measure their position relative to the top of the orifice) they are closing all the way. I cant really think of another way to prove that. I have thought for quite a while that there is some problem in this region though as usually an engine will tolerate a little choke without dying. That said, these are not "conventional" choke mechanisms. Id like to know what another VTR is like in this respect and have requested someone test their for me but not seen any replies.

Edit> I should also say that without the "choke" applied she will not start from cold so that makes me wonder about the part the enricheners are playing in this. Surely if she was that rich she may start from cold albeit a bit rough but she will not run even on a hot day.

I have too many other distractions (AKA money drains) at the moment and don't have the time, patience or budget for this one so I think I'm going to cut my losses and sell this one as is.

Cheers

Dave

[kenmoore]

One thing I know for sure instead if a VTR is running O.K eave it alone.

I have recently changed my exhaust so I will sync the carbs and do an Idle Drop.

That oughta do eh?

Going for a ride will tell.

Cheers from down under and everyone have a good one.

Merry Xmas From Oz .