Jet sizing.

[AllanB]

I have read up a little on jet sizes and some of the information is slightly conflicting, so I am looking for some help from someone who really knows the subject. IYP.

Mod 1. Okay first of all I cannot get a paper air filter for this 30 year old NV400 anywhere, so have removed the paper element and replaced it with foam.

Mod 2. The silencer was a mess when i bought the bike and I am unable to find any data to put the baffles back to standard. So after a lot of twoing and throwing, I have settled on a 4cm bore, straight through baffle.

The exhaust system is 2 pipes into one silencer and all but the baffles, is standard.

The bike is fitted with 2 Keihin VD carbs and as far as I know these and their jets are still standard. I could probably measure the jets as I think the sizes are based on 1/100th mm, i.e. #80 = 0.80mm.

The bike is running a little lean overall, even though I have the mixture screw (fuel screw) opened up a little and the exhaust pops a lot under de-acceleration and sounds a little crude (for a Honda V-twin) especially at low rpm.

I am worried about overheating pistons, so what do I need to do, I am pretty sure I can get some jets locally?

For some reason this particular engine was designed to achieve 43bhp (1983-1989), whereas later models were detuned in some way down to 30bhp. I am only looking for some decent grunt from the engine and 30bhp is probably more than I will ever use.

Thanks for your help.

[namdocmai]

I guess nobody can tell you exactly which jet will be perfect. The bike is modified so i would buy a few different jets and just try them out. Also try different hights of the needle. Just trial and error method untill it runs smooth.

[KRS1]

a cbr 150 uses 115/35 ...so 118/38....120/40...and up might be a good start if you have 26mm-28mm carbs...the numbers are stamped on the jets....you can also fit a small washer under the needles top to lift it up to run richer if your needle is not adjustable. Hard to find though, remote control car kits usually are a good source to find these washers.

the rear cylinder is usually one size larger do to hotter operating temperature.

knock off jets and original jets also perform differently, cheap jets dont have the holes edge beveled and flow less.

btw: all keihin jets are NOT interchangeable, make sure you get the right ones. Be careful in your choice of foam, some do not flow very well.

Edited March 27, 2014 by KRS1

[Woodsie888]

I'm uncertain as to why you still play around with the mixture screws Allan. As mentioned to you many times before, the mixture screws only affect the 'idle circuit' of the carb.

Changing needles or jets, which ever is the case will alter the mixture as mentioned.

I recall advising some weeks ago, that it sounded very much like a lean mixture.

[seedy]

Generally. the slow speed circuit and throttle cutaway handles up to 1/4 throttle.

Needle position 1/4 - 3/4

Main jet 3/4 plus

With unleaded fuel it is harder to read spark plugs than in the day of leaded fuel. Purchase a lighted magnifier so you can look down into the base of the insulator to detect mixture changes. Beware of small dark specks on the nose of the insulator - this will be molten aluminum from the piston crowns and is a sure sign of a lean mixture.

A hand-held temp gauge - looks like a handgun - can help with on-the-spot readings of cylinder head and exhaust system temps. Has a laser pointer to help you aim.

When in doubt - start rich and work down.

[VocalNeal]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

[VocalNeal]

Double post. True ISP

Edited March 27, 2014 by VocalNeal

[AllanB]

Generally. the slow speed circuit and throttle cutaway handles up to 1/4 throttle.

Needle position 1/4 - 3/4

Main jet 3/4 plus

With unleaded fuel it is harder to read spark plugs than in the day of leaded fuel. Purchase a lighted magnifier so you can look down into the base of the insulator to detect mixture changes. Beware of small dark specks on the nose of the insulator - this will be molten aluminum from the piston crowns and is a sure sign of a lean mixture.

A hand-held temp gauge - looks like a handgun - can help with on-the-spot readings of cylinder head and exhaust system temps. Has a laser pointer to help you aim.

When in doubt - start rich and work down.

Thanks for that Seedy, I was just looking for those figures and I think the mixture screw possibly helps with the popping issue since it occurs with the throttle closed. Maybe.

I am finding it quite difficult to read spark plugs and if as you say unleaded petrol is the reason, that would explain it compared to the old days. There is certainly no soot on any of them.

I tried fitting a 1mm thick washer under the needle and the needle retainer wouldn't refit, not enough spring compression, odd really. So it would mean machining something, which I would like to avoid.

I didn't mention that the bike is water cooled and the temperature gauge reads normal to low, I only switch the fan in heavy traffic on a hot day, would that be the case if I had a serious weak mixture problem?

The bike will start without a choke, but will not start if I twist the throttle, even a little.

I am not trying to get it perfect just heading in the right direction, as it does sound a bit rough and I am pretty certain not 43bhp, but that is at 9000rpm which I never do.

The foam I have used is reasonably open celled and is not that dense, (but lightly oiled)10 baht from a builders merchant, couldn't find it elsewhere. A real shortage of motor factors and bike shops here in Khon Kaen.

[seedy]

Temp gauge hand held is nice, very accurate, you can check the temps of the cylinder head and exhaust port right at the juncture with the head.

Then when you make a change, jump on the bike, ride and then get off and do another temp reading. Tells right away if you are running cooler or not with the change done before.

Try this website - carbparts dot com slash keihin. Lots of tuning advice, and parts available.

They may have different taper needles available. I know they are available for HD's, which use Keihin carbs too.

K and N filters make universal oiled filters, their website give dimensions. So if you know the intake diameter of the filter, you can check and see if they have one you can make fit.

[AllanB]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

Not 100% no, but here is what I have so far on the Keihin VD carb:-

1. The pilot screw is on the air-box end of the carb, which would point to air screw.

2. The screw itself is short and pointed, rather that than a long taper, so a fuel screw.

3. Quote from the Shadow forum on the Keihin VD carb "I leaned in the pilot screws from 2 1/2 turns to 1 3/4 no change." So fuel screw.

4. I even had a carb manual and that is not definitive, it calls it a "pilot screw" and describes the set up as follows:-

a) Set the pilot screws at 2 turns.

Warm up the engine and set the throttle speed at 1000rpm

c) Turn each pilot screw 1/2 turn out.

If the rpm increases by 50rpm or more, turn each pilot screw out by a further 1/2 turn until the speed drops by 50rpm.

Adjust the idle speed with the throttle stop.

So to me that looks like fuel screw.

5. Another website says this of the all CV Keihin carbs including models VB, VD and VE:-

Idle & Low Speed System

An adjustable pilot screw controls the idle mixture. The pilot screw is located on the back of the carburetor in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. It has a limiter cap on it so adjustment is only slightly above three-quarters turn. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system. The slow jet on the 1985-88 CH250 is #38. To fatten up the idle and slow speed circuit, it may be necessary to raise the slow jet to a #40 or #42 when the main jet is increased.

So completely ambiguous.

Does anyone know for certain, as it would be good to know?

With regards to the Supertrap exhaust with adjustable back pressure, yes that is on the list, but it would be nice if the bike sounded good too.

[VocalNeal]

Different for a lot of carbs but one method is to turn in until engine slows or hesitates, turn out until engine runs better then out some more until engine slows or hesitates then turn it back to the middle of the two rough running settings.

[seedy]

The website I mentioned is not your run of the mill money making store. Just a guy with lots of knowledge and a very basic site.

Worth giving him an email with explanation of your problems.

Not familiar with that model carb myself. Now if it was a Linkert / Schebler ...

Still feeling the effects of those Mo Fo's !!!

[Woodsie888]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

Not 100% no, but here is what I have so far on the Keihin VD carb:-

1. The pilot screw is on the air-box end of the carb, which would point to air screw.

2. The screw itself is short and pointed, rather that than a long taper, so a fuel screw.

3. Quote from the Shadow forum on the Keihin VD carb "I leaned in the pilot screws from 2 1/2 turns to 1 3/4 no change." So fuel screw.

4. I even had a carb manual and that is not definitive, it calls it a "pilot screw" and describes the set up as follows:-

a) Set the pilot screws at 2 turns.

Warm up the engine and set the throttle speed at 1000rpm

c) Turn each pilot screw 1/2 turn out.

If the rpm increases by 50rpm or more, turn each pilot screw out by a further 1/2 turn until the speed drops by 50rpm.

Adjust the idle speed with the throttle stop.

So to me that looks like fuel screw.

5. Another website says this of the all CV Keihin carbs including models VB, VD and VE:-

Idle & Low Speed System

An adjustable pilot screw controls the idle mixture. The pilot screw is located on the back of the carburetor in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. It has a limiter cap on it so adjustment is only slightly above three-quarters turn. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system. The slow jet on the 1985-88 CH250 is #38. To fatten up the idle and slow speed circuit, it may be necessary to raise the slow jet to a #40 or #42 when the main jet is increased.

So completely ambiguous.

Does anyone know for certain, as it would be good to know?

With regards to the Supertrap exhaust with adjustable back pressure, yes that is on the list, but it would be nice if the bike sounded good too.

It's not ambiguous at all Allan.

You just don't seem prepared to take any advice.

[AllanB]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

Not 100% no, but here is what I have so far on the Keihin VD carb:-

1. The pilot screw is on the air-box end of the carb, which would point to air screw.

2. The screw itself is short and pointed, rather that than a long taper, so a fuel screw.

3. Quote from the Shadow forum on the Keihin VD carb "I leaned in the pilot screws from 2 1/2 turns to 1 3/4 no change." So fuel screw.

4. I even had a carb manual and that is not definitive, it calls it a "pilot screw" and describes the set up as follows:-

a) Set the pilot screws at 2 turns.

Warm up the engine and set the throttle speed at 1000rpm

c) Turn each pilot screw 1/2 turn out.

If the rpm increases by 50rpm or more, turn each pilot screw out by a further 1/2 turn until the speed drops by 50rpm.

Adjust the idle speed with the throttle stop.

So to me that looks like fuel screw.

5. Another website says this of the all CV Keihin carbs including models VB, VD and VE:-

Idle & Low Speed System

An adjustable pilot screw controls the idle mixture. The pilot screw is located on the back of the carburetor in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. It has a limiter cap on it so adjustment is only slightly above three-quarters turn. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system. The slow jet on the 1985-88 CH250 is #38. To fatten up the idle and slow speed circuit, it may be necessary to raise the slow jet to a #40 or #42 when the main jet is increased.

So completely ambiguous.

Does anyone know for certain, as it would be good to know?

With regards to the Supertrap exhaust with adjustable back pressure, yes that is on the list, but it would be nice if the bike sounded good too.

It's not ambiguous at all Allan.

You just don't seem prepared to take any advice.

So what is the answer fuel screw or air screw, you don't have any or you would explain? You rattle on about weak mixture, when we all know that is the issue, so please go way in short sharp jerks and troll someone else's thread and let other people lend a hand. Try reading the answer from Seedy regarding the effect of the idle circuit, anyway any further replies from you, I will simply ignore.

[Woodsie888]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

Not 100% no, but here is what I have so far on the Keihin VD carb:-

1. The pilot screw is on the air-box end of the carb, which would point to air screw.

2. The screw itself is short and pointed, rather that than a long taper, so a fuel screw.

3. Quote from the Shadow forum on the Keihin VD carb "I leaned in the pilot screws from 2 1/2 turns to 1 3/4 no change." So fuel screw.

4. I even had a carb manual and that is not definitive, it calls it a "pilot screw" and describes the set up as follows:-

a) Set the pilot screws at 2 turns.

Warm up the engine and set the throttle speed at 1000rpm

c) Turn each pilot screw 1/2 turn out.

If the rpm increases by 50rpm or more, turn each pilot screw out by a further 1/2 turn until the speed drops by 50rpm.

Adjust the idle speed with the throttle stop.

So to me that looks like fuel screw.

5. Another website says this of the all CV Keihin carbs including models VB, VD and VE:-

Idle & Low Speed System

An adjustable pilot screw controls the idle mixture. The pilot screw is located on the back of the carburetor in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. It has a limiter cap on it so adjustment is only slightly above three-quarters turn. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system. The slow jet on the 1985-88 CH250 is #38. To fatten up the idle and slow speed circuit, it may be necessary to raise the slow jet to a #40 or #42 when the main jet is increased.

So completely ambiguous.

Does anyone know for certain, as it would be good to know?

With regards to the Supertrap exhaust with adjustable back pressure, yes that is on the list, but it would be nice if the bike sounded good too.

It's not ambiguous at all Allan.

You just don't seem prepared to take any advice.

So what is the answer fuel screw or air screw, you don't have any or you would explain? You rattle on about weak mixture, when we all know that is the issue, so please go way in short sharp jerks and troll someone else's thread and let other people lend a hand. Try reading the answer from Seedy regarding the effect of the idle circuit, anyway any further replies from you, I will simply ignore.

No problem.

Enjoy your day

Edit: You probably have forgotten that I was the first person to raise the lean mixture issue whilst you were concerning yourself with 'stator test readings'

Can lead a horse to water etc

Edited March 28, 2014 by Woodsie888

[AllanB]

Temp gauge hand held is nice, very accurate, you can check the temps of the cylinder head and exhaust port right at the juncture with the head.

Then when you make a change, jump on the bike, ride and then get off and do another temp reading. Tells right away if you are running cooler or not with the change done before.

Try this website - carbparts dot com slash keihin. Lots of tuning advice, and parts available.

They may have different taper needles available. I know they are available for HD's, which use Keihin carbs too.

K and N filters make universal oiled filters, their website give dimensions. So if you know the intake diameter of the filter, you can check and see if they have one you can make fit.

Optical pyrometer is what you are referring to, but they call them something different now and I doubt if I can get one here, so will pick one up in the UK this summer. About £20, I think. This is a very strong engine, but don't want to wreck it. I am not 100% certain about water circulation, although that should show up on the temperature gauge.

I looked at www.carbparts.com and there is some good information there, just need to spend some time reading it. I would still like to know whether the VD has a fuel, or air screw.

The trouble with the carb and the bike in general is lack of information, I could do with someone who speaks Japanese as this is home market model and I am sure there is a lot of stuff on PDF files.

Anyway thanks

[AllanB]

Alan,

Are you 100% sure the idle mixture adjustment controls the fuel? On most Keihin carburettors the idle mixture adjustment controls air, so screwing it out would lean out the mixture NOT richen it. Depends if the mixture screw is upstream of the throttle valve (air) or downstream (fuel)

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Exhaust? have you thought about saving up for a Supertrapp with adjustable back pressure.

Jets. As a rule of thumb older jets will be worn so will tend to richen the mixture.

Isn't that statement a little out of date, with unleaded and gasahol fuels? Here is my thinking:-

A jet size changes the internal diameter by only 0.002", so a 1 thou build up of crud (not much) on the wall would make the jet smaller by one size.

Any way of cleaning the crud of the inside of a jet, I haven't seen a set of jet cleaners over here, so I use a single strand of a wire brush wheel, two strands is too big.

I also remember that bikes built for the home market, where they had tight emission controls in the 80's, they aired on the side of lean when the bikes left the factory. Any truth in that?

Someone said that the front and rear cylinders of a V-twin need different jet sizes, is that true for water cooled bikes? If so I may have them the wrong way around.

Edited March 28, 2014 by AllanB

[AllanB]

Woodsie, I am sorry about losing my rag, but if you are going to make a contribution, make a contribution.

I think the manual is ambiguous, if you think it is clear then you would know whether they are referring to an air screw or fuel screw, so kindly say so.

I do remember that it was you who raised the issue of "leaning out", but it wasn't the cause of the misfire, a faulty float valve was, as it turned out. We were all (educated) guessing at that point and thanks to all of you, that particular problem is resolved, now I have another.

My own opinion about this bike is that it is "sensitive", if they are getting 43bhp out of 400cc it probably goes out of tune quite quickly and maybe that is why they detuned the Steed, that followed, down to 30bhp.

[Gonsalviz]

Look at your spark plugs to determine fuel/air mixture. Google up some photos on what they should look like if you don't know.

[AllanB]

Okay I think I have got to the bottom of the the mixture/pilot screw problem and this may help others who have Keihin VB, VD and VE carbs. These are all CV type carburetors, CV standing for "Constant Velocity".

From 3 separate sources, they all have fuel mixture screws, even though they are located where you would expect to find an air screw, on the filter side of the carb.

Jeeze this is hard work, why didn't they put "fuel screw" in the manual?

Gonsilvis, They look quite neutral and I tried taking photos before and they showed nothing.

[AllanB]

http://www.ebay.co.uk/itm/Motorbike-Monkey-MX-Pit-Bike-Scooter-Carb-Jet-Cleaner-Cleaning-Tool-Kit-FREEPOST-/121293732014?pt=UK_Motorcycle_Parts_13&hash=item1c3dab78ae

This would seem a pretty important piece of kit, given that Gasohol is gunking up everything, will get one in the UK, haven't seen them here.

[VocalNeal]

http://www.ebay.co.uk/itm/Motorbike-Monkey-MX-Pit-Bike-Scooter-Carb-Jet-Cleaner-Cleaning-Tool-Kit-FREEPOST-/121293732014?pt=UK_Motorcycle_Parts_13&hash=item1c3dab78ae

This would seem a pretty important piece of kit, given that Gasohol is gunking up everything, will get one in the UK, haven't seen them here.

Alan,

That is a oxy-acetelyne welding nozzle cleaner. But I guess it could be used for jets. With care.

Or you could strip some copper electrical cables for the same purpose.

Edited March 30, 2014 by VocalNeal