Can 1983 Honda VT-500 Meet the Vetter Challenge?

[Varn]

If you are too rich on the low end you can adjust the mixture with the pilot screws. They are usually covered with sheet metal covers but are easily removed.

I thought the honda 500 was water cooled. In which case 95% of the cooling is through the radiator and probably one cylinder doesn't run much hotter than the other.

I think you probably have some tuning to do. Perhaps using a O2 sensor or EGT if you need the tools.

I have a Virago 700 that is mechanically stock except for the carbs. I made my own single carb manifold and run a dirt bike carb on it. All the tuning jets are available from most any dealer or Sudco.

[redyaris]

As I have mentioned in privieus posts there are a series of solution options any of which may do the trick. I will try them all in order of dificulty of implementation. The most difficult is the relocation of the radiator and/or the addition of a second radiator positioned at the rear of the bike dumping the hot air into the wake. In the end I also want to run the same smaller main jet size in both carbs. how effective the relative cooling of each cylinder is could be contoled by restriction of flow 60/40 so that the rear cylinder gets more of the flow and or by the order of flow through the system...
The Yamaha 250 V-twin runs a single carb that feeds both cylinders so they seem able to avoid the problems that Honda solves with two differant main jet sizes. What Yamaha does I have no Idea so I may have to look into it.

[Varn]

If you are running at 10% throttle you are not running on the main jets. No amount of leaning there is going to help. It doesn't make any difference if you change a part that is not part of the problem.

Look for a round 1/4" sheet metal cap that usually has a small round hole in the middle. It can be removed and it will expose a small adjustable needle jet. This is on the engine side of the carb and is either on the top or bottom. You might want to remove the carbs from the engine to find them.

If you have done this already then, "never mind "

[Varn]

Might say that my 700 virago runs a single 34 mm standard Mikuni VM carb. I don't make any attempt to get extraordinary mileage like you do. I get around 50 mpg. The carb sticks out the left side of the bike. More than once I have adjusted the idle speed or mixture while stopped at a sign. Just reached down and turned the appropriate knob.

At any rate the VM carb has a simple slide throttle. Mine will run 75 or 80 mph with the throttle open 25% giving me a throttle opening of 34x8 mm. In other words I probably could run that fast with a 17 mm slide carb. If you choose to do something similar make your intake runners fairly long and choose nothing bigger than 21 mm carb or carbs. That way you could hold it open nearly all the way and you could be on your main jets. I would also make sure that there are steps on intake side from small at the carb to bigger at the intake ports to prevent reversion.

One reason the Mikuni VM is still pretty popular after nearly 50 years is that it has a full range of jetting. Just look at the Sudco online catalog.

Had to snicker when I read that you were going to consult with an engineer about what to do with your carburation. Most engineers that I know I wouldn't ask about such mundane things as tuning. They wouldn't have a clue. You probably can tell that by the way that I write I am not an engineer.

[redyaris]

Varn
You may in fact be right about the main jet not being involved, because the CB250 slow speed jet is a #35 and the VT500 slow speed jet is a #38. I suspect however that both circuits are partly at work delivering fuel; mostly as you suggest from the slow speed jet. Changing the slow speed jet to a #35 may be something I have to try.
With respect to engineers; my friend started as a heavy duty mechanic for many years and knows both ends of the process. In fact he intimidates many of his younger coleges with his knowlege of the practical and the theoretical side of things. both my daughter and son in law are chemical engineers and are very helpful in getting my understanding of issues clear even if they know little about the details I am working on. Engineers like mechanics and others come in the full range of compitance...

[renault_megane_dci]

Stupid question :
If you P&G in 6th with 50% or more throttle, does your rich running problem disappear ?

Because what you are trying to solve maybe is an issue only in the Vetter challenge where P&G is forbidden ?

[redyaris]

On relativley flat ground I do very little P&G. I mostly run at a constant throatle. I Glide when ever I have to slow down or stop or when going down an incline. The problem as I see it is that the reduction in road load has moved the carb into a part of the fuel schedule that is not very well metered. In the stock condition this part of the fuel schedule is only used for a few seconds in each gear when accelerating. With the stream lining the carb ends up in this part of the fuel schedule for hours on end! I am trying to improve the situation as much as posible at the lowest cost. If money was not an issue there are a host of expensive and very effective solutions. I am going through the less expensive options until I eather solve the problem or find that it can not be solved with out lots of money.
The problem is not related to the vetter challenge conditions. The problems are related to the effects of stream lineing the bike... I have made radical changes to a bike that worked quit well as it came from Honda. These changes have effects on many other systems of the bike and all the resulting problems need a solution to get the full benefit of the original streamlining.

[renault_megane_dci]

Sorry if I didn't made myself clear.
I understood what you were saying previously, I just added an other option.

What if the cheapest solution was "more P&G" instead of constant throttle at a throttle opening that is not appropriate ?

It would also be a good test to narrow the problem to the carb.

To be honest my favorite solution would be to build a single carb manifold and use only one of your carbs.

[Varn]

Hi
Have you adjusted the mixture needles yet? The slide needles may have to be lowered as well. Is there a leak in one of the diaphragms

You mention that one cylinder runs hotter than the other. Have you synchronized the carbs throttle plates?

Was your bike one that came with two spark plugs per cylinder? Are both sets of ignition working? Have you tried advancing the timing?

Would also add that the starter enrichment (choke) might be leaking.

Originally Posted by renault_megane_dci

Sorry if I didn't made myself clear.
I understood what you were saying previously, I just added an other option.

What if the cheapest solution was "more P&G" instead of constant throttle at a throttle opening that is not appropriate ?

It would also be a good test to narrow the problem to the carb.

To be honest my favorite solution would be to build a single carb manifold and use only one of your carbs.

[redyaris]

Originally Posted by Varn

Hi
Have you adjusted the mixture needles yet? The slide needles may have to be lowered as well. Is there a leak in one of the diaphragms

You mention that one cylinder runs hotter than the other. Have you synchronized the carbs throttle plates?

Was your bike one that came with two spark plugs per cylinder? Are both sets of ignition working? Have you tried advancing the timing?

Would also add that the starter enrichment (choke) might be leaking.

mixture needles are as specified 2 1/4 turns, slide needle is non adjustable, no leaks in diaphrams, rear cylinder on almost all V-twins is hotter, thus the larger jetting of the rear cylinder, carbutetor are synchronized, starter inrichment OK, Ignition OK, changes to timeing do not change the over delivery of fuel. bike has much improved and very good performance except for fuel consumption.
The VT500 cooling system was enlarged by Honda in 1985 from 1.7 L to 2.0 L and a row was added to the radiator; which indicates that Honda knew there was a cooling problem. In 1986 the main jet for the rear cylinder carb was reduced from #122 to #120 most probably resulting from better cooling of the rear cylinder.
The cause of the fuel system problems is the stream lining which moved the carbs to the worst metered part of the fuel schedule. I am sure that if I removed the stream lining the fuel schedule problem would go away... as I have said I will be testing a smaller slow speed jet and a smaller rear cylinder main jet with improved cooling of the rear cylinder.