Can 1983 Honda VT-500 Meet the Vetter Challenge?

[renault_megane_dci]

Great trip.

A change in elevation probably has the effect of changing the jetting from an oxygen point of view ?
I remember that carbs are velocity sensitive more than anything else so getting higher could be richening the mixture ?

[redyaris]

my suspision is that the stream lining has caused the fuel schedule to be off by as much as 30% due to the reduction of drag [by as much as 50%] at all speeds over 60km/h. I will have to talk to an engineering friend to see if I am correct.

[renault_megane_dci]

Am I correct understanding you're saying that the engine doesn't see enough load to make the carb work properly ?

If so, I have only one answer : gear ratio is not tall enough !

[redyaris]

I have abandoned the trip, after 2 days, to Ohio with Allen and Vic and returned home from northern Wyoming, due to the affects of heat and variouse other small problems. I did not want to have to U-Haul the bike back over 2000 miles from Ohio. I will be relocating the cooling fan and replacing the stock main jets with smaller ones to see if that helps to get the bike consistantly into the 80+ mpg rang. the exhaust pipe is covered on the inside, with a black soot, as is the panel beside it, suggesting that the bike is over fueling. I will also be reshaping the cargo section to reduce the affects of side winds. When i have fixed these issues I wil try again next year.

[jkv357]

Sorry to hear that you had to turn back.

Thankfully it wasn't a crash or anything like that.


Jay

[renault_megane_dci]

Maybe just a matter of fuel valve stuck open ?

[redyaris]

The basic problem is that in stock configuration in 6th gear at 100 km/h [60 mph] the throatle is between 50% and 80% open, and running on the carb main fuel circuit, after streamlining the bike the throatle is only 10% open and running on the slow speed and the main fuel circuits. in stock configuration this part of the fuel schedule is only used for a few second at a time as the bike is accelerting. the slow speed fuel circuit in the carb is there to deal with fuel management from idle to up to the point when the flow of air through the carb is enough to keep the main fuel circuit happy. The streamlining doesn't change this condition. The streamlining changes the throatle position required to maintain 100 km/h thus putting the carb into a part of the fuel schedule that is very poor in that the fuel/air ratio is maybe 9 or 10:1 rather than 14:1. the best solutions if you have the money are fuel injection or smaller carb or a single carb. reducing the jet size will work up to a point, after which the carb will provide a overly lean mixture that will cause engine damage and poor performance... but changing jet size is easy and cheep and it may be enough. [?]
So long as the leaning out occures at a speed high enough it won't matter I only need the speed range up to 150 km/h, not 210 km/h.

[redyaris]

...ferther to the problems of the VT500 V-twin; the rear cylinder is inadequitly cooled because it is shielded by the front cylinder, the solution Honda uses is to put a larger main jet in the rear cylinder carb [Honda VT 500 front #112, rear #122] this reduces the heat generated by the rear cylinder [cheep and effective]. the normal main jet size for a 250cc cylinder is #110 [Honda CB250].
So by improving the cooling of the rear cylinder the main jet can be reduced to #112 the same as the front. This will also improve fuel econonomy. may even be able to go to #110 main jets.
The way I plan to improve the rear cylinder cooling is with a small 1.9 watt fan [solar powered] aimed at the rear cylinder & head, from the side. This may work well because the cylinder and head are fined, just like an air cooled engine! In fact when at the Honda dealer I looked at the new V-twins; they are all fined like an air cooled engine even though they are liquid cooled?! With this set of modifications I may be able to get into the 80 -90 mpg range consistantly, and no poor/marginal cooling isseus...

[renault_megane_dci]

Originally Posted by redyaris

The basic problem is that in stock configuration in 6th gear at 100 km/h [60 mph] the throatle is between 50% and 80% open, and running on the carb main fuel circuit, after streamlining the bike the throatle is only 10% open and running on the slow speed and the main fuel circuits. in stock configuration this part of the fuel schedule is only used for a few second at a time as the bike is accelerting. the slow speed fuel circuit in the carb is there to deal with fuel management from idle to up to the point when the flow of air through the carb is enough to keep the main fuel circuit happy. The streamlining doesn't change this condition. The streamlining changes the throatle position required to maintain 100 km/h thus putting the carb into a part of the fuel schedule that is very poor in that the fuel/air ratio is maybe 9 or 10:1 rather than 14:1. the best solutions if you have the money are fuel injection or smaller carb or a single carb. reducing the jet size will work up to a point, after which the carb will provide a overly lean mixture that will cause engine damage and poor performance... but changing jet size is easy and cheep and it may be enough. [?]
So long as the leaning out occures at a speed high enough it won't matter I only need the speed range up to 150 km/h, not 210 km/h.

Smaller carb, more velocity, bump up low speed tractability as well as clear your rich running issue.

Or even better, use only one of the two carbs !

[renault_megane_dci]

Originally Posted by redyaris

...ferther to the problems of the VT500 V-twin; the rear cylinder is inadequitly cooled because it is shielded by the front cylinder, the solution Honda uses is to put a larger main jet in the rear cylinder carb [Honda VT 500 front #112, rear #122] this reduces the heat generated by the rear cylinder [cheep and effective]. the normal main jet size for a 250cc cylinder is #110 [Honda CB250].
So by improving the cooling of the rear cylinder the main jet can be reduced to #112 the same as the front. This will also improve fuel econonomy. may even be able to go to #110 main jets.
The way I plan to improve the rear cylinder cooling is with a small 1.9 watt fan [solar powered] aimed at the rear cylinder & head, from the side. This may work well because the cylinder and head are fined, just like an air cooled engine! In fact when at the Honda dealer I looked at the new V-twins; they are all fined like an air cooled engine even though they are liquid cooled?! With this set of modifications I may be able to get into the 80 -90 mpg range consistantly, and no poor/marginal cooling isseus...

I don't know a thing about this engine in particular but what if, due to packaging / design issues, Honda prefered the bigger jet solution over a properly designed cooling circuit ?

Is the water from the rear cylinder coming straight from the cold side of the rad ?
If not, I would mess with the water pipes first.

The small fan concept is not ideal because those fan become useless at speed and I believe it is where you have your cooling issue.

How about a bigger rad now that you have room ?