Too much jetting?
03-23-2006, 08:00 AM
#1
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SuperSport
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Join Date: Nov 2005
Posts: 547
Too much jetting?
I am running a K&N airfilter without the blockoff plates, partly because I don't feel like buying replacement plates after I discovered them missing. Here's the concern:
I have to jet richer to compensate for the extra air getting in the airbox from the bottom. The air that comes in those bottom holes does not go through the filter and could presumably bring just about anything into the airbox manifold. Granted, the holes are small (1/2" at most) and they are oriented upward, and under the tank and frame. Still, I'm concerned about disrupting the smoother airflow coming from the snorkel side and making my mixture unpredictable. Is anyone running this setup?
I have run 190/185 K&N Jets (Also known as dynojet), but I found there was a lean stumble around 2k, so I rejetted to 195/190 mains and #50 idles. This would normally be rich as hell, but considering aftermarket pipes and air filter, it might be spot on.
I guess it all comes down to just how much fuel and air you can cram into the cylinder before you hit the breaking point. Without massive mods to the airbox and carbs, though, I'd think you could tweak it a little here and there without getting into the red-zone. (not to be confused with the Danger Zone by Kenny Loggins). Does anyone have any thoughts about this?
I have to jet richer to compensate for the extra air getting in the airbox from the bottom. The air that comes in those bottom holes does not go through the filter and could presumably bring just about anything into the airbox manifold. Granted, the holes are small (1/2" at most) and they are oriented upward, and under the tank and frame. Still, I'm concerned about disrupting the smoother airflow coming from the snorkel side and making my mixture unpredictable. Is anyone running this setup?
I have run 190/185 K&N Jets (Also known as dynojet), but I found there was a lean stumble around 2k, so I rejetted to 195/190 mains and #50 idles. This would normally be rich as hell, but considering aftermarket pipes and air filter, it might be spot on.
I guess it all comes down to just how much fuel and air you can cram into the cylinder before you hit the breaking point. Without massive mods to the airbox and carbs, though, I'd think you could tweak it a little here and there without getting into the red-zone. (not to be confused with the Danger Zone by Kenny Loggins). Does anyone have any thoughts about this?
03-23-2006, 12:21 PM
#4
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SuperBike
Join Date: Sep 2005
Location: Buffalo, NY
Posts: 1,572
Re: Too much jetting?
your spark plug color should tell you if your running too rich, to lean or just right. If the bike is running good and the plugs show the proper color leave it be... You could use a rubber plug or something to plug those holes and go back to the other jetting but like I said if its running right leave it but your gas mileage would be less than normal and we all know they aren't the most efficient to begin with 
.
Rick B
02 SH
.Rick B
02 SH
03-23-2006, 02:39 PM
#6
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SuperSport
Join Date: Dec 2005
Location: Sunshine Coast, Australia
Posts: 963
Re: Too much jetting?
G'day Loco.
Generally speaking, more air and more fuel is all good! So your little experiment indicates that getting more air through the carbs may be a good mod on the VTR. I would however try to achieve this by another means other than the unfiltered holes in the bottom of the airbox. I would be blocking them off.
Experiments have shown that increasing your filter area and intake area will assist in producing more power in a track bike. Whether this works well on a street bike is not really known it seems. Keep experimenting and get on the dyno.
Of great interest to me at the moment is the HRC jet kit. An Australian rider has just got his hands on one of these, and we have had a look at it in detail to try and come up with some theories as to how it works. He has installed it and reports more power across the rev range. With a trick exhaust and airfilter he was running 190/195 Keihin jets, and is now down to 160/165 with the HRC kit, however the dyno showed this as being too rich.
The trick to the HRC kit appears to be a mod they get you to do to the main air jet. You remove it and install a blank to block it off. The needles also are much richer in their taper compared to the standard needles and my Dynojet needle. You can spot the different taper by eye from a mile away. BIG difference. Experimentation is still under way and final tuning specs have not been found, but it is proving interesting.
My theory is that by blocking off this air jet, the air speed through the main body of the carb is increased, and is therefore promoting good cylinder filling. Torque figure reached on the dyno was the same as an R or GSXR (can't remember which one) that had just been done, at 75ft/lb (I think). It now lifts the front wheel off the ground easily in 2nd with no clutch and standard gearing, which it didn't do before he says.
He has also found a few sets of FCR Keihin pumper carbs for sale to suit the VTR, and they are all 41mm that are being used. Once again my thoery is that the smaller carb body is increasing air speed, and therefore more power.
So there you go, all very interesting. Final tuning has not been done yet, so final specs and results are not known, but it is looking good so far. Honda Racing Corp know their stuff though, so I will be surprised if it doesn't work well as early indications suggest it will.
Time will tell on this, but it is definately different to conventional thinking in regards to the VTR. Unfortunately the HRC kit is virtually unobtainable now, at least in Australia.
Final results will be advised when known, whenever that may be. Development takes time.
Generally speaking, more air and more fuel is all good! So your little experiment indicates that getting more air through the carbs may be a good mod on the VTR. I would however try to achieve this by another means other than the unfiltered holes in the bottom of the airbox. I would be blocking them off.
Experiments have shown that increasing your filter area and intake area will assist in producing more power in a track bike. Whether this works well on a street bike is not really known it seems. Keep experimenting and get on the dyno.
Of great interest to me at the moment is the HRC jet kit. An Australian rider has just got his hands on one of these, and we have had a look at it in detail to try and come up with some theories as to how it works. He has installed it and reports more power across the rev range. With a trick exhaust and airfilter he was running 190/195 Keihin jets, and is now down to 160/165 with the HRC kit, however the dyno showed this as being too rich.
The trick to the HRC kit appears to be a mod they get you to do to the main air jet. You remove it and install a blank to block it off. The needles also are much richer in their taper compared to the standard needles and my Dynojet needle. You can spot the different taper by eye from a mile away. BIG difference. Experimentation is still under way and final tuning specs have not been found, but it is proving interesting.
My theory is that by blocking off this air jet, the air speed through the main body of the carb is increased, and is therefore promoting good cylinder filling. Torque figure reached on the dyno was the same as an R or GSXR (can't remember which one) that had just been done, at 75ft/lb (I think). It now lifts the front wheel off the ground easily in 2nd with no clutch and standard gearing, which it didn't do before he says.
He has also found a few sets of FCR Keihin pumper carbs for sale to suit the VTR, and they are all 41mm that are being used. Once again my thoery is that the smaller carb body is increasing air speed, and therefore more power.
So there you go, all very interesting. Final tuning has not been done yet, so final specs and results are not known, but it is looking good so far. Honda Racing Corp know their stuff though, so I will be surprised if it doesn't work well as early indications suggest it will.
Time will tell on this, but it is definately different to conventional thinking in regards to the VTR. Unfortunately the HRC kit is virtually unobtainable now, at least in Australia.
Final results will be advised when known, whenever that may be. Development takes time.
03-27-2006, 08:20 AM
#7
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Join Date: Nov 2005
Posts: 547
Yeah it does and I'll let you know how the bike rides now that we finally look to have good riding weather. My seat of the pants test will be a 3rd gear wheelie. That seems to indicate available torque and I'm pretty uniform in my technique. If the bike comes up without a ton of effort, it's a good mod, if it doesn't come up at all, I'm changing back to my old setup. Not an exact science, but that's my life motto.
03-27-2006, 09:48 AM
#9
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Join Date: Nov 2005
Posts: 547
heh. Obviously I will be checking other things out too, like the idle/off-idle transition and general smoothness. But since my mod has to do with adding power into the engine via the carbs, there is no more solid test for me than the immediate throttle response required for a power wheelie. Granted, I will probably have to have this same conversation with the police officer who pulls me over, but IT'S IN THE NAME OF SCIENCE!
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