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Followup To
Question -
Hi Mark, I ride a 1996 Honda Shadow ACE 1100 (VC1100C2) I've had it for two seasons now. When I first bought the bike it had only about 10,000 miles and very little modification done to it. It had been very well maintained with no major breakdowns that I am aware of. The stock pipes had been punched out and then a baffle welded into the pipe. The baffle was a piece of thin pipe about six inches in length with a one inch diameter and several holes drilled through it. Performance was good except for some back firing and sputtering when backing out of the throttle.
I was getting up to 45 miles per gallon. Of course I wanted more power. In the first season, I installed a K&N filter, re-jetted the carbs using the big jets from the dynojet kit three stage kit. I removed a screen from the air box for better air flow and removed the baffles that the previous owner had spot welded into the stock pipes.
I was very impressed with the difference in power at any rpm. The backfiring and sputtering was still there. Maybe even a little more than before. However my fuel mileage dropped bigtime. Now I was getting no more than 30 mpg. I'm not so concern with the consumption as I am with the range. The tank is small, only 3 gallons. Note my gallons are Canadian gallons, just a touch bigger than an American gallon.
Season #2 remove the big jets and installed the smallest jets from the dynojet three stage kit. Everthing else was left alone. I still get close to the same performance, backfiring and sputtering are almost gone but the fuel mileage is just as bad if not worse than before, 30 mpg or less. I installed a brand new set of plugs and checked them after a few miles, they look great with no indication of lean or rich fuel mixture.
Your opinion would be appreciated. I've always wondered if I have too much air flow or if there is such a thing.
Thanks for your time,
Merv Cawkwell
mervc@canada.com
Answer -
Hi Merv,
The fuel consumption will drop noticably with the modifications and higher speeds. The amounts are not unreasonable for a high reving multi-cylinder engine, especially if you enjoy twisting the throttle.
If no fuel leaks are observed, another cause of excessive fuel consumption is worn needle jets. These jets wear oblong from jet needles raisng and lowering into them. The expanded diameter of the needle jet's mouth allows extra air/fuel to pass and enter the cylinders.
Respectfully,
Mark Shively
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Thanks Mark,
I noticed the fuel consumption difference immediatley after installing the brand new jets, therefore am ruling out worn jets. Am I correct in that diagnosis?
I assume then that from what you say the only way to get better fuel mileage would be to re-install the factory jets. I am puzzled though, because my understanding has been that rejetting often improves fuel consumption as well as performance. Maybe I've been talking to the wrong people.
Regards,
Merv Cawkwell
ACE Rider
AnswerMerv,
Merv: I noticed the fuel consumption difference immediatley after installing the brand new jets, therefore am ruling out worn jets. Am I correct in that diagnosis?
Mark: You can rule out worn needle jets.
Merv: I assume then that from what you say the only way to get better fuel mileage would be to re-install the factory jets. I am puzzled though, because my understanding has been that rejetting often improves fuel consumption as well as performance. Maybe I've been talking to the wrong people.
Mark: If larger jets are installed, fuel consumption will increase. Exactly how much is difficult to measure. Same with cars. Put on a big Holley 4 bbl carb and gas goes bye-bye. Especially true if you step on the accelerator. : )
Rejetting may or may not improve performance or milage, and sometimes detunes one or both. A poorly tuned carb is inefficient. Depends on the tuner's skills, choice of adjustments, and environmental variables.
Rejetting is performed to correct air-fuel mixtures. Changes to the A/F mix occur by altering the original design with parts, fabrications, defects, and normal wear. The A/F mix varies between the ranges of carburetion. Each circuit is tuned with respect to A/F mix of that circuit. Each circuit overlaps adjacent circuits a tad.
Installing the factory jets would start the diagnosis process of elimination. If MPG range returns, you have the answer. If not, continue the process until you find the source. Keep an eye on the plugs and tune the A/F mix (jet)accordingly.
My experiences with drop in jet kits have been okay. They get the carbs close, but I find additional tuning is uaually necessary. Product engineers calculate jet sizes in their kits based on a few combinations of air filters and exhaust systems in good working (new) condition, and in a controlled environment. Not considered are modifications and fabrications owner's make and the specific climates and elevations were we live and ride.
It's important to make one carb adjustment at a time to know what effects each change makes.
Valve clearances and cylinder compression must be within service limits for the carbs to work properly. If out of specs, tuning is most difficult and sometimes impossible.
Respectfully,
Mark Shively
