Question2003 Honda Ace 750CC, rejetted with Dynojet kit, 130/126 jets, needles on 2nd. notch, modified the air box to increase air flow, K&N filter, Vance & Hine Cruzers w/ baffles.
Had a problem with the bike surging when going into gusty head and cross winds. Installed a carb. cover and the problem has decreased, now while at 55mph. if I give it full throttle, the bike stalls. If I back off a hair on the throttle she goes, but still sputters a tad.
If I give it WOT from a dead stop, it'll run great, only while cruising does it not want to accept full throttle. Any ideas?
I live @ sealevel in South Central Louisiana / high humidity.
AnswerHi Barney,
From the information provided, I did the math and figured the following corrected jetting; at sea level at about 92 degrees average summer temperature:
Main jet Sizes: 125 & 120
Jet Needle & Needle Jet: no change.
Air-Fuel Screw: 1/2 turn out from current position.
No data on the pilot jets was provided:
My calculations are subject to additional tuning, too. I would start with my recomendations as a tuning base line. Valve clearances and cylinder compression must be within service limits for the carburetors to work properly.
I hope you and your familiy are well after Katrina.
Respectfully,
Mark Shively
Dayton,Ohio
(sample tech file)
Drop-In Jet Kits
By Mark Shively
With respect to the engineers who designed these products, my experience with jet kits is they usually require additional tuning to get the correct air-fuel mixtures. Jet kits are engineered to make power improvements for the intended model application.
Many of my customers buy these kits, drop them in per instructions and expect a well running machine. Why doesn't every engine with these kits installed perform as expected or proclaimed? The reasons may be many and include: environmental variables, interpretation of instructions, skill level of installer, available tools, condition of the engine, and more. If it were that easy, you would not likely be reading this.
It should be known that factory motorcycle engineers do an outstanding job designing engines to operate under a variety of elements and variables. Think about it, a stock motorcycle must and does runs very well in a variety of climates and other elemental conditions. If not, their products wouldn't sell well. Dealership service departments would be overwhelmed with unsatisfied new motorcycle owners.
When we make changes to the stock air intake, carburetion, and exhaust, we unknowingly de-tune the engine in most cases. Who cares when the result is that desirable new exhaust sound? Most riders would not notice that their engine was de-tuned unless the problem was an obvious change from what they were used to hearing, feeling, or seeing.
What I'm trying to explain is that making changes to the OEM design may require on-going tuning. For example, at the Daytona International Speedway, I would have to tune the Yamaha TZ250's carbs (Lectron carbs) for the morning practice air temperatures. By noon the temps have changed and the engine requires new air-fuel mixture tuning to produce optimum performance. Tuning changes are required constantly as the environmental elements change and as engine components are changed or modified.
The same can be said for a street motorcycle engine, though not as important to tune as often as a roadrace engine. In any case, the air-fuel mixtures must be very close or performance problems will likely be noticed.
If you're about install a jet kit: Review the instructions in advance of the installation. When ready, follow installation instructions carefully. Work patiently and be prepared for the task. Expect to perform additional tuning to dial-in the carburetors.
If you've already installed a jet kit and are experiencing problems with performance or installation: Review the instructions and review your work step-by-step. Verify each step is properly installed. Having a second person verify your work can be helpful. Review the illustration below and learn about tuning each range of carburetion. Make necessary tuning adjustments one at a time to know what effect each change makes. Doing otherwise is fruitless causing you to waste time un-doing and changing things. If all fails, have a local tech perform the work for you.
