Performance Tuning Guidelines
This is the tuning sheet that was included in many of our performance tuning products. We have posted it here so that you can reference it any time you need!
EDIT: In the 8 years or so since i have written this I have tuned countless dual carb bikes, pushed them hard and never had a problem with removing the fuel pump. I have only encountered two separate instances that resulted in re-installing the fuel pump to attain acceptable performance of a bike with open exhaust and stacks. Both bikes were VT750 dual carb models and both were beat to hell, ridden hard and hung up wet! The valves were sloppy, compression test revealed leaks, and it was pretty obvious previous owners hadn't given the bikes much TLC. Both bikes were fitted with velocity stacks, and open exhaust.
I currently ride a 2006 VT750 Spirit with dual carb stacks, and wide open exhaust, and I have zero issues with running in any throttle stage. WOT I can hit around 100mph.
Now, in the past year or so I have gotten feedback from customers encountering the same problems and I feel these are more than just a handful of random cases, to where Its time to offer folks some further feedback. So, I dug up my tuning log and some of these symptoms from customers matched what I encountered about 4 years ago.
The symptoms and responses were as follows.
- Initial issues around 1/2 throttle to WOT, stuttering, flat spots, etc, further tuning was done, plug chops, some symptoms could be tuned away pretty easily
- Difficulty maintaining WOT
- Inability to tune out the 3/4-WOT shudder/stutter/miss feeling
- Feeling like fuel ran out intermittently at high speed.
- Side winds exacerbated the issue.
- Float heights were increased
- Needle tapers were changed
- Machined the float needle seats inlet to a larger size, this did make a pretty big change but unfortunately I couldn't source a larger size float valve so this wasn't a viable long term solution for my customers
- Re-installing the fuel pump and removing the tiny pre-filters from under each float valve were the only thing that resolved the problem completely.
From what I have written in my tuning log and from what i remember, the draw from the bowls was more than gravity feed could keep up with. larger hose, higher flow of fuel, larger float valves removing the small pre-filters from underneath the float valve were solutions that solved the issue, however the only thing that I tried that was a viable solution for a customer who might not have access to machining or tools that i did, was to re-install the pump. This ONLY applies to dual carb models VT600 and VT750, however i believe its worth bypassing the pump to see if your bike can be tuned without the pump.