Hey guys. There are a lot of extremely knowledgeable folks on this forum (I am not one of them!), and I have been thinking about gathering this knowledge into a general guide to blow-thru carbs.
What I was envisioning is a system-by-system, part-by-part overview of a blowthru carb, formatted like so:
(System name)
=(System purpose)
-(Component A)
-(picture of component)
-(Component purpose)
-(Component modification reason and effect)

Example, and it's probably wrong but you get the idea:

System : Accelerator Circuit
= Purpose : Deliver additional fuel as throttle is applied
- Accelerator Pump Cam
-(picture of an accel cam)
-Determines the rate at which fuel is added as throttle position increases
-(picture of accel cam curves would be useful here)
-To correct a lean stumble off idle, change pump cam to a more aggressive profile and vice-versa
- Accelerator Pump
- (picture)
- Pump cam linkage depresses the pump diaphragm, forcing fuel through the accelerator nozzles/sprayer whatever
- 30cc and 50cc pumps are available. If lean stumble persists even with aggressive pump cam & nozzles, a 50cc pump may be required.

etc etc etc.
I am hoping that this would demystify some of the inner workings of a carb for myself and everyone else. I know my main issue with tuning is not knowing exactly what to change to solve what problem without messing everything else up, or at least knowing what side effects it will have and how to go about mitigating them. If you don't feel like digging up pictures but want to contribute a description, that is fine, I could always google up some images to throw in there. Another question is what to do with "serious" modifications like vent tube extensions and BRPV's and such, since they are (somewhat) optional parts. Should there be an addendum which includes these mods, or should they be thrown right in the mix with all the 'standard' parts?
I'm not sure if this should go in the Lounge or not, feel free to move the thread if it would fit there better. I figure we can hash it out in this thread, then I'll put it all together in a fresh one and maybe a mod could sticky it for future reference
I guess the first thing to do is to form a master list of all the major systems and components. Idle circuit, accel circuit, mains circuit, PV circuit... what am I missing?

Re-reading, perhaps the "system description" should be expanded to describe the overall order-of-events of a system functioning, to explain how all the parts interact? Like: "The accelerator circuit is responsible for adding fuel as throttle position increases. The accelerator pump cam is attached to the throttle shaft. The accelerator linkage rides on the pump cam; as the throttle turns, the cam causes the linkage to depress the accelerator pump diaphragm, forcing fuel from the bowls through the accelerator circuit and into the venturis through the accelerator nozzles."
Thoughts?

 

Here is what I put together about a yeat ago.

http://members.tccoa.com/392bird/tuning.htm

 

Tom, the blow-through guide sounds like a great idea and a big undertaking. Maybe add a troubleshooting section with links back to relevant parts of the narrative/photos. This could also link to special mods which might be suggested for that particular issue such as the vent tubes, BRPV's, etc. Maybe also provide a link section to other related guides such as Hanger18's, 393Bird's, etc. - with their permission, of course.

 

Excellent, I've only breezed through it for now but I'll go through in detail and try to pull out the bits I need. Thanks!

I actually bounced back and forth between a carb troubleshooting flowchart and the general description. I think what you described is the best solution, sort of a "Tuning FAQ" section, which links back to the parts which need modifying.
I'm actually moving in the next few days, but over the holiday I'll try to get a start on this. I'll need it myself in the spring when I get back to tuning my disaster

 

i think is a great idea. my blowtru carb was the first carb i ever touched beside on small motors. and i had a hard time just finding info most guys dont even think about when they are working on a carb.

 

I also think this is a great idea. I could certainly use it