cliby

After doing engine rebuild this winter (stage 1 cams, HC pistons, porting) I had made several jetting attempts and had it pretty close with a DJ kit running 180/185, needles at 3rd position, 48 pilot jets. It made good power (115 hp) and the AF ratio was very close to right on. Then I found a stage 1 full exhaust and put it on. My sense was it felt like another couple of hp from the exhaust. The dyno confirmed that, but to my suprise indicated everything had become very rich - AF in the 7-10K rpm range went from about 13 to about 11. I ended up having to go down to 170/175 mains to get the AF correct, and got even more power doing so. But, with the dynojet kit I can not get rid of a very rich spot starting at 4.5 to about 6K rpm. I can not go leaner on the needles, or I get 1/8 throttle surging at 4-5K. Been pulling my hair out trying to get rid of it, making lots of little changes one at a time to see the effect: pilot jet, fuel mixture screws, needles. Finally, I've replaced the stock springs instead of the lighter DJ springs thinking maybe the rich spot comes from the slides moving too fast. Seems better by seat of the pants but will get some more readings in the future to see.

I've learned a lot, particularly that there may be some general rules about jetting, which circuit controls what etc, but there are a lot of grey areas and overlap, particularly when modifications have been made and there aren't many shortcut, or hard and fast rules to shorten the process.

mikstr

Have you tried the shimming the stock needles?

I find your results interesting, it goes to show how each angine and the actual state of tune affects the engine and its fueling needs across the rpm range. For instance, I have noticed a few owners who reported similar problems with Factory kits on slightly modded engines while they work fine for others. Similarly, the DJ kit on my VTR works perfectly but that may change after my next round of mods.

Incidentally, I was speaking with Kevin Cameron (he of Cycle World fame) yesterday and we got on the topic of cam timing. He mentioned how he noted on some carbureted engines running aggressive cam timing (with lots of overlap) that a "flat spot" often develops at an engine located at about 70% of peak torque, due to a pressure wave coming from the exhaust which pushes the intake mixture out. This mixture then gets pushed through the carb venturi where it picks up fuel twice (once on the way out, then on the way back into the engine). What you are left with then is a mixture that is overly rich in fuel, along with being lean in oxygen (contains a significant proportion of inert exhaust gases) so torque suffers. This sounds like it may be your problem.

gboezio

The needles are shaped so they enrich vs air flow. But the flow is not a flow like we could imagine, it goes both ways and pulse like crazy. So most of the jetting developments are made with the stock cams, but performance exhaust and maybe an air filter. So displacing the torque peak (max VE) higher in the power band can overcome the design of the needles.
Remember that hole that DJ had us to drill on the slides ??, well I think it's exactly that. It seems to be a vacuum bleed hole, the way it's placed it will bleed more air when the airflow is low than when the slide is saturated (high flow) pulling the slide up and avoid some choking.
But it wasn't clear if your throttle surging was a lean out, but the way I understand it you are stuck between 1/8 throttle leanout and WOT mid-RPM over rich.
The possible fix, I think you should reduce the hole size, try expoxy and file it until smooth, this should remove the bleeding at low flow and pull the slide higher, So if the needle sits higher at low flow and about the same at WOT, you are just too rich but linear, then you will have to lean the needle. If it's too much you can try to drill the bleed hole with a smaller bit.
On e thing for sure, you got to be good by now at taking the carbs apart

cliby

Thanks for your thoughts guys. Yes, I could open a shop just to remove/replace VTR carbs! I have the entire job including changing a jet and moving needle position down to 45 minutes from bike off to bike on the road. That is working at a pace I won't drive myself nuts having to do it again 2 hours later.

Mikstr, I had heard that too and that some spots are impossible to get around without also adjusting cam/ignition timing. I think the DJ instructions even refer to this. These cams are so relatively mild that I was hoping not the case.

Gboezio, your thoughts on the DJ kit were similar to mine. And yes, the surge was definitely a lean surge that only went away with raising the needle 1 notch - I'm just worried that may have amplified the rich spot later in the rev range. That is why I replaced the stock spring, thinking the slide was moving up too fast in the lower midrange. I hadn't thought about the hole much, but that may also be amplifying the lean problem. You are both right about what a messy dynamics of airflow, gas flow, slide movement it is. Just hook up an auxillary fuel tank and run the engine and watch those slide bouncing around in the lower rpm range its amazing they run at all!

My next step it to get to a dyno with AF and do a run where I'm at now and see what change replacing the stock spring had. If I'm going in the right direction, I still have some stock slides that I can put back in next to see what effect that has too. My goal at this point is to even out the AF curve and reduce the dive into the rich range that occurs between 4.5 - 6.5K.

Hawkrider

Sounds like a needle profile issue to me. You might want to give DJ a call and see if they have or can make you a slightly different profile. More than likely they have something in stock that'll work for you....or you can call FactoryPro.