Findings from messing around with ACV

After checking the valve clearances on the two bikes (all sweet!), I decided to have a play around with the ACV circuit on my 450.

First of all I put in a new spark plug, set the fuel screw and went for a strap for a baseline.

The ACV is the cover on the left side of the carby, held on by 2 screws.

I turned the diaphram around, and blocked off the port where the rubber o-ring goes with a tiny bit of plastic.

I adjusted my pilot circuit jetting, I ended up going from a 52 to a 50 in the end, I tried 45, 48 then 50. Fuel screw set at about 2. I didn't have to change the needle position or main jet. I dont have any pilot air jets, I really need to get some. I feel I could get the pilot jet back down to about a 45 with the correct PAJ.

I have found that the first noticable thing is that the bike pops a fair bit more on decel, not because of lean jetting (pilot circuit set correctly), but because of the run on. I believe this is from the really heavy flywheel driving momentum and keeping rpm's up, even with the throttle closed. The popping stopped when I tried dragging the rear brake a bit, simulating lighter flywheel of a YZ (which gives more engine braking). The run on did decrease from when ACV was intact, less fuel available for the engine to suck in.

The thing that I really noticed is that the throttle response was a lot better. The bike seemed to rip a bit more through out the rev range. My jetting was right on before, but the less air and fuel from the pilot circuit really seemed to help.

It is hard to explain, but throttle response was remarkably the same, every twist of the throttle. It seemed to be cleaner. That may puzzle you, but the ACV circuit is not constant, the ammount of air being sucked in and when it shuts off the air always varies. This is what screws around WR's jetting, and every jetting circuit is effected.

My bike idled very sweet, remarkably cleaner. I found I could turn revs down a lot more than I could before. This could be to do with smaller pilot jet being used.

I need to get some pilot air jets (hmmm, 52 pilot to start with, PAJ must be too big!) and have a play around a bit more with this, I think if you could put up with the increaced popping, there is gains to be made.

Yes, the ACV works, it does it's job of stopping "lean condition" when you get off the throttle. It is a pitty that it is not constant, because the bike seemed to run better with out it.

I definately think with a quieter pipe, it wouldn't pop so much, my procircuit seems to have great scavinging effect, which doesn't really help the run on.

I must try disabling the ACV on my missus' WR250, her bike isn't so loud. It was fun to try, maybe you guy's with quieter pipes might have a bit more benefit.

All I can really say is BRING ON THE FUEL INJECTION!!! :thumbsup:

EFI, EFI, EFI, EFI .......................! Cant wait. :bonk::thumbsup::eek:

Note if you bypass the ACV circuit you can go to a much smaller pilot.

Your pilot seems richer than most. Your carb must have a circuit defect somewhere since most of us TT members use a 48 & 45. I tired 50 in the winter but 48 at 2.5 turns was just as good. :thumbsup:

My ACV is currently blocked off, and at this weekends ice race, I was running a 40PJ with FS approx. 1.5 and I had no popping at all once the FS was set. I don't have the same pipe as you, but I am running a YZ pipe with the PMB end cap, but without the spark arrestor and silent insert (and all free mods). The temperature was about -7C in the morning during practice (20F) and around 0C during the afternoon races (32F).

One difference may be that I'm running my needle richer (JD Blue C5) and my MJ is a 168. I was experimenting with the MJ and I found the best at those temps to be somewhere between a 168 and a 172. I couldn't really tell the difference. 178 was much too big. It felt slower on top with the 178. One thing I did notice, was when I dropped back down from the 178 back to the 168, I had to turn up the FS by at least a turn, otherwise it would pop on decel. Next week, I hope to go up a day early to have more time to practice and try to narrow down on a good setting. I plan to go leaner on the needle (probably try JD Blue C4 or C3). Also, I may try to drop the MJ lower. I'll keep on eye on the pilot too. I suspect if I do drop the MJ, I may have to go up to a 42PJ. I'd have to double check my notes, but I think I'm currently running either an 80 or 85 PAJ. Working on the bike on a cold windy lake is not pleasant, so I may skip trying differnt PAJs though. I think an 80 or 85 is probably a decent match for a 40-42PJ. I will concentrate on MJ and needle since oval ice racing, it's WOT or no thottle, but I'll keep on eye on the PJ and FS, and adjust accordingly.

Oh, before I forget!! EFI, EFI, EFI, EFI .......................! Cant wait either. :thumbsup::bonk::thumbsup: Those EFI Raptors sure seemed to run sign of the Quadrace 450 yet though. Hope to see one of those at the races too.

EFI, EFI, EFI, EFI .......................! Cant wait. :p:bonk::bonk:

Note if you bypass the ACV circuit you can go to a much smaller pilot.

Your pilot seems richer than most. Your carb must have a circuit defect somewhere since most of us TT members use a 48 & 45. I tired 50 in the winter but 48 at 2.5 turns was just as good. :thumbsup:

With the stock pipe de-restricted, I had to run the 48, or 45 with fuel screw way out. The procircuit is way less restricted and needs the bigger PJ. Reading the plug confirms this too. When I first rejetted for ACV disabled, I put the 45 in first but way too lean.

This is the only thing that is really different to you guy's, I run 168 main, on red #4 at the moment, red #5 still a tiny bit rich. It is constant 90's here with humidity anywhere between 85 - 100+, which requires a lot less fuel than you guys in the cold, but I am using the same.

I would like to get some smaller PAJ's, as I think the stock one might be too big, forcing me to use the big PJ. I have to order them in, I can't just walk into a shop, so it has to wait. :eek:

I will keep the ACV intact, the decel popping is way too annoying, but I will try again with smaller PAJ's. I did like the difference, but I think I might be able to get the same result with smaller PJ and PAJ. I believe too much air is my problem, not the fuel. I might try a bit stiffer spring in the ACV, that might help, depending on if the pin is pushed in to cut air off or not. :p

I am going to steal my mates YZ 450 carby and throw that on and see if I have the same results / problems. He has the same pipe as me, so it will be interesting to see if it is my carby. I have been reading his YZ manual flat out, but as you know, unless you try something for yourself..........

It is keeping me occupied anyway, what else is there to do on 4 days off besides riding? :eek:

What are we all going to do when we have all got fuel injection? :thumbsup:

Jetting will be a lost art! :lol:

What are we all going to do when we have all got fuel injection? :thumbsup:

Jetting will be a lost art! :thumbsup:

Oh no we will have way more to talk about when EFI hits the scene:

Owning my R1 we have a similar forum as TT and the discussions around EFI set up and programming as well as dyno tuning and ignition timing changes are extensive! :bonk:


I have one of these tied into my EFI and with my lap top I can download or modify different ignition and fuel maps. We share them on the forum. Most people have air box and pipe mods on their R1's. Power Commander will most likely get into the dirt bike market when it is larger and more viable.


That sounds like way more fun indeed! It is also what I have invisioned here on TT to be like in the not so distant future!

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