jetting

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freddog
Posts: 2
Joined: Fri Aug 07, 2015 5:43 am

jetting

Post by freddog »

Need help changing to round slides from power jets need jetting advice
freddog
Posts: 2
Joined: Fri Aug 07, 2015 5:43 am

Re: jetting

Post by freddog »

Sorry 1987 vMax 540
Bob Vehring
Posts: 659
Joined: Tue Feb 24, 2009 12:42 am
Location: Milw. Wi

Re: jetting

Post by Bob Vehring »

I have several different set ups in my notes, different due to changes in engine as I worked it out and changed things. Several other things to note. Outside air temp is big differance, dialed in at 40 deg is most likely not safe at 0 deg. ALSO, more heat will build in piston on a 3/4 mile run on a lake then what will happen on a 500 ft short blast.

Using stock 38 carbs I have
Main 350
needle jet Q2
needle 6f4
pilot 50
cutaway 3.0

I have another set of notes for used with 39.5 taper bored carbs
Main 320
needle jet Q0
needle 6dh2
pilot 40
cutaway 3.0

I would guess this was somewhat old and probably a test for a day on a lake or racing between my own sled. I have several sets of these carbs, my guess this was just my SRV carbs swapped to the VM.

Here is the set up currently in that sled altho it has some major changes. The first two have Yamaha's update porting otherwise mostly stock. Currently this sled is 618 cc's porting is completely different, ports were welded up and changed. port timing is mild, it has more compression and uses stock pipes. It is set up for the trail which these jets reflect.

Main 370
Needle jet Q0
needle 6f4
pilot 40
cutaway 3.0

I do not know how much you know about jetting, please don't take that the wrong way, but alot effects it. I use recording EGT systems on all my sleds. This means I can download to a laptop any or all runs during testing or races. I can then graph EGT at any RPM. You can precisely see what is happening at all times. It is entirely possible to have a needle jet/needle combination that at least at some throttle opening positions, will not flow as much fuel as the main is capable of flowing. That can mean you will have lean spots regardless of what size main you put into it.

You can buy used EGT gauges for sometimes around 100 bucks, that's alot cheaper then a rebuild if you miss it. It doesn't mean you can't do this without it, but you need a very good ear and experience to hear when its two rich or too lean. Plug color can help a little but that really is only good for racing at wot. Plug color shows a snapshot of everything that has happened to those plugs since they've been in there. That will include any idling, trail running, slow riding, etc so it does not clearly pin point any one thing.
Remember one thing. even if it seems crisp and clean in the middle of the day at 35 deg, that doesn't mean its safe at 0 deg later that night if the temp drops.
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