Bigger Bore--Changes to jetting?

I am going to be putting 1135cc bored cases and Wiseco 74mm/10.25 pistons onto my M2. I'd like to hear recommendations on what I should do with my carb jetting.

I currently have BS34 carbs with 120 mains and 40 pilots.

Any advice from those who have done a bore/piston upgrade would be appreciated.

Jeff Saunders at Z1E thought I should bump up my mains 3-4 sizes (127.5-130) So I went ahead and ordered some 130s. He thought I might want to also step up my pilots one size from stock. But I know my pilots are already one size above stock for my bike. So I may leave them at 40.

He also said I may need to retard my timing a few degrees. That's not something I have experience with, so any comments on that would also be appreciated.

Thanks guys.

p.s. I can't wait to put this together.

The displacement bump isn't the biggest factor... it is about 10 percent if your airbox/pipes are STOCK. You need to mention which airbox/pipe combo you are using as this affects jetting far more dramatically than does a displacement bump.

As far as retarding your ignition advance, I assume Jeff suggested this so your bike doesn't ping on the high-test that it will need once you install the higher compression pistons. The way you do the retard is to cut slots in the mount holes on your pickup coil backing plate and then rotate the plate a tad to retard. The OEM plate has only round holes and doesn't rotate. I personally wouldn't do this until after assembly and see if I had pinging after a couple fill ups of high test (93 octane).

Thanks, George.

I do indeed have stock airbox. But non-stock 4-2 turnout exhaust.

I want to resurrect this thread to ask some more questions.

I've been riding this bike for several months now. It rides really nice, but not perfect.

One change I made since the original post was that I added a set of 1100 GPz cams which gave it a nice little boost in power.

I have been running it with 130 mains and 40 pilots.

The problem is that I get a slight hesitation on the low end, just as I roll on the throttle. It coughs for a couple seconds, then WHAM!, it kicks in and rockets off.

The mid range seems just fine. Plenty of power, and no stumbles.

Then on the high range, WOT, it starts to cough again.

So I'm assuming that I need to change my pilots and mains.

Given this description, is it more likely that the jets are too lean or too rich? Or is there no way to tell without doing a full plug chop workup?

FYI, I did check and adjust the fuel service level, and that did seem to help the low end a very little bit.

Skyman wrote:

The problem is that I get a slight hesitation on the low end, just as I roll on the throttle. It coughs for a couple seconds, then WHAM!, it kicks in and rockets off.

I'd try raising the needles for that before buying pilot jets.

bountyhunter wrote:

I'd try raising the needles for that before buying pilot jets.

Okay, I'll try shimming the needles up a bit to see if that makes a difference (they only have one clip position).

What about the mains? Any opinions as to whether to go up or down in size?

Here are the results.

I found some small nylon washers, 1.25mm thick, which I used as shims. This should be equivalent to 1.25 clip positions.

I then went for a test ride. There seemed to be no change to the low-range hesitation. However, there was a significant change to the mid range. Before, the mid range ran smooth and strong. Now it boggs down and stumbles in the mid range.

So I think the problem is with the pilots, not the needles.

Here's a little history on my carbs. This set is not original to this bike. When I bought the bike, the PO had thrown on a set of VM28 carbs. Those were not orginal to this model, so I bought a set of BS34s on Ebay. These carbs were advertised as having come off of a KZ1100 bike. Jetting was 120 mains, 40 pilots.

As I wrote at the begining of this thread, when I put 1135 pistons on the bike, it was suggested that I bump up the mains 3-4 sizes and pilots 1 size from stock (which for the M2 is 122.5/37.5) So I bought 130 mains (and already had 40 pilots).

So since I now have slight hesitation on the low end, and coughing on the high end (WOT), I am thinking that maybe I should try going back to 37.5 pilots and maybe 125 mains.

Does that sound right? Any other opinions?

I will tell you from personal experience that I have no clue. I would wait for WiredGeorge. That being said, I thought hesitation and bogging meant too much fuel and/or not enough air. Just my $.02

Shimming the jet needles won't help the issue you describe. I would consider first trying to adjust the idle mixture with the #40 pilot jets and if that doesn't cure the problem, buy some #42.5. The off-idle transition is lean by the sounds of it.

wiredgeorge wrote:

Shimming the jet needles won't help the issue you describe. I would consider first trying to adjust the idle mixture with the #40 pilot jets and if that doesn't cure the problem, buy some #42.5. The off-idle transition is lean by the sounds of it.

Ah, the Sage of Mico weighs in!

Thanks, George.

I had tried adjusting the mixture screws before, and it didn't seem to help. But I will try once more, turning the air screws in to enrichen the circuit. If that doesn't work, I'll get the 42.5 pilots.

What about the mains? When it starts to bog at WOT, is that also usually indicative of a lean condition? Or too rich? Would you recommend that I go up or down from 130s?

I'm not overly concerned about the high end because frankly, I don't spend a lot of time up there. But I would like to have it available if I need it. :side:

One more data point to consider. This may be irrelevent.

As I opened the carbs back up to remove my trial shims, I took a note of the jet needle code and compared it against the stock needle according to the FSM.

I hadn't noticed this before, but my needles are not stock. According to the FSM, the M2 should have 5FL51 needles. Mine are 4CG74 needles. I guess I shouldn't be surprised, as I know these carbs did not come off of my model bike.

I have no idea what the needle code numbers mean, but they are obviously different.

George (or anyone else), should I try to get a set of stock needles? Are they even available? Or since my bike does not have stock configuration, should I even try to mess with the needles? Since my mid-range seems to run strong, I'm guessing I should just leave the needles alone, right?

Still hoping to hear some advice on my main jets. The engine bogs at WOT. Should I go with larger or smaller mains?

Thanks.