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Message started by HefyJefy on 07/06/16 at 15:53:48

Title: Carburettor pilot jet size s40 Boulvard
Post by HefyJefy on 07/06/16 at 15:53:48

The bike is new (2015) done around 3,000 miles. From the get go it backfired when closing the throttle and when switching off. Did the idle mixture mod and the backfiring only occurs occasionally. The bike goes well, good acceleration and smooth at high throttle openings. The problem is at low throttle settings (30 - 40 mph) it seems to "hunt" and does not run smoothly. I have checked the vacuum petcock and it seems OK. We are pretty much at sea level here so I am guessing a larger pilot jet might be in order. Any advice on where to get jets and by how much to change the size?
Geoff

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by striknein on 07/06/16 at 16:19:13

7F72716E7D72716E170 wrote:

Any performance mods done to the bike? If stock, I'd probably start with a #50.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by batman on 07/06/16 at 16:48:47

As above a 47.5 is stock a 50 is the next step up . If this doesn't seem to help and the motor 's still "hunting" it is running lean and you need to do the needle jet spacer mod. seeing your at sea level I'd replace the stock spacer with 3 #4machine washers (.75 mm ,the stock being 1mm)this should make your mixture richer and solve the problem .You should adjust your air screw after any of these mods.If it still hunts take out one more washer moving to .50mm .Next the main jet (stock145) to 147.5 if needed.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Kris01 on 07/06/16 at 17:02:16

I can't remember the stock jet sizes (50/145?) I'm a little above sea level and didn't up size my jets by much. She runs beautifully!

See my signature.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Dave on 07/07/16 at 05:34:49

5E53504F5C53504F360 wrote:

The problem is at low throttle settings (30 - 40 mph) it seems to "hunt" and does not run smoothly. We are pretty much at sea level here so I am guessing a larger pilot jet might be in order. Any advice on where to get jets and by how much to change the size?
Geoff

That is outside of the range that is controlled by the pilot jet. As batman said.....you need to raise the slide needle slightly to get more fuel in the mixture as the slide is opening. Remove the white spacer at the top of the slide needle, and replace it with 3 washers that are #4 machine screw size. If there is still a hesitation....remove 1 more washer and go with 2 on the top of the needle.

I would start with a #50 pilot and # 150 main....and the washer mod. Those changes should get you in the ballpark and make the bike run better.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by HefyJefy on 07/07/16 at 07:46:59

Thanks guys I will try these ideas and let you know. Couple of things I forgot to mention:
1. Yes the bike is stock.
2. We are in California and I note that the CA carbureter has different part number. Could this be a factor?
Thanks again.
Geoff

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by batman on 07/07/16 at 09:11:34

YES! THE JETS ARE MUCH SMALLER DUE TO CAILF. EMISSIONS!I'd check what you have and move up from there(your main may only be a 125 as I recall)(the numbers are stamped on the base of the jet) but moving the jet needle with three/two washers is standard to all carbs.(see technical section fuel&exhaust stock carbs&jetting,casb specs) your jets are 47.5 pilot 125 main.Dave's 50/150 is right on your bike will run completely different (very well)

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by verslagen1 on 07/07/16 at 09:47:10

34393A2536393A255C0 wrote:

Where's about? I'm in Duarte, east of LA.
Tell us what your stock jets are, I never believed they were actually 125.
My '96 was 145, going up to 150 actually improved mpg.
The pilot jet has always been right for me.
The "hunt" is due to lean mixture in that range of throttle.
After you check your main jet size, then a judicial trim of the white spacer should cure that.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Kris01 on 07/07/16 at 19:35:54

I hope 125 is wrong! I can't see the engine even running that lean.

Dave, when you tuned your bike with an A/F meter, did you happen to notice the AFR with stock jets? I'm just curious how lean Suzuki considered "just right".
/off topic

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Dave on 07/08/16 at 04:00:44

5F667D672425140 wrote:

My A/F meter is a "narrow band" meter. It uses a 2 wire Oxygen sensor, and it was designed for use in a fuel injection system to adjust the mixture. It is supposed to be looking for a 14.7/1 ratio.....and it can only go a few points above or below that ratio. It can only be used for gasoline. It is a great tool for jetting - but it does not read out any ratios or percentages or have any numbers.....you can only tell rich/correct/lean.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by HefyJefy on 07/08/16 at 11:20:09

OK well...
I just got both the Pilot and Main Jet out, the pilot is 52.5 and the Main is 145. Not what I was expecting, however I think a 150 main might be a good move.
In case anyone is following this the bike was a new California model last year, stock. It had the initial service (done by the dealer) and I did the 3,000 mile.
The problem has I think, been around from the start but the bike belongs to my wife so I only just got my sticky fingers on it.
I will report back once I have changed the main.
Geoff

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Dave on 07/08/16 at 11:35:46

The main jet is not going to have much affect on the 30-40 mph lean surge. You are really going to have to work on the slide needle to correct that issue. Replace the white spacer with the washers as batman, Verslagen and I have all suggested in this thread.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by verslagen1 on 07/08/16 at 12:01:58

58636E7968647F79626A67780B0 wrote:

Main jet feeds the needle jet, so increasing it increases the whole range.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by Dave on 07/08/16 at 12:11:08

160512130C0107050E51600 wrote:

[quote author=58636E7968647F79626A67780B0 link=1467845628/0#11 date=1468002946]The main jet is not going to have much affect on the 30-40 mph lean surge. You are really going to have to work on the slide needle to correct that issue. Replace the white spacer with the washers as batman, Verslagen and I have all suggested in this thread.

Main jet feeds the needle jet, so increasing it increases the whole range.[/quote]

My hunch is still the need to raise the slide needle. Going from a #145 to a #150 will have almost no effect on the 30-40 mph throttle range.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by batman on 07/08/16 at 14:06:39

I'm with dave! be careful ! All the screws on and in the carb are jis (Japanese industrial standard) a Phillips head screwdriver wont fit deep into the slots unless the point is blunted,and may strip them! it's best to pick up some jis anyway.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by HefyJefy on 07/09/16 at 19:16:02

Yep that's where I am going next, it was just a bit easier to swap jets first - need to get the tank off to do the needle valve...
Geoff

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by batman on 07/10/16 at 10:16:49

Your better off removing the carb and placing it in a vise between two pieces of wood to remove the screws.

Title: Re: Carburettor pilot jet size s40 Boulvard
Post by verslagen1 on 07/10/16 at 11:59:53

never use an impact driver on an unsupported tab... you'll break it off.

if you're not going to reuse the screws, grab the head with a vise grip like using a wrench on a bolt.

if it's from the factory, they use some sort of permanent loctite. use pen oil or heat or both... curse words optional. ;D