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Message started by DrunkenDwarf on 05/01/10 at 07:24:09

Title: Newb Carb Question
Post by DrunkenDwarf on 05/01/10 at 07:24:09

Will too rich an idle jet cause occasional afterfire? Specifically after the engine is warm and choke is pushed in.

I don't seem to get any afterfire during the first 2 miles, but after I push the choke it seems to happen. Even after I've done a little highway and know the engine is hot.

My typical start up procedure is to start the S40 with choke, put on my gear, ride away with choke, after 1/4 - 2 miles push choke in and let the engine finish warming without choke.

Details are in sig.

-D. Dwarf

Title: Re: Newb Carb Question
Post by LANCER on 05/01/10 at 08:11:30

IT IS POSSIBLE BUT NOT PROBABLE

WHAT JETS ARE IN IT AND WHAT ARE THE PILOT SCREW AND NEEDLE SETTINGS ?

Title: Re: Newb Carb Question
Post by Serowbot on 05/01/10 at 09:03:51

yes, it can,...
if popping increases at higher elevations... and on humid days... you are rich...

Title: Re: Newb Carb Question
Post by DrunkenDwarf on 05/01/10 at 13:11:20

505D525F594E0E0B3C0 wrote:

IT IS POSSIBLE BUT NOT PROBABLE

WHAT JETS ARE IN IT AND WHAT ARE THE PILOT SCREW AND NEEDLE SETTINGS ?

My signature says:

Quote:

Pearl White 2007 S40; Dyna exhaust; Current jets: 55 (1 turn out) / half spacer / 150

My profile says:

Quote:

Rochester, NY

For those not familiar with upstate New York, Rochester is on Lake Ontario. It's about 500 feet above sea level and always moderate to high humidity.

5741564B53464B50240 wrote:

yes, it can,...
if popping increases at higher elevations... and on humid days... you are rich...

It's pretty flat here, I don't change elevation very often. Less than 100 ft normally.

My gas mileage is lower than I'd like it to be (mid 40's) and I'm considering going down a size on my pilot jet. Just curious if everything I'm seeing points to slightly rich or if there's something else I should be looking at. It runs fine. Maybe I shouldn't mess with it.

-D. Dwarf

Title: Re: Newb Carb Question
Post by DrunkenDwarf on 05/04/10 at 13:29:32

Slipped off the first page, so a bump to get Lancer's and Serowbot's attention again.

-D. Dwarf

Title: Re: Newb Carb Question
Post by bill67 on 05/04/10 at 13:37:50

I would put the spacer back in and I think you will be alright and your mpg should go back up.If your jetted for max power you will be to rich for normal driving.

Title: Re: Newb Carb Question
Post by Serowbot on 05/04/10 at 16:07:11

Simple enough to try it,... drop down to a 52.5 on the pilot and see if it's better...
Also consider what Bill said,... most people go to 2/3 thickness on the spacer... 1/2 may be wasting a bit of fuel...
;)...

Title: Re: Newb Carb Question
Post by DrunkenDwarf on 05/05/10 at 17:35:05

I'm running three 0.5 mm washers. So it's a bit more than half the stock spacer. Four washers will be a bit more than three quarters stock.

The reason I reduced the spacer was a mild surging at 1/8 to 1/4 throttle (35 mph in third gear).

When I bought her the carburetor had 55 pilot, full spacer, 155 main. I haven't noticed a difference putting the 150 main in. That's probably due to my inexperience/ignorance.

It does make sense, now that you point it out, that the needle would have a large impact on the mpg. I imagine I spend most of my time at throttle openings where it's the primary source of fuel.

I did try to start her this afternoon with no choke. It was about 80 degrees and the engine was not happy. It started, but barely. So the pilot isn't excessively rich.

Final question:
Can someone remind me why our engines backfire in the exhaust on throttle close. I remember it has something to do with the engine pulling a high vacuum in the carburetor.

-D. Dwarf

Title: Re: Newb Carb Question
Post by DrunkenDwarf on 05/05/10 at 20:28:46

Found the answer to my own question. This is for a Mikuni BS28. Slightly different than ours, but I think the concept applies.

Quote:

The idle circuit has a fuel jet and TWO air passages to feed it during normal running. As modern bikes are very lean jetted due to emissions, the high vacuum created during deceleration causes an over-lean condition that leads to backfiring in the exhaust. The air cut-off valve reacts to the high vacuum cutting off one of the air ways and so enriching the idle mixture momentarily and avoiding popping in the silencer.

Here's the source thread: http://www.mychinamoto.com/forums/showthread.php?755

The author also mentions something called "rich stumble". I don't hear that term thrown around this forum.

-D. Dwarf

Title: Re: Newb Carb Question
Post by Serowbot on 05/05/10 at 23:15:02

don't know about that, but... big thumpers are renowned for decel backfires...
...all that on/off puffing, is hard to smooth, with no other cylinders for balance..
but ours has a decel enrichment valve, that's supposed to reduce it.. and someone recently posted a softer spring mod for it....
I've grown kinda' fond of it, myself... :-?..