Backfire!

Hi M8
Yes it could be too much air getting into the mixture, but it is often caused by air sucking onto the exhaust joints, particularly at the union to the cylinder head.
Worth checking.
good luck
Regards
Ady

I'm No Expert On Jetting,Just Starting To Learn Myself.
I'd Have Thought That If This Is A Dellorto Carb That You'd Need Something Like This
190 Main
35/38 Pilot Depending On Exhaust
Clip In The Middle Somewhere Again Depending On Exhaust.
Maybe Have A Look And See If Yours Has A CD1 Jet As Well :lol: :wink:

Deceleration popping is usually caused by too lean of a pilot jet. Increase the pilot one size at a time and then test. If it doesn't help it probably is not a jetting issue.

Obmar said:

OK... I have changed the pilot jet to 35. I adjusted the mix screw to 1 1/3 turn(highest rpm) the turned the idle screw so that the bike idles at about 1800rpm.

When opening the throttle 1/8 quickly it dies.. as it is getting flooded.

is the pj to large?

What does ady mean "exhaust joints"?
stupid question maybe but I'm not so good at mechanical english

Click to expand...

Does It Die When Your Riding Along?
Think The Pilots About Right.
Ady Means Where The Header Pipes Join The Mid Pipe And Mid Pipe Into Silencer,Sometimes It Leaks And Causes All Sorts Of Problems.
You Can Usually Tell Because There Will Be Back Soot Coming From It.
Have You Changed The Internals Of The Carb As Some Of The Parts Do Wear Out?

obmar

a dead spot is typical of the way you and i set it for best idle speed, however if you adjust the fuel screw 1/4 turn either way and try it again you'll get rid of that flat spot!

secondly, i can't agree with husabutt about the bikes running lean on the overrun.

1) the ignition timing is set wrong at the factory on my bike and year
2) they are well rich even when leaned off considerably so there are unburnt gases in the exhaust that burn and cause this noise
3) i have put it in an old post but the best position i once worked it out for the needle taper to start was at 45mm. this means that if the taper starts at 42mm it needs to be on C2 etc. i only ever really used K51 and K35 needles. that is where i got the best response!
4) pilot jets are for 1/8 to 1/4 throttle so if it's dead at the bottomit ain't yer pilot jet! it's the needle clip!
5) get the MJ and atomizer right first THEN worry about acceleration, then worry about snap, idle and startr.

regards

Taffy

Obmar said:

But she Backfires as hell when getting from full throttle to closed throttle.

Click to expand...

Did going up one size on the pilot jet help with the backfiring?

Taffy said:

i can't agree with husabutt about the bikes running lean on the overrun.


2) they are well rich even when leaned off considerably so there are unburnt gases in the exhaust that burn and cause this noise


regards

Taffy

Click to expand...

What do you mean that you can't agree with me :?

As a courtesy for not jumping on the sheep thread bandwagon you should of at least acknowledged the possibility that I may be correct. I will see you at the sheep thread later.

From the Mikuni site

When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.

Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.

When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.

eric

i don't want to start a war ( and i've started a few here at UHE :cry: ) it's just my personal thoughts on it and i'll tell you why! and with you i think it would take a lot to PYO. bu....t

i ran a 170MJ in my WR400 and it popped back terribly on the overrun. everyone said the same as you!

so i went and loaded it up with fuel again at low revs or wherever and then i decided let's say to drop the needle in another part of the fuel curve. then the MJ. then the PS was in then out. then i dropped the MJ, then the needle. you get my drift?

at the end i was on 155MJ with a fatter needle (leaner) and a 35PJ (the smallest!).

the bike never 'popped-back!'

that's it. the only way i can describe it.

there was a book when i was a kid and i think it was called 'alice in the looking glass' and this girl walked into a mirror and into a different world. now i've done it i know that i'm 'on the other side'.

it goes with my story of the white spark plug as being reality over hyperboly.

regards

Taffy

No war here Taff just good ol' discussion and yes it takes an awful lot to ruffle my feathers. Skin like leather.

If I needed jetting advise you would be the first one I'd look to (maybe the second) :wink: .

Real world beats out book smart every time. The trick here is figuring out which world you are currently in :twisted:

..."there is barely enough time to make changes,I can figure out the why part later"....Erv Kanemoto