I just put the new Dyna CDI on and I love it great power all the way through! Pulled my plugs after a short ride just to make sure and they were a very very light tan color on the electrode so I decided while I had it apart to do the shim mod. I used 2 number 3 steel washers drilled out to accomodate the needle and they measured .040 together. They were just a hair larger than they top hat on the needle which did not allow them to fit into the plastic stand as well as I would have liked but I went ahead and used them anyway. First ride I didn't really notice a difference until I let off the throttle at 60 then it started to backfire out the exhaust, ran it back up to 60 let off and the same thing happened. From what I have read that means that it is running lean so I parked it and haven't had a chance to remove them to see if the washers are at fault. Has anyone else had results like mine or have any idea what I may have done wrong. Thanks for any info.

 

My 650 did the same thing after I installed the cdi, cranked my airscrews out a 1/4 turn each and no more backfire. I did not do a thing to the needles.

 

I was playing around with my needles last night and although I didnt measure shims I was putting in, the thicker shims caused some pretty serious backfiring ( not the backing off you here when running lean) when decellerating. removed thicker shims and backfire went away----IMO backfire was coming from being to rich

 

Thanks for the info I will try and pull them shims out this morning before I leave for the big state of Minnesota. It will drive me up the wall if I don't before I leave. Thanks for the quick reply.

 

Backfiring through the CARBS is lean, backfiring through the EXHAUST is rich, it started doing this after you richened the needles, take the shims out, set the air screw's 1/4 turn out more than recomended I think 2 1/2 will work and it should be fine after that......

 

you are exactly right.

 

Depends on the conditions.
Backfire in intake on tip in....is lean.
Backfire in exhaust on declleration...is lean too.

From the mikuni carb website.

3: Backfires Through Carburetor

Common Causes:

Ignition: The factory Evolution engine's ignition can contribute backfiring through the carburetor.

Cam design: Long duration cams with early opening intake valves can contribute to backfiring.

Intake manifold air leak: A lean condition due to an intake manifold air leak can cause backfiring.

Carburetor jetting: An overly-lean low-speed circuit, non-functioning accelerator pump or clogged pilot jet can contribute to backfiring.





4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) 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.
2) 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.
3) 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.
4) 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.

 

I have heard the max needle shim for these carbs is .032-.035