backfire
Mar 30, 2004 | 12:07 AM
#1
Thread Starter
|
Trailblazer
Joined: Nov 2003
Posts: 59
Likes: 0
backfire
hi all. i installed a alien pipe on my son's z250. now it backfires sometimes so i figure it must be lean. i am not familiar with this carb so can anyone describe where the air/fuel screw is at? also, turning which way increases the fuel flow?
Anyone with an Alien on the Z250 rejet? Any other tips appreciated.
Thanks
Anyone with an Alien on the Z250 rejet? Any other tips appreciated.
Thanks
Mar 30, 2004 | 04:51 PM
#3
Elite Pro Rider
Joined: Dec 2003
Posts: 5,599
Likes: 0
backfire
If it backfires on throttle let off, then adjusting the mixture screw is your best bet. Go richer. Also check for intake leak and exhaust leaks, they can cause it too.
carb The mixture screw is #27, and its under the cover#23.
This is from the mikuni website.
"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."
carb The mixture screw is #27, and its under the cover#23.
This is from the mikuni website.
"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."
Thread
Thread Starter
Forum
Replies
Last Post
fordfaithful21
Polaris Ask an Expert! In fond memory of Old Polaris Tech.
9
Dec 7, 2015 05:52 PM
Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)