Thursday, January 26, 2006


The skinny on backfire

I’ve been involved in two separate discussions this week concerning carburation and fuel mixture, specifically with regard to what exactly causes backfires in carburated, high performance motorcycle engines. The popular misconception seems to be that, all free-flowing engines that backfire when the throttle is snapped closed from higher-than-idle revs must be running lean across the rev range. This is not always the case.

Like any good sportbiker, the first thing I do whenever I buy a new motorcycle is to store away its portly, restrictive stock exhaust and install an aftermarket pipe. In the case of my ZX7R, the bike was already lean from the factory, and freeing-up the back pressure made it go even leaner. Crackling backfires on roll-offs were common.

Next, I installed a BMC filter and a big, fat, Factory Pro jet kit. Doing so improved the bike's mid- and high-range performance dramatically, and it's actually slightly rich all the way through the rev range now. But you know what? It backfires just a bad as it ever did. Why? Mikuni has the answer:
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.

5. Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.
If you’ve ever been to a superbike race, you know that backfires are commonly heard as racers slow down for corners. BIG backfires. As the Mikuni guys explained, backfiring is one of the characteristics of a properly tuned, carburated race engine. So be thorough and don’t assume. You know what that does.

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