Fuel Injected KCAB Backfires

G

gofast711

New member
I have a 1970 KCAB with a fuel injected IO320. It backfires (pops) when I taxi. Not continuously. Leaning or richening the mixture doesn't seem to make a difference although I keep it pretty lean. Plugs check good, no apparent injector issues, mag timing is good. Any thoughts???
 
I've figured it out. When bringing it to an idle during stalls, spins, or landings, go full rich. Lean it while power is on or when taxiing. A friend with an injected Decathalon taught me this and it works.
 
Leaning on the ground is always good but it should not be necessary to prevent back fires or popping. Make sure your fuel system is set up correctly so you are not masking another problem with you "technique". If the servo is adjusted properly, you should see a 50-75 RPM rise when you pull the mixture to idle cut off from full rich when shutting down. Adjusting the servo for the proper RPM rise is simple. You can down load the SB from Precision Aeromotive's website.

Bill
 
Hi, i have a 72 8kcab with the io320 and i have been experiencing the same problem as well. I spent well over 15K after eventually overhauling the engine and fuel servo and the pop remains more so when the ambient temp is high.
 
Do you get a rise in RPM when you pull your mixture to idle cut off? You may be too lean.

Also look for leaks in your intake tubes.
 
Hi, yes. Intake tubes are sealed well and i get the rise when i pull the mixture. I sometimes wonder if its not the design of the exhaust (i have the dual exhaust).
 
Do not have experience with a dual exhaust 8kcab so I am unable to comment there.

When is the popping or backfire the worse? On the ground or when you significantly reduce power like on downwind? I still think it is induction related, but have you tried to eliminate some funky going on with your mags or ignition?
 
As wpbarnar says, check for rpm rise when pulling the mixture to cut off at shutdown. You should see about 50 RPM rise just before the engine quits.
Also, check the exhaust manifold flanges/gaskets/security. Leaks can draw in air at the cylinder flange/gasket area and cause a "pop" as unburned exhaust gasses are reignited with the extra oxygen.
Bill B
 
When the engine was gone thru last May at Zephyr engines, they installed new Bendix mags and also had the fuel servo overhauled. It seems that the popping becomes worse when both the engine is hot and there is high ambient temps.If you have extended idle/taxi times it also seems to pop worse.
 
First of all a disclaimer. I am NOT a mechanic. I have a fuel injected Citabria and experience similar conditions when the weather is hot.

Here is my suspicion of what may be going on. First of all while operating on the ground, the fuel flow is very low. The injector lines sit on top of the hot engine with very minimal "cooling" airflow from the hot ambient air. This all leads to perhaps some vapor bubbles forming inside of the fuel injector lines. This does not promote smooth running.

During taxi after flight on a hot day, I will leave the fuel boost pump on. This provides higher pressure which should mitigate fuel vapor bubbles from forming. Seems to help for me.

Anyone with "real credentials" care to comment on this.
 
The electric fuel pump only increases pressure to the servo, it has no effect on the pressure in the injector lines.
Bill B
 
Frankford, did they rebuild your spider at the same time they overhauled your servo? There is a small piston and spring in it which helps even out the distribution at low fuel flows. You may be getting an uneven distribution hence the "lean pops" at idle or low power.

There is a good article in this months "Kitplanes" magazine on fuel injection and Lycoming. Maybe worth a read.

Bill
 
Frankford, any luck on this? I'm experiencing the same issue when taxing after a flight...popping and periodic backfires at idle when taxing. I've played with the mixture to see if running lean vs. rich improves the symptoms. Boost pump doesn't seem to improve things either. My IO 320 E series has always performed like a tank but really dislikes being restarted after a shutdown unless the engine is completely cool. Refueling between legs of a long cross country, for example, always results in a jumpy idle taxing back to the runway. I wonder if the two symptoms are related to the same issue.
 
If your problem coincides with hot engine and hot ambient temperatures, it sounds like fuel vaporization in the lines or uneven fuel flow.
The previous comments are correct in that the boost pump does not directly influence fuel pressures to the cylinders unless you have a problem earlier in the system.
I would think that ensuring your spider is functioning correctly would ne paramount.
 
I had an engine rebuild, and two flights later the engine quit just a minute and a half after take off. I got back to the cross wind runway ok, but it was a great experience!
The problem was the spider was not rebuilt! It wasn't "part of the engine" they said!!!
We got the rebuild done only to learn it had not been touched since 1976 (when it was installed new)!
We purchased in 2007 and this happened in 2008. Have not had any backfiring or engine issues since!
The original rebuild had to be tweaked since the first mech did not adjust the Lean setting correctly. Engine would never start. Now after one setting adjustment by another mech (different shop) it runs well. Leaning from the mixture whenever called for works well. No Backfiring so far.
 
Interesting, I have the same problem on a 8KCAB w/160hp. After a flight the engine pops when taxing. I live in a warm country 30 °C
Never though this was a common issue to many of us
Gerardo
 
The problem on my Decathlon was fix, it was an small air inlet on the manifold gauge piping. Hope this help you all.
 
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