[mfleming]

Two things.

First question. Does anyone have advice or a written procedure for replacing the intake tube seals on the sump end of the tube? Photos below.

I have gotten some great advice from a respected member of VAF about trouble shooting my problem but just casting a wider net.

Second question. Can an intake leak on the intake tube cause a massive backfire? The backfires during hot starts have blown out the air filter several times. Always on a hot start. It happens whether I start on the left impulse Slick, right P-Mag or both. It happens when the mixture is being used in as it's trying to start.

I've had experienced mechanics and pilots watch me hot start and find nothing wrong with my technique (I've tried every technique I can think of.

 

[Carl Froehlich]

Not sure why you thing intake seals are the root issue.

As far as backfire, before doing anything else I’d check ignition timing. Remember for this engine the:
- Mag is 25 degrees BTDC
- pMag is also 25 degrees, but to set this timing you move the crank to TDC, then blown into the pMag manifold pressure tube to tell the pMag to set the timing. Do not make the common mistake of setting the pMag timing when the crank is at 25 degree BTDC. If you forget to do this, the pMag will actually be at 50 degrees BTDC.

Carl

 

[Full Throttle]

To install. It's Just a big orange silicon O-ring . Be careful not to cut the o-ring on the sharp edge of the tube o-ring keeper. Any intake leak lowers the demand of fuel from a mass flow sensor. Meaning less fuel to the engine at all times.

 

[Full Throttle]

If your able to crank the engine over a few times before turning on the mag or EI, it clears the engine of residual fuel and should help to not backfire.

 

[mfleming]

Not sure why you thing intake seals are the root issue.

As far as backfire, before doing anything else I’d check ignition timing. Remember for this engine the:
- Mag is 25 degrees BTDC
- pMag is also 25 degrees, but to set this timing you move the crank to TDC, then blown into the pMag manifold pressure tube to tell the pMag to set the timing. Do not make the common mistake of setting the pMag timing when the crank is at 25 degree BTDC. If you forget to do this, the pMag will actually be at 50 degrees BTDC.

Carl

I don't know what the root cause is. This intake leak is the only thing I've found out of order so far.
Timing - Been there, done that. Any other suggestions? This darn engine has 160 hrs on it and it's been backfiring on start up since new.
As stated in the OP, the backfire can happen with any combination of ignition being on or off.

 

[mfleming]

If your able to crank the engine over a few times before turning on the mag or EI, it clears the engine of residual fuel and should help to not backfire.

I have thought of that and it might help...but why my engine and not others??

 

[Carl Froehlich]

I don't know what the root cause is. This intake leak is the only thing I've found out of order so far.
Timing - Been there, done that. Any other suggestions? This darn engine has 160 hrs on it and it's been backfiring on start up since new.
As stated in the OP, the backfire can happen with any combination of ignition being on or off.

I do not see how an intake leak can cause this problem. For that matter how do you know you have an intake leak?

Not trying to piss you off, but after many hot starts I finally gave into following the guidance on how to do hot starts. Even so I can advance the mixture too soon and end up with an engine with fuel/air too rich to run. The cure is always to open the thottle, mixture at cut out, and crank. In a few blades the engine purges the excess fuel and runs. At that point an easy advance of the mixture and we are off.

Carl