I've heard and read tons about the 470's poor induction system, but I'm beginning to think most of the issues with fuel/air distribution are due to induction leaks, and after taking my system apart and doing some repairs and detailed inspection and testing, am going to not worry about tiny leaks at 7 psi, and hope that this will be goo d enough. I will update this thread after I get my flight testing done.

I did find some gushers, and thank goodness for the engine monitor, because I knew where to look. But even then, with the carb still on, I couldn't pressurize the system enough to find them. It wasn't until the carb came off and I used a blanking plate that I found the biggest leaks.

Here are the things I've learned along the way:
* The torque specs for the clamps go from 30-45 in lbs depending on which manual and/or service bulletins you find.
* Most of the 470s I've seen have the wrong clamps, and that can cause real problems, as can the lack of a torque wrench.
* Without putting a blanking cap on where the carb goes, it's hard to find leaks because you don't have enough pressure for long enough to feel around, and also any gushers will blow the soap away so you won't see any bubbles ( ask me how I know)
* At spec torque with about 10 psi in the system, almost every joint will start to leak
* At sea level ambient (14 psi), and at idle say 15" map, there is a 7 psi vacuum inside the system.
* Some of my induction pipes have flanges, some don't
* The best seal comes from the clamps being right next to the flanges, and spreading the clamps apart makes things worse.
* Positions for clamps are critical: it takes time to find the right place.

So a few lessons:

* Somebody here shared that Steve Knopp told him to ensure that the induction pipes are aligned before you finalize their placement (which one on which side and cylinder). I am guessing, based on testing with regulated pressure and a blanking plate, that if this wasn't done, there will be little leaks, because even small offsets pipe to pipe can cause leaks at 7 psi.
* The heat shields cause induction leaks because unless you bend the the mounting feet, they force the clamps too far apart to be effective. I am guessing this might a major reason most people think the 470 has poor distribution! Funny!
* I found a bunch of deformed pipes and I bet you have them too.
* You will likely find gushers where people have overtorqued the clamps or flat spots caused by the wrong clamps.

I know that pressure vs suction are different, but this is the best I could do.

I used a sand bag support, mallet, and ball end steel similar to the end of a ball peen hammer to bang out the dents I found. That helped fix most of the gushers.

Hope this helps somebody else.

Below see the blanking plate, photo of a hose showing the square indentations caused by the head shield showing how far apart they force the clamps, and the spark plug air inlet adapter.

I agree with you and am surprised that I don't hear of this issue more often. In order to properly seal the gaskets to the intake manifold at the rear of the engine you need to either use a very narrow clamp such as an Oetiker Stepless, or grind one of those Breeze type clamps. Also the dual bridge clamps used for the front near the cross over tube are often spaced wrong for a good seal. I've made my own. ...and yes the heat shields can cause problems.

The bubble test is not always effective. Also you almost need to have a vacuum dedicated for the purpose, other wise you could easily be contaminating your engine, even with the filter removed.

An inflight induction leak test is the way to go.

How are you pressurizing the intake? Through a spark plug hole? I have my carb off right now and getting checked out by ly-con so I want to do this as well as these induction systems really like to leak.

I was thinking of building a carb block off plate with an air nipple, that way I wouldn't need to pull a plug.

How are you able to secure the prop and check for leaks at the same time? Do you have a helper or does the prop find a home and stay put?

schu

Mine was very leaky also. I tied the tail, removed the lower cowl, restrained the nose bowl, and idled as lean as it would go. The leaks perked up with a propane bottle and nozzle, right to the nearest inch or so. I made a small amount of castable two part hi temp silicone (smooth on brand), and repeated the exercise with a syringe of the stuff and let it run for 10 minutes around the boots and flanges.

The propane test was negative for all of them but one (there were a lot of leaks). That was fixed on the second time around.

My large roughing pumps for my vac chambers cant stay ahead of the carb leaks down to 70 kPa absolute at ~4l/min rated. That is roughly .04% of the total flow at 2500 RPM. I'm not worried. One of the cylinders with a flange leak did cool down a bit after the fix, though. So apparently even small leaks have an effect.

Thx for the good Info Slo.

How did the test flight go?