air inlet tube leak

[Radioflyer]

Engine idling rough and can't get 50 rpm rise at idle cut off. Found several possible causes. After doing an induction system pressure test, one big cause was an air leak at the inlet to the #4 cylinder. Dis-assembled, inspected, cleaned, reassembled with new, dry gasket. Still leaks. Will take apart again and insure that lip of intake flange is properly sitted in the retainer flange recess. However, I was very careful on the first attempt and am surprised it didn't remedy the situation. Maybe the retainer flange is warped.

Anybody else have trouble re-sealing these intake tubes?

 

[Kent Ashton]

Sometimes the inlet tube leaks where it is swaged into the sump.  If the tube can be wiggled where it goes in the sump, that might be it.  Then the rubber joints have to be tight.  I have heard of checking for leaks there by spraying a little unlit propane around the joints and seeing if the rpm increases.

 

Of course, the engine has to be running a bit rich before the 50-rpm-rise check.  If it the idle mixture was set too lean to begin with, then the rpm would not rise as the mixture is leaned and the engine might go from rough to rougher.  Marvel carb, I presume?

[Radioflyer]

Yes ma4-spa. The induction leak would presumably make adjusting the rich/lean mixture screw difficult. As I was hoping to describe above, the leak is at the cylinder inlet port, not the sump swaged tube. Finding the leak is not the question. A watery soap solution and 10psi pressurized induction system seems to do a good job of that. The question was what troubles, if any, has been had with re-acquiring a good seal where the tube joins the cylinder intake port. Maybe I just did a bad job and have to redo. It wouldn't be the first time!

[Kent Ashton]

Guess #2:  The retaining ring can sometimes warp, won't make good contact with the gasket.  

[Radioflyer]

Yeah the retianing ring or clamp was somewhat warped. Easy enough to remedy by sanding the contact face on a very flat surface. However, the real problem is that the lip or flange of the inlet tube rests too deeply in the recess of the clamp ring. I can't believe this is purposely or carelessly done this way by the manufacturer. So, I'm going with the idea that after 1500 hours, enough vibration has cause the lip of this steel tube to "dig" deeper into the recess of the softer aluminum clamp ring. At any rate, further sanding to make the lip of the tube rest proud of the recess should allow for proper clamping and cure the air leak. That my story and I'm sticking by it.

[Lukeman]

Be sure the tube flange is even and flat, also use a little Red Hi Temp silicon on the new gasket. That solved my intake leak problem. By the way, do not over torque the nuts.