What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

How to find an intake leak ?

Larry DeCamp

Well Known Member
My carb 0320 has run rough on startup recently when temp is between freezing and 40F. Boost pump, magcheck,mixture leaner, switch tanks , none of these affects the VERY rough running. I heard some exhaust popping recently. However, as the engine warms up, it smooths out, takes a full power runnup and runs normal thereafter.

Could it be thermal expansion closes an intake leak ? If a visual inspection and checking nuts for tightness dont fix it, how do you find intake leaks.

Also, is it possible a lean idle mixture can cause this magnitude of rough running. The idle cutoff rise is about 20 RPM, not 50’ but I have not had this problem for 250 hrs until temps turned cold this year.
 
I did an exhaustive look for intake leaks this year. Ultimately, the best solution for me was to make a plate (from plywood) that bolted to the sump and had an air fitting in it. I connected the air fitting to the compressor and ran about 10 PSI into the system. Gotta be careful with the prop when you do this. It'll want to move, just like on a compression test.

At that point, I squirted all of the joints in the intake system with soapy water to find the leak(s).

Comment - intake leaks usually get worse as your engine heats up, not better.
 
Back in the automotive days, we did something similar to Kyle's process. WE ran the engine, and took carb cleaner and lightly sprayed it at the suspeced area. If it were leaking, the spray would richen the mixture, RPM would rise, vacuum would rise and stabilize, and the engine would be noticeably smoother.
Now with a prop swinging in front of the engine, it promotes all kinds of safety issues, but it works.

Tom
 
Also, is it possible a lean idle mixture can cause this magnitude of rough running. The idle cutoff rise is about 20 RPM, not 50’ but I have not had this problem for 250 hrs until temps turned cold this year.

Yes. The relationship between intake air temp and fuel required increases exponentially as temps drop. I did a custom FI map on a Porsche and was simply shocked at how much more fuel was required at 20*F compared to 60*F. Could never understand why I couldn't start the car at 30* untill I upped the fuel delivery by a significant amount at those temps.

After fires in the exhaust are not uncommon when running too lean at idle.

Larry
 
Intake leak testing back woods style
My dad used to lay a thick bead of axle grease around the intake ports of the old cars, one runner at a time. Idle for a short period, if no change move on to the next runner.
What a mess, but he found the leaks.
 
It's fairly inexpensive/easy to replace all flange/cyl head gaskets and the rubber intake tube couplers. Mine were 6 years old and the crusty, heat-hardened couplers were the source of the leak. Did an idle mixture adjustment at the same time.

Quite an improvement in both cold/hot starting and idle quality.
 
Carb cleaner

Back in the automotive days, we did something similar to Kyle's process. WE ran the engine, and took carb cleaner and lightly sprayed it at the suspeced area. If it were leaking, the spray would richen the mixture, RPM would rise, vacuum would rise and stabilize, and the engine would be noticeably smoother.
Now with a prop swinging in front of the engine, it promotes all kinds of safety issues, but it works.

Tom

I still do it that way on cars but not sure I would stand that close to a prop!
 
It's fairly inexpensive/easy to replace all flange/cyl head gaskets and the rubber intake tube couplers. Mine were 6 years old and the crusty, heat-hardened couplers were the source of the leak. Did an idle mixture adjustment at the same time.

Quite an improvement in both cold/hot starting and idle quality.

^^^ This!!!!
By the time you are done messing around you can be done by just replacing. Parts are cheap.
 
If you can borrow a smoke checker from an auto repair shop it makes finding intake leaks pretty easy. You block off the intake air at the air box and inject the smoke. I have found hundreds of intake leaks at my old Porsche shop with a smoke machine. Works great for EVAP systems on cars but we do not have them on airplanes. Yet😳

Got first run of engine in my 3B two weeks ago. Minor issues but nothing scary
 
If you don't want to spend $ on smoke machine - Ductape shop vac hose from vac output to the cowling intake...open throttle...spritz intake tube joints with slightly soapy water...any leaks will bubble up.

If there's no leaks then then you may have a sticking valve and need to do a wobble check
 
Last edited:
It may also help to tape off your exhaust. At that point, the only normal leak will be past the rings. And instead of the cowl intake as mentioned above, connect to the carb intake.
 
It may also help to tape off your exhaust. At that point, the only normal leak will be past the rings. And instead of the cowl intake as mentioned above, connect to the carb intake.

Is this necessary? Is there any valve overlap on these engines? I'm thinking no, but I'm only 98% sure. ;-)
 
Is there any time either all of the exhaust or intake valves closed at the same time?

The carb does supply all cylinders.

Right. But if either the intake or exhaust valve is closed on a cylinder, air isn't going to flow through the cylinder. I think one valve or the other is closed at any moment in time, and that's true for all 4 or 6 cylinders.
 
Classic Case of Morning Sickness

I’m pretty sure your description is a classic case of “morning sickness.” I would focus my investigation towards the possibility of a stuck exhaust valve.
 
Right. But if either the intake or exhaust valve is closed on a cylinder, air isn't going to flow through the cylinder. I think one valve or the other is closed at any moment in time, and that's true for all 4 or 6 cylinders.

The intake ports from the carb to the 4 cylinders are interlinked. So if one cylinder (out of the 4) has an intake and an exhaust valve that is not seated, you will not be able to pressure the intake system enough to check for leaks. You must plug off the exhaust in order to close the system.
 
The intake ports from the carb to the 4 cylinders are interlinked. So if one cylinder (out of the 4) has an intake and an exhaust valve that is not seated, you will not be able to pressure the intake system enough to check for leaks. You must plug off the exhaust in order to close the system.

If blowing in the intake results in a leak out of the exhaust, or vice versa, then that would mean that something is wrong (valve is stuck open) with a cylinder, correct? I understand that OP is however trying to find if there is a leak somewhere in the intake manifold and has not indicated that a valve may be stuck open?

I have tested for exhaust system leaks by pressuring the exhaust pipe and it is a closed system, i.e. there is no air flow taking place out of the intake manifold when the exhaust is pressurised.
 
Thanks to all

Great ideas. I will apply them asap.
First trial will be reach in and richen idle screw.
BTW, Larry vetterman advised to use 4oz of two stroke oil per ten gallons gas. Select darkest oil available and just fly. The color will reveal a leak in the system.
 
The intake ports from the carb to the 4 cylinders are interlinked. So if one cylinder (out of the 4) has an intake and an exhaust valve that is not seated, you will not be able to pressure the intake system enough to check for leaks. You must plug off the exhaust in order to close the system.

Right, but our engines never have both the intake and exhaust valve open at the same time on one cylinder. The system is effectively closed - you cannot blow air in the intake and have it come out the exhaust (internal failures excepted).
 
It does..

Right, but our engines never have both the intake and exhaust valve open at the same time on one cylinder. The system is effectively closed - you cannot blow air in the intake and have it come out the exhaust (internal failures excepted).


There are times in the cycle when both valves are off their seat at the same time in a 4-stroke engine. Check the valve positions between exhaust and intake strokes and you will see the overlap.
 
Vacuum leak

A friend tried a clever method on his car recently. He blew smoke into a vacuum line and looked for the smoke exiting. He had a hole in the brake booster hose. He uses a Vape but said cigar smoke works equally well. Been meaning to try it on my car. I've had a pesky vacuum leak for quite a while.
 
Last edited:
Back
Top