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Carburetor experts, riddle me this

skukum12

skukum12

MEMBER
The Last Frontier
2nd to last flight: run up, mag drop, carb heat, op etc all normal. Idle and climb out rpm normal. Normal as in same numbers for this plane last 6+ years.

Last flight: run up, mag drop and op all normal. Carb heat only drops 50. I depart, reach altitude and notice the position of the throttle is straight up(mid throw)at cruise instead of about a 1/4" ahead of the window trim. Additionally, full power typical climb out she is showing 2600 rpm where before 2450-2475 was the norm. On the other end, she will now happily idle just over 600 where before 750 was the norm.

Hmmm, precautionary landing, all checks out. I thought the cable may have slipped but it was tight. Motor runs great, never missed a beat.

It all sounds bassackwards to me. Fire away your opinions!
 
Sounds like it's breathing better for some reason. Maybe something to do with the carb heat mechanism????

FWIW, I get about 2600 at 70 mph indicated in climbout, 8241 prop.
 
skukum12 said:
2nd to last flight: run up, mag drop, carb heat, op etc all normal. Idle and climb out rpm normal. Normal as in same numbers for this plane last 6+ years.

Last flight: run up, mag drop and op all normal. Carb heat only drops 50. I depart, reach altitude and notice the position of the throttle is straight up(mid throw)at cruise instead of about a 1/4" ahead of the window trim. Additionally, full power typical climb out she is showing 2600 rpm where before 2450-2475 was the norm. On the other end, she will now happily idle just over 600 where before 750 was the norm.

Hmmm, precautionary landing, all checks out. I thought the cable may have slipped but it was tight. Motor runs great, never missed a beat.

It all sounds bassackwards to me. Fire away your opinions!
Click to expand...

It sounds like the similar performance increase you get when you replace a muffler having collapsing baffles.
Unless yours is gutted it may have been unknowingly going bad and a piece of the baffle core finally broke off or repositioned to free up the exhaust flow. Performance loss from bad mufflers can come on very slowly. Worth checking.

Only getting 50 rpm for carb heat is weak.

The engine is trying to tell you something, so keep looking until you understand what’s going on.
 
Check for broke or cracked primer line, not just the cylinders but that T in the back of the engine. That would cause the engine to run lean and account for the carb heat change and increase power. If you have 4 cylinder CHT/EGT you should have seen some increased CHT on climb and cruze. DENNY
 
Gordon Misch said:
Sounds like it's breathing better for some reason. Maybe something to do with the carb heat mechanism????

FWIW, I get about 2600 at 70 mph indicated in climbout, 8241 prop.
Click to expand...

If it's breathing better, I don't understand why the physical location of the throttle at cruise is so much further forward.
 
cubpilot2 said:
It sounds like the similar performance increase you get when you replace a muffler having collapsing baffles.
Unless yours is gutted it may have been unknowingly going bad and a piece of the baffle core finally broke off or repositioned to free up the exhaust flow. Performance loss from bad mufflers can come on very slowly. Worth checking.

Only getting 50 rpm for carb heat is weak.

The engine is trying to tell you something, so keep looking until you understand what’s going on.
Click to expand...

Flying buddy told me to check the muffler as well
 
DENNY said:
Check for broke or cracked primer line, not just the cylinders but that T in the back of the engine. That would cause the engine to run lean and account for the carb heat change and increase power. If you have 4 cylinder CHT/EGT you should have seen some increased CHT on climb and cruze. DENNY
Click to expand...

Good idea, poor little primer lines have a bunch of influence on the motor but are often overlooked.
 
If it's breathing better, I don't understand why the physical location of the throttle at cruise is so much further forward.
Click to expand...
Yes, that doesn't seem to fit with improved climbout power and idle.
 
I am thinking that your carb heat box is broken. You rpm drop on the carb heat check is low. You are getting more power. Your engine appears to be breathing easier. That would indicate you are bypassing the air filter. Check out everything on the intake system prior to the carburetor.
 
Has anyone on Supercub.org used the Knots 2U Carburetor Air Filter for the PA-18. UniVair is out of stock on paper filters and this one from WI is advertised as more fuel efficient.

However, there are no online reviews.

It is at least 40% more expensive that the paper filters but the marketing hype indicates that that will be recouped in .3 to .5 Gallons less fuel burn thus paying, over time. for less paper filters.

Just looking for information from anyone that has tried this filter.

Jerry
 
i can see how a less restrictive filter could lean the mixture some, and I can see how max power could be increased. However if one already leans carefully I don't understand how the air filter could change fuel economy. Opinion, of course.
 
I looked at the K2U add and see K&N elements (Challenger) are used. They are oiled cotton gauze material meshed in a wire maize. You can see through them...shine a light through them. So can dirt. They will work to trap some dirt as advertised as long as they are properly oiled. I've used them on a variety of vehicles including aircraft and they flow well. I prefer Donaldson elements if they are available as they flow as well as or better than K&N.

Gary
 
Back to the OP. I would be most concerned about the new throttle position. Is the throttle hitting both stops on the carburetor?
 
skukum12 said:
2nd to last flight: run up, mag drop, carb heat, op etc all normal. Idle and climb out rpm normal. Normal as in same numbers for this plane last 6+ years.

Last flight: run up, mag drop and op all normal. Carb heat only drops 50. I depart, reach altitude and notice the position of the throttle is straight up(mid throw)at cruise instead of about a 1/4" ahead of the window trim. Additionally, full power typical climb out she is showing 2600 rpm where before 2450-2475 was the norm. On the other end, she will now happily idle just over 600 where before 750 was the norm.

Hmmm, precautionary landing, all checks out. I thought the cable may have slipped but it was tight. Motor runs great, never missed a beat.


It all sounds bassackwards to me. Fire away your opinions!
Click to expand...

I missed the part or misunderstood the throttle position change due to not knowing its relationship to the “window trim.”

In a normal cub installation, when the throttle lever is almost, but not quite to its aft end of movement; the idle stop at the carburetor is in solid contact with the idle set screw. Conversely when the lever is almost at, but not quite to its maximum travel, the carburetor throttle is solid against the “wide open” stop.

Can you verify if yours was / is set up his way?

I suspect that you are currently hitting both stops due to the lower idle speed and full throttle performance.
Based on the previous performance, you were neither hitting the idle setting or full throttle stops, and perhaps were not for a long time. Why?

Im having concerns about the integrity of your throttle cable. Has it been in some sort of internal bind that was limiting travel and has suddenly freed up?

Throttle cable integrity is a big deal. Had a good friend with a nice cub which experienced throttle cable breakage at idle during approach to land. He went down on the tundra and flipped over. His neck was broken and became paralyzed as a result.
 
It has always bumped both stops. I was going to turn the idle down before the next flight then it happened on it's own. As to the lower full power climb rpm, I was told way back that is what I should expect from a Catto due to its fatter profile. The new higher rpm at climb and the lower idle are both quite pleasing but the sudden change has me clinching.

Since it has always bumped the stops, but had incorrect rpm on both ends, could it be there was some sort of physical blockage of the butterfly plate(are they called that?) that has just now become dislodged?

A mechanic is checking the situation tomorrow as I am away for the next 10 days. Naturally I will pass on your ideas...
 
Check the throttle cable travel - confirm carb heat travel - check the airbox flapper for looseness and get fixed - overhaul the carburetor at ak aircraft engines. I just did all that during an annual so no sympathy :lol:

Gary
 
Turns out that I had a couple of problems. One was the throttle cable housing position was incorrect. The upper end of the housing has a "notch" where a clamp is supposed to live. The clamp holds the housing to the fuselage under the panel. The notch is there to keep the housing from moving fore and aft during throttle adjustments. In my plane, when the clamp was in the notch, the housing was too far aft, thus not allowing the throttle handle to travel far enough to bring the carburetor to slow idle. So moving the housing forward approximately 1/2 inch and re clamping solved the travel issue. The housing is also trapped with safety wire on either side to prevent movement.

The second problem was the carburetor shaft had a fairly large air leak. Replaced the carb and noticed my cht dropped a solid 15° in all power configurations. 20° at cruise. All you folks chasing lower cht should get your carburetor checked for leaks.
 
Thanks Skukum12 for the follow up!

So many times these trouble shooting threads just die and we never get a chance to learn!

Is it possible that someone installed the wrong throttle cable at some time, or the tubing where the welded tab is was replaced and the tab incorrectly positioned? By design you should not be having this issue.
 
Last edited:
I believe the throttle cable is incorrect for the plane but I don't have enough proof yet to say it's a fact. In the wag aero catalog they have a cable that "fits all piper tandems except the J5." Their cable is 54", my cable is 54". The 12 is really an later model J5. Sooo, time for me to dig a little deeper. While my cable is secure for now, it is relying on clamp pressure and safety wire, not the "notch" for which it was designed.

I want to give a big thank you to Charlie Center for going above and beyond rummaging through his inventory to find a cable to compare mine with.

I do concur, these threads often trail off without a conclusion. I wanted to provide resolution to this not so common but very annoying issue because we can all learn. So many of you spent time contributing ideas that it would have been disrespectful to cut and run.
 
I thought I was saving some money by buying a Wag throttle cable for my PA-18. It did not fit correctly. The Univair cable at 4x(?) the price does fit as it should. I don't remember exactly which dimension on the Wag was off, but I remember it having something to do with the clamp under the panel.
 
Univair and spruce are both over $500. My jaw hit the floor. On the other side of the coin, if it fits....
 
I saw an issue with a throttle cable from univair a few years ago where one end of the cable was not swaged correctly. Caught it in time as you could pull it apart by hand. Have had mixed results on parts from all the vendors. Have to watch them closely.
 
How does one check our carbs for leaks? remove it and send it to an overhaul shop? Is that the same as sending it in for overhaul?

skukum12 said:
Turns out that I had a couple of problems. One was the throttle cable housing position was incorrect. The upper end of the housing has a "notch" where a clamp is supposed to live. The clamp holds the housing to the fuselage under the panel. The notch is there to keep the housing from moving fore and aft during throttle adjustments. In my plane, when the clamp was in the notch, the housing was too far aft, thus not allowing the throttle handle to travel far enough to bring the carburetor to slow idle. So moving the housing forward approximately 1/2 inch and re clamping solved the travel issue. The housing is also trapped with safety wire on either side to prevent movement.

The second problem was the carburetor shaft had a fairly large air leak. Replaced the carb and noticed my cht dropped a solid 15° in all power configurations. 20° at cruise. All you folks chasing lower cht should get your carburetor checked for leaks.
Click to expand...
 
There's a couple of field options I've witnessed. One involves looking for external fuel stains on the intake system airbox to cylinder. That assumes fuel is used that contains a colored dye like 100LL. It can sometimes be simulated by pumping the accelerator pump to wet the area around the carb then look for wet external leaks of fuel. Another is to introduce a flammable fluid or gas to the potentially leaky components while the engine is idling. Dangerous business of course. Harley Davidson recommends using a propane bottle attached to a flexible hose with that ending in a copper tube. Expelled propane from the tubing if ingested through leaks will be burned in the cylinders and slightly increase engine rpms. Brake cleaner sprayed on leaks will do the same. But the best is proper removal and inspection of the parts/gaskets/couplings for leaks.

A carb overhaul will accomplish that. I just had my MA3-SPA overhauled for $900. New bushings, gaskets, float valve, and accelerator pump parts plus others.

Gary
 
soyAnarchisto said:
How does one check our carbs for leaks? remove it and send it to an overhaul shop? Is that the same as sending it in for overhaul?
Click to expand...
I wasn't able to witness the procedure. The mechanic did tell me he pressurized the system and used soap and water outside the affected areas. He said there was a massive bubble around the carburetor shaft.
 
Overhaul kits usually come with new shaft bushings. Good test is wiggle the throttle plate shaft...if it wobbles much fix it now. That's what that PFD check coming in few is for every year it seems.

Gary
 
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