Vexing C90 problem?
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Vexing C90 problem?
Hi,
I wonder if anybody has any ideas on a problem I'm having with a C90?
I had to make a quick landing the other day as she started to run very rough at the top of the climb. I put it dow to carb ice as I had no time that day to investigate further. Having warned the group, another member went to fly today and despite a perfectly ok run up and power check, she started to play up just as he was getting airborne, so he aborted.
Now, I'm sure it's not a sticky valve as I had one recently which I sorted and this is nothing like that. I guess it's possible, but I'm pretty sure it's not. The timing is OK and anyway the engine doesn't miss a beat throughout the rev range under no load. This then brings us to carberation, which is always a bit of a black science unless you have test equipment. She was running a bit rich a while back, but after adjustment (a little each flight until all seemed right and the sooty exhaust pipes cleaned up) she has been running fine for the last 20 hours or so.
So to sum up ...
Runs throughout the rev range perfectly ok on the ground during power checks, timing seems ok, mixture "seems" to be about right, but .... at full power once flying she starts to miss and run rough!
I did wonder about fuel supply? could the fuel pump be getting weak? or the carb float (if it has one? "marvel carb") be fuel logged? This might show itself as demand for fuel increases under high power conditions?
Any ideas? I have a reasonable mechanical experience, but I'm certainly no expert and I'd appreciate the thoughts of those who are.
Thanks ... SS
I wonder if anybody has any ideas on a problem I'm having with a C90?
I had to make a quick landing the other day as she started to run very rough at the top of the climb. I put it dow to carb ice as I had no time that day to investigate further. Having warned the group, another member went to fly today and despite a perfectly ok run up and power check, she started to play up just as he was getting airborne, so he aborted.
Now, I'm sure it's not a sticky valve as I had one recently which I sorted and this is nothing like that. I guess it's possible, but I'm pretty sure it's not. The timing is OK and anyway the engine doesn't miss a beat throughout the rev range under no load. This then brings us to carberation, which is always a bit of a black science unless you have test equipment. She was running a bit rich a while back, but after adjustment (a little each flight until all seemed right and the sooty exhaust pipes cleaned up) she has been running fine for the last 20 hours or so.
So to sum up ...
Runs throughout the rev range perfectly ok on the ground during power checks, timing seems ok, mixture "seems" to be about right, but .... at full power once flying she starts to miss and run rough!
I did wonder about fuel supply? could the fuel pump be getting weak? or the carb float (if it has one? "marvel carb") be fuel logged? This might show itself as demand for fuel increases under high power conditions?
Any ideas? I have a reasonable mechanical experience, but I'm certainly no expert and I'd appreciate the thoughts of those who are.
Thanks ... SS
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I've had further thoughts on the problem, what do you think? The syptoms I think are conducive to fuel starvation at highest revs (no popping, but sounds almost like pulling the mixture back). Therefore perhaps the first suspect (which I hadn't thought about until this morning) has to be a blocked filter or gauze in either the gasolator (some have them), fuel pump or tank finger strainer. Would this make sense? Seems like the first and easiest place to look to me.
SS
SS
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Leaking inlet manifold allowing unmetered air into the induction side and causing an over weakened mixture. this would become more apparent at higher power settings. Checking the electrode colour of each spark plug might give a clue as to which cylinder is affected. If they are all roughly the same colour, it could be a leak downstream of the carburettor.
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Thanks,
I know it's not the induction tubes as I've checked all those. Incidently, it was a lose induction tube that caused the sticking valve on the cylinder that I did have to free. I suppose the induction spider? casting could be leaking ... something else to check when I go up.
SS
I know it's not the induction tubes as I've checked all those. Incidently, it was a lose induction tube that caused the sticking valve on the cylinder that I did have to free. I suppose the induction spider? casting could be leaking ... something else to check when I go up.
SS
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As it has a fixed pitch prop, the amount of power produced static and airborne are almost the same, the main difference being dynamic air pressure on the air filter, a marginally higher RPM and improved engine cooling.
If I recall correctly, when I worked on light aircraft, we had a Piper Warrior with similar problems, yes I know different engine and all, but a similar problem. We tried all sorts to fix this beast, went down the route of sticking valves, carburettor float and main jet etc, it turned out to be an air filter problem, although the exact filter problem escapes me, it was eventually resolved by replacing the filter scat hose and element.
In summary, what I am trying to say is that the problem is related to either air pressure or flow.
It's been a while since I worked on light aircraft so I wouldn't like to guess any further without direct involvement. If i do remember anything similar of the problem you are having, then I'll let you know.
This is definitely, As I See It
If I recall correctly, when I worked on light aircraft, we had a Piper Warrior with similar problems, yes I know different engine and all, but a similar problem. We tried all sorts to fix this beast, went down the route of sticking valves, carburettor float and main jet etc, it turned out to be an air filter problem, although the exact filter problem escapes me, it was eventually resolved by replacing the filter scat hose and element.
In summary, what I am trying to say is that the problem is related to either air pressure or flow.
It's been a while since I worked on light aircraft so I wouldn't like to guess any further without direct involvement. If i do remember anything similar of the problem you are having, then I'll let you know.
This is definitely, As I See It
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This problem is getting more and more vexing!!! I think "As I see it" may have a point ... but I really am at a loss to know for sure.
Here's what I (and my group members) have found, and done, but to no avail.
Sticking valve? .... had one before and the symptoms don't match.
Symptom. Runs OK to full power on static, but breaks down as T/O speed is approached. Leaning the mixture by the cockpit control helps, but leaning the mix via the mixture screw does not.
Induction tubes leaking? ... Nope, all OK.
Timing? ... OK
Mag problems? ... what both, together??? don't think so?
A fuel line was badly perished and actually leaking fuel! .... problem solved! ....... errr no
So, off with the carb to check the float (seems ok, but hard to tell for certain I guess, without replacing) Jets, airways ect all blown through and a new gasket .... but the problem is still there.
I/we have thought of all sorts? Weak valve spring and a bouncing valve? .... only ever had that in a two stroke? obstructed air feed? could that do it?
The Marvel carb has a venturi and bleed holes to create a vacuum and suck fuel. The mixture screw simply blocks one of these holes to a greater or lesser degree to alter the vacuum. Adjusting this makes no great difference. However, I think the cockpit control actually restricts the amount of fuel allowed and therefore leans in a different way. Whether this is significant to the problem or solving it, beats me?
Any more idea's? Please help .... I'll be bald soon at this rate
SS
Here's what I (and my group members) have found, and done, but to no avail.
Sticking valve? .... had one before and the symptoms don't match.
Symptom. Runs OK to full power on static, but breaks down as T/O speed is approached. Leaning the mixture by the cockpit control helps, but leaning the mix via the mixture screw does not.
Induction tubes leaking? ... Nope, all OK.
Timing? ... OK
Mag problems? ... what both, together??? don't think so?
A fuel line was badly perished and actually leaking fuel! .... problem solved! ....... errr no
So, off with the carb to check the float (seems ok, but hard to tell for certain I guess, without replacing) Jets, airways ect all blown through and a new gasket .... but the problem is still there.
I/we have thought of all sorts? Weak valve spring and a bouncing valve? .... only ever had that in a two stroke? obstructed air feed? could that do it?
The Marvel carb has a venturi and bleed holes to create a vacuum and suck fuel. The mixture screw simply blocks one of these holes to a greater or lesser degree to alter the vacuum. Adjusting this makes no great difference. However, I think the cockpit control actually restricts the amount of fuel allowed and therefore leans in a different way. Whether this is significant to the problem or solving it, beats me?
Any more idea's? Please help .... I'll be bald soon at this rate
SS
