Folks,
I have an old Lyc O-360-A1A of uncertain provenance that I've been flying for about 500 hours in my E-AB GlaStar with great performance and reliability until now. I hope you don't mine me posting here to get the benefit of this huge community -- I am helping a friend build his RV-8A!
Recently on engine start, Cyl #4 wouldn't fire -- very rough engine, no CHT or EGT rise on Cyl #4. The first couple of flights, it cleared up after a minute or two and run-up was normal. The situation progressed so now neither Cyl #3 or #4 will fire -- engine very rough & shaking like a dog. CHT's & EGT's rise on Cyl #1 & #2 but not on #3 & #4. I have to shut it down after a couple of minutes.
Here is what I have done to isolate the problem and the results:
1. Compression check: All Cyls at 75 PSI or higher.
2. Timing: Correct on both mags at 25 degrees BTDC.
3. Spark Plugs: Cyl #1 & #2 look good -- nice tan color. Cyl #3 & #4 have carbon build-up, but not too bad. (Swabbed exhaust stack with finger -- some carbon soot but nothing I think is abnormal. No noticeable black smoke from exhaust.) I cleaned/blasted and checked the gaps on the plugs and tried another ground run. Same results. I don't have a spark plug tester so I wasn't able to test the plugs themselves. Next ground run, I'll switch plugs from rear cylinders to front cylinders to see if problem follows them.
4. Ignition leads: Will test those tomorrow with borrowed lead tester. They are about 4 1/2 years old and 500 hours since new (Champion).
5. Mags: Older Bendix, again of unknown provenance. Impulse coupled mag on left and shower-of-sparks on right (shower-of-sparks not used).
6. Carb: Marvel Shebler (sp?): New/overhauled 500 hours (and about ten years) ago. Mixture and throttle linkages and throws are fine. On one ground run, I got a slight backfire. On another, as I pulled the mixture to idle cut-off, as Cyls #1 & #2 started to die, Cyls #3 & #4 seemed to fire up for a few engine revs before the whole engine died.
7. Intake manifold tubes. Inspected rubber hose connectors between intake tubes and oil sump. They look OK. Inspected attach points of intake tubes to cylinders for any evidence of air leaks (which would lean mixture). No evidence of air leaks found.
I lean aggressively while on the ground but don't lean in flight until at 5,000' MSL or above.
My first hypothesis (which was wrong): I have a Van's primer system with the solenoid valve on Cyls #3 & #4 only. Since it seemed like the engine (at least Cyls #3 & #4) was running rich, I thought the solenoid valve had failed and additional fuel was being sucked into Cyls #3 & #4. (I also confirmed that the solenoid was installed correctly re "in" and "out" ports.) I completely disconnected the primer system and blocked the primer inlets in the intake manifold. No change in behavior during subsequent ground runs.
So, other than spark plugs and ignition leads (which I'll test tomorrow), it looks like I'm running out of "cheap parts" as the root cause of this behavior. It seems to me that it is coming down to either the mags or the carb.
My question to you experienced hands out there: Are there any diagnostic procedures that I can run that will definitively point me to whether it is the mags or the carb? I don't want to just start replacing or overhauling them willy nilly to see if that fixes the problem ($$$). A neighbor A&P/IA suggested pulling all plugs and looking for sparks by grounding each plug in turn to the cylinder while motoring the engine with the starter. Could this be a definitive test?
Or it is something else entirely that might be causing this? The consistent, but mystifying, behavior I see is that neither Cyl #3 nor Cyl #4 appears to be firing at all based on lack of CHT and EGT rise. But that implies the failure of two mags, two sets of ignition harnesses and four spark plugs (or some combination of them), or a carb/intake manifold that is feeding a too-rich mixture to two cylinders and the correct mixture to the other two.
Thanks for any help or suggestions you have for me.
Best regards,
Bob Falstad
GlaStar N248BF
~500 Hours
I have an old Lyc O-360-A1A of uncertain provenance that I've been flying for about 500 hours in my E-AB GlaStar with great performance and reliability until now. I hope you don't mine me posting here to get the benefit of this huge community -- I am helping a friend build his RV-8A!
Recently on engine start, Cyl #4 wouldn't fire -- very rough engine, no CHT or EGT rise on Cyl #4. The first couple of flights, it cleared up after a minute or two and run-up was normal. The situation progressed so now neither Cyl #3 or #4 will fire -- engine very rough & shaking like a dog. CHT's & EGT's rise on Cyl #1 & #2 but not on #3 & #4. I have to shut it down after a couple of minutes.
Here is what I have done to isolate the problem and the results:
1. Compression check: All Cyls at 75 PSI or higher.
2. Timing: Correct on both mags at 25 degrees BTDC.
3. Spark Plugs: Cyl #1 & #2 look good -- nice tan color. Cyl #3 & #4 have carbon build-up, but not too bad. (Swabbed exhaust stack with finger -- some carbon soot but nothing I think is abnormal. No noticeable black smoke from exhaust.) I cleaned/blasted and checked the gaps on the plugs and tried another ground run. Same results. I don't have a spark plug tester so I wasn't able to test the plugs themselves. Next ground run, I'll switch plugs from rear cylinders to front cylinders to see if problem follows them.
4. Ignition leads: Will test those tomorrow with borrowed lead tester. They are about 4 1/2 years old and 500 hours since new (Champion).
5. Mags: Older Bendix, again of unknown provenance. Impulse coupled mag on left and shower-of-sparks on right (shower-of-sparks not used).
6. Carb: Marvel Shebler (sp?): New/overhauled 500 hours (and about ten years) ago. Mixture and throttle linkages and throws are fine. On one ground run, I got a slight backfire. On another, as I pulled the mixture to idle cut-off, as Cyls #1 & #2 started to die, Cyls #3 & #4 seemed to fire up for a few engine revs before the whole engine died.
7. Intake manifold tubes. Inspected rubber hose connectors between intake tubes and oil sump. They look OK. Inspected attach points of intake tubes to cylinders for any evidence of air leaks (which would lean mixture). No evidence of air leaks found.
I lean aggressively while on the ground but don't lean in flight until at 5,000' MSL or above.
My first hypothesis (which was wrong): I have a Van's primer system with the solenoid valve on Cyls #3 & #4 only. Since it seemed like the engine (at least Cyls #3 & #4) was running rich, I thought the solenoid valve had failed and additional fuel was being sucked into Cyls #3 & #4. (I also confirmed that the solenoid was installed correctly re "in" and "out" ports.) I completely disconnected the primer system and blocked the primer inlets in the intake manifold. No change in behavior during subsequent ground runs.
So, other than spark plugs and ignition leads (which I'll test tomorrow), it looks like I'm running out of "cheap parts" as the root cause of this behavior. It seems to me that it is coming down to either the mags or the carb.
My question to you experienced hands out there: Are there any diagnostic procedures that I can run that will definitively point me to whether it is the mags or the carb? I don't want to just start replacing or overhauling them willy nilly to see if that fixes the problem ($$$). A neighbor A&P/IA suggested pulling all plugs and looking for sparks by grounding each plug in turn to the cylinder while motoring the engine with the starter. Could this be a definitive test?
Or it is something else entirely that might be causing this? The consistent, but mystifying, behavior I see is that neither Cyl #3 nor Cyl #4 appears to be firing at all based on lack of CHT and EGT rise. But that implies the failure of two mags, two sets of ignition harnesses and four spark plugs (or some combination of them), or a carb/intake manifold that is feeding a too-rich mixture to two cylinders and the correct mixture to the other two.
Thanks for any help or suggestions you have for me.
Best regards,
Bob Falstad
GlaStar N248BF
~500 Hours