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Strange engine problem

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Gino230

Well Known Member
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2 weeks ago, all of a sudden my RV8 (IO360) became harder to start. Even cold, used to start of the first blade, and hot maybe on the third but suddenly it's taking up to 2 cranking attempts cold and worse when hot.

When I failed to make a flight 2 weeks ago due to failure to hot start, we took out all of the plugs and cleaned them, (some lead fouling but not too bad) but #3 and 4 had lots of fuel in the plugs, it was actually dripping out. Plugs have about 35 hours on them.

So now my start procedure has "evolved" to basically more throttle opening (i.e. more air) and less prime to get it to start, hot or cold.

Once running, I noticed today that #2 is running higher EGT than all of the other cylinders. It's always been the hottest but now it's the hottest by 150+ degrees.

CHTs are all normal and within 10 degrees of each other. This plane runs cool, I've never seen over 350.

Fixed pitch (Catto 3 blade) so power settings are a bit of a dance. Old school EI engine instruments so I need to click back and forth to see each cylinder temps. NO fuel flow info.

Generally run ROP 75% PWR.

Today in cruise. Leg 1 6500'
22.2" MP
2670 RPM
67F (OAT)
Cyl #1 EGT 1276, CHT 302
Cyl #2 EGT 1420 CHT 324
Cyl #3 EGT 1264 CHT 309
Cyl #4 EGT 1339 CHT 310

Leg 2 5500"
22.75" MP
2620 RPM
69F OAT
Cyl #1 EGT 1297 CHT 300
Cyl #2 EGT 1450 CHT 319
Cyl #3 EGT 1288 CHT 310
Cyl #4 EGT 1356 CHT 312

Plane is flown weekly, so any time something suddenly changes I'm wary.

My current theory is #2 injector is dirty (has the most exposure to the airstream due to its position in the cowl opening) and forcing more fuel into 2,3,4 making it rich on start. And of course #2 running leaner.

Any thoughts?

I was attempting to find LOP operations today and #2 was getting to 1499 before peaking- (it's the first to peak) and RPM is dropping smoothly at this point. I know 1500 is about the max you want to see and with the current spread I don't think I can run LOP as #2 will be well LOP while #s 1,3,4 will be peaking. But that's a discussion for another day?
 
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My limited experience is a change to starting reliability, especially when cold, are often ignition related, not fuel.
Are you Mags or EI? .
 
Mags
No sniffle valve

Mag timing was my second thought, perhaps the mag timing is off? CI it was checked with the buzz box and that was 60 hours ago- April 2025.

Have not turned the prop through by hand in a while to see if I hear the "snap" to make sure the impulse coupling is firing.

Once engine is running, Run up, mag drop, etc. seem normal
 
Gino230 said:
Mags
No sniffle valve

Mag timing was my second thought, perhaps the mag timing is off? CI it was checked with the buzz box and that was 60 hours ago- April 2025.

Have not turned the prop through by hand in a while to see if I hear the "snap" to make sure the impulse coupling is firing.

Once engine is running, Run up, mag drop, etc. seem normal
Click to expand...
A bad or weak impulse? Even if you hear it click through, it may not do so when the starter is swinging it. I had this happen and had the same start issues you did. Ended up being an improperly manufactured impulse on the Mag that took years to start to fail.
When I spoke with a well known injector guru, he told me start issues like this are rarely fuel which led me to send my mags in.
He was right. However, my servo was due for overhaul and the spider and servo went in for a refresh. Nothing wrong with them.
It was the Mag impulse.
 
A 150* rise in egt above normal or typical is a classic sign of running on one plug. If the bad plug, wire, etc is on the ignition that you are starting on, that could explain the harder starting, but should also be rough before adding in the second ignition. Once on both ignition, you can’t really feel the difference, but the 150 egt delta confirms it.

Check your #2 plugs and wires for issues. Anin flight mag check should help. If the #2 is dead on one, it will be very obvious on that test.
 
lr172 said:
A 150* rise in egt above normal or typical is a classic sign of running on one plug. If the bad plug, wire, etc is on the ignition that you are starting on, that could explain the harder starting, but should also be rough before adding in the second ignition. Once on both ignition, you can’t really feel the difference, but the 150 egt delta confirms it.

Check your #2 plugs and wires for issues. Anin flight mag check should help. If the #2 is dead on one, it will be very obvious on that test.
Click to expand...
If you confirm that #2 egt is 150 high and all 8 plugs are firing, then next step is to clean out the #2 injector. Likely a partial blockage.
 
lr172 said:
A 150* rise in egt above normal or typical is a classic sign of running on one plug. If the bad plug, wire, etc is on the ignition that you are starting on, that could explain the harder starting, but should also be rough before adding in the second ignition. Once on both ignition, you can’t really feel the difference, but the 150 egt delta confirms it.

Check your #2 plugs and wires for issues. Anin flight mag check should help. If the #2 is dead on one, it will be very obvious on that test.
Click to expand...
Both harnesses look good on the outside, but the white insulator(s) are a little beat up and we planned to replace them this CI.

I will pull #2 plugs and see which one looks worse- with 1450 EGT they should not be fouled.

Impulse is on the left mag so theoretically this would be more likely to be the problem plug / wire?
 
lr172 said:
A 150* rise in egt above normal or typical is a classic sign of running on one plug. If the bad plug, wire, etc is on the ignition that you are starting on, that could explain the harder starting, but should also be rough before adding in the second ignition. Once on both ignition, you can’t really feel the difference, but the 150 egt delta confirms it.

Check your #2 plugs and wires for issues. Anin flight mag check should help. If the #2 is dead on one, it will be very obvious on that test.
Click to expand...
If this were the case, that would be a hella rough engine on the run up mag check? Mag check has been normal.

Appreciate the advice as always.
 
Gino230 said:
If this were the case, that would be a hella rough engine on the run up mag check? Mag check has been normal.

Appreciate the advice as always.
Click to expand...
Yes, that would next point you to a partially blocked #2 injector. As lomg as the indicated egt is at or below the official peak egt for that cylinder, the problem can be fuel. If it is above the peak, it is typically ignition.
 
How long since the mags have been internally inspected/adjusted?

Incorrect internal timing (E-gap adjustment) causes a reduced output voltage. This is most noticeable during start because of the slower rotation speed of the magneto.
Once the engine starts, the issue tends to have a much lesser effect on engine performance. If the mag is correctly timed to the engine, the mag check rpm drop and overall engine performance typically seems normal.
The internal timing drifts because of points wear.
This is the primary cause for needing to readjust the mag to engine timing.
Once that has been required a few times, the mag internal timing has probably drifted enough to cause a noticeable change in starting performance.

This is not necessarily the cause of your starting issue because there are other potential reasons as well, but this is a fairly common one.
 
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