Rotax 914 exhaust temperatures

Mike G

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What differences do you see in 914 exhaust temps?

After a long break I finally got my 914 back together. It started OK but won't run below 2000 rpm without shaking itself so badly it hurts. At 2500 it smooths out but is still vibrating and at 3000 it is a bit smoother but still seems to be missing.

I get the following :
rpm 3000 3200
cyl 1 600 590
cyl 3 616 660
cyl 2 628 647
cyl 4 730 738
degrees C

I compression tested the cylinders and they are all good although cyl 1 is a fraction lower than the others. What compression pressure would you expect from a 914? I got between 10.5 and 11 bars (152 to 160 psi)

I balanced the carbs at 2000 yesterday when it ran much smoother and they are balanced.

I suspect the idle mixture screw but it's unaccessible with the old style drip bowl under the carbs. When I rebuilt the carbs I set them at the recommended one and a half turns open.

I'm trying to establish how important these different temperatures are and can they point me towards the fault.

Mike G
Any comments or ideas would be welcome.
 
What differences do you see in 914 exhaust temps?

After a long break I finally got my 914 back together. It started OK but won't run below 2000 rpm without shaking itself so badly it hurts. At 2500 it smooths out but is still vibrating and at 3000 it is a bit smoother but still seems to be missing.

I get the following :
rpm 3000 3200
cyl 1 600 590
cyl 3 616 660
cyl 2 628 647
cyl 4 730 738
degrees C

I compression tested the cylinders and they are all good although cyl 1 is a fraction lower than the others. What compression pressure would you expect from a 914? I got between 10.5 and 11 bars (152 to 160 psi)

I balanced the carbs at 2000 yesterday when it ran much smoother and they are balanced.

I suspect the idle mixture screw but it's unaccessible with the old style drip bowl under the carbs. When I rebuilt the carbs I set them at the recommended one and a half turns open.

I'm trying to establish how important these different temperatures are and can they point me towards the fault.

Mike G
Any comments or ideas would be welcome.


Maximum Exhaust Gas Temperature
950ºC / 1742ºF
Normal Exhaust Gas Temperature
900ºC / 1652ºF

raton
 
If your EGT at WOT exceeds 1550 F (about 840 C) you need to be alarmed!
Stoichiometric is about 1475 F (800 C) . 200 F to the rich side will give you better climb power. 100 F to the lean side at cruise throttle will give you better fuel ecconomy.
Note that EGT readings can be affected by probe positions, and to a lesser degree by fuel quality and blends.
 
Thanks
I'm not alarmed by the temperatures, I'm curious to know if the differences I see between cylinders at a given power setting (all be it low) are usual or should they be telling me something about the cause of the engine vibration?

Mike
 
Raton
Thanks, I knew the website but hadn't seen that paticular section.

Mike G
 
Check the balance tube and connections. Check the carb sockets for condition and proper seating. Did you replace the o-ring or omit the o-ring between the socket and the intake.

If you idle mixture screw is hex head as well as screwdriver slot, you might be able to adjust the screw using a wrench.

Just some thoughts.
 
Jeff
What do you mean by carb sockets, the rubber mounting between the carb and the intake manifold? If so I didn't take it off, I just inspected it in situ.

I'm going to try to make up a tool to get to the hex head on the idle mixture screw, although I doubt if it's that since the engine is still rough at higher revs.

Raton what's FI?

Mike G
 
Yes, the carb sockets are the rubber mounting between the carbs and the intake manifold. They sometimes break and a leak to the outside pressure develops. To test for this, you need to take them off and inspect them. The rubber is quite stiff, and tears or leaks difficult to detect.
 
I've stripped the carbs down again to make sure I didn't do something stupid or that they were full of sh1t. They're still very clean so I'm more and more convinced my problem is not carburation. I don't think its the rubber mountings either (but I'll take them off and inspect them thoroughly next time I'm at the field) because I get such different exhaust temperatures from the same carb or rubber mounting. I'm now thinking that the problem may come from the hydraulic tappets since the oil system was completely emptied when I fixed the oil pump leak and perhaps I didn't manage to bleed all the air out of the cam followers. I'll carry out the Rotax test next time I'm at the field.
Does anyone have experience with changing the hydraulic tappets/cam followers, I notice that the engines made just after mine have a different tappet that is "pre-
charged".
Mike G
 
I've stripped the carbs down again to make sure I didn't do something stupid or that they were full of sh1t. They're still very clean so I'm more and more convinced my problem is not carburation. I don't think its the rubber mountings either (but I'll take them off and inspect them thoroughly next time I'm at the field) because I get such different exhaust temperatures from the same carb or rubber mounting. I'm now thinking that the problem may come from the hydraulic tappets since the oil system was completely emptied when I fixed the oil pump leak and perhaps I didn't manage to bleed all the air out of the cam followers. I'll carry out the Rotax test next time I'm at the field.
Does anyone have experience with changing the hydraulic tappets/cam followers, I notice that the engines made just after mine have a different tappet that is "pre-
charged".
Mike G

http://www.sportpilot.org/magazine/...ks for the Ideal 912 Installation, Part 2.pdf

raton
 
Raton
Thanks for the link they confirm most of the things I've learned recently which is reassuring.
I've carried out the hydraulic tappet check but had to do it cold instead of hot per the instruction (I'm giving the carbs a second strip down to ensure I didn't do something stupid the first time). The results were promising, the hydraulic tappets (in fact they're hydraulic cam followers not tappets) showed no signs of being soft.
In fact with the engine cold there is absolutely no rocker arm movement at all. I applied the specified 15 lb force for 3 seconds and it made no difference at all. In fact it took a considerable force for a longish time (about 10 secs) to get any clearance at all. Does anyone know if this is normal?

When I finish the carbs and get back from my travels next week I'll warm up the engine and do the check again.

I seem to have a slight oil leak from the turbo into the intake system but the plugs aren't oiled up. Any thoughts or comments??

Mike G
 
0F to 80F can be normal depending on your setup, carb health and throttle setting. A bad carb sync will affect temps or a needle clip in the wrong position. If I see 100+F I start to wonder whats going on. It could be something simple, like a wire, bad connection, bad EGT probe. It can even be header wrap cloth that covers an EGT probe. For some reason header wrap cloth has something in it that causes swings of 60F-120F and quite rapidly. I talked to Dynon and Rotax and neither had an explanation. I have duplicated this several times so it can happen. If you wrap your exhaust, wrap up to, but not over the EGT probe then start back up on the other side. It could be a throttle cable that has a little friction and now doesn't perform like it used to. It hangs up ever so slightly
 
First, in all honesty, you’ll probably get much more useful info about Rotax-specific issues by asking them on rotax-owner.com than here. No offense meant to anyone.

Second, a quick observation is that only #4 looks hot, as if, for example, it alone is running lean. Maybe an intake leak there? And it’s the difference between it and the others that’s more significant than the absolute temperature, IMHO
 
Guys this is a pretty old thread. I no longer have this gyro.
Mike G
 
Ed
My memory isn't that good. I seem to remember that the problem was due to the ignition trigger coils being too far away from the cam lobes on the flywheel. I do remember modifying the trigger coil mounting to get a smaller gap but I'm not 100% sure that this was related to the problem discussed here and not another of the many problems I had with that 914.
Mike G
 
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