Carb sync for beginners

by Roger Lee in 912 / 914 Technical Questions 10 years ago

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Re: Carb sync for beginners

by Roger Lee » 9 years ago

Roger Lee

125 5.12k 4.53k

So long as the mixture screw on the bottom of the carb is 1.5 turns out, the carbs are synced and as you say the EGT's are within 0-60F of each other you should be fine. The EGT temp spread will change with throttle position. The plugs should have a gap between .023 and .027 with thermal paste applied to the threads. (no anti-seize) The dark plugs on one side is probably after you have been idling. This is normal on many engines and it can be on different plugs. If the engine has been run up around 4000+ rpm for about 3-4 minutes then the dry black soot from idling will go away and if you shut the engine off immediately after that higher rpm run all plugs will most likely be clean. Plug color can vary slightly from a very light tan to a dark tan for normal burning. Some of this has to do with where the outside air comes from, plug gap, cold dry air verse hot humid air, cruise rpm, ect...
Your needle clips should be in the 3rd notch for a 912ULS. Not sure which engine you have. Have you moved the needle clips no matter which engine model you have.

A new NGK DCPR8E plug that is fairly new will look very clean and almost white. I think maybe you might want to leave things alone for a while and just observe.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Carb sync for beginners

by Przemysław Wiśniewski » 9 years ago

Przemysław Wiśniewski

65 247 3

Hi Roger, thanks for answer.
My engine is 912UL (80hp) and spark plugs dcpr7e, new re-gapped before installation.
I'll try tomorrow to heat 4 min @ 4000 and turn of w/o idling.
I didn't touch clips after overhaul of carbs and I think it's on default position.

Many people told me, that my previous problem with swapping pressures on random throttle was problem with throttle controll but today I have well working throttle system with no effect. Effect gives me leaning about 1/4turn of only one carb. Now pneumatic synchronization is ideal.

I'll make photo of each plugs and send. I'm not sure if I'm not running on to lean mixture on white plugs side (this side is on default tuned carb - 1,5turn mixture by service).

Can you explain "EGT's are within 0-60F of each other you should be fine." ?
I have about 750*C on cruise but spread on one side is about 30-40*C and on second side is about 30-80*C.
This side with bigger spread is hotter for about 30*C than other side.

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Re: Carb sync for beginners

by Roger Lee » 9 years ago

Roger Lee

125 5.12k 4.53k

Your 750c (1382F) is not hot by any means. It is a tad on the cool side. Any EGT's within 15F of each other are considered equal. A 30c (86F) is about as far apart as I would prefer to see an EGT temp, but this can happen depending on several factors listed above in the previous post. Once in a great while someone calls with a 110F spread. This could be engine factors or even a EGT probe starting to go bad. This would need a little trouble shooting to pin the exact cause down. Many times (especially with a Vernier cable throttle system) the carbs don't go exactly back to the exact spot every time you pull the throttle back and push it in. This can happen because of bends in the cables are a little too sharp or too much friction within the cable sheathing and even a tad of slop in the duel cable setup. When this happens the EGT temps can be different from time to time and even swap temps as far as one being a little hotter than the other.

This may be what you are experiencing. I certainly wouldn't worry about an EGT of 750c.
If this is a brand new plane and engine fly it a few times and see if things settle in a bit.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Carb sync for beginners

by Przemysław Wiśniewski » 9 years ago

Przemysław Wiśniewski

65 247 3

"Any EGT's within 15F of each other are considered equal. A 30c (86F) is about as far apart as I would prefer to see an EGT temp"

Yee but I had about 80*C (176F) spread on one side. It's quite big.

I make today test on 4000 and check colors.

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Re: Carb sync for beginners

by Roger Lee » 9 years ago

Roger Lee

125 5.12k 4.53k

If your EGT temp spread is 176F between the left and right side then you need to find out why. You need to determine if this is a real temp spread or a false gauge indication. This may be something in the line of a throttle on one side no opening equally to the other side. You can ground check this, but make sure the plane is wheel chocked and tied down since you need to run it up in the high rpms. An EGT probe may have a bad connection, if you have header wrap cloth on the exhaust pipes make sure that isn't touching or covering the EGT probe, check the connections at the gauge, make sure you have good grounding (earthing) connections, you may have a bad EGT probe. One easy check is to swap EGT probes and see if the problem follows the probe to the other side. This can be done at the connection for the probes and you don't physically need to move the probes. The EGT probes need to be the exact equal distance from the cyl. head which should be 100mm.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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