Rotax 447 surging at part throttle

[newairly]

I have a Rotax 447 fitted to a Tyro Mk2

 

The Rotax is single ignition, single carburettor. It has a single EGT sensor and single CHT sensor on the cylinder nearest to the flywheel.

 

My problem is that at part throttle the RPM will not stay steady.

 

I see this both in the air and on the ground with the A/C restrained with a tether.

 

The RPM will vary from, for example, 5200 to 5700 with steady throttle. As the RPM goes up the EGT also rises to about 1150F. When the RPM drops so does the EGT . This is on the ground so I have plenty of time to watch gauges. This suggests to me that it must be a mixture effect, but what?

 

The full throttle RPM, on ground, is about 6350 which sounds about right. Fuel is 50:1 premium unleaded.

 

Where can I find information which might help resolve this?

 

Phil

 

 

[Guest ozzie]

Definatley start with the fuel system sounds like the float levels could be low or the pump cannot keep up with the demand. Blocked jets or passage ways.

 

be where i would start looking.

 

 

[facthunter]

I'd go with Ozzie. Check that there is enough fuel flowing to the carburetter bowl. The rate of fuel flow should be well above the max you need. IF the engine used 12 litres at full throttle, you would want to flow more than 18. Nev

 

 

[newairly]

I am with Nev and Ozzie. It sounds like a lean out at half throttle which is a little unusual since you say it runs fine at full throttle. Full revs stable at full throttle indicates the fuel flow is fine. Unless the fuel pump is not working properly at half throttle I would be looking at a carburation issue.The rise in egt as the revs rise indicates a lean out; as it leans the revs will always rise if the mixture is correct in the first instance, and as it leans and the revs rise I would expect the egt to rise. It sounds like it then gets enough fuel again, the mixture returns to normal and the revs and egt drop accordingly.

 

I would be looking at the pump and carburation. Is the pump getting a good pulse from the crankcase at half revs? What condition are the hoses and clamps that connect the crankcase pulse signal to the pump. Is it the square or round pump you are using?

 

David

The pump is a square type. I also have an electric boost pump so I can try that to see if it changes anything. However because full power is fine, even for several minutes, I feel that it is something other than fuel availability.

 

I have seen some references to surging when the mid range fuel circuit in the Bing is nor working correctly. Also effects from the exhaust tuning. However the exhaust is Rotax standard. As you comment the change in EGT is what is expected with a leaner mixture, as is a rise in RPM. It used to be OK so what has changed!. Very annoying.

 

The next thing I will check is input manifold leaks.

 

Phil

 

 

[pudestcon]

What happens if you apply the choke when the engine is surging?

 

Pud

 

 

[Yenn]

Rich mixture will cause hunting, but this sounds too much for that. Try the choke to see if it gets worse.

 

 

[facthunter]

The choke set-up is such that it is only effective when the throttle is near closed. Check that the needle in the throttle slide is properly retained by the clip. It might be wise to overhaul the mikuni pump if it hasn't been done for e few years. Put a kit in it and they CAN be assembled incorrectly, so be careful... Nev

 

 

[Guest Maj Millard]

Does sound like a fuel problem..however....I purchased a brand new 447 FC back in the late 80's for my homebuilt Bi-plane. Same as yours, single points ign, single carb, fan cooled. I had a faultless run out of it for 175 hrs, other than the CHT always wanted to creep up close to 375-400 F at full power. It always came back when throttle was reduced after takeoff. I'd have a look at what grade fuel Rotax recommends for that engine...I don't believe it is Premium UL...more like 92-95 octain only. 50 to 1 oil mix is correct.

 

After an early morning take-off and about 10 mins into the flight mine spat a top (dykes) ring which put me into a field. I'd watched the temps come back that morning, and I'd always used quality oil. (Penzoil 2-stroke ashless in those days)

 

The EGT is your most valuable guage on that engine, and the one to watch, as you will most certainly get a rise if things are not right with fuel/fuel supply. EGT tells you what the temp of the fuel burn is. Rich mixture should give somewhere between 900-1100 F. Ideal burn is around 1150-1250 F, and anything above that is too hot. A rich mixture will give the low temps, and a lean mixture the hot temps. EGT responds on the guage much quicker than CHT, so thats the one I'd be watching if it is a fuel supply problem..

 

If you have an internal problem with rings binding against the bore etc., then the CHT will rise accordingly...

 

Also suspect a mismatch of oil and fuel, it's rare, but some 2-stroke oils didn't want to mix well with some fiuel types causing lack of adequet lubrication internally and improper combustion..............................................................................................Maj...

 

 

 

 

 

[newairly]

Maj. That is in interesting bird! Is it based on any WW1 plane? Is it still around?

 

The fuel I used for the recent ground test is premium unleaded (95 octane) with Pennzoil at 50:1 It was mixed just before use. Pennzoil still seems to be the best 2 stroke oil around for air cooled engines. The tests that I found 3 years ago rated it the lowest ash and best performing on a comparison test using a Rotax 503 (I think). Rotax say that 91 octane is OK but certainly no ethanol! Not too sure about the 91 octane. Incipient detonation can cause a sharp rise in temps so higher is safer, especially as octane number drops on storage with mixed fuel/oil. I may even go to 98 octane unleaded.

 

The surging is quite slow. A period of 10-15 seconds. EGT at full throttle is 1150-1200. at part throttle, 5500RPM EGT is 1050-1150. My CHT also approaches 400F at full power.

 

I am trying to get the Tyro going again after 2 years of not flying. One of the reasons I stopped flying it was because I was not happy about the engine, especially the part throttle problem.

 

Phil

 

 

[robinsm]

Did this occur just after you had the carbie apart? Is the needle mounted above or below the slide. (I assembled mine wrong in the early days and had the same sort of problem)

 

 

[Guest Maj Millard]

newairly, Yes I still have the bipe in my shed. Did around 275 enjoyable hours in her (mostly air-combat against lesser machines !) Because your engine was stored you may have rust on the cylinder bores, the engine is fighting to break through it. The 400 egt needs to be watched closley...not good. remove a manifold and check for rust on the bore..........................Maj...