Losing power at takeoff

[pauldblackmore]

Hi All,

We have just completed first flight of Sonex 898 but are having a lot of trouble getting the aerocarb working right. I am getting 3050 RPM static with the required EGT spread but when the aeroplane starts flying the engine starts running rough and revs drop back. In climbing flight the Max RPM was 2880 with 250 fpm climb rate. Its a tri gear and I'm using the number 2 needle and the throttle is restricted 3/16" from wide open. I dont want to pre-suppose a cause, but it sounded like it is leaning out during the take off run - but why?

Can anyone offer suggestions for the possible cause and fix?

Thanks in advance

Paul
Sonex 898 (0.8 hrs)

[gammaxy]

Hi Paul,

Make sure you are talking to the factory and following the tuning procedure in the Aeroinjector manual. Everything I'm about to say is based on my limited experience of tuning one Aeroinjector.

I'm surprised you need to restrict the throttle that much. I haven't restricted mine at all, but have no idea how many other builders need to restrict their throttles. Do you think there's a chance that the throttle restriction is masking a mixture problem at WOT that is somehow being unmasked when you take off? Maybe spending some more time adjusting the mixture for WOT will make the throttle restriction unnecessary?

I suspect you are lean, so I'd start by twisting the needle 1/4-1/2 turn counter-clockwise without the throttle restriction. I wouldn't try flying it again until I was satisfied it runs well at WOT. Do you get any RPM rise when you pull the mixture out at WOT? I get a perceptible rise of probably 30 or so RPM. I think my EGT change is more than the 100 degrees called for, but it depends on the cylinder.

Have you done a fuel flow test? When I disconnect the fuel line from the aeroinjector I get about 20gph. I used to get 12gph when I had a fuel flow sensor in the system. Both flow rates seemed to work fine at wide open throttle.

I get around 2800 or so rpm from my Prince P-Tip 54x46 propeller at 75mph on climbout, but I'm climing at ~700fpm.

[marsolgp]

Paul,

Reading your post brought back vivid memories of my first flight…. all your numbers were the same as mine. And I did mine on a 2600' grass strip. I was at the 2000' mark when I finally lifted off and that 250 fpm climb barely got me over the trees at the end of the runway. (in hindsight I should have aborted earlier in the T/O roll, but….). I had spent hours prior to this trying to get the AeroInjector 'right' and finally figured I got to get this thing flying.

It wasn't til I had a little altitude that I retarded the throttle a little (CHT's were climbing) and noticed my RPM increase. I was overcarbed and later adjusted the slide so I could get max RPM. And like you ended up with about 3/16" of the slide still showing when at full throttle.

My only advice here is keep flying and adjust as necessary after each flight. Don't get caught in the trap of keeping acft on the ground til you have carb 'perfect'. Ground running is not good for an engine that's not broken in. One good thing I can say about the AeroInjector is, my engine NEVER quit running…. it may have run badly at times, but fiddling with the mixture usually made it better. Then when on the ground, make an adjustment and go fly again.

I'm not bashing the AeroInjector here. It is what it is, a simple, elegant, cheap way to get fuel into and engine. Some folks get a 'good' one and claim no problems, some get a 'not so good' one and play with it til it's 'O.K.' and then others get one that just won't run right no matter what.

I played with mine off and on for about a hundred hours. Finally got it to idle, kinda sometimes, got T/O power when needed and cruise was alright. I always had to play with the mixture but it got me from point A to point B….. Then summer comes and I started getting the 'burps'. Sometimes on T/O, sometimes in cruise, sometimes on approach. I did everything that was suggested and still it would burp. It was very disconcerting to say the least. And I guess that's the biggest problem with anything, once you don't trust it anymore, it's time to change.

I had ordered the turbo kit and looking forward thought that I definitely didn't want to go thru all the AeroInjector issues again, so I bought a Rotec TBI. I made a throttle stop to limit slide travel (being 34 mm it too was a little too much carb for the AeroVee) I adjusted the idle mixture and haven't an issue since! Note that I had this on the Aerovee when it was normally aspirated and when the turbo was installed I stuck it on that, only needed to adjust idle mixture screw. It's run perfect since.

Now, I hope all my carb concerns have been addressed. I added a fuel pump for vapor lock (haven't had to use it year) and I added a intake plenum with carb heat (haven't needed it yet, either).

So, to sum up, keep playing with it, you may get it right as many people have…. but if not, there are other options out there.

[pauldblackmore]

Hi, G

Thanks for your encouragement - I'll persevere a bit more, but I must confess to more than one visit the Rotec web site. This might be a strange question, but may I ask the location of your fuel vent. I have the vent as shown on the plans which runs a 1/4" tube down to the bottom of the firewall which is then cut off at an angle facing the airflow. I have a suspicion that the airflow in this region may be de-pressurising rather than pressurising the tank which might explain why it goes lean as the airspeed increases.

Anyone had an issue with fuel tank vent location/design and aerocarb tuning?

Thanks

Paul

[marsolgp]

Paul,

My fuel vent is just a 1/4" I.D. Tube straight up out of the tank and bent facing into the airstream just 6" above the cowling.

[billmaxmcw]

The fuel vent would be an unlikely source of problems. I think you are too lean at WOT; my needle was the most lean there, and got richer pulling it back. That lead me to initially think (wrongly) that I needed to restrict the throttle opening.
Bill

[gammaxy]

may I ask the location of your fuel vent. I have the vent as shown on the plans which runs a 1/4" tube down to the bottom of the firewall which is then cut off at an angle facing the airflow.

I have the same plans fuel vent you have and it seems to work fine.

[SonexN76ET]

I think that your mixture might be too rich. If you are getting the 100 degree range of EGTs from full rich to lean peak on a new engine while ground running you have to be fairly rich. A too lean engine will run strong, but cause high EGTs. Too rich and it will bog down. (I tested my AeroInjector at both extremes while trying to tune it).

Might I suggest you try leaning it to see if that works? Also, I don't think it is wise to restrict your full throttle setting. I suspect that only masks another problem.

Also, check to make sure you do not have an induction leak or dirty air filter. A little bit of engine oil on an air filter will greatly restrict air flow. Induction leaks are difficult to pin point. I resealed my intake elbows and tightened down my clamps and went with the new silicone hoses from Sonex.

What kind of prop do you have?

I have a Prince Prop and I see 2950 - 3000 static. On takeoff roll it drops to 2880, then on climb out it bounces between 2800 and 2950. I do not let the RPM on climb out drop below 2800 so I don't fall behind the power curve. I am climbing at 80 to 95 MPH and at 700 fpm. At wide open throttle and in level flight my RPM is 3250 but I am seeing 138 mph (tricycle gear).

I will also say that I spent a lot of time tuning my AeroInjector, but now it works perfectly. I don't have to adjust the cockpit mixture except as you would normally with changes in altitude. I am very pleased with the AeroInjector.

Good luck and keep us posted on your findings! Most of all, be safe. Don't take any chances. I would rather run my engine on the ground even if that caused unnecessary wear while I adjust things and get everything sorted out than take off with an unsafe situation. Stay in touch with Sonex Tech support. They can give you the best advice.