Fuel burn with larger diameter carb jets

[cleve_thompson]

I have a new Lyc O320 with 68 hours and I am still having high CHT's (as high as 450 on climb-out on #3) especially since we are now getting ambient temps in high 90's down here in South Georgia. I have worked on my baffles and seals, dam in front of #1, etc and still no joy! When I level off the CHT's drop into high 300's. I am climbing out at 130 MPH.

I think that I will have to make the engine run richer by installing larger diameter jets in my carb. I am getting good fuel burn numbers now and with the price of gas I wonder if my fuel burn will go up appreciably.

[LifeofReiley]

Shed the rubber and install a plenum is on my mnd.

[jonbakerok]

I suffered with the same problem for several years before I finally got up the nerve to drill out the nozzle. Afterwards, climbout temps rarely broke 400 and cruise temps were below 350. It must have increased the fuel burn but not enough to notice and I never bothered to measure it. The only downside was that afterwards I occasionally had to lean for hot starts. There's been a lot written on this pro and con but I never heard of anyone who did it and regretted it.

The certified carb on a Lyc 320 is jetted for the convoluted exhaust and muffler on a Cessna or Piper. Any biker will tell you that if you bolt on a set of drag pipes you have to re-jet the carb.

[Bob Martin]

Originally Posted by cleve_thompson

wonder if my fuel burn will go up appreciably[/b].

The fuel burn will only rise during takeoff where you need it most. The rest of the time you control fuel burn with the red knob.

Once you see the jet and how it works, you will understand the main jet can and does restrict the max allowable fuel flow at WOT-full rich. The only time you use this condition is at Take Off. Otherwise you lean to get the proper fuel air ratio for good combustion and cooling.

Drilling the main jet should not be the first thing you do, it should the last thing you do after checking all other possibilities of improper set-up.
But once it determined you need more fuel, then it is Very important that
it be done to achieve a proper set up. Heat in your air cooled engine can be hard on it.
Good Luck.

[mannanj]

I know this has been posted before but just in case you missed it...... Make sure that you have at least one washer between the aluminum baffle and the rear of number three cylinder on the AN-3 bolt that holds the baffle to the cylinder. That washer seperates the baffle from the cylinder and allows cooling air to pass this area where there are no cooling fins. Hope this helps.

[victor]

Also, sealing intake ramps made a big difference for my oil temps, and I am sure it would help cyl temps also. A lot of air can escape under the ramps.

[cleve_thompson]

Thank you all for your help. I had already inserted the washer behind #3 and sealed my inlet ramps. I took my bird up to Ray Lawrence in Sandersville, GA yesterday. Ray has helped build over 20 RV's I think. So I had him look at my baffles and seals and he said that he saw nothing wrong with them. We then took 2 of the GRT EIS CHT probes out and held them in boiling water and they were right on. I had never gotten a rise in RPM's when pulling the mixture control during shut-off. We tried to adjust this but were not able to get the desired rise even with the screw almost all the way out (even if this had fixed the idle mixture we couldn't have left the screw there).

I called Lycoming tech support and they said that it could be an induction leak or engine running to lean. Ray and I checked clamps, gaskets, etc. and could not find any obvious leaks. Then Lycoming suggested we pull the carb and send it back to Precision since it is still under warranty and suggest that they install larger diameter jets if they find nothing else wrong.

We are pulling the carb so I am without a bird right when I had gotten all the bugs out of the coupled approaches using my 430W, GRT, and Trutrak.

[dmeloche]

Bart at Aerosport opened up the jet on my O-320D1A after I installed the baffle washer and installed a plenum on my RV6 to no avail. Opening up the jet makes a real difference. The only drawbacks i have encountered are that I can't do a run-up at full rich or the engine loads up and flying around in the pattern at low power settings and full rich results in surging and rough running. To compensate I do the run-up with the mixture knob out a little bit and don't push the red knob to full rich in the pattern until I'm on short final. I've noticed no measurable increase in fuel burn however I am quick to pull it out a little bit once the power settings are less than 75%. I have also learned that if you have a CS prop, reducing RPM to 2400-2500 and climbing at 120kts and WOT/full rich gives good climb rates and reasonable CHT's on all but the hottest days (90F+).

Doug
RV6