Carb swap, performance drop

I exchanged my Marvel Schebler 10-5009 with a 10-5217 because of an issue with an occasional in-flight stumble. The new carb solved that problem, however we have seem to be losing some top end performance. Before the swap the plane (RV-4) was cruising around 160mph. 2500rpm @ 5K. Now with the same settings it's running around 150mph. At 8000' WOT the engine is making about 100rpm less than it was before. Leaning LOP and ROP makes no difference. I know that there are all kinds of variables when making comparisons, and that real testing is needed for conclusive results, but my question here is a simple one. Can a carburetor swap affect performance in the manner I'm describing?

I would assume you installed a new gasket at the time of the new carb.

Did you check that the new gasket had the correct size hole for the throat, and that the carb and sump holes were the same size, and matched??

It is amazing how much effect a gasket overhang, or slight ledge in the bore can have.

Hmm, the new carb did come with new gaskets. I assume they sent the correct ones. One was bigger, it went on the bottom toward the FAB. The top gasket looked like it fit the throat without any overhang, but I really didn't pay that close attention. It's been off once to drill out the main jet. The only difference that made was lower temps and more fuel flow. The speed and RPM remained unchanged. I know my Catto 66x70 3-blade prop is under pitched, which is why we noticed the RPM drop. Before at WOT level flight you could easily redline the engine. Now she tops out at around 2750. It should be 2850-2900. That's what we were seeing with the old carb.

Did you verify that both throttle and mixture controls operate the carb arms stop-to-stop?

Did you verify that both throttle and mixture controls operate the carb arms stop-to-stop?

Click to expand...

Verified. Also visually checked butterfly is opening wide open.

Arlie, do you have manifold pressure? Mixture, air volume, and cylinder to cylinder distribution will affect power. You have made a change to mixture, but should still quantify ROP degrees to verify it has all the benefits. Air volume change could be indicated by manifold pressure, and distribution by recording egt peak for each cylinder. If you do this at 7-8000 ft, then you should not be concerned about engine damage. With that information, then you could call MS and talk to their tech guys with your data and issue. You probably know these already, so just a reminder.

A reduction in airflow would decrease performance at WOT. A reduction in air flow can be caused be several things. Assuming you didn't create a blockage or restriction with the installation, you can look at the carb throat diameter on the upstream side. These carbs came with various different exit throat sizes and a mismatch with the sump can cause turbulence. In the range of designed capacity, turbulence can cause a reduction in mass flow.

I wouldn't think that mixture should really be a factor given your controlling it externally (the jet is only metering when at full mixture. Once you start pulling back the red knob, the metering is done before the jet), except for distribution issues and those are usually from poor design of sump/plenum/runners, not the carb. If anything, the improved nozzle should create finer atomization and better power and consistency. However, the mismatch I discussed above can account for a worsening of distribution due to the turbulence. Do you notice similar behavior in your EGT's around Peak?

Larry

It's actually running a lot better now. I took it up tp 8000' and did full power runs in 4 directions and averaged the speed. GPS shows an average of 186.25mph @ 2750rpm. It's not the fastest but we can live with that. I don't know what the deal was but it seemed to have worked itself out. Who knows? The reason for the carb swap in the first place was that it had an intermittent stumble. That problem is fixed.