Help--Max. RPM is 2500

[Onex107]

After 80 hours of good flying I couldn't get more than 2500 rpm on the next runup. When I hit 2550 it runs rough and I have to reduce throttle to 2300 to recover a smooth engine. I have richened the needle for the cold temp. with no result. I even backed the needle out to the start position, where I was controlling the rpm with the mixture control, and still couldn't get past 2500. It does the same thing on both ignitions individually. It almost acts like it's cross firing with a timing problem. That tends to tell me it's a fuel problem but I haven't been able to change it that way.
How did the engine act for you guys that had bad magnitrons? What's 2500 rpm have to do with anything? Voltage regulator? That's about where you get 13 volts from the generator.

[Gripdana]

What are your CHT and EGT temps when running on each ignition and on both. That could narrow it down to an ignition problem with one or more cold cylendars. I would also check the valve adjustment.

[fastj22]

Could it be a fuel flow restriction, not giving the carb enough juice at the higher RPM? Plugged filter?
I'd do a flow test to eliminate it.

[Rynoth]

I would also check the valve adjustment.

This was my first thought also, if your gaps are too large the valves won't be opening fully and the engine will be starved for air especially at higher rpms. Be sure the engine is cold when you adjust them (otherwise the valves may be longer and indicate a smaller gap when warm versus when cold.) You mentioned that this happened on the next runup, meaning the engine was likely colder than during your previous flights. If the gap is too large, as the engine warms and the valves elongate, the problem would tend to be less apparent than when cold.

[Onex107]

Thanks for the responses guys. When I run it on the individual ignitions the temps don't change. The ignition check at 2100 is normal. I can't hold it there very long but there are no low or cold cylinders. I took the fuel line loose and did a visual flow check but did not measure it. I checked the valve clearance by shaking each rocker arm but again did not measure it. I was looking for a tight one. It acts like a fuel restriction. Too much air, not enough fuel. I need to measure fuel flow and remove the fuel line to see if it's deteriorating inside. I have had the AeroInjector all apart to look for something that could be blocking the needle orifice and it was clean. If there is a flow restriction, it's further upstream. Has anyone had a problem with the hoses connecting the intake manifold? Are they fuel proof?

[Rynoth]

I checked the valve clearance by shaking each rocker arm but again did not measure it. I was looking for a tight one.

You should measure them, I think if the valves are off it's more likely that the gap is too large given your symptoms.

[fastj22]

If you had your needle out of the aerocarb, did you happen to install it 90 or 180 degrees off? The flat side needs to face the engine. Don't ask me how I know, but the symptoms were very similar.

[mike.smith]

Things to check:

- When running up, try moving the mixture in and out and see what happens (RPMs and temps). Even with the colder temps, I have found I still need to lean the mixture a bit on the ground to get maximum RPMs.
- When shutting down, pull the mixture all the way out and verify you get an increase in RPMs (to prove you aren't running too lean).
- Check the air filter. In fact, you can run the engine with it off to verify that you do or do not get the lower RPMs
- Check fuel valve at the fuel tank to make sure it's opening fully.
- Check that the finger strainer in the fuel tank is not clogged (you should be able to shine a flashlight down there and get a look).
- Check fuel vent for any restriction. This has been a gotcha for more than one aircraft engine of any type.
- Check the throttle linkage at the carb; make sure it hasn't slipped.
- My throttle and mixture cables run through the firewall, to the front of the cowl, then loop back to the carb. I have an aluminum bracket that holds the cables in place. I once forgot to tighten the bracket and it let the cable housings move with the cables, and so I could not get the throttle and mixture to work through their full range. Tightening the bracket solved that.
- Having either ignition not working probably would probably not keep you RPMs down. I can run on just the mags or just the coils and get full RPMs.
- Check the full path of the intake system and make sure you don't have any leaks (like loose clamps, or intake manifolds that are loose or not sealed).
- If all that checks out you may need to do a fuel flow check (remove the hose from the carb and do the same test you did before the airworthiness inspection)

Try checking only one thing at a time so you can narrow it down to only one issue.

[Onex107]

Thanks for all the replies and suggestions. Sonex, Joe Norris, also responded and agreed with all your suggestions plus adding a few more. I'm working through the list and will give a full report when finished. One logical thing that was not mentioned is, when you are only getting 3/4 of the expected rpm, maybe only 3/4 of the cylinders are doing their thing. I'm currently pursuing that thought after eliminating all the other possibilities.

[gammaxy]

When a cylinder isn't firing, you should be able to tell pretty obviously by sound. If you've never heard it before, you might not realize how obvious it is; I purposely disabled one spark plug just to see.

If you turn the propeller by hand (with ignitions and fuel off), does the compression feel the same on all cylinders? Did you try running without an air filter? Did you test the fuel flow rate--it seems 12-20gph at the input to the aeroinjector is typical.

Thinking outside the box... Did you check that the mixture arm on the aeroinjector is at the correct angle at full rich? I bet if it somehow shifted, it would be pretty difficult to troubleshoot unless you were looking for it.