Why won't my engine achieve full power?

Limbach L-2000, basically an air cooled VW Bus block with Stromberg carbs.  You're supposed to do a run up, that is lock the brakes, apply full power and hope it ends well.  They want to see 2750 rpm, I can't get past 2400.

It used to live at 50 ft. above sea level, now at 450 ft.; I don't think the jets are an issue.  I get 2400 rpm at half throttle, the rest of the throttle movement does nothing, the linkage seems fine, at full throttle the butterflies are fully open and the linkage is extended to its end.

Plugs?  Factory plugs are four electrode things that cost $104 each.  I installed Bosch with a medium/high heat range.  They come out honey colored so assume they do the job.  No restrictions in the air cleaner.  Fuel delivery? 

Can you try a different tach?  You're roughly 10% off, which is not unexpected for a tach.\

Watching the Mazda tach vs. the Autometer tach vs. the data coming from my computer is interesting, the Mazda tach can be 15% off and the Autometer is about 5% off.

I’d guess fuel just based on what you’ve already ruled out. 

I'm not a 100% on the test procedure; you run the car up to x mph and then apply the brakes to hold back the car and floor it to see what RPM it will pull?

Seeing as the motor falls flat above 2450 it can only be a couple of things provided you don't have a misfire and I'm assuming the motor is worn out

Ignition timing retarded several degrees. 

Cam timing retarded.

To much valve lash I would think you'd hear the clatter.

Fuel flow but that would also cause the motor to go lean which should show up on the plugs but possibly not. 

Compression

So check the ignition timing. Check the cam timing, pop the valve cover and do a cursory check I.E. it's within a few degrees of what it's supposed to be. While you have the valve cover(s) off check the valve lash. Pull one of the fuel lines and see how quickly it pumps out a gallon. Finally see what the cranking pressure is for each cylinder. 

EDIT: I just thought off another thing ..........plugged exhaust.

Is the prop the correct diameter and pitch?

Prop is correct, it's a variable pitch unit.  Fine, or shallow helix for take off & landing, slightly steeper for cruising and then flat, parallel to the centerline for soaring.

Tom, fuel?  Delivery or too low of octane?  It requires ROZ 87 (German)  which is 'Merican 81 octane.  I'm running 91 octane ethanol free.  As I said, the factory plugs are four electrodes; the factory rubs off the Bosch, NGK number and applies their logo,  >$100 each.  Hate to spend $400 for a guess.

Dan

Is the prop automatic feathering?  are you sure the pitch is adjusting as it is supposed to?  if it is not switching all the way to the shallower pitch that could explain the lack of revs on the run up. 

Is that dual mag? If so, what happens with the mag switch test?  I too am wondering about the prop pitch when doing the run up.

Why the expensive plugs ?   Is it because it is an aircraft engine ?

Are they shielded ?

Single mag.  Expensive plugs for job security and they're shielded.  Plug type would hold it back?  Prop feathering is manual and properly set, can't be changed easily on the ground.

I know jack E36 M3 about airplanes, however the choke on a stromberg is a mixture enrichment device and not the typical butterfly choke.  If you are running lean at run-up, perhaps give it a tiny bit of choke to fatten up the mixture.   If that works, you may have a carburetor issue or your ignition is weak.

OK first I'm a dufus..........I didn't read what the motor was in. The basics still apply.

I think Doc Brown's suggestion for testing the mixture. 

It is possible that the non standard plugs may result in the mixture not fully burning and therefore it runs a little lazy. I'd ask if any one has ever run back to back comparisons.

914Driver said:

Single mag.  Expensive plugs for job security and they're shielded.  Plug type would hold it back?  Prop feathering is manual and properly set, can't be changed easily on the ground.

so, why can't it be changed on the ground and only in flight?

I came to suggest that maybe your seat is positioned too far back in order to get the pedal to reach the floor, then saw that the concern is in an aircraft engine. 

Shielded plugs are required to prevent radio interference.   Should have no effect on performance.

Is it possible that 2400 is with in the permissible operating range ?

Those are CV carbs, right? Zenith Strombergs?

Have you checked that the rubber diaphragms are in good shape and that there is oil in the piston wells? Tears in the rubber diaphragm can make these things behave in "interesting" ways. Especially when the damage (or also the stiffness of the rubber) affects the way that the pistons lift even when the throttle is wide open.

Problems with the carb(s) might explain why you don't get any difference between half throttle and full throttle.

I'm with Tim.  Never actually worked on these carbs but they are close enough to the SU that I did.  Might want to check out this link  Tuning Stromberg Zenith carbs.

Strizzo said:

914Driver said:

Single mag.  Expensive plugs for job security and they're shielded.  Plug type would hold it back?  Prop feathering is manual and properly set, can't be changed easily on the ground.

so, why can't it be changed on the ground and only in flight?

I don't understand yet.  I can feather it such that it's parallel to the centerline, as in soaring, and lock it there; but the "cruise" mode won't lock unless the engine is >2200 rpm.  Some kind of friction lock?

Looking into the diaphram this morning.

Thanks guys.