What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Momentary RPM drop at climb power

BJohnson

Well Known Member
What would cause a momentary loss of power at 85% power about 5 minutes after takeoff on a IO 320 using a Silverhawk fuel injection system? I was step climbing to clear Class B floors around Seattle and added power, full rich and started to climb with boost pump off. About 40 seconds later the engine lost about 200 rpm, then recovered over a 10 second period. During the same event fuel flow dropped from 13.4 to 11.6 gal/hr, fuel pressure dipped slightly from 25 to 24 psi, three EGT?s rose about 12 degrees, and one EGT dropped 54 degrees. I had just refueled and sumped the tanks. A few weeks prior I had a smaller but similar event that occurred just after refueling while a level on downwind departure with a 50 RPM drop but this time 3 EGT?s stayed the same and one EGT dropped 20 degrees, a different cylinder from the other event.

I pulled the cowling and checked for loose fuel connections, ignition wiring, or signs of any loose injectors, throttle body, intake manifold. Everything looked fine. Subsequent flights have not repeated the same issue (on the same tank of gas).

Could this be a water ingestion issue? Or what else could cause a transient low of RPM?
 
What was your altitude and DA at the time?
Do you maintain EGT of 1250-1300 while climbing?

Wondering if the mixture was too rich for the altitude...
 
It was rich

All EGTs were below 1200F. I typically do this to keep CHTs in check during climbing, but probably don't need full rich. Would that cause a momentary rpm drop?
 
Hmmmm, first things that come to mind are sticky exhaust valve, since one EGT was way different in response. The flow drop could have been due just to the drop in rpm and lowered airflow.

How many hours on the engine? 400+?

The other possibility is bubbles in the fuel due to low pump inlet pressure of warm fuel from takeoff, and it diminished the fuel flow. This is really does not fit the engine data though. All cylinders should have been the same in this case unless one went way lean. Is that possible at 11.6 gph? Does not seem likely.
 
Do you have an alternate air system? On my -7A I originally had the Van's magnetic alternate air door. It would come open at high power settings and disrupt the flow of air into the AFP fuel injector servo, causing erratic fuel flow - in my case a lot higher - and noticeable power changes. Then it would fall back closed and the problem would go away. However mine was more abrupt.
 
More info

The engine has 260hrs since new. The OAT was below 80 in both cases. The odd behaving cylinder was different in both cases. Would that happen with sticky valves?

I do have an alt air door slightly modified from the pivoting Van's door to support it more. The door is outside the FAB. The less severe event had the boost pump on and the plane was in level flight. The other event occurred with the boost pump off while climbing.
 
Time to ask Don Rivera

The "different cylinders had decreasing EGT" suggests (at least to me) a spider issue. There is no harm asking the professional (Don at Air Flow Performance) to inspect and test it.

A buddies RV-8A recently had a problem that had all the indications of a stuck valve, until the problem moved to another cylinder. Don found a piece of crud in the spider.

A fuel problem will not fix itself.

Carl
 
I also would suspect fuel blockage at the spider or downstream to a nozzle. I have an IO-320 and at 85% power in a climb (2500) am usually around 11-11.5 GPH (slightly fine pitched FP prop). A reduction from 13.5 to 11.5 increases RPM and power for me as 13.5 is too rich at 85% power. If I remember correctly, lycoming is looking for 13.5 - 14 GPH at 2700 and 29" for a 160 HP 320.
 
Last edited:
Great tips

Thanks for all the help. It seems like either fuel quality or the injector system/spider/nozzles. I called Precision Airmotive and they were very helpful with insights and information. Looks like I need to pull the filters, and clean the injectors, and check the spider.
 
Thanks for all the help. It seems like either fuel quality or the injector system/spider/nozzles. I called Precision Airmotive and they were very helpful with insights and information. Looks like I need to pull the filters, and clean the injectors, and check the spider.

Sounds like a good plan. Valve sticking is not a likely failure mode with your added event comments and hours. Hard to go wrong with Mahlon's advice.
 
Back
Top