RPM drop

[Aerosan]

Hi all
I have problem with the Rpm on my motor dropping slowly. If I do a static run-up the motor will go to 6400 rpm (it is a 618) and then slowly drop down to about 5000 rpm I have to stress this happens over 90 seconds or even 2 mins Has anybody got any idea????

I think it is the pump not supplying feul fast enough so it is running dry to the point where it balances out.

Please help Im grounded untill I can get it right!!

Cheers San

[RudiGreyling]

Check and flush ALL your fuel lines for dirt before you check the pump.

[Aerosan]

Thanx greylingr

I spoke to a Gideon this morning and he sugested that I tighten the RAVE valves in the open position and give it a go.

Let me see maybe!!

Thanx Ill do the feul lines anyway
San

[Duck Rogers]

Are all the fuel lines good?
Is the pulse line good?
Check for air leaking, most likely at the rubber boots on the carbs.
What happens to the CHT/EGT during this time?
What happens if you leave it to cool down for about an hour?
Is the exhaust not maybe compromised....ie. plugged?
Have you changed your prop lately?

Duck Rogers

[Microwave]

Have you checked whether one of your mags may be dropping when the engine heats up? I had a faulty mag once that run fine when cold, but started failing as it heated up. It would start failing intermittently and eventually it would fail completely, resulting in a general decline in engine revs. I guess not the daily occurrence, but it may be your problem too. Jut do a mag test when the revs are down and see.

[Leprachaun]

I have had similar experiences on two occasions, 1 being a BMW which ran perfectly well when cold , but when hot it just wanted to keep on revving , dropping slowly - checked all and found throttle cables to have no freeplay - BMs also use Bing carbs - check freeplay at least 1 mm :D
My good Buddy ZU - AAS alias David Mentz had the same problem on his newly aquired Raptor 503 - same problem after sicronising the carbs he suddenly had a runaway Raptor - check freeplay and mixture setting . and whala all sorted - Two Kilkennys please

[Aerosan]

Hey hey!! all

I think Ive got it nailed, it is a combination of the RAVE valves not being able to open fully and some carb settings.
Now I have to confirm this and hope to do so this afternoon, but you know with work and all interfering with flying you never know what might happen!
I have screwed the valves in to to lift them more and it made a day night difference! the rpm still dropped but now only to 5800, it seems to be the valves equalising and moving slightly back causinf the drop. Im giong to override them completely this afternoon so lets hope for the best!!

Thankx fro the replies :D

still dont know who Duck Rodgers is!!

[Duck Rogers]

Neither does he!

[Aerosan]

well well well DR so we meet again!

Im happy to report that I have my problem sorted, I fixed the valves in the full up position and there we go no problem

thanx everyone who helped

San

[Duck Rogers]

Sounds like a stop-gap measure to me. Is it really fixed?

[Microwave]

Why is there a Rave valve there to start with if you can just jam it fully open? I agree, you have not actually solved the problem yet, even if it does seem like it! Look deeper

[Aerosan]

Hi guys

Bit is "according to those in the know" that the valve dont work over 5500ft anyway. There is a lever that is supposed to be operated when going over whatever the alt is, to do just that override the valve. Aparently the pressure drop causes the valve to close and then loose rpm. Im just told all of this so not sure myself.

Have to trust the guys at Aviation engines and hope for the best

Thanks guys
San

[Duck Rogers]

Aparently the pressure drop causes the valve to close and then loose rpm.

I don't quite agree with this. What really closes and opens the Raves is the spent exhaust gases and in this way adjusts the back pressure in the exhaust system. Atmospheric pressure is going to be overruled in this case by exhaust pressure. If you crank them open manually all the way, you'll find that the engine will be sluggish at low RPM, there won't be enough back pressure in the system. You've probably just traded one problem for another.

I think what the real problem is, is that the Raves are probably sticky. I would suggest cleaning and decarboning the guillotine slides instead.

Remember one of my previous questions? I asked if you have changed your prop lately? An overloaded prop is gonna do much the same thing if you have an oil injection system fitted because the engine will be lugged down and pumping too much oil and too much oil will eventually cause sticky Rave valves.

Have to trust the guys at Aviation engines and hope for the best

Trust me....I'm a Lawyer!

[Duck Rogers]

Just to clarify, in case I get flamed!
The ALTITUDE does, to a certain extent, have SOME role to play in the operation of the Raves..... that's why it's better to have a cable connected to the lever you mentioned, to operate the Raves manually, from inside the cockpit. But the real point is this: Your 618 hasn't always done this rpm dropping thing... I THINK...and the altitude hasn't changed at your airfield

[Aerosan]

OK DR
Im going to go with your reasoning and remove the previously mentioned override!! See the thing is that there was rather a lot of gunk (oil) that came out when I removed the back of the valves the first time.
Also when I tries to remove the valves I had to yank them out as there was a lot of build-up behind and on them. So after doing this I never tried it I just went ahead and did as I was told! this is out of caracter for me as Im normally a pain in the but questioning everything!!

So yep you gut your way !! Ill try it and if it works Ill uhm... wel OK

Cheers San

P.S. Ill never flame anyone for trying to help!! that is how we learn isnt it So to everyone who took the time to try and help.. thanx