Hi Bob.
I got my new Jett 46 a few days ago and ran it in. When I changed to the 10/6 prop from the 9/6 as the dirrections told to do, the motor revs high and low.( 12,000 to 15,000) Rpm. I changed to an OS # 8 and the motor runs better,but the rpm is around 14,000 Rpm. (10% fuel)
I was wondering what plug came with the motor since it isn"t marked at all.

When I mounted the 46 on the plane the muffler hit the wing so I had to order an extension for the muffler. Jett said that I would loose a few RPM by using this extension.(3/4"). Can you tell me what RPM I should see when the extension comes in and I get to run it up.
So far this is the best performing 46 that I have ever had and I've been flying RC since 1958.
Thanks

Hi,

Thanks for writing...

I am glad you are pleased with the engine. Lets see if we can work out some of the issues.

The plugs we ship with are Merlin plugs. Usually Dub includes one of the merlin business cards in with the engine shipment. Any good medium range or medium/cold plug will work fine. The OS 8 is good. The K&B 1-L is pretty much the standard plug we base all others by. The K&B HP plug is also a good choice. Thundervolt, McCoy, Rossi 4, H9 performance plugs ... all seem to work well. With any of these, the glow plug will rarely affect performance, unless the engine goes lean and blows/damages the plug. The only thing that sometimes is affected by the plug is idle/transition if the plug is way off in heat range.

What aircraft do you have it installed in? Can you send me a photo of the installation? That may help a bit.

The RPM surge you noted, was that after you installed it in the airplane ?? Did you run it with both the 9x6 and the 10x6 on the test stand ? If not, please install it on a test stand so we can get baseline rpm numbers. Also, run the engine on the test stand without the extension, and then with the extention installed so we know what the delta rpm truely is.

The 3/4" extension will cost you a few RPM. How many..... sorta depends. Somewhere around 500-1000 rpm drop would be about my estimate, but you should still be able to get the 10x6 easily over 15K rpm with the 10% fuel. You basically end up with a longer 'pipe' tuned for the slightly lower RPM. This also shift the torque curve back a bit which means you can load on a bit more prop if desired... maybe a 10x7... after some more run time. Prop selection will depend on your aircraft type and speed range.

In the short term, the surging can be addressed in this way. I understand you have a number of years in the hobby, and may have already address these... but just for the record as they say......

IF this is happening while installed in the plane (let me make this assumption for a moment), there is a good chance you are getting fuel foaming. These engines exert a lot of energy, at a higher frequency than most. Make sure your fuel tank is completely wrapped in foam, and does not touch any of the airframe structure. Also make sure your prop is balanced. Both of these will help prevent foam from forming within the fuel tank.

Surging or inconsistant needle settings are almost always a symptom of air getting into the fuel somewhere. Foam is one source. Double check all of the fuel and vent/pressure lines for cuts/kinks... make sure your fuel tank clunk line is free to move, and is not cut/kinked as well. On the needle valve, ensure the packing nut is snug (no need to lock it down) so the needle can be turned with a little effort. If you can, watch the fuel lines while the engine is running... big bubbles are easy to see...foam is sometimes not as easy.

What you may have noticed, is that if you richen the mixture up a bit (down to 14K rpm) , this a) makes up for the extra air in the fuel b) drops the RPM down out of that harmonic range where it was causing the foam in the first place.

We typically find that 90% of the issues we receive email or calls on can be traced back to fuel system and fuel tank problems.

Let me have a follow-up on this when you have the opportunity.

Bob Brassell

Hi again Bob,

First of all the tests are done on the test stand. I got my extensions yesterday and I have gone with 1/2 inch rather than 3/4 inch.

I will work on the engine tomorrow as I have it mounted and ready to go. If I have any problem I will take it back to the test stand and work with you on it!
The plane that I really like is the basic Morris SU DO KOI. Although not stock anymore it basically looks the same as the orgional. This is #9 build out of the 12 that I have scratched built. Each one is better than the last. I usually get between 200 and 250 flights before they show the stress of what I put them through,so I retire them and "fire up" a new one. (It takes about 4 days from stick to ready to fly)
The tank is in the wing and covered with foam. Do you think that perhaps I should mount it on the Fire Wall?
I'll get back to you tomorrow!
John (tailspin)