Author Topic: Jett 60 - Too much compression?? (Read 1393 times)

Matt Piatkowski · « **on:** May 23, 2015, 04:06:28 AM »

Hello,
My Jett 60 ,that is the main character of my latest stories, showed exceptionally high compression yesterday.
When the glow starter was attached in the engine head up configuration, I could barely flip the prop using my chicken stick, the engine was kicking back and did not want to start for a long time.

When the glow starter was disconnected, I could flip the prop feeling that the compression level was rather high.

I have removed the glow plug, put the engine head down and flipped the prop several times assuming that perhaps the engine was flooded but it was not.

After some more flipping in the engine head up configuration, the engine finally fired and then run ok for ~4.5 minutes with good RPM with the Intrepid XL standing on my garage floor ( please see my previous messages re: Jett 60). The temperature of the head immediately after run was ~180 deg.F and the temperature of the header very close to the exhaust port was 260 deg.F.

The engine has still relatively new liner and piston so perhaps the tightness and very high compression will simply go away after, say, another 5-10 runs?
There are 3 copper shims separating the button from the liner - they were installed by Dub Jett last year together with new button, liner and piston.

Is it possible to damage something somewhere forcing the engine to fight high compression?
Perhaps I should add another shim, wait for the engine to finish the break-in process with slightly less compression and then remove this shim?

What is the acceptable compression level for Jett?
Some R/C sources claim that 50 to 60 psi. is ok for glow engines but 20 psi. is not enough.
I could not find any data regarding too much compression.

Lastly, I am assuming there exists a device using the glow plug thread that can measure the compression. What is it and where to get it?

Your comments and suggestions are appreciated.

Thank you,
Matt

Brian Hampton · « **Reply #1 on:** May 23, 2015, 10:08:30 AM »

I generally raise compression on my engines to around 13:1 (because I don't use nitro) and have no problems starting them. I'd be certain your Jett wouldn't have anything near that much compression so your problem seems more like having too far advanced ignition causing the kicking back. Cut back on nitro (even go zero nitro) or if you use a power panel then dial back the current so the plug isn't glowing as hot. If no power panel then try different brands of plug that don't glow as bright. Compression by itself puts very little load on the rod.

Dave_Trible · « **Reply #2 on:** May 23, 2015, 04:05:11 PM »

I have a bunch of experience with these and I'd suggest you leave it alone- Dub has them set superbly as is without tinkering. High performance engines like these are best started using this technique:

1. Don't prime but rather choke the Venturi and pull the engine over about three turns cold and maybe 4 turns warm.
2. Flip the prop five or six times without the battery to vaporize the fuel charge up in the cylinder.
3. Battery on, back-flip the engine to start. It will come up on compression, fire, and reverse rotation to run forward. If the fuel charge is close to right it will start every time.

Dave

Brett Buck · « **Reply #3 on:** May 23, 2015, 08:00:56 PM »

Quote from: Matt Piatkowski on May 23, 2015, 04:06:28 AM

Hello,
My Jett 60 ,that is the main character of my latest stories, showed exceptionally high compression yesterday.
When the glow starter was attached in the engine head up configuration, I could barely flip the prop using my chicken stick, the engine was kicking back and did not want to start for a long time.

When the glow starter was disconnected, I could flip the prop feeling that the compression level was rather high.

I have removed the glow plug, put the engine head down and flipped the prop several times assuming that perhaps the engine was flooded but it was not.

After some more flipping in the engine head up configuration, the engine finally fired and then run ok for ~4.5 minutes with good RPM with the Intrepid XL standing on my garage floor ( please see my previous messages re: Jett 60). The temperature of the head immediately after run was ~180 deg.F and the temperature of the header very close to the exhaust port was 260 deg.F.

The engine has still relatively new liner and piston so perhaps the tightness and very high compression will simply go away after, say, another 5-10 runs?
There are 3 copper shims separating the button from the liner - they were installed by Dub Jett last year together with new button, liner and piston.

Is it possible to damage something somewhere forcing the engine to fight high compression?
Perhaps I should add another shim, wait for the engine to finish the break-in process with slightly less compression and then remove this shim?

What is the acceptable compression level for Jett?
Some R/C sources claim that 50 to 60 psi. is ok for glow engines but 20 psi. is not enough.
I could not find any data regarding too much compression.

I answered Matt in a PM, but for reference, a typical peak cylinder pressure for these engines is around 100-130 psi depending on how you set it up. Mine is around 105, and my PA was something around 130. Pressure readings are not terribly good indicators of the compression ratio.

The fact that it managing to develop an astonishing 2.8 HP in this condition tells me there is nothing wrong with the engine. If it was overcompressed it would be detonating and wildly misfiring, and not able to put out any significant HP. Its putting out nearly 3 hp on something less than 1 oz/minute. It's proof of the miraculously good engineering that you can get more power than some lawn mowers in something this small without it flying apart. In even higher-power applications, the glow plug and head button might be *glowing visibly red* in a bench test.

The issue was/is the prop, which to my astonishment is absorbing nearly 3 hp. It's running at 10250 peaked out lean on the ground. Mine also launches around there (10000-10100) but it's in a deep 4-stroke, and as of about 2 hours ago, it will easily rev up to 12,000 one the ground if I restrict the tank inlet. That's much less power overall that we are talking about here, it gets reasonably warm (to the point you cannot put your fingers on the head or header, maybe 200-ish degrees. Based on previous experience it's putting out around .8-.9 HP on the ground and substantially less in the air.

Brett

Author Topic: Jett 60 - Too much compression?? (Read 1393 times)

Matt Piatkowski

Jett 60 - Too much compression??

Brian Hampton

Re: Jett 60 - Too much compression??

Dave_Trible

Re: Jett 60 - Too much compression??

Brett Buck

Re: Jett 60 - Too much compression??