Zman39

My Feedback: (6)

Hello,

I just put back together a SJ50 I got from a guy. I needed a few new parts and Dub helped out big time. My only question is the when I disassembled engine, I noticed there was two head shims and I am wondering if this is how it is suppose to be stock or did previous owner add some?

Any info much appreciated.

Z

bob27s

My Feedback: (19)

It is unlikely he added a shim. Most of the engine have two.

The shims are added at assembly to achieve the correct deck height. All depends on how the engine fits up. So some engines come with .003" of shims up top, some have .015" .... of course extremes, but the point is... it is a custom fit-up.

In any case, the shims will not make a big difference. Adding or removing .005 from the head clearance on the sport-jett engines will not affect performance or any noticable traits. Only when you go to extremes of running zero-nitro FAI fuel, or going up to 30+% nitro fuel would any changes in shims be considered.

When you re-assemble the engine.... ensure the head bolts are snug. And check them again after the first run while the engine is still slightly warm.

Bob

still4given

Can I ask if there is much additional power to be had by adding a shim and running 30%? I run 30% in my YS 1.10. It would be nice to just use one fuel at our warbird races.

Thanks, Terry

bob27s

My Feedback: (19)

You can develop a bit more power on the 30% fuel. Nitro is liquid power. A number of jett owners run 30% just for that convenience factor of hauling around the one fuel jug. No problem there.

Usually you do not have to decompress it when using 30% heli fuel. But if you have an extra .003-.005 gasket hanging around, sure - toss that in.

Bob

still4given

Hi Bob,

Do I understand that the SJ50 will run fine on 30% as is, without an extra shim? I don't have any shims "hanging around" and would have to order them. If it won't hurt anything then I will give it a try. Anything I need to watch for?

Thanks, Terry

bob27s

My Feedback: (19)

just run it.... the sport engines are not set up that critical.

Set it rich an extra 100-200 rpm from normal (ie, for flight .... 600-800 down from ground peak).

still4given

Hi Bob,

I got a chance to fly the SJ50 this weekend with 30% Cool Power Heli fuel. The first thing I noticed was that I had to back out the HS needle almost a full turn from peak to get it to drop 800RPM. That sounded too rich on the ground but it did get on the pipe once in the air. I'm not sure it was any faster than it had been on 15% and it was murder on my glow plugs. After each flight, I pulled the plug and found the coil distorted. I'm thinking I either need to add shims or just go back to 15%. I don't want to have to install a new plug for each flight.

Blessings, Terry

bob27s

My Feedback: (19)

Ah... the reply from yesterday got dumped when they reloaded the backup.....

The extra nitro alone should not be blowing the plugs -- but it is possible. As long as the engine was set rich it generally does not make much difference. If the engine is going lean in flight and that is causing the plugs to go --- time to check the fuel system.

Easy way to check --- add a second glowplug washer to it for a flight. Be sure you are backing off an honest 800 rpm or so. That should be pleanty down.

The only other reason it would distort the plug is if the engine was overloaded --- running as if the pipe were set too short. You noted the needle did not adjust well - it should not require a full turn out to drop just a few rpm unless the needle was turned in too far to start with, but that can sometimes be the case if the engine is running at the "minimum" tuned rpm --- the engine will keep trying to stage, and will not change in rpm until it fall completely off the pipe. So, something to look for there.

Always start on the way-rich side until you see how the engine runs. Adjust in from there to briefly find peak rpm. Once you have a needle setting that works, it will not vary more than 1/8 of a turn on any given day.

still4given

Hi Bob,

Here is what I did,

I started with the needle turned out 5 full turns. I have the fuel line run to the outside of the cowl for filling and that gives me a good place to pinch it as I lean out the needle. Once I don't hear any increase in rpm after the pinch I call that peak which in this case was 17,600 rpm on the APC 9x8 prop. I then opened up the needle until it read 16,800 on the tach. It was starting to get a little blubbery at this point so I stopped there. It still transitioned nicely so I flew it like that. It unloaded quite a bit in the air and was quite fast. I only flew the 10 laps and then landed. All seemed fine, but when I started it the next time it had a rougher time transitioning so I pulled the plug and saw that it was distorted. I put another fresh MC-59 plug in and all seemed well until I started it again after the next flight. Same thing, the plug coil was distorted. It would read good on my meter but wasn't reliable once the glow driver was removed. This wasn't the only engine having that problem that day. My YS 45 and TT46 were both hard on plugs running the 30% fuel. I ended the day with a handful of bad plugs. I backed all of them down at least 500 RPM from peak. They all flew the entire course without a problem. It wasn't until they were restarted that the problem would show up. The TT was the least problematic as in I could get three flights out it before I had to change the plug.

Anyway, I'll try the two washers on the plug idea and see if that helps. I do like the additional speed I'm seeing, especially on the TT46.

Blessings, Terry

bob27s

My Feedback: (19)

Let me know if the plug washer helps.

I can send you a few head shims.

still4given

Thanks Bob, will do.

Blessings

still4given

Hi Bob,

I tried the double washer and no joy. Still blowing plugs. I'm going back to 15%. My Strega with the SJ50 is kind of stuck in the middle right now. It is turning about 2:15 for ten laps. Break out for Bronze is 2:30 and Silver is 2:00. I'm going to have to play with prop selection and see if I can get it a bit faster. At 2:15 I have to hope someone goofs up in order for me to win.

Blessings, Terry

bob27s

My Feedback: (19)

Hi Terry......

a few questions....... what size race course are you flying? 2.5 mile ? 2 mile ? Other?

The RPM you noted originally sounded ok. Take a few steps on the props. Try a 9x7 first to unload the engine. The SJ-50 has also been very successful in sport racing using a 10x7 prop also. That was my choice for warbird and AT-6 racing. Seems to get out of the turns better than the 10x6 does. Your strega is more along those lines.

Im still a bit concerned that it blew plugs regareless of the fuel. (my own 8 year old SJ-50 engine ran fine on 25% last summer.....)

Id look again at the fuel system. The only time I run into blown plugs is if the engine goes lean in the air for some reason.... and its usually because I was not paying attention and flew the fuel tank dry or something.

If you have not tried mounting the needle valve on the airframe, do that too.

Just a few suggestions .... hopefully we can get your times down a bit.

Bob

still4given

Hi Bob,

First of all, the course is a two pole, 700' course. Secondly, I flew it today and went back to 15%. I used a 9x9 APC and ran 1:47. What a difference. I think it is really starting to run in. If this continues, I'll be a solid Silver. Thanks for all your help.

Blessings, Terry

bob27s

My Feedback: (19)

excellent...... good job!

Two pole 700 foot..... yeah, hedge to the higher pitch..... you spend more time in the straight/dash mode than in the corners. The 9x9 should work well