Glorfindel

Hi, i ordered a Jett q500. I did the break-in as the manual. When i tested 'peak' on the ground i only got 18500rpm on a 8.8x9, Nelson hd plug, and PowerMaster 15% fuel.

It was on a cold day (13deg.C) i'm at sea level (about 100'). I looked at the sleeve-piston, everything looks good, plug too. I called Jett he said that my engine was tested at 19500...


So why am i 1000rpm short???

Thx....

daven

My Feedback: (1)

You are definately 1000 rpm short on that prop. A few ideas for you to check would be:

Try a new glow plug
Make sure your Carb is opening completely
Is it a balanced thrust washer? Is it set properly?
Is your prop balanced?
Are you using a bladder tank?
Is the fuel old?
Is your tach accurate?

MaxQ

Can somebody educate me requarding the statement" Is it a balanced thrust washer? Is it set porperly?" Is it really critical the clocking of a thrust washer?

Thanks for any input.

MaxQ

bl10

Nelson Q40 and Q500 along with Jett Q40 (I don’t think Jett Q500 engines do) use a thrust hub which is balanced to offset the out of balance caused by the intake opening in the crank. The balanced hub really helps reduce vibration on the Q40 engines. In a prior post putting it on wrong was mentioned. Since our engines don’t have keyed hubs it is necessary to align the balanced hub carefully.

As to the Q500 engine being down 1000 rpm I wouldn’t worry about it too much. Depending on weather conditions and prop it is very possible you will only see mid to high 18k rpm numbers. A couple of suggestions. 1st try 3 or 4 props. (The same size) I get as much as 800 rpm difference between props. 2nd try running it when hot and dry as opposed to cold and damp. Most of us are down as much as 900 rpm early in the morning especially if it’s damp. The reason for this is even though the engine has more horsepower in dense air the extra drag on the prop more than makes up for it. 3rd make sure the engine mount is rigid. Any unnecessary vibration or movement can absorb power.


Barry

R.Bridge

I wouldn't be using the 8.8 x 9.0 at all, that is the bigest prop we use.. Trying finding a 8.8 x 8.75 or the 8.75 x 9w.... maybe the motor just isn't broke in all the way, you don't want to hurt the motor tyring to pull the biggest prop out there...

and Barry's post is right on! read his note...

Good luck...
RB

daven

My Feedback: (1)

Now that I reread your post, I wouldn't say you are down 1,000 rpms at all.

I misread your prop for a 8.8x8.9 which would turn over 19,500.

18,500 on a Jett with a 9 pitch prop is not completely out of line. Depending on the weather, my Nelsons will turn that prop somewhere around 18,500-19,500 (peaked).

Also, as Randy stated you may want to go down to a 8.75 or even a 8.5 pitch, a 9 should be saved for a well broken in engine under the right conditions.

Lastly, I'm not a Jett guy, but I believe alot of the guys are running smaller props on them at highter rpms.

As to the balanced thrust washer. If you take it off the engine and look at the back you will see a small crescent shaped indentation where material has been removed. If the back of the thrust washer is flat, it is not balanced. The balanced washer should also have a notch on the outside. You want to set the notch at approx 10 oclock to the engine head while the piston it is at top dead center.

SSAN

Daven,

You are correct as to the Quickie Jett run on smaller prop at higher rpm. From recent conversation with Dub Jett, I was informed that the Quickie Jett mufller is shorter than the Nelson and will unload in the air more, but we have to use smaller prop (i.e. 8.8x8.75) in order for it to do that. However, a 8.8x9 is not out of the question if condition is right, but it's a little tricky to set the needle. Once you get it right, it run like a bat out of hell.

Sam S.

Glorfindel

Thx for all the valuable info. i ordered 8.8x5, 8.8x8.75, 8.8x8.9 and a 8.75x9w for testing, thx again