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Bob, I have aquired a whole bunch of Jett motors & have some questions

2 Quickie 500's with Red air valve carbs & Red mufflers

2 Quickie 500's with Black air valve carbs & Black mufflers

1 60LX with Red carb & Gold muffler

What is the difference in carb colors if any?

What is the difference in muffler colors other than length?

The Quickies seem to run considerably more RPM'S with the black mufflers

They all seem to scream on the test stand but need more info.

Thanks, Mike

bob27s

My Feedback: (19)

Hi Mike...

Simple enough to answer....
Yes, each muffler is a different lenght - and that is indeed the important part. They are tuned for different applications.


Lets start with the 428 engine. Uses Nelson glow plug.

Black carb - airvalve only - shutoff only. No real throttle capability - no idle capability.

Black muffler - tuned for 19,000 rpm and higher.

The 428 engines require the black muffler, and do turn quite a bit more rpm.
Prop selection is 8.75x9, 8.8x8.5, 8.8x8.75, 8.8x9 all which will exceed 19,000 rpm easily. Back off 600-800 rpm for launch.

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Sport Q-500 engine, or SS-40 engine - offered both with and without the Nelson plug head. They run pretty much the same.
This is a regular SJ-40 engine, just provided in a configuration for sport pylon racing. (CAPS, SEMPRA ran this configuration)

Red SS carb - looks like an airvalve - this has no low end mixture adjustment.
However, this carb will allow for throttle, and the engine actually will idle (for a bit of time) with a racing prop on it.

Red SS muffler - this red muffler has an enlarged section in the middle.
(the standard jett muffler is straight cylinder on the outside - no raised mid section)

Red SS muffler is designed for over 17,000 up to about 20,000 rpm.
For best operation, prop selection should be such that peak ground rpm is around 18,000 - back off 600-800 rpm for launch.
Prop selection - std 9x7, 8.75x8W, 8.8x8, 8.8x8.5 sometimes

(Although we found it has another tune point with a 8.5x7 and 8.5x7.25 prop up a little over 20,000 rpm - runs well like that)

-----------

SJ-60LX engine

Carb is the standard red jett carb - includes full throttle, idle and transition - has adjustable mid/low mixture needle (resist temptation to adjust it though).

Gold LX muffler - it is actually the SS muffler noted above. Same raised mid-section.
It is just supplied in Gold these days since it is primarily for the 60LX.

On the 60LX engine, prop the engine for no less than 17,500 peak ground rpm. High end rpm prop selection around 19,000 peak ground rpm.
Runs best of all with the 8.8x8.5, 8.75x9W, 8.8x8.75 props --- also std blade 9x8, 9x9 and 10x6 props.
Same as the others... find peak rpm briefly ..... than back off 600-800 rpm below that for launch.


And....

You can use the black racing muffler on the SS-40 or 60LX engine to turn a lighter prop at higher rpm.
But prop selection is based on the higher rpm design for the muffler.
Select a prop that turns no less than 19,000 peak ground rpm.
(for example, on a 60LX, black muffler, 8.8x9.25 clipped to 8" diameter works here)

I hope this is not tooooo much info

Bob

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Bob,

Thanks for the quick response & great explanation of the different combinations.

Mike

fizzwater2

My Feedback: (61)

So, Black "magic" then for the higher RPM....

Bob, with the black muffler on a SS 40, what props might be suitable?

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Bob, I forgot to ask if the SJ 40 Quickie with Red Carb & Muffler has the same port timing & internal parts as the Race Quickie 40 with Black Carb & Muffler?

My 40's all have the Nelson style heads on them.

Sorry for all the questions

Mike

bob27s

My Feedback: (19)

Questions are how one learns .. no problem at all....

The SJ-40 has different port timing than the 428 racing engine. Different head button/chamber design too (even though they both have nelson plugs). At one time I believe the crankshaft timing was different - not sure if it still is. Also, most of the 428 racing engines have been build as AAC construction ... sport engines are ABC.

bob27s

My Feedback: (19)

For just sport/speed use -- the 8.5x7 and 8.5x7.25 D1 series would definately work.

Some have taken the 8.8x8.5 or 8.75x9W and clipped it in diameter a bit to around 8" diameter.

stock 8.75x7.5 should be ok.

8.75x7.75W might work nicely right from the package --- have not tried that one in recent years.

All is rpm relateted. Takes a little testing to find one best for your engine/aircraft combination.

A word of caution.
If you are going for the rpm approach, please use either good wood racing props (zinger or rev-up Q-500 props) or D1 series APC. Standard blade props and their hub designs are not designed for these flight loads (in other words, a stock C2 series 8x8 is not capable - a D1 series 8.8x8 cut to 8" diameter would be)

Bob

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Thanks again Bob,

As usual you are a great help & really know your Jetts!!

Mike