Johnson qd-17 compression

[Tech Tony]

Hi, all...I just rebuilt the power head on a qd-17. Honed cylinders, and new rings. Did a compression check today and got 74 top, and 76 bottom. I was a little disappointed I thought 90 psi was more like it. Wdoes anyone know what is good psi for these motors?

 

[boobie]

Try a different compression gauge.

 

[Tech Tony]

OK, I'll try a snap on gauge from a friend of mine, but if anyone knows the compression on these motors please let me know.

 

[racerone]

If ring gap was good and head was resurfaced the compression is very good.-------No other factors come into it.

 

[lindy46]

My QD23 has about the same compression as yours and runs fine.

 

[Chinewalker]

With fresh rings, it may come up after some run-in time.

 

[Tech Tony]

Ok, thank all of you, I was not sure what the # was but I feel better now , and the rings are new and have yet to seat.

 

[boobie]

Have you ran it yet ??

 

[HighTrim]

I have heard this many times lately. I get brought a motor that was "rebuilt" with still poor compression.

What did you do in your rebuild?

Were the cylinders checked for out of round, and that they are still within spec and not worn?

You cannot throw new rings in an worn out bore and expect compression to increase right? Not saying that you did, but seen that many times. Sometimes, a block just simply cannot be rebuilt, unless you go oversize.

 

[racerone]

Grooves in the piston can / will be worn.---Air does not needs much of a defect to " leak away "

 

[thatone123]

What type of recoil started does it have? I know the unusual started on the Q-10's 9.5's have a type of starter that because of the small diameter of the spool, turn slower than most starters. Hence, you get lower than normal compression readings. These motors seem to run fine at 60 psi. But maybe the real compression is higher?

 

[HighTrim]

Realone,,,,,you are referring to a 9.5hp, much different than the 10hp in question. Different animal all together. The 9.5s are lower compression as you mentioned. A good 10hp should be in the 85 to 110 psi range however.

 

[racerone]

QD = 10 hp-------------MQ = 9.5 hp

 

[Tech Tony]

Well high trim, I didn't check for out of round, I thought a little bit of hoining would fix that, I the only reason I rebuilt it was the wrist pin on the top cyl. Was bad. Did the upgrade to the more modern wrist pins. Put in new rings for good measure. I just didn't know what the compression was for these motors, and wanted to find out. You say 80 to 110 that's good enough for me. I think I am lucky to be 75psi on such an old motor. Took it out last weekend and It ran very well, just needs a little more fine tuning at low speed idle. Wish I could figure out how to post pics on here.

 

[HighTrim]

Well that's great.

What is it doing at idle?

IF the carb is set up properly, linc n sync done, and still poor idle, did you set the points timing? If so, how? Setting them with a timing fixture or the like, and buzz box/meter will get you the absolute best idle. You need to get the cylinders to fire exactly 180 degrees apart for best idle, which is hard with feeler gauge alone. Don't get me wrong, millions of people use feeler gauges, but for best idle, a timing fixture is best.

What plugs are you running?

DId you pull them after your run? Check colour? Check for any water droplets?

 

[Tech Tony]

I set the points at .21. autolite303 plugs the motor is a little hard starting cold but once it starts and runs it starts first pull. Has an occasional miss at low speed idle. I don't have anything but feeler gauges to time it. I have champion j6c plugs if you think they will be better for it.

 

[Crosbyman]

try this

http://www.youtube.com/watch?v=ga1Cjymj6ms&feature=youtu.be