Compression on a 1946 3.6 HP Firestone?

twocyclemania

Chief Petty Officer
Joined
Aug 10, 2010
Messages
505
Re: Compression on a 1946 3.6 HP Firestone?

Nice looking motor. I'll bet the compression is the least of your worries. I have about 30 motors of varying makes and I can't remember the last time I had a compression issue. I rarely check compression if the motor turns over. Most of these old motors were not used enough to wear out. Not that it's a bad thing to check and it's easy. See if you have any reasonable compression. Sometimes I put my thumb over the spark hole and pull it and that usually tells me what I need to know for the time being. Compression problems are rare. What you want to find out is does it have spark (use a spark tester - cheap). Clean the carb and tank and make sure it's getting fuel. With those two it should fire up. If it does; go from there: new plug, ignition wire (metal core) if needed, points cleaned and set, lower unit grease (use Lubriplate 105 available at NAPA). I predict the carb will be the most pain. I have 4 Scott/Firestones of the same era and all with carb problems. Anyway, if you get it running don't go looking for too much trouble. Do what has to be done and run it. By the way- it's the same as a Scott Atwater.
 

nwcove

Admiral
Joined
May 16, 2011
Messages
6,293
Re: Compression on a 1946 3.6 HP Firestone?

with all do respect tcm, a compression test is a must before throwing $$$ at any vintage motor. sure most have never been ran enough to be worn out, but many have spent decades in an old shed....corroding to the point of being nothing but parts. a thumb over the plug hole means absolutely nothing ! jmo
 

ronboonville

Petty Officer 1st Class
Joined
Jun 6, 2012
Messages
287
Re: Compression on a 1946 3.6 HP Firestone?

I'm kinda from the same school as 2 cycle. spark-fuel-start. but, would like to be enlightened as to what the compression should be on a 3.6 scott atwater. Maybe nwcove could give us the acceptable figure. I've yet to get a working compression tester at o'reilly's.
thanks for more input.
 

nwcove

Admiral
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May 16, 2011
Messages
6,293
Re: Compression on a 1946 3.6 HP Firestone?

from my experience on other brands of vintage motor, they will start at 50psi, but wont idle. at 60 psi they will idle, but not great. get above 70 psi with a well tuned motor, and it will run slow and smooth.
 

ronboonville

Petty Officer 1st Class
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Jun 6, 2012
Messages
287
Re: Compression on a 1946 3.6 HP Firestone?

Not wanting to jack the thread nw but would low compression also make them hard to start? my 3.6 wizard won't start unless I give it a shot of fuel in the carb and I can't get it to idle. on the other hand my 46 3.6 starts easy and idles fine but feels like it has more compression. thanks for more info!
 

nwcove

Admiral
Joined
May 16, 2011
Messages
6,293
Re: Compression on a 1946 3.6 HP Firestone?

Not wanting to jack the thread nw but would low compression also make them hard to start? my 3.6 wizard won't start unless I give it a shot of fuel in the carb and I can't get it to idle. on the other hand my 46 3.6 starts easy and idles fine but feels like it has more compression. thanks for more info!

absolutely! you can have good timed spark, perfect air fuel ratio.....but without the third piece of the puzzle ( compression) any ic engine will be hard starting...or non starting. just an example.....i have an old seagull , 20 psi comp. only way to get it to make smoke is with a drill on the flywheel nut at 1750 rpm.....still wont run , just kinda sounds like it untill i pull the drill off. i have another identical gull with 67 psi comp, and after its warmed up....i can start it by "palming" the flywheel ! basically you need the big three to make any ic motor run.....strong timed spark, proper fuel air ratio and compression.
 

ronboonville

Petty Officer 1st Class
Joined
Jun 6, 2012
Messages
287
Re: Compression on a 1946 3.6 HP Firestone?

Thanks, I may go with a new set of rings.
 
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