Upshur Twin 4 strokes RUNS

This Forum is dedicated to the Hobbyist I.C. (Internal Combustion) Engine Community.

Moderators: Harold_V, JackF

Post Reply
Posts: 1424
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Upshur Twin 4 strokes RUNS

Post by tornitore45 » Thu May 15, 2014 7:56 am

I searched for my previous post asking advice regarding my Upshur twin sputtering but refusing to run but could not find it.

I am elated to report that yesterday I got it to run very well, with plenty of power at full throttle and smooth idle albeit very sensitive to throttle.

Problem 1) I installed the Rimfire spark plugs just as it came gapped without a second thought. After countless fiddling with the mixture and pulling the cord it dawned on me that the gap may be to small to provide the required energy.
After raising the gap to 0.040" the sputtering time increased.
After much head scratching I notice in the corner of my vision a sparking on the PC board.
Explanation: As the engine revved up the dynamic compression increased (less time to leak past valves and rings), that raised the spark voltage to the point a second path became preferable.

After improving the creepage distance on all secondary circuit the engine really sped up to sustained running.
Then it blew out the two 90* pipes on the exhaust, they were simple elbows about 1.25" per side friction set into the head. As a result speed increased substantially. Did not realized how much breathing obstruction they presented.

While on one side I was happy that it finally worked, I was humbled that a guy that spent 50 year in power electronic could do such a shoddy work on the electronic ignition assembly.

Off to the next engine we go.
Mauro Gaetano
in Austin TX

Posts: 1525
Joined: Mon Jan 26, 2009 3:56 pm
Location: Caldwell, Idaho

Re: Upshur Twin 4 strokes RUNS

Post by JackF » Thu May 15, 2014 1:16 pm

Good work on diagnosing the problems :D , even if some of it was accidental :roll: :wink: :lol: .


Post Reply