hello all, I am new here and have some questions. thanks in advance for any help. I recently purchased a stock 1970 r75/5 motorcycle with 33,336 miles on the clock. compression with warm engine is 119 psi on each side, is that to low?
Hey, welcome aboard !!
In my experience, it should be fine. It's low, but that's to be expected in it's present (unstarted) condition.
Once the engine is cranked, the compression will continue to rise for several hundred miles. Each time you ride the bike you'll swear it's running better and better... for maybe 6 months or more ! And it's true, the valves and rings will need to completely re-seat. This engine more nearly represents a factory fresh engine in its present state.
• The optimal restart would be where you could ride it immediately for 25 miles or so at varying speeds ranging from 35 to 60MPH (no stop'n go), with fresh high-test gasoline and fresh 20W50 oil which meets API SF and/or SG oil specs.
• If you are really OCD and you intend to dump the engine oil after 10 or 15 miles, then a cheap substitute you can use is 15W40 diesel oil (API CH-4). This is a good bit cheaper than the specialty 20W50 and meets the requirements good enough for short duty.
• If this is a shop situation and other parts of the bike are in a state where the bike can't be ridden, then don't neglect to put a blower or fan in front of the bike. First starts tend to go on longer than intended and people forget that these are air cooled engines.
• I would not adjust the valves until the engine is run at least 5-10 miles. But you definitely want to readjust the valves after each or the first several rides.
Let us know how it goes !
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Thank you wobbly,
the engine is not rebuilt, so braking in the engine is not needed.
i adjusted the valves as per your wonderful tutorial, installed new carb to head manifold boots, new air filter, new spark plugs, new points, cleaned carbs. rubber diaphrams look good, The left spark plug is wet, however it is firing and the spark looks the same when both plugs are layed on the head and engine is cranked. Starts right up when cold and idles but runs rough when throttle is opened.
Thanks, blind lemon pie.
The engine is not rebuilt, so braking in the engine is not needed.
Mr Lemon -
Read again. I did not say your engine had been rebuilt. I said in its present condition it would be LIKE running in a new engine, due to the fact that rings and valves will need to thoroughly re-seat.
I adjusted the valves as per your wonderful tutorial, installed new carb to head manifold boots, new air filter, new spark plugs, new points, cleaned carbs. rubber diaphrams look good, The left spark plug is wet, however it is firing and the spark looks the same when both plugs are layed on the head and engine is cranked. Starts right up when cold and idles but runs rough when throttle is opened.
Your carb's throttle valves aren't synchronized. As the compression rises, the carb synchronization will change rapidly over the first 50 miles. Try to get it closer before your ride, but don't spend a lot of time getting it precise until you ride it.
After you come back from your first 25 mile ride, the carbs will be ready for their first really good sync job.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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