850 Commando Right cylinder POPS upun decelaeation

Hello again. After rebuild and break in of a 1974 850 Commando I am having a slight problem that is annoying. When I decelerate with the throttle backed off fully my right cylinder occasionally makes a POP-POP POPPITY _PO.P Maybe I just need to re-synch my carbs. I am not the best at fine tweeking yet. The valves are set correctly and it has a Pazon ignition . Pretty sure the timing is set correct. Did it by the book. It has the stock Amals and the engine has only 3200 original miles on it. Any ideas as to what I should do or what the problem is??? The bike runs strong . It does not smoke. Looking at the plugs they are pretty white on the insulators. Not much tqan or grey. Thinking about raising the needle jet one notch. It is on the leanest notch now. Looks like the previous owner had the intake polished and some material removed to open up flow in the heads??? Thanks for any help===Mark C

BTDT, probably either an exhaust leak (make sure you've got a good gasket in there, and torque up the exhaust header nut) or -what I had - an intake leak. You can test for the latter by spraying WD-40 or penetrating oil around the intake runner (I had a bad gasket) and other possible sources of the leak with the engine idling. When you find it, the idle will change noticeably - probably increase but at least "smooth out."

Failing that, maybe that cylinder is lean because of blockage in the idle circuit. Perhaps run a can of carb cleaner through your next several tanks of gas.

I'm betting you'll find an exhaust or intake leak, though.

Good luck, and do report back to let us know how you make out! - Brian

You might also check to ensure that the carb bowls are cinched up--I finally put in Allen screws and carried a key with me because the bowls would loosen and produce mystifying engine misses, pops and erratic behavior.


Tim Kraakevik
kraakevik@voyager.net
'72 Combat

I had exactly the same post-rebuild symptoms and exactly the same issue/solution that Brian mentioned - intake gaskets between the head and manifold. I had installed new ones w/o sealer. I pulled the carbs/manifolds and put a very thin coat of Permatex gray on the head side of the gasket - none on the int manifold side and that eliminated the problem.

Wow what quick help--Thanks for the info and I will seal off the intake as advised. I will get back to you with the results once it warms up in the next few days---Mark Cigainero

Try turning the right side air screw in a tad.
Cash

kraakevik@voyagernet said:

You might also check to ensure that the carb bowls are cinched up--I finally put in Allen screws and carried a key with me because the bowls would loosen and produce mystifying engine misses, pops and erratic behavior.

Click to expand...

Be careful with those allen screws, especially the stainless ones. I don't know if it is a slight mismatch in thread profile or just the sharp edges, but I have had a couple strip out the threads in the the pot metal of the carb body with very little torque.

I had the same problem once, turned out to be a loose jet holder

Once again you were right on both counts. First when checked the right bowl screws had loosened up AND the right carb to manifold bolts were loose. I disassembled the carbs from the engine and manifolds and used some grey permatex gasket maker to seal it. It all worked out great--Problem solved. I now no longer have that annoying popping when the throttle is backed off. I thank you and my ears thank you--I do have a rich mixture issue now but I will inquire in another new post.--Thanks again---Mark Cigainero

I love a happy ending!

I have the same problem on my 850 - right carb running lean. I think I'll take the carb off and apply some sealant too.

Debby