Onan B48 twin 18h.p.

ray hofstetter · Jan 27, 2013

Runs rough at low rpm, but smooths out at high rpm.
Almost trying to backfire, but only one cyl.
Any thoughts or suggestions? I've tried about everything, but maybe I'm missing something obvious?

farmerjohninpa · Jan 27, 2013

how are the plugs,if old replace them also check points for burn or gap issue

ray hofstetter · Jan 27, 2013

New points, condenser, and plugs....no change....timing, right on...

joe csuti · Jan 27, 2013

Older onan eng split intake manifolds are prone to developing air leaks due to gasket going bad or loose/broken rivets.

To check if manifold sucking air,run eng at idle/slightly above,with a can of carb cleaner & plastic ext tube on nozzle,slowly spray around manifold at joint,if eng speed changes,manifold sucking air due to leaking gasket,if you can pinpoint leak you can plug with red rtv sealer or take manifold apart,drill out rivets & replace with screws,seal split with red rtv sealer or jb weld before reassembling.

ray hofstetter · Jan 27, 2013

Joe,
That's about the only thing I haven't tried.
I've heard that the Onans were prone to that...
Other than this problem, this Gravely has been one heck of a good tractor for the last 30 years....

Old Roy · Jan 27, 2013

Joe could this possibly the same with my engine.. don't really run rough like missing , but at less than half throttle (Not under power) it will surge bad.16 hp Onan with load on it runs great!

joe csuti · Jan 28, 2013

If your eng an onan(wh used both onan & kohler on 416)the most likely cause of surgeing is intake manifold sucking air at split or idle passages plugged/restricted in carb body,a left field possibility is previous owner was fooling with govenor setup.

Try the carb cleaner at split trick 1st,if no change in eng operation then consider disassembling carb,clean out all passages in body with a fine wire after soaking in carb cleaner,then blow out all passages with compressed air,get a kit for carb & rebuild or check on ebay for a new oem/aftermarket carb.

A quick & dirty trick you might try(I've had a 50/50 success rate with this method)pour 3-4 oz of seafoam gas treatment into tank & run 1-2 tankfulls of gas thru eng,if no manifold air leaks & eng operation doesn't improve with seafoam then consider cleaning/rebuilding or replaceing carb,you might also want to check on ebay for an eng service manual to check out govenor setup/operation.

kevinsstuff · Jan 28, 2013

my case 446 16hp b43 did same thing. ended up it had burned up intake valve due to debri blocking air flow around head and clyinder. mine was starter side. do a compression test to see if u have low compression . unlit propane torch also works for manifold leak points should set to .016 gap which is 16 degree timing

BushogPapa · Jan 28, 2013

Valve clearances been checked..??

Ron..

Onan B48 twin 18h.p.

ray hofstetter

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farmerjohninpa

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ray hofstetter

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joe csuti

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ray hofstetter

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Old Roy

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joe csuti

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kevinsstuff

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BushogPapa

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