Post by allgreen on Feb 11, 2021 10:08:43 GMT -5
I had a long battle with the Onan surge, and thought I'd share my findings. I tried all the usual fixes, but none of them helped. After replacing all the fuel lines, fuel intake, new carburetor, complete electrical troubleshooting and repair, replacing the short block with a rebuilt unit, all new gaskets, checking and adjusting the governor, and more, I still had a surge. Now, many of the above mentioned repairs were needed anyway, and were part of fixing up the 318, so I didn't do all that just to fix a surge. But the point is, even with so many new parts, the surge was still there.
The clue that led me to the repair took place after installing the rebuilt short block. The surge was gone for a short while, but then came back after 5-10 hours of operation. I went through the intake completely (again) replacing gaskets, making sure the intake joint was sealed, and after putting it back together the surge was gone (again) but then came back after 5-10 hours of running (again). Maddening? Yes.
A note here on sealing the intake joint. I don't believe it's necessary to split that in half to re-seal it. If you thoroughly clean the joint with a wire wheel and brake cleaner, you can apply Permatex Ultra Black sealant around the outside and make a very long lasting seal. I had more than one intake, and swapped one I fixed with the above mentioned method and another that was fixed by splitting and sealing. No difference. Sealed is sealed. However, not just any gasket making material will make a lasting seal. Permatex Ultra Black hangs in there for a long time, years, and I've seen other sealants fail in much less time. No matter what method you choose, use of a long lasting, strong bonding, correctly rated sealant is key.
Back to the surge fix. I eventually realized the problem was due to the shape of the intake manifold tract and that the "ears" by the bolt holes had been slightly bent/warped over the years by over tightening of the intake manifold bolts. My opinion would be that this is something of a design flaw - the intake fastening bolts are too far apart, it's very easy to have the ears get slightly bent down, and you can't easily re-machine it flat due to the curved "chute" being in the way. So, what was happening to me is the intakes would seal with new gaskets (I tried several kinds), but after a short while the gaskets would leak slightly along the long sides of the oval shaped intake tracts. The leak induced is very minor, and it's almost impossible to detect using the method of spraying around it with the motor running. This is partly because the Onan engine (my B43G at least) is set up a little lean from the get-go, even with the factory correct new carburetor with the correct jets. Point being, you don't need a big intake leak to induce a lean surge.
Solution? I cleaned thoroughly around the intake tracts, installed new gaskets and then used Permatex Ultra Black to make a bead of sealant all the way around the outside of both intake tracts after they were bolted down. A pipe cleaner or similar is handy to help spread the material into tough to reach places. Surge gone. Over 100 hours of running on this fix and it hasn't come back. My engine still runs a shade lean, so I can get a surge to come back if it's really cold in the winter, choke is off, and engine is not fully up to temperature. But once warmed thoroughly, it goes away, and in summer it's gone completely. Plugs look good now, idle is perfect, engine runs smooth at any speed, and power output is excellent. A lot of hassle, but well worth it for me. My 318 is a primary workhorse for me, so it needs to run right.