1998 E8RLEUC Carb issue

[w2much]

I needed a carburetor for this motor but could not find one. The proper carb 0436878 is obsolete, it fits between a 1994 and 1998engine,finding a used one has been difficult. This motor can be fitted with either a 6 or 8 HP carb. I purchased an older carburetor for a 6 HP 1992 on ebay, it seemed to be plug and play as far as linkage is concerned. I soaked and rebuilt it before install . I can not get this thing to run correctly. Difficult to start and a high idle which will stall out if I try to lower it. My question should this new older carb be expected to perform the same as the the original carb? If they both fit on the engine the same and have the same linkage should they not perform the same? Or are there internal differences within the carb, leaf valves or block which prevent the 1992 carb from performing on the 1998 engine.
I have found a 1997 carb recently for sale, do not mind spending the money on it but hate to be hasty and wastey.

 

[w2much]

I noticed a hairline crack in the carb cover. I covered the crack ,which is barely noticeable, with JB Weld. The hairline fracture does not appear on both sides of the cover, only the top. It is still running about the same, hard to start and no low idle. It will run for a while then as I lower the throttle it will shut shut down and again be hard to start.
Might the fracture of the plastic carb cover be the sole culprit or does the 1992 carb deliver a different amount of fuel than the 1998 original carb. ? I would think a six horse carb be it 1992 or 1998 delivers the same amount of fuel no matter what on this engine. The linkages are the same and it installs the same. I guess there is a difference due to part numbers being different but it is not a visible one.