Re: 8 HP Johnson Carb
You will find that during this time-frame & later, that the factory was playing with carbs. Apparently the bodies were different (throat sizes) for different HP motors, but the tops were interchangable. There were considerable top numbers superceeded, ultimatley trying to utilize one or two different tops for many motors. That said, the one you have may just be OK, just a replacement. However you say you found your top was for a 93, I have doubts of that, because it only has 5 bolts holding it down, the 93 I have uses 6 bolts, with the 6th on the left rear by the roller looking forward.
What I have found on the 9.9/15hp pre 93 they used the needle jet in the front plugged hole on yours, where you could adjust the idle using the knob thru the front cowling. In 93 that jet was plugged, (88 on your 8hp) using the needle like yours on the rear where th cover has to be removed to adjust the needle. This style of top was designed to be used on either , however if being used on the older motors, it needs the right front needle (longer than the older original front adjustment covers) & at a cost of $26 just for ther new late needle itself. I am using the 9.9 as a comparision because if you look up the covers, they may say it covers motors from 6 to 25hp in those years.
I am currently battling the same on my 92 9.9. My top was broken at the roller, & was an early version where repair parts are only available as a kit at $99, while the later style top is $13. I bought a late top, but found my old needle is shorter by what appears to be about 3/16", so am now waiting for a new one to come in. So I understand your frustration.
I think I would check your existing needle. Is it straight, or possibly has a broken tip, these usually come to a very sharp point. That may well effect your idle problem & should be a cheaper repair.
Thanks for the photos, If you don't mind I will use one in my updated carburator article.
