1987 48 spl carb upgrade options

**ALBASS** · 08-11-2018, 08:23 AM

Hello everyone; I have searched these and other forums quite a bit, still I need help. I just got a 87 Fisher Gray Thunder 16ft with an 87 48SPL. After I got it home (no test drive) I start noticing the outside of the transom oily, then I took it to the river and it started right up with a shaky, smokey idle, I let it run while I parked the truck, then idled right out of the harbor (about 1/8 mile) shakely, upon hammer down all I got was a big bog, then little by little it started gaining rpm, planed off and sounded right to my tachometer ears, guessing above 5k and 32mph per gps. My first thought was that it was loaded up with fuel and needed to clear it before it would go, and that it would probably go away with use, it didn't. I brought it home pulled and cleaned the carbs (not too bad), when I was putting it back together I noticed the bottom carb has a much bigger bore than the top, even the screws through the airbox were larger on the bottom. Also the fuel enrichment line to this carb was plugged with a pop rivet. I don't suppose this setup can be linked and synked? And these are non adjustable, maybe 100:1 carbs. So I have been looking at used carbs, and have found a set off a 93 48spl that are adjustable and look pretty good at about half what 87 carbs cost from the same seller. I like the thought of being able to tune, but don't know if this is an upgrade or even possible. I would not mind a little more performance as long as its dependable and reasonably smooth. By the way the compression is 120 on both, non vro. Thanks for reading my windy post, even more if you reply. Don

'

**Mark Poole ModVP** · 08-11-2018, 02:22 PM

No doubt it has been 'shade treed'. Don't run it anymore until you have two matching carbs on it. The two cylinder 40 and 50 went through a total redesign I think in '87 or '88, different exhaust housing, lower unit, etc. I would try and find the carbs for that year model. OMC was notorious for changing carbs and/or jets from one year to the next. On the newer ones most of the carbs were plastic and used air bleed jets to control idle mix.

**flabum1017** · 08-11-2018, 02:45 PM

with that much boat on that engine, you will need the small bore carbs to give you the bottom end to get on plane...... as said, look for the carbs it came with.

**ALBASS** · 08-11-2018, 06:16 PM

Thanks for the help guys. I don't mean to ask for help then not take it but.. I believe 87 was the year the 48 spl was introduced. It was a non vro 100:1 premix motor, so wouldn't the original carbs be jetted for 100:1? I think they went to 50:1 very soon after. Would I not be better off with a 50:1 non vro carb? even if it was from a slightly newer year, but still a 48 as to be non vro?

**flabum1017** · 08-11-2018, 08:24 PM

the premix level has no bearing on the jetting...... they do not jet it for 100:1 or 50:1 etc...... they made that motorfrom 40 up to 60 hp..... so they had a couple different carbs for it and jetted for each horsepower range.

**ALBASS** · 08-11-2018, 09:58 PM

Somehow earlier I was able to get just the 48s from 87 to 96 to come up on a parts site, and yall are right the jets changed almost yearly from 49d, 50d, 55d on the mains maybe a few more sizes in between. I was able to confirm that the intake, air box and reeds were the same between 87 and 93, and around mid 92 the jetting was different top and bottom. So are the jets tuned to the exhaust tube? i.e. larger jet = better tube= more hp rating?

**BOATMANBLUES** · 08-12-2018, 07:30 AM

The jets size is determined by the amount of air being pulled into the motor...optimum air/fuel mixture is about 14.5 to one..the amount of air a motor gets is determined by carb bore size,intake ,ports,exhaust ,etc.....if you use bigger bore carbs on a motor you would need to bump up the jets to compensate for the extra air...but to big of a carb on small ports make velocity go down and the makes the motor lazy at low speeds...the best bet for getting a stock motor to run decent is to go with the same size venturi /jets combo that the motor came with....in your case the 48 /50 hp were the same ...one with VRO and one pre-mix...you can find some later model carbs if you want,just find some with the same bore and jet them for your motor....the best way to see if your right is do a high speed shut -off and read your plugs....you can make them little motors run on just about any carbs that will bolt on if you do your jetting like that ,but they usually run best on what came on them....or a bit smaller for bottom end help....I would pre-mix at 50:1 for that motor which ever carbs you use...

**ALBASS** · 08-12-2018, 09:05 AM

The bottom plug looked a little dry when I did the compression test, of course that was from the low speed jets. I did find my way back to the place where I could compare ipls by year and hp and started looking at 50hp, things really get complicated then as different shaft lengths come into play. I guess all 48hp motors were long shaft. Anyway I did pull the trigger on what I thought were a set of 93 48hp carbs, which upon further study of the pictures are actually 1995 carbs. The 87 carbs had a #36 top and a #50d main. The 95 has a #56d top, and a #55d main. I don't know what jets are in my carbs, or really which if either came with it. I am hoping at least having a matched set is a good starting point.

**ALBASS** · 08-16-2018, 03:24 PM

The carbs came in yesterdays mail. I pulled the bowls and they were clean so I went on and bolted them on. Started right up in the back yard and sounded pretty good, and took gas with no detectable bogging. River tested today, right out off the box idled a lot smoother, still was stumbling on hammer down, so I tied it to the dock and adjusted it in gear idle I didn't count the turns to seat them but they were at least 3 turns out. I started at 2 and turned them in some more, then set the idle down some more. Dang near perfect hammer now, and I feel like it would rope start very easy if need be. Then I started experimenting with thrust rod (third hole's best) and in the middle of a turn it laid down on me. One of the carb linkage adjustment screws came out and hid from me. I looked some more after I had it on the trailer still haven't found it, so I used one of my 2 spares which were at home 12 miles from the river. Next trip I do plan to read plugs from the high speed jets. The only issue now is the bottom carb primer hose still has a rivet in it couldn't find a small enough T locally but it starts so good do I even need it?

**ALBASS** · 09-02-2018, 05:03 AM

I got back to the river finally, the boat is running pretty good, smooth idle, seems to have gained a mph, planes great, still I wonder if there is a little more in it. What is happening now is I seem to be reaching top speed before I reach maximum rpm, seems the last 3 or 4 hundred don't increase speed. Pushing in the key at wide open slows the motor so I think I am rich enough. My prop is a 11 something x 17p Michigan Vortex with a little leading edge rash which seems to be about as much prop as I can find for this motor. This is a 600lb aluminum pro bass type hull that in the third hole (no trim) is right on verge of porpoising. How can I tell if its prop related, or maybe exhaust restriction, or fuel? Maybe reeds?