Opinions on the need for re-jetting 85 'Rude 90hp

[Reggie08]

I'm hoping I can get some opinions on the need for re-jetting carbs for simple fact, I'm trying to determine if I have a need for it.

I have an 85 'Rude 90hp that I completely rebuilt the powerhead on. In the process I replaced the block because the old one was bored .030 over already and I had some pretty substantial scoring on the cylinder walls. The current block is bored .030 on three cylinders and .040 on the 4th (#3 cylinder to be exact). A local mechanic here told me I would probably need to re-jet because as he put it, "it'll idle like a 3 legged dog". His reason being, the cylinders were larger requiring more fuel/oil. Since re-accomplishing my link-n-sync, the motor idles very well/smooth in my opinion (better than the last 2 outboards I owned). So re-jetting for that purpose I think, is unnecessary. Another mechanic in the same shop suggested I bore the current jets out "4 thousandth's" which I'm uncomfortable with doing.

However, I'm concerned with the possibility of a lack of lubrication issue arising. Will the larger cylinders require me to have to re-jet up a size or two? My current jet size is 56c which is the standard size per the OEM manual/parts diagram.

I'm still in the break-in period and as such, I'm using a 25:1 mix (bypassed the VRO system) so lubrication right now shouldn't be an issue. But once the break-in period is over, I'll go back to a 50:1 mix. Should I considered re-jetting? Is there ever a time when re-jetting is necessary?

 

[Dhadley]

Re: Opinions on the need for re-jetting 85 'Rude 90hp

It'll be fine as is provided it's functioning as it should. IE - carbs clean, ignition working correctly etc.

The weight on a .064" over piston is the same as a standard. There will be no balance issues. The guy you spoke to is obviously a 4 stroke guy, probably a car guy - not a 2 stroke outboard guy.

Fuel is introduced into the intake by the piston moving toward the head therefore pulling fuel thru the carb. A larger piston will, if anything, draw more fuel thru the carb.

Drilling the jets can (it's very possible) actually make the motor run leaner. Once you drill a jet you change the ramp angle on both ends. You also change the rifleing in the barrel of the jet.

Since you have a well idling motor with one hole only .010" larger (that's .005" on each side) you now have more experience on the subject than the "mechanic" you spoke to. Congrats!

 

[Reggie08]

Re: Opinions on the need for re-jetting 85 'Rude 90hp

Thanks Dhadley!

The mechanic however is an outboard mechanic not a car mechanic.

The motor seems to run great. Following each trip with the boat to date, I pull the plugs just to see if they're ok and not fouling or appear to be running lean, etc.

 

[F_R]

Re: Opinions on the need for re-jetting 85 'Rude 90hp

Goes to show a complete lack of understanding of how a carburetor works. Yes, if you were to put a 10hp carb on a 3hp, it wouldn't work because of the difference in cubic inches of air and fuel that it breathes.

However, here we are talking about increasing the displacement only about half a cubic inch for the whole engine. A miniscule amount as far as the carburetor sees it. Carburetors increase the fuel flow as the air flow increases, doing a pretty good job of maintaining the fuel/air ratio constant throughout the speed range.

I just crunched a bunch of numbers and found that your motor, bored at .010" and running at 600 rpm, breathes a total volume of fuel/air equal to a standard bore engine running at 604 rpm. Certainly, the carbs can handle that miniscule difference in fuel requirements. It isn't even the proverbial drop-in-the-bucket.