350 to 305 swap

[pugwash sunray]

Hello everyone , I'm new to boating ...so I know very little about them .Please be patient.
I swapped engines in my 1992 21' sunray. It had a 350 chev omc and I replaced it with a 305 chev omc{Marine} .The boat still has the 5.7 drive and had a 14 1/4 4 blade 19 pitch prop. It would only rev to 3000 rpm with just me {140 lbs}in the boat so I bought a 15 by 15 pitch prop .It now revs to 4000 rpm with the trim up .

Do I need to change my drive or just keep trying different props until I find one that works ?

Any info is greatly appreciated !!

 

[Bifflefan]

Re: 350 to 305 swap

it depends on what your trying to do.
if all you want is to get the RPMS up then just go to a shorter prop.
My real questions is why did you goe from a 350 to a 305?
you lost a ton of power....

 

[jtybt]

Re: 350 to 305 swap

ditto!

 

[pugwash sunray]

Re: 350 to 305 swap

After searching for 2 months this spring and no luck finding a 350 that I felt was worthy of rebuilding I went with the 305 ,I had it bored .30 and installed flat top pistons it has a marine cam and a high rise intake .
And for the little horse power difference in the 2 ,I thought it would work fine .
Seems I was wrong.

 

[IVAZ]

Re: 350 to 305 swap

The HP might be slightly different, but the torque is considerably higher in the 350 and that?s what gets you out of the hole. Big step down in my opinion.

Do you know if the 305 and 350 use the same gear ratio? What was the reason for the engine swap? Does the new engine backfire or run rough?
The "adults only" section has a list you can go thru if you can?t reach WOT. Try troubleshooting it using that list, ask here anything that you don?t understand.

 

[pugwash sunray]

Re: 350 to 305 swap

The HP might be slightly different, but the torque is considerably higher in the 350 and that?s what gets you out of the hole. Big step down in my opinion.

Do you know if the 305 and 350 use the same gear ratio? What was the reason for the engine swap? Does the new engine backfire or run rough?
The "adults only" section has a list you can go thru if you can?t reach WOT. Try troubleshooting it using that list, ask here anything that you don?t understand.

The 350 uses a different gear ratio as far as I know .
The previous owner did not winterize it and anything that had water in it froze and broke... motor, manifolds, oil cooler even the thermostat housing.
The engine runs smooth and has never backfired .

jason

 

[Bifflefan]

Re: 350 to 305 swap

And for the little horse power difference in the 2 ,I thought it would work fine .
Seems I was wrong.

As you read, I didnt say HP i said power.
I wouldnt say wrong, just uninformed..
You should be fine tho,
but those block are still a dime a dozon. i cant belive you couldnt find one. I dont even like GM and i can find a ton of them every where.

 

[pugwash sunray]

Re: 350 to 305 swap

Ouch ....
Look guys I did not come here for sarcasm on my engine choice or to have my choice of words picked apart ,I'm only looking for some help here!
I also do not like gm engines but that's what I have {sorry} And as far as your briggs and stratton idea I'm sure you can find a better place for it then I.

I'm going to try a 15 x 13 pitch prop I'll let you know what happens.

 

[cooter2506]

Re: 350 to 305 swap

I think you are barking up the wrong tree. Dont change props find out why the engine is not running right. Youo should be in the 17-19 pitch range. I think 4000 rpm is all your engine has. Go up to the adult only sticky and read and do ALL the suggestiions under cant reach WOT. You will be happier once you find out why engine is not running up to snuff. As far as the gear ration I think they are the same for the 305 and th 350 but I will try to find out and post back.

 

[dan t.]

Re: 350 to 305 swap

305-2 bbl,1.65 ratio
305-4 bbl,1.50 ratio
350-2 0r 4 bbl,1.50 ratio

 

[pugwash sunray]

Re: 350 to 305 swap

thanks guys , I'm going to re-check my timing and go from there.
jason

 

[Gary H NC]

Re: 350 to 305 swap

I think he has other problems...that 305 should still turn 4800 rpms..
Did the old 350 have a 4bbl carb?

 

[pugwash sunray]

Re: 350 to 305 swap

Yes, the 350 had a rochester 4 barrel. I used the intake and carb on the 305 and even tried another carb with no difference in performance .

 

[bruceb58]

Re: 350 to 305 swap

I had a 20.5' Bayliner with the 305 engine, Cobra drive, 4bbl carb and I swung a 17" prop at sea level and a 15" prop for high elevation.

if you have a 1992 OMC the distributor will more than likely have an electronic pickup in it. If you happen to have the older distributor, make sure your dwell is correct and your timing is set correctly and that it also advances to spec. I am assuming the advance curves are the same between a 305 and a 350 but I may be wrong here.

What is the history of the boat? How did it run with the old 350? Was the boat not used for awhile and have old gas in it? The more info the better.

 

[dan t.]

Re: 350 to 305 swap

the 1.5 ratio should work just fine, with a 305,I had a 21 ft Campion with a 305 2 bbl,1.5 ratio,and swung a 15x17 prop with no problem,worked even better when I put on a q-jet

 

[pugwash sunray]

Re: 350 to 305 swap

I had a 20.5' Bayliner with the 305 engine, Cobra drive, 4bbl carb and I swung a 17" prop at sea level and a 15" prop for high elevation.

if you have a 1992 OMC the distributor will more than likely have an electronic pickup in it. If you happen to have the older distributor, make sure your dwell is correct and your timing is set correctly and that it also advances to spec. I am assuming the advance curves are the same between a 305 and a 350 but I may be wrong here.

What is the history of the boat? How did it run with the old 350? Was the boat not used for awhile and have old gas in it? The more info the better.

The boat sat for 3 years .I just bought it this spring so I have know idea how it worked with the 350 . I changed all the fluids and drained the gas tank the topped it up with high test .

thx for all replys
jason