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View Full Version : What jets on carb for a 81 S&S w/ 351W


98aggie
10-18-2010, 12:31 AM
I got the motor rebuilt this year, and installed a new fresh and raw water pump along with a new thermostat housing with a 140deg thermostat.

The distributor has no advance in it so I have timing set at 25deg.

But after running above 3000 rpm for a while the temperature will creep up to 160-165 range, when I push it harder it would go to 180 degrees.

So I pulled 2 spark plugs and they were white. So I assumed I'm running to lead with the fresh motor.

So the carb had 66 jets in it, I bumped them to 69. Took the boat out again and it ran cooler - stayed around 160degrees. Pulled a plug and still white, but with a slight tint on it. (I never got above 140 degrees last year, so I am not sure what it should really run temperature wise)

I am going to bump the jets again to a 71 or 72. Just want to check with y'all does these jet size match what y'all are running or am I going the wrong direction or need to go even higher.

Thanks for any guidance - since I don't have a PCM manual on where to set timing, etc for this engine.

Thanks
Kurt

98aggie
10-18-2010, 12:54 AM
I found something when searching on temperature 140-160 for direct cooling and 180-200 for fresh water cooled.

I have the heat exchanger below the header on the driver side so what cooling is this considered.

98aggie
10-29-2010, 11:47 AM
bump on the last question

Luv2Ski
10-29-2010, 11:57 AM
I suggest you first get the correct distributor in there. That is a very high base timing and will cause a variety of problems. Do you have the stock carb? Re-jetting after a rebuild should not be necessary unless you did some major changes.

There is no heat exchanger in a standard cooling system. If you have one, you must have closed cooling.

You will not hurt anything by running at 180 degrees.

thatsmrmastercraft
10-29-2010, 12:01 PM
I'm not sure what to think with that much timing advance either.

Do you really have a dist with no mechanical advance?

98aggie
10-30-2010, 10:11 PM
Yes it is a closed loop system on the engine. The distributor guts were replaced with pertronix. Doesn't look like there is any mechanical advance in it.

This was the original motor and was pretty worn down - heads pitted real bad. So don't know if the place got the motor back to the same specs are not, since they replaced the heads with new cores. I am just looking at the plugs to guess on fueling. it stays around 160-170 now but it used to run 140 last year.

Laurel_Lake_Skier
10-30-2010, 11:47 PM
I'd check to see what is (or isn't) happening with your timing advance before looking at changing your jetting (66 are the stock jets and likely on the rich side unless you've done some pretty major engine work).

You can check advance by connecting a timing light and watching your timing marks while reving the engine. As the engine rpm increases, you should see the timing advance up to about 2500-3000 rpm. Total advance should top out somewhere around 30-32 degrees. If you are at a base timing of 25 degrees, I'd expect you might have problems with hard/slow cranking when you start the engine.

thatsmrmastercraft
10-30-2010, 11:50 PM
The distributor advance weights and springs sit under the plate on the distributor where the Pertronix electronic takes the place of the original points. Get your face right up close with a shop light or a flashlight. Ste your timing to 10 degrees at 600 - 700 RPM in gear (in the water only) or 800 RPM in neutral (that will get you close). Once the timing is set properly, when you increase the RPM's you will see the advance increase. That should get you back to the 150 - 160 degree operating temp.

JimN
10-30-2010, 11:51 PM
I got the motor rebuilt this year, and installed a new fresh and raw water pump along with a new thermostat housing with a 140deg thermostat.

The distributor has no advance in it so I have timing set at 25deg.

But after running above 3000 rpm for a while the temperature will creep up to 160-165 range, when I push it harder it would go to 180 degrees.

So I pulled 2 spark plugs and they were white. So I assumed I'm running to lead with the fresh motor.

So the carb had 66 jets in it, I bumped them to 69. Took the boat out again and it ran cooler - stayed around 160degrees. Pulled a plug and still white, but with a slight tint on it. (I never got above 140 degrees last year, so I am not sure what it should really run temperature wise)

I am going to bump the jets again to a 71 or 72. Just want to check with y'all does these jet size match what y'all are running or am I going the wrong direction or need to go even higher.

Thanks for any guidance - since I don't have a PCM manual on where to set timing, etc for this engine.

Thanks
Kurt

You'll never get the best performance or fuel economy if you don't vary the timing advance. Get a different distributor and set the base to whatever works best with the cam and compression you're using. Don't even bother with the carb until you get your timing issues straightened out. The needed total advance can be in the 40 degree area and you'll never get that. What is the compression of the motor? You can't jet it or set the timing until you know this. For that matter, you can't even know what gas to use unless you know compression. What plugs are in it now? IIRC, the 351 uses NGK BR6FS or equivalent.

98aggie
10-30-2010, 11:59 PM
Right now when I tried to set it at 10 base it wouldn't idle, it starts real easy set at 25. and when rpms increase timing doesn't change. So I guess i will pull the pertronix to look to see if something is broke.

right now running autolite 25, going to step it down to a 24.

JimN
10-31-2010, 02:00 AM
Right now when I tried to set it at 10 base it wouldn't idle, it starts real easy set at 25. and when rpms increase timing doesn't change. So I guess i will pull the pertronix to look to see if something is broke.

right now running autolite 25, going to step it down to a 24.

It should have weights and springs under the mounting plate for the points/electronic ignition. These are rarely lubed, so they rust and freeze in place, which makes it sound just like what you're experiencing. The other problem is that the springs break and you'll only see the advance being the same at all RPM. Neither is acceptable.

If the timing mark jumps around, I would suspect that the distributor bushing is worn, too. If it is, think about just replacing the distributor- it'll be a lot less screwing around. Don't change the plugs unless they're not the OEM part number.