more carb questions for the gurus

[chopwatchagot]

was just wondering why the 79 has larger main jets than the 80 and can I use the jets from my 79 in the bodies of the 80 units?

[DiverRay]

The '79 and '80 carbs have a different design internally. The jets will screw in, but will be TOO LARGE in am 80 carb.
Any jet changes should be from STOCK ON THE YEAR THE CARB WAS DESIGNED TO WORK WITH. You can put a set of 79 carbs in place of the 80's, and tune from there if you want. It will make it easier to tune, and you may find a little better responce.
Ray

[chopwatchagot]

Thanks Ray. I think I'll go ahead and rebuild the carbs with new kits. Might as well do the '79 carbs while I'm at it.

[TADracer]

Yes, the 79 carbs are completly different and have a pressed in pilot air jet. Not sure about this on the 80's but the mains cannot be just swapped out because it will upset your air/fuel ratio.

If you are wanting to re-jet because of modifications, check the tech tips out on the site. I remember seeing a posting that gives guidelines for adjusting the size of your main and pilot jets for various stages of madifications. I have aftermarket 4 into 1 header and air filters. The guidelines allowed me to hit the mark right on.

[randy]

Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

3 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

[chopwatchagot]

So if my math is correct, I need to go up 4 jet sizes on the mains for having a 4into 1 and individual filters, [(3+2)-1] and go up 1 or 2 on the pilot jets. Next question, how do I adjust for altitude. I live at about 5200 ft. Since the air is thinner here than at sea level I should reduce accordingly the amount of fuel getting into the mix. Do I need to guess at this or is there a formula? I suppose the guessing and experimenting is part of the fun. Ok I'll stop rambling.

Dean

[DiverRay]

Dean,
At 5200', I would drop 1 size on the main and pilot jets. The pilot jet CAN be compensated with the adjustment from the pilot screw. You should be able to adjust as needed from this starting point.
Ray

[chopwatchagot]

Thanks Ray, All this math makes me wish I hadn't already bought the rebuild kits!

Dean

[skids]

The main air jet is pressed in. The pilot air jet screws in for all models. I think stock for 78 and 79 models is #210, and stock for later models is #180. It seems that the thread pitch is different, but Blane Hoops said that the threaded distance is so short that you can force them into the wrong pitch.

Originally posted by TADracer
Yes, the 79 carbs are completly different and have a pressed in pilot air jet. Not sure about this on the 80's but the mains cannot be just swapped out because it will upset your air/fuel ratio.