Main jet question.

[Yard Dogg]

I had my carbs apart earlier today and noticed something that doesn't seem right. The Clymers manual shows my bike having a 137.5 main jet stock. I just switched to Harley mufflers and have read that normally you would go up one size with the pipes but it's not needed. Well I went ahead and ordered 140 main jets, but discovered today my bike had 110s in it. Last year when checking my plug color they did look white, and I was getting right at 40 mpg. I thought I just had the floats set too lean, but now I think it may have been those jets. I decided that since I have the 140s I would install those and see if it does good. I'm just wondering what main jets others have in their 80 Specials. I think I would rather foul a few plugs than to burn up my engine, whats your take on this? Are 110s normal?

[gonzo]

I ran into the same thing, I needed a carb rack and bought one off ebay and expected the bigger jets but these were 110"s. I,m running them on a 1981 with stright pipes, stock air box and getting tan plugs, air screws out 2.75 turns. runs fine so far did not even run a tank of gas thru it though so not sure of milage.

[petejw]

hey yard dogg,
the 80 model carbs are a bitsa model,
they re a cross between the early model and later model carbs,
if the carbs had 110 main jets the 140 will be way to big,

the early model carbs had the pilot system pick up fuel from the
main jet resulting in a bigger main jet, the tower where the pilot jets are fitted are blocked off,
the later model carbs uses the pilot jet tower to pick its fuel up, resulting in the smaller main jets.


if your running a bit lean and every thing else is stock u can always richen the mixture screws out a bit.


also if your only changing mufflers theres no need to rejet as youve already
stated.

[dbeardslee]

they re a cross between the early model and later model carbs,
if the carbs had 110 main jets the 140 will be way to big,

There is one particular model of carbs on some early '80 models that are the 'bastardized' variety. On the outside they look like 80/81 carbs, but on the inside they're different. They use a rubber plug in the pilot jet tower, and their pilots draw their fuel from the main tower, and use larger main jets, just like the earlier model carbs. I've seen at least one set with the plugs removed and the jetting all boogered up. You can easily tell if you have this carb type by looking down into the pilot jet tower. If you see a little hole about 1/2" down into the tower, pointing towards the main tower, then you've got the screwball carbs and need the plugs (if you don't have them already) and the larger jets. No hole - then you've got the later carbs and the smaller jets are correct without the plugs.

[rpgoerlich]

I had work on an 80 special recently which had 110's on 1&4 and 120's on 2&3
The pilots drew directly form the float bowl, no plug.

Georgefix on Ebay has a kit for an 80 Midnight special which has the 110 & 120 jets.

Every Carb fiche for the 80/81 midnight special I've seen shows 110 main jets. They also show a plug/screw over the pilot jet though... Go figure.

In the back of your manual, check see if there is a supplement for the 80/81. Mine shows the LG & LH to have 110's on 1&4 and 120's on 2&3.

[mro]

1980 Yamaha changed to the BS34 III from BS34 II carbs.
There has been a few who have reported a "cross over" version carb mentioned above, but I've yet to run across a set.
To meet "emission" regs. here the 80's ran on the lean side. Also some 80+ experience a knock (ping/pre-ignition) mostly under hard acceleration which Yamaha mentions as normal and said it was not a problem.

I’ve found 110, 115 and 120 mains in G’s and SG’s.

That suggests to me that Yamaha was trying to find the best running spot in there carbs while still trying to meet regulations and were not happy with 110 mains. Some have the inner carb pair mains up one, some don’t.

I experimented on a G with BS34 III carbs that I thought was too lean. Slowly increased the fuel level without getting good results to the point that it affected performance.

On that particular bike (stock everything) I found going up to 115’s seemed the best I could get. Bike had very good performance with the 110’s but white plugs plus the knock. With the 115’s still ran very good, mileage was the same and the “knock” was slightly diminished.

If it runs good with non stock mufflers but seems lean I’d try a few different mains and keep the floats set to original spec’s. If the performance is still there after a change then check plug color and mileage. Short of a dyno tune not much else you can do.

mro

[Yard Dogg]

You know I'm such a dummy. I've got a the carb flow diagram right on my desk top to show the insides of these carbs and how they work. My carb is the later model one which does feed directly from the main jet. I'll put my 110s back in it. It's kind of funny one of you mentioned switching 2 and 3 to a little larger jets. I noticed when tuning mine, I had a real hard time with 1 and 4 setting the pilot screw, in fact, they had almost no affect. 2 and 3 ramped right up when I hit the sweet spot. All my mains were 110s. I set the fuel level last night so they are all gnats ass. I'm hoping a smooth sync will happen this time.

As far as the pipes, I haven't had a chance to ride it much except for a few miles. I've got a feeling it'll be dialed in good today.