Carburetor Jetting

by David Schwarze and Barb and Eric

David Schwarze writes:

A while ago someone on the list mentioned that the stock 72-74 bus dual Solex carbs used the same jets as Mikuni carbs and that they might be available at a motorcycle shop. This in response to someone (maybe me?) who was expressing dismay at the fact that jets for the Solexes are very hard to come by. Well whoever that person was, I want to buy them lunch! I stopped by a Yamaha dealer today and asked to see Mikuni carb jets, and the guy brought out a box from behind the counter which contained hundreds of shiny gold jets in at least 50 different sizes, which looked to be functionally identical to one of my old Solex jets that I brought with me for comparison. Eureka!!! Only drawback was the price - $9.95 each. Ouch. Fortunately the guy cut me a quantity discount on four jets (two 132.5 and two 142.5) and I left without my wallet stinging too badly.

I have to give credit to Dave Kautz, if he's still on the list, for getting me started messing with the jets. I hate to admit it but before I heard him talking about jet sizes on the list I don't think I even knew what sizes of jets were in my carbs. Jets were something that I didn't need to mess with, or so I thought.

Eventually, in the quest for a better running bus, I saw the error of my ways and started experimenting with jet sizing. I'm only talking main jets here, the air correction jets are something I still haven't figured out entirely. Anyway, I started with 137.5 jets, which were the largest I could find. My reasoning was that it was better to run rich than lean, since it seems to be generally well known that a lean-running engine will run hotter than one that runs rich.

The bus ran okay with the 137.5's but I had a set of 130s also so I tried them for a while. I found that my mileage increased substantially and the bus ran well up to about 7000 ft elevation, although the CHT increased somewhat. I ran the 130s until last month when I put the 137.5s back in because of driveability problems in the colder winter weather. My guess is that the 130s combined with the cold air caused the mixture to lean out to the point that the engine wouldn't run right. I believe Dave Kautz told me that he runs 132.5 mains and I gathered that those would be about perfect for me as well if I could find them. Today I found them.

But why the 142.5s? Last Spring I got my original tranny rebuilt, which has rather short gears in it. In order to maintain 65mph on the freeway the engine has to turn about 4000 rpm. I haven't tried to figure it out, but 1 liter displacement at 4000 rpm pulling through a 26mm venturi (in the carb) seemed like a large bottleneck. The engine seems to run best in the 3500-3800rpm range now, and falls flat on it's face just before 4000 rpm.

So soon after the tranny was rebuilt, I started thinking about ways to make the engine breathe a little better up top. I considered converting to fuel injection, getting bigger carbs, or trying larger venturis in the stock carbs. Since the last solution was the easiest and cheapest, I tried it right away. I got a set of venturis out of some junk carbs and hollowed them out to about 28mm. The extra 2mm would get me 8% more airflow, which I figured should move my powerband up about 300 rpm. Put them back in the bus and it idled fine but had no power. I was getting ready to leave for a trip to California so I just put the stock venturis back in and forgot about the whole thing for a while.

Some time later I started thinking about why the bus wouldn't run well with the bigger venturis. It didn't take me long to realize... I was still using the 130 jets! There was more air going through without more fuel, which meant that it was simply running too lean. How could I have been so stupid to have overlooked that? (don't answer that!) I did some quick figuring and came up with 142.5 jets with 28mm venturis being an approximate match for 130 jets in 26mm venturis. I was only sort of excited about this since I didn't have a source for the 142.5 jets, but that problem has been taken care of. I probably should have gone for 145s since if my calculations are correct the 142.5s will be a little lean, but if it looks like it's going to work I can always go back and get the 145s and still come out way cheaper than a set of dual Webers.

So tomorrow the Grand Experiment will take place. I will yank the stock carbs, put 28mm venturis in place of the 26s and 142.5 jets in place of the 130s. If all goes well, I should end up with more horsepower, a higher freeway cruising speed, and slightly worse mileage. I will report back to the list with the results, good or bad, as soon as they become obvious.

Barb and Eric write:

You are right that running rich is better than lean, but over time running rich can ruin the rings by washing the oil away with the extra fuel. Checking your spark plugs for how your bus is running is important. You mention a main jet size you went to that worked well except in the high RPMs, the main jet probably was correct. When the engine enters the higher RPMs the air correction jet goes to work adding more fuel. You might want to try a higher # air correction jet, this has a smaller dia. hole in it's top which allows less air into it. Also important to have your volumn control screw adjusted correctly. There are three jets and two adjustable screws jets and all must be taken into account not just one, for each affects different operations of the carb. It is a bit of a juggling act, but if you look at the whole picture it is easy to adjust here and there for the desired affect. You might want to check all your jets and compare them to the recomended stock jets first then experiment a little with each.

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