Ken,
You are going to need to rejet if you are not going to run the stock airbox. If you are racing this bike then are you going to be changing the exhaust then that will also require jetting changes. All that being said, here are some suggestions. Assuming that you will be running a different exhaust here is where I would start. First off I would slightly richen up the air screw about a turn. The problem is that the 81 Yamaha has the fuel screw sealed. Usually by a plastic plug but sometimes by ****. The fuel screw is on the bottom of the carb on the engine side in front of the floatbowl. If it has the plasic plug, or some of them were even aluminum, if I remember correctly, drill a hole in the plug, screw in a sheet metal screw and pull the plug straight out. If it has goop in the hole, start digging. The next thing I would do is raise the needle in the slide. I don't believe those carbs have adjustable needles so you will need to place a shim under the needle to raise them. I have used very thin washers. Honda used to have a shim kit for the 82 magna that was a warranty fix that worked great. If you have access to a friendly Honda dealer you might try and find that service bulletin and try and see if any of those shims are still available. The next problem is going to be main jets. I suspect they may not have to be changed much, but may need to be a size or two larger. Since you have a spare set of carbs I might try and use the spare set of mains and open them up a little. I have successfully used welding torch tip cleaners to do this. I found the size cleaner that would just fit in the jet and then use the next larger size to ream the jet out. It isn't very precise, but it does work. It would give you a starting point without buying a million jets that you won't ever use.
Based on your description you are running very lean. The 81 carbs were set very lean to start with for emission reasons and I havent found cv carbs to be very happy until rejetted.
I am going to be setting up an 82 Seca 550 as a race bike this year too. I plan on finding a set of carbs off a Seca 650 or 750 on the bike with a 4-1 header and see if I get get it going from there. So I am anticipating the same exercise soon. Good luck and report back.
Ken Essex
AHRMA 412
Vintage racing - old guys on old bikes
You are going to need to rejet if you are not going to run the stock airbox. If you are racing this bike then are you going to be changing the exhaust then that will also require jetting changes. All that being said, here are some suggestions. Assuming that you will be running a different exhaust here is where I would start. First off I would slightly richen up the air screw about a turn. The problem is that the 81 Yamaha has the fuel screw sealed. Usually by a plastic plug but sometimes by ****. The fuel screw is on the bottom of the carb on the engine side in front of the floatbowl. If it has the plasic plug, or some of them were even aluminum, if I remember correctly, drill a hole in the plug, screw in a sheet metal screw and pull the plug straight out. If it has goop in the hole, start digging. The next thing I would do is raise the needle in the slide. I don't believe those carbs have adjustable needles so you will need to place a shim under the needle to raise them. I have used very thin washers. Honda used to have a shim kit for the 82 magna that was a warranty fix that worked great. If you have access to a friendly Honda dealer you might try and find that service bulletin and try and see if any of those shims are still available. The next problem is going to be main jets. I suspect they may not have to be changed much, but may need to be a size or two larger. Since you have a spare set of carbs I might try and use the spare set of mains and open them up a little. I have successfully used welding torch tip cleaners to do this. I found the size cleaner that would just fit in the jet and then use the next larger size to ream the jet out. It isn't very precise, but it does work. It would give you a starting point without buying a million jets that you won't ever use.
Based on your description you are running very lean. The 81 carbs were set very lean to start with for emission reasons and I havent found cv carbs to be very happy until rejetted.
I am going to be setting up an 82 Seca 550 as a race bike this year too. I plan on finding a set of carbs off a Seca 650 or 750 on the bike with a 4-1 header and see if I get get it going from there. So I am anticipating the same exercise soon. Good luck and report back.
Ken Essex
AHRMA 412
Vintage racing - old guys on old bikes
