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Hello, This is my first time on the website and I've really enjoyed it. Now I'm here at the forum to ask for help with a situation. It's a 1981 Yamaha seca 550 that I'm building into my racer. The bike had been sitting for a long time,the carbs were junk. I found a set of used carbs tried to start the bike without the air box because I want to run pod air filters. The only way I could get the bike to start was to prime the carbs and hold my hand over the throats. once it starts it will slowly take throttle but doesn't idle. What I have been told by the self proclaimed motorcycle experts here in town is that i will have to rejet and syncronize the carbs to run these filters. I hope someone if not several of you can point me in the right direction.
Thanks,Ken
 

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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 gook. 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
 

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If you are planning on saving money on a race bike, then you might as well stop now. It is gonna cost you. You just don't know how much more than your budget was, yet. I am going to go with 650 or 750 carbs but don't plan on it being any cheaper to jet, it might end up being a little closer to jetted or maybe not. I think I will want the bigger carbs for the top end performance on the racetrack. If you are planning on racing, then what other chanes are you planning to make? I don't care for 19" wheels so I got an 18" one off a 550 Vision. One off a Seca 400 or RZ350 would have worked too. I also have a wider rear off a FZ750 but it isn't a straight bolt on like the front wheel. I stll need to work out the rear disc setup. I might go for dual discs and front forks off an FZ750 if I can find a set off e-bay cheap. Don't forget fork springs and oil if you are keeping he stock forks. What about rear shocks? the stockers are wimpy and sucked when new. Lots of ways to spend money before you even get it on the track.

Ken Essex

AHRMA 412
Vintage racing - old guys on old bikes
 
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