Larger Jet on Stock '79 CB650?

[hapless]

Greetings, wise forum-folk!

I've recently come into a red 1979 CB650 with the stock PD carbs and the usual idle and tuning issues. I am into the carbs for the second time, having made all the usual mistakes, and now fully resolved to do a proper cleaning. And yes, the tank is clean, thanks to an EvaoRust, water flush, and alcohol job. There's a new inline filter between the petcock and the carbs now, too.

I have the stock 35 press-fit jets out. They seem to be a match for press-fit Keihin 3Ls, which are available from vendors in a few sizes up to 45.

The Question: Is it worth replacing with larger idle jets to compensate for the lean factory tuning? I'd gladly trade a bit of mileage for easier starts and a more controllable idle.

My bike has the stock US airbox. The only non-stock item is a set of Kerker 4-into-2. Judging by the patina, I'm guessing they're about as old as the bike.


Thanks in advance for all who take time to respond. I've been pretty deep into some twins of the era, but this is my first serious work on a four. I'd be roadkill without resources like this website!

[tario]

New jets would obviously help with fuel mixture problems. Dirty jets may be the problem though. There is also a fuel mixture screw that IIRC can adjust the mixture on each carb. Mine are 2-1/8 turns out, although there is a procedure to properly adjust this setting. I'd do everything you can to get the bike running correctly with the stock setup first. Then, ideally, you would ride the bike at about 3000rpm for a while, cut the engine and check the plugs. This should tell you what adjustments need to be made to the slow jets. Then with the slow jets sorted out you should be able to do the same thing at 7500rpm or so. this will give you an idea of what you need for main jets.

Because there is usually gas coming from both jets at all times you may have to repeat this process a couple times. I'd check it under load and while sitting still. Also if you have poor compression or other problems you could end up tuning the bike for any current issues. If you ever fixed these problems in the future the jets would not work as well.

I assume you have checked the plugs and they are very white? No brown/tan?