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Greetings,

I have a 1978 Kz650B with a stock air box and a 4 into 1 Vance & Hines exhaust. What size main jets should I be using?

Take a look at this, it's on this site. Since we don't know what you have in there now, you'll have to open the carb bowls and check.

kzrider.com/filebase/download/32-kz650-c...ing-guidelines-kz650

102.5

I thought 102.5 was the stock main jet size?

Check your manual.
From memory I think the KZ650s were 92/95 mains.


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Wow I give you a straight answer and you question it. If you know the answer why ask the question?

102.5 s should be what you have stock.... you probably should check first as stated........ Baldy's probably right as just the header doesn't need much ..... 105s at most.

Adding pods would change things and maybe need 107.5s and with my set up I used 110s to 112.5s.

Baldy- Im not questioning your answer i just thought that 102.5 was the stock size and since the header was added i asuumed that the jet size would need to increase. I'll try anything to get the bike running smoothe for once. When i bought the bike the PO had 120 mains installed and the bike would barely run. Since then i've dropped down to 112.5 and the bike will run good for about 20 minutes then start acting real sluggish. When i check the plugs they are black.

I ran a Kerker for several years using stock jetting on a 550, so it is possible to run stock jets with a 4-into-1. It was a tad lean and would take longer to come off choke, but it really ripped on the highway. I have since rejetted differently to get slightly better cold-weather smoothness.

Have you checked the fuel level in each carb?

Everyone who doesn't know what their doing thinks when they add a header they must also run big jets. Not true unless you do other mods also.

Baldy,

Appreciate the help. There's a motorcycle shop close to my house that carries mikuni jets. Ill grab a set of 102.5's and put them in.