CB360 Primary Main Jet Blues


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I have a rebuilt 74 CB360 with stock air filters, stock carbs running stock 68 Primary mains and 100 secondary mains and stock headers going into aftermarket mufflers.

The problem is the bike runs super rich at small throttle openings at 3500-5000 rpm, right where you ride around town, at a third throttle and above it runs great with plugs that look great, and it idles fine. Dropping the floats from standard 18.5 to 21mm helped as did leaning out the idle mixture(at the expense of the idle) but it still runs bad around town. On a bike with normal carbs I'd say the transition onto the needle was an issue and drop the needle one notch, but these carbs with that extra circuit are something else.

So will putting in smaller primary main jet help? If i put in smaller primary mains will I need bigger secondary mains? Or should I be putting in smaller secondary mains and shimming the needle?

Will these jets fit and work, they seem to be the closest I can find but thay are 8.5 mm long, mine are 7.2mm?

Will they work the same as the stock jets ie will a 65 be the next size down from my 68?

I can source these in Australia, otherwise any suggestions where to get the right ones?

There seems to be a lot of similar threads on here but usually with bikes with pod air filters running lean, or otherwise the threads go off on a tangent beforre addressing the issue. I hope someone here can help.

Common Motor has jet tuning kits for the 360 carbs. You may find yourself swapping the shorty mufflers for long mufflers. The way these carbs are set up, you get better performance going longer instead shorter than stock. They like back pressure, so going more open and short is probably the root of your issues.

IS PJ still doing modifications to those carbs? He is an expert when it comes to CB360 jetting. I think he has to modify the air circuits to increase air through that circuit. On the race 1000 motors, Honda fitted a zero sized jet ie blocked the primary main off and tuned on the regular main jet, but I am sure that PJ had a more street worthy solution.
If I run a stock carb on a 360, I get PJ to modify them every time. It's big performance jump for little investment. Otherwise, I run VM carbs. For a stock style bike, I think reaching out to PJ is worth it. (CrazyPJ) is his forum name.
It should be fine with stock jetting although you may need 105 secondaries.
If it's too rich at low rpm, most likely cause is pilot jets or primary mains have been enlarged (oversize from 'cleaning'.)
If I modify jets I always remove the numbers so there is no confusion 'down the line' but others rarely bother.
The 8.5mm long jets will work, get 68's if you can or 65 if you must try leaner.
I've tested 70's, too rich
All the aftermarket pilot jets have wrong size cross drilling and some are not the size stamped on jet,
I always check pilots and modify to 1970's Kei-Hin spec. You should have 38 pilots, could try 35 from a CB350 (350's go to 360 jerts if richermixture is needed to compensate for exhaust modifications)
I've got a fistful of jets on the way to try, I think the pilots are OK as I can get a rev drop adjusting both lean and rich. I'll start with the primary mains and see how it goes, expect it will make the secondary mains too lean but I have a fistful of those jets coming too.
I do love plug chops.
Thanks everyone for your help
Have you installed new aftermarket diaphragms? Mine wasn't even close to runnable until I installed new diaphragms from Honda.
I believe the obstacle is w the cut of the needles. Like PJ says, move down as much as 105 main primaries.
I found if you go smaller w secondary mains, it will want to die on deceleration to a stop. (Going too lean).
I cut the new secondary mains down to match factory length. In order to work w float height.
If you go larger w the secondary mains, its defeating the purpose of the smaller primary mains. As your back to adding more fuel right in the rpm spot your trying to lean.
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I know this is pretty old now but I recently did a set of carbs where primary mains were missing so I made some 'short' 65 primary main jets. (quicker than waiting for the 'correct' too long ones to shorten, he already ordered wrong ones and had them sent to me) Bike is running better than ever but due to weather has only done about 10 miles until spring comes around and weather warms up. (plus roads are clear)
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