Topic: Just changed main jets

[Pinhead]

I just changed the main jets on my '79 CB650 from the stock 90's to 102.5's. It is quite a jump in size, but all I have to say is, "WOW!" It has a lot more power at 1/2 to WOT. It'll idle beautifully at 1000rpm all day. There is a small problem, though. At a little over 1/4-throttle it stumbles, almost sounds like it's missing out. I'm assuming the idle jets need to be changed to better match the larger mains. Would I need to re-synch the carbs after simply changing the main jets? Also, after riding it for bout an hour today I came back smelling like gas; the exhaust smells rich.

My questions:

Would raising the needle clip (lowering the needle) smooth out the transition from idle jet to main jet? Or do I need to look for different idle jets?

I'm planning on drilling my exhaust for better flow and a little more sound. I've heard that less restrictive mufflers will lean out the mixture. Could this possibly bring the mixture closer to "normal?" I'm also going to remove the stock airbox cover to try to lean it out a bit, too.

I know these aren't exactly scientific changes to change the tune of an engine, but I basically got slightly larger mains (102.5's vs 98's) to counteract the effects of the above changes.

[eldar]

Well the exhaust may change the mix a little but not really much. Old fart says he can count the number of pipes that require rejetting on his hands. So drilling them out will only make a small change.

Have you tuned your low speed mix with a dwell/tach or shop tach? If not, do that and see if things stumble or if you still smell like gas.

If you do, then your mains are too large. Drop to 100s. A change from 90 to 102.5 is BIG jump on a relatively small motor. I suppose of you have slots on your mains, which I would be suprised since the 78 750s do not, try dropping the needle then and see if it helps.

[nickjtc]

I thought that the 'normal' increase in main jet size size for a 650 was up to a 98, everything else being equal and unchanged........

[Short Round]

it sounds like the larger jets are not getting enough air.  removing the air filter cover and holding the air filter in with washers might help a little if it doen't then you might want to try pods.

[Pinhead]

I tried pulling the air cleaner and it bogged out and died. I'll have to try the needle jet adjustment and exhaust before I test anything else. Hopefully the exhaust combined with the filter cover will lean it out enough to make it run smoother.

[eurban]

Pinhead-  As Eldar alluded to, have you actually checked to see if your main jet needles have multiple slots?   The last year for the adjustable slots in the 750s was 77 and I would guess the same antitampering restrictions would apply to the 650.  You may need to use small electronics washers to shim up the needles. . . . . You should probably get your mains set prior to messing around with the needle position.  It does sound like you have gone a couple of steps too far in your main but the best test would be topgear WOT plug chops.  This can be a bit scary!  Good luck.

[Pinhead]

I was afraid I went too big. But like I said, it runs great 1/2 throttle on up. No sputtering, no missing, just good constant power. I'm going to take a highway ride tonight to see how it does closer to the "top end" (65-80mph or so, don't want to get pulled over). I've only riden around town, 50mph max. If it's missing at WOT, high speed, I'll first try removing the air cleaner cover and if that doesn't work I'll be ordering a new set of mains.

98's or 100's. The site I ordered my mains from jumped from 75 to 102.5 so I ordered the closest to 98's that I could find. Where can I get the 98 mains?

[Pinhead]

I guess I got lucky with my choice of jets... I pulled the filter cover off the airbox and HOLY BEJEEBUS!! Front wheel comes off the ground in 1st and 2nd gears!!!  The rich exhaust smell is gone, it idles smooth at 900rpm, and once it's warmed up the "stumble" at 1/4 throttle is almost gone. Now I just need to drill the exhaust for a little more sound (it's quieter than my car right now) and I should be all set engine-wise. After that, let the cleaning begin!!

[Pinhead]

btt