Hey Earlfor et. al. Exhaust & jetting ?

[KEITH KRAUSE]

Yes, the carbs synch could cause your problem. I also wonder if your throttle plates are opening completely. Normally, you follow your manual to make any fully open and fully closed position adjustments first. This would "manually" synch the carbs. Then you adjust the mixture screws and do the far more precise vacuum synch.
Your problem could be fuel starvation or poor venting too. Some easy checks are to be sure the fuel line is 5/16", the vacuum line is new, the petcock is completely clean, and the gas cap venting is clean.
Again, these are all normal maintenance things that need to be done anyway.
Do these checks, synch the carbs, and the bike should run well. If not, then things have to be re-checked more carefully.
If the Mac pipe is flowing better than the stocker, I would try one full size (5) up on the mains and test. You can make a small adjustment to your mixture screws to help with any "around town/lower speeds" issues. I really doubt any needle adjustment will be needed.

[KEITH KRAUSE]

By the way, are you sure the stock main jets are supposed to be 102?
What brand of carbs are on the bike? Keihins? Mikuni jets increase in 2.5 increments. Mikuni doesn't make 102, but they do make 102.5
DJ and Mikuni size their jets differently, I'm not sure about Kiehins, so I wonder what brand of carbs you have and if the E-bay jets are sized the same way as the jets that your bike has now.

[nabrams]

Kieth,

Yes, they are Keihin carbs, and I verified that the stock jet is 102 (according to my Clymer manual). It remains to be seen if the eBay jets are in fact for my carbs (the seller indicated that they are). I disconnected the fuel line from the carbs to verify unrestricted fuel flow (and it is 5/16").
My throttle plates move freely and open fully at full throttle (I verfied this when I had the carbs off the bike). I also verified that I do NOT have an adjustable advance mechanism (it's CDI).
I'll do the carb synch, but I suspect I'm going to put 110 jets in (when I get them) because I found someone on another site who said that was what he had on his 1978 CB750 that had a 4-into-1 exhaust. It is true that his bike is 3 years older than mine and his 4-into-1 exhaust was not the same as brand as mine, but I'm starting to suspect that increasing the jet size is allmost "standard operating procedure" for 4-into-1 exhausts. Even my Clymer manual states (just found this last night) that changing to a non-stock exhaust will likely require re-jetting.
I'm guessing the previous owner got the 4-into-1 exhaust because it was cool, but never had the patience to properly seal the pipes to the header and deal with the re-jetting. That's probably why he let the bike sit for three years. I can't confirm this guess because I bought the bike from the PO's father (he didn't have much history info).

Wish me luck. I'll let you know how it goes.

Thanks again.

[KEITH KRAUSE]

OK. Good luck. Like I said before, if the pipe flows better than the stocker, I would try (5) up on the mains, which would be 107. Of course, you don't have 107's, but 110's are fairly close.
And are you sure the advancer is working on the bike? Can you shoot it with a timing light and see movement?
Let us know how it goes.

[nabrams]

Hi All,

Well, I installed the larger jets (110) and the bike runs great now. I get great acceleration at all RPM ranges and the bike idles smoothly after it's warmed up. The plugs are light tan. All this is without the carb synch. I guess it will run even better after I synch the carbs. Luckily, I got this done before the snow storm here.
I got this bike with the intent of riding it for a bit and then selling it (I already have a 1975 Honda XL250 dirt bike and my trusty 1980 GS450), but it handles so well and now I want to keep it. I've been restoring a 1973 Datsun 240Z that will take up most of my tinkering time.
Decision, decisions - what to do?

Thanks again for the advice. Now if winter would just pass quickly so we Northeners could do some nice riding!