my "new" 81 cb750f

[chrwilkins30]

Well now that I have them out I might as well clean it. You think I should just clean everything or rebuild it??

[shinyribs]

I dont really know alot about the DOHC carbs,but if it is running good I would think just a good disassemble and clean should do it. But if your rubber bits seem dried out or cracked you might as well replace them while it's apart.

[chrwilkins30]

it ran really good before pulling. Just the idle was kinda high when warm.  I think that im going to just clean everything really well and get rebuild kits on order. I need this bike running to get to work so I cant really wait until the parts come in. Thanks for the help.

[shinyribs]

Sounds like a plan! Always welcome,just wish I could help more. I'm still learning the DOHC's

[slownugly007]

Hey bud I've got a set of black drag bars that I've drilled for the stock switches. If you want them I'd take $20 + shipping, let me know

[chrwilkins30]

where are you located?

[slownugly007]

Jupiter,  fl

[chrwilkins30]

I cleaned the carb out really good and got the bike to start except every boot was leaking go it airtight and the bike runs great now. No more popping and I set the idle at 1500rpm.

When i took the seat of to do it all when I was done adjusting I took it for a ride without the seat on and I really like how it felt( if it had some cushion).

And I'll think about the bars. Whats the specs of them.

[shinyribs]

Glad you got it sorted out! Yeah,I rode mine the same way once. it does feel nice sitting down low like that. Much more like you are sitting in the bike,rather than on top of it.

[chrwilkins30]

yep.. Exactly.

On another note, I got stuck in a really bad thunder storm tonight. Was soaked!! but it was still pretty fun first experience.

[Johnnylevi]

Hey wilkins, where would you hook up the vacuum hoses on the DOHC? I've got one myself and have been wondering for awhile.

[Johnnylevi]

and what i mean by that is the vacuum hoses for the carb sync

[chrwilkins30]

http://www.youtube.com/watch?v=rge7Totfrio

Click the link. You can see where they go.

On another note. My bike started running like shit when I got off work to go to the gym. I've determined that its the gas I got after going for a 150mi trip the day before. It has since stopped and is back to normal.

It was backfiring at 6grand in neutral and slow acceleration throughout the rpms. My friends was doing the same thing and backfiring while cruising.

[chrwilkins30]

I drained the gas and put fresh stuff in. The bikes better but not 100%. If I rev it to redline backfires pretty bad. I'll prob have to pull the carbs again.

Also seem as if the timing chain needs adjusted. Any info on that.

[shinyribs]

Hmmm. A load of bad gas (water in it,usually) should be gone after a drain and refill. It IS possible that you picked up debris from the filling stations tanks. I learned that the hard way on my diesel pickup. I wont buy fuel from a station that doesn't have an in-line filter anymore! There's a sight chance you could still have some water lingering. Maybe some Stabil or something similar can ''grab'' the water and help it work it's way out. I would give that a shot and try running at least one more tank of fuel through it before I pulled the carbs. Pulling carbs SUCK! but you already know that!

There was some talk not long ago on here about adjusting the timing chain on the DOHC's. You should be able to do a site search and find that pretty easy. Just be sure that whatever post you read IS for DOHC and not SOHC. It's sooo easy to get that confused on here sometimes.

[chrwilkins30]

It actually seems like its all better. I filled it and put some sea foam in it and after 20 or so miles of beating on it, no more backfiring and no lag in power. We will see what it does tomorrow when I go to work.

I called a local shop on a chain and the cheapest one they had was $130. and a RK chain was $150. Seems pretty expensive just for a chain.

Also passed 10k miles today.

[sgtslag]

SeaFoam will absorb water, as well as dissolve varnish in the fuel system.  It's the cheapest/easiest thing to try.  Good call!

The DOHC CB750 bikes use a 630 O-ring chain, and, yes, they tend to be expensive -- wasn't used a lot, so minimal demand.  A lot of people swap out the sprockets, and convert to 530 chains.  I left mine stock, no regrets, as with lubing it every 300 miles, it should last 20,000 miles.  Cheers!

[chrwilkins30]

The guy on the phone said it was a 530 series chain stock.?

Also do you swap both sprockets?

[sgtslag]

My bad!  The 630 was used on only a few years of CB750's, I think...  Best to look it up in the FSM.  With regards to the sprockets, officially, yes, replace them both with the chain.  Unofficially, only change them if they are worn.  My mechanic only changed out my 630 chain, leaving both sprockets in place (2,000+ miles ago), and everything is good so far.  Cheers!

[chrwilkins30]

alright. I'll change out both then. Is it pretty easy to remove them.