Topic: 1975 CB750 Build! "Kona" - Sold and gone.

[Ravie]

I still have tuning to do. I drive the bike to the grocery store a couple weeks back and it only runs good in the upper registers. Died on me at a stoplight and doesn't seem to have a lot of power from a stop. It's nice having the forum but not having a guy here with me to tune who knows what he's doing really sucks. I will get it eventually.

Pulled the front end off so I could replace the fork tubes. Dave B sold me complete tubes minus the lowers (I'm reusing mine...I even stripped and polished them!) which I'm cleaning up and rebuilding. Turns out they're from an F, which is fine by me! Actually it's funny...he sent them to me and I got to looking and the parts were different...so I researched and found out it was an "F" tube which has a longer spring and a different damper rod...So I got ahold of him and bought the other one! 

Anyway, I got the one I have so far rebuilt today. New damper piston ring, new copper crush washers, etc.. I put about 160cc of fluid in it from dry. Seemed right from the reading I did.

Anyone up for a trip to Topeka?  :-D

[Ravie]

Anyone up for a trip to Topeka?  :-D

Only if thats what it takes to get my gauges back?   

YES!  Will that work?  :-D  :-D  :-D

[edwardmorris]

Bogging at stop lights after warming up tells me its still running rich. Where are you now with the jet sizes?

[Ravie]

110 mains and 40 slows. I had 42 slows in and realized after some reading that big bores tend to pull more gasoline for a given jet size so if anything I should have gone smaller. I didn't have smaller so I went back to stock size. I am about 3/4 turn out on the idle mix screw right now.

[Davez134]

110 mains and 40 slows. I had 42 slows in and realized after some reading that big bores tend to pull more gasoline for a given jet size so if anything I should have gone smaller. I didn't have smaller so I went back to stock size. I am about 3/4 turn out on the idle mix screw right now.

I agree that you'd idle/slow circuit seems rich. Try a quarter turn out at a time (mine are set at just under 1 1/4 turn out) I had the same issue for a bit.

[Ravie]

I agree that you'd idle/slow circuit seems rich. Try a quarter turn out at a time (mine are set at just under 1 1/4 turn out) I had the same issue for a bit.

I bet I went the wrong direction. Out leans it out, doesn't it?  I seem to remember reading that somewhere.

[edwardmorris]

Out makes it rich, in makes it lean, but the air screws' working range is only between 11/4 and 3/4 turns, no effect after that when controlling the mix. Type up ALL the carb info here, lets get you sorted out

Carb body numbers (later EPA crud carbs work differently than the early ones)
Air screw type (solid/holed)
Air filter kind (choked up stock/EMGO or free flowing K&N)
Needle position
Baffled/Open/Semi-baffled exhaust
Stock coils?
Standard or hotter plugs?
Anything I missed?

[Ravie]

657B
Holed
Emgo
2nd from bottom
Semi-baffled. I've drilled it much bigger than it was. I can get a pic up if necessary.
Dyna 5 ohm coils with their standard copper wires and brand new NGK resistor Caps
D8EA plugs
110 Mains
40 Slows

[Davez134]

I'm sure that turning mine out leaned out the idle circuit. Both by symptoms and plug color

[edwardmorris]

Needle clip needs to go up a notch (to the middle one) to lean it out some more. That's what I'd start with, you may not even have to drop to #38 idles after that, but lets see.

@Davez, mine did/do the opposite, but mine are the early 7As, before all the EPA crap got in there...I'll have to dig up the post about that setting again, maybe tomorrow

[edwardmorris]

My convo with our well known legend




The description of idle shift with temperature matches a too-rich scenario. Remember this: despite the posts you read in the forums, these carbs are LEANER when the screw is turned inward, RICHER when out. The trouble with most people's "diagnosis" comes when the hoses are leaking vacuum: then they get past the 7/8 to 1-1/8 range of the screws. Once that happens, all bets are off: the whole range of these air screws is 3/4 to 1-1/4, after which the whole system goes dead rich for lack of vacuum control (beyond 1-1/4) or air availability (pinched off below 3/4 turn). This has always seemed to confuse everyone. So remember: 7/8 to 1-1/8 turn, and if you are outside that range, you need a different idle JET to get you back into that narrow range. (This is the foible of simple carbs like these, with no moving parts!)

[Ravie]

I have to admit, I'm a bit tired of working on it at this point.

I will get the carb rack off sometime in the next week or two. I got the new front forks together and installed and got the front end back together this evening. Decided to start the bike and waddaya know, number 1 isn't firing. *huge eyeroll* 

Pardon my stupid question but - if my issues seem to be idle and off idle, why do you think I should bring the needle up to the middle position? Doesn't that mainly just affect the higher ranges? I will do so, I'm just curious.

As a side note - I've noticed a lot of pressure building up in the oil tank. Like, so much that I can hear air seeping past the cap seal when I turn the bike off and if I open the cap I get a similar sound to opening a pop bottle...so there's a decent amount of pressure there. Why is this happening? I can guarantee you I made sure all of the passages in the tank were free and clear when I cleaned it.

[Ravie]

Okay. A fellow Corvair guy got me motivated to get out and get on the bike stuff. I pulled the rack off, raised the clip to the middle position (was 2nd from bottom), rechecked the float heights (Because....I realized after RTFM that I've been setting them incorrectly and they were all several millimeters off), pulled the plugs and cleaned them all (saving the new ones for plug chops). I also installed two small clear fuel filters with bronze filtering stones in them in each line from the petcock to the carbs (I was getting sediment in the bowls and I think that was key in my floats not always closing the needles). I also used some good band clamps for the carb side of the inlet rubbers because the CruisinImage ones were stripping with any amount of force.. 

While I was at it I drained out the 10W40 I've been using for tuning and break in (not much breaking in as the cylinders/rings were about all that got brand new parts) and switched over to some 20W50 Honda 4 stroke moto oil (just conventional for now)

Got it all back together and started her up and BOOM!  HUGE (or as Trump would say YUGE) difference. It'll now rev up to 7k without any problem. It struggled to do that before. It was still a bit rich at idle so I did end up going to about 7/8's of a turn out (had them at 1 full turn) on the idle screw.

I'm about to do a test drive. Will report back.

Ry

[Ravie]

Dear god, yes. Yes. Yes. Yes.

Took her for two short drives this evening. I was GIDDY. Ran so great! Rafi, I owe you a cool beverage.

Washed and waxed her, too. :-D

[edwardmorris]

Ride it some more and check a couple times, but really glad to hear its doing better. Check after a lousy stop and go city trip, and a fun freeway trip to make sure its all good.

[Ravie]

Cool. Will do. Tomorrow I think I'll pull the plugs and see what they look like after todays couple of rides. it's still rich enough at idle that I can smell it but I'm not as worried about that. It smokes much less now.

It's crazy how fast this thing is. What's crazier still is I haven't even had it past 3rd gear. 40mph is about 4k rpm which is the speed limit near my house. I'm not looking for any road stops...lol

[Ravie]

I'm not sure why I continue to update this thread. Certainly not because it's a popular build. Meh.

Rode the bike to work today. I couldn't NOT. It was beautiful sunny blue sky and already in the 50's when I left for work. It ran GREAT!  I'm hitting about 4k at 40mph. I don't even go past 3rd gear in town...lol

It still doesn't seem perfectly smooth just cruising there, really I could be nitpicking. It is still breaking in a bit afterall, too. I pulled a plug this morning before I left and it was a beautiful tan color...so I've got to be damn close.

Next door to me is the world famous Wolfe's Camera where a buddy of mine works. I alerted him I'd finally driven the bike to work so he snuck out and snapped some photos. Here's one:

[SOHC4 Cafe Racer Fan]

I'm not sure why I continue to update this thread. Certainly not because it's a popular build. Meh.

Rode the bike to work today. I couldn't NOT. It was beautiful sunny blue sky and already in the 50's when I left for work. It ran GREAT!  I'm hitting about 4k at 40mph. I don't even go past 3rd gear in town...lol

It still doesn't seem perfectly smooth just cruising there, really I could be nitpicking. It is still breaking in a bit afterall, too. I pulled a plug this morning before I left and it was a beautiful tan color...so I've got to be damn close.

Next door to me is the world famous Wolfe's Camera where a buddy of mine works. I alerted him I'd finally driven the bike to work so he snuck out and snapped some photos. Here's one:

Good looking bike, Ryan. Way to get out there even if it was a frigid 50s!

[70CB750]

Sweet!

[seanbarney41]

Keep updating...even when you think no one is reading, we are reading.  It is great to hear that you are sorted, or close to it.  Many others only come back  with more problems and questions.  You have done well at a task that many are unsuccessful with.  Excellent job.

[SKTP]

I feel my thread has also become unpopular...Your bike looks great

[SOHC4 Cafe Racer Fan]

I feel my thread has also become unpopular...Your bike looks great

Nonsense, Chris.  You have a bunch of members waiting to see your bike as it makes its way through the shakedown process.

[Retro Rocket]

I feel my thread has also become unpopular...Your bike looks great

Nonsense, Chris.  You have a bunch of members waiting to see your bike as it makes its way through the shakedown process.

Yep, fifteen and a half thousand views, stop whinging and get on with it....

[Tews19]

Glad to see it on the road! I remember when you first came to the site and posted pics. You have come a far way!

[Buckskin Bandit]

Enjoyed reading through this build thread! Subscribed so I can watch the future progress.