Author Topic: 1980 CB 750 K Not sure of problem- (Read 2666 times)

deanofall · « **on:** March 31, 2006, 03:50:25 PM »

So I bought this bike from a guy who had "restored" it. It ran when purchased and then later but it was leaking through the overflow drains on the carbs. I removed them and replaced the needles and cleaned inside and that seemed to stop the leak. (I couldn't find the adjustment for the float bowl heights and still can't.) I've read a bunch of posts on here and it seems that it should a be a tang that I adjust but I still don't see it.

Nevertheless, I got the carbs back on with the throttle cables in place and started it up. It started(with a backfire and then ran for a few moments and puttered out. I tried to start it again but no luck. I've tried spraying carb cleaner up into the airbox(a friends recommendation) and will try starter fluid next. I opened carb 4 float bowl and it was full of gas.

I've also tried using the vacuum hose (carb breather hose) so suck gas into the carbs and tried it then but no luck. I opened carb 4 float bowl and it was full of gas. I know that it should run because it did both before and after the carbs came off. After was only for a few moments, however. Any thoughts on some possible troubleshooting tips?

I know (and have read) multiple posts exist regarding things like this but none matched my situation exactly and I figured if I posted maybe I could find something new.

Big Jay · « **Reply #1 on:** March 31, 2006, 04:25:41 PM »

Is this an 80? A twin cam?

Jay

cb650 · « **Reply #2 on:** March 31, 2006, 04:33:37 PM »

Also if it is a 80 twin cam its cv carbs and there is no float adjustment. Not 100 percent sure.

Terry

MetalHead550 · « **Reply #3 on:** March 31, 2006, 04:42:51 PM »

Yours is a DOHC. You may try this site: http://www.cb900c.net/. Ill bet it has CV(constant volocity) carbs on it, which Im not too familiar with. As far as the tangs on the floats, look at the picture Ive included. That little tab in the middle is whats considered the "tang" and it makes contact with the needle which governs the amount of fuel allowed in the bowl. Mind those arent floats for your bike, but itll look very simular. With the carbs upside down, you'd bend the tab up, or towards you, to lower the float level, and therefore the amount of fuel allowed in the bowl before the needle cuts the fuel off. I think Ive got that right, kinda hard to visualize. Be careful bending those tabs and use a pair of standard pliers to grip just before the tab, and a pair of needle nose to bend the tab. Dont wanna warp the shape of the float! You may find the correct float hights for your carbs on the above site, but a generic way of setting float hights is to level the floats with the face of the carb body. That is, if you look at the carbs with the bowls off it would be the surface that the bowls mount to. Hope some of that makes sense!

One more thing, you know you have fuel at the carbs. Crank it over for a couple seconds, choke on, then pull your plugs and see if they are wet with fuel. You could be flooded? Good luck man.

MetalHead550 · « **Reply #4 on:** March 31, 2006, 04:43:50 PM »

Oh yeah the pic. And too slow!

cb650 · « **Reply #5 on:** March 31, 2006, 05:00:01 PM »

I believe the CVs had plastic nonadjusable floats. I know the 650 cv carbs are. Another reason they are not liked.

Terry

deanofall · « **Reply #6 on:** March 31, 2006, 05:13:21 PM »

Yes, as far as the floats go, I've questioned my sanity trying to find anything to adjust on it and its a single piece. That makes me feel better. I'm not nuts. At least not as far as the floats go. Time will tell on the decision to buy this bike, however.

If I did flood the engine, what should I do?

deanofall · « **Reply #7 on:** March 31, 2006, 05:14:19 PM »

BTW - thanx to you all for quick feedback.

BobbyR · « **Reply #8 on:** March 31, 2006, 07:13:50 PM »

Your essential problem is that it is not a SOHC.

Killer Canary · « **Reply #9 on:** March 31, 2006, 07:41:48 PM »

I've got a tear-down manual for the DOHC 750s; if you need specs, let me know.

cb650 · « **Reply #10 on:** April 01, 2006, 07:12:15 AM »

Turn off gas take out all plugs and cycle engine through a few revolutions. For safety turn kill switch off. Check the air cleaner also. might have a nest of something blocking air.

Terry

deanofall · « **Reply #11 on:** April 06, 2006, 07:47:54 PM »

Still won't start,

I made sure that they are in the heads securely. The engine is not flooded as I checked. The plugs are carbon fouled, cleaned w/wire brush.
They are NGK J10B.
=Checked coils resistance, 2.4 & 2.5 ohm, manual didn't specify what that resistance should be.

Voltage at coils testing has been problematic. Plan to check compression, also.

Anybody got any ideas on what to try after that?

prsman23@hotmail.c · « **Reply #12 on:** April 06, 2006, 10:12:44 PM »

i've got a dealer manual for the 80 750k it says "To adjust the float level, bend the float arm carefully until it just contacts the float valve." Float level : 15.5mm (.61 in) Note: Before adjusting remove the adjacent chambers.
Hope this helps. Also, the people over at cb900c.net are pretty helpful as well.

prsman23@hotmail.c · « **Reply #13 on:** April 06, 2006, 10:15:45 PM »

Also, spark plugs are supposed to be ND X24ES-U or NGK D8EA
Regards,
Jay

byidesign · « **Reply #14 on:** April 07, 2006, 08:56:27 AM »

Carb floats on CV- 1980 and up {plastic} are Non Adjustable type,Only Brass ones can be adjusted..
if you leave the fuel on, {direct feed to carb, not thru the Vacuum shutoff}
and they pee out the overflow, then,needle/seat, crud problems,{clean carbs}
Sometimes if you drain the bowls dry to clean or check for water,,and then try to start it up,
{models with Vac shutoff} you will not get enough fuel to start and run.
not enough manifold vac, when cranking to let enough fuel in.......{the Vac shutoff}
I took Mine cb650 {vac shutoff} out because if you let it sit 3-4 days it was a
pain to start{ after some fuel evaporated from carbs}crank+crank, now i
just shut the petcock off. bike fires instantly when I start..
Hope it helps
Bruce

deanofall · « **Reply #15 on:** April 07, 2006, 10:14:42 AM »

Hey, thanks for the feedback again.
I'm not trying to buck the system (I reposted my original post on cb900c.net back when the DOHC was brought up and have gotten a couple of responses. $:-\$ ) But this site seems more trafficked and the feedback has been good.

I have a question about the other tube coming off of the carbs. I assumed it was a breather hose to allow air to get out or in (as well as gas) more easily. However, it is attached to piping that rotates and goes to all four carbs.
My question, does the position of this tube leaving the carbs have an effect and what is the best position for it?

Thx

deanofall · « **Reply #16 on:** April 07, 2006, 10:15:39 AM »

Can anyone recommend a good site for a Yamaha 400 1983?

bill440cars · « **Reply #17 on:** April 07, 2006, 01:01:37 PM »

On that Yamaha, I assume you are talking 2-stroke? If so, try usa2strokers.com

I've got a 73' RD350 and I was sent there by one of the guys here.

Check it out. Later on, Bill

deanofall · « **Reply #18 on:** April 07, 2006, 01:06:59 PM »

thanks, I'll check it out.

cb650 · « **Reply #19 on:** April 07, 2006, 06:59:14 PM »

Quote from: deanofall on April 07, 2006, 10:14:42 AM

Hey, thanks for the feedback again.
I'm not trying to buck the system (I reposted my original post on cb900c.net back when the DOHC was brought up and have gotten a couple of responses. $:-\$ ) But this site seems more trafficked and the feedback has been good.

I have a question about the other tube coming off of the carbs. I assumed it was a breather hose to allow air to get out or in (as well as gas) more easily. However, it is attached to piping that rotates and goes to all four carbs.
My question, does the position of this tube leaving the carbs have an effect and what is the best position for it?

Thx

Well there is a vent tube and a vacuum tube. Vent on the 650 splits into each side then T's into the carbs.
The vacuum is a pos safety shut off that is best removed.

Terry

deanofall · « **Reply #20 on:** April 16, 2006, 03:24:13 PM »

The tube is attached to a "rail" that makes it to all the carbs.
I sucked on it when I thought the bowls were empty and tasted strong gas fumes. Moving the tube forward or backward seems to rotate the "rail" (piping that goes across). Does this rotation have any affect on the air/fuel mixture?

I have a picture of it. How you load pictures on here?

also, I replaced the plugs to D8EA correctly gapped at .028(I believe that should be correct).

I'm recharging the battery now to give it try.

Thanks for all the help.

Doug

cb650 · « **Reply #21 on:** April 16, 2006, 03:39:59 PM »

Heres the 650 cv carbs setup. 1 main fuel line that goes to one rail that goes through all carbs.
2 vent lines that go from 1/2 and 3/4 then into a T that goes to the air box. A vacume line that goes to a port in #2 carb I think then to the auto shut off POS.

Terry

deanofall · « **Reply #22 on:** May 01, 2006, 07:45:19 AM »

thx to all that helped, this beast finally fired. With the air filter out and some starter fluid, we had liftoff.

what is a good way to inexpensively tune these carbs?

Also the air filter looks like an oil filter but I couldn't find one on ebay. Is there a good used air filter source?

OR

Is there a good method for restoring an air filter(i've seem vacuuming and compressed air - I already tried vacuuming it but the crap is dried on)

cb650 · « **Reply #23 on:** May 01, 2006, 10:04:25 AM »

You should be able to get new for not to much. Maybe try a K&N.
Those carbs arent to bad once you get used to them.

Terry

deanofall · « **Reply #24 on:** May 02, 2006, 01:55:18 PM »

Yeah, just remember the petcock or you're out a tank of gas.

Thanks again.

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Author Topic: 1980 CB 750 K Not sure of problem- (Read 2666 times)

deanofall

MetalHead550

MetalHead550

deanofall

deanofall

deanofall

prsman23@hotmail.c

prsman23@hotmail.c

deanofall

deanofall

deanofall

deanofall

deanofall

deanofall