Topic: I need some Carb advice!!!!

[rallyegolf]

I just recently picked up a 79 CB 650, that wasn't running, the previous owner ( who claimed he was a decent mechanic) had taken the carbs apart, cleaned them and rebuilt them. But when I go the bike home I noticed that, the carbs where put together wrong. So I took them part and put everything back together.

Well the bike now runs but it will only run with the choke fully closed. I got it to idle a few times with out the choke, but as soon as you give it a little throttle it dies. So here is what I have done to the bike so far maybe you guys can give me some ideas on what to check.

The bike has a 4 into 1 header, along with pod filters. The main jets that where on the bike when I got it where 90 (but the manual says it is supposed to have 92) so I replaced them with some 102, (Based on a jetting guide I found online). I took the carbs apart scrubbed every inch possible with a tooth brush, and followed it up with some compressed air. I readjusted everything based on the manual, I bench sync'd them(drill bit tip) ( I even bought a set of Sync gauges) But I can not for the life of me get the thing to idle.......

If anyone has any ideas Please let me know maybe there is something that is staring me in the face that I am missing.....

[jason teamshralp]

not sure if the '79 cb650 carbs are the same/similar to my '78 cb550k carbs but the slow jets were a problem on mine until today. If they are like mine you have to actually pull them out, mine took a little force, then i cleaned them with carb cleaner, and a small singal strand of wire from a bike brake cable. Helpped with idel and the first 1/2 of throttle. this is the thread that helpped me figure it out.

http://forums.sohc4.net/index.php?topic=34058.0;topicseen

[TwoTired]

+1 on the slow jets.  Newbies just don't know to pull them, as they are a wedge fit into the carb bodies.  But, they provide nearly all the fuel for idle operation...

when they aren't plugged.


Cheers,

[voraciousc]

While I agree about the slowjets, I am not so sure that would be the problem here.  If it will only run with the choke full on and it dies as soon as you touch the throttle, it sounds like you may need to changed the needle jet setting.  This is usually something that is done when the switch to pods is made. 

Also it just occurred to me, after you bench synced those carbs did you lower the barrels down?  1/8 is what is recommended for the drill bit, but that is a pretty large gap as far as carbs go.

[BlindJoe]

The needle position shouldn't be much of a factor in determining the idle, the bike's not getting enough fuel through the slow idle circuit, the most likely culprit would be a blocked slow jet.

[voraciousc]

If needle jets have been moved up to attempt to adjust for the increased airflow, created by the pods, then that would cause the main jet to open up at too low of RPMs.  I do not mean to argue, I am just stating a possibility.

[TwoTired]

If the slow jets are plugged, operating with the choke on will force fuel through the mains/slide needle orifice, since the carb throat vacuum is abnormally high with the choke on.

Opening the throttle suddenly reduces the carb throat vacuum, and fuel starvation results.

Cheers,

[BlindJoe]

If needle jets have been moved up to attempt to adjust for the increased airflow, created by the pods, then that would cause the main jet to open up at too low of RPMs.  I do not mean to argue, I am just stating a possibility.

sorry I didn't mean to sound smug, its 2 am and I'm studying for a differential eqn. test; You're right about that but I don't think that's the case because if it were the carbs would be drawing in more fuel which in turn would require the choke to be somewhat open instead of fully closed.

[rallyegolf]

not sure if the '79 cb650 carbs are the same/similar to my '78 cb550k carbs but the slow jets were a problem on mine until today. If they are like mine you have to actually pull them out, mine took a little force, then i cleaned them with carb cleaner, and a small singal strand of wire from a bike brake cable. Helpped with idel and the first 1/2 of throttle. this is the thread that helpped me figure it out.

http://forums.sohc4.net/index.php?topic=34058.0;topicseen

hey thanks for the advice, I didn't pull the slow jets. I though a good soaking and a shot of compressed air would clear any blockage,  I'll pull em tonight just to make sure there is nothing clogging them.

[rallyegolf]

While I agree about the slowjets, I am not so sure that would be the problem here.  If it will only run with the choke full on and it dies as soon as you touch the throttle, it sounds like you may need to changed the needle jet setting.  This is usually something that is done when the switch to pods is made. 

Also it just occurred to me, after you bench synced those carbs did you lower the barrels down?  1/8 is what is recommended for the drill bit, but that is a pretty large gap as far as carbs go.

OK I didn't mess with the needle clip position. If the clip is in the wrong position will it affect the idle? I was under the impression that it made more of a difference at different throttle positions? Is there a clip position that should be used for a bike running filter pods and a 4 into 1 header?

[rallyegolf]

Oh and after  I benched sync them  I did close the throttle, there is probably a 1-2 mm gap in the slide. Do you guys have any tips for pulling the slow jets?

[BlindJoe]

look over this post http://forums.sohc4.net/index.php?topic=34058.0 it should help with determining your jetting.  As for pulling the jets, spray around the base of the jet with WD40 allow it to sit for a few min., get a pair of needle nosed vise grips, wrap some cloth around the jet to prevent marring it, set the vise grips to firmly grip the jets but not too tight. Twist the jet while pulling straight up and they should come out pretty easily.

[rallyegolf]

I'll give it a shot, thanks for the advice.

[Frankenkit]

...how do you get them back in properly?
It's probably really obvious, but I've never taken mine out and I'd like to know how to put 'em in before I try...

[Gordon]

...how do you get them back in properly?
It's probably really obvious, but I've never taken mine out and I'd like to know how to put 'em in before I try...

Just gently tap them back in until fully seated.

[jason teamshralp]

If the slow jets are plugged, operating with the choke on will force fuel through the mains/slide needle orifice, since the carb throat vacuum is abnormally high with the choke on.

Opening the throttle suddenly reduces the carb throat vacuum, and fuel starvation results.

Cheers,

This was the same thing that was happening to my bike and after the slow jets were cleaned she will run(once warm) with a quick twist of the throttle.

[rallyegolf]

OK I finally had a chance to pull the slow jets, they where a little dirty. So I cleaned them, re installed  everything, and its still doing the same thing.

It will only run with the choke fully closed, there has to be something else, that I'm missing!

[BlindJoe]

did you adjust the float height?

[TwoTired]

The slow jets are but one part of the slow circuit in the carb.  There are four holes in the carb body  for the slow circuit and fluid passage must be verified between all the holes.  The slow jet, the air jet, the adjust Screw hole, and the carb throat exit.

Are all your head pipes hot?

Cheers,

[capriccio]

are there air/idle screws on the carb?  if there are, try turning them out (or in, depending on whether it meters fuel or air) and see if it runs any better.

[rallyegolf]

Thanks for the responses guys.......  Ok I checked the float height before I put everything back together everything checked out 12.5mm.

The Slow Jets where a little dirty, so let them soak in some carb cleaner, I also shot some compressed air through them. there pretty clean now.

The Airscrew/Pilot screw is adjusted 2 turns out at the moment.

Since the slow jets where fairly dirty is it safe to say the slow jet circuits are dirty as well?

Some one mentioned the needle clip position, would this have an effect on the idle?

Thanks

[rallyegolf]

So I took the carbs apart again last night, and picked up some Yamaha carb dip. I'm going to clean every thing just to make sure nothing is clogged. ....... I 'll keep every one posted when I get everything back together.

[rallyegolf]

Ok the Carbs are cleaner then they where when they came from the factory.......The Yamaha carb dip works great.....

But it still has the same problem, runs with the choke fully closed, as soon as you let it out the engine  dies quickly. I am thinking  I may need to up the slow jet size, but I am not sure. The bike has some popping/backfiring out of the carbs and I think that means it's running lean!

Any thoughts

[jason teamshralp]

have you checked your timing?

[bender01]

Pretty clean wont work. 7 hours to soak clean and put back together is fast. Ive found that the third time is a charm. Twice!...so it will be running fine late monday or tuesday.