Carb Questions

[gaillarry]

My 79 has been difficult to start and bogs down until warmed up - ever since I got the bike. Since its winter, time to clean the carbs. Pulled them this morning, no green crud, look pretty clean. Some questions:


1- mixture/mixture screws were all at 2 3/4 turns out - seems too much to me - book states 1 1/2 turns out, my bike is all stock.

2- got replacement slow jets - how do I get the old ones out, they are pressed in.

3- the diapragham on each carb - should there be an o-ring - circled area in red?

[Mainerider]

Hi,

Given the age of the carbs and their unknown history, once the cleaning/rebuilding is completed I would re-set the pilots as though they had been replaced and follow the procedure outlined in the service manual (using an external tachometer and adjusting the pilots one at a time). Do you have the factory or Clymer manual?

The press fit jets will come out with some careful pulling and twisting but are you sure you need to replace them? Your running issues don't really sound like a slow jet problem; because of the tiny passageways, the pilot circuit is much more often the root of the problem. Did you buy the replacements to address a known problem? If not, I would just clean the originals and see how it runs after re-setting the carbs.

Yes, there is an o-ring that sits in the carb body recess that you circled. Look closely if you haven't already; sometimes they stick in the hole and you overlook their presence. It may be there.

[Volker_P]

My 79 has been difficult to start and bogs down until warmed up - ever since I got the bike.

More or less the same with mine.
In fact a CB650 engine typically behaves poor as long as it is cold. Careful adjustment of everything and afterwards carbs synchronizaton may bring some improvement.
If you really want a bike that runs smoothly and has a useable throttle response from the beginning on, have a look at pinhead's experiences in the Project 100mpg thread.

[gaillarry]

Thanks for the replies.

Couldn't find an o ring for the area circled in my pic. Looked on the parts diagram can't find a part number for it - anyone have it? I guess I'll order 4.

I'm using the factory manual, don't have a Clymer. I ordered the pilot jets but can return them to the dealer if not needed.

[Mainerider]

Thanks for the replies.

Couldn't find an o ring for the area circled in my pic. Looked on the parts diagram can't find a part number for it - anyone have it? I guess I'll order 4.

I'm using the factory manual, don't have a Clymer. I ordered the pilot jets but can return them to the dealer if not needed.

Are you missing all 4? If you need only one, I have one from a spare 650 carb rack that I can give you. Just FYI, very recently I refreshed my carbs. As stated above, these bikes are known for being very slow to warm up and my bike ran fantastic once fully warmed up but needed the choke for a fair amount of time during initial running. However, once I had cleaned everything in a dip tank and had re-set the pilots using my timing light tach, the bike runs much smoother right from the get-go. Keep in mind though, though, that these are mechanical slide carbs and as such, off-idle throttle response will never match a cv carb; the fueling transition from pilot to slow jet circuits is cruder (a cv carb's butterfly throttle valve allows a much more precise and smooth transition). Ride an '81 or newer CB with the CVs and you'll see what I mean. The only real advantage that the mechanical carbs have over CVs is that, all else being equal, a slide carb will flow more air than a CV carb of the same venturi size, because even when fully open and laying horizontal, the butterfly valve downstream of the venturi does restrict air flow. Other than that, the CVs have it all over the slide carbs (especially the CVs much lighter throttle action and the nice 1/4 turn throttle movement-the slides need tall linkage to move the slide and needle fully which means a lot of throttle turning, and stiction/wear on the slides caused by engine vacuum requires a very heavy throttle return spring).
That being said, get the carbs cleaned, pilots set properly, and these carbs will work fine for you; Ii will make you a great bike.

[gaillarry]

I have to look at the other carbs and see if they are missing an o ring.

Looking at the carbs I suspect I have to separate them to get at each one or I may try needle nose pliers.

I'm hesitant on separating the carbs. Just my luck I'll need o rings on the T fittings.

[Mainerider]

I have to look at the other carbs and see if they are missing an o ring.

Looking at the carbs I suspect I have to separate them to get at each one or I may try needle nose pliers.

I'm hesitant on separating the carbs. Just my luck I'll need o rings on the T fittings.

DON"T separate these unless absolutely necessary. That's opening a potentially huge can of worms; it's a nightmare putting them back together... don't go there

Less is more in this case; spend $20 at NAPA on a 1 gallon carb dip tank, clean everything as outlined in the factory manual, reset pilots using the service manual procedure and see how it runs.

[gaillarry]

Agreed, no need to tear them apart. I have a small ultrasonic cleaner and the jets,needles are cleaning up really nice.

I'll finish cleaning them. I have a spare o ring. I'll make sure the washer/oring for the pilot screws go in proper sequence.