Topic: '78 CB550 idle problems

[Big Tom]

Hey,

Been ages since I've worked on this old bike... hell it's been sitting in a garage for over 2years now. Anyways I thought it's about time I pull it out and have another go at fixing it and possibly riding it (was in an accident with another bike... havn't riden since)

The story goes as this, the bike was owned by a friend of mine who drove it for a number of years but it never really ran right in that it wouldn't rev up over 3500RPM (probably just needed a carb clean) so after while he got mad and parked it... outside beside his garage without removing the gas/oil or anything... 6 years later I told him I'd like it and after alot of pestering he gave it to me for free providing I install a stereo into his car! Nice deal.

The bike is 90% original, couple pieces broke/scraped up but nothing severe. So my brother/dad and I tore the thing apart, and after 6 years of sitting those carbs were horrid... took me an entire weekend of cleaning to get them freed up and looking good. changed the oil, 4 new plugs, adjusted the valves, set the points with a dwell meter, cleaned/checked the brakes, replaced the chain/sprockets... basically all the little tune ups we could do without removing the engine.

Got the bike fired up, runs pretty damn good after being sitting so long, we kept tinkering with it and just slowly taking it out for a couple quick drives around the block etc before eventually taking it out onto the highway and letting it loose (oh, the no high RPMs problem is now fixed)

Everything seems to be going well except for one problem with the idle, at rest it would sit around 1200RPM (I think that's where the '78 are sopposed to sit???) and then for no reason after a bit of time it would shoot up to 1800RPM and not go back down until you drove around for a bit then it would settle for a time... then shoot back up.


Soooo... I thought maybe there was a leak between the carbs and the cyl but we tried spraying cleaner around the boots and the engines rev's didn't change and no bubbles seen so they seem good. The boots from the carbs to the intake air box also seemed good. The only thing I didn't get the chance to do and what i'm thinking the problem is... I never did sync the carbs, don't have the tools to do it and not 100% sure how to.

So right now I'm thinking it's the carbs need to be sync'ed. I've also heard someone say it could be the plug wires being faulty but I'm not sure how much merit I give that considering it seems to be running fine besides the idle.

Anyways, figured I'd post this problem up and see what you guys have to say. I'm heading back home this upcomming weekend and I'd like to try and fire it up and see how it still runs if I can (it's sopposed to be about -30C where I'm going)

Anyways, any help is appriciated.

Cheers,
Tom

[TwoTired]

So right now I'm thinking it's the carbs need to be sync'ed.

Good thinking!  Do it!

Enjoy the ride!

Cheers,

[SteveD CB500F]

Lloyd said it all.

We see a few similar questions here and the answer is usually "sync the carbs"

Do you have any guages?  There are threads here on that too. I use a Morgan Carbtune (eBay £20) which I prefer to the dial guages I had when I was a kid.

[Big Tom]

Lloyd said it all.

We see a few similar questions here and the answer is usually "sync the carbs"

Do you have any guages?  There are threads here on that too. I use a Morgan Carbtune (eBay £20) which I prefer to the dial guages I had when I was a kid.

Yeah I used to check out another forum on Greenspun or whatever it was called and got that impression any little quirk with the bike the usually solution was to sync the carbs. I'll maybe fire up the search feature later on tonight when I'm not at work and see what info I can bring up. I don't have any gauges for testing the vacuum BUT I do have some left over cash in my paypal account that needs to be depleted... might just have to look into a sync kit.

[SteveD CB500F]

Greenspun archives are available here:  http://www.glennstauffer.com/smf/  but unfortunately are broken at the moment.

[Jay B]

I use a Twinmax electronic manometer to do synchs. Mainly because I have one to synch the throttle bodies on my BMW. I bought some adapters that screw into the vacuum ports when I bought the unit. It only compares 2 inlets at a time. I pick a carb, usually #2 as my baseline carb and synch the other 3 to that. Works great if you like such gadgets.

[n9viw]

Boy, you happened into a grand deal there if you got a Morgan for £20! Only ones I've seen are $60USD and up except this one (http://cgi.ebay.com/CARBTUNE-II-CARB-INJECTION-BALANCER-CARB-TUNE-2_W0QQitemZ8037804406QQcategoryZ25644QQssPageNameZWDVWQQrdZ1QQcmdZViewItem) which still has 5 days to go! I'd LOVE a Morgan, but I'll stick with the merc sticks that I could afford. Someday... 

[SteveD CB500F]

Well this was 2 or 3 years ago.  As always with eBay, the longer you hang out, the more likely you are to find what you want.

[fotoguy79]

Big Tom,   there are little screws on each carb.  If you don't have a manometer to sync them try closing all of he screw (witht the bike off) and then open them each an equal number of turns.  Try 2 full turns on each first.  See how it runs.  From there, if the bike is still idleing bad leave it on and tweak them with small turns, but make sure if you give a quarter turn to one that you do it to the rest.  This isn't the best way by any means and if you can get a manometer then you should.  They aren't terrible expensive.

[TwoTired]

burris79,
I applaud your willingness to help.  However, you seem to be describing idle ar bleed screws found on the earlier carbs and not on the 78 Cb550K carbs.  Further, the air bleed screws do not synchronize the carbs, only the idle mixture of each carb at small throttle openings.

The 78 carbs have idle mixture screws that face downward on each carb.  These meter a fuel/air mixture directly before they enter the carb bore and are set with a very sensitive RPM meter capable of distinguishing 50 RPM changes.  They are then adjusted for RPM peaks at idle speeds.

The synchronization adjustments for the 78 carbs are found under the top covers of carbs 1, 3, & 4.  These are adjusted so that all carbs match the vacuum produced at #2. 

The earlier carbs are synchronized in a similar manner.  But, all carbs have an adjustment as part of the actuating linkage.

Cheers,

[fotoguy79]

Oh I see.  I was unaware of the change.  I have the 76 CB750 and I was unaware of the differences.  Thanks.

[jdpas29]

burris79,
I applaud your willingness to help.  However, you seem to be describing idle ar bleed screws found on the earlier carbs and not on the 78 Cb550K carbs.  Further, the air bleed screws do not synchronize the carbs, only the idle mixture of each carb at small throttle openings.

The 78 carbs have idle mixture screws that face downward on each carb.  These meter a fuel/air mixture directly before they enter the carb bore and are set with a very sensitive RPM meter capable of distinguishing 50 RPM changes.  They are then adjusted for RPM peaks at idle speeds.

The synchronization adjustments for the 78 carbs are found under the top covers of carbs 1, 3, & 4.  These are adjusted so that all carbs match the vacuum produced at #2. 

The earlier carbs are synchronized in a similar manner.  But, all carbs have an adjustment as part of the actuating linkage.

Cheers,

should the idle mixture screws be adjusted before or after synching the carbs?

[fotoguy79]

That's just what I did before I had my manometer and it was only a temporary fix.  The best thing to do is get a manometer.

[TwoTired]

should the idle mixture screws be adjusted before or after synching the carbs?

They should all have an equal setting before synch, and then fine tuned afterward.  However, if you know they were correctly adjusted prior to out of synch condition and you are not trying to correct some problem, leave them alone during synch and revisit afterward.

The proceedure order can vary a bit per the situation and bike history.
Completely stock bikes follow the settings in the manual.

Idle Mixture Screws and idle air bleed screws behave differently during adjustment.  IMS has a much more limited range of effectiveness and finer control of mixture.

Cheers,

[jdpas29]

thanks two tired.. i'll leave them all at 1 1/2 turns out until after the synch then do my idle thingy.