Topic: 2 sets of 1974 CB550 carbs, drastically different results *update*

[cleveland]

I posted a while back about a 1974 CB550 I picked up.  When I start it the RPM's go through the roof!  So I checked the idle screw, as well I backed out the other screw 1 1/2 turns like the manual says.  The damn thing still shoots up to over 6K RPM right away when I start the bike.

So I pulled out an extra set of carbs I had laying around, cleaned the jets and put it in the bike.  Runs fine with the exception of gas pouring out of the bowls.  I know how to take care of that, as it is an easy fix.

So my question is why would 2 different sets of carbs, with the same size jets (138) give such drastically different results?  I am sure there is something I am overlooking, but what?

I doubt it will help, but here is a side by side of the two.

http://i92.photobucket.com/albums/l30/cb750caferacer/100_3017.jpg

http://i92.photobucket.com/albums/l30/cb750caferacer/100_3019.jpg

[paulages]

well for one thing, 138 jets?  i can't see why you would possibly need jets that large for a 550. sure you're ot talking about #38 pilots?

second, without knowing anything else about the carbs except that they run differently as is diagnosis is hard. i'd say the leaker at least has sticky needle valves, and that the high idler probably needs a proper cleaning or rebuild and a sync. why they run different? IF all of the internals are identical, then it's all a matter of proper specs and synchronization.

[cleveland]

I may have misread them, but I though they said "138" on the side, it could have been "#38". 

[jsaab2748]

Make sure the bell crank for the push cable isn't bent.
It don't take much, but if it's tweaked towards the center
of the bike, it can interfere with the eccentric adjuster for
closed throttle overtravel. And while you're at it, check to make
sure that the overtravel adjuster itself isn't out of whack. Either
condition can interfere with complete throttle closing, and you
end up not having enough adjustment at the throttle screw.
Goin by memory here, but had a similar issue with my 550, and it
turned out to be a bent bellcrank. Whatever it is, hope you get it sorted..

[cleveland]

Make sure the bell crank for the push cable isn't bent.

I think that may be the problem.  It does not appear to be bent though.  Is there a way to adjust it?

[cleveland]

I guess I was looking at the arm that the idle screw adjusts.  I went over the other carbs and the pilot screws on the other carbs were turned out 1 1/4.  I adjusted them, but it made no difference. 

[754]

With aircleaners off you can see if they are closing all the way or make a mark on the linkage.. sounds like they are sticking open... hanging up somewhere..

[cleveland]

With aircleaners off you can see if they are closing all the way or make a mark on the linkage.. sounds like they are sticking open... hanging up somewhere..

I think you may be on to something there.  I just shined a flashlight in there and the damn things are all the way open. 

[cleveland]

OK, so I saw that they were open quite a bit.  I took them off and unscrewed the idle adjustment screw all the way.  This adjusted them so they are open the same amount that the "working" carbs are, but in doing this the arm that the throttled cables hook up to are 1/3 inch farther away then the "working" carbs.  By default they open back up when I hook up the throttle cables.  So I fired it up without the cables attached and it idled at 4.5-5RPM's (as compared to 6+ RPM's before).  So it's a step in the right direction, but I am still rather clueless as to how to solve the problem. 

I know what your thinking, "just use the other carbs."  The problem with that is I would like to walk away from this knowing more then I do now.

 

[TwoTired]

Each carb has its own travel adjuster, used for vacuum synchronization.

Back the idle knob out so there is no limiting effect at all.

Peer into the carb throats and you should be able to completely seat all the carb slides.  If not, then loosen the travel/sync adjusters until you can close each and every one of them.  Now use the big idle knob to lift all the slides uniformly.
You will need to re-synchronize the carbs with manometers.  Just pick one as a master and adjust the others to it.  Set your idle with the big idle knob.

Cheers,

[cleveland]

TT

First of all, thanks!

I don't see the "Travel adjuster" on my parts list though.  What other name does it go by?  Did I just give away my secrete that I have a great lack of knowledge. 

Is it unusual for the same gap in 2 different sets of carbs to give such different results?  Here's a pic of each so you know what I mean.

http://i92.photobucket.com/albums/l30/cb750caferacer/100_3022.jpg

http://i92.photobucket.com/albums/l30/cb750caferacer/100_3024.jpg

Oddly enough, the cleaner set is the one giving me trouble.  Although, after reading you post I wonder if someone didn't clean them and screw something up. 

[Lumbee]

...theres one for each carb...its the threaded screw thats takes a flat head screw driver...should have a lock nut on it.  The 2nd pic looks like the slides are way up, and yes, it doesn't take much for the rpm to run off the scale if they are adjusted high...see pic...

[cleveland]

The big one on the bottom of the float bowl?  You can see it in the pics.

[TwoTired]

The big one on the bottom of the float bowl?  You can see it in the pics.

The big screw at the bottom of the float bowl is a bowl drain plug.  In the last picture it is facing the wrong way.  That bowl should be on the left hand carbs so the screw can be accessed while on the bike.

The Honda shop manual calls the part an "Adjusting screw".  See page 59.

Here's a pic.

[cleveland]

Sweet, thank you so much! 

I seem to remember a post some time ago that the person said they put a small screw driver in the throat to "bench sink" them.  You are suggesting that instead of doing that, I set them so that they are closed all the way, right?  If I am understanding this correctly, then I will be relying on the idle adjustment screw to open the slide slightly to allow air into the carbs.  Once opened slightly I can hook up my CarbTune (still in package ) to make the final adjustments.  Am I getting all this?

I wont have any more garage time until Wed, so I will post results then.  I might be able to sneak out sooner, but I doubt it.

Thanks again!  There is no way I would have been able to get half as far in that garage as I have if it wasn't for this forum.

Bobby

[Lumbee]

...you got it...you don't have to close them all the way...leave a small crack...1/8" maybe...you want them equal, thats why people will use a nail or something to "bench" sync them.  This will get you close, then u'r carbtune will finish the job...

[TwoTired]

Sweet, thank you so much! 

I seem to remember a post some time ago that the person said they put a small screw driver in the throat to "bench sink" them.  You are suggesting that instead of doing that, I set them so that they are closed all the way, right?  If I am understanding this correctly, then I will be relying on the idle adjustment screw to open the slide slightly to allow air into the carbs.  Once opened slightly I can hook up my CarbTune (still in package ) to make the final adjustments.  Am I getting all this?

Yes, you have the idea.  A few finer points:
 With the carbs off, you can put a strong light behind the bore and adjust each slide to where it "just" blocks the light you can see through the bore from the opposite side.  This is an effective "bench sync".  If you backed out the idle knob beforehand, now you know you will have enough idle knob adjustment range to make the idle as low as you care to have it, after the vacuum sync.

Cheers,

[cleveland]

I fixed them!  No way I could have done this with out you guys.

I used a drill bit.  The one on the right is before the adjustment and the one on the left is after the adjustment.

http://i92.photobucket.com/albums/l30/cb750caferacer/clevelandsvintagecycleparts/100_3108.jpg

http://i92.photobucket.com/albums/l30/cb750caferacer/clevelandsvintagecycleparts/100_3109.jpg

I picked up some gas line to set up the gas tank away from the bike to sinc the carbs with the Carbtune I picked up.  After hooking up the Carbtune and getting ready to sinc then I realized they were perfect!  What are the odds of that?  I guess it's beginners luck. 

http://i92.photobucket.com/albums/l30/cb750caferacer/clevelandsvintagecycleparts/100_3111.jpg

Here she is running!

http://s92.photobucket.com/albums/l30/cb750caferacer/clevelandsvintagecycleparts/?action=view&current=100_3112.flv   

I have a bit of white smoke coming out of the exhaust and is smells rather putrid.  I remember reading somewhere here about white smoke, but I don't remember what it said.  So I will do a search and that is the next issue to address. 

THANKS AGAIN!!!  YOU GUYS ROCK!!!

[754]

Funny, i read about the drill bit method in a Clymer manual for Z-1.. but they never mention it in the 754 one..

[TwoTired]

I have a bit of white smoke coming out of the exhaust and is smells rather putrid.  I remember reading somewhere here about white smoke, but I don't remember what it said.  So I will do a search and that is the next issue to address.

It's not an issue.  Just condensation in the exhaust turning to vapor as it heats up.

However, if after a thorough warm up, you see smoke with a bluish tint, then you have an oil issue.

Cheers,

[cleveland]

I just read your post to someone else saying pretty much the same thing. 

Next....

The bike has ape hangers and the controls have longer wires.  So I figure I'll redo the whole front end at once.  Rebuild the brakes, add a second set and a bigger MS, polish the forks (brakes too), replace the ears with after market ones, polish the triple, replace the gauges, paint the headlight bucket (hammered gray), and chop the fender.  Got my work cut out for me, eh.     

I'll start a thread for the project once I have done enough to show some progress.