Topic: Vacuum leak on carb body? '78 550K/PD carbs

[Scott S]

 Bike is a '78 CB550K with the PD46C carbs.

 I've been chasing an idle/timing issue for a little while now. The bike wanted to idle high and no adjustment on the timing, idle mixture screws, idle screw, etc. seemed to solve it. I even tried two different mechanical advancers to make sure the advance mechanism wasn't the problem.

 I had checked for vacuum leaks around the manifolds, head and boots on a few occasions and found nothing. Then, today, I accidentally found this:

CB550 carb bushing leak?

 This is the only spot ANYWHERE that has an effect when I spray carb cleaner or WD40. It appears that the balance tube (?) has an air/vacuum leak where it enters the #2 carb body.

 The video shows a normal idle (I have a CB650 cam and CB750 tach, so I'm not 100% sure it reads correctly. When it shows ~1400-1500, the engine sounds best at idle. I don't think it's really quite that high). I can demonstrate killing the bike by spraying carb cleaner around the tube/bushing. Then, when I restart the bike, the idle rises and hangs.

 That's just a balance tube, right? No moving parts there. Is that bushing or O-ring or whatever replaceable? Can someone describe or show me what I'm dealing with in that area of the carb?
 Lastly, I know it's sort of a ghetto solution, but why couldn't I just seal that up with some RTV? Just to stop the vacuum leak?

[flybox1]

Looks like the rod that turns your butterfly valves, when you actuate your choke.  There is an oring there. Check the parts fiche for part number or get a generic one

Check this.  It's the same
http://forums.sohc4.net/index.php?topic=122137.0

[Scott S]

 Ahh, yes.....I think you're right. Still, that tube itself doesn't move. The choke butterflies must be inside that tube.

 EDIT: Yes, in fact, that "tube" does move when the choke is opened/closed. Dang. Looks like I'll be pulling the carbs in the near future. That sucks, especially on the later bikes. Those things are a ROYAL P.I.T.A to R&R on those models.

[uksparky]

I just posted a problem i have #3 cyinder lean...spraying carb cleaner on the plate bolted to the carbs....starts to run rough....also i did what you did on the throttle shafts ..same thing runs rough..I do have steady idle, not like yours high....check the rubber hoses in front of the carbs to intake, and spray on that plate....

[Scott S]

 I did. No leaks around the boots, manifolds, connectors, etc. Only at that one spot in the video.

[Scott S]

 I contacted SOHCDigger on the carb rebuild thread and he said that he was unable to locate those O-rings for the 550 sets he's rebuilt.

 Anyone have a measurement and/or source?

[bollingball]

First off I have a 78 750k and that o-ring came with the kit from the dealer so check with them. If only one is bad they most likely are all the same age. I also found them in the metric o-ring kits from northern tools. I see they only have two types now and don't know how they are with gas and e-gas. If you don't know the history of the carbs I would go with the dealer and go through the rack.
http://www.northerntool.com/shop/tools/product_200324966_200324966
Ken

[lucky]

That is what happens when people boil and blast carbs without thinking what they are doing. The throttle shaft seals take a beating.

[Scott S]

 These haven't been boiled or blasted....at least not by me.
 I was informed that on the 550 this is a felt seal, not an O-ring. True?

[chickenman_26]

I'm not sure of the cause of the high idle, but I'm sure it isn't that choke shaft seal. A leak on the air cleaner side of the venturi wouldn't cause that issue. But the fact that such a short blast of carb cleaner kills the engine tells me that it's likely one or more of the other three cylinders isn't firing at all, or maybe very weakly. That #2 cylinder is contributing so much of the total power that choking it with the carb spray kills the engine like turning off the switch.

[Scott S]

 Well, let me get the electrical issues sorted and I'll get back to work on this one!

[brewsky]

These haven't been boiled or blasted....at least not by me.
 I was informed that on the 550 this is a felt seal, not an O-ring. True?

There are both rubber seals and felt washers used.
Outside carbs use a steel bushing in the carb body and felt washers to seal.
Inside carbs have no steel bushing and use a rubber "seal"....not really an o-ring.
I don't remember the size right off.
You might be able to find a replacement seal at a bearing store, but the felt washers are unobtainable as far as I know. Some cut their own.
The only purpose they serve is to keep unfiltered air fron entering the inlet side of the carbs.

[Scott S]

 Thanks, Brewsky. But the pics you posted show the throttle linkage holes. My leak is on the choke rod. Still use the same type seals there?

[brewsky]

Thanks, Brewsky. But the pics you posted show the throttle linkage holes. My leak is on the choke rod. Still use the same type seals there?

OOOPS...sorry, still on my first cup of coffee!

Now that I think about it, I believe the choke shaft uses all felts, I can check later today to be sure.
I did try to replace them with o-rings, but, make sure they are a very loose fit on the shaft, or the choke spring will not be able to return the choke shaft to the off position.
I was unable to find a size that wouldn't hang the choke shaft.

[Lostboy Steve]

That is what happens when people boil and blast carbs without thinking what they are doing. The throttle shaft seals take a beating.

Learned that lesson quickly! Again I replaced all mine with orings out of an assortment from harbor freight. #$%* rubber but its working well for now.

[chickenman_26]

Well, let me get the electrical issues sorted and I'll get back to work on this one!

yes - carbs after all other tuning issues. But there's something I see in your video that's throwing up a red flag. Your #1 manifold rubber isn't completely on the manifold. In fact, it appears to be not even close to seated. Maybe one or more of the others are the same? If what I think I see is true, you actually have significant leakage that you just haven't found. Of course, I may be mistaken. Could you show us a clear photo of that area?

Stu

[uksparky]

I agree with chickenman..spraying one carb should not stall the engine....how about the other carbs too low idle/throttle closed...and #2 cylinder idle set too high....spraying that #2 carb would stall the engine....if that was the case

[Scott S]

 All four manifold boots are fully seated. This is a '78 bike and uses different manifolds than the '71-76 CB500/550's. There's a ridge or ring there. I'm positive the manifolds are seated. The boots have a ridge on them that fits into a ring on the manifolds.

 Once I get the electrical issue sorted (which started AFTER I found this issue), I'll do a full vacuum sync to make sure everything is kosher.
 I plan on working on it this weekend.

[Scott S]

 You know what....you're absolutely correct. Kept digging and looking and found this pic of the carbs from when I first brought home the bike.



 We ported/polished the manifolds and removed the rubbers. I think I have them installed backwards. Crap. One more thing to fix.
 Funny, though.....rode it for many a mile like that. I guess that explains why my airbox is so damn difficult to get on!

[chickenman_26]

You know what....you're absolutely correct.

Yes, I knew that. I was just trying to be tactful about it. The rubbers in your before-I-fixed-it photo are installed completely and correctly on the manifolds - the end with two ridges goes forward. At the carb end, the rubber should be touching the stay plate. You might find that new rubbers are a lot easier to install and seal better. Honda still sells them. IMHO, you need to correct this issue before attempting any more "adjustments," or you'll drive yourself nuts and accomplish nothing.  Have fun.

Stu

[Scott S]

 It's on the "to-do" list. The ever growing, ever changing to-do list.....

[Scott S]

 You know, it's amazing how easy it is to get the carbs and air box on when the manifold rubbers are installed correctly.

[TwoTired]

Felt on the choke shaft would act as not only an air seal seal but also a small oil reservoir to keep shaft to body wear minimized and rust free.  An o ring serves only half that function.

[chickenman_26]

You know, it's amazing how easy it is to get the carbs and air box on when the manifold rubbers are installed correctly.

Excellent. If there aren't any other hidden issues, synch those carbs, and the engine should idle at 1000 rpm smooth as a sewing machine.

Stu

[Scott S]

 Fixed the electrical issue and got the bike running. However, she still ain't right.

 I went UP on the mains while I had the carbs off because the bike wouldn't pull in high gear/RPM's.
 Started the bike with IMS screws at 1.5 turns. Still some snapping, popping and hanging idle.

 Loosened the throttle cable to make sure nothing was sticking or hanging.

 Crept up on the IMS screws...first 2 turns, then 2.5. The idle is much better and not hanging, so I think it was lean at idle.

 However, it's still snapping and sneezing a little on take off. Runs OK once I get off the idle mixture screws until I hit ~4.5K. It will pull to redline now, but is very rough. Almost like it's dropping a cylinder or something, but temp readings on all three pipes are good.

 Removed the snorkel on the air box. Seemed to help the idle but don't really think it was causing any issue.

 Did a plug chop. Had it in 3rd gear, wound it up to ~7-8K and hit the kill switch/pulled in the clutch. Coasted into the garage and pulled a plug and it looked good. Dark tan color....definitely NOT lean....but not too rich either.

 I'm frustrated and tired right now. I'm wondering if I damaged the Pamco when I pinched the wire (see my other thread: http://forums.sohc4.net/index.php?topic=122418.0 ) and it's not working correctly at upper RPM's?

 Current set-up:

 CB550 with 1.00mm overbore.
 ported/polished head.
 match ported manifolds.
 CB650 cam.
 Jardine 4-1 with baffle.
 Uni filter in stock air box.

 PD carbs with 110 main, stock press in pilot (42, I think), needle raised one clip. Idle mixture screws at 2.5 turns out.

 It seems like it was lean at idle and may still be, with the sneezing and snapping just after take off. But in the mid to upper RPM's, it almost seems ......rich?

 With 105 mains, it wouldn't pull to redline. With 110's it will, but it seems to run poorly after ~4,500 RPM up to redline.
 Is there a way to see if I'm getting spark/running on all four when I'm actually riding the bike?