Topic: Carb Vacuum Leaks on Top Cover. Should I RTV It?

[uncle_fester]

This winter, I've been working on getting a new (to me) '74 CB550 running again. I've done a full 3k service on it (including a good carb cleaning) and got it running, but I'm fighting some vacuum leaks on the carburetor. It usually idles okay initially at start, but the RPM is inconsistent after 30 seconds or so. If I rev it, it has problems getting back down to idle and floats around 3k, then often moves around from there. The RPMs are just really erratic.

I used some carb cleaner and found that the top covers for cylinders #1 and #3 directly cause the RPMs to drop. The cover surfaces are clean with new felt gaskets. Not sure what else I can do to keep it from leaking. Do the scratches on the surfaces look suspect? Can the top cover screws be too tight? Are these gaskets known to be difficult for leaks? Not sure what to do here other than maybe just do a DIY RTV gasket.

I may be hitting the linkages with the felt gaskets next to the covers with the carb cleaner, too. Are these common areas for vacuum leaks, too? Those were not replaced with the carb cleaning.

Just wondering on how best to tackle this... Any input is appreciated.

[DaveBarbier]

Don’t RTV them. Those surfaces seem good and the gaskets look to be in good shape.

In syncing carbs, most of the time (not on these carbs though) you remove the top caps to get at the adjusting screws. That doesn’t cause any issue with RPM raising or anything so I bet your carb cleaner is hitting another spot that’s causing the RPM change.

[uncle_fester]

Don’t RTV them. Those surfaces seem good and the gaskets look to be in good shape.

In syncing carbs, most of the time (not on these carbs though) you remove the top caps to get at the adjusting screws. That doesn’t cause any issue with RPM raising or anything so I bet your carb cleaner is hitting another spot that’s causing the RPM change.

Hmmm.... Okay. I'll keep digging. Thanks!

[Flyin900]

Don't shoot carb cleaner on the felts around the throttle shafts, as these will be compromised with age and some shrinkage already and a harsh chemical won't do them any favours. Most old carbs will suffer some leakage around the shafts, yet I don't feel enough to cause problems with hanging idles or running issues.

The top surfaces as noted seem quite clean and if the gaskets are new there shouldn't be any issues from there with leakage. Make sure there is a good seal at the insulators and check that all the cables are routed without any binding, or that the bellcrank where the cables are attached is resting in the closed position at no throttle.

[Bankerdanny]

The only reason to take off the carb tops on '74 550 carbs is to get at the slides and needles. Adjusting the carbs during a synch is via the slotted screws visible between the 1-2 and 3-4 carbs.

[uncle_fester]

Make sure there is a good seal at the insulators and check that all the cables are routed without any binding, or that the bellcrank where the cables are attached is resting in the closed position at no throttle.

The push and pull throttle cables seem to be functioning fine, and the throttle snaps back well.

What are the "insulators" you're referring to? When I did the cleaning, I replaced most of the o-rings with a kit from 4into1.

The pilot jet is original and was thoroughly cleaned as well.

EDIT: I assume you mean the rubber insulation boots between the carb and the intake manifold. Yes, I am going to replace these. They are old and kind of rough. Carb cleaner really didn't make a big difference on them, but they may be part of the problem.

[Flyin900]

The insulators or manifolds are the rubber part that the carbs are pressed into when mounted to the head. When they are old and hard from age they can sometimes leak air after reinstallation of the carbs. Test while running with propane or some other flammable material sprayed around the outside. I like propane as it doesn't leak liquid all over the place.
If the idle increases while running the test you have air leaks around the manifolds that need correcting. If you replaced the O rings on the manifolds then they should be good as far as a seal at the head.

[seanbarney41]

550's also have o-rings between the manifold and the cylinder head that chronically fail.

[my name is nobody]

Hey Unkle Fester. The 550 carbs are pretty picky about how things are adjusted, and if things aren't just right,
they don't function well. Have you tried bench syncing them? If they're out of sync with one another,
they will give you problems. Anopther thing that will cause erratic idle is the timing advance weights
being stuck wide open, or going to full advance too soon. Check for  weak springs in the advance mech.
as well. 550's are known for that condition. There should be no looseness, tensioned even at rest.

[jlh3rd]

I’m curious as to why the number one gasket is not showing the “sealing” ridge/ line  on a section of the gasket as the other 3 show all the way around. Could that show the #1 is not sealing good. I’d be suspicious.

[DaveBarbier]

I do not think the caps are leaking into the carbs. Even if they don’t seal that well, it shouldn’t cause an issue.

I would test this by removing the top caps completely and see how it runs. As I said before, to sync a lot of carbs you have to have the top caps off as the adjusting screws are inside. Doing this doesn’t cause any problems.

[uncle_fester]

Hey Unkle Fester. The 550 carbs are pretty picky about how things are adjusted, and if things aren't just right,
they don't function well. Have you tried bench syncing them? If they're out of sync with one another,
they will give you problems. Anopther thing that will cause erratic idle is the timing advance weights
being stuck wide open, or going to full advance too soon. Check for  weak springs in the advance mech.
as well. 550's are known for that condition. There should be no looseness, tensioned even at rest.

Hey, Jim. Thanks again for selling me that throttle cable bracket!

I did do a bench sync and was able to start a manometer sync. The fluid levels were pretty close before the bike started revving.

I have not checked the advance springs yet. That is a good idea.

[uncle_fester]

I’m curious as to why the number one gasket is not showing the “sealing” ridge/ line  on a section of the gasket as the other 3 show all the way around. Could that show the #1 is not sealing good. I’d be suspicious.

Good catch. Maybe the screw just needs to be a little tighter?

At this point, I think I'm going to place an order for some new insulators and the manifold cylinder head o-rings to see if that makes much of a difference. I'll update in a week or so....

[TwoTired]

The carb top caps are just dust covers.  They just need to seal well enough to do that.
Now if the slide or slide bores are worn out, particularly unevenly, perhaps a tighter seal could make some improvement.  But, then the carb is worn out!

For me, a fluid check, without first performing the whole tune up checklist, just on the top caps, seems a waste of time to me.

And, if the carb connectors aren't sealing...THAT seems a much bigger fish to fry.

Cheers,

[uncle_fester]

UPDATE.

I replaced the intake manifold o-rings and insulators, including a thorough cleaning. Same RPM issue. I used a propane torch around the carbs, and no RPM fluctuations were observed.

I'm suspicious of the jets. I think they're original, but I don't know for sure. I want to order a new set of genuine jets to be sure. Jet-R-Us appears to have some, but they list a primary main, a secondary main, and a slow jet. What's going on with primary and secondary mains? Isn't there just one main jet? They have different dimensions: primary (9.4mm x 4.95mm) vs secondary (9.6mm x 6.0mm).

Would I need a primary main jet or secondary main jet? It's for a '74.

[flybox1]

Only look at the slow/pilot and main jets listed here.....https://www.jetsrus.com/a_jet_kit_street/honda_550_CB550K_1974-1976.htm
Ignore the 'secondary' jets on the bottom of the page.

If your jets are OEM, they'll have the Keihin K logo stamped into them...

[uncle_fester]

Only look at the slow/pilot and main jets listed here.....https://www.jetsrus.com/a_jet_kit_street/honda_550_CB550K_1974-1976.htm
Ignore the 'secondary' jets on the bottom of the page.

If your jets are OEM, they'll have the Keihin K logo stamped into them...

Thanks. I think I'm just going to get new OEM jets regardless for piece of mind. Who knows how they may have been handled or cleaned before my ownership...

Out of curiosity, do you know why there are secondary jet part numbers listed for this model bike?

[flybox1]

no idea....they are the same part. 

Got your jets loose? post up a picture showing any markings on the outer edge.
If you are keeping the bike stock, the current ones probably are fine if all the thru holes are clear and undamaged....

[uncle_fester]

no idea....they are the same part. 

Got your jets loose? post up a picture showing any markings on the outer edge.
If you are keeping the bike stock, the current ones probably are fine if all the thru holes are clear and undamaged....

Through holes are clear and appear un-modified. They look new. None of them have the K, so they must be reproduction. Pilots are 38, mains 100.

I did measure the diameter of the mains, and they are definitely the 6.0mm diameter "Secondary" mains.

EDIT: Looking at the Jets R Us site now, the primary jets only go up to 92 in size. It appears larger jet sizes require the "Secondary" main type?

[flybox1]

Most likely they are aftermarket KEYSTER brand parts.  I would be suspect as too often their specs are different from OEM.
They are cheap and readily available.
Buy the pilots and primary mains from Jetsrus and be confident in them.

[uncle_fester]

Will do.

I'll report my results.

[uncle_fester]

UPDATE: I installed the new jets from Jets R Us, and that appeared to have fixed it!

At first, it started with an RPM around 1900, I rev'd it to about 4000 and it hung around 2500. I mumbled some curses and set the idle screw down to around 1500. After that, no RPM hangs or lazy returns. I let it warm and tested everything again. It all appears to be functioning great. Good throttle response and the RPMs are settling at a reasonable rate. After the bike cools down, I'm going to do a carb sync either tonight or tomorrow. From there, work on the A/F ratio.

I'm now a believer in genuine parts. The other jets must have had an anomaly or something else going on. FWIW, the Jets R Us jets did not have a "K" stamped on them.

[DaveBarbier]

If they don’t have a K stamped on them then they’re not genuine Keihin. But either way, if it performs well and your plugs are showing a good mixture then you’re good to go.

Edit: Did they advertise these as genuine Keihin and send you aftermarket? If so, I’d contact them and ask what the deal is.

[uncle_fester]

Here's the page I bought them from. They don't say genuine, but Keihin is in the jet name.

https://www.jetsrus.com/a_jet_kit_street/honda_550_CB550K_1974-1976.htm

[flybox1]

jetsrus sells keihin replicas....but the quality control is much better than the aftermarket brands.