CB550 Revving Back Down to Idle Too Slowly

titan5699

titan5699

Been Around the Block
Hello All!

I have noticed that my 76 Honda CB550 revs down slowly. The throttle response when revving up is great, the throttle snaps back great, but the engine takes too long to rev back down to idle.

I know I have seen an older posting about this topic but I can't seem to find it.

If you might know of a possible cause for this problem, please let me know. Or if you know of which older posting I am talking about please send me the link to it.

Any help would be greatly appreciated!

Thanks!
 
Thanks for your input man!

That does sound like it could be a possibility. I just recently synced them, but the bike wasnt running at her best when I did the sync, so I wouldn't rule it out.
Didn't think of that man, definitely worth checking again!

Thanks again! Maybe I'll get a chance to check it tomorrow and I'll reply back.

Until then, cheers!
 
Another thing that caused that on my 550, depending on how your bike is running or if you've recently done any work to make any changes, is the idle screw being set too high.

What would happen on mine is it was running rough and having a hard time keeping idle, so I'd adjust the idle screw to where it was a steady, normal RPM idle, but then once it's revved up above 3 or 4 it would hang and then slowly drop back down.
 
Weeburn said:
Another thing that caused that on my 550, depending on how your bike is running or if you've recently done any work to make any changes, is the idle screw being set too high.

What would happen on mine is it was running rough and having a hard time keeping idle, so I'd adjust the idle screw to where it was a steady, normal RPM idle, but then once it's revved up above 3 or 4 it would hang and then slowly drop back down.
Click to expand...

I was having a similar problem with a rough idle and I had to do the same thing by keeping the screw further in and keeping the idle around 1,500 so it would be somewhat smooth. It was also running rich with throwing out black smoke. And while at a higher RPM, 3K-4K, it would rev down about 500 RPMs then back up and keep pulsating like that while I was holding the throttle steady.

I fixed the idle problem by cleaning out the carbs again and the petcock to make sure it was getting fuel properly and double checked all my float heights.
To fix the idle problem further and the running rich problem I actually had to take off the cone filter I bought from DCC. For some reason it seems that the bike doesn't like those filters. I can see light shining through them but I guess they are too restrictive. Either way, that's a different problem for a different post lol.

Anyways, after doing all of that, I started the bike up while forgetting to back off the idle screw, which was pretty much in as far as I could get it. Once she fired up her RPMs shot up so I quickly shut her down, backed off the screw and fired her back up. Now she runs great and can smoothly idle around 1,250 or so with no smoke. I still have plenty of room in the screw in either direction. But if the screw was still the problem I wouldn't think the throttle would snap back as it does. As the throttle is all the way back into idle position the engine is still hanging where I left it at then it comes down. But it doesn't seem like the screw is holding it there because it does eventually make it's way back down to a smooth idle, just takes a full couple of seconds to do so.
 
It could also be an air leak.
If your intake boots are cracked or over tightened it happens sometimes.
Remember that your carb synch needs to be done at normal operating temperature.
If you synch a cold engine, it will rarely be completely happy once warmed up.
I would firmly suspect either a carb issue or an air leak.
 
Bozz said:
It could also be an air leak.
If your intake boots are cracked or over tightened it happens sometimes.
Remember that your carb synch needs to be done at normal operating temperature.
If you synch a cold engine, it will rarely be completely happy once warmed up.
I would firmly suspect either a carb issue or an air leak.
Click to expand...

I will have to inspect for air leaks. Thank you. It is possible the boots might be too tight and I'll check the screws for the vacuum ports.
I did warm up the bike before syncing, but thank you for the reminder. I'll have to recheck the sync, I did do it when she still wasn't running too well, that could be a factor.

Would the crankcase breather be of any concern? I'm not running the stock airbox, so I don't have the hose going from the breather to the air filter. I have tried running with a little cone filter over the breather and with no filter so that the breather is completely open and venting out unrestricted. I can't say that I've noticed a difference either way.
 
Update.

I have tried to sync the carbs these 2 past days. Yesterday, the battery died while syncing. Today, the rain rudely interrupted my second attempt.

So far after what I have done I have a few questions. This is my first time syncing carbs, and although the process seems easy and straight forward enough I just want to make sure I am doing it right and am open to advice.

I am using an older mercury PCCC vacuum style synchronizer. There is a 'CM' ruler on the side of the synchronizer.

-Should the carbs be synced so that the mercury level is at a particular cm? It seemed that the throttle response was better at a higher cm. When at a lower cm, idling at about 1000-1250 RPM, it would seem to run decent, but the vacuum levels would fluctuate. When you gave it throttle it would make a sucking sound and bog down before raising RPM.

-Should they be synced at idle or at a higher RPM? When I first started, the carbs were out of sync. I got them leveled at about 25cm at about 1500 RPM. As I lowered the idle down closer to 1250, the levels were reading at about 15cm. Then I lowered the idle down to 1000 RPM, it wasn't running it's smoothest. It ran it's best around 1250 RPM.

-Is it normal for the carbs to go out of sync as the throttle changes or should it stay level at the way through? No matter which RPM I sync it at, as I roll on the throttle the carbs wouldn't pull in unison. So I tried to set it at around 2500 RPM but when I let it idle back down the carbs were out of sync again. And when I sync it at 1250 RPM and lower the idle down to 1000 RPM it would again go out of sync. It only seems to stay synced at the RPM I sync it at, if that makes sense. Also, cylinders 1 and 4 seem to constantly float between a range of about 4cm, while cylinders 2 and 3 seem to stay more steady.

I am just trying to make sure I am on the right path and get some advice for the next opportunity I have to hopefully finish syncing the carbs.
 
Help if I read the manual more before posting..

The book says the carbs should be uniform at 16-24cm, and they should all be within 3cm of each other. Seems like it should be a little higher than 1000 RPM because it states when all carbs are synced to set the idle screw so it idles at 950-1050 RPM.

Just FYI, if you were interested.
 
the condition of individual cylinders can possibly cause this changes you describe
the physical condition/sealing quality of the valves/rings ie compression and or leakage at valves
the clearances of valve lashes if considerably off between cylinders as well as ignition tune of each cylinders
if individual exhaust pipes are in different tune
do an accurate warm engine compresion test to get going
 
xb33bsa said:
the condition of individual cylinders can possibly cause this changes you describe
the physical condition/sealing quality of the valves/rings ie compression and or leakage at valves
the clearances of valve lashes if considerably off between cylinders as well as ignition tune of each cylinders
if individual exhaust pipes are in different tune
do an accurate warm engine compresion test to get going
Click to expand...

I just want to clarify so I can try to fix it, which of the changes I described would be a result of the condition of the cylinders?

I hope it's nothing that serious! I haven't checked the valve clearance yet. The exhaust I dont think is a concern, its the same system the previous owner had on there for a while.
I dont own a compression tester but I do want to check the compression to see how the cylinders are.
 
if you synch your carbs at say 1500 then synch changes when you try to drop idle down to 1000 it is possible part of that could be caused by cylinders in verying condition
...was my point
first and easiest is comp test,i hope it isnt serious either but it is a machine hoping doesn't matter to the machine
 
A carb synch should only be done after a valve adjustment and compression test.
THe carb synch doesn't need to be to a specific CM setting, but rather to ensure that each carb is in relative balance with the other 3.
Valve adjustment/valve timing will greatly affect the volume of air each cylinder ingests per cycle.
This is why you should check the valve adjustment before you synch.
(if you synch THEN adjust valves you will need to synch again after)
Compression testing will also tell you something about how each cylinder is running. Ideally they should all be within 10% of each other, iirc.
If compression is low in one cylinder (or more) check valve adjustment first, then when correct recheck the compression.
If valves are in adjustment and compression is acceptable, then you are ready for a carb synch.
Because of all the variables, it is impossible to maintain synch throughout the rev range.
Synch at or near idle is the goal, as this will greatly affect the smoothness of the idle as well as throttle response/acceleration from low RPM.
 
Bozz said:
A carb synch should only be done after a valve adjustment and compression test.
THe carb synch doesn't need to be to a specific CM setting, but rather to ensure that each carb is in relative balance with the other 3.
Valve adjustment/valve timing will greatly affect the volume of air each cylinder ingests per cycle.
This is why you should check the valve adjustment before you synch.
(if you synch THEN adjust valves you will need to synch again after)
Compression testing will also tell you something about how each cylinder is running. Ideally they should all be within 10% of each other, iirc.
If compression is low in one cylinder (or more) check valve adjustment first, then when correct recheck the compression.
If valves are in adjustment and compression is acceptable, then you are ready for a carb synch.
Because of all the variables, it is impossible to maintain synch throughout the rev range.
Synch at or near idle is the goal, as this will greatly affect the smoothness of the idle as well as throttle response/acceleration from low RPM.
Click to expand...

Thanks much man! I'll check the valves first, then do the sync. I would still need to pick up a compression tester, but when I do I'll check that as well.

xb33bsa: Thank you as well. Your explanation was informative as well.
 
Bozz said:
A carb synch should only be done after a valve adjustment and compression test.
THe carb synch doesn't need to be to a specific CM setting, but rather to ensure that each carb is in relative balance with the other 3.
Valve adjustment/valve timing will greatly affect the volume of air each cylinder ingests per cycle.
This is why you should check the valve adjustment before you synch.
(if you synch THEN adjust valves you will need to synch again after)
Compression testing will also tell you something about how each cylinder is running. Ideally they should all be within 10% of each other, iirc.
If compression is low in one cylinder (or more) check valve adjustment first, then when correct recheck the compression.
If valves are in adjustment and compression is acceptable, then you are ready for a carb synch.
Because of all the variables, it is impossible to maintain synch throughout the rev range.
Synch at or near idle is the goal, as this will greatly affect the smoothness of the idle as well as throttle response/acceleration from low RPM.
Click to expand...

I checked and regapped the valves where needed. I got the sync pretty much all dialed in. It is revving up, down and idling very well now! :)

One thing that I noticed is that I synced them without any air filters on. I replaced the stock filter with individual cone filters from DCC. It runs fine with the filters off but once I start to put on the cone filters it bogs down and run terrible. I understand that with having to pull air through the filters it will drop the rpms because of the extra load on the engine. So my next step is to try to sync the carbs with the filters on and if that doesn't work I'll be looking into getting some velocity stack filters. Those would be the closest thing to running no filters. So we'll see how the sync with filters goes. But already an improvement from before! :)
 
Be aware that certain carbs have air jets right on the edge of the throat. (where the pods attach)
Some pod filters have an inside edge where they slide onto the carb.
Sometimes this inside filter edge blocks the air jets in the carb and messes things up royally.
If it runs well without the pods, and runs worse with them I would inspect those pod filters carefully.
Ideally pods shouldn't affect anything and should only filter the air.
(this is on direct linkage carbs, not Constant Velocity carbs. Pods on CV carbs are extremely challenging)
 
Bozz said:
Be aware that certain carbs have air jets right on the edge of the throat. (where the pods attach)
Some pod filters have an inside edge where they slide onto the carb.
Sometimes this inside filter edge blocks the air jets in the carb and messes things up royally.
If it runs well without the pods, and runs worse with them I would inspect those pod filters carefully.
Ideally pods shouldn't affect anything and should only filter the air.
(this is on direct linkage carbs, not Constant Velocity carbs. Pods on CV carbs are extremely challenging)
Click to expand...

Very good point! I didn't think of that, it will definitely be something I look into. I honestly don't know which they are, I believe the direct linkage, but I'm not sure the difference. I know I have seen carbs with CV printed on the side, mine do not have that, so I assume direct linkage?..
 
If you carbs are stock I believe that those are Direct Link carbs.
If the tops of your carbs are roughly the size of your thumb, you have Direct Link.
If the tops of your carbs are about the size of a tea cup, those are CV carbs.
 
Bozz said:
If you carbs are stock I believe that those are Direct Link carbs.
If the tops of your carbs are roughly the size of your thumb, you have Direct Link.
If the tops of your carbs are about the size of a tea cup, those are CV carbs.
Click to expand...

Gotcha! Yea mine are direct link then. I'm guessing direct link is the lifter style and the CV is the diaphragm style, that would make sense.
 
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