Author Topic: '77 CB550 Over revving (Carb issues) (Read 7638 times)

ATZ · « **on:** February 19, 2012, 07:30:59 PM »

Hey there, I'm new to this forum and to SOHC 4s. I have dealt with two strokes mostly but I have worked on some 4 strokes. So I recently picked up a 550 and got work on it. It ran when I bought it but not well.

The first issue I noticed is that the #4 cylinder doesn't seem to firing. It's sparking outside the engine regularly and when I put an inline "light tester" on it you can see its firing. All the carbs are getting gas. I went through all of them multiple times. After running the bike, when I pull the #4 plug its wet but doesn't look like it has been firing. There is very little sign of combustion. The plugs are new and all the others have some sort of brown/black carbon build up. (see photo) I of course can tell its not firing by feeling the header pipes. The #1 and #3 are super hot. I noticed that the #2 is not as hot as the others and the #4 is just warm. I also popped the tops of the carbs off and hooked up my synchronizer to see if they were way off but they seem okay. With the tops off I raised the slide on the #4 carb and nothing changes engine wise. It doesn't cause it to rev up. Whereas on the other 2 I can get the engine to rev higher/lower- if that makes sense.

Things I have checked:
Timing
point gap
valve clearance
carbs: jets, valve, float level, synchronized
The plugs seem to be too dark

This leads to the second problem, I believe the two are related.

When I pull the throttle after the engine is warmed up and get it to about 3k on the tach it will jump up a bit to about 4k and linger there after the carbs have been closed. I can hear the slides hit and everything. I have tried to read up and search this forum for answers but I cant seem to find anything. Sorry for the long post I have just been thinking this over for some time and really need some help. These four cylinders really can be a #$%*! I appreciate any help or advice you guys can give.

Other things: the bike came with an aftermarket 4 into 1 exhaust already installed, I don't know how long its been there or if the guy who installed it jetted accordingly. Could this be related?

thanks alot!

my78k · « **Reply #1 on:** February 19, 2012, 08:34:10 PM »

Choke on/off make any difference? Also, I would check for airleaks around the carb boots. Spray some wf40 around the intake side. My guess is dirty idle jets and also the airbox/carb leaking at the header.

Dennis

TheSaint · « **Reply #2 on:** February 19, 2012, 09:42:33 PM »

You've checked everything but compression. So...............

nancy · « **Reply #3 on:** February 19, 2012, 09:48:06 PM »

Swap the coil leads around and see if the fault moves..1 to 4 4 to 1....

TwoTired · « **Reply #4 on:** February 19, 2012, 09:53:08 PM »

The 77 Cb550K model has PD style carbs. The 77 CB550F model has earlier style carbs. The specs. are different between them and you didn't mention which specs you used for the carbs.
Better advice could be offered if we knew which you have, and what exactly you applied to the carbs.

Also, has the air induction path been changed from stock?

Have you swapped plugs between the dead cylinder and one that is known to fire?

Spanner 1 · « **Reply #5 on:** February 19, 2012, 09:53:41 PM »

Do you know the correct float level for your carbs?.... are the floats set correctly to the spec. setting ?
Waay too high float level ( too high fuel in bowl ) would be my first impression as the problem

dave500 · « **Reply #6 on:** February 19, 2012, 10:15:49 PM »

check or replace the plug caps,are the plugs the correct ones?

BrockSamson · « **Reply #7 on:** February 20, 2012, 07:26:21 AM »

Quote from: my78k on February 19, 2012, 08:34:10 PM

Choke on/off make any difference? Also, I would check for airleaks around the carb boots. Spray some wf40 around the intake side. My guess is dirty idle jets and also the airbox/carb leaking at the header.

Dennis

+1 I had an issue with my 750k last week reving straight to 4k RPM once started and it was an air leak. What condition are your boots in?

ATZ · « **Reply #8 on:** February 20, 2012, 07:42:37 AM »

I checked for air leaks around the boots I could again and be more thorough. I also have the 550K with the PD carbs. I set the float levels at 12.5mm. The boots seem to be in good shape. I just checked the plugs, apparently I bought the wrong ones! I just went ahead and pulled one from when I got the bike and brought it in and bought four. Guess I should have checked. How much of a difference can that make? I believe with the choke on it doesn't want to over rev like it does with it off. So I guess possible air leak causing me to run lean?

crotella18 · « **Reply #9 on:** February 20, 2012, 08:55:17 AM »

Float height should be 14.5 mm for a 77 550 w pd46a carbs. Check the 77 addendum on this site, page where you can download the manuals.

Edit: Here is the link http://www.sohc4.net/?page_id=207

TwoTired · « **Reply #10 on:** February 20, 2012, 11:23:42 AM »

Stock carb specs, are different between PD46a (or X) and PD46c carbs. Carbs are different between 77 and 78 K models. But, neither of the stock settings will be correct if the exhaust and air induction aren't stock.

Like people, there is far more detail to describe them than just the first name.

ATZ · « **Reply #11 on:** February 20, 2012, 03:06:32 PM »

So you guys think that there might be too much gas? I guess setting the height 2mm more would close it off causing less gas to enter the bowls, but would that really cause it to suck it up more and continue to rev after the slides are closed? I also noticed someone put a tap around the intake flange on the carbs, I would guess to get a better seal, could this cause an air leak rather than forbid one?

TwoTired · « **Reply #12 on:** February 20, 2012, 03:46:57 PM »

I think we have such a small picture of the problem bike. We can send you in 12 directions and only one may be fruitful.

My stock CB550K with PD 46C carbs was set at 14.5mm float height when I got it. I didn't change it during cleaning, and it runs well.
It also has the stock air filter and exhaust.

Spanner 1 · « **Reply #13 on:** February 20, 2012, 04:22:36 PM »

Atz... you are upside-down on your thinking about the float level

. Actually a float level of 12.5mm( when the spec. is calling for 14.5mm ) will allow more fuel in the bowl and a higher fuel level than intended with both jets giving an over-rich mixture. Time to double check the correct setting for your year/carbs. Lots of info here....

my78k · « **Reply #14 on:** February 20, 2012, 04:40:53 PM »

I would take 30 seconds and rule out the airleaks by spraying wd40 around the boots.

alot faster than pulling the carbs off again....

Just my 2 cents.

Dennis

ATZ · « **Reply #15 on:** February 26, 2012, 01:02:31 PM »

Alright, I finally got around to check some things and wanted some input before I pull these carbs again. I Checked for airleaks and nothing seemed to changed. I sprayed all around the intake boots with WD-40. I noticed the revving seemed to get worse with the choke on. Anything else I should check before taking these things off. One last question about the coils- Can I just take the plug cables from one coil and try them in another? If so how do I take them out of the coils?

Thanks

Scott S · « **Reply #16 on:** February 26, 2012, 04:53:25 PM »

If they're stock, the cables are molded into the coils.

Do you have the stock airbox or pods or velocity stacks or

?

If you pull the carbs, see what is stamped on the jets and determine what size you have. Over-revving can be an indication of a LEAN mixture but, as has been pointed out, we need more info.
Have you adjusted the valves? Points? Set timing? Carbs synched?

ATZ · « **Reply #17 on:** February 26, 2012, 05:48:14 PM »

I pulled the carbs again, the float was set right at 14.5mm The main jet is 90 the idle jet is 42. I am using the stock air box. I have adjusted the valve timing and ignition timing. One other thing I noticed was when I pull the cam vent hose? off of where it goes into the air box there was some fluid in the hose. Is this normal? it wasn't a lot but there was something in there.

ATZ · « **Reply #18 on:** February 26, 2012, 05:49:58 PM »

It seems like its running lean but the plugs seem to say otherwise. Also the over revving seemed to get a bit worse when I put the choke on

TwoTired · « **Reply #19 on:** February 26, 2012, 06:13:16 PM »

Quote from: ATZ on February 26, 2012, 05:48:14 PM

The main jet is 90 the idle jet is 42.

Measured? Or, just reading the numbers? What is the confidence the carbs weren't disturbed from stock factory dimensions?

What IMS setting do you have? It will not like to be in stock position, with an exhaust change.

Quote from: ATZ on February 26, 2012, 05:48:14 PM

One other thing I noticed was when I pull the cam vent hose? off of where it goes into the air box there was some fluid in the hose. Is this normal?

Water condensation droplets can happen normally. Atmospheric condition during previous run and after shut down dependent.
At the bottom of the air filter box is a water separator and filter with a drain hose to the bottom of the bike. The end of that hose should be a pinch tube to allow collected water to drain out.
The breather filter is foam and can clog with collected water. It should be cleaned and reinstalled.

Quote from: ATZ on February 26, 2012, 05:49:58 PM

It seems like its running lean but the plugs seem to say otherwise. Also the over revving seemed to get a bit worse when I put the choke on

The choke knob is coupled to a fast idle cam on the carbs. Adding choke should make the idle higher.

ATZ · « **Reply #20 on:** February 26, 2012, 08:57:12 PM »

I would guess the carbs may have been disturbed by some PO's. I can tell someone along the line tried to do some revamping of this bike. New components, engine paint, etc. So someone definelty could have monkeyed with the carbs. Are you referring to the slide needle position when you say IMS setting? If so I haven't checked that. In the bottom of the airbox I have all the components that go around the foam filter but no foam. And I didn't measure the jets I just read them. I'm off at school again this week but hopefully I will be able to clean everything up carb wise again and re-install everything and see what that does

TwoTired · « **Reply #21 on:** February 26, 2012, 09:50:08 PM »

IMS = Idle Mixture Screw

Hard to diagnose and recommend a change if the current configuration is not known.

ATZ · « **Reply #22 on:** March 02, 2012, 05:26:28 PM »

I set the idle mixture screw to stock settings of one and a half turns out. I went through everything on the carb again and still its having the issue. It does seem to idle better with the choke half on about. My guess is there is either a hidden airleak I can't find or its just running too lean because of the aftermarket exhaust. I don't want to but I think I might have to buy some main jets and try and jet it. Any thoughts?

TwoTired · « **Reply #23 on:** March 02, 2012, 06:38:14 PM »

The 77 IMS is 2 turns out stock. (see Honda booklet pages I posted earlier).

However, the exhaust changes for this model negates stock settings, as the stock 4 into 4 had higher pressure in the exhaust. This allows leaner mixture settings in the carbs, as scavenging isn't a good.
I wouldn't be surprised if you had to turn the IMS screws out even more than 2 turns.

How's the loaded throttle response from low RPM? Can you twist the throttle half way and still get smooth, reliable pickup?

It wouldn't surprise me if the carbs needed slightly larger mains for just an exhaust change with those PD carbs. May have to raise the slide needles a notch, too. Air induction changes will need even more carb changes.

Did you ever tells us all the non-stock items on the bike?

ATZ · « **Reply #24 on:** March 02, 2012, 07:23:02 PM »

Everything else is stock, I can get decently smooth reliable pickup but it back fires and spotters at some points.

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Author Topic: '77 CB550 Over revving (Carb issues) (Read 7638 times)

BrockSamson