Topic: 75 cb550f idle issue

[02z06dave]

Hi everyone. I will start off with some background. I just finished getting this bike back on the road after it had been parked for 10 or 20 years. The carbs were rebuilt, new air filter all the usual tune up maintenance done. Its 100% factory stock with mostly still the original parts, though it does have new air box rubbers and intake rubbers. It had been running great for a couple weeks, then the other day it started having an idle issue. I would get to a light and the idle would be fine then drop down and either die or it would come back up. It would make almost a coughing noise right when it would drop and start to stumble. So today I tore it down to go through everything again. The carbs had some debris in them so I cleaned them out pretty well and added an inline filter. I followed that by checking the valves, cam chain, and timing again then finally the carb sync. The sync wasn't really successful because it just wouldnt idle steadily unless I set the idle at 2k or higher. I thought maybe in was a vacuum leak, so I checked around all of the rubber parts and the exhaust and found nothing then checked again and nothing again. I did also check the timing advance mechanism and it seems to be functioning properly, as well. At this point, I'm thinking maybe its weak coils (they are still the originals), but I'm not sure if weak coils would cause an issue with the idle. Any suggestions are very much appreciated.

I've also noticed that the spokes seem to maybe be loose on the front wheel. They make an occational little pop noise. I didn't check them when I cleaned up the wheel and got the tire put on. It's not something I've ever had an issue with before, so I didn't really think about it. Can the spokes be tightened/trued with the tire still on and the wheel still on the bike?

[uksparky]

Not a good idea in line filter.... in tank filter is enough.... another filter ...in line  will slow the fuel flow... check the  spokes that are loose, give the loose ones half a turn at a time, did that with my front wheel...setting up the spokes done with no tire on rim.... cost around $100....

[02z06dave]

After going through and syncing the carbs again today, that seems to have have settled it down fornthe most part. With the idle set atv1100, it doesnt die. The idle will dip down to 900 or so then come back up still. With the vacuum gauge still hooked up, I would hear the cough noise, and see the #1 cylinder would suddenly dip way down for a second then catch back up. There is no other surging in the idle. The throttle seems very responsive when accelerating under load and it drops right back down to idle when I let off. Maybe its a non issue that it has that little fluctuation. It just seems weird. My other bikes dont do that and they are all carbed bikes as well.

[TwoTired]

If you found debris in the bowls, you d=need to check the pilot jets.  These are only 0.016 in. in diameter at the orifice.  Floating about stuff in the bowls can intermittently block/restrict these.

Very unlikely the coils themselves are bad.  They are pretty much like minerals rock.  Don't hit them with a rock hammer, and they work longer than a human lifetime.   The exception is the wire insulation going into them.  If old stiff wires are bent severely at the join to the body, they can crack there. and let the spark energy escape.
Measure the resistance of the secondary from spark plug cap to spark plug cap.  The 550 has 10K ohm caps from the factory and the coil is in the realm of 15K, so you are looking for ~35K total in that loop.  If someone has replaced the caps with 5K, then only ~25K ohms is reasonable.

Don't overlook the condensers for the points.  If you can narrow the issue to 1-4 or 2-3 cylinders, swap the condensers to see if the issue follows the condenser.  There has been a spate of posts about failing condensers from Diachi.  Some don't work properly even when brand new.  These must work reliably to have good reliable spark.  You reported symptoms do follow a failed/failing condenser.


Do check the relative head pipe temps.  A colder one tends to narrow down focus. to that cylinder.  As will looking at the spark plug deposits to "read" firing condition in the cylinders.

I think you added another potential problem with the inline filters.  "Cheap" isn't the way I normally solve issues, though inexpensive is good.  Inline filters interfere with gravity feed fuel delivery.  Now if you had an in tank fuel pump...  Anyway, do check that the in tank filter is still intact.  Filtering is good at that point in the system.

Lastly, the motor will run odd at the point where reserve is needed.  Don't be chasing an issue with fuel supply.  Run on reserve or fill the tank to see if that makes a difference in symptoms.

Cheers,

[Stev-o]

Hey Dave...good advice from Two tired above.

I'd flush the tank to remove any debris and remove the inline filter to eliminate that possibility.  Clean the low jets too.

Good luck.

[02z06dave]

The carbs have already been completely cleaned again including the slow jets. After i ran it last i checked in the bowls again and still clean. When i syncing the carbs, it was running on a auxiliary tank that was perfectly clean as well with no inline filter and it ran the same as with own tank and inline filter.  I dont believe the inline filter  is the cause of the problem. I will check the condensers, though.

[uksparky]

If the carbs are ok it got to be spark related... coils/points etc... your cough sound could be one of the coils/condenser  failing...