Topic: 1975 cb550 still having fuel flow problems

[gcpemberton]

My 1975 cb550 loses power/fuel flow on number 2 and 3 cylinders at the same spot on my afternoon commute on hot days.  Sometimes it recovers and sometimes not.  I just rebuild the carbs and petcock to solve this problem, but to my surprise, the bike did it again today: at the same exact spot! 

The bike sometimes recovers when I open the throttle for about 30 seconds and get the bike up to speed without stops.  However, sometimes after it recovers it will just recover for a short time, then lose power again.

It has never done it on my morning commute, which is about 15 degrees cooler and requires few stops.

I have no outboard fuel filters on the bike.  It is a completely stock bike.  The screen in the tank which joins the petcock is clean.  The tank has been cleaned and kreemed.

The only thing I think of is that it is not venting properly or I have some sticky valves.  The tank was about 1/2 full when it stumbled today.

Any other suggestions?

Thanks.

Grant 

Spokane, WA 

[ProTeal55]

Sounds like a vent issue, or your hose routing from the Petcock might be alittle jacked.
Take the time and start from scratch. Make sure you dont have crap in your tank/petcock/lines/etc. Make sure the carbs dont have crap in them as well...

[Gordon]

How did you determine that those two cylinders are not getting fuel when the symptoms occur?  Did you remove the float bowl drain screws and nothing came out, or are you basing the diagnosis on the way the bike is acting? 

I'm asking because the symptoms you've described wouldn't normally lead me to think it was a fuel flow problem unless you've determined that there is actually no fuel in those two carbs when this happens.  Just from the symptoms it sounds more like an ignition issue to me.

[TwoTired]

I agree with Gordon.
The fuel feeds 1&2 from one hose and 3&4 from the other hose coming from the petcock.

If it was a fuel starvation from the tank issue, it should be along those boundaries.

The ignition system drives 1&4 and 2&3 respectively.  Checked your points lately?

While possible to screw up carbs 2&3 for a problem, the odds seem against this occurrence.

If 2&3 are indeed being starved for fuel, the spark plugs should show a lean pattern compared to the ones that were still operating.

Cheers,

[gcpemberton]

I do find it strange that 2 and 3 are having problems, since 1&2 and 3&4 are tied through  the carbs.  When the this problem occurred before, I did drain the bowls and found that 2 and 3 had significantly less fuel than 1 and 4.  But I did clean the carbs very thoroughly and spent $60 on a rebuild kit.  So, perhaps ignition is a consideration since it is 2&3 acting up.  But the plugs looked fine when I checked then before rebuilding the carbs.

I will look at the points again to see.

Many thanks.  I will keep you posted on what I find.

[tonycb650]

make sure your floats are set correctly and you have no air leaks around the rubber boots when you reinstalled the carbs.

[gcpemberton]

Well I have check the timing and points on the bike.  All are where they need to be.  However, I do think it is an electrical issue, not fuel.  How long do Honda coils last?  Has anyone every known their coil to over-heat.  Again, my bike only has issues when it is very hot outside.  I just took it out and raced around on a cool evening and it performed perfectly.  But I can garuntee that it will have issues tomorrow afternoon as it will be in the 90s.  How can I test if my coil is faulty.

Thanks.

Grant

[billybobobrain]

I just went the rounds with this same problem with my old 500. I replaced the fuel lines, added filters, cleaned the carbs, adjusted the floats, and cleaned and coated the tank. Check the vent in cap. If it’s plugged it will not allow the fuel to come out if the lines are hooked up. If it’s just the two carbs you are having problems with I would check the floats. Electrical ether works or it doesn’t in my opinion.

[Walt]

I'm not sure if coils can overheat, but I had a problem on my 71 750 where the bike would die, but once it cooled off, it would start again. I didn't know what the problem was until one day it died for good. Once I got it home I realized there wasn't any spark. I got new coils and put 10K trouble free miles after that. I don't remember about temperature, or whether one coil went and the other one didn't, I just replaced them both.

I think I have a similar problem with my 400F and posted a question about it.

Good luck.