Topic: 1974 CB550 dies under load

[adamh]

Hi all,

I bought a 1974 CB550 back in November and have been attempting to get it back into good running order since then, mostly by following the advice here and on youtube etc. I have previously 0 mechanical experience in working on vehicles of any kind so I'm a total noob, and in my desperation I'm asking this group for help

When I got the bike, it started and ran, but quite poorly. First thing I did was set the timing and adjust the valve clearances. After that, I used an infrared thermometer and saw that one of the exhaust headers was always cold while the other three were hot. I pulled and rebuilt the carbs using a kit from 4into1, cleaning them in an ultrasonic cleaner with a diluted mix of simple green and water. I bench synch'ed the carbs, threw them on the bike and went for a ride without the airbox and it ran OK for 30 minutes as I rode around the block at ~30mph. It didn't run great, but it ran and the thermometer showed me all 4 headers nice and hot (450+ degrees F). It for sure pulled harder and with better and smoother acceleration than before I started working on the carbs. Occasionally the bike would struggle but I'd just crank the throttle and it would be OK.

I let the bike sit a few weeks, but finally got the stock airbox back on, air filter etc. When I tried to take it around the block, the bike would die as soon as I tried to let out the clutch and give it some throttle. I had to rev it up to 5k+ rpm while holding the clutch halfway in to get it to move without it stalling.

When I got it back to my house, the headers for the 1st and 3rd cylinder were at ~280 degrees while the others were 450. Something clearly isn't right here.

A few other potentially relevant details - the bike has two inline fuel filters, one going to each half of the carb rack. I don't know the state of the tank filter, but do know that the previous owner had the tank sealed at a reputable local vintage bike shop. The fuel line does dip a little bit before feeding the carbs, but when I ran the bike for 30 minutes without the airbox on it wasn't dying, but it did feel sometimes like the bike was lacking fuel.

The bike has some sort of aftermarket 4 into 1 exhaust on it - not sure what kind. I'm guessing the gas in the tank is likely a year old, based on when the previous owner bought it  - not sure if that's an issue here too.

I'm a little lost on what to focus on at this point to get this running well. Any advice on where to focus efforts would be greatly appreciated!

Thanks,

-Adam

[Deltarider]

What I have learned from experience, is that the CB500 and early models CB550 are very sensitive to have the correct fuel lines. If you have the standard petcock*, length should be 18 and 30 cm resp. with an ideal internal ⌀ of 5,5 mm. Outside ⌀ will be around 10 mm. Route the fuellines correctly using for fuelline to carb 3 + 4 the little clamp you will find in the front of the carbs. Make sure this particular fuelline runs at least horizontal and will not have an 'up and down', but will facilitate gravity to do its work unhindered. No extra inline fuelfilters. They'll bring trouble on these models.
* Later models have a new style petcock. Fuellines then are 17 and 28 cm resp.

[adamh]

Thanks for the feedback guys. I found the issue but don't know the cause. I took the tank off the bike, and although I can see a decent amount of fuel in the right side of the tank, no fuel comes out when the petcock is on open or on reserve. If I shift the tank around so I know gas moves to the left side of the tank, gas will come out of the petcock, although it appeared to come at an uneven rate from each of the outlets. Should I drain the tank of fuel and pull the petcock to inspect at this point? Gas certainly used to come out of the petcock when the tank was more full - prior to rebuilding the carbs gas was always leaking everywhere

To all of the other questions, before I noticed the gas wasn't coming out of the petcock, I pulled the carbs and installed 5.5mm fuel line without inline filters. I also used the little clip on the carb rack (didn't even notice it before!) and routed the lines so they were always heading downwards. I confirmed that the ignition coils were feeding the correct cylinders. When I rebuilt the carbs I did set the float height to I think 23mm using a digital caliper.

[Deltarider]

I took the tank off the bike, and although I can see a decent amount of fuel in the right side of the tank, no fuel comes out when the petcock is on open or on reserve.

Well, what would you expect, there's a 'dam' in between. The fuel left in the right part is the reserve reserve so to speak. When you're out of gas, you can still lower your bike to the left so the fuel reaches the left side of the tank and you can make it to a gas station.

If I shift the tank around so I know gas moves to the left side of the tank, gas will come out of the petcock, although it appeared to come at an uneven rate from each of the outlets.

This is normal, dynamics of fluids which is a study in itself. Just check they both are open, by closing one with your finger. Isn't that wonderful?

Should I drain the tank of fuel and pull the petcock to inspect at this point?

No.