74 R90/6 running rough

[bluecycle3]

Tried to get the new to me 74 R90/6 going today since it was such a nice day for Michigan. I got it to start fairly easily but when i went on my test ride it was running very rough and wouldnt pull very well, got home and had exhaust coming out of both pipes and gas coming out of the right carb.


The bike had been sitting before i bought it for about a year, im thinking the carbs are gunked up and want to know if i should just clean them out or get the rebuild kit and start from there.


Any ideas or suggestions appreciated.

Thanks Greg

[jmerlino]

Get a rebuild kit. It's not that expensive or difficult, and it's probably a good idea to replace the o-rings and stuff. You might also consider cleaning out the petcocks and replacing the fuel line.

[sgborgstrom]

A bit more history on the bike would help. High or low mileage, pristine or ratty?

Before you go spending too much money, drop the float bowls and make sure they are clean, the leaky right carb is probably a stuck float valve. I would then pull the main jets and make sure they were clean, a few shots of carb cleaner in the various other orifices will tell you whether they are blocked or not.

Next would be open up the air filter box and make sure nobody moved in over the lay-up period. I'd also check the valves for proper lash and the timing while I was at it. Those old style spark plug caps screw onto the leads, the connection there can be renwed by trimming about a 1/4" off the wire and screwing things back together. Those caps can go bad too.

If you decide to rebuild the carbs, do them one at a time and when you put the throttle butterfly back on it's shaft, make sure it is centered up in the hole.

An old friend (who drove Alfa Romeos and Fiats from the '60's to the '90's) once told me, "90% of all carb problems are electrical in nature"

airheads.org has a wealth of tech information and resources too.


Steve

[pmdave]

The pre-1980 engines have very small ignition points to fit into the smallish space available. Points gap that closes will mimic a fuel problem, as I discovered years ago. The advance mechanism must also be free to move, and the tiny springs in place. So, after you get the carbs cleaned up and adjusted, it wouldn't be a bad idea to get down there on your hands and knees and clean up the points and advance mech.

Remember to always disconnect the battery negative wire before removing the front cover, to avoid shorting out the diode board.

I also discovered that water can find its way into the points cavity, but not find it's way out. If the water freezes overnight, the advance mechanism becomes an ice grinder, and it's hard to get sparks. On my '78, I drilled a small weep hole up through the casting into the points cavity to allow water to drain out the bottom.

I second the motion to set the valves. The current spec is .008" ex and .006" in. But I prefer .008 and .005, since the intakes don't heat up like the exhausts. The symptom of tight exhaust valves is after a hard, hot run, the engine refuses to idle, and it's abnormally quiet. Clattering valves are happy valves.

The stock braided rubber gas lines tend to disintegrate internally over time. I now use snowmobile fuel lines--made from some very tough flexible plastic. As a minimum, replace all the fuel lines with new stuff.

The stock (metal shrouded) spark plug caps have 500 ohm resistors, which can fail over time. The symptom is that the bike runs OK at speed, but cuts onto one cylinder as you slow down. I prefer 500 ohm NGK plug caps, which you can find at most any oriental bike shop for a modest price. They screw onto the wire, and have very tight fitting rubber boots.

After a few years of corrosion, the connectors to the coils can be dirty. It doesn't hurt to unplug each connector one at a time, clean the metal with some fine sandpaper, grease with Vaseline or silicone grease, and if necessary tighten up the female connector with needle nose pliars carefully used to pinch the metal a smidgen tighter.

Caring for a vintage airhead is an ongoing task, so don't put your tools away too quickly.

pmdave

[sgborgstrom]

The stock (metal shrouded) spark plug caps have 500 ohm resistors, which can fail over time. The symptom is that the bike runs OK at speed, but cuts onto one cylinder as you slow down.
pmdave

Having struggled with this over several hundred miles one tour... the opposite can be true too. My R75/7 ran fine at low speeds and cut out at high RPM.

Sorting electrical gremlins is like :banghead

Steve

[Friedle]

pm dave, isn't it 5000 ohms resistance caps, not 500

blue, start with the easy stuff first. Check your air cleaner cavity and filter. Little critters love to take up residency in there, especially for winter months when the bike isn't being used. Look for a chewed up grid on the air intake on top of the motor, under the gas tank, a sure sign you have some tenants who are not paying rent. The air filter, air cleaner cavity and even the starter motor cavity can get packed with nesting material and food supplies.

If that's okay, make sure the valve clearances are set correctly. Then clean up all the electrical connections on the bike. The you can clean and adjust the carbs. You can find all the necessary information at www.airheads.org

The motor only needs three things to run; spark, fuel and air. At least one of them isn't right.

Friedle