CB 350 idles for 3 seconds

tenseventythree

Active Member
This has been a build from scrap and frame. I have been building this since April and I have never been able to start it. I have rebuilt the carbs wired it for kick start only, replaced the coils and plugs. So after replacing the plugs and about 30 kicks it starts and idles for about three seconds. I then go through another barrage of kicks and carb spray down the intake and then.. It starts again and dies after a few seconds. I pour gasoline into the carb and kick it a few time and it continues to run for about 3 seconds.
What am I missing here?
It's a 1971 cb350


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
I recently started my 350 for the first time. I had everything apart and cleaned but it just wouldn't start. Like yours it was kick only and it used to almost start but wouldn't stay running. Finally remembered that since having the carbs apart I had the idle screws backed out too much, they weren't even touching the stop. I turned them in a bit and it literally fired up first kick. Give the carbs a quick once over to ensure they are in a bit and also any other adjustment point is set correctly. Also, did you remember to close the choke? The choke levers on the 350 are a bit misleading looking so make sure you have them closed and not open.
 
I would start with the plugs out and check that you have a nice bright spark on both sides. That's the easy part. If you don't, then address the spark.

For the fuel side, it sounds to me like you have none. Basics. Gas in tank. Petcock open. Carbs, first is the float working and is the float valve opening. At the best, your bike should run a lot more than 3 seconds regardless of where any jets are set at. Even if its belching and burping, or running only with the choke on.

My bet from your description is no fuel because the float is either not moving or the valve is stuck closed.
 
Have you checked the timing? I was going through the same thing with mine. It had lots of spark and fuel but wouldn't run. I had set the timing according to the manual but the timing chain had skipped a tooth. Needless to say it out for a rebuild now.
 
CB 350 idles for 3 seconds

duff said:
Have you checked the timing? I was going through the same thing with mine. It had lots of spark and fuel but wouldn't run. I had set the timing according to the manual but the timing chain had skipped a tooth. Needless to say it out for a rebuild now.
I'll check it again before I clean the carbs again.
I need to figure out how to do it. At least with a static light or something.


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
CB 350 idles for 3 seconds


TheCoffeeGuy said:
I would start with the plugs out and check that you have a nice bright spark on both sides. That's the easy part. If you don't, then address the spark.

For the fuel side, it sounds to me like you have none. Basics. Gas in tank. Petcock open. Carbs, first is the float working and is the float valve opening. At the best, your bike should run a lot more than 3 seconds regardless of where any jets are set at. Even if its belching and burping, or running only with the choke on.

My bet from your description is no fuel because the float is either not moving or the valve is stuck closed.

It does feel fuel related. (I just resolved a similar issue with my karmann ghia). I'll probably have to get some carb dip.


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
CB 350 idles for 3 seconds


HerrDeacon said:
I recently started my 350 for the first time. I had everything apart and cleaned but it just wouldn't start. Like yours it was kick only and it used to almost start but wouldn't stay running. Finally remembered that since having the carbs apart I had the idle screws backed out too much, they weren't even touching the stop. I turned them in a bit and it literally fired up first kick. Give the carbs a quick once over to ensure they are in a bit and also any other adjustment point is set correctly. Also, did you remember to close the choke? The choke levers on the 350 are a bit misleading looking so make sure you have them closed and not open.

The idle screws are cranked. I thought you need an open choke to start. Maybe too much air not enough gas?


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
Cam timing and ignition timing are not the same. One will turn your bike into a maraca within one cycle, the other takes longer.

1. Have you pulled your bowls off to see if they are filling? As an idiot gauge, basically when you pull it off a little should spill out and the fuel should at LEAST be over the small ledge in the bowl.

2. Turn your idle stop screws in until they first seat. Then continue to turn them for one full turn. That should get you started.

3. Your advance may be stuck. Pull off the points plate. Pull off the advance. Clean the cam on the advance and oil it. Now open it (turn the cam part anti-clockwise) and it should immediately snap back. Secondly, take the little chunk of fiber off of the points plate, dip it in oil, let the excess run off then install that last. It should contact the cam when it's installed. It cleans and oils the cam.


Timing these is hella easy if you're into that kinky stuff. Why you're trying to (KICK!!!) start a bike before it has been done is an indication of just that.

If you're going to set it static, probably better to set it between the F and T marks. If you have no light, you'll check the plug for temperature and it will tell you if you should advance it any further.

Take your feeler gauge (you do have 'em, right?) and set the left breaker gap at it's widest point. I believe the widest point is when the dot on the points cam is 180º from where the heel contacts the cam. If I recall, it should be between 0.3-0.4mm. Check the manual. Once the gap is set, make sure the screws on the left breaker-arm are tight!!

Now that the gap is set for the left, you set the timing by rotating the points plate as a whole.

Pull the plugs before setting timing. Compression complicates things.

If you have a continuity checker function on your multimeter or a test-light:

Unplug the yellow and blue leads from the points housing. Connect one end of your continuity checker/light on the yellow (left point/cylinder) wire and the other end on a good frame/engine ground. I find that tucking the lead under the rubber insulator at the end of the points leads holds it in place against the metal connector just fine. Then shove the other end in a crevice in the engine.
The way this works is when you have continuity, the coils are powered. When you lose continuity, the power is lost creating electromagnetic induction that gives you spark. So the moment continuity is lost is when it sparks.

If you don't have any way to check continuity:

Leave everything plugged up to the ignition as normal.
Put a spark plug into the left spark plug boot and rest it securely on a frame/engine ground. Exhaust pipes or the spark well are fine, just be sure you could clearly see it spark. This is a bit more difficult to do than continuity tools because you have to turn the ignition on as you rotate to the timing marks, but remember to never leave the ignition on for more than a minute as you're constantly heating up the coils and using battery power.
The way this works is simple, when you see a spark, that's when it sparks. Just remember you're going to be switching the ignition on and off a lot, and to not forget to turn it off!

Remove the round plate on the left side of the engine. Now you have your rotor with marks on it and a brass indicator.

Place a box-end wrench on the nut that holds the rotor, and rotate anti-clockwise only! Rotate the rotor and look at the valves through the spark hole until you reach the LT mark, since we set the left side first. Did you see the exhaust valve closing as you reached this mark? If not, you are on compression. If you did see it close, you're 360º out or on exhaust stroke and need to rotate the rotor another 360º.

So, now we're on compression on the left side. Look at your points cam. Notice that the heel of the left (yellow side) arm is in contact with the points cam just clockwise of the big depression. Now you will always be able to tell whether you're on exhaust or compression. Compression will always have the heel close to the depression. Exhaust would be 180º from the depression. Same will be true with the right (blue side) heel when we move on to that side.

Now we're oriented. You're on LT on the compression stroke, correct? Cool, now rotate 360º so that you're on exhaust. (I know, right?)

Now that you're at LT on the exhaust, turn on your tester or ignition, whichever method you're using.

Continue rotating the crank until you're within 10º or so from the F mark and slooooow down. You want to observe where the spark is or where you lose continuity.

Now that you know if the spark is late or advanced, you can turn the points plate accordingly. Just barely crack the screws that hold the plate so that you can turn it. If these screws are too loose, they'll throw off your timing when you tighten them. Just so you know, there are two notches at the top of the plate. I use a flat-head screwdriver to leverage the plate when adjusting.

Turning the plate clockwise will advance it and anti-clockwise will retard it, since the cam rotates anti-clockwise.

Keep adjusting the plate, checking timing, adjusting plate, checking timing until you've got it right where you like. Carefully tighten the points plate screws and check twice more to be certain the timing did not wander on that cylinder.

Now we repeat for the right cylinder using everything we learned here with only one difference: you will be adjusting the point-breaker arm instead of the plate.

Check the gap on the right breaker at 180º from the dot on the points cam, just as a starting point. Anywhere close to or around 0.3-0.4mm is an ok starting point.

Rotate the rotor until you find the right cylinder's current timing.

Now adjust the arm accordingly.

To advance, you bring the heel closer to the cam. Retard it by doing the opposite. So rotate the arm clockwise to advance and anti-clockwise to retard. You'll be doing such minute adjustments that you will literally not even see the arm moving as you adjust it. Loosen the screws that hold the arm in place, then gently tighten them down. I do this because I make my adjustments by gently tapping the arm one way or the other with the end of a plastic screwdriver handle and you don't want a lot of movement. Again, you won't even see it move when you're adjusting it, it's very fine.

So after you've cranked the engine over 20 times and tapped the arm a thousand times, you should have this one aligned to whichever degree you chose for the other. Tighten down the screws on the arm, TIGHT, and pray the timing isn't affected. Rotate the rotor over and check the timing two more times.

It's very frustrating when you do all of this work and then tighten a screw and it's all thrown off and you have no idea as you're kicking the thing. Save yourself the anger and check it again before buttoning up. I stress that the screws for the arms should be tight because they will wander. These twins vibrate. No, they buzzzzzz and anything that isn't secure will shake loose and wander.

Finally, check the gap for the right point.

Don't melt your coils.

Long write-up, but I've been away from the web for days. Too much work here. You're welcome. ;D
 
Sounds like a fuel supply problem. Either the bowls have insufficient fuel or perhaps it's just cold and needs more choke.
 
Re: CB 350 idles for 3 seconds

tenseventythree said:
The idle screws are cranked. I thought you need an open choke to start. Maybe too much air not enough gas?

APPLY the choke. When you use the choke it should CLOSE the choke plates to start it, not open them.
 
Get a factory manual.
Has a step by step section on a tune up...do that.

More then likely other "mods" are having a negative effect on the bike.
Short exhaust and pods?

Have you checked compression?
 
CB 350 idles for 3 seconds


teazer said:
Sounds like a fuel supply problem. Either the bowls have insufficient fuel or perhaps it's just cold and needs more choke.
I noticed that it ran longer when i put gasoline straight through the carb body into the intake. I already broke down the carbs with a rebuild kit so all the jets and passageways are clear. I noticed that there was no fuel in the bowl at all even though that passage(float needle) is clear too.
It has stock air boxes and stock exhaust.
I tell you though when I dropped more gas into the intake. It ran longer.


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
Pull the bowl off of the carb, whichever carb is on the same side as the petcock.

With the bowl off, hold it below the float to catch fuel as it pours. Turn the petcock to on just enough to determine if fuel is making it down that far.

Tell us what you get with the bowl off.
 
CB 350 idles for 3 seconds

Ok. I got it.
The float was way tweaked so that the needle didn't get to release. I adjusted and boys does it work. She's a "one kicker". Thanks to all who chimed in for this diagnosis. This forum rocks.
ejupuhar.jpg



1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
CB 350 idles for 3 seconds


Redliner said:
So no need to adjust or check the timing ;D
That was one of the things I had to rule out. This was like my 69 beetle all over again.


1969 Suzuki M15-2
1971 Honda CB350
2012 Triumph Bonneville T100
 
I think that Red's point is that you must make sure that timing and valves are also spot on according the the manual.
 
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