Topic: 2 and 3 not getting hot but still getting fuel and spark

[cbcoker]

my cb350f is not getting hot on 2 and 3. I have set timing, replaced the points and condensers and the coils and plugs, still no love. When I check the plugs I find gas on them so they are obviously getting fuel. Argh!

The carbs have been rebuilt but the only thing I can think of is that the fuel is, for whatever reason, not being properly atomized. There is consistant compression across all cylinders so a blown head gasket is not the issue. Although I would suspect the ignition, as 2 and 3 run off the same coils, I have replaced, rechecked and regapped everything and am unsure about what to do next.

I plan to swap coils with 1 and 4 just to see but other than that I guess I will go through the carbs again to look for anything obvious. 1 and 4 are running a tad rich but then the bike is only being ran at idle as it dies whenever I try to grab any gas.

Anything helps and thanks in advance.

[Tim.]

you say you've confirmed spark at the plugs, or are you assuming you have spark because all the components have been replaced?  You need to confirm the plug is actually firing and at the right time.

[cadler]

replace the plugs first, just because they fire outside the bike does not mean they fire under compression.

[TwoTired]

Sounds like, despite replacing components, you still have not visually verified that 2&3 spark plugs actually fire across the spark gap.
Take one and only one out and lay the connected plug on the engine block.  To verify proceedure, do 1 or 4 as a demonstration proof and confidence builder.

Is power verified getting to the 2&3 coil? 
Does power get to the open 2&3 point set?
Does power drop to zero when the 2&3 points actually close?

Cheers,

[736cc]

Probably something ez to fix but you have to find it. Double-check all the connections on all the parts you replaced, check timing on the 2-3 points. Are you POSITIVE the carb floats aren't sticking or that the 2-3 carbs are getting fuel?

[cbcoker]

I have confirmed spark at all plugs, one by grounding out the electrode and two by the fact that my induction gun does off when the pickup is placed along the plug wire. The plugs are very new, under 10 miles, so I do not believe that is a concern.

I am getting power to the coils, and to the points open, and not when they are closed.

The carb floats is possible but I have cleaned the carbs to a high degree of cleanliness but will check on the floats sticking. I was going to swap coils just to see if that helps at all and from there I was going to disassemble carbs two and three looking for anything of interest. The gas is also a bit old so maybe I'll through in a gallon of fresher fuel.

Carb three will overflow if I leave the petcock open which is another reason I am probably going to need to disassemble carb three.  By the way, even if my float were stuck open, that doesn't mean that more gas is going to get sucked up into the venturi does it? I mean there is still only going to be so much pressure and whenever there is a pressure difference fuel is going to get sucked up into the jet stream. But since there is only a finite amount of fuel that can be drawn through the various jets, how could more fuel in the bowl result in more fuel being drawn into the engine, I understand that the converse, if too little fuel is getting in the bowls, how that WILL affected performance but miss the connection when it is the other way around.

If I am full of it just let me know. I just figure this stuff out the best information I have at the time.

[750goes]

Possible weak spark on 2 & 3 - can you trim back the plug leads - also check the plug caps are screwed on nice and firm - if they are loose at all they may not be working properly.

is static timing on 2&3 OK...

check fuel flow to 2&3 by releasing drain screws - lots of fuel - clear or contaminated ??

[Jonesy]

Did you check the resistance of the plug caps? They could be way out of spec. Also, check the connections in that supply voltage to the coils. Look for corrosion or loose bullet connectors. A loss of just one volt on the primary side of the coil can mean a loss on the order of around 1,000 Volts to the plugs.

Also, you say the plugs are wet with fuel. You mean just damp or dripping wet? Check the float heights. When the air passes through the carburetor, it goes through a narrowed section called a venturi. The velocity of the air increases through this reduced area, causing a pressure drop (the Bernoulli principle) This pressure drop is what draws fuel up through the jets and into the airstream passing through the venturi. Fuel level in the bowl is critical as this pressure drop can only act on a very small area and it is fighting the weight of the column of fuel being drawn up through the jets. If the fuel level is low (but high enough to reach the jet), the fuel must be drawn up quite a ways through the passage to the venturi and require a greater vacuum to do so, making for a lean mixture. If the level is too high, The fuel doesn't have to travel as far as it normally would, so it does not need as much vacuum to pull it, making for a richer mixture. Think of a soda straw in a glass of water. When the glass is full and the straw only sticks up a bit above the water, it takes very little suction to draw the water up, but if the glass is almost empty it takes much more.

(Hope I explained it well...)

[cbcoker]

What do you mean by "resistance of plug caps?" Is that the same as checking primary and secondary coil resistance? Because I have done that and was within specifications. I will check coil connections but believe they are alright. Good point about losing one volt on the primary side and its effect on the secondary voltage to the plugs. I guess I never thought about that.

No, the plugs are not literally dripping with gas, but they are damp. Mind you the bike only runs for ten minutes or so while I am tuning the thing so a little soot and the like is should be expected.

I am experiencing some overflow out of carb three and plan to remove the carbs tomorrow after I swap the coils to see if two and three will get hot then. I will go through the carbs again and recheck float levels. Thanks for the description of the fuel being drawn-through the carb as it simplified what I had already suspected.

I'll post up whenever I have some results...

[Jonesy]

When I mentioned checking the plug caps, I meant removing the plastic caps that fit to the plugs and measuring their individual resistances. (You unscrew the caps off the wires for removal) I *think* the stock caps are around 10Kohms, Aftermarket NGK ones measure 5Kohms. These can break down inside over time and get way out of spec. They are easy and cheap to replace if found defective.

Another thought, do you have standard or resistor-type plugs installed? Since there are already resistors in the plug caps, you don't need resistors in the plugs. Having both will make it tough for your igniton system to function properly.

Good to hear the plugs aren't dripping with gas (seen it happen). If the plugs that aren't firing are the least bit carboned up, try cleaning them before re-running, as NGK plugs (if that's what you are using) have a tendancy to short out rather easily when carbon fouled and damp with fuel...

[Hockers Choppers]

Do you have the accelerator pump carbs? My 78 cb750f did the same thing, cleaned the carbs 4 times! Finally pulled the pressed in idle jets and found little pieces of the o-ring for the idle air screw stuck in one of the passages. Blew out every little passage in the carbs, sprayed carb cleaner thru them all and blew out again a few times. Now it starts right up on all 4. I'd say that is your problem. Sounds like you have spark. Check compression?

[n8andles]

Had a similar issue with my 77 cb750.  Everything checked out with my tests...spark, float was not stuck and would flow gas when I would lift it up, air flow was fine.  I eventually noticed that the float on the cylinder I was having issue with was in upside down, thus not letting in enough fuel.  Just a thought.

[tmht]

Do you have the accelerator pump carbs? My 78 cb750f did the same thing, cleaned the carbs 4 times! Finally pulled the pressed in idle jets and found little pieces of the o-ring for the idle air screw stuck in one of the passages. Blew out every little passage in the carbs, sprayed carb cleaner thru them all and blew out again a few times. Now it starts right up on all 4. I'd say that is your problem. Sounds like you have spark. Check compression?

350's do not have accelerator pumps.

[Hockers Choppers]

Hey Coker, Ever get the 350 to fire on all 4?