1981 CB750C Build

[iGreek]

Didn't want to update the thread like this, but here we go.

The bike is currently sitting like this:

I decided to tear it down, because I was going to install new parts and because it started running like crap. It was cold blooded since I bought it, but it wouldn't even idle once it warmed up and just wouldn't stay on. So, I decided to take the carbs apart and cleaned them. I followed this guide religiously: http://deuceswild.yuku.com/topic/903/Carb-Cleaning#.Tq7-XZsr2so

Put the carbs back on and installed the following things: New NGK Spark Plugs, New Gauges (Speedo with dummy lights and Tach from DCC), New Brake Light and Rear turn signals. I also changed the oil. Other than that I changed nothing.

Put in new gas, new fuel filter, and fuel line. Started it up and it was running decent, but somewhat rough. I noticed that it wouldn't idle, unless I was holding the throttle open slightly open. Turned it off and quickly touched the headers. 2 and 3 were hot and 1 and 4 were cold! So, I assumed that they aren't getting spark, since those two are on the same coil

Did a bunch of research, until I couldn't read anymore. Took the coils off and tested them as I read online. Got 2.4-2.5 ohms on both, which seems real low from what I've read. Swapped coils to see if that made a difference, but same thing; 2 and 3 warm, 1 and 4 cold. Swapped igniters and same problem. Tested the black/white wire that goes to coils and got 10.5 volts.

My brain is racked and I can not figure out what the problem is for nothing. Is is possible that the carbs are still the problem, but why would it be that 1 and 4 are cold? Is it electrical? Is the battery possibly bad? I've always have it on a charger.

What else can I check or what do I do at this point? I'm pretty discouraged.

 

[wahvtec]

Sweet bike its what i want when iget a big boy bike. But on to the topic. You need to know if your lacking spark or fuel in the cylinder. I would remove the plugs from the cylinder in question, and attach the wire to the plug. Ground the plug to the block and turn the engine over. If it produces a spark u know it is fuel. Also the plugs may be wet with fuel when removed. You can also undo ur carb bowl drain plugs to ensure they all contain fuel for starters to rule out priblems in fuel lines

 

[iGreek]

Sweet bike its what i want when iget a big boy bike. But on to the topic. You need to know if your lacking spark or fuel in the cylinder. I would remove the plugs from the cylinder in question, and attach the wire to the plug. Ground the plug to the block and turn the engine over. If it produces a spark u know it is fuel. Also the plugs may be wet with fuel when removed. You can also undo ur carb bowl drain plugs to ensure they all contain fuel for starters to rule out priblems in fuel lines

Tested and I'm getting spark on all 4 cylinders. All bowls contain fuel when I drained them.

I'm gonna copy a post over that I made on another forum, so I can show what I've done so far:

I'm going to go over what I've done since I've this problem started.

Went to get the battery tested at Advance Auto Parts. Computer said that the battery was fine. I've always had the battery on a trickle charger and when fully charged it reads 12.45 volts. Quick question; when you turn the ignition on and everything is on, before you turn the bike over and on, the battery reads around 11.65 volts. Is it suppose to drop almost one volt with the headlight on, before turning over?

I did the relay bypass mod. After the mod, the voltage at the black/white wire, for the coils, is 11.63, which is pretty much exactly the same as what the battery reads when you turn the ignition on. So, I assume that's a good thing.

I received and installed the CBR F2 Coils and Wires I got off ebay. Those coils read 3.3 ohms, so it seems they are good.

When I pulled the plugs 1 and 4 looked completely clean like the one on the left and 2 and 3 looked black like the one that is on the right; which I'm assuming is from running rich. Remember that these are new and just put in before I found this problem. Looking at this does it mean that 1 and 4 aren't getting fuel and the carbs are to blame?

I did the test on top of page 17-4 in the CB750 FSM dealing with the pulse generator, after I installed the new coils, wires and bypass mod. All 4 were getting strong spark, visible spark.

Also, when I drain the bowls from the carbs, all 4 bowls have fuel run out quite evenly.

So, I assume that I am getting spark on all 4 cylinders, that is not the problem and I don't have electric problems and that the pulse generator is good. Can I assume that? Am I right to assume that spark is not the problem now? It really seems like the carbs now...

 

[wahvtec]

Yes i believe you have thoroughly tested your ignition circuit and she is good to go, at least to produce some fire. All of the plugs should have something on them or that cylinder should have smelled of fuel once the clean plug was removed. I would remove the carbs in questions and give them a good going through.