Topic: No fuel on cylinders 3 & 4?

[Lamp]

Well I think I've got my electrical side taken care of, but now I'm not getting fuel into cylinders 3 & 4. I can get it running but it sounds like a Triumph. I can cover the carbs for cylinders 3 & 4 with my hand and it doesn't change how it runs. So I'm only running off 1 & 2. I'm hoping this is just going to be a common problem with an easy fix. Any ideas?

[harisuluv]

You need to give all the specs on your bike first.  The more the better.

You can probably eliminate the coils as they operate 1-4 and 2-3.  That does not mean it's not electrical.  Do you have spark on 3 and 4?

What's the situation with your carbs?  Did you clean them recently or have you opened them up?

[Lamp]

1972 cb750, stock carbs. I know the coils run 1-4, 2-3 which is why I've ruled them out. I haven't checked spark but I'll get it started, then shoot some starter fluid down carbs 3-4 and see what happens. I imagine it will fire on all 4 for a second, then back to 1-2. I just wondered if there was an easy fixes this problem.

[Deltarider]

Well, maybe it's a dumb question but does fuel reach the floatchambers of 3 & 4?

[Lamp]

Not much if any does. When I pulled the carb bowls to check float height, there was verrry little fuel in carb 3-4 bowls, but 1-2 were up right where they should be.

[harisuluv]

How do you have your fuel lines routed?  For a 72, there are two fuel lines, if it is routed incorrectly they won't get fuel correctly.  Just an idea.

[Lamp]

I think I'm just getting antsy and over looking simple stuff now that I'm oh so close to being "done." Probably just a simple blockage somewhere. I'll find out more Saturday when I have a little more time to mess with it.

[lucky]

First take a timing light and clip it onto each spark plug wire one at a time and make sure you see a steady pulse.

No need to take the spark plugs out.

Please let us know what you find.

[Lamp]

Well go it fixed. The oring I was using on the petcock was a little to thick, and I've been running it on reserve while I work all the bugs out, since there's only about a gallon of gas in the tank. Anyways, the oring swelled and blocked the reserve pick up. Swapped a smaller oring in, choked it a touch and it fired right up.