Topic: Strange firing problem

[troppo]

G`day guys
I took the tank off my 76F750 to fit the freshly painted one, a little prematurely as i now need to get a petcock O ring first, so i fitted the old tank back on....
Started her up and it seems she isnt firing on cyl`s 3 and 4, i`ve done a search and didnt find anything i thought could help...
I fitted new plugs a month or so ago, it was running fine before i took the tank off but now it runs like crap.
I did the touch test and the pipes for 1 and 2 are getting hot but the pipes for 3 and 4 are cold.
While the tank was off i hit the carbs with some carb cleaner but didnt remove the pods or take the carbs off the engine, now before you say "rip the carbs off and clean them" carby work is the dark arts as far as i`m concerned (bloody scarey and way beyond me, i`m a hammer hand not a grease monkey lol) i dont have kits for the carbs and the local honda shop is staffed and run by snot nosed twerps that think my machine should be scrapped not rebuilt (no help in other words).
So, is this something simple or do i have to sell my soul for the ability and take off the carbs?
Please help, i was hoping to get it rego`d this week
cheers guys
troppo

[oldbiker]

Don't hit me if this seems stupid, but are you sure you didn't accidentally switch the plug leads to 3 and 4 when you had them off? That would certainly result in 3 & 4 not firing.

[troppo]

never went near the plug leads mate, didnt touch anything apart from spraying some carb cleaner on the carbs.

[SteveD CB500F]

Do the F's have two fuel feeds?  1+2, 3+4?

It can't be ignition so must be fuel. Either too much or not enough.

On my K2, the one of the inline fuel filters was blocked. I could blow through it but no way would fuel flow under mere gravity.

[johnny_from_bel]

I'd say fuel. It could be that the overflow tubes are piched somewhere.
Floater stuck, give it a gentle tap.

Unscreew the drain plug to see if there is any fuel in the bowls.

[troppo]

i was thinking fuel too johnny, but its happening on two cylinders, would there be something causing both to get stuck, or both tubes to get pinched?
I didnt fiddle with anything that i can think would cause it, but i will go out and unscrew the drain plugs tomorrow to check if theres fuel
cheers mate
troppo

[750goes]

Hey troppo,

sounds like a simple thing but the F should have a single outlet from the tank and then use a "T" piece to split off to carbs 1&2 then 3&4, remove the float bowls off 3 & 4, then remove the fuel line going in between carbs 3 & 4, spray some of that lovely carb cleaner in the pipe and hopefully it should easily flow out the bottom of carbs 3 & 4, if it does all is good so far...
now turn on the fuel tap and catch the fuel that comes out of your tank from that pipe you just took off - hopefully it will run as well....and the fuel is nice and clean too... hope it is..

if all is well with the above two tests, put the float bowls back on, connect the fuel pipe ...and make sure you have a good gravity feed to the carbs with no "ups" in the piping and kick her in the guts.....

let us know...cheers
 

[troppo]

thanks 750
going to have to print all this out and take it with me out to the bike, carbs are scarey unpredictable things that make your car/ bike stop............
hopefully you are right and if you are i owe you a beer
cheers mate
troppo

[hymodyne]

Low speed fuel jet clogged?

I can get my carbs unattached in the time it takes to get those two middle #%& nuts off...

thanks to countless opportunities to re-do work on them and the rest of the bike. 

[troppo]

OK
now as i said i know nothing about carbs, i have manuals and i have this forum, i trust this forum over the manuals....
What are the low speed jets and where can i find them?
any pictures anyone?
this would help me and probably others in the future

[Patrick]

Check for a pinched or kinked fuel line.

[OldSchool_IsCool]

Trop,

On the bottom of each carb, there is an overflow tube.  There is also a recessed screw that opens a drain to let out the contents of the bowl. 

- turn off your petcock
- insert the free end of the drain tube from carb 4 into a clean, clear container
- open the drain screw
- Does anything drain out into the container? Is it clean, clear fuel? is it about 3-4 ounces?  Any water in it?
- repeat for carb#3
- If you have a good volume of clean and clear fuel:
  - put both drain tubes into the catch container
  - briefly open the petcock
  - do you have a steady flow from BOTH drain tubes?
  - if YES, then fuel starvation is not the issue
- if the bowls had only a small about of fuel, trace your fuel line(s) back to the tank looking for a pinch, blockage or high spot that might trap air.
- if the fuel is dirty, try 750's procedure of removing the bowls and cleaning

It seems to me that this event following so closely behind tank work that you have a fuel line blockage in the tube that feeds carbs 3&4.