Topic: 750 FO carb work

[Don R]

My 75 F has a problem with gas fouling plugs, It will foul a new set in 2 miles running hard but idles like a taxi. I won't bore you with all the things I checked, fuel, floats, needle & seat etc. On this model carb can the metering rods be moved and if so which groove should they be in?

[Spanner 1]

Don, if their 7A carbs the needle should be in position 3.........if plugs foul in 2 miles it must be smokin' rich ! Float level @ 26mm ? Is it stock filter/airbox and exhaust ? Do all the plugs foul equally or just one or two ?  If your confident of the needle valves being clean, try taking-out each float and test for any leaks by submerging them in a glass jar of gas.....

[Don R]

Everything stock except for a custom muffler. I have run it without a  baffle, no help. New filter timing etc OK. This bike sat for 15 years, the carbs look new inside but it has had a series of owners, none willing or able to bring it back to life. It's a really nice one but the carbs are starting to frustrate me.  Plugs 1,2,3,black 4 brown I'm also draining the tank to try fresh gas. I was able to run it on the road for a while, about 18 mpg over 80 miles. Thanks for the input. Back to the shop.

[Jerry Rxman Griffin aka MuthaF'er]

Needle jets in middle position.

75 is F or F0. 76 is F1. At least in the states.

[Don R]

Ok, F0 then. Right now it's F0Oey. I'm spoiled by my K0 carbs where I can pop the slides right out. I'm gonna have to pull them the rest of the way apart later. Enough for one week.

[TwoTired]

Check the actual fuel height in the carb bowls. (The clear tube up side trick.)

Check the jet numbers as well as the hole actual size to see if it matches the number stamped on it.

I won't bore you with all the other possibilities.

[Don R]

Thanks, I didn't think about the jets being drilled. These jets say 105 but as we have learned, never trust a P.O. 
 How does the clear tube hook to the bowl?  Custom fittings? Maybe I could drill a couple drain plugs and solder copper tube into the holes then slip clear hose over the tubes.
  My drag car has glass sight windows in the 1150 carb, that would be nice too.

[TwoTired]

Some carb bowls use the overflow nipple at the bottom as a drain also.  Opening the "drain screw" allows fuel to drain from the bottom of the carb through that nipple.  Just attach clear tubing to the bottom tube, open the drain and the tube should fill with gas from the bowl.  Run the tube up the side of the carb bowl and you can clearly see the level of fuel translated from inside the the bowl.

If the carb bowl has separate drain screws at the bottom of the bowl where, when removed the fuel comes out, you either modify a screw to accept a nipple for the clear hose, or you jam a clear hose into the bowl screw threads temporarily, to fill the clear tube from the bottom of the bowl and determine internal bowl fuel level.  As long as it doesn't leak faster than the bowl can fill, you should be able to determine internal levels from your makeshift sight gauge.

You are looking for internal fuel levels slightly lower than the where the bowl base gasket sits.  Higher levels than that can more easily trickle extra raw gas into the carb throats.

You have checked for sunk floats, haven't you?  Brass floats can get holes in them.  They don't float very well, then.

[Don R]

 I gave up brass floats after sinking six with my K0, these are the newer type and look good, I have a set of junk carbs, the float posts are broken off two of them. I could cut off the inside overflow at the bottom of a bowl and use a clear tube on the outside of that one to check all my floats, one at a time, right? A little information and inspiration is all we need. Thanks. I will also check the spare carbs for jets to compare these 105's to.

[Don R]

The needles are in the middle position, are stamped 271307, the 105 jets fit a pin that measures .0042 sound right so far?  10.5 mm= .413"(found on a chart here) so,  .105mm= .00413" ?
Next I will drop the floats in gas and also make float level test bowls. All passages are open and were cleaned again anyway.

[TwoTired]

Just checking the math..
A #100 jet is 1 mm diameter.  A #105 jet is therefore 1.05 mm, which equals 0.04133858267721 inches (using an on line converter).

I hope your report only has decimal issues...

If you can find a #59 drill bit (1.0414mm, .041"), the shank should to "just" fit into the orifice. And a #58 bit (1.0668mm, .042") should be a no go (if the #58 goes in, then the jet has been drilled).

Make sense?

[Don R]

Yes, thanks decimal issues on my part. These carbs check out really well, I have to assume now there were issues with the fuel in the bike. I did find some liquid in the bottom of the glass container I dumped fuel into, probably water. The next time it is not raining I'll try another test ride with fresh gas. Thanks for all the input. I know a lot more about these carbs now, I won't have to pay to have any cleaned again.
 If this was easy the PO's would have had it running and I wouldn't have bought it so cheap, so I'm gonna have to do a better job than they did to enjoy this F-er. Thanks again for the info and encouragement.D.

[Don R]

Can anyone shed any light on adjusting the idle screws? It is a 75F and I have the black plastic screws that can only be turned one turn stop to stop. The manual says 1 + or - 3/8 turns which I can't do. ? When I rev it from idle I get black smoke from the exhaust.

[Don R]

OK I set the plastic idle screws to the halfway point. Put some gas and seafoam in it and went to the station for a fill-up. It is way better but still not quite right. Timing checked with a light spot-on. and the advance worked well.  I trimmed the plug wires but I think I left my meter in the race car trailer when I stored it for the winter so I can't check the wire caps. Should I swap condensers with another bike to test them? It runs well under power and pulls well to 80mph when I back off the throttle it breaks up and has a mid range steady speed miss.

[Don R]

 Rode this beast over to the most recent PO's house, he said it ran rich for him too. But he didn't have the brakes done yet so he could ride it. He also said also the plastic  carb adjustment screws come off and there are more adjustable screws under them. I knew that a long time ago and forgot. This is the best it has run so maybe I'm getting close.