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1979 CB750F carbs

Old Air Force Guy

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Hey All!

I'm new to this site, this being my first post. A while back I bought what was to be a completely restored 1979 CB750F. For the most part that seems to be the case, with one exception. I found after riding the bike a couple hundred miles that the fuel tank was terribly rusty. I took the bike out of service until I had coated the tank. I re-assembled the fuel tank to the bike, and found that then every carb leaked from the overflow. So, I'm in the process of cleaning and replacing needles and jets etc. My first carb kit came with parts I assume to be from a later vintage than mine. Some parts are the same, but the needles are significantly different. I went back online and ordered the correct ones. Then, when I went to replace the secondary main jet inserts, I found that my original parts are stamped with the number 102, and the replacements are 100's. Should I use the new 100's or clean and re-use the original 102's? They look identical to my eye. Also, are the needle seats replaceable and available, or does a guy only have the option to clean them? Anybody who could use my incorrect needles can have them for the asking. They have a "T" type head without a plunger. Any help you folks give me would be greatly appreciated.

Old AF Guy
 
'They look identical to my eye.'

LOL, like one could ever hope to see a couple of ten-thousandths difference there. No insult intended of course.

Use the 102s if they have clean openings. But such a small difference either will work, I wouldn't mix them though unless it was the 102 in the middle two cylinders and 100 on the outsides. True jet changes are around 10 at a time not 2. And even then you likely can't see the difference by eye.

The wrong needles? Likely no telling what they fit, a thousand different ones there. No, the needle seats do not replace.

Rust in tank and carbs flooding out overflow when sitting scream ethanol used in the local fuel, these hate that and give trouble in as little as two weeks. Ethanol fuel is transparent when the vehicle gets used every day but oldschool stuff with no sealed carbs will condense water in the fuel in literally hours since the carbs vent straight to the air. I wash parts in fuel and it picks up water in it in as little as 5 minutes here in Texas on a humid day. The carb needles stick at the rubber to flood when the ethanol dries up the fuel in bowls to lower the level, ethanol having the property of increasing the evaporation rate like 400% too. The superfine red rust you may find in the carb bowls is more evidence of it too, practically no filter on the planet can stop the finer stuff from getting into the carbs.
 
I am a member of the VJMC and in the current issue of the club magazine is an article about non ethanol fuel that is designed for long term storage of engines. It's for farm machines, lawn mowers and so on. here in the UK it's called Aspen fuel.
It's a highly refined Alkylate petrol. it's very pure and contains no ethanol. :) It's expensive though :( . When your bike is laid up for the winter drain the fuel tank and carbs dry. Put about half a gallon of Aspen in the tank, run the engine for a few minutes to fill the carbs and switch off, leave the carbs full. |Result no rubbish in the carbs. :D Here it can be bought in garden machinery stores. I don't know if its available in the U.S. Probably is.
 
They have that here in like quart containers for the price of a gallon, super high. For like small lawn equipment.

You can only buy e-free gas in certain sporadic areas far removed from the beaten path as most cities violate EPA pollution standards to force the sale of the ethanol fuel as an oxidizer to lower pollution. My closest point is like 75 miles away.

They DO make a product to add to fuel to 'stabilize' it and it worked on pure gas, later modded to cover e-fuel too. We HOPE. It works by greatly lowering the ability of fuel to degrade by it evaporating the more volatile compounds into the tank airspace, doing it by coating the top surface of fuel with something that changes the ability to vapor off as easy.

Cars don't have the problem as much since they have the heavy evap systems to prevent vapors getting out and relatively closed tight as compared to bikes with vents all over them and short paths to atmosphere. Let them sit long enough though and it happens there too, just had one that sat for 3 years with no issue then a bit later went out to start it (the spare car) and no fuel pump, a brand new one as of 3/4 year ago was so eaten up it was beyond belief. The old one coming out had nothing wrong other than old. Man, when that stuff flips over it happens quick if conditions are right. If you do tank work you MUST seal the tank opening if say yanking pump under back seat to leave tank in place or 24 hrs. later the tank fuel is garbage. Yet I can still run the phase separated fuel in the lawn mower although it spits a bit when it hits the water droplets.

On the bikes and lawn equipment like 2 strokes I learned a while back to simply run them totally out of fuel to have empty tanks and carbs to stop the issue.

Use the device everyday and no issues since the fuel gets used before it can go bad but let it sit and you are asking for it. Once the water gets pulled into the ethanol part then the water combines with other components to make acid and then it goes wild. The ethanol also makes gasoline evaporate so much faster you can't believe it. spill some in the summer and a big puddle will be bone dry in like a minute, why vented carbs like these dry up inside so fast. Why they clog too, the smallest passages instead of drying open dry with a film over them and the pilots on these are that, the high point so they dry and clog fast and then you have no idle fuel and fits getting them back open again. Even worse on the early pressed pilot models as you cannot get to the cross passages for air in top of jet without pulling them out then you lose your pressfit doing it.

I have several devices that sit a lot and HATE ethanol fuel for anything other than passing emissions inspections.
 
I have rebuilt both kehein v43 and the 42... And have never change Jets in 10 size increments unless there were major mods done... Stick with what came out of the bike.. and stick with genuine kehein parts... There's a reason those aftermarket rebuilt kits are so cheap... Their junk.
 
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