Topic: 1972 Honda CB500 - Cyl 4 Running Rich

[b52bombardier1]

Hello,

  Yes, as in the other thread about cylinder #4 being the worst, mine is also the worst of the four on the bike.  The main jet carb body tower is somewhat corroded and I think that is what makes the main jet a little wobbly to the point that unmetered fuel is likely getting past the main jet o-ring.  The fit is almost to the point where the main jet could fall out into the float bowl.  All jets are currently aftermarket but my Honda OEM Keihin jets are in hand and I will install them this weekend. I have no manifold vacuum leaks, the valves and points are adjusted and the advancer functions well when viewed with a strobe timing light.  The petcock is new, the fuel tank is very clean and has been lined with POR-15. 

This is a bike that I bought from an ex boss for $800 back in May and he was the first owner - he bought the bike new and rode it in college. He got it going again in about 1978 for a short while but it had been parked since that time.  I bought the bike with a little under 7400 miles on the odometer and I have put about 250 miles on it since it roared back to life in mid summer.  The carbs were absolutely horrible and the fuel tank the same all gummed up with nearly 40 year old fuel.

  The #4 spark plug soots up after a mere ten miles or so. It will even soot up at idle after a few minutes. The idle struggles to slow down to a normal RPM.  I have a 40 idle jet and a 100 main in all four carbs - aftermarket, not Keihin. All four pipes are within 5-10 degrees of each other on my infrared temperature gauge but number 4 is consistently the coolest. Compression is around 145 psi across all four cylinders on a cool engine vigorously cranking with the throttle held wide open.  I replaced all four spark plug caps with NGK 5000 ohm caps because mine ranged quite wildly on resistance with one nearly at 12,000 ohms.  The #4 spark plug cap was at 8300 ohms before I replaced it. I have attempted to sync the carbs but in this running condition, I'm not sure my effort was valid.

   So you think that JB Weld to tighten the fit on the #4 main jet is a bad idea?  I wonder if it would work that I soldered on some extra metal around the circumference of my new Keihin jet to take up the loose slack?

Who would have a decent quality #4 replacement carburetor?

Many thanks,

Rick

[scottly]

JB weld won't last submerged in fuel. Have you done the "clear tube" test on the #4 carb?

[b52bombardier1]

Yes. Fuel rises to about 2 mm below the bowl edge. Just about right I think.

Rick

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[flybox1]

Nothing can tighten the fit of a main jet.  Get a good viton oring on there.
If its not cracked, you will be fine.

Is it sooting up after idle only?

[b52bombardier1]

No, not just at idle. It will soot up after ten miles or so on the road.

The tower in the carb body that holds the main jet is somewhat corroded. I'm thinking out loud here about adding solder to the base of my new Keihin main jet to tighten the fit and make up for the lost and corroded metal.

Rick

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[b52bombardier1]

Now that all four carbs have Keihin jets installed, #4 has stopped sooting up. The engine also idles better but the plugs are still blacker than they should be and will eventually foul again. But at least I'm headed in the right direction.

  I think I will lean them all out with raising the needle clip one notch.  I've never done this but there's a few You Tube videos out there.

Rick

[calj737]

You'd be better to lower you fuel in the bowl than to raise your needles. Since the sooting also occurs at idle, its an indication the problem precedes the needle clip. 2mm below the seam is too high. And yes, 1-2mm makes that much difference.

[Deltarider]

And yes, 1-2mm makes that much difference.

Not on anyone's.

[b52bombardier1]

I'm going to attempt to buy four new spark plugs today and see how these changes play out.  In my first time doing it, it only took me an hour to make my needle height changes on all four carbs so undoing the change in height is not so bad if I have to.  My worry is whether I might eventually need to replace the sheet metal keepers that embrace the two small bolts on each throttle slide arm. Metal fatigue is not kind to those small tabs.

Does Honda or David Silver still stock these items?  I will have to check and buy them as future insurance spares. 

Making changes to the float height is also easy to accomplish and will be next on my to-do list if necessary.  In any event, I thank you for reading my posts and providing advice.  With your help, I think I'm finally on the verge of making this a dependable bike.

Rick

[Deltarider]

Making changes to the float height is also easy to accomplish and will be next on my to-do list if necessary.

If you check float levels with the clear tube test I recommend to have the bike idle (no, these engines don't overheat in 10 minutes) to have at least some useful vibrations. I have the suspicion that many here mistake an insufficient fuel supply and/or an intermittent sticking floatneedle (the more with todays gasolines) for an incorrect position of the floattang that most probably nobody has ever touched before.

[b52bombardier1]

Well . . . she's running pretty good.  Four new oem Honda float valves have stopped my bowls from leaking. Four sets of oem new Honda main jets and idle jets have gotten the bike to where it idles well and accelerates nicely. A 100 main and a 38 idle jet seems to be the trick.

I learned a lot about these carbs along the way. Putting the needle clip on the top notch is way too lean but the middle notch is too rich. So I'm at the second clip from the top and doing nicely.

I will continue to monitor for overflow leaks but I think I'm done there and I will keep an eye on the plug color.

Rick

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[BomberMann650]

Is it the bodies or the brass getting corroded?  Might be able to find a suitable replacement body on ebay.

[b52bombardier1]

It was the #4 carb body at the main jet area that was corroded but I worried there for nothing.  My problem was that I did not realize how the main jet butterfly spring worked to hold the jet in.  The aftermarket o-ring around that aftermarket jet was a little loose fitting but that problem is solved now with a Keihin main jet and its good o-ring.

  I think this bike is finally headed in the right direction but I will check the plug color tomorrow.

Rick