March 2011 6

I have a Seca 750 1982. Once bike gets up to temp the idle stays high. When you let off the gas and pull the clutch in the RPMs stay high. this will drop back down if you let the clutch back out or work the throttle. If you turn the idle screw down the bike will idle to low. Any help?

If turning the idle screw down causes the bike to idle too low, and turning it back up causes it to idle too high, then I’d bet you just need a simple tune up.

When was the last time you adjusted your valves?

I bet it’s been a while since you synced your carburetors too!

Doing those two things I’m almost positive will solve your problem (assuming the bike is running good otherwise).

It’s also possible you have a clogged pilot jet or two. Have you had the carbs opened up for a cleaning in a while? When it’s idling too low is it firing on all 4 cylinders or just 3?

Cheers.

I have recently just rebuilt two carburetors for a Honda CB500T. I am having problems keeping it running below 2,000 rpms as well. I have noticed that the air cut off diaphragms are torn a bit and probably should be replaced, but I can find no new replacements ANYWHERE! What exactly do these do for the carbs and what are my options in fixing these? I am thinking about trying to patch them somehow with something light as an epoxy or something. Are there any replacement OEM diaphragms anywhere that will also work in this carb?

Thanks!
Andy

It will be hard to find brand new replacements for those diaphragms. However, if the tears or holes aren’t too big you can indeed patch them with a thin coat of epoxy. I have read of others using Plasti-Dip to repair them as well with good results.

The diaphragms are used to raise the slides and needle in the carb. The intake vacuum created when the intake valves open acts on the diaphragm and lifts ups the side allowing more gas to be released. The wider the throttle is open, the more vacuum there is, the higher the slide is, the more gas there is, the faster you go!

Looking for a Clean SR 500…

If anyone knows of one for sale please contact me!
Will pay top dollar as long as it’s worth it…

I’ll keep my ears open for you, but don’t have contacts with anyone selling one at the moment. There are however dozens of them on craigslist at any given time – including right now.

I have a 1976 Honda 550F that needs a set of carburetors.  I’ve been looking on ebay and have bid on a couple of sets.  My question is most carb descriptions only give limited info regarding if the carbs are from a 550F (98 main jets/38 idle jets) or 550K (100 main jets/40 idle jets).  Are these carbs interchangeable-will the different jets much much of a difference?

Great blog-thanks for your help.

As far as I know bustoff21 the carbs are exactly the same with the exception of jetting, but I’ll see if I can do some checking for you and find out. What is wrong with the carbs you have?

When I got bike the carbs were missing.  I did pick up a set of carbs on ebay with 100 main jets/40 idle jets instead of the original 98 main jets/38 idle jets.  Will the slight difference in jet size make much of a difference.

It likely won’t make a big difference, depending on your elevation and bike setup. Only one way to find out! Or you can just swap the jets.

I’ve been intently going over the facrory service you posted for the 1978 Cx500. By the way,  AWSOME!! Thanks SO much. I was wondering if  you could possibly hook me up with chapter six. The link is no good and that’s the one I  really need at the moment. So, if you can hook it up I’d appreciate it.   Thanks, Joe

Hi Philly –

I’ve had dozens and dozens of requests for chapter 6. I don’t have another copy from that manual that works. I do have an entire different manual for the bike which I will get online real soon. But it’s huge so will take a while to upload.

I’ll post back when I get it up. If you have any questions about something you’re working on I’ll help as best I can.

Cheers

Hello all,

I am in the restoration of a 1975 Honda CB500T. I got it running yesterday, but noticed I only have one cylinder firing. I tested the points and they were in spec and ran a Ohmmeter on the coils independently and  one was pooped out the other was weak. I also noticed they had broken spots on the spark cables and since they were “no good” I cut the rubber on the cable and sure enough the copper wire was broke and was to blame for the coil failure. I retested from the break point and got a better reading. My question is since these are no longer available from Honda OEM what are my options? A used one from ebay sounds risky seeing they are old as well. Can the wires be repaired if the coils are fine? Would an after market 5 ohm coil work?

Thanks for any responses,
Andy

On most of the old Honda CB’s the coils and spark plug leads are molded together so the wires are not replaceable. If this is the case you do have a couple options.

I have read reports more than once from people who have converted their coils to use replacement wires. You can cut back the wire so there is just a little stub then solder/jb-weld on a cap so replacement wires can be used. This can be done with both motorcycle or auto parts. I haven’t personally done this, but see no reason why it wouldn’t work, and would certainly be the budget conscious choice.

The other option is of course to buy new coils. Classic Motorcycle carries generic/universal coils for old Hondas and I’m sure you could dig them up other places as well. 5 Ohms is correct as long as you are using a ballast resistor. I can’t recall for sure if the CB500T had one factory or not, either way it can be easily setup if not.

Are you sure your bike doesn’t already have replaceable plug wires? They appeared at different times on different models, but here is a coil from a CB500T with replaceable wires. It also has a ballast resistor wired in.

New spark plug caps and wires for the CB500T can be found many places, Parts n More is one such place.

Thanks for the help!

That looks just like the set I have, but I don’t think mine are removable as far as the plug wires go. I have heard about the ballast resistors before. What exactly are those in the picture? I am not too familiar with how those work or where they are. Would my set have one wired in already then? I can upload a picture, but it looks just like the photo you posted.

Pardon my haste – the photos above show a condenser (bronze tubular piece), not a ballast resistor.

A ballast resistor is a simple device used to add resistance to a line. In the case of moving from 3ohm rated coils to 5ohm rated coils you need to use a ballast resistor to insure the coils don’t overheat. A ballast resistor rated at 1.5ohms would be suitable for this application because they tend to over perform once they’ve heated up. It is simply wired directly inline with the leads to the coils.

Now, we are talking about all of this, but I’m not sure for certain what the factory coils were rated at. Most points ignitions have factory coils at 4 or 5 ohms. It is typically electronic ignitions which run at 3. You should verify for certain what factory coils are rated at.

Here is a Honda CB500T wiring diagram to help:

Thanks for the posting! It is nice to see the wire colors as well that really helps keep things straight. I have 4ohm coils on the way. Since I need 5ohm resistance for the honda CB500T set up here to keep them from burning out early I will need a 1 ohm resistor wired in. The correct placement for the resistor is in line on each coil on the blue and yellow leads after the connection with the condensers correct? I would have liked to find a resistor at .75 ohms or something just less than one to compensate for the over performance, but i can’t find anything rated at least 20 watts or higher at that low of resistance.