Topic: Tried to start her today....no luck

[Scott S]

 First attempt at reviving the CB500 didn't go so well this morning. At least two of the carbs are overflowing, one pretty steadily. That obviously means I need to go back inside them and address the floats and/or needle and seats.

 I was a little disappointed that she didn't even TRY to ignite, though. I have compression. I have spark (at least on #1 and #4, the only two I checked). I have air ( I put my hand over the mouth of the #4 carb and it's sucking HARD when I crank it over).
 I've also changed the plugs, set the points gap, new coils/wires/caps. Fresh oil and filter. I have oil pressure. Valves adjusted.
 I even squirted a bit of starter fluid into the throat of the #4 carb and got no reaction.

 The plugs are dry, so I'm 99% sure it's a fuel problem. Any ideas on where to look when I pull the carbs off again?

 I cleaned them thoroughly, bench synched them and set the mixture screws 2 turns out. I have not messed with the floats yet.

[scondon]

Spray starter fluid into the throat of each carb, not just one, and see if you get any sign of ignition.

[Scott S]

 Just tried spraying starter fluid into the #4 carb and spark plug hole. If I hold the throttle open while I kick it, I can hear some slight rumbles...like it's sorta trying. I also got a little flame out of the #4 carb.
 I'm wondering if the throttle plates are too closed from the bench synch?

 I'll try starter fluid in all four cylinders. Gonna let the battery sit on the charger for a bit first, though.

[xenoscr]

First attempt at reviving the CB500 didn't go so well this morning. At least two of the carbs are overflowing, one pretty steadily. That obviously means I need to go back inside them and address the floats and/or needle and seats.

 I was a little disappointed that she didn't even TRY to ignite, though. I have compression. I have spark (at least on #1 and #4, the only two I checked). I have air ( I put my hand over the mouth of the #4 carb and it's sucking HARD when I crank it over).
 I've also changed the plugs, set the points gap, new coils/wires/caps. Fresh oil and filter. I have oil pressure. Valves adjusted.
 I even squirted a bit of starter fluid into the throat of the #4 carb and got no reaction.

 The plugs are dry, so I'm 99% sure it's a fuel problem. Any ideas on where to look when I pull the carbs off again?

 I cleaned them thoroughly, bench synched them and set the mixture screws 2 turns out. I have not messed with the floats yet.

I also got a little flame out of the #4 carb.

Are they stock or aftermarket coils? Are they backwards if they are? I ask because these symptoms sound a bit familiar. I just installed Dynatek coils and hooked them up backwards by mistake and I would not fire and I got some interesting smoke that came out my pods when I tried to crank it.

XeNoSCR

[Scott S]

 How do you mean "backwards"?
 I'm using these coils from Z1:
http://www.z1enterprises.com/detail.aspx?ID=2857

 I was pretty careful about hooking them up just like the stock coils.

 I was able to get a few rumbles of life using starter fluid in all 4 carbs. It even ran for a couple of revolutions. I'm still not so sure I'm getting fuel.....

[scondon]

Yeah, the bike won't want to start on just one cylinder. Starter fluid on all four will at least tell you if the bike wants to run.

With two carbs overflowing badly it could just be that you're flooding the cylinders with gas. Shut off the petcock and drain the bowls before trying the starter fluid. No need to spray any down the plug hole, just the air intake of each carb. If your airbox is attached, remove the bottom half and filter, then spray into opening.

xenoscr points out a mistake I have made as well. If all you get is THUNDEROUS backfire from starter fluid on all four, it's possible that coils or points are wired backwards. Shot my unconnected exhaust can clear off learning that one

[xenoscr]

The yellow and blue wires are what I'm talking about on my 750. Not sure if the color coding is the same on your bike. There should be on wire that splits off and goes to both coils (black on the 750) and then two separate wires (Blue and Yellow on the 750). If they are backwards the bike won't fire when it's supposed to. Explosions coming out the wrong end was the result in my case.

XeNoSCR

[scondon]

 I was able to get a few rumbles of life using starter fluid in all 4 carbs. It even ran for a couple of revolutions. I'm still not so sure I'm getting fuel.....

Excellent. That's what it should do when things are set right. You're almost there

[Scott S]

The yellow and blue wires are what I'm talking about on my 750. Not sure if the color coding is the same on your bike. There should be on wire that splits off and goes to both coils (black on the 750) and then two separate wires (Blue and Yellow on the 750). If they are backwards the bike won't fire when it's supposed to. Explosions coming out the wrong end was the result in my case.

XeNoSCR

 Yes, I was very careful to match those wires up when swapping the coils. I'm pretty confident they're right.
 No thunderous explosions. One or two small flame fronts out of the carb throat, but that's it. None when it was trying to run.

 I'm new to SOHC's and their carbs, but I understand the float level is very important? I didn't mess with the floats at all. I did find one sticky needle/seat when rebuilding the carbs but thought I had it moving freely....guess not.
 What other settings/internal passages, etc. would keep it from getting fuel?

[xenoscr]

The yellow and blue wires are what I'm talking about on my 750. Not sure if the color coding is the same on your bike. There should be on wire that splits off and goes to both coils (black on the 750) and then two separate wires (Blue and Yellow on the 750). If they are backwards the bike won't fire when it's supposed to. Explosions coming out the wrong end was the result in my case.

XeNoSCR

 Yes, I was very careful to match those wires up when swapping the coils. I'm pretty confident they're right.
 No thunderous explosions. One or two small flame fronts out of the carb throat, but that's it. None when it was trying to run.

 I'm new to SOHC's and their carbs, but I understand the float level is very important? I didn't mess with the floats at all. I did find one sticky needle/seat when rebuilding the carbs but thought I had it moving freely....guess not.
 What other settings/internal passages, etc. would keep it from getting fuel?

I guess since you have fuel spilling out your carbs the float level would be the obvious thing to look at. When you cleaned out your carbs did you blow out all the passages with compressed air? Make sure you do a bench sync before reinstalling as well if you hadn't already. The instructions are in the Carb FAQ if you need them. The instructions linked to in the FAQ for rebuilding carbs has this in it, http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm. It helped when I was trying to rebuild my carbs the first time.

XeNoSCR

[scondon]

For starting the bike, your idle circuits need to be clear. These consist of the four small pilot jets, air mixture screws, and air intake jets(in the airbox side of each carb throat. If you didn't remove the pilot jets and air screws when you cleaned your carbs it is likely they are still clogged.

The carb bowls fill up at a somewhat even rate. Try emptying them of gas and putting the bike on the centerstand. Then open the petcock for about 10 seconds, then shut it back off. Try starting the bike. Repeat petcock open/close for another 5 seconds if bike doesn't start. The idea is to bring fuel level in bowls to proper height without going over. If you can get the bike to start and run well doing this, it will at least tell you that your idle circuits are clear enough and that your float heights are the likely problem.

[Scott S]

 I will try all of these things. I did the petcock open/close things a couple of times. One carb ( I think it's #3...hard to trace that line with it in the bike) doesn't stop overflowing if the petcock is on. It's not overflowing out of the carb throat.It's coming out of the hose that runs underneath the bike.

 This bike sat for 28 years. The carbs were in pretty gummy shape. I cleaned them as best I could (see my project thread here: http://forums.sohc4.net/index.php?topic=67907.0 ), but I was fully expecting more problems with them.
 They have been bench synched. I sprayed air from my air compressor through every orifice I could, as well as lots of carb cleaner. I may not have gotten everything....like I said, these are new to me.
 Wonder if there's anyone semi-local to me that could ultra-sonic clean them?

[scondon]

Is your #3 overflowing the moment you start filling it with gas, or after the petcock has been left on for a little bit? If you start with a empty bowl and gas comes out as soon as you open the petcock, then either your drain screw is not closing properly or your overflow tube is cracked.

[Scott S]

 Not immediately. It only overflows once the bowl is full and the petcock is on. It is coming out of the hose on the bottom of the float bowl, second from right in this pic:


 Again, I "THINK" it's #3. I have the hoses zip tied together and snaked down across the engine/down the frame. It's hard to trace it back up to the exact carb with them in the bike.
 I can fix a needle/seat valve or set a float....my biggest question right now is why I'm not getting fuel.
 I need to sit down with some exploded diagrams and study these carbs a bit more, then pull them apart again.

[thehammer]

those pilot jets get real dirty. take off the float bowls and blow brakleen through the pilot and main jets till you see juice squirting out the openings in the venturis. also make sure the bottoms of the floats sit 22mm above the mating surface of the throttle body with the floatbowl. for the bench sync, i believe you should use a 1/4 inch drill bit as a feeler guage (you should be able to slide the drill bit freely between the cutaway in the slide and the bottom of the venturi opening at full close). now tell me, did you do a spark test?

[Scott S]

 I do have spark, for sure.

 I blew carb cleaner through the pilots and mains when cleaning them, but I'll do it again. These things sat for a LONG time.

 Is 22mm the correct height for the CB500 carbs? I saw 26mm somewhere else, but that was for a CB750. Just double checking....

 I bench synched the carbs but the drill bit I used was smaller than 1/4 inch. Think the slides may be TOO closed for it to start? But...I tried it with the throttle held open and the choke in various positions.

 The biggest clue to me are the dry spark plugs. I just don't think fuel is getting into the combustion chamber yet. I ordered 4 rebuild kits today and will start the cleaning process all over again.
 Keep the tips coming, though!

[steam-powered man]

look for a post by "Hondaman" dated aug 18, 2006. 

on my '72cb750 (last inspected in 199?) i also had a no start.  she wanted to, nothing but pops once in a while. 

i had cleaned the carbs, bench sync, full tune with new plugs/points, set timing perfectly, checked/replaced spark plug caps, new air filter, good boots, good compression, 12.28v at the points, set the floats precisely, new o-rings here/there. and ............ nada.  felt like no fuel. 

removed carbs, took off throttle return spring and fully opened slides to pull up the jet needle.  removed the mains.  cleaned the idle circuts by pushing twisted-strand copper wire in/out until i could see the wire end near the jet needle.  used shots of carb clean and kept working at it until i had enough.  zoom, starts easily now and lotsa power.  i'd try cleaning the idle circuit with a wire, sometime spray just won't do it.             

[Scott S]

 I can't find that thread via search. It would be great if you had a link.
 I downloaded the exploded carb diagram from the manual, but if there are pics on that thread of what I need to check/clean, that would be awesome.

[steam-powered man]

here's HM's post, but w/out pics.  pics show the carb bowl and circut path.   

"This is for CB750, but roughly applies to any slide-type carb.

You already know how to "clean" the carbs normally: remove the jets and spray Brake Cleaner or Carb Cleaner through the holes until they come out clean. This usually works. Some of you are soaking the whole disassembled bodies for days to get the last passage clean: this will speed up that process. After all, mechanics only get paid a fixed amount to "clean carbs"...   

We have all suffered through a (finally gone) period of MTBE-laced gasoline, which attacks the metal of these carbs. The result is a fine white powder (corrosion, actually) that migrates everywhere. If it is allowed to thoroughly dry (like 2 years or so), it hardens inside the passages and narrows (or blocks) them. This alters the mixture properties of the passages.

I was sorely reminded of this when I recently fired up my long-sitting 750, post rebuild. The Emulsifier Air Bleed passages in 3 of the carbs acted like they were blocked (black sparkplugs), but they would pass Brake Cleaner through them. This implied (correctly) that they were PARTIALLY blocked, which is hard to clean. This is a very common problem on bikes that have been sitting long or are ridden infrequently after being fueled with MTBE gas.

The following few pix show how to clean this problem. The first one shows the entry to this passage: it's a little brass jet in the air horn. The second pix shows the passage's direction, and how long the wire has to reach inside to get all the way to the Needle Jet in the center of the carb. The third pix, if you look real carefully, shows the wire up against the side of the Needle Jet, having pushed its way fully through. Since there is a Z-shaped dogleg at the beginning of this passage, use a wire that will bend, not a stiff piano wire. Push, pull, push again: the wire will progress and the crud will break up and work its way to the edges of the Needle Jet, where it can then be flushed away. Be patient and straighten the wire several times, trying again until you can look down into the Main Jet Holder's hole and see the tip of the wire bumping into the Needle Jet's side. You'll be able to feel it opening up the passage."

good luck, worked for me.

[steam-powered man]

here they is

[thehammer]

I do have spark, for sure.

 I blew carb cleaner through the pilots and mains when cleaning them, but I'll do it again. These things sat for a LONG time.

 Is 22mm the correct height for the CB500 carbs? I saw 26mm somewhere else, but that was for a CB750. Just double checking....

 I bench synched the carbs but the drill bit I used was smaller than 1/4 inch. Think the slides may be TOO closed for it to start? But...I tried it with the throttle held open and the choke in various positions.

 The biggest clue to me are the dry spark plugs. I just don't think fuel is getting into the combustion chamber yet. I ordered 4 rebuild kits today and will start the cleaning process all over again.
 Keep the tips coming, though!

sorry for the late reply. actually the float height at 22mm is spec for the 550, wich i suppose might be differrent. real sorry if you had to take the carbs off again because of my advice. i suppose i had assumed they would be the same, and they may be . . . anyone on here know the proper float height?

the bench sync is done with the carbs at full close, but again, the 1/4 inch drill bit is what i use on my 550 . . . A clymer manual is always a good investment. I'll shut up now.

[flybox1]

float height = 22mm for the CB500

[Scott S]

 No problem. I actually found a local guy (like, 1/2 a mile from me) that rebuilds carbs. I feel like I have some stuff deep in these carbs. A couple of the float bowls looked like they had molasses in them. I have four rebuild kits and will let him take a shot at them. He can deep clean them better than I can.
 I used a very small drill bit to bench synch them since I read the 1/4" bit may make them idle high upon initial start up.
 I need to close the mixture screws from 2 turns down to 1 1/2. I think 22 mm is right for the CB500. I think.....clarification anyone?

[Scott S]

float height = 22mm for the CB500

 We must have been typing at the same time! I'll let you all know how it goes when I get the carbs back.

[flybox1]

trust me, do them yourself and keep at it until yo've got them dialed in. it'll save you $$$$ and you'll know exactly how they look/operate.