Fresh rebuild and hanging idle

[jchek779]

Hi guys,
I recently restored a 69 Honda CL450. The motor was completely torn down, cleaned, refreshed and rebuilt with gaskets, seals, piston rings and a fresh cylinder hone. Since completing the restore, I'm having trouble with a hanging idle when I fire it up. I'll blip the throttle and the bike will continue to rev at 3,000 or 4,000, or whatever. It will also pop through the exhaust occasionnally after letting off the throttle.
Since this started, I have verified my ignition timing, cam timing, checked to ensure my vacuum carb slides are returning when I let off the gas, carbs have been cleaned and rebuilt with fresh brass, and I just moved my valve lash from the 0.0012" that the manual calls for to 0.004" intake and 0.005 exhaust.
The valve lash changed moved my cylinder compressions from 120 per side up to 140psi per side.

Valve timing pics are in Reply #44 in my build thread here: http://www.dotheton.com/forum/index.php?topic=44487.0

As a next step, I was thinking about moving the lash out to 0.007 intake and 0.008" exhaust to see what happens.

Has anyone experienced this before? The bike starts fairly easy and runs good once running, but the hanging idle is obviously incorrect and needs to be fixed.

I appreciate any suggestions you guys can provide.

Jay

 

[HerrDeacon]

I doubt the hanging idle is due to the valve clearances, I would put them back to stock and leave them there. Have you checked for air leaks around the the intake manifolds? A air leak there can cause it to run lean and make the idle hang like that. Also, if the carbs are not sync'd it will cause this as well. Also, have you adjusted the air (or fuel) screw as described in the manual?

 

[Rich Ard]

Agreed completely with the above...having chased that same gremlin on my 450

 

[Redliner]

Before you start, pull the throttle all the way open and see how the throttle levers on the outsides of the carbs returns. They might hesitate for one reason or another. It's more likely than not for the left carb to get hung up on the clutch cable when reassembling these carbs.

With the bike running, you can also just pull the clutch cable and release it and see if that has any effect. If so, there's your problem. the cable should be routed on the right side of the frame and be held by a cable-stay on the rear cam cover. If you don't have a cable stay, just let it rest on the breather hose. Whatever it takes so that the cable is not interfering with the throttle operation.

If that doesn't solve it:

Check valve lash. Don't take my word for it but I believe it should be 0.03mm? If so, go to 0.05 and make it only a tad tight. Should be able to slide it around still. I don't recommend going tighter than this. 0.03mm is a tiny gauge and leaves little room for a slip up. Continue sliding the gauge around as you tighten the adjustment lock nut or else!

Set the idle and mixture to the benchmark.

The idle: Turn the adjustment screw all the way out until it lifts from the idle screw stop. Turn it in until it just lightly touches this stop, then add one half turn. Maybe 3/4 at the most until it's warmed up. Don't let it idle over about 1,200 for long, they will get HOT.

The mix: Turn this screw in until it lightly seats, then turn it one turn out. For reference, turning it in will make it lean and turning it out will make it rich.

Finally, check that the two throttle levers lift at exactly the same time and adjust the throttle cable height as necessary. Slack should be minimal and allow about a millimeter of throttle movement before engaging. A keen way I've learned to balance is to open the throttle until you see the throttle lever on one of the carbs hit the stop. Adjust the one that hit first by giving it slack until the other one hits the stop at the same time. Pull the throttle again until you see one of them contact the stop, and press your finger against them to see if you feel it move at all.

1. Don't perform ANY carb adjustments until the engine is HOT. Spit on the veins on the jugs and when it evaporates without hesitating, it's hot. Or lick your finger and wipe it on the top vein, near the spark plug, to be more polite to her.

2. Adjust the idle to about 900-1,000 RPM's by evenly adjusting both carb idles. If you turn the left one in 1/8th turn, repeat for the other. It's important to keep them balanced. You want a low idle because the next adjustment will raise it.

3. Adjust one carb by turning the air screw in or out about 1/4 turn. If the RPM's drop one way, turn back 1/4, then try to turn the other way. You want the RPM's to go up. Wait 30 seconds after each 1/4 turn. Repeat until you turn it and the idle does NOT change. Go back 1/4 to confirm it isn't changing, then continue back by another 1/8. This will keep you safely above lean condition.

4. Now you will have to adjust the idle evenly again to bring it back down to 900-1,000RPM. Do so just like in step 2.

5. Repeat step 3 for the other carb.

6. Finally, evenly adjust the idle screws until you get her right at 1,100 on the clock.

Just finished a CB450 and couldn't get off the thing. Started first kick every morning. Prefer it over the CB750 DOHC I just fixed. And now I get to tear down a CB500. My neighbors must think I'm terribly indecisive :

 

[mark13018]

+1. almost guarantee it's intake/carb issue. I think Redliner covered all your bases.

 

[jchek779]

Guys,
Thanks for the quick replies. To answer a few of your questions -
1. Carb Sync - I did sync these on the bench, such that each butterfly starts moving at the exact same time.
2. Throttle Return/Closing - they both snap back to the idle position as soon as I let off the throttle grip. I have check/rechecked.
3. Initial idle settings - this is weird. When I first start the bike, it will rev/idle around 1200-1500. The engine will respond to adjustments to idle settings, but as soon as I blip the throttle, it starts to hang at the 3k-4k that I described earlier. If I blip the throttle when the idle is hanging, it will climb higher if I had opened the throttle far enough.
4. Mixture screws - I had these at 1.5 turns out. I tried making adjustments with the engine running, but it didn't seem to respond. It has been difficult trying to make adjustments to the bike because of the high idle. I can't really touch the throttle to keep it at idle, but sometimes that means the bike keeps stalling out on me.
5. Air leak - I used a propane torch (obviously not lit) trying to find an air leak. I didn't have any luck with that approach. Tonight or tomorrow night I'll try using some carb cleaner around the carb boots just to verify.
6. Valve lash - in my experience with hot rods and 4st MX engines, things are sensitive to valve lash. Hard starting, popping through exhaust, etc. I went from having to kick this thing for 10 minutes to being able to start it on the center stand with my left foot last night after adjusting the lash. I also went from 120psi per cylinder to 140psi per cylinder, just by loosening the lash slightly.

Again - thanks for the input. I'm going to head home tonight and put your suggestions to work. We'll get this thing running.

 

[DeanJ]

Bench sync is a good start, however you need to do a proper sync at warm idle with a set of gauges or manometer . A clogged pilot fuel / air circuit will also cause this.

I looked through your build thread- nice work sir

 

[Redliner]

1. Carb Sync - I did sync these on the bench, such that each butterfly starts moving at the exact same time.

3. Initial idle settings - this is weird. When I first start the bike, it will rev/idle around 1200-1500. The engine will respond to adjustments to idle settings, but as soon as I blip the throttle, it starts to hang at the 3k-4k that I described earlier. If I blip the throttle when the idle is hanging, it will climb higher if I had opened the throttle far enough.

1. Um, as I'm understanding your reply, that doesn't sound like a proper bench sync. You just want the butterflies to be open at the same degree when closed. If they're off the bike, adjust one so that you can just see half of the pilot fuel circuit. There's a little hole or two on the bottom of the bore, covered by the butterfly. Then adjust the other to match. Must be bang on!
If it's still on your bike, use my method as a start. Shouldn't be more than 3/4 turn in once it touches the idle stop.

3. Sounds like one or both idle is set high. When you start the bike, adjust one carb until the idle drops and nearly stumbles. Then come back up until it's about 1,000. Repeat with the other carb. If you open the throttle and it races or doesn't return immediately, it's: lean, dirty, leaking air, or that damned clutch cable on the left carb.

Stuck slides would not cause it to race. I forget exactly what it causes, but since it's a CV it won't raise the idles. It would be more likely to cause a stumble.

 

[mark13018]

Bench sync only gets it so close. You still need to sync them with the manometer.. The problem is the 450 models don't have the vacuum ports to do so. You'll either have to tap the cylinder head just after the carb boots or the carb boots themselves with a 1/8" brass barb then get rubber caps when your not using them... There are several "how toos" on this online. A couple maybe on here.. Sorry, I'm on my phone or I'd send you the links.

 

[jchek779]

Thanks Redliner - I'll put this advice to work later tonight or tomorrow.

 

[jchek779]

I finally got a chance to work on the bike yesterday and today. It turned out to be I had the carb idle screw settings way too high. I kept backing them down 1/8 turn at a time until I finally reached a stable idle around 1100-1200rpm. It sat there and idled clean enough that when warm I didn't have to work the throttle, and was able to get my mixture screws dialed in.

I think the deal was I had the throttle blades opened so much that when the bike was cold, it was enough to keep the bike running at a low idle. When things got warm, the bike was running where the throttle blades were opened to, hence the high R's when I was thinking the bike should have been idling. It didn't take much from those throttle blade idle screws - I'm at maybe 1/4 turn in from when the screws just start to move the blades.

Thanks again for all the help guys. I really appreciate it. Now if it would just stop raining everyday in the southeast...

 

[Redliner]

Oh...the rain. I have had to move four bikes indoors, and two of them can't even roll XD

Glad that got sorted out. I bet you can't wait to whip around the block ;D

 

[Joseph New York]

I finally got a chance to work on the bike yesterday and today. It turned out to be I had the carb idle screw settings way too high. I kept backing them down 1/8 turn at a time until I finally reached a stable idle around 1100-1200rpm. It sat there and idled clean enough that when warm I didn't have to work the throttle, and was able to get my mixture screws dialed in.

I think the deal was I had the throttle blades opened so much that when the bike was cold, it was enough to keep the bike running at a low idle. When things got warm, the bike was running where the throttle blades were opened to, hence the high R's when I was thinking the bike should have been idling. It didn't take much from those throttle blade idle screws - I'm at maybe 1/4 turn in from when the screws just start to move the blades.

Thanks again for all the help guys. I really appreciate it. Now if it would just stop raining everyday in the southeast...

Sorry to open such an old thread. I'm experiencing hung-up RPMs with the same conditions you had on my CB500/4. Can you touch on HOW you remedied it? Are you saying it was it caused from bench syncing with a high idle setting? Thanks!!

 

[Redliner]

A set of vacuum gauges would show you right away which one is thrown off. A higher vacuum reading means a more closed throttle. On a 400, I believe that the #2 is non-adjustable, so you match the other cylinders to that one all while dropping the idle again and again until you're at a cool 1,000-1,100. Wait until the bike is fully warmed up and complete the sync, then move to the mixture screws and idle speed.

As a hint, that popping out of the exhaust is the cylinder with the more-closed throttle firing off now and then due to the increased vacuum of the higher RPM's but low throttle position. Same thing that causes backfiring on engine braking.

 

[Joseph New York]

A set of vacuum gauges would show you right away which one is thrown off. A higher vacuum reading means a more closed throttle. On a 400, I believe that the #2 is non-adjustable, so you match the other cylinders to that one all while dropping the idle again and again until you're at a cool 1,000-1,100. Wait until the bike is fully warmed up and complete the sync, then move to the mixture screws and idle speed.

As a hint, that popping out of the exhaust is the cylinder with the more-closed throttle firing off now and then due to the increased vacuum of the higher RPM's but low throttle position. Same thing that causes backfiring on engine braking.

Thanks for the response.

I vacuum synced again today and everything is beautifully in-sync to #2 as usual. Should I ignore the PO's issue as being the culprit since #2 is fixed on the CB500/4? BTW I'm not having any carbie pops or backfires. Thanks!

 

[Redliner]

If all of the carbs read evenly, then we can rule out a rogue throttle valve.

Make sure you have slack in the throttle cable. You should have free play in the throttle, about 1-2mm of rotation measured at the switch housing.

Lower the idle set screw to the point you see that the screw is no longer touching the plate that pushed the throttle shaft.

If that doesn't drop your idle, we gotta go stage 2. Stage 2 will cost you 78¢ per minute prepaid, and if I can't fix it we can settle up with the bike

 

[Joseph New York]

If all of the carbs read evenly, then we can rule out a rogue throttle valve.

Make sure you have slack in the throttle cable. You should have free play in the throttle, about 1-2mm of rotation measured at the switch housing.

Lower the idle set screw to the point you see that the screw is no longer touching the plate that pushed the throttle shaft.

If that doesn't drop your idle, we gotta go stage 2. Stage 2 will cost you 78¢ per minute prepaid, and if I can't fix it we can settle up with the bike

Hah! She idles beautifully at 900/1000.

I didn't want to hijack the thread especially since it had a solution, so I didn't post all the details. But what the hell, 78c a min!

I have no vacuum leak to the boots/manifold; no sticking of carbie slides, throttle cable, or timing advance; cutaways about-face; no more hole in my 4-1 exhaust; and no leaks around UNI pods. Went through many main jet sizes; adjusted pilots without RPM changes. Left at 1-1/2; I just increased slow jets to #42 as I was thinking the hangup could be caused by a lean idle circuit. But that didn't change. It hangs only 1/2 OT to WOT. Drops back to idle if my acceleration is low. This has me thinking that it could have a lean main circuit or needle circuit (if that makes sense). I had #120 mains in there and the needle clip to 3rd groove before I just set em to #115 and 2nd groove. Could put back the #120s or raise the needles... BTW I have a CB550F rack on here.

I have so many changes going at once, I'm very tempted to roll up to the dyno up the road.

 

[Redliner]

Could easily be hanging timing advance cam. Could be low fuel in the bowls.

With the bike COLD, start it and let it idle with your hands on the pipes. See if half are remaining cold at idle. If so, that means when you rev, the two dead cylinders are coming on and thus raising the RPM.

Check that the carbs are all the way in the insulator boots.

If you're not adjusting the idle speed when the bike is HOT then it will be set high. The bike should idle low or not at all when freshly started if there's no choke.

Did you watch the vacuum gauges AS you rev it and it hangs?

 

[Joseph New York]

Could easily be hanging timing advance cam. Could be low fuel in the bowls.

With the bike COLD, start it and let it idle with your hands on the pipes. See if half are remaining cold at idle. If so, that means when you rev, the two dead cylinders are coming on and thus raising the RPM.

Check that the carbs are all the way in the insulator boots.

If you're not adjusting the idle speed when the bike is HOT then it will be set high. The bike should idle low or not at all when freshly started if there's no choke.

Did you watch the vacuum gauges AS you rev it and it hangs?

Checked the float heights; checked timing advanced action; advance cam??

I'm always touching the pipes. Running on all 4.

I didn't let it hang when syncing. I'm going to sync again right now but this time I'm going to note everything and let it hang. I'll start off at a very low idle. I actually end up dropping it to 900 after a while.

I don't know if this is useful info, but if I blip the choke I can stop the hangup.

Thanks!

Edit::

I just hooked up my synchronizer and realized two things:

1) All of my slide adjusters ARE adjustable. All my other CBs aren't adjustable on #2. How did I miss that? It's not to say that it should ever be adjusted though, but in researching, it's not to say that it's even #2 that's needs to be adjusted to! This makes my involvement in this post more warranted at least

2) Since my last sync and the re-installation of my rack from the rejetting, I seemed to created a leak at my #4 boot :/ Ive never been able to to seal up my manifold without using a liquid sealer, and my last installation I may have broken it loose. I also noticed that my recent exhaust leak fix wasn't perfect so I need to either redo it or add another patch to it.

So it's looking that I need to seal both leaks before I sync again. I have to go back to work now though :/ I'll be back soon for an update! Thx