85' cb700sc will not idle - Wrist Twisters
 
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post #1 of 35 Old 04-14-2013, 05:10 PM Thread Starter
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85' cb700sc will not idle

So my buddy owns a 700sc. Two summers ago we replaced the valve cover gaskets, and a gasket in the shifting linkage. It still leaked and he ended up not riding it that much anyway. So last summer we replaced on o ring in the linkage and stopped the leak. We also pulled the carbs cleaned the bowls and sprayed it down in carb cleaner. Without taking the carbs fully apart. We didn't mess with any adjustments. (That we know of.) Now it will run ok if you keep it above 4000rpms but it won't idle worth a crap. He can even ride it if he keeps the revs up. We tried playing with the idle adjustment and that didn't help. Also the bike has new gas in it, so thats not the problem. Maybe the carbs have to come back off and be completely cleaned? Do the carbs have to be synced after removing them? Just trying to help and give it a couple more tries before taking it somewhere. Thanks for any input.

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post #2 of 35 Old 04-14-2013, 05:14 PM
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Yup, carbs gotta be taken off again and thoroughly cleaned. You shouldn't have to separate the carbs in order to thoroughly clean them. Take off all rubber pieces and strip the carbs down, then boil them in lemon juice (can get expensive) or water for 10+ minutes, clean it all up, blow out all fuel compartments, lube the diaphragms (automatic transmission fluid works well), clean all the jets thoroughly paying special attention to the pilot and low speed jets (where your problems are at). Reassemble and test again.

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post #3 of 35 Old 04-14-2013, 05:19 PM Thread Starter
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Wow that was fast. Thanks I'm going to send him a link to this. Maybe he'll join as well. I really appreciate the response. I'm not of the carb generation and the niner was godsend with it's fuel injection.

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post #4 of 35 Old 04-14-2013, 05:22 PM Thread Starter
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No syncing?

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post #5 of 35 Old 04-14-2013, 06:31 PM Thread Starter
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I have to ask why lemon juice? The acid in it?

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post #6 of 35 Old 04-14-2013, 06:31 PM
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Quote:
Originally Posted by NH919 View Post
No syncing?
Any time you do any mass tinkering with carbs, its always best to sync it. However, as long as you don't separate the carbs it shouldn't be mandated to sync.

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post #7 of 35 Old 04-14-2013, 07:05 PM Thread Starter
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Quote:
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Any time you do any mass tinkering with carbs, its always best to sync it. However, as long as you don't separate the carbs it shouldn't be mandated to sync.
Cool I'm hoping to avoid it if possible. Thanks again.

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post #8 of 35 Old 04-14-2013, 10:16 PM
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Do these have rubber or plastic floats?

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post #9 of 35 Old 04-14-2013, 10:27 PM
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Oh, and i have done my fair share of carb rebuilds... and ive used the pinesol dunk method....its surprising how well it does work.

http://www.advrider.com/forums/showthread.php?t=560117

Used to log into cb750c.com all the time, when i was working on my cb823cf project. Good read on dohc Honda's. Velodesign was my name there too.

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post #10 of 35 Old 04-14-2013, 10:51 PM
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Quote:
Originally Posted by velodesign View Post
Oh, and i have done my fair share of carb rebuilds... and ive used the pinesol dunk method....its surprising how well it does work.

http://www.advrider.com/forums/showthread.php?t=560117

Used to log into cb750c.com all the time, when i was working on my cb823cf project. Good read on dohc Honda's. Velodesign was my name there too.
Good to know. I'm about to tear down carbs for my 78 CB750. I'll give this a try. Thanks for sharing.

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post #11 of 35 Old 04-15-2013, 10:04 AM
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Hi All, I'm the guy who owns the 700sc. Thanks for the advise. I will try it and let you know how it works out. I hope it does cause I need to get this bike on the road. My truck gets 12 MPG and not great for daily commuting to work.

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post #12 of 35 Old 04-15-2013, 04:03 PM Thread Starter
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Quote:
Originally Posted by velodesign View Post
Do these have rubber or plastic floats?
Plastic we talked about it today. Which means if they get boiled the floats have to come out. I would think the high temps would melt or deform them.

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post #13 of 35 Old 04-15-2013, 04:10 PM
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Quote:
Originally Posted by NH919 View Post
Plastic we talked about it today. Which means if they get boiled the floats have to come out. I would think the high temps would melt or deform them.
And the acids would pull out any oils in the plastic and potentially cause them to crack or fail prematurely, if they didn't melt that is.

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post #14 of 35 Old 04-15-2013, 05:57 PM
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Yes, pull off anything non-metal. This includes the floats, diaphragms, rubber/plastic o rings etc. Ideally you want to boil just the carb bodies themselves (linkages and springs are fine) as well as all the jets. After you're done boiling, allow them to cool and then blow out all the pathways with compressed air and a thistle from a stiff brush or a needle if needed. If necessary, boil again to flush out all the crud you broke up the first time around.

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post #15 of 35 Old 04-15-2013, 06:09 PM
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Of just submerge the whole thing in pine sol. I have my CB750 car s soaking right now.

Spoiler:

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post #16 of 35 Old 04-15-2013, 07:03 PM
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One other thing, you did connect the choke cable correctly and its not stuck open....right? That'll mess up your idle good too

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post #17 of 35 Old 04-15-2013, 07:14 PM Thread Starter
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Quote:
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One other thing, you did connect the choke cable correctly and its not stuck open....right? That'll mess up your idle good too
Yeah I think so. If the choke was stuck open wouldn't it rev like crazy. This is more of a low idle problem.

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post #18 of 35 Old 04-16-2013, 07:21 AM
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It's not the choke. Holding the choke wide open and still giving the bike throtle is the only way it will run. I will start removing the carbs and let you all know how it turns out. Hopefully, this is all it is. It must be right?

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post #19 of 35 Old 04-16-2013, 10:10 AM
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Here she is. 1985 Honda 700SC
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post #20 of 35 Old 04-16-2013, 11:59 AM
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Does the bike have the full stock airbox installed? Most old Kehein CV's run like garbage without the complete stock airbox.

Also check the intake boots for leaks, spray some carb claner generously around the intake boots on both sides and see if the revs creep up.

On my 83 CBX550 I swapped engines and rebuilt and jetted the carbs, last fall. Bike worked great for for a few days, then wouldn't run at all, had to be on choke and was miss firing and fouling plugs. I blamed the carbs since that was the last thing I messed with, and had them installed and removed, swapping jets for another few days solid.

Turned ouy to be the ignitiokr pick ups in the engine, OHM'd out unservicable. Swapped them out and it fired right up.

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post #21 of 35 Old 04-17-2013, 06:47 AM
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Yes, it does have a full stock airbox. I will check for leaks when I reassemble the bike. I'm making progress, got the carbs off last night and I will break them down tonight. First I gotta go find a pot big enough to boil / soak them in.
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post #22 of 35 Old 04-17-2013, 04:41 PM
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The choke tubes can get stuck, that happened with mine and I broke off one stem trying to get it out.

The choke fuel passage is very small, I had one clog so bad a carb shop couldn't get it cleared. I ended up replacing that carb.

After you boil them, I'd blow carb cleaner thru each passage just to make sure they are all clear.

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post #23 of 35 Old 05-02-2013, 06:53 AM
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Alright, I did the pine sol soak for 5 days instead of the boil because there was two breather tubes inbetween the carbs that were plastic and I didn't want to do any damage to them. Bike went back together fairly well. Then of course, took the battery off the tender and it was junk. Got a new one and started her up yesterday. The Honda book says the bike should ideal between 900 to 1100 rpms. The bike is backfiring in that range. The bike ideals perfect at 1400. I checked the boots for leaks, didn't find any. Any idea's on how to correct the backfiring???

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post #24 of 35 Old 05-02-2013, 10:59 AM
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Did you actually replace the intake boots or just inspect them?

I (obviously) have one of these and have done the carbs several times - they're nothing unusual and not terribly complex.

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post #25 of 35 Old 05-02-2013, 11:30 AM
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Quote:
Originally Posted by JC1 View Post
Alright, I did the pine sol soak for 5 days instead of the boil because there was two breather tubes inbetween the carbs that were plastic and I didn't want to do any damage to them. Bike went back together fairly well. Then of course, took the battery off the tender and it was junk. Got a new one and started her up yesterday. The Honda book says the bike should ideal between 900 to 1100 rpms. The bike is backfiring in that range. The bike ideals perfect at 1400. I checked the boots for leaks, didn't find any. Any idea's on how to correct the backfiring???
Quick questions man: How did you check the boots for leaks? Did you spray WD-40 on em with the bike running and look for smoke out of the exhaust, or just visually inspect?

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post #26 of 35 Old 05-02-2013, 11:40 AM
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I visually inspected them before putting them back on and I sprayed WD-40 around all 8 boots while the bike was running and nothing changed. No smoke, no Rev change. I was thinking I should inspect the few vaccum lines this bike has tonight and see if I can find anything. Any more ideas? Add sea foam to the gas?

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post #27 of 35 Old 05-02-2013, 12:30 PM
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Did you spray air or carb cleaner thru the passages?

It sounds like a passage is clogged up. You can check other things like filter and plugs, but it sounds like one of the carb passages is clogged up.

You can do this with a can of carb cleaner or compressed air.

I assume you removed the idle screws and shot some cleaner thru there and checked that it was coming out behind the butterfly.

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post #28 of 35 Old 05-02-2013, 01:36 PM
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Quote:
Originally Posted by JC1 View Post
I visually inspected them before putting them back on and I sprayed WD-40 around all 8 boots while the bike was running and nothing changed. No smoke, no Rev change. I was thinking I should inspect the few vaccum lines this bike has tonight and see if I can find anything. Any more ideas? Add sea foam to the gas?
It only has one vacuum line, it goes from the intake to the petcock. Unless it's a CA model, then you've got some abortion of an evap system.

I'd also point out that the current formulation of WD-40 is no longer suitable for hunting down vacuum leaks. Use starting fluid or an unlit propane torch. Better yet, replace the boots between the carbs and the head just on general principles - they're old.

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post #29 of 35 Old 05-02-2013, 01:53 PM
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+1 on the propane and new boots. I loved mine when I had it, I purchased all 4 carbs rebuilt and bench tuned from a rebuilder in Texas for $130 plus my carbs as a core.

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post #30 of 35 Old 05-02-2013, 01:56 PM
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+1 on the propane and new boots. I loved mine when I had it, I purchased all 4 carbs rebuilt and bench tuned from a rebuilder in Texas for $130 plus my carbs as a core.
Which rebuilder did you use?

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post #31 of 35 Old 05-02-2013, 02:36 PM
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I think it was classic carbs or something like that. I might have found them on eBay, it was 7 or 8 years ago.

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post #32 of 35 Old 05-16-2013, 06:01 PM
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I checked everywhere around the carbs for leaks with starter fluid. Nothing, no leaks. I took the bike for a ride and it was lacking substantial power. I changed the plugs today. Performance was better but still not like she should be. I noticed the plugs from cylinders 2 and 4 were black and wet. I also noticed the bike was sucking down a lot of gas. I suppose I should try adjusting those carbs?

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post #33 of 35 Old 05-17-2013, 08:21 AM
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I recently did the pinesol dunk process on my nt650 carbs and I have to admit I was impressed with the results. The only thing I would recommend is just do not put any of the springs or any other steel parts in because the acids in the pinesol will cause them to rust if you do not dry them off and put some light lube on them afterwards I screwed up on the lids that go on top of the carb diaghram and I ended up just hitting them with the wire wheel on my grinder and throwing some silver paint on the outsides.
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post #34 of 35 Old 05-20-2013, 10:20 AM
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I found the only Carb adjustment on my 85 nighthawk is the pilot screws. My honda manual says these are factory set and cemented in place. It states do not adjust unless you do a complete carb overhaul. I reached in there and i was able to adjust both of the carbs that are running rich. So, these must have been adjusted by the previous owner. I ran out of travel because the caps were still on the screws. I will remove the caps and adjust/ride until it's right. I sure hope this is the culprit.

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post #35 of 35 Old 05-20-2013, 10:39 AM
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Usually the caps are pressed into place. Little aluminum caps. I can not say for sure on your specific bike but I know they were originally on my nt650. But if they are very similar then I just used a small drill bit to get a small hole but large enough to thread a drywall screw into it a tiny bit to catch the thread and give me something to clamp on to with vise grips, then I just worked it back and forth until it pulled out. You want to pull all of them out and remove the needles with the oring's, washers, and springs before you soak them in any kind of cleaner as well, and then you want to make sure and replace that o-ring (should come with your rebuild kit)

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