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Old 04-26-2011, 09:46 PM   #1
btcn OP
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Super high idle?

Ok, so I have a 1985 Honda Shadow 700 that I just finished restoring.

Its running GREAT for the most part. I hit 85 MPH in 5th gear just cruising, and it has 6 gears and 6 is OD so its power is good. Its a blast and I'm enjoying it.

BUT theres one little thing that bothers me. When the engine is warm, it idles VERY high! Somewhere around 3,500 RPM or so, sometimes 4,000! But thats SOMETIMES, other times it idles just a little high at around 2,000 RPM or so.The choke is off as well.

So whats up with this? I got the idle screw all the way out. When I start it up, it idles nice and smooth at around 800-1000 RPM.

Also it revs down slowly. If I put in in neutral and rev it, it will rev up real quick, but the RPMS come down VERY slowly.

Any ideas? I really appreciate ANY help. What should I look at? Air leaks in the intake system?

BTW heres a pic before the paint job:

[IMG]Photobucket[/IMG]
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Old 04-27-2011, 06:10 AM   #2
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Your symptoms describe an air leak to a T. It sounds like there is an air leak between the carb and the engine causing it run way lean
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Old 04-27-2011, 11:03 AM   #3
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+1 on air leak. I'd consider, in order-

1) too much air compared to fuel being delivered (lean)....for whatever reason (high float, air leak, clogged main jet, etc)

2) stuck throttle cable or throttle springs very weak and not closing butterflies/gates

3) stuck choke? I've done this myself.....but then, high-speed performance would probably be affected.
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Old 04-27-2011, 12:21 PM   #4
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Not familiar with that bike in particular, but I've seen that type of problem caused by carburetion that is too lean on the pilot circuit.
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Old 04-28-2011, 01:59 PM   #5
btcn OP
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Thanks for the responses guys. Yes I was thinking something like an air leak. Maybe check for any tiny leaks in the intake manifolds?

I think your right it is running a bit lean. I'll also check the throttle cables.

Also I notice the diaphrams in the carbs go down VERY slowly when I push them up with a finger. Could this be related to the slowness of the engine revving down?

Since its stock I've been assuming it should be jetted correctly. But maybe not?

I'll also try and turn the choke on to see if it does anything. Thanks for the tips and I'll let you know what happens. And if anyone has any other ideas I'd appreciate them.
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Old 04-28-2011, 02:18 PM   #6
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That symptom is called "hang idle" and is usually from a big air leak causing a very lean condition.

Spray some carb cleaner or something around the carbs and boots and intake plumbing while the engine is idling. Listen for changes in engine speed indicating some of the cleaner got sucked into the engine. This will help you narrow down if/where there is an air leak.

Your comment about the slides being slow to return to position after lifting with a finger-- I'm not sure? Can anybody comment on this?
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Old 04-28-2011, 02:24 PM   #7
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Quote:
Originally Posted by btcn View Post
Since its stock I've been assuming it should be jetted correctly. But maybe not?
The other guys may well be right about an air leak. A bike that old could have intake boots (between carb and cylinder head) that are hardened, cracked, etc... But, I've seen 2 bikes, with stock factory jetting, unmodified, that had hanging idles like you describe. In both cases opening the fuel screws and/or installing larger pilot jets fixed it right up.

Have you cleaned the carbs, jets, etc?
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Old 04-28-2011, 07:24 PM   #8
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Ok, a little update. I just went down and tinkered a little with the carbs.

First as mentioned I tried the choke. I haven't really used it yet. So I started it up and turned the choke on [as in enriching the fuel mixture like you would when its cold]. The idle soured up, and by the time it got warm it idled SUPER high, at around 6,000+ RPM. Turned the choke off and it went down to it's regular high of 4,000 RPM or so.

Then I noticed I could see the idle air/fuel mixture screws. It appeared that they have been drilled out as you could see where a screw would be in the hole. On most Japanese bikes in my experience these have a piece of medal welded or screwed in so you can't mess with the mixture.


So I said what the hell and started turning each one 1/2 a turn one way. I wrote the # of turns and which way down. So I started to go one way, it might have been left and the idle increased even more. So I did it the other weigh, and the idle started to come down slowly. I kept doing it until they were all the way in. The idle is now decent at around 1,300 RPM or so, and the backfiring is almost all gone!

This must richen the mixture? So I think you guys are right, it must be running very lean. It COULD be jetted wrong. Maybe someone had a different exhaust setup at some point and the stock ones were installed back on and it wasn't jetted back? Just an idea.

Or more likely it has a big air leak. The intake manifolds do seem pretty hard to me.

DevilNinjaDog thats a great idea with the carb spray! I'll definitely give it a try!

Skowinski yes I have cleaned the carbs. Yea I'm definitely getting into the carbs and manifold more. I bet it is the manifold.

I also have a Suzuki DRZ 250 that does the same thing! Its only about 4 years old and its always done this. I never really payed much attention to it as its a high revving single and I thought it was normal, but now I bet it has a problem or something. It also seems more common on CV type carbs, my other dirt bikes with slide carbs don't do anything like this.

I have seen a lot of bike do this, particularly older Japanese bikes, but I've never minded it until now, as I want that V-Twin sound like when it starts cold!

Thanks for the help, and yea can someone who knows comment on the sticking carb slides? I'll look into it more and keep taking in any ideas. I'll keep you updated.
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Old 04-29-2011, 05:58 AM   #9
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Hmmm. Turning the mixture screws IN is supposed to LEAN the mixture (can anybody confirm this?). Typically to set them, you turn them all the way in, then back out 1.5-2 turns depending on the bike.
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Old 04-29-2011, 07:42 AM   #10
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Quote:
Originally Posted by DevilNinjaDog View Post
Hmmm. Turning the mixture screws IN is supposed to LEAN the mixture (can anybody confirm this?). Typically to set them, you turn them all the way in, then back out 1.5-2 turns depending on the bike.
Yep, on 4-stroke carburetors they are almost always (to my knowledge) called fuel screws - because they adjust the amount of fuel to the pilot circuit. Opening them allows more fuel and a richer pilot circuit jetting. On my KTM 2-stroke carburetors they are called air screws, and do the opposite. Not sure why the difference , that's for someone smarter than me to answer.
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Old 04-29-2011, 11:59 AM   #11
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Thats strange! I wander why leaning it even more would lower the idle.

I wander if it is blocking so much fuel that it has to idle lower?

Because turning on the choke makes it rev almost to redline, and this is just adding more fuel.

When it idles high it will chug along at about 15 MPH in first gear without any throttle input.

I am now pretty sure its an air leak. Probably in the intake manifold.

I'll probably go ahead and look for it and just order new intake manifolds and see what happens from there.

I will also take a look at the jetting.
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Old 04-29-2011, 12:07 PM   #12
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Quote:
Originally Posted by Lomez View Post
+1 on air leak. I'd consider, in order-

1) too much air compared to fuel being delivered (lean)....for whatever reason (high float, air leak, clogged main jet, etc)

2) stuck throttle cable or throttle springs very weak and not closing butterflies/gates

3) stuck choke? I've done this myself.....but then, high-speed performance would probably be affected.

Also check the fuel filter.
I was experiencing high idle on my 82 seca and found out that it was the extra inline fuel filter that I installed to prevent dirt into the carb, removed and problem solved.
It was restricting fuel flow to the carbs.

Good luck
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Old 04-29-2011, 08:33 PM   #13
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Mixture screw

If the mixture screw is on the air cleaner side of the carb it is most likely an air screw. Adjust out =more air=lean Turn in= less air =rich. If the mixture screw is on the intake side of the carb it is the reverse. The mixture is controled by the size of the idle(pilot)Jet and the idle air bleed. The mixture screw in this case controls the volume of fuel/air mixture. So adjust out =rich adjust in =lean. There may be some exceptions but this is a good rule of thumb. Hope this help. Racer07
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Old 04-29-2011, 09:05 PM   #14
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Originally Posted by racer 07 View Post
if the mixture screw is on the air cleaner side of the carb it is most likely an air screw. Adjust out =more air=lean turn in= less air =rich. If the mixture screw is on the intake side of the carb it is the reverse. The mixture is controled by the size of the idle(pilot)jet and the idle air bleed. The mixture screw in this case controls the volume of fuel/air mixture. So adjust out =rich adjust in =lean. There may be some exceptions but this is a good rule of thumb. Hope this help. Racer07

+1

If on the opposite side, it's called a fuel mixture screw. Most people interchange the lingo.

But I just replied to racer 07's 666th post, so evil could have me backwards...
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ignatz72 screwed with this post 04-29-2011 at 09:15 PM
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Old 04-30-2011, 07:36 PM   #15
btcn OP
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These are three great ideas.

So I took it for a little ride and it overall runs MUCH better with the screws turned the way they are. It lacks a little bit of low end power [below 2,500 RPM], but it runs cooler and idles pretty nicely at about 1,000-1,500 RPM. The hang idle is also a WAY better, it still hangs a little in the high revs, but not enough to be a huge deal and doesn't do it in gear anymore.

achtung3 thats interesting. I never even thought about that. I do actually have an inline filter for the same reason you did! Wander if this could have to do with anything? I'll actually go and take it off right now just to see if it has any affect! Although it does have an electric fuel pump I'll do it just so I can eliminate it or not.


achtung3 and ignatz72 that sounds more like what this screw is doing! If it is restricting the air it would make much more sense.

Do you guys mean closer to the air intake side of the carb or closer to the intake manifold side? I'll go take a pic of it right now and post it and see if you can tell from that.

Thanks for the ideas!
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