Idle low with choke, high after warmup?

[Field]

Hello! I just got myself an 81 750k. It has some after-market exhaust mufflers, maybe some carb stuff? I dunno. It runs strong, very fast, with only 27k miles. Problem though...

When I first start it, choke on (always unless engine is warm), the idle will be like 1k, then it'll dip below and try to die unless I give it some throttle and hold it just about 1k. I do this for awhile, then take off. After taking off it starts to hold its own, idle around just about 1k.

Funny thing... after I zoom around for a bit the idle hits 2k. It stays at 2k the entire time riding afterwards. I don't get it. This seems high, and I think I'm going through gas faster than I want to be!

I'm going to look into buying a vacuum get to sync the carburetors, but first oil change (maybe a filter converter), and some DR8EIX spark plugs (need to get a gapping tool and socket end). Thought about having local place do the syncing but it'd probably cost me very little more to get a kit.

[LukeM]

Welcome to the forum.
On the home page, you'll find a pinned link to Manuals for these bikes.  Get yourself one.  You'll find all sorts of information and troubleshooting tips.  Pretty handy.

I'd go through the Tune-up section of the FSM, and get everything set there.  Then you can worry about carb sync procedures and such.  You may be looking at a carb issue, but I have noticed that after I've ridden my bike for a while, the idle will be up about 10% or so.  It eventually settles down to 1K, where I set it originally.  If you're idling at 2K all the time, that's not horrible.  These bikes are pretty efficient if they're in good tune.  I remember getting between 40-47mpg with my 79 CB750. My current Shadow gets about that as well.

Find a deep well 18mm 6 point socket to remove the spark plugs.  If you're lucky enough to have the factory tool kit, there's an 18mm spark plug wrench in there as well. I know some on this forum like the Iridium plugs. They deliver a hotter spark, which can help with a weaker ignition system.  If you decide to sync the carbs yourself (you can...it's a VERY easy process), you can expect to find the 4 gauge carb sync package on EBay for between $50-100 USD.

I hope this helps.  Keep us in the loop.
Luke M

[rrgunslinger]

You can find the carb sync guages on this page.

http://www.cyclexchange.net/Carb%20System%20Comp%20Page.htm

[rrgunslinger]

I have found high idle usually means vacuum leak or air screw or screws out of adjustment but I think your airscrews are sealed. But do what Luke says first.  Some like to use sea foam in the gas so that may help over time to clean the jets and such.  

Good luck.

[Field]

I do have manual, will look into get a shop manual.

I was looking at kits on this page, even though they cost more,

http://www.cb750partsdepot.com/Tools_c2.htm

Perhaps they are not worth the extra $, I don't know.

These are the carburetors on it,

http://www.ebay.com/itm/1982-Honda-CB750SC-Keihin-Carburetors-CB-750-/190707416469?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item2c670c3d95

would I use the same kit?

Mac Exhaust and Keihin carburetors, wonder what else I'll find! I thought it had a lot of scoot for a 750...

[Field]

New spark plugs make for a much better start. The old ones were not gapped correctly even though they were DR8 NKG's. The DR8EIX gapped at 25 are much better. The old ones are happed at like 28 and 33. (looks like two seperate sets).

It idles better too. However it has a little bit of a weird knoch-ish marble rolling around sound too until the rpms go up a little, to get the idle just over 1k (although the idle might be lower, my spedo runs almost 5mph under). Thoughts on that?

[Piute]

   After my bike hit's running temp 200deg+ the idol stays up at around 1500ish but is set at 900,this is cool with me.  i'm getting 45 to a gal.at 73mph(5500 rpm),Just covered over 300 miles today 43 avrg.(city n hwy) on that short run,no complants here.

[Field]

My idle is down to 1300 or so after warm up now! It was all spark plugs. I gapped to 25. I run premium non-ethenol. Power is up too, especially low end power. It takes off way better.

Oil change is next, I suspect it'll all work better after that. Oh and I'll check the chain.

[icerigger]

Had that rattling noise coming off idle too. Figured it was running lean at idle so I opened the pilot screws a half turn and the noise went away.

[Lucky 1]

You cannot just ignore this kind of stuff.
QUOTE:
"Hello! I just got myself an 81 750k. It has some after-market exhaust mufflers, maybe some carb stuff? I dunno. It runs strong, very fast, with only 27k miles. Problem though... ?"

What kind of "carb stuff" ?

If it does not have the original air box and filters AND you have an aftermarket exhaust the
carbs need re-jetting. It has a high idle because it is LEAN.

No need to start turning ALL the screws to correct the problem except for the mixture screws.
If you open a mixture screw 1 turn then you have to make all of them ONE TURN OPEN.

A carb sync just gets all of the slides to be the same height with in about .007 thousandths.
That is not needed yet. Because if that was off it would probably not idle at all.

You just have a high idle from a mixture that is too lean. And that is because of the intake and exhaust.

So tell us..Have the stock carb intakes been modified from stock?

[Field]

Intake is 100% stock. It just has aftermarket mufflers. I thought maybe there carburetors were aftermarket but I guess not.

Now when warmed up the idle varies between 1300-2000. It seems like the more gas I give it, the higher the idle will be when I stop again.

The idle had been lower until I gave it seafoam and oil change. Then it went up a little again. The choke often isn't enough to keep it from dieing even when it isn't cold out. It still starts up way easier and better than with the old un-properly gapped spark plugs! It shifts nicer, less noise too (unless RPM's are really high, then shifting is a bit louder). It runs a lot smoother now, that easy going pitter-patter sound, less vibration, etc.

[seestheday]

Slow return to idle often means a vacuum leak.  Have you tested for this yet? It's a very easy/cheap test.

Just spray carb cleaner, starter fluid, propane or something similar (I've heard that wd-40 even works) around the carb boots.  If the idle goes up when you spray in certain areas, you have a vacuum leak.

[Field]

I suspect this to be the case. The bizarre part is that it does it when the bike is warm instead of cold. When winter comes and I'll probably redo the carbs or send them off, I'll purchase those super clamps from that one place (forget which one, bunch of CB750 sites). I'd like to test it but I'll wait till I see one of those items laying around (believe it or not I own none of them).

The more it runs the better it seems. I still wish the weird sounds when it is cold would stop but whatevs I guess. Just hard to understand the weird sounds that come from the bottom. My mileage is up, but not where I want it yet. Although that may have a little to do with testing the top speed, and I can confirm that I've been within 500rpm of the top speed.

The most annoying thing is the gas cap lock buzzes if I don't put something under it. I think the rubber is just old or something.

[seestheday]

Sometimes vacuum leaks only show up after the bike gets warm.  Makes sense, when things are cold they shrink, when they warm up the expand.

Top speed you say.  Are you taking it up to places that only shineyribs has gone to?: http://honda-cb750-s.456789.n3.nabble.com/High-speed-questions-td3866686.html (138 MPH!)

[Field]

I was at 9,000rpm in 5th, whatever speed that happens to be. I guess I could calculate it if I knew the gear ratio and sprocket sizing etc.

[robs750]

How do the carb insulators look?
Are they hard,cracked?

[Field]

No, but they are old.

[Field]

Ok so I think I know the problem. The issue is with the timing.

http://forums.sohc4.net/index.php?topic=95593.0

My air filter, I have an uni now. I assume you push it all the way to the back?

Well anyways, it only idles high after giving it some throttle. This likely means the advance isn't returning properly.

Oh if only I had $500 for electronic unit, and $500 for progressive shocks and springs....