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Irratic idle...

Spyder

Spyder

Forum Sage
Past Site Supporter
I am having much difficulty figuring out my idle issues on my '83 750T.

What I know about it is that it has K&N pods, a mystery header into a singe D&D baffle. Only thing I know for certain about the carbs is that they have 140 main jets in them. I read that, a "stage 3" kit has 130 jets. So I tried to drop down to a set of 135s. Wouldn't even start. Back to 140s. It runs pretty good, strong, and fairly quick. It was dyno'ed at 67hp at the wheel.

My issue (among many I think) is the very irratic idle. I MUST keep it at around 13-1400 and it is bumpy. I have to choke it hard to cold start it, and keep it choked for some time.

I've done a bible full of reading here and nothing seems to nail it. I cant really afford to do much trial and error on it.

I just got a sync tool and got them pretty close and still bumpy weak idle.

Sorry for the novel, but the more info up front the better my answers should be. Right?

Thanks in advanced for any of your help...
 
I have varied them, at the moment 2 turns out. runs like hell at 1.5, not much change at 2.5...
 
I guess I'm on my own with it, huh?

Well I'll start with a carb rebuild and go from there. For some reason (as an old gearhead and 9 year veteran automotive technician), I don't understand why these carb need to be rebuilt so often. To a newcomer it might seem that it is the answer to all GS problems. (of course I know better) I'm just not convinced that a rebuild is going to solve my idle issue.

I guess my question should be, do pods cause a bike to idle rough from the lack of vacuum pressure from the airbox and crank case?
I don't know anybody with a bike like mine, and don't really have a solid reference. For all I know this "condition" could be perfectly normal...
I have gone through Cliffs site and made my own service manual, and have been reading as many posts, and the advice there in as I can.
IDK, I'm just thinking it runs to strong to demand a full scale carb tear down...

I think I'm going to try a few more little things before the rebuild. I haven't done a compression test yet. hmmmm....
 
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Your erratic idle has nothing to do with you main jets and the thing will tickover fine with standard jets and no air filter. The erratic symptoms suggest that it's leaking in air somewhere - any chance of perishing on your inlet rubbers? Or are the pilot circuits blocked (and you know what that means.....).
 
I don't think these carbs " need to be rebuilt so often", but once they get gunked up idle and low speed problems will pester you to clean them.
You said it wouldn't even start with 135 main jets- this makes me think you have clogged idle circuits, after all the main jet does nothing for start up. Plus having to leave choke on makes me more suspicious of gunk.
 
Well, the boots "look" good. doesn't mean they are. I have had this bike for almost 4 years now, and its pretty much the way I got it. It always starts, and rides pretty good. It even seems fast to me for a 750 of almost 30 years. I would think it should idle stronger, steadier. I have an extra set of brass "taps" for my sync tool coming (today or tomorrow) I think I'm going to use them to set up the 1/4 2/3 vacuum mod I've read about here. I will pull the air screws out and have a look at them, make sure they're not broken and check the o-rings. (I can do that right?) they just unscrew?

I would really like to know exactly what it is. Not just through a "blanket" rebuild at them. I don't have a problem with the rebuild, new is always better in such cases. I'm just one of those that have to analyze the hell out of things to learn why things happen...

Thanks for your interest and reply...
 
tom203 said:
I don't think these carbs " need to be rebuilt so often", but once they get gunked up idle and low speed problems will pester you to clean them.
You said it wouldn't even start with 135 main jets- this makes me think you have clogged idle circuits, after all the main jet does nothing for start up. Plus having to leave choke on makes me more suspicious of gunk.
Click to expand...

Ok, I will start there. Play with the idle circuit some, and likely end up doing a total rebuild.

And please let me correct myself, (my memory is getting gunked) it started, it just ran like hell. Couldn't snap the throttle, it was just bad. It does run a bit rich when riding hard. I've already thought about trying like a 137-139ish. I'm not sure what my options are as far as size increments.
 
The mixture screws do 'just unscrew' (sometimes they can be sods to get out but if you have shifted them enough to turn them you are well on the way). Sometimes / usually when you unscrew them they leave the o rings behind in the carb and they can be a fiddle to get out, with the tiny little washers as well.

Your main jet increments are available in 2.5s - so 135, 137.5, 140 etc.
 
If the bike is ridden regularly, the carbs will likely never need to be rebuilt.
Sitting idle is what messes them up.
 
Well, I'll try to get the mixture screws out tonight and see whats happening there. I will deffinately have to pull the carbs out again and do a proper inspection of what has been done to them. I couldn't even tell you if they changed the needles or the pilot jets or anything. I just know the mains are 140's. I may just have to do a needle adjustment for the rich issue. Maybe?

Thanks for the input, I will check these things out this evening and post my findings. If I find something really cool and out of the ordinary I'll take a few snaps of it...
 
I don't like the term "rebuild", makes me think of all sorts of new parts. Except for o-rings, no carb parts are likely to wear out- they just get dirty. tiny passages will clog easy- and this ethanol fuel makes things worst. If you just ran the thing full out (like a generator), you could ignore low speed and transition problems, but unfortuantely you can't.
 
tkent02 said:
If the bike is ridden regularly, the carbs will likely never need to be rebuilt.
Sitting idle is what messes them up.
Click to expand...

As far as I know this bike hasn't been "parked" in a long time (if ever). Since I've owned it it gets a healthy dose of Sta-bil before I park it for the winter. the last couple winters I've been able to ride it more than usual... No matter how long she sits, she always starts. Varnished fuel is the #1 killer of induction systems... I won't have it.
 
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SUSPECT, 1 UGLY o-ring in #1 mixture screw. Replaced...

However, I do need to rebuild. They still have the stock needles in them. (at least I assume so, no taper at all and pretty blunt tip)

I have to say I'm surprised she runs as well as she does. I'm anticipating a marginal gain in power and performance after a proper dip and rebuild...

NEXT question, when ordering from the o-ring guy (name slips my mind while typing this), do I need to order 2 sets of screws for the caps?
 
Hi,

Send an email to Robert Barr. He's the (http://cycleorings.com) guy. I'm not sure of which screws you're talking about but he will be able to answer your questions.

Have you also replaced the intake boots and/or the intake boot O-rings? They are a common source for air leaks which cause an unstable idle. The thing is, to ensure proper operation you pretty much have to do it all - clean carbs properly, air tight intake and exhaust systems with no leaks, float height, proper jetting, bench sync, valve adjustment, vacuum sync, etc. If you don't know the complete history of the bike it's always a good idea to take care of all of the neglected maintenance from the last 20 years so that you have a good starting point for further troubleshooting.

And so you know, Dynojet and Mikuni use different measuring standards for their jets so you can't do a side-by-side comparison. There is a conversion spreadsheet calculator on my website (Thanks salty_monk!) and conversion charts all over the internet (http://www.jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm). Basically, the Mikuni jets are measured by the volume of gas passed (:p), i.e. a 130 Mikuni jet will deliver 130cc of fuel per minute. The Dynojet jets are the actual diameter, i.e. a 130 DJ has a 1.30mm orifice. A 130 DJ is smaller than a 130 Mikuni.


Thank you for your indulgence,

BassCliff
 
Yes it is time to break down and do it. Where is the best place to get everything in one shot? i.e. intake boots, needles, and jets... I will get the o-ring kit from Cycleorings. The cap screws I'm refering to are the 20mm stainless allen heads he adverises on his site. it mentions a set of 8, I need 16, no?
 
Spyder said:
Yes it is time to break down and do it. Where is the best place to get everything in one shot? i.e. intake boots, needles, and jets... I will get the o-ring kit from Cycleorings. The cap screws I'm refering to are the 20mm stainless allen heads he adverises on his site. it mentions a set of 8, I need 16, no?
Click to expand...
Those are to screw the intake rubber boots to the head. 2 per cylinder, 8 total.
 
ok, put it back together with the o-ring fixed and also connected the vacuum ports together. (1-4 and 2-3) Wow, night and day difference. runs so smooth now. I would say she went from iratic idle to nevous idle. but so smooth and powerful on the low end throttle. I'll hunt down all of the parts I need for the rebuild and take care of it after Brown County run...
 
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