Author Topic: Best way to get my bike to not run rich......??? (Read 2208 times)

zoo mob · « **on:** June 27, 2009, 12:28:22 AM »

So, my 73 750 has always run rich, it had a mac 4-1 and stock air filter with drilled out airbox when I got it. It now has a k&n filter, and HM341's with a couple of small holes, and still runs pretty rich main jets say either "12G" or 120, forgot to check the smaller ones, they were synced last summer. So my question is; just start messing with jet sizes or can this be fixed by moving the needle or messing with the air screws? I don't really know what the best way to approach this is. On a similar note, should I be able to just rip the throttle open all the way and not have it bog down, or is just a touch of hesitation normal? Thanks

Hush · « **Reply #1 on:** June 27, 2009, 01:14:18 AM »

Hey Zoo dude, because your bike is running rich, when you give it a fistfull it can't swallow all that gas so it chokes on it a bit hence your hesitation.
Not having played with pods or oversize jets etc I would only be playing with the air (mixture screws), turn them all "in" 1/8 or a 1/4 turn and see if that helps...sorry my only suggestion.

torrinh · « **Reply #2 on:** June 27, 2009, 04:47:11 AM »

Stop spending money on it...?

zoo mob · « **Reply #3 on:** June 27, 2009, 10:01:52 PM »

That's a riot. In all seriousness, just the air screws? Should I even mess with the jetting?

Hush · « **Reply #4 on:** June 27, 2009, 10:16:29 PM »

Sorry dude not really my field as I'm into stock running but your bike has had some serious "mucking around" done to it already.
You can change jets if you want but if it's running rich it probably already has huge jets and you should be looking to come back closer to stock ones.
I'm a cheap bastartd so I would adjust the air screws to add more air thus balancing up the rich petrol mix it is getting.
That would be a free fix, hey it might not work but it would be my first "free" step.

Spanner 1 · « **Reply #5 on:** June 27, 2009, 10:24:16 PM »

Your bike may be needing ( stock ) 110 Main Jets depending on the carb type( stamped number )...also needle may be pulled-up from 3rd. position...floats may be set too high...general tune-up may be overdue....air screws need to be 1 turn out from seated, for starters !!

zoo mob · « **Reply #6 on:** June 27, 2009, 10:39:39 PM »

Forgive my ignorance on this one, but will adjusting the air screws in anyway effect the the sync on the carbs? Just to be clear, I was thinking of going down to 110's as the other step.

Spanner 1 · « **Reply #7 on:** June 27, 2009, 10:49:47 PM »

Unless your carbs are 657A's you need to keep the 120 Mains. Carb sync. is not effected by air screw position....but air-screw turns are more relevant than most folks think on these carbs, as their effect is within your normal riding-around-range...rich/lean....

zoo mob · « **Reply #8 on:** June 27, 2009, 11:35:51 PM »

Cool, I'll fiddle with them tomorrow, get some new plugs and see how it goes, thanks.

Hush · « **Reply #9 on:** June 27, 2009, 11:52:37 PM »

Carb sync really only lines up all the carb slides so they are opening at the same time and thus delivering gas at same rate to all cylinders.
As long as you don't take the tops off your carbs and fiddle with the slide heights you will be OK.

mlinder · « **Reply #10 on:** June 28, 2009, 06:00:41 AM »

You can't grab a handful of throttle from low rpm and have it work because your 73 cb has mechanical slide piston carbs, not VM's or the later mechanical slide piston carbs with an accelerator pump. Mechanical slide piston carbs (or any carbs, for that matter) don't shove fuel down into the cylinder, the fuel and air is sucked in by the vacuum created by the cylinders piston going down and the intake valve opening, paired by the amount of space the slide piston of the carb has open.
When engine speed is low, and you whack open the throttle, you reduce vacuum, meaning that vacuum is decreased too much to pull in enough fuel to run properly. Mechanical slide pistons without accelerator pumps require a bit more finesse than VM or or accel pump carbs. You must keep the vacuum up to continue pulling in more fuel for the amount of air available. That requires a more delicate understanding and feel of engine speed vs. throttle position.

Short version: You can't whack open mechanical slide pistons.

As far as your rich condition, what's your altitude? Remember the k0 and k1 came stock with 120's, jetted for just about sea level. Prolly a tad rich, but worked well. Just check your plugs, and jet accordingly. I'd try 115's first. What do your plugs look like? Just remember, no jetting will allow you to whack open the throttle on this bike with these carbs, though, ever.

zoo mob · « **Reply #11 on:** June 28, 2009, 08:23:01 AM »

Well the throttle response seems pretty good, and not having ridden someone elses 750 before, I didn't reall have anything to compare it to, good to know that the carbs work ok. So, the real question here is how much of a result can I see from just messing with the screws?

mlinder · « **Reply #12 on:** June 28, 2009, 08:43:51 AM »

Quote from: zoo mob on June 28, 2009, 08:23:01 AM

Well the throttle response seems pretty good, and not having ridden someone elses 750 before, I didn't reall have anything to compare it to, good to know that the carbs work ok. So, the real question here is how much of a result can I see from just messing with the screws?

Not much of anything above an 8th throttle, and nothing at all above a quarter throttle.

Again, what do your plugs look like? Why do you think it's running rich?

zoo mob · « **Reply #13 on:** June 28, 2009, 09:26:56 AM »

The plugs are carbon dust black. I'm not getting what you're saying about 1/8 throttle and 1/4 throttle.

OldSchool_IsCool · « **Reply #14 on:** June 28, 2009, 09:37:12 AM »

I think he means 1/8 or 1/4 turn of the air screw.

mlinder · « **Reply #15 on:** June 28, 2009, 03:17:37 PM »

I'm talking about the air screw.
The air screw does not affect anything above 1/4th throttle. It does very little between 1/8th and 1/4. It affects mostly between 0 and 1/8th throttle.

Spanner 1 · « **Reply #16 on:** June 28, 2009, 09:22:06 PM »

I would disagree about the relevance of the air-screws vs. more than 1/4 throttle. The air-screws 'set the scene' for the slow speed jet and if not allowing enough air to mix with the fuel being sucked out of the slow jet bleed hole will blacken the plugs with a rich condition. A great deal of riding is done under the influence of the slow jet...think about it, if the valves are say, 1/3 of the way up you have 'exposed' the main jet and are accelerating ! O.K. you reach the desired speed and you shut-off the throttle to a minimum point to maintain your speed...back on the slow jet ( which doesn't turn off because of bigger throttle openings ). If the slow jet/ air set-up is rich the brief 'excursions' into say 1/2 or more throttle ( adding main jet ) are not long enough to burn off the soot on the plugs. You could do an idle to 1/8 throttle plug chop !, why not?...

mlinder · « **Reply #17 on:** June 28, 2009, 09:48:11 PM »

Quote from: Spanner 1 on June 28, 2009, 09:22:06 PM

I would disagree about the relevance of the air-screws vs. more than 1/4 throttle. The air-screws 'set the scene' for the slow speed jet and if not allowing enough air to mix with the fuel being sucked out of the slow jet bleed hole will blacken the plugs with a rich condition. A great deal of riding is done under the influence of the slow jet...think about it, if the valves are say, 1/3 of the way up you have 'exposed' the main jet and are accelerating ! O.K. you reach the desired speed and you shut-off the throttle to a minimum point to maintain your speed...back on the slow jet ( which doesn't turn off because of bigger throttle openings ). If the slow jet/ air set-up is rich the brief 'excursions' into say 1/2 or more throttle ( adding main jet ) are not long enough to burn off the soot on the plugs. You could do an idle to 1/8 throttle plug chop !, why not?...

Again, the air screw does pretty much nothing above 1/4 throttle position. You haven't said the contrary, either, though your argument appears to be doing so on the surface. Going to 1/3rd then back down to 1/8th to maintain speed is, again, mostly below 1/4 throttle. It's true that the cb750's will cruise at close to 70mph at 1/4 throttle, but if the bike is running rich in the main circuit, no manner of air screw turning will rectify this, and your plugs will soot up every time you pull past 1/4 throttle.

zoo mob · « **Reply #18 on:** June 28, 2009, 09:51:51 PM »

I wish I could chime in one way or another on this, but I have yet to do any experimenting. Just to be clear; turn the screws all the way in and then one full turn back out for stock setting correct? Turning in is less air and out is more correct? Thanks, and I'm glad to have inspired some lively debate.

mlinder · « **Reply #19 on:** June 28, 2009, 09:56:03 PM »

It's not a debate, it's fact

Yes, turn them all the way in, but don't force them. Just snug. Then turn them out. I'm pretty sure stock was like 1 1/4 turn out, or somesuch. Don't have the manual on me right now.
And yes, in = less air, out = more air.

(that being said, a plug chop at wot and a plug chop at 1/4 throttle would be a good way to determine where your A/F problem is. If it indeed soots up at 1/4 throttle but not at wot, the air screws can help you with this.)

Spanner 1 · « **Reply #20 on:** June 28, 2009, 10:08:31 PM »

Zoo, your correct....what I am saying is that a rich low speed carburation will soot-up the plugs and that throttle maintained past 1/8 ( say ) will bring-in the main jet which now has to burn-off the soot on the plugs which will re-deposit as soon as the throttle is closed enough to bring in the slow jets again....air-screws have only an effect on the slow jet, but the slow mix has effect on the plugs at greater throttle openings, ie. plugs already sooted.
I agree the main jet/air mix could be too rich..but I think a lot more riding is done in the range of the slow jet.

mlinder · « **Reply #21 on:** June 29, 2009, 05:56:58 AM »

Remember, the idle circuit on these bikes is supposed to be rich. No accel pump. Jetting idle circuit to stochiometric mixture may give you a nice tan plug if you just let it sit there and idle, actually getting th ebike moving will be a different issue altogether. The initial opening of the throttle requires more fuel to keep the engine running, let alone accelerate.
Dunno, it's true that I may have been a bit of too aggressive a rider before teh accident, but I surely never had any problem burning off the carbon deposits that were created from having the required rich idle circuit.

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Author Topic: Best way to get my bike to not run rich......??? (Read 2208 times)