Topic: Air screws all the way in, bike still runs...

[LeoLegendATL]

I turned my idle screws, the ones next to the air box all the way in, and the bike seems to idle fine.  What is this an indication of my mixture? too lean? if so how do I correct, btw this is for a 75 750f.  I read somewhere that these need to be about 1 and a half turns out. I pulled them out about 4 turns, then the bike starts to die.  I'm thinking my mixtures a bit off, Help!  Also the bike seems to run great up until 5000 rpms, then after it starts missing and coughing a bit.

[Hondalubr]

Are you setting this with the airbox on or off? Sounds like you have too much airflow to the intake for the little screws to make any difference. If it is on have you inspected the seal to the carbs to your airbox? Maybe your slows are too big? Just a guess.
Other than that, I am stumped as well!
Mike

[LeoLegendATL]

airbox is on, the slows are stock, the seal around the airbox, the little rubber gasket is broken in places, would this make a difference?  Also, do these screws just mess with the idle, or do they have an overall effect on how the bike runs?

[Bob Wessner]

Here's a pretty good write-up on the inter-relationship of various carb parts, thought you might find it useful. Are all the carb parts (jets, needles, etc.) stock for your bike?

http://www.speedandsport.com/jet.html

[Hondalubr]

They are supposed to make a difference. That is why they are there. If you are not seeing any change in the performance while adjusting them, something alse is causing them not to work. They themselves may be dirty or mucked up and only when you open them up 4 turns they are allowing a bit of air flow change. As for the rubber not sealing, you need to fix or replace that. It will have an adverse affect on your bikes performance. Plus, dirt particles that get by the holes could eventually cause you a big headache.

Mike

[LeoLegendATL]

also, I noticed the choke has almost no affect on the bike when it's cold, but when it's warmed up and the choke is applied it dies like it's supposed too.

[78 k550]

I would try the easy first. I would count the turns in on each before you take them fully out. Then I would squirt some carb. cleaner in each hole and see if that help's. If not you might be looking at a rebuild on the carbs.

Paul

[TwoTired]

The carbs for the bike are not set up properly if you can close off the pilot screws and the engine keeps running at idle.  Turning them out too far usually effects low speed throttle response.

The screws on your carbs meter air from the air jet to be mixed with the fuel from the Slow jet, primarily for throttle settings of idle to 1/8 open.

The Carb slide and attached jet needle set the mixture for 1/8 to 3/4 throttle positions.  And, the Main jet sets mixture for WOT position.

I caution you that carb adjustments should be made after all other tuneup items are addressed; Points, timing, spark plug cleaned/gapped, and correct heat range plug verified, tappets an cam chain adjusted, and air filter cleaned, renewed.  Also any, leaks in the rubber couplers will make carb tuning "fun" (if you like repetative and futile tasks).

All the book value carb settings apply to machines as Honda sold them. Changes to exhaust or air filter may required a unique setting for your assembled components.

You might also want to learn to read your spark plugs, as the deposits can tell you whether your combustion mixtures are lean, rich or just right.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
Cheers,

[vhenle]

I just rebuilt the carbs on my bike ( Stock airbox, 115 mains, stock slow jet, Mac 4into1). The bike runs great, but I can also adjust my air screws in or out with no change in the idle?

P.S. I'm new to the CB750 and to this forum, you guys rock!

Chad

[TwoTired]

I'm not sure this applies to the Cb750 carbs.  But, many of the CB550 carbs types have Idle Air Screws that are hollow tipped and cross drilled.  The effect of this is that, even though the air screw is fully seated, it cannot totally shut off the air for the slow circuit.  If you cannot make the slow circuit rich enough with this type, larger slow jets are the intended remedy.

Cheers,

[pmpski_1]

Can pilot screws affect how the bike runs at all throttle positions?

My 74 CB550 was running lean, I turned the pilots in until they were 1 turn out. Bike ran like crap, slow throttle response, felt like it was working hard to go whenever I opened the throttle. I started turning the screws out 1/8 turn at a time, did that about 4 times. Now it runs much better.

That was the only change I made.

[TwoTired]

Can pilot screws affect how the bike runs at all throttle positions?

To some extent, yes.  The slow circuit is a separate fuel delivery passageway to the carb throat from the main and needle jet source.
It's like you with your garden hose standing next to the firemen with their equipment shooting a stream up to a third story window.

My 74 CB550 was running lean, I turned the pilots in until they were 1 turn out. Bike ran like crap, slow throttle response, felt like it was working hard to go whenever I opened the throttle. I started turning the screws out 1/8 turn at a time, did that about 4 times. Now it runs much better.

The book value for your Idle Air Screw setting is 1 and 1/2 turns out +/- 3/8 of a turn.   I think you just told us that your bike works best at 1 and 1/2 turns out.

Okay....
However, turning the screws out, lean the idle mix even more.  If you began with a lean mix, as stated, how did going leaner help?
One of us just doesn't understand something.

Still, I'm glad your bike runs better.

Cheers,

[78_SaltLick]

Can pilot screws affect how the bike runs at all throttle positions?

My 74 CB550 was running lean, I turned the pilots in until they were 1 turn out. Bike ran like crap, slow throttle response, felt like it was working hard to go whenever I opened the throttle. I started turning the screws out 1/8 turn at a time, did that about 4 times. Now it runs much better.

That was the only change I made.

How many turns total would you say the screws are out?

[78_SaltLick]

Nevermind.......i think two tired say 1 1/2 turns out total......that is the stock setting.

[pmpski_1]

It was lean in the first place so I decided to turn them in to richen it up. I turned it out one turn to see how it ran at that setting. Too rich. After that I turned them out 1/8 turn at a time to get the mixture right. I was just surprised that the pilot screws affected it across the entire range.

In the carb faqs it mentions getting to know how too lean and too rich feel. Now I know how too rich feels

[78_SaltLick]

i got mine set 2 turns out.....i have 4 into 1 exaust adn it runs rich, so turning it half turn out from the stock settings seemed to make it run a bit better.