Topic: not sure what to do

[Rsnip988]

76 CB750 K

the bike as i bought it had the air pods, overjetted carbs (not sure of the size, is there a way to find out?), and there a 4 into 1 header to a megaphone pipe gutted of the baffle, ran like a champ when warmed up.... 

Upon complaints of noise from the wife and others i bought an aftermarket JC whitney exhaust and put it on, looks and sounds a lot better and when the choke is on it runs fine until you stop at a sign and idle, but as soon as you try turning off the choke it dies as if it was choked to death.....

the header exhaust OD was 2 1/2 in  and the muffler OD was 1 3/4 so i put on a reducer to fit it on...  the exhaust manifolds going into the headers were smoking (could have been from the oil leak i fixed) so i was thinking that maybe its putting out too much exhaust for the baffled pipe and exhaust sems too rich to me.

Any suggestions on what is causing it to die upon turning off the choke...   Sorry if this is a stupid question, or a newbie one but i have exhausted all i know to do to it

is it better to be too lean or too rich?

thanks all RKS

[bryanj]

Take a jet out and it will have a number on it and that is the size, sounds like you may need to go down a size

[Rsnip988]

thanks much

[TwoTired]

Has the bike been in hibernation?  How clean is your fuel supply?

The needing-choke-for-idle thing is often found to be plugged idle jets.

The idle jets have a parrallel fuel path in the carb with the main and throttle valve fuel path.  The main and throttle valve fuel path doesn't usually do much at idle unless the vacuum in the carb throat is very high at idle such as when the choke is applied.

When you get around to looking at the jets, check both of them and make sure you can see light through them.

Another player in the idle fuel mix is the Idle Air bleed Screws.  At least find out where they are currently set,  Turn them in til they gently seat, counting the number of turns in the process.  You can always set them back to where they were.  And, if you end up needing a mixture adjustment for idle you can make some adjustment here.

For the home mechanic without exhaust sniffing equipment, your best window on the combustion process is the color of the deposits on the spark plug.

Also, if you do have one or more clogged idle jets, those cylinders will usually run at a different temp than the others; cooler at idle and hotter when the throttle is more opened.

So, get to work, and give us some real data to work with!

Cheers,

[Clutch Cargo]

The stock jets in a 1976 750K are 105's.

110's or 120's were for earlier years.

[KB02]

For the home mechanic without exhaust sniffing equipment, your best window on the combustion process is the color of the deposits on the spark plug.

Plug Chops!!

[nuke]

So when you change mufflers, from stock to after market slip-on, what effect does or can that have on idle. I've covered all the carb, spark, valve issues (6 days 5 hours 6 minutes of sohc4.us) and still have choke no choke idle probs. I have a modern sport bike slip-on (Hindle) and if this could be the culprit because of some sort of backpressure issue, than I fix.

Sorry Rsnip, don't want to steal your thread but you bring up a good point....ie..effects of different exhaust components on idle.

thanks
nuke

[KB02]

Pretty much trial and error. Plug chops are the best way to tell if you're running rich or lean at a given throttle range.

[Rsnip988]

Nukes question was prety much the same as mine, the bike ran perfectly the day before i put on the aftermarket muffler from the straight megaphone pipes, if I do change the jets  wont i have to take the airpods off too to keep the air inpup/output ratio back to the origional?

plug chop?

[KB02]

Just do a quick search for "Plug chops" and you'll find tons of info. To make it easier, I'll quote TwoTired:

Inspection process:
Reading the plugs is done after a "Plug chop".  The engine is warmed to operating temperature, stopped, and clean plugs are installed.  Then the engine (under load) is run for 15 seconds to 5 minutes at a specific throttle setting.  (The variance is due to the heat generation during combustion and the number of firing cycles in a given period. WOT operation needs less time to collect deposits than idling.)
Simultaneously, the kill switch is engaged, and the clutch pulled in, to stop the engine.  The plugs are then extracted and inspected with a magnifying glass for deposits and condition.  If not within desired parameters, adjust carbs for compensation of that operating range, and do another plug chop.  The process is reiterative for the various throttle settings.

This is from his post:
Reading Spark Plug Deposits for Mixture Readings

[Rsnip988]

I was thinking...   I guess i can take the carbs w/110 jets and put them on with the airbox from the bike that needs new tires and i cant ride now anway and put them on the rideable bike



comments and concerns welcome
rks