Topic: Syncing carbs or replacing main jets

[aminemed]

Hi

Mounted on my CB 750 K&N pod filters and a 4 into 1 exhaust, bike is not running well at all, no power at all in 1st and 2nd gear. This might be a stupid question, but how do i make the difference and know if it is carbs that should be synchronized OR if it is main jets that need to be replaced ?

[riverfever]

Not a stupid question. Sounds like you need to revisit overall carb tune after making the changes. You're likely going to spend a while doing some plug chops and making changes to idle jets, mains, IMS screws, etc.

[Duke McDukiedook]

Jetting and needle positions first then bench synch and final with gauges or manometers.
Hopefully your bike ran well before you made these changes and these are the only things you need to modify.

[flybox1]

If you can get it to idle, do a vacuum sync, followed by an idle plug chop to assist in you getting the right idle A/F mixture/mixture screw setting.

Main jets have little to no effect on riding at 0-1/4 throttle position riding.

[trebot]

What is "idle plug chop" ?

[evinrude7]

the basics of it: you run the bike within a certain throttle range for an allotted time then cut the bike off and check the plugs to see what kind of explosion you're getting inside the cylinders.  the color and state of the plug helps determine how well you have tuned your carbs.   this is flybox's specialty.  he should pipe up soon. 

[carnivorous chicken]

If you can get it to idle, do a vacuum sync, followed by an idle plug chop to assist in you getting the right idle A/F mixture/mixture screw setting.

Main jets have little to no effect on riding at 0-1/4 throttle position riding.

This -- if your carbs are out of sync swapping jets to try to fix it is going to be an exercise in frustration. Once your carbs are synced, go from there. Dialing in out of sync carbs can make a huge difference in how a bike runs.

[trebot]

Have '72 cb500 four. Stock intake, but has 2 into 1 pipes with no baffles.
 Will this affect the syncing of carbs with vacuum gages or do I go with  Service Manual specs (16 - 24 cm hg) with carbs within 3 cm hg of each other

[flybox1]

Have '72 cb500 four. Stock intake, but has 2 into 1 pipes with no baffles.
 Will this affect the syncing of carbs with vacuum gages    or do I go with  Service Manual specs (16 - 24 cm hg) with carbs within 3 cm hg of each other

Ignore this^^
Adjust your carbs so they all draw the same vacuum.  Period.

[Duke McDukiedook]

A bench sync should more than be able to get you close enough to make your jet and needle position changes. Save the gauge/manometer sync for the very last after you have sorted the previous changes out.

[carnivorous chicken]

A bench sync should more than be able to get you close enough to make your jet and needle position changes. Save the gauge/manometer sync for the very last after you have sorted the previous changes out.

Carbs are already on the bike and it's running, albeit poorly... why not sync first?

[Duke McDukiedook]

Manometer/gauge sync should be the last thing this guy does. Bench sync with the drill method should get you close enough to where you can make jetting and needle position changes to get your A/F mixture correct. That's assuming your crab's orifices are clean and clear and your float levels are right too.

[flybox1]

Manometer/gauge sync should be the last thing this guy does. Bench sync with the drill method should get you close enough to where you can make jetting and needle position changes to get your A/F mixture correct. That's assuming your crab's orifices are clean and clear and your float levels are right too.

I hate crab orifices. 


...a vac sync now wont hurt a bit.   It'll allow carbs to more closely pull the correct fuel into the mix, making jetting choices more 'educated'
It can later be revisited, once all jetting and tuneup items are assured correct.

[Keith]

How do you know it's the carburetors? Did you check for proper dwell and timing on both sets of points? (If not changed to electronic) Compression? Coils? The carbs should be the last thing  checked, after eliminating all else.

[eigenvector]

The carbs would have to be pretty far out of sync to noticeably affect power.  And even then it would idle badly.

It could be something as simple as the valves or those pods you have on there in which case rejetting is probably it.