Topic: Another Carb/Fuel Mix/Exhaust Question

[wjustinleigh]

I have an ongoing idle and acceleration from stop problem.  It's a 72 cb500 with a stock airbox, 4 into 1 exhaust, and stock sized needles/jets with air screws 1 3/4 turns out. 

I rebuilt the top end after picking it up in a non-running condition.  Once rolling it runs strong in all gears with no noticeable flat spots, the problem is while sitting and starting.  Once warm the idle 'sticks' at high RPMs, I can force them down by engaging the clutch slightly, then it will hold a normal idle until the throttle is blipped at which point it sticks again.  Also, again when warm, it's difficult to start out in first.  Unless I over-rev it will kill it.  With the clutch fully open it's almost impossible to start out in first without killing it.

Here's the weird part.  Since experiencing this, I've swapped almost everything out and it runs exactly the same.  Compression was a little low so I pulled the head and had it rehoned, new valves, etc.  Checked the cam timing, re-cleaned the carbs, new jets, tested then swapped the carbs with another set for another bike that I had cleaned and rebuilt.  Replaced the points with dyna, replaced the coils with dyna, new plugs, new plug wires, new air filter.

From the first ride to now the only thing that's the same is the exhaust and it performs exactly the same.  A #$%* when sitting at the light and getting going, then runs great down the road/highway. 

I can try bigger pilot jets, although I don't know why it would run this poorly using the stock config.  I'm wondering if maybe the exhaust is causing too much back pressure or something?  I have a set of 4 into 4s that could be cleaned up and tested, at that point I will have replaced everything that touches fuel, fire, or air.

[flybox1]

did you use aftermarket jets?   
Jets R Us, or Sirius are the only retailers i trust to be accurate.....

[wjustinleigh]

Originally I used aftermarket jets, some kit from ebay.  After it ran terribly I pulled the jets out, cleaned the crap out of the originals using guitar string and such and installed the originals.  When I rebuilt the second set of carbs I used the originals thoroughly cleaned.  I've run it now with three different sets of jets in two different sets of carbs with no noticeable difference in the problem.  So I think it has to be an exhaust or an exhaust to jet problem.  All the jets were the #40 pilots and 100 mains, the mains don't seem to be a problem.  I'm thinking the exhaust needs bigger pilots.  Seems odd that it would cause this big of a problem. 

[flybox1]

what do your plugs tell you?
check this thread for the plug color chart....
http://forums.sohc4.net/index.php/topic,146178.0.html

[flybox1]

D7EA plugs, yes?

[wjustinleigh]

Yep - D7EA.  I'll pull the plugs again and check them out.

[flybox1]

Checking plug color after a ride and many throttle positions wont tell you anything.
Letting the bike warm up with old plugs, installing new plugs, letting the bike idle under a fan for 3-4 minutes, and THEN checking the plug color, will give you an accurate idle mixture deposit.

[wjustinleigh]

The problem I'm having is going from idle to enough RPMs to generate power in 1st.  When throttle is applied it bogs down and will even die if enough throttle is applied.  I have to wait a moment until the RPMs wind up before engaging 1st.  Since adding fuel bogs it down it seems like it's lacking air, but opening the choke further/all the way - adding air - makes the problem worse.  What I really want to know is what is the mixture doing when I'm rolling on the accelerator when the motor wants to die and it's tough to run it at that condition long enough to do a plug chop.

I had the exhaust ceramic coated, I'm wondering if somehow the airflow is restricted.  So when I roll on the throttle it wants to push more air into the exhaust and causes back pressure.  If I let it wind up it creates a flow which might still be restricted but at higher rpms is sufficient.

If I leave the headers on but remove the muffler to see if it's a back pressure issue will that cause any engine damage/other concerns?

[ANDY W]

.  Since adding fuel bogs it down it seems like it's lacking air, but opening the choke further/all the way - adding air - makes the problem worse.

Are u saying that u don't have the choke open all the way when it is warmed up?

[flybox1]

does the condition get better if you RICHEN the mixture screw setting by 1/2 turn on all carbs?
I have feeling that your aftermarket 4:1 exhaust has made you too lean.  if the symptom gets better by richening up your air screw (turn it in 1/2 turn), we're on the right track...

[wjustinleigh]

.  Since adding fuel bogs it down it seems like it's lacking air, but opening the choke further/all the way - adding air - makes the problem worse.

Are u saying that u don't have the choke open all the way when it is warmed up?

That's correct - when warm I have the choke 3/4+ open, if I go full open it's possible but difficult to start up from a full stop.

[wjustinleigh]

does the condition get better if you RICHEN the mixture screw setting by 1/2 turn on all carbs?
I have feeling that your aftermarket 4:1 exhaust has made you too lean.  if the symptom gets better by richening up your air screw (turn it in 1/2 turn), we're on the right track...

Will give this a go and report back.

[flybox1]

.  Since adding fuel bogs it down it seems like it's lacking air, but opening the choke further/all the way - adding air - makes the problem worse.

Are u saying that u don't have the choke open all the way when it is warmed up?

That's correct - when warm I have the choke 3/4+ open, if I go full open it's possible but difficult to start up from a full stop.

this ^
more evidence to support you are too lean in the idle circuit.
the choke closes off air supplied to your carbs....you should be able to run NO choke at all after warm up.
choke is only needed at start-up to momentarily richen the mixture

From the first ride to now the only thing that's the same is the exhaust and it performs exactly the same.  A #$%* when sitting at the light and getting going, then runs great down the road/highway. 

and this ^ too    as you get going and are above idle, to say, 1/4 throttle turn, you engage the needle jet and main jet incrementally (above 1/4 throttle turn) and your combustion improves as they provide a more balanced mixture.

(I have a feeling you'll run out of air screw adjustment and need to go up one size in your pilot jets)

[ANDY W]

First I would check for any intake leaks and the fuel level in the bowls

[wjustinleigh]

Air screws definitely helped, I kept turning them in and am now at just 1/2 turn out and seem to have the best performance, but that's pretty closed - seems like I would want to change the pilot jet or something so I can run with a bit more air than that?

I haven't gotten it very hot yet due to weather, but it got up to temperature and would start off the line with the choke fully open.  Had some surging at between 0 and 1/4 throttle at lower speeds.  The idle seemed improved but it wasn't hot enough to be definitive yet.

[harisuluv]

Have you synced your carbs?

I didn't see you answer the question about what your spark plugs look like?

Did you use the needles from the kit you used first?

[wjustinleigh]

Thanks for the advice on this, I finally swapped out the four into 1 exhaust for a set of stock 4 into 4 pipes that were sitting in the corner and it solved all of the issues.  Put the air screws back out to 1.5 turns and it's running well and idling great.

Either something is wrong with my 4-1 pipe or it needed a significant slow jet change to work.