Topic: cb750 carb help

[capnspif]

I've only recently put my project bike back on the street and I'm now trying to tune in the carbs. The motor is a '75 750K and the carbs are '74. All parts were picked up used, so everything needed some work.

The info on the site has been extremely helpful, I couldn't have gone this far without all the instructions but I think I've hit my ceiling. I'm looking for some feedback on why my #2 & #3 might be running so lean.

So far I've:
Broke the carbs down completely and dipped them, checked all passages and cleaned out with compressed air. Carbs were bench synced and mercury synced on the bike.
Stock motor, compression is between 141-150 through all cylinders.
Don't have the stock air cleaner box so I'm using Uni foam pod filters.
Needle jet clips are set at the lowest position, slow jets #45, mains are now #120. Mains are new from siriusconinc.
Ignition is 3ohm Dyna coils and Dyna S pickup.

Bike idles nicely at around 1200-1400 after it's warm. Plugs seem to foul at idle or low rpm pretty quickly. I'm looking for a set of 40 slow jets.

I did a plug chop test, the bike had been running 10-15 minutes at this point, then 3 or 4th gear, WOT, kill the motor. These were the pics taken before the local police showed up and asked me to 'move along'. 
Cyl #1


Cyl#2


Cyl#3


Cyl#4


The question is, how do these look? Does 2 look lean? 3 seems obvious to me but I'm a noob at reading plugs. And if 2 & 3 are lean, what am I looking for? Something overlooked in the carbs, maybe an air leak in the intake boot? Should I go to 125-130 mains? Thanks for any feedback.

[TwoTired]

What plug number and brand are you using?

What you have there is carbs that have different mixtures at different throttle positions.  Plus, you have some variability between carbs.

The first questions to answer is why aren't each of the cylinders he same?  And how do I make them so?

1- do you have the same oil volume in each of the uni pods?
2- are all the tappets adjusted properly and equivalent.
3- do you have any leaks in the rubber couplers.
4- do you have worn parts or possibly jets that were drilled by a PO?  Throttle valve orifice or needle valve wear?
5- What are your air screw settings?
6- Could any of the carb passageways be restricted in any way.  Air jets? emulsion tubes?

Step two:
Next find out why the mixtures are proper on a per throttle setting basis.

For example: If it idles rich, and has a slightly rich mid throttle setting, the carbs will soot up the plugs.  A short Lean or perhaps proper WOT setting would burn off the soot near the tips.
Your #2 seems to appear this way and #3 plug, moreso.  # one clearly isn't getting to proper temp.  And, I question if #4 (out of focus) is still firing.  Is it carbon fouled?  Do all the head pipes seem to heat up?

Finally, you need to revisit your plug chop technique.  Since you have changed all three maybe four of the fuel metering devices in the carbs, you need to isolate each one for proper mixture.  Buy a couple sets of new spark plugs.

Drive to where you can do a throttle run.  Plug chops are done while the engine is at operating temperature.  Stop, put in new spark plugs.  Run through all gear at WOT. Ignition kill and clutch in simultaneously.  Remove plugs and take pictures.
If the plugs still look clean.  Put them back in and do another run or two at, say 2/3 throttle. (You HAVE marked your throttle, yes?)  Ignition kill and clutch in any time you need to reposition throttle settings.  You want a specific throttle position at max load condition to get a decent readable deposit on the plugs.  Pull the plugs and take pictures.  With clean plugs, do the same thing at 1/3 throttle position.  Pull plugs take pictures.  Note that at lower throttle settings, the cylinder temp is not as high, so deposits can take a while to build for reading purposes.  Idle setting are the hardest to do for this reason.

When you come back from your testing, you will have some run data vs. throttle position, so you can make adjustments to each of the carb metering devices in a methodical way.  Then you can go back out and repeat the test to see the effect of your changes and any requirements for further changes.

Many people will realize the value of a Dyno after doing plug chops, as it will give you mixture vs throttle position data very quickly, as well as magnitude deviation from ideal.  But, we didn't have prevalent dynos 30 years ago.  Plug chops and timed test runs were the tools of the day.

Cheers,

[capnspif]

The plugs are NGK D8EA.
Oil in the Uni pods could definitely be an issue, I will pull them off, clean and re-oil properly. All the internal carb parts looked good to my eye but I wouldn't rule it out. I was very carefull to check all the carbs, passages, jets, etc. to see that they were clear before reassembly.

Thanks for all the other questions, I will go through and re-check. I had the worst problems with the ignition firing, certain cylnders were just dead but the dead one or two seemed to jump around. New plugs and the Dyna coils and ignition seems to have solved it, all the headers are getting warm now, but if you're suggesting one may not be firing I may need to re-check. I've rewired the coils direct to the battery through a switched relay and rewired the fuse box. If I still have a cylinder dead, what could possibly be the cause other than the plug fouling out, or is that it? This drove me nuts for a couple weeks. I have an inductive timing light to check if a particular cylinder is firing. Timing is set very slightly retarded from the full advance setting, it needs to be adjusted.

Thanks for the tips on the testing procedure. As you say I need to take a methodical approach and go through everything again carefully but especially in the testing. I've got a new box of plugs waiting for the rain to stop.

[TwoTired]

If number 4 plug is fouled ??.  It behaves as a shorted plug.  Current still flows, triggering the inductive light.   But, no arc across the electrodes.  However, a cylinder with a fouled plug won't heat the exhaust header.

Have you checked/replaced the spark plug caps?  Stock resistance is 5K ohms.

Cheers,

[capnspif]

If number 4 plug is fouled ??.  It behaves as a shorted plug.  Current still flows, triggering the inductive light.   But, no arc across the electrodes.  However, a cylinder with a fouled plug won't heat the exhaust header.

Have you checked/replaced the spark plug caps?  Stock resistance is 5K ohms.

Cheers,

When I was just getting it running again all the cyl were dead at one point or another. Now, the ignition is all new, Dyna 3 Ohm coils, Accel 8mm supression core wires and DynaS. 4 didn't seem to be fouled, I'm not sure the picture represented them correctly, 1 & 4 both seemed to be tan on the insulator and electrode. No luck with the weather this weekend, I will need to schedule some time later to do some runs.

[TwoTired]

Suppression core wires?

These have resistance built in to suppress radio frequency interference.  Each spark plug cap of the stock bike had resistors built in for the same purpose.  Are you still using the stock spark plug boots, or non resistor types?

I don't like the suppression core wires on these bikes, as the core deteriorates with use.  Copper and solid core wire work better over the long term, as do the stock spark plug caps.

1 & 4 both seemed to be tan on the insulator and electrode.

Really? I dunno then, all the plugs look to have black deposits on the center insulator except #3, which appears white on my monitor (LCD).

Cheers,