Plug chop results.

Loctite77

Loctite77

Yeah Son!
Went out and did plug chops the other day after installing new plugs wires coils and Pamco ignition. Here are the results. The first two photos are 1 and 3, the look lean right at the tip but really sooty around the threads. 4 (last photo) looks rich and sooty. What would cause something like this?

I have ordered vacuum gauges and am going to sync the carbs this week, maybe 4 is getting too much vacuum? I'm also going to go up one jet size to 120 from 115.

I am running KandN pods and a 4 to 1 headed with a fairly open pipe. The rest is stock.
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Yeah Son!
 
What is the bike?....Your on the right track with the vacuum gauges, but if your going to go bigger on the mains you'll want 125 main not a 115.... You may be thinking of idle jets which the holes get smaller when the size gets larger....anyways i try not to do tooo many changes one at a time when plug chopping so you can get the most from each small change....Huge Gains!
 
What revs and throttle position? If that was flat out, they are all rich, but I suspect that was low load and doesn't tell us much of anything.

Plugs get much hotter at high load than at low load and at those higher temps they can self clean. At lower loads they don't tell us much and not enough to fine tune. In addition, at much less that wide open, the needle and needle jet are doing the fuel flow modulation. Up to about 1/2 to 3/4 throttle you could run with the main jets still in the bags they shipped in!

Desmo is correct that all main jets use larger numbers to indicate more flow. Some are mm (diameter) sizes and some are flow rates but in all cases, more is more.
 
That was at 3/4+ throttle pretty heavy load. I agree that they look rich around the base, but on 1/3 the tips looked really white and lean, plus I'm getting hesitations at 3/4 throttle and higher which to me feels lean. I'm just wondering if it could run rich at lower speeds/revs and lean at higher?

It's a 76 cb750f, sorry. And I am going up from 115's to 120's. Needle currently set with clip in the lowest (richest) position.


Yeah Son!
 
FWIW, stock jet size for a 1976 CB750f is 105. You're getting really far from that. Running K&N would have a tendency to lean it out. Because the 750f ran a more open pipe, they were down on the main jet from the K model, depending on what casting # the K carbs had. Most 69 - 76 Ks are jetted to 110, but some were 120 and some were 105. Also, where are you setting your timing at? I run Dyna, which requires you to time the bike at full advance. 10° of advance equals about 70°F on plug. You need to be at 450°F to burn off the carbon, which is why it's often suggested to be running at highway speed to clean the plugs and why it's better to do plug chops there as well.
 
deviant said:
FWIW, stock jet size for a 1976 CB750f is 105. You're getting really far from that. Running K&N would have a tendency to lean it out. Because the 750f ran a more open pipe, they were down on the main jet from the K model, depending on what casting # the K carbs had. Most 69 - 76 Ks are jetted to 110, but some were 120 and some were 105. Also, where are you setting your timing at? I run Dyna, which requires you to time the bike at full advance. 10° of advance equals about 70°F on plug. You need to be at 450°F to burn off the carbon, which is why it's often suggested to be running at highway speed to clean the plugs and why it's better to do plug chops there as well.
Click to expand...

I just installed a Pamco ignition and timed to the F mark at idle (1000). Are you saying I should set the timing differently? Not sure I follow, I'm a novice, obviously.


Yeah Son!
 
Loctite77 said:
I just installed a Pamco ignition and timed to the F mark at idle (1000). Are you saying I should set the timing differently? Not sure I follow, I'm a novice, obviously.


Yeah Son!
Click to expand...
You set initial timing correctly, but you also need to check the advance timing at 2500 rpms. There are two advance marks about an inch from the F mark. Also, where's your air/mix screws? The stock air/mix setting for your bike is 1 turn from lightly seated. The 69-76 richen as you turn in, while the 77-78 lean out as you turn in.
 
Ah right, I follow you now. I was reaching full advance a little later, about 2700-2800 due to tightening my advance springs a half turn. Currently have the mixture screws about 3/4 out.


Yeah Son!
 
Let the ruckus begin, when doing a read of plugs pulled from a bike this article tells you how to read them right http://kawtriple.com/mraxl/carb/plugchop.htm. Now I do not suggest you cut the thread section to reveal the colour band on the insulator down inside the thread section, but with a magnifying glass and a flashlight you can see the band. The exposed tip colour tells you very little and if you are relying on just that good luck tuning the fuel/air mixture. ???
 
cqyqte said:
Let the ruckus begin, when doing a read of plugs pulled from a bike this article tells you how to read them right http://kawtriple.com/mraxl/carb/plugchop.htm. Now I do not suggest you cut the thread section to reveal the colour band on the insulator down inside the thread section, but with a magnifying glass and a flashlight you can see the band. The exposed tip colour tells you very little and if you are relying on just that good luck tuning the fuel/air mixture. ???
Click to expand...
His pics are a pretty good indicator that he's way too rich. Good luck seeing a band of any kind with a fouled plug.
 
doing plug chops with iridium plugs is a waste of time you are way rich and the iridiums are masking it

the iridiums burn way hotter at the tip and should never be used for chops

put a set of conventional plugs in and repeat your drive then you will see what is really going on
 
cxman said:
doing plug chops with iridium plugs is a waste of time you are way rich and the iridiums are masking it

the iridiums burn way hotter at the tip and should never be used for chops

put a set of conventional plugs in and repeat your drive then you will see what is really going on
Click to expand...


Ah! Now we are on to something. Maybe I'll just bring this beast to you and let you tune the carbs for me?


Yeah Son!
 
Loctite77 said:
Ah! Now we are on to something. Maybe I'll just bring this beast to you and let you tune the carbs for me?


Yeah Son!
Click to expand...


I think learning how to tune is easier than getting the carbs in with the stock airbox
 
Just letting you know what worked for me.... K1 cb750 stock engine, 4 into 1 exhaust without restriction, v stacks... And here is what works for me! Stock pilot jet, needle last clip position raising it most, 130 main jet, idle screw 2 turns out. Air fuel ratio gives me 12.8-13.5 cruising in town, 14.1 to 14.9 at cruising at 60-70 mph. And 12.7-13.5 WOT. Check out air fuel ratios for gasoline and this is pretty much as good as it gets! 14.7 burns the cleanest. In the 12-13 range is the power band for strong engine performances without overheating.
This is on a fully restored bike pis 300km since rebuild.
Hopw this helps!!
 
What heat range are those plugs? If they are "colder" than stock they will not get hot enough to self clean. I can't see any sign on the electrodes of heat and that tells us teh plugs are too cool either mixture or advance or heat range of the plugs. Try a set of cheap stock OEM type plugs first. When you do a plug chop the reading has to be right there and then. Don't ride home after a plug chop and then expect to read the plugs when you get home.

As someone pointed out earlier, the place to read mixture is on the so called mixture ring deep down inside the plugs. Read tip temperature to read timing and heat range.

And to your earlier question, main jet is really only the main event at 2/4 to full throttle. Up to that point the needle and needle jet are moderating fuel flow. You could take the main jets out and leave them on the bench and still ride OK up to about 1/2 throttle on most bikes.
 
I have since installed regular OE plugs. Still running rich which puzzles me since I only have 115 mains. I also have the clip in the lowest position as mentioned above by mikeelem with the same set up. Others have mentioned they are running larger mains with a similar setup, 125-130's. How could I be so rich at WOT with only 115's? Could something else be the issue here?


Yeah Son!
 
yup weak spark at maxload can make it look rich as the fuel charge is not fired completely
 
cxman said:
yup weak spark at maxload can make it look rich as the fuel charge is not fired completely
Click to expand...

How would I know if the spark is weak? Timing is dead on and advancer is moving freely.


Yeah Son!
 
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