Topic: Sparkplug Health

[Coltww12]

Hey Everyone, I've been tuning my 72 750 for a little bit now and I'm very close to getting the needles right. My question is, based on the look of these plugs, should I move the needle position up one more time or are these good?

Before the needles were one space lower and it was running very rich. Lots of random back fires. After bringing the needle up one more time, the sound cleared up a lot, but still backfires when reducing throttle quickly. I put about 100 miles on it then checked the plugs and this is what I found:

I haven't done a full sync yet, because I didn't want to go through the trouble until I knew the preferred needle placement, but it is benchsynced.

I'm new to these bikes, so if a little backfire is normal when throttling down, let me know.

[evinrude7]

don't look too bad to me although i'm no expert on plug color.  sync it and see what happens.  i get a little back fire on hard deceleration.  i think i have a small exhaust leak on mine.

[bryanj]

When you are talking up and down do you mean the needle or the clip?
If you move the needle UP by moving clip DOWN it makes it richer not weaker

[Coltww12]

When you are talking up and down do you mean the needle or the clip?
If you move the needle UP by moving clip DOWN it makes it richer not weaker

I'm referring to moving the clip up, needle down, making the mixture leaner. I'm sure I'm not too lean, my debate is if I'm still too rich or if moving the needle one position leaner would be too lean. What do you think?

[PeWe]

Change needles on stock CB750 carbs a real pain. Carbs off and new synch after clips adjusted.

Read plugs for needle you need to ride in the needle area for a 1-2 km, ca  120-140kmh. Kill switch at speed before throttle off + clutch
Where you know you can park beside the road, roll to park.

Also possible to ride at 120-150kmh and very slowly roll off the throttle. If you hear and feel a D-D-D-D sound, too rich.
If it instead accelerates a little, too lean.

If it runs fine at full speed and from idle to cruising around 100kmh.
If here weak acceleration, needle clip need to raise the needle.


Which carbs?
There is a number stamped on them.
Clip usually at 3 or 4.
If using stock air filter or not.
And not stock exhaust that flows better also effect.

If you can tell which carbs, stock needle a must. Cheap aftermarket very different.
You can get recommendations from others.

Make sure that fuel hoses are not bent and fuel will not fill bowls in time.
Enough that one  hose is bent, 2 carbs will act lean at high speed.  Easy to bend bad where the hose is attached to carb. The pipe can be tilted wrong.

[newday777]

Welcome to the forum Coltww12
I think this should have been put in your 1st thread on carb rebuild rather than starting a new topic so all the information is together (a mod might merge them together)
Your thread on the carbs says you have 4into1 exhaust and pods.....
The Pods may be your problem child....Did you read through this thread on pods??
http://forums.sohc4.net/index.php/topic,182280.0.html


And did you put in aftermarket brass carb parts??

[PeWe]

My K6 had pods on the stock carbs stamped 086A.
836cc, ported head, hotter but rather modest cam and a rather open 4-1.
It ran fine in the 1980's with small conical pods. 4 long tours thru Europe. 5000-6000km each time. A lot of heavy rain riding. Plus high altitudes over the Alps and Pyrenees.

Fuel screws opened 1/2 turn
Pilot jets: Stock 40
Needle: 4th notch from top
Main jet was huge. I should try at least 130-135 today.

If carbs should have been a set of 064A, needles at 3rd notch from top.

My almost K2 had those 086A carbs 2019 to last year.
It has 392 pistons and cam. A little bit higher compression.

Good flowing Yamiya no numbers 4-4, stock airbox with K&N filter.
Pilot stock 40
Needles had to be at 4th notch.
Main jet 115.

It got a set of 064A carbs last year.
Pilot stock 40
Needles at 3rd
Main jets 120
It runs better with these carbs.

Air screws around 1 turn out.

[BenelliSEI]

Welcome! Looking at your plugs, you are probably running leaner than you think. Following PeWe’s advice, they may be a lot “whiter”. Ideally what you want to see is “coffee with milk” colour. Personally a slight bit darker works for me too.

Careful pulling and reinstalling hot plugs. Easy to get one crossed. In my area, we have a really nice eatery on a back stretch of highway. I occasionally wind it up hard about 5 kilometres from the place and then coast the last 200 meters into their parking lot. Let the bike cool for 45 minutes while I’m having a nice lunch!