Topic: exhaust wrap= leaness? '78 cb750k

[Kev Nemo]

Jut wondering what could be causing some recent leaness. I had the carbs synched and they seemed ok for awhile, now burning lean/ whitened plugs. Here are the details:

-4-1 free flow exhaust w/ header wrap
-pods
-synched carbs with 127.5 mains
-new plugs

seems to bog at 3/4 maybe right before. Not as bad as when I got it, so maybe I need to go up to 130-135s?

[Gordon]

Synchronizing won't have any effect on the air/fuel mixture.  The more likely culprits of your lean condition are the 4-1 exhaust, pod filters, and inadequate jetting to compensate for these changes.   

[Johnny5]

Where's the needle set at?

[Kev Nemo]

I want to say the needles are set 1 down from lowest, but not sure. What I'm really trying to say is that the bike has started to run lean AFTER being synched and setup with 120 mains (up from the stock 115). It seemed to run fine like this for a few months, and THEN began running lean. Warmer/wetter weather maybe? I went ahead and ordered 130s and 135s and Ill see if one of those sets doesn't fix it.

[BobbyR]

If it is a 78K, there are no needle settings.

[gribbs]

If it is a 78K, there are no needle settings.

What do you mean?  I admit I know nothing about jetting....

[Johnny5]

If it is a 78K, there are no needle settings.

What do you mean?  I admit I know nothing about jetting....

If youre gonna run pods, better start researching now.

Most of the CB750 carbs had needles with slots to move the needle higher and lower. You'll need to find small washers to shim your needles.

[gribbs]

Actually I plan to get the CR 29mm carbs, or upgrade the ignition system totally...just need to figure out which will clear up my issues of having random cylinders firing and unfiring at will ...but that is another story

[BobbyR]

On the 1978 the adjystment screws are actual mixture screws not air screws as in pre 77 carbs.  The needles are fixed, but can be shimmed if you understand why you are doing it. That will only effect the motor up to 1/4 throttle or less. My bike came with 110 and the PO had moved them up to 120 without changing exhaust or airbox and while I am a tad rich, I like the way it runs.

After market exhaust and pods change the engine breathing. I would sugget you search on Pods, you will find a lot of info on making them work well. If you change carbs, you are walking down a road you may not to go down. It gets more complicated. 

Do a search on PODS and you will learn everything you will need to know.

[Kev Nemo]

Just out of curiosity, if my exhaust had a crack under the wrap, would that cause a lean condition? It cracked in the past and was welded up professionally, then wrapped. I'm trying to keep the pipe under wraps if possible, cause it such a pain to re-wrap.

[BobbyR]

A crack could make a difference, but you would hear it i guess