Topic: Noobie Carb question.

[drackett_19]

Hey folks, hopefully I don't get too roasted for this question, but I am pretty new to carb tweaking and don't really know where to start. I bought a '75 cb550k last year. It has a MAC 4-1 exhaust on it and I am putting in a UNI air filter this year.

I am pretty sure it was running lean last season because the plugs were light and it was getting really hot after long rides. My question is, do I need to rejet with different sized needles because of the mods to exhaust and intake? if so, what size would I start with? or can I just move the stock sized needle up a notch?

Right now it is at stock setting, second from the top.

Thanks for any tips on this.

[moham]

a ton of info here...

http://forums.sohc4.net/index.php?topic=5410.0

should be most of what you need...good luck.

[drackett_19]

Thanks, I have my carbs apart for cleaning and am going to put in some new gasket kits. I might raise the needle one notch before reinstalling them, then start doing some plug chops.

[moham]

there's also many other threads that highlight the importance of the other types of adjustments (valve clearance, point gap and timing, etc) that need to occur before the final carburetor adjustments are made. Also, if you don't have the shop manual for your bike, then check here:

http://forums.sohc4.net/index.php?topic=17788.0

[TwoTired]

Hey folks, hopefully I don't get too roasted for this question, but I am pretty new to carb tweaking and don't really know where to start. I bought a '75 cb550k last year. It has a MAC 4-1 exhaust on it and I am putting in a UNI air filter this year.

I am pretty sure it was running lean last season because the plugs were light and it was getting really hot after long rides. My question is, do I need to rejet with different sized needles because of the mods to exhaust and intake? if so, what size would I start with? or can I just move the stock sized needle up a notch?

Right now it is at stock setting, second from the top.

I don't think different profile slide needles are easy to find.  I have never seen any. 

The chart in the FAQ shows your slide needles were 4the groove from the top (blunt end) when stock.  If yours is truly at 2nd position the midrange is set leaner than stock.  With the Uni over a paper filter, stock would be better, methinks. 

Does your MAC still have the baffle in it?  The stock setting worked well for this combination (Uni NU-4055 and MAC), on a bike same as yours I once had.

You should be aware that there are different metering devices in the carb beside the main needle.  The slow or pilot jet and the main are used in addition to the Jet Needle.  They make dominant contributions depending on the throttle setting or position.  Learn to read plugs for home adjustments.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
  The chart below gives an idea what pary of the carb to adjust relative to throttle position.

[drackett_19]

Woops, sorry about that, I meant to say fourth from the top.

[hapsh]

I have found that at my altitude (2600 ft) stock jetting and needle positions work for my 550 with Uni's and a Mac 4-1.  That is partially due to the fact stock jetting is a bit on the rich side at that altitude.  It seems that the Pods mostly effected my full throttle mixture more than anything.  It still ran fine but seemed a bit lacking in power at WOT so I went one main jet higher than stock and I was just fine.  From my reading of most of the jetting posts I have seen that most people at or around sea level have the jet needle one to two positions richer and 2 main jets richer, with minor adjustments to the airscrew for idle mix.  If your bike is running well other than running a bit lean I would first check timing to make sure it is not too far advanced at 2500-3000 rpm and test raising your jet needle (lower the clip) and see how it runs.  If it is now too rich you will start noticing 8 stroking off idle where the usually over rich idle circuit mixes with your now too rich needle jet mixture.  Remember changing your main jet will not change your mixture at normal cruising speed or highway speeds, it only changes you 3/4 to full throttle mixture.  Also, remember to only make one change at a time in jetting or timing before moving on.  You will make yourself a mess if you change needle clip, main jet, airscrew and timing all at the same time.  You wont know what change did what.

[drackett_19]

Thanks alot for the info. I thought I set the timing last season, but I'll have to give it another go when I get the carbs back on. I was having trouble seeing the marks with my timing light, so that may be the culprit.

[hapsh]

Get an inductive timing light at Harbor Freight, you can get them for as low as $9.  The older style that go inline with your spark plug wires are really hard to see.  The newer Zenon (sp?)  bulb type that clip to your battery and have an inductive clip for your plug wires are super bright you can even see the marks in full daylight.