Topic: 1976 750k jetting with MAC 4-1

[76CAFE]

My Dad, brother and I are restoring a 76 K into a cafe racer,  we just got it running.  It sounds better than I thought it would.  It fired right up on all 4 and sounded decent.  It's a stock motor, we re ringed it, cleaned up the head, resealed the top end, and put a 4 into 1 mac with a baffle.  The carbs are ultrasonic cleaned and rebuilt with a stock rebuild kit with stock jets 40 pilot and 110 main.  Needle in 2nd to lowest clip. Air screws adjusted 1 turn to 2 turn.  Also has the stock airbox with a replacement air filter. It idles decent, still needs the carbs syncd, but if you give it a quick blip of the throttle it cuts out.  If you slowly rev it there's a flat rough spot between 3k and 4k, then if you rev it from there it revs quick and sounds great.  So it seems very lean.  Anyone else have a similar combo, what jetting do you have?  Which jet should I change first.  I messed with the mixture screws, and it didn't make a difference.
thanks

[Really?]

Getting on this thread to see what others say.

[GammaFlat]

I've got a K6 with a MAC 4-1 with baffle.  I have a stock airbox and K&N filter.  My mains are 110s (I believe the factory jets were 105s.).  I believe my bike to be close to "good" but change is in the air.... I plan on modifying the airbox (more holes).  I also plan on taking off my MAC 4-1 (already spoken for by another of my critters) and going with the HM341s (brand spankin' NOS).  Right now (last fall), bike ran great.  It friggin' screamed, idled well, started nice... it's my sweetheart. 

[KingCustomCycles.com]

Midrange lean condition, move the needle clip down on the shaft one notch and try again.  You are in the ballpark.  Jets control idle and full throttle.  Midrange is all on the needle.

[Johnny5]

I agree with King about the needle. Try finding a small washer (i found mine at Ace Hardware) to shim the needle to 4.5 and if that is still too lean, put the needle on the lowest setting. It's crazy how a little adjustment can make a huge difference.

[PJ]

I wouldn't start making too many changes until you sync the carbs and guarantee there are no vacuum leaks. Some of that off idle stuff may go away.
Oh yea, and recheck the timing too

[Duke McDukiedook]

Tune-up first before you tinker with the carbs (you didn't mention a full tune-up). Make sure those ducks are in line first, then you can monkey with the carbs last.

[76CAFE]

thanks for all the replies.  I forgot to mention.  We installed a new dyna ignition, we did reuse the factory coils and plug wires but put new plug ends on.  Timing is right on when using a timing light, both 1-4 and 2-3.  The clip on the needle is on the 2nd from bottom clip, so next one is the last slot.  I know the problem isn't a mechanical engine problem or a timing issue because if I give it just a little bit of choke then turn the choke off and then rev from an idle it will rev quick, then next time I try to rev it, it bogs and sounds flat. 
As for a vacuum leak, I don't really know where it could leak since the flanges screw into the head there's little chance of a leak there, and the boots between carbs and cylinders look like new.  I didn't spray any carb cleaner around to check for a leak, because we repainted the bottom end and blasted the cylinders and cleared them, so I really don't want to mess up the finish.  I guess I will sync the carbs next, if no better I'll raise the needle to the last slot.  Also anyone know what kind of vacuum these bike have at idle at the vacuum plug on the carb.
thanks

[76CAFE]

Update; we put the needles back to the middle slot, mixture screws all 1 turn out.  we then sync'd the carbs, at least withing 1 in. of vacuum from eachother, What a difference, the bike revs nice and quick from idle, and no more popping in the exhaust.  We will fine tune it a little more, but it's much better, at least rideable to get a basline for jetting.
Number 1 and 2 cylinders had 3-4 in. of vacuum while 3 and 4 were at 8-9 in. of vacuum,

[Duke McDukiedook]

Vacuum with the standard airbox should be about 8-10 psi.

[Johnny5]

Interesting. It was running lean or at least that's how you described the flat spot, and you leaned out the needle by moving it up one notch.

Do you think it was running rich before and not lean? Did the flat spot feel boggy? This has my curious.

[76CAFE]

Yeah I now have 8-9 in. of vacuum at each cylinder.  As far as the needle goes.  Initial startup the needle was set with the clip in the middle groove.  It seemed lean because it would bog when you would crack the throttle.  So we lowered the clip in the needle one notch to raise the needle to make it richer.  the bike still ran like crap, still bogged when you would crack the throttle, and it also smoked pretty good and smelled real rich.  The next thing we did was sync the carbs, and move the needle back to the middle clip to lower the needle since the bike seemed to be running rich.  As it was running and we were adjusting each carb you could tell it idled better, and slowly stopped popping and burbling out of the exhaust.  I reved it a couple times and it reved quick and sounded great.  We didn't ride it yet, since the bodywork isn't on yet,  I'm sure we'll still have some issues, but it seems rideable, that way we can get a basline plug reading before messing with anything else.

[Johnny5]

Good stuff. Glad to hear it's running better. Are you in one of the northern States? You're probably dying to get that bike on the road!

[Duke McDukiedook]

Yeah, the way you described it, sounded like you were running the needle a little rich.

[76CAFE]

I live in PA, so there's still salt on the road,  can't wait to ride it.  I'll post some pictures when it's done