Topic: Do any of you run 130 or larger main jets on a stock motor?

[judder]

I've got a 74 cb750 (stock motor), 4 into 2 exhaust w/ baffles and pods (ya I know pain in the ass, but I think they look cool). I went from 120 mains to 130 mains today and man what a difference in a good way. Do any of you run larger mains on stock motors? I'm thinking it may still need some more jet. I need to do a WOT pull and look at the plugs to know for sure.

Any help would be appreciated.

Thanks

[seaweb11]

78K 750 4-2 exhaust/pods I run 138's, but the carbs on the 78 are different than yours.

[bwaller]

When I raced my K3 with a hi-comp 810 I ran 130's, can't imagine you should need mains that large.

[MRieck]

That is to large IMO. Check for vacuum leaks at the carb manifolds. Are the manifolds original?

[mlinder]

130's were required for pods and slightly opened up 341's on my K0.

[tattdadd]

if your running 130's, and the bike still seems lean you might want to check your float level. 

[judder]

I sprayed some WD-40 on the intake manifolds and #1 is leaking so that is one of the problems I've got. The other rubber manifolds seem OK, at least the WD-40 didn't change the idle speed.

Yes, the rubber manifolds are stock so they are old. 

I've now got jets from 105 up to 130. I'm getting quite a collection. I may have to make a necklace out of them.

Thanks for your help.

Jud

[judder]

I just rebuilt the carbs a couple of months ago so the float should be good. I made sure to measure them accurately when I put them together. I have to check again when I put new rubber manifolds on.

Thanks

[vanillagorilla]

Hmmm Wonder which jets to run with velocity stacks and an open 4 into 1?

[mlinder]

Hmmm Wonder which jets to run with velocity stacks and an open 4 into 1?

Well, yer only 500 ft above sea level, looks like not terribly humid or hot, for  k4 with the stock carbs, my guess is 132.5.
There's not as much overlap on the cam, if I remember correctly, on the k4. Exhaust changes don't affect mix as much with less overlap.

[bucky katt]

brand new carb rubbers are fairly cheap and well worth the money spent. i think i paid 60 or so for all 4. new airbox rubbers are worth the time and effort as well.

[Terry in Australia]

Yeah, I agree Fuzzy, my "stock as a rock" K0 had K1 carbs when I bought it, so I bent over and grabbed my ankles, logged onto EBay and "won" a set of the correct cable carbs when all the other (sensible) bidders decided that "enough was enough" and stopped bidding against me.

They were in great condition, but were missing main jets, so considering that the book said to use 120's I fitted some, but it does run a tad rich, probably 110's or even 105's would have been closer to the mark. I'm running HM300 pipes, one of Hondaman's ignition amplifiers with D7 plugs and a freer flowing K&N filter, the bike starts easy and idles pretty smoothly (for a K0) and I've had it over 100 MPH, but the pipes are pretty sooty. When I pull the carbs off next, I'll jet them down a tad. Cheers, Terry.  

[vanillagorilla]

Hmmm Wonder which jets to run with velocity stacks and an open 4 into 1?

Well, yer only 500 ft above sea level, looks like not terribly humid or hot, for  k4 with the stock carbs, my guess is 132.5.
There's not as much overlap on the cam, if I remember correctly, on the k4. Exhaust changes don't affect mix as much with less overlap.

Oops... This is on my K8 with stock carbs. Also have an R1 cam(I think its the REAL one) I may throw in. Deciding if I should use it now, or wait for head work...  Thanks for the info mlinder!!!

[mlinder]

OH. i dunno, vanilla. Never worked on the k7/k8's. Little different animals. You'll have to do it the hard way. Buy the range 12.5 to 20 larger mains. Also, the k7/k8 carbs don't have 'rea' air screws, right? So you'll need to change idle jets too.
These are just guesses. I had to go from 97.5's to 120's on the kz650, but stock idle jets, though those carbs had real air screws.

[vanillagorilla]

I believe they have air screws... Thanks for the tip on the main sizes!

[55fbomb]

i run 130 mains in mine, stock motor, open 4-2 pipes and a breadbox air cleaner.

[vanillagorilla]

i run 130 mains in mine, stock motor, open 4-2 pipes and a breadbox air cleaner.

Cool, I think 130s will work well for my setup... Good enough anyway, at some point I would like to throw some CRs at her anyway!!!

[lunchandmurder]

i run 130 mains, with the same filters and same exhaust set up on a cb650 and it runs great

[754]

All you guys with 130s..is your exhaust tip black, or more grey than black...

 I am finding it hard to believe that they are all jetted correct, that would seem rich to me.. Over rich, may provide a tad more response, at certain throttle openings, but beyond that, it is a waste of fuel money, and you are probably giving up a few horsepower up top..

JMO...

[eurban]

The main jet has a good amount affect at mid throttle openings but for proper jetting should be evaluated at WOT.  I know this may be a touchy subject, but I would be surprised if some of those running jets in the 130 plus range are not over jetted and actually using the main jet to compensate for improper main jet needle position and profile or perhaps other issues with the carb's function or settings.  While it may "work" this way, performance may not be optimized.  Of course I could be wrong but my own personal experience is that less is often more with jet sizing.  . . .BTW, the 77/78 750 carbs have idle mixture screws (ims) and they control fuel rather than air.  The hot rodded 77/78s that I have messed with were all happy with the stock #35 pilots as long as they and their passages are completely clean.

[mlinder]

All you guys with 130s..is your exhaust tip black, or more grey than black...

 I am finding it hard to believe that they are all jetted correct, that would seem rich to me.. Over rich, may provide a tad more response, at certain throttle openings, but beyond that, it is a waste of fuel money, and you are probably giving up a few horsepower up top..

JMO...

Mine was a KO with pods and an opened up set of hm341's. I'm at a couple feet above sea level, and it's usually cold here.
Plugs were perfect. I usually was riding at 6 to 10k rpm. I've jetted enough bikes to know how to do it. 127.5's were lean. It's not like I started at 130's. Started at stock, 120's, and went up in every increment until ti was right.

[754]

I am not suggesting they are all wrong, but some may be..

 At 1100 ft with a header, pods or stock box, 811 or 836cc, and hotter cams , a few of us did not need 130S..

[kpier883]

I run 105 in my 74 750 with 4 into two aftermarket (no name) exhaust, stock everything else including air filter.  I do have Dyna ignition though.  Runs good and doesn't blacken up the plugs like it did with larger jets.  Gets over 40 mpg most of the time too.  I also went down some on the idle jet, but I can't recall right now what size is in there.  If I can find my notes I will post. 

[754]

I ran 120s with a baffled Hooker header , and pods on a K2 & K3 750..

 I do notice on here that a lot of folks get amazingly low gas mieage , compared to what we would get.. from 50 -60mpg to a US gallon..

[mlinder]

I am not suggesting they are all wrong, but some may be..

 At 1100 ft with a header, pods or stock box, 811 or 836cc, and hotter cams , a few of us did not need 130S..

It depends on the carbs.