Topic: Another jetting question...please help!!

[hevykevy420]

I'm trying to get the jetting sorted out on my '78 CB750K 836 rebuild but im having difficulty getting the jetting sorted out.  I've read the other posts, but thought somebody maybe could help?

The PO had 120 mains in the carbs so I left these in.  The only mods to the bike are the 836 kit, .370 lift cam and heavy duty springs in the head, and 4-1 exhaust.  The PO had pods on but I am trying to go back to the stock airbox set up.

With the 120's, the bike didn't feel like it could go faster than 60-65 mph tops.  It was just wheezing and felt weak after that.  I even tried the pods with no improvement.  I tried 130's, and with the lower half of the airbox removed and no filter the bike ran and felt better on the highway.  But, with the whole airbox in the bike, it hits a brick wall at 5,000 rpm around (65mph or so) when cracking the throttle.  what gives? the only other adjustment i made was increasing the gap between the plunger that shoots gas directly into the carb throats and the actuating arm on the carb.  Previously, there was no gap set at all by the PO, any throttle movement would push down on the plunger (but the PO also had the plunger spring in upside down, not sure what this was all about or if the assembly even worked at all).

It seems there are different philosphies with jetting bigger vs. smaller with larger displacement, but does anybody have similar mods and can tell me what main jet they used successfully with an airbox???

Also- what is the gap supposed to be for the plunger arm thing (not sure what its called, sorry)?  Could this be affecting the whole mixture???  Any advice is appreciated!!!

[jdpas29]

the plunger deal you're talking about i think is the accelerator pump.  this deally mostly affects your quick acceleration.  if you can't get above 65 mph at all you have a different problem i would think.  i would, however, find out what the stock setup is for the accelerator pump and set it to that.  i toiled over my 78 750K for 6 months and invented a lot of new curse words and drank numerous adult beverages in the process.  i will tell you that my bike never ran worth a crap until i returned everything to stock settings/parts.   

stock for that bike on the mains is a 110 jet.  i have 110s, and an airbox and a fairly unrestrictive set of 4-2 exhaust.  it sounds like you may be flooding the bike with the airbox on if it seems to run better with the pods.  make sure that your float height is correct and the same on all 4 carbs.  clean the crap out of your main jet and emulsion tubes.  set your breaker point gap and timing.  try it again, Sam!

the one think i know nothing at all about is the 836 mod and your lift cam.  i don't know how much that stuff affects the stock settings.

good luck.

[eldar]

kevy, these bike run very lean. thats they way they like it.
Here is a test. Get on an open road, now when at speed, pull the choke and try to whack the gas. If it sputters right away before you hit the gas, you are probably very rich. If it dies when you hit the gas, you are also rich then.

First, set your accel pump. It is mainly used off the line or off idle. The gap between the tab and rod is .008. Make sure the spring is between the cap and the diaphragm. 

Now set your fuel screws to about 1.75 turns out. Normally start is 1.5 but with the 836 I wuold say 1.75 but then I dont have the 836 yet.
Reinstall you 110 mains. Clean your plugs and kick it over. See what it does at this point. Run it for a bit and while on general indicators, do a couple of plug shops. Now I an using d7ea right now and standard is D8ea BUT my d7s are almost completely white and the bike still acts rich. If you ahve the D8s, your plugs should probably be only SLIGHTLY tan on the insulator, the outside ring of the plug will still be black.

Now try going on the road like this. See how you go and if you still have lag, then pull the choke a bit at higher rpm and see if it picks up. If it does, try it at lower rpm and see if it pulls harder there to.

If it pulls stronger once the choke is pulled in a bit, turn out the fuel screws to 2 turns. If it does pretty good, leave it till you can fine tune it with a shop tach.

If it pulls at higher rpm with some choke, them pull the tops off your carbs and pull out the needle. Get 4 washers that are the same thickness and put them between the slide body and the E-clip on the needle. this will raise the needle and make it slightly richer.

[hevykevy420]

thanks eldar and jdpas. i'll follow your advice.  i'll try going leaner, i think i have some 110's and 115's.  I'll try the 115's first and see what gives.  i'll double check the fuel screws too.

will post with results.

Thanks all!

[dusterdude]

damn eldar,go on with your bad self.

[eldar]

Hey evill is an artform!

[hevykevy420]

UPDATE-  I installed the 115 leaner main jets per your suggestions, put the whole darn airbox and carbs back together (wow!, the bowls don't leak this time ) and took if for a test drive.  (I also shortened the gap on the accellerator pump somewhat).

It ran MUCH MUCH better, i am really stoked now that it is running so much better on the highway with the airbox!! totally stoked, thanks ELDAR and jdpas!!    it was responsive down low, and the stumble at 5K when whacking the throttle at speed was almost all but eliminated.  After putting years into this frame off rebuild, it was a great feeling riding the bike and having it 90% or so "there".  I am thinking about going down to 110's to see if this is the perfect main jet with the airbox, 836CC kit, high lift cam, and 4-1 exhaust.

This brings me to a new question:  Does anybody run bigger main jets on the inside two cylinders compared to the outer (1&4) cylinders?  My brother has jetted some of his ninjas, and he said it was common to run bigger main jets on the inside cylinders.  Is this just a water cooled thing not applicable to air cooled inline 4's???  Anybody set up their main jets this way???

[Caferacernoc]

The reason some people run a leaner jet on the outside is because of the greater cooling on the outside two cylinders.

[eldar]

Newer bikes might benefit from this more but I dont know about ours. I dont know if new bikes use 4 carbs yet or if they use 2.
Something a person could try I suppose but it may make tuning somewhat goofy.

Now that you have dropped in smaller jets, make sure your low speed fuel mix is set, that could finish taking care of the stumble.

[hevykevy420]

eldar- well i dont' want to make my tuning any goofier than it already is!!  I'll let someone else venture into running different outer jets.

eldar- how do i set the airscrews/fuel mixture???  I don't have a shop tachometer, is there anyway else to do this without a shop tach?  I think i set them at the 1 1/2 or so setting a while back, but haven't touched them since.  I synchronized the carbs as best I could after the carb overhaul using a vacuum tool at this time too, but haven't messed with anything else except the main jets since.

I think i'm going to drop 110's in tonight to see if they're the best mains, maybe i should do the fuel mixture too if there's anyway to adjust without expensive tools!!

Thanks all!!

[eldar]

Well, some people claim to be able to do this without a tach. I say bull to that! You might be able to hear slight changes but that does not mean you are getting it properly in tune.  A while back, I was going to get a sunpro 2 tach. The one I wanted was about $30+- a few bucks, at my local autozone. It is accurate to within 30 rpm. Not too bad really and good enough for this job. Otherwise, if you still have points, it would be to your advantage to get a dwell/tach from sears for about $50. Even if you have electric ign, it would still be good as it is also a volt meter too.

You hook up whatever tach you can find and start adjusting the #2 carb first, it is your base carb.
This site has a shop manual you can download, get it if you can and follow the steps in it.

Nice thing about carbs is that they work on the vacuum created by your engine so you will be tuning based on what your engine is doing.

A lot of people rip on our year bikes and carbs, but you know, we really do have good machines that take well to tuning but they just need some different tricks to do it.

[dusterdude]

hey!!!,i resemble that remark.

[eldar]

I know you do, or would like to! See, I know you have K8 envy!