1980 cb750k full throttle problems

Kolasanick

Kolasanick

New Member
Ok so I have a 1980 cb750k. I recently turned it into a cafe racer. Eliminated the air box and rejetted the carbs. I run a 4-1 header with cycle x race slip on. I used velocity stacks for intakes. I jetted the carbs with cylinders 1,4 at a 127.5 main jet and cylinders 2,3 with a 125 main jet. All pilot jets are 80's.

My question is when I hit about 6000 rpm's my bike seems to stumble acting like its missing and or not enough fuel. It's not either one of those problems and I'm coming here for advice.

Let me know what you guys think.
 
http://www.dotheton.com/forum/index.php?topic=45863.0

and once again
That's got CV's? Take a #2 phillips screwdriver, heat the tip with a torch until cherry red and plunge it into your eye. It will hurt less. Really. Bikes with CVs have airboxes. Bikes with slide needle carbs have pods. Even Mark Dobeck, former owner of Dynajet, cops to a loss of midrange using his kits.

And for the other eye heat a 3/4" drill bit chucked up in an air drill ...

I can get BS34's to almost work on the 998-1100 with one serious caveat . They will pull hard with the throttle arbitrarily pinned OR they will mix fuel cleanly at part throttle cruise . Pick one I don't know how to give you both . Mark Dobeck and I went back and forth for most of a summer getting several of my customers bikes and my 998 urban terror two wheeler running right . The customers are probably still ****ed even though I neither recommended nor supplied this serious impediment to performance . In the end I knew neither was right (thanx for the flash runs Ron !) But the one that was the best and closest to right got me the most grief . That guy thinks I'm an idiot and in ten years hasn't wasted an opportunity to say so . Funny that the last two guys to touch it asked how I was able to get it so close . The one that isn't so good , that guy thinks I'm magic . The preceding applies even more so to your stock Honda carburettors and individual filters . CV carburettors require an airbox .Anything less falls short of a compromise in a horrible , frustrating and purposefully ignorant manner .

just a partial list

various attempts at
springs
needles
needle jets with and without discharge shields
pilots
mains
main air bleed
air horns within the air filters
closer to or further away from the head
adjustable cam sprockets
different headers ....

All because in one case the owner wanted to look like all the other cool kids
and the other thought it was a pain to get the carbs in and out with the stock aribox

coincidently both of these mental midgets went from 35-36 mpg to 30-31mpg and might have gained a couple of hp on top at the expense of having me spend untold hours filling a huge hole around 4k .

The stock needles are fine for all but full on racing . Why people insist on a "jet kit" is beyond me . I had many conversations over a period of several years with Mark Dobeck about individual filters on constant velocity , diaphragm carbs . Here are the high an low points of that discussion .
Great full throttle response , great part throttle cruise , pick one . You just don't get both .
The larger the volume within the individual filter the better your result .
Finding a velocity stack that will fit over the carb and inside the filter is never a bad thing until its opening is shrouded or interfered with by the filter .
Most needles supplied with jet kits are aluminum for ease of production and not steel for durability .
Find the main air bleed/jet on the inlet to the carb . This will be a fixed brass jet that unmistakably leads to the main down well that the emulsion tube , main jet , and main discharge to the throttle bore reside in . Remember this is a fixed jet and difficult (not impossible) to change . Small diameter changes in this jet can cover the usual giant hole between idle/transfer and the beginning of the midrange .

Last but not least are the ignitors , a well known problem whatever Honda they are used on . Their failure mode spans doa to intermittent as well as high rpm sign off . This last easily confused with coil degradation .

Your stock Honda carburettors REQUIRE an air box .

~kop
 
Kop is correct. Those carbs are notoriously difficult to convert to run without a filter and air box. Kaz Yoshima from Ontario Motor works kept the stock airbox until he swapped the carbs for a set of CRs.

Leon Moss used a back bleed tube to change pressures inside the float bowl on the first 900 he played with.

HRC on the F1 race bikes used to lift the slides more than stock and blanked off the primary main jets.

In other words, those carbs had the experts losing their hair. That's why a certain Honda 4 forum will no longer entertain the missing airbox how do I jet it postings any more. The solution over there is CRs or BS34SS from a GSXR.

According to the FSM, main jets are 68 Primary and 102 Secondary.

Does the bike go through that 6k flat spot or does it stay bad all the way up to 9500? Does it happen at all throttle openings?
 
I have the keihin 54mm carbs, idk if those are the cV's you guys are talking about.

It's run strong all the way up to 6000 and then just stops right there. It doesn't happening In all throttle openings just full, but it does happen in every gear.

The sad thing is the bitch ran great for like a month. Then I rode in the rain one time and she never ran the same since.

One guy I was talking to said it could have something to do with my ignition. Another forum said alternator. I'd just like to find the problem so I'm not dancing around my bike with money.
 
Talk to me about timing of events. Did it run OK with the new jets before the rain or did it stall in the rain and you jetted it while you were cleaning it up?

How did you come up with that 54mm number? Carbs on a CB750 are 30mm CV. :)
 
Kolasanick said:
It ran great with the new jets before the rain.
Click to expand...

In which case it seems to be water related and that sounds odd. If it revs beyond 6000 on part throttle but not on full throttle, it's most likely to be the main jets rather than ignition. Or it could be something in the main air jet s that's causing the problem. Did you clean the pods and dry them out and clean out the carbs after the deluge?

It's possible you got water in there and water bubbles will typically not flow through a jet.

I'd pull the carbs and clean them and inspect every passage. I use a can of spray carb cleaner to spray through one drilling on one carb. Repeat for the same drilling on the next carb etc That way you can compare flow across all 4 - one circuit at a time.
 
Stacks on the street with no rear fender or air filter isn't too effective for long engine life :-(

Pull the carbs and see if that fixes things
 
Fender - Good. Screens - not so much. I have a set like that for the strip.

Let us know how you get of with the carb cleran
 
Not likely but those are pretty large jets. I think you said it ran fine with those jets and you got the bike wet and it's been problematic ever since. What else changed?

Did you flush all the drillings through with a can of carb cleaner and were all four more or less identical or did you just remove jets and clean them? It takes me hours to strip and clean a rack of carbs.
 
Yeah I took every jet out blew it with air, and cleaned every passage way with carb cleaner, double checked my air mixture screw and cleaned the float needle and passage way.

Flipped them over and cleaned the slides and everything up top.

I found that when you touch my plug wires you can get them to arc from your hand to the motor. So I tried gapping them all again and go the same thing. Cleaned up the ground wire on the battery and on the coils.

Still getting the same results, I even tried a whole different set of coils and wires off another cb750 of the same year and got the same thing.

Double checked my timing and still same result.

So as for now I'm burnt out of ideas.
 
Well, if anything, I just learned a crap ton about the carb work I'll need to do on my bike when I get to that point. Very informative everyone!

I had a couple questions for you Kolasnick. Do you have any pics of your ride anywhere I can take a look? Also, that slip on exhaust, is that on the stock 4 into 1 that comes with some of the CB750's? My basket case came with a 4 into 1 header and I'm looking for a muffler to complete it. I'd be interested in hearing how it sounds too.
 
MAC or Kerker are your best bet. You need one with a "step" in the pipe between the actual muffler part and the end that mates to the header. Otherwise, you'll be scraping your muffler every time you turn right, trust me. As far as sound, crank your engine with no muffler, now imagine it without the raspy sound. That's what a 750/4 sounds like with a muffler :D But seriously, it really does sound a lot like an open header, just without the rasp and "pinging", maybe about 10dB quieter.
 
Ill try and upload some pictures soon. I had bought a full exhaust kit from cycle x. But since you already have the 4-1 headers then you can buy just the slip on from cycle x also. Just made sure the end of the headers is 2.5"

Check out steel bent customs they use the same slip-on on some of there bikes and they have videos so you can hear how it sounds.


Good luck man
 
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