Topic: My Bike V2.0

[Sandcast 2356]

Alrighty then, I have figured out what the deal is with my bike.... It runs on 3 cylinders at low RPM (low being under 5000). over that however, # 4 kicks in and the thing takes off for the split second at the tip top of every gear. Well, after replacing the points and careful modifications, the spark on all 4 cylinders is great. yet, #4 is still cold unless run and really really high rpm. So, I have concluded it is in the carbs. So, tonight I'm letting carb cleaner sit in the bowls until morning, then I'll see what happens. I must say, for running on 3 cylinders, the CB750 has a lot of power, almost does wheelies n stuff.

[Bob Wessner]

I think you are on the right track re: carbs, sounds like the idle circuit on #4. As I understand it, the 750's will actually "run" but not start on 2 cylinders, allbeit not so well. The theory being some redundancy in the engine design that would allow one to limp in off the road.

[Sandcast 2356]

yeah, I turned off the gas and ran the bike for a while and bowls 1,2, and 3 were equally low, and #4 was full. Therefore, carbs get gas, cylinder doesn't YES?

[Bob Wessner]

Sounds right.

[cben750f0]

a good carb clean sounds like the deal, you have a blocked air jet, and a bloked main by the sound.....you will be surprised what a good clean does.... good luck and keep us posted... peace

[Caferacernoc]

Your number 4 idle jet is blocked. Simple as that. If soaking doesn't work, pull it out and clean it with a skinnt guitar string. You have to be able to see through it or it can't pass any gas.

[CHUNG]

My DYNA III burned a channel out and I rode it home of 2 holes for 20 miles. Can't recall the speed but it was on the freeway. Unlike the 350-360. Bout 30mph max on one hole.

[Sandcast 2356]

welp, I finally got the gas back on the thing after having the fix a leaky fuel line connection.... AND!! IT RUNS BETTER THAN EVER!!!!! on 3 cylinders. All kinds of junk in the bottom of the other 3 carbs, but not a thing in #4 GRRRRRRRRR So, now I need to pull the carbs...again. Can someone elaborate on the proper procedure for cleaning them? Don't want to mess tis up. I believe the guy that I bought it from said they were '75 carbs.

[Sandcast 2356]

Work work work, whoops, got carb cleaner all over the keyboard 

[Sandcast 2356]

SOOOOOOOO, I FIXED IT!!! kinda. Idles on 2,3,and4 flawlessly, and found out it was only idleing on 2 and 3 before hand. Cleaned all kinds of good stuff out of the carbs through an array of methods, soaking, vibrating with 12" sub-woofer, and finally, cleaning jets out with a thread of copper out of a wire I found laying around. After tinkering, and poking and riding and riding, only #1 has problems idleing, (not sure why) but kicks in around 1000 RPM. So, I'm satisfied for tonight/morning/yeah I need some sleep. Only real problem is, now if I want to accelerate at a reasonable speed, I can't keep from burning rubber    No, really, I can't stop it, it's kinda scary, I'll havta get you guys a video. ok, well, Cheers ya'll (sorry, had to do it)

[cb750_chris]

Sounds like it's still the idle passage on that #1.  Are you using the Yamaha cleaner?  Make sure you bump the starter as per the instructions to get it up in the passages.  I'm sure after all these cleanings it will work its way out sooner than later. 

[Mark M]

I have a theory on cleaning carbs out.

You know how when you pull the carbs for the first time, it's a real pain in the @@@@.

As you do it a few times, it gets easier and easier until you can do it in your sleep, with nothing more than a broken biro in about 10 minuites flat.

Well thats how many times you need to take them off and clean them out!

[Bob Wessner]

Mark,

You're absolutely right. Last summer I had them off and back on, and I was in and out of mine so often I think I could do it in my sleep. Unfortunately, I guess the shame of it all is I had to do it so many times to in the first place. Oh, well, learning is fun.. most of the time.

[cben750f0]

i agree with the pulling carbs statement... i find i am doing them about once a week atm... still cant find the hesitation problem with mine....

[Sandcast 2356]

I have pulled mine lots too. Only problem with the diagnosis of the blocked idle passage on #1, is that up to this point (#4 idleing) #1 idled...............stumped

[Sandcast 2356]

Welp, brought back the old "My Bike" thread.  Given, this was a long time ago, so the stuff you read up to this point is old news. Now, I am a little more in tune to the inner working of a bike. So, after playing around with EVERYTHING, I have decided that my choke isn't working properly. On a cold start, I have to put it on full choke, and hold the throttle wide open in oder for it to fire, after about 7 short fires and 10 minutes, it will start and run fine on 3 cylinders. After 5-7 minutes, #4 (far right) will pick up and it will idle, but rough. Ride it for a while, and it runs and idles on all 4 cylinder great. Now, the way I got it this good, was trying my best to synch the carbs, (did bad job) and this improved it very much, but #4 wouldn't fire. I have always had problems with #s 1 and 4, these being the outside cylinders. As of right now, both of the cylinders, especially #4 are run much richer than the other 2. #4 is as rich as it will possibly get and still runs lean. I still has hesitation as well. I now have reason to believe it may have a gas-flow problem. I am led to believe this even more so, because someone cut the fuel line and stuck a fuel filter in it right after the petcock. THis also might make sense being this would make gas flow to #s 1 and 4 more difficult than to 2 and 3. And now that I think about it, I kinda adjust my carbs while paked on side stand seeing as my bike no longer has a wheel stand, therefore gravity wouldn't help # 4's chances any. As a side note, I would like to point out that I have had the bike idle on #s 1,2, and 3 cylinders, and 2,3, and 4 cylinders, but never all 4. Anyone have an opinion and diagnosis? Help would be greatly appreciated.

[kghost]

All my 750s feed the fuel to 1&2 and 3&4 from the petcock (K2).

As I read through the posts I get the feeling that people are not disassembling thier carbs all the way. Would it be a correct assumption?

What I do is Strip them all the way down. I'm talking choke w/ lever, Idle jet, Main jet, the works. The only thing left is the aluminum body.

 All the metal parts (Aluminum, Brass, etc) go into a 1 gallon bucket of carb cleaner. Not the spray kind I mean the kind that looks like a paint can and has a cool metal catch bucket to put the parts into the solution. Just don't put plastic into it.

After it has soaked for 30 -90 minutes take it out and blow air through all the passages of the body.
blow air or spray type carb cleaner through all the jets.

Pay attention to what we used to call the emusifier tube that goes with the main jet and idle jet. Plug any of those holes and improper fuel atomization will result.

Reassemble.

Remember that most carbs have three circuits: idle, Midrange, and full.
Idle - runs the idle and transition to the midrange. Idle passage and fuel level is important. The idle screw controls airflow to the Idle jet. Thus controlling Air/Fuel mixture.
Midrange - Controlled by needle position, venturi effect (airflow thru slides), fuel level, and emulsifier tube. Remember it gets its fuel through the main jet but in the mid range the main jet size is not restictive, needle height is. Air orifice must be clear or fuel will not atomize correctly and mixtures will be off. Also remember the balance tubes are there to let ambient or atmospheric pressure reach the fuel bowl. Plug it up and it will run wrong.
Full - Wide open slides. Main jet meters fuel. Too small a main jet and the engine runs better at slightly less then full throttle.

Just a little ramble hope it makes sense

[Sandcast 2356]

I could probably go a bit farther with the break down of the carbs, but I''m afraid to mess something up. Can I rule out a fuel flow problem??

[kghost]

Take the screw out of the bottom of the bowl. If gas comes out then its probably not a fuel flow problem. remove the hose at the filter outlet unless the filter is clogged or partially blocked it should pass fuel at a pretty good rate.

 It sounds to me as if all the passages in the idle jet or the emulsifier tube aren't clean or the float levels are off.

What I could do one day is dissassemble and clean a carb while taking about a 1000 pictures if that would help.

[kghost]

Here's a picture. Pay attention to the part labeled 19. Its under the main jet.

[cben750f0]

kghost, just wondering, you mentioned not to put plastic in the carb cleaner, well how does the nylon carb slide guild go in it, i was told if you stuff these through cleaning, then throw the carbs, they carnt be replaced... just a thought.. ben

[kghost]

Don't know which guide(?) you mean. 750K1, K2 is metal. As a rule I don't soak any composite material in the dip bucket.

If you can't disassemble it with out breaking it I wouldn't put it in there.

You would think there would be a way though.

[Sandcast 2356]

Say kg, the disassemble with 1000 pictures would be great, and I bet would make it to at least the FAQ. If time permits, and you are up to it, this would help tons. Only outdone by a video in a series of short clips .

[kghost]

Sandcast, I would be willing to do a carb off a 70's vintage 750 if it would help and the Board gods are alright with it.

Just noticed the pics above don't show real well. If you save them to your computer you can blow them up for better details.

[Sandcast 2356]

I can see um alright, just never been too good at the whole diagram thing, don't get me wrong, I use them and they help alot, just better at step by step stuff, with pictures, not a picture for evey step, but for the big stuff. Thanks a ton if you can do it, if not, it's alright, I'll figure it out.