• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Just bought "78 K 750 Runs ONLY with Choke full on

From whom? This forum is hosted by CB750.com. I am not aware of a parts supplier that uses the "CB750.com" domain.

Are the rubber bits made of Viton rubber?


Bought them from CB750SUPPLY.com........the link was posted earlier on this thread by DirtDigger. Here is the link I used............http://www.cb750supply.com/.........did not say the kind of rubber used.
 
Another carb/ fuel delivery question just cmae to mind.....

I have noticed a few hoses on my CB'78 that are emissions related, some of which are attached to a sort of circular thingie on the frame under the seat.
Are these hoses etc "necessary"? and can they be removed without any bad effects? ( other than possibly getting into trouble with the air quality police"
:)
 
should be able to eliminate it all, I dont run and emissions stuff, carbs just need vent hoses. I am trying to remember what was all on the later bikes as far as emissions but it has been forever since I have seen the stuff I dont remember. Maybe post pics of the sruff you want to eliminate and can tell you for sure.
 
The "circular thingie" under the battery box is a water vapor separator. The vent from the valve cover passes through this and then the oil vapor is drawn through the air cleaner. There would have been another hose from the bottom of the separator terminating near the rear wheel on the right side. Part of the routine maintenance tasks described in the manual involve removing this cap to drain any moisture that has collected. You can eliminate the water vapor separator, but you still have to vent the crankcase breather at the valve cover.

DO NOT CAP THIS VENT. It must be vented to the atmosphere somehow.
 
Thanks guys........will pst some pix just to be sure, and will not mess with any of it till I get toe old girl running reliably.
 
Here's a pix of the "round thing"....it has a hose going to it from the engine and another to the airbox.

I think it is what you kind folks say can be eliminated......BUT must keep the breather hose, got it! thanks

round thing.jpground thing 2.jpgeng hose.jpg
 
OK Gang......here's the latest on the choke/carb issues of my 78 750.......

I cleaned all the carbs ( best as I could without removing them from the "rack"
I installed all new gaskets and float needles from CB750supply.com. Hopefully made out of the right kind of rubber.....

The good news:
1) The carb that was leaking fuel no longer does so
2) When warm the bike will idle at aobut 1500 - 2000 RPM.


The bad........
1) The left 2 cylinders ( # 1 & 2) are much cooler than the right two (# 3 & 4) ........yet they are getting spark and there is good compression.
2) Plug on # 1 cylinder seems wet........not sure about #2
3) Plugs on # 3 & 4 look like they are running hot
4) If I roll on the power S-L-O-W-L-Y.........she will rev nicely, except for hanging up a bit at 3K RPM
5) I thought these problems might be related to the mixture control screws??..........I have no idea what the "base line" should be for these screws.
All the info I find is for carbs with the "round top"........mine ( 1978) do not have the round top........
Any suggestions greatly appreciated........( yeah I know I have to balance the carbs, for now I'm just trying to get it to run ......OR MUST I balance the carbs in order to get it to run??
 
5) I thought these problems might be related to the mixture control screws??..........I have no idea what the "base line" should be for these screws.
All the info I find is for carbs with the "round top"........mine ( 1978) do not have the round top........
Any suggestions greatly appreciated

I would suggest you get a proper manual. The proper manual would tell you what to set the mixture screws to. http://manuals.sohc4.net/cb750k/

My second suggestion would be to consider seeking professional help: http://harisuluv1.imgur.com/

Carbs suck. You either get them right or you don't. If you can't get them right then they are the most frustrating parts on the entire bike. I hate them. I've gotten pretty good at them, but I still hate them. I once did an engine swap just because I was in denial and did not want to rebuild the carbs again. Well, the replacement engine (with the same carbs) ran exactly the same as the motor I took out. At that point I rebuilt the carbs again and eventually got them right.

I have dozens of carb rebuilds under my belt, have studied numerous publications regarding the operational theory of carburation, and can usually get them right the first time now. I tear them down when I have to, but I still hate them.

Did I mention I hate rebuilding carbs? :wink2:
 
madmtnmotors.......thank yo so much for your response. I am getting a bit frustrated..........actually been wondering if cylinders 1 & 2 are firing at all, perhaps not even carb related.
After a 5 minute warm up they are a good 100 degrees lower in temp than 3 & 4 ( which are at about 220 degrees just above the header pipes)..........wondering if it is a poor spark issue.

Also wondering if it is the accelerator pump that needs replacing.....as it is supposed to kick in when the throttle is turned quickly. PS: I just ordered that part.

I do appreciate the links.........will study the manual further and SERIOUSLY consider professional help ( psychiatric might be the answer!)..........thanks again.

PS: I was just wondering, do you HATE rebuilding carbs???
 
actually been wondering if cylinders 1 & 2 are firing at all, perhaps not even carb related.

It's easy enough to check for spark. Might be a good idea to do that before you tear back into the carbs.


PS: I was just wondering, do you HATE rebuilding carbs???

Because they require such an incredible attention to detail, small parts that are easily lost or broken, small passages that are easily plugged, you can do everything "right" and they still leak or run lean! :mad:
 
take some carb cleaner or something to spray a little gas and spray it into 1 and 2 and see if it picks up any, if it does then they are lean, if it doesnt then it is spark or a mechanical issue.
 
I echo the carb subject matter. It just so divides the guys doing it into those that do and those that try forever but don't. I personally have no trouble doing them all day long and the more complicated the less I get bored by it, but most cannot do that.

I personally enjoy working on them and even more if I've never seen the type before. According to most that would mean I'm crazy......................and I really can't argue with that.

Ethanol effect on rust in the tank often has one thinking the carb work is flawed when it's just that superfine rust that ethanol leads to. That stuff gets through the best filters in the world and microscopic particles of it can drive you nuts with fuel level issues. I learned in the late '80s when it first started being used to bypass OEM filters on cars and go to the biggest inline ones I could find or even two of them, that is problematic on a bike with tight room.

On my 550 I went to a one-off setup that had two tightly coupled filters to stop ethanol rust issues from flooding out all the time but it lead to another issue that plagues these in things like that, the fuel passages are not big enough. The 550 ran fine until you let it run out of gas, but then the filter restriction prevented fuel weight from filling the carbs back up. It might take 5 minutes before the engine would crank and reliably run again. Even worse on these 750s which use the same puny '60s passage size first used on S-90 engines, and why Honda around '81 or so whoppingly increased the size of the fuel outlets in petcocks. The 900 and larger engines were having fuel supply fits. The later vacuum valve setup increased the line length to make more troubles.

If a silt filter in petcock bowl I dump the long tube filter that surrounds the tank standpipe (the holes are way too big to stop that rust ever anyway) and go to a finer non-stock filter somewhere after the petcock. Tight space is the word there but it works to stop a lot of fuel flooding issues.
 
If the plug on #1 is wet then spraying fuel into the intake of it will not help. Wet plug will not fire from no spark or too rich already.
 
I gave up trying to clean the carbs...I decided it is above my pay grade ( and skills level ) :)
Sent them to a "PRO" here on Long Island. I think its $200 well spent, assuming it fixes the issues for which I started this thread. I will know in a few days when I put them back on the bike.
 

Attachments

  • IMAG0247.jpg
    IMAG0247.jpg
    205.8 KB · Views: 148
  • IMAG0248.jpg
    IMAG0248.jpg
    193.7 KB · Views: 141
  • IMAG0249.jpg
    IMAG0249.jpg
    198.4 KB · Views: 165
Last edited:
Bummer...........Got the carbs back from the shop, bad news, some of it rather odd.

First, now ALL FOUR carb over flow pipes are spilling fuel....a LOT of fuel.

More interesting however is this. Before the "mechanic" cleaned and adjusted the carbs cylinders # 1 & 2 were cold, presumably from clogged idle circuit. ( they had spark) but # 3 & 4 were OK ( #4 running a little hotter and #3 )

NOW however.........1 & 2 are are running OK, BUT numbers 3 & 4 are cold ( but betting spark) Any thoughts as how to proceed? I have contacted the "mechanic" but have not heard back. $200 bucks down the tube!~ :banghead:


EPILOGUE TO THE CARB STORY.......

It turns out the solution was a VERY SIMPLE!..........can ya figure it out with out reading the next comment.....:wink2:

I was taking off the carb bank to bring back to the mechanic......
I went to loosen the float bowl drains .......
All four of them were open.........closed the screws, no more leaks....
AND all four cylinder firing, AND she seems to be running well........

I feel much better........
 
Last edited:
I didn't wanted to start a new thread for this, but any of you guys are aware of a carb rebuild manual for SOHC similar to the excellent one SeanG did for the DOHC bikes?
 
Back
Top