Question: 1980 CB750F has carbs from a 900 - yes or no?

[spartan8ball]

The bike runs pretty well as is.  Something definitely needs adjusted, because the bike will stall when the clutch is pulled and the engine returns to idle speed.  It also lacks the "ball-kicking" power that everyone tells me it is supposed to have.

so here's my question.  My bike has had the carbs replaced with carbs from a 900.  I also have the original 750 carbs as well for spares.  the bike will not even start with the original carbs (they need rebuilt) but it runs good with the 900 ones on.  I noticed the fuel rail is bigger on the 900 carbs, other than that, seems to be the same size (although I am sure the needles, etc are different sizes.)

So... Should I rebuild the original carbs?  Or take it to a bike shop as is and let them adjust the 900 carbs? (synch, adjust idle speed)  Will running it with 900 carbs foul/burn the plugs/pistons/rings becuase of too much fuel?

and if rebuilding the original carbs is the best option, where is a good place to get the rebuild set for all four for cheap?  

[gonebiking-2]

By hell you pose a tricky question. Its like how long is a piece of string. Remember that the CB900 is 150cc bigger in capacity and the gear ratios are different from the 750. So when  modifying an engine that wasnt designed ( technically speaking ) to take bigger carbs, other factors have to be taken into consideration, such as valve timing, compression ratios exhaust valve timing and many other items.The 900 has bigger inlet ports and inlet/exhaust valves. For what its worth, I would personally rebuild each of the four carbs off the 750 taking your time and make sure everything is 100% spot on. Then  put them back and I see no reason why it wont run properly. One thing that you may have overloked is the number of teeth on the front and rear sprockets. There seems to be an anomally with the CB750s. In the workshop manual for my 1981 CB750K it says the rear sprocket has 38 teeth and the front has 15. Problem is when I bought my bike it had 44 teeth on the rear sprocket and 15 on the front. I replaced the chain and sprockets with a 15 front and 40 rear. Made a hell of a difference in that the RPM dropped by a 1000 at 70 MPH from 4000 to just under 3000, fuel consumtion improved , top end performance was better, but bottom end required swopping cogs frequently to keep up the power up in the 3000+ area. So if I were to replace the chain and sprocket with a 15/38, I`ll have no bottom end torque at all and starting on hills will become a problem. Suggest you get someone who knows about Honda carbs and get the 750 carbs stripped and overhauled. Also check that the air filter element isnt clogged.

If you get the 900 carbs re jetted for the 750 motor, there shouldnt be any problems with burning out valves, rings, pistons etc. Its only when they are set too lean that everything runs too hot and eventually engine failure happens. Hope that is of some help and somewher to start.

[spartan8ball]

Thanks for the reply.  I have decided to clean the original 750 carbs and try them.

Dunno who had these before, but they had two of the float bowls in the wrong place, one had no gasket, and the other had two gaskets.  I have since taken them apart according to the clymer's manual and cleaned/reassembled everything.  Hopefully the bike will run better once there on.

I'm going to start a new thread to keep you all updated with the history of my bike and pics and such.  Thank for the help thus far!

[gonebiking-2]

Thanks for keeping me updated. I`m always interested to see how people are going with their Hondas. But nothing surprises me when it comes to carb problems. You get folks who are little bit mechanicly minded and they think they know everything. The problem with the floats being back to front is nothing new ! What happens is owners take the carbs off , turn them upside down to work on, remove the floats , and because they have forgotten that the floats are upside down they put them in back to front , and then wonder why the hell the bike wont run properly. Make sure you follow the instructions word for word in the manual, check that the butterflies are all opening at the same time, connect four vacuum gauages to the carbs, also make sure that the idle mixture screws are turned all the way in and then back them off one and half turns.  Start the bike and get it up to normal operating temp. Then start to adjust the throttle settings making sure that all four readings on the vacuum gauge are exactly the same. ( Clymers should tell you what the vacuum readings should be) Once thats done, some very minor fine tuning  of the slow idle mixture screw will be required to finish off the tuning.  The idle speed should be around 1000 RPM +/- 50 RPM.  While I think about it, check the number of teeth on the front and rear sprocket. For reasons best known to Honda, the CB750s had a 15 / 44 tooth set up. They went like hell and revved al the way to the red line on the open road. But I changed my rear sprocket chain and front sprocket when they wore out to a 15/40 set up. Big improvement all round. Any thing smaller then a 38 tooth rear sprocket, and she problably wont get off the the start line. But then Honda in their infinite wisdom may have altered the gearbox ratios for the USA market where as in Australia and South Africa  they all had the 15/44 set up. Weird !!!

[spartan8ball]

Okay, update time.  It's taken me a little over a week (about a half our a day) to get the original 750 carbs cleaned out.  Everything looked pretty good, with the exception of one of the idle needles (cant quite remember the proper name, but its the one that you turn until it lightly seats, then back out 1 1/2 turns) is slightly nicked up, like its been graduated.  I did notice a few other problems with them.

1:  The acceleration pump is not functioning.  on the 900 carbs, when you move the throttle by hand (i mean, the linkage with the carbs taken off the bike) you can see a little bit of fuel being squirted out of little brass nozzles near where the choke plates are.  Not so on the 750 carbs.  The pump "plunger" is being moved by the throttle in the same manner as on the 950 carbs, and I tried spraying carb cleaner/compressed air into the holes, and I was able to get carb cleaner to squirt out those nozzles, so I know they are not blocked/clogged.  There is a little rubber "boot" that covers the long metal bar on the plunger that is torn, maybe it needs that to vacuum to create suction?  The 900 carbs do not have this boot, but merely a little o-ring were the plunger passes through the carb body.

2:  Also, the choke plates do not close fully.  You can move them freely by hand, so they're not stuck, but if you move the cable linkage, it only closes about 75% of the way, im thinking maybe a weak spring, as I couldnt see any other way it is actuated?

3:  The needles in the 750 carbs are slightly loose, even when I ensured the needle set screw was fully tightened.  on the 900 carbs, once the needle set screw was tightened, they wouldn't budge.  So, either the 750 needles are worn out, or its just a design difference?

So, the bike is now carbless.  I'm pretty pissed, considering that the original 750 carbs are now clean as a whistle, and just as useless, as the bike wont even start with them on.  Some people on the cb1100f boards said that they use 900 carbs on their 750's all the time.  I was wondering if I could remove the jets from the 750 carbs, put them on the 900 carbs, if that would prevent them from running too lean?  I [i]reallllllyyyyy[/i] want to get this bike running right in the worst sort of way.  What do you guys think about it?

BTW:  Here is the beast in question:


Doesn't look like much, but it was FREE!!!

[gonebiking-2]

Ok, It seems that all that work was for nothing. Next question is - how  deep is your wallet and how much money are you prepared to get it running right ????? With the 900 carbs, my sugestion for what its worth ,is to reinstall them ( after giving them a good clean out ) and as it is a very time consuming exercise taking them on and off all the time, take it to an expert or preferably someone with a dyno and let them do the tuning to get the 900 carbs to perform in the same way as the 750 carbs would have done. To me that would be the easiest way out.  I would question - if possible from the previous owner -  why did they put the 900 carbs on ???? I ask that, because one wouldnt go to all that trouble if there wasnt a serious problem with the 750 carbs. Try and find out what went wrong with the 750 carbs that made the previous owner go to the 900 carbs. Get as much information if possible to make an informed decision as to where you go from here. Dont give up as there is a solutin to every problem, and by the time you have it all sorted out , you will be a genius on the carb situation with these bikes. If all else fails and the $$$$$$$$$ question is not a problem, there are aftermarket carbs that are better than the Honda carbs that come as complete set, and just slide straight on - seen them on one of the many websites I have regarding carbs for this make and model bike.

To date, and I think I put it down to sheer luck that I havent had any problems with the my CB750K carbs, other then a full clean and replace seals and gaskets - normal routine servicing, but I suspect and have done so for quite a while, that the accelorator pump is not working and hasnt done so for years. Not that it matters , as once its warmed up and I dont need to have the choke out, she then goes like a rocket. No flat spots or hesitation- just goes. Keep me posted as I am sure your feedback will be of benefit to all of us who have these great bikes