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honda cb 750 k2 .......not too well

met2

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hi everyone.

i have had a 1972 honda cb 750 for just over a year now.
its a good looker and quite original in that it doesnt look as if its been tinkered much.
in the time that i have owned it, i havent ridden it too much but
it felt always aquite a strong bike, but always a tad difficult to start.
lately and as a consequence of this, i decided to get to the bottom
of it and get it looked at, as the starting problem was somewhat getting worse.
the mechanic who has a good reputation, has pulled the carbies and re seated them.
he also relpaced all the rubber tubes to and from the carbies and balanced them.
fuel tank was also checked for rust which there was non of.
new spark plugs and it ran like a dream for the first 15 minutes or so whilst on the expresway back home.
i could not believe how well it ran with plenty of power...plus.
almost like new!!!!!
after about 15 minutes, i heard a little backfire but i continued home.
i didnt think much of it and garaged it. the next day i went to start and it did but then it died. started again then died.
soon, i couldnt even start it.....i took it back to the mechanic,
he checked the compression which was good. he took out the new plugs that he installed previously and replaced them with new ones
as they were fouled up already.
it started first kick....i then decided to leave the bike with him and after a week or so he called me to tell me that it might need a top end
job, which would include valve stems, maybe rings and obviously a timing chain....
this came to me as a surprise because i rode the bike and it ran like new with plenty of power to boot. besides, i could never see any smoke
uphill, or downhill with this bike since the day i bought it....
has anyone gone through this experience or does anyone have any suggestions that might give me some direction....

cheers
met
 
What fouled the plugs? Was it oil fouled or fuel fouled? If it fouled from oil that quickly you would have seen blue smoke. If it is just really black from excess fuel, I would start to take a look at the ignition failing. If it runs good then gets bad when the engine warms up you need to start looking at ignition coils and condensor and/or point issues. Coils did go bad after many miles on the 750s and I have had running problems with a bad condensor. Stock Honda spec for compression is 150 to 170psi. A lot of things can cause backfiring too. Tight intake valves will backfire through carb. Incorrect ignition timing, wrong plug gap, incorrect point gap and dwell and weak or bad ignition coil will cause backfiring too.
 
Incorrect float heights can cause a lean or rich condition. Bad valve stem seals can cause oil to pass. Run your finger around inside the muffler, the residue should be black and dry if it feels oily then you are passing oil into the cylinders.
 
whats new with the continuing saga on my cb 750?

well....... the mechanic who has performed the initial carby re-seat, jet and clean,
and who has later suggested a top end rebuild because i complained of on-going starting and running problems after his work was done, has now decided to pull the carbies apart for the second time.

with this overhaul, he has now discovered that i might need new carbies....
he couldnt tell me exactly why but he will know more in a little while.

not much i can do for now but give him more time and just wait to see what he says.
(ohhhh i hate this)

anyway............stay tuned for more drama !!!!
 
Sounds to me like you need a new mechanic. These bikes are not all that complicated and the carbs are dead simple. Very seldom are the carbs to the point that they need to be replaced unless they are worn from dirt or corroded away from moisture. Sounds to me like he is looking for an easy way out of trying to fix the carbs or he broke something and is covering it up. Either way maybe you should consider looking into the problem yourself...you cant do any worse job then what has already been done. JMO
 
latest according to this mechanic of myn, is that he has tried working with these carbs and according to him everytime he screws something in, the body is so corroded that it just brakes and falls apart.

he says the only way out is to buy another 4 .......new or second hand.

i have no way of knowing whether he is telling the truth or not but he says that he will do the work from now on......for free.

i guess i have to look at it logically and think that it is a 40 old year old bike......and i cant prove otherwise, so i will have to surcumb to getting another set of carbs.

if anyone out there in this big wide world has a set of 4 to sell...............i will buy.
 
Tons of them on ebay if you are able to get them shipped to you. Couple of people sell rebuilt carb sets on there also.
 
thanks to both dirtdigger and cyclebuster.

yesterday i decided to visit my mechanic and finally see exactly what he is doing.
he didnt expect me which was what i wanted.

he showed me the row of 4 keihins in a stack which he had already taken of the bike.
he had already taken out their bowls and it was very clear that these carbs were corroding very badly.

he had at some point tried to tighten the screw and he had broken the shaft, but i am convinced this was due to corrosion and not negligence.....i just wish he had picked this up way before when he first put the needle and seats in, but anyway thats the latest.

i had asked him whether he thinks whether this was why the bike had not been starting due to fouled plugs, backfiring and flooding issues, etc. and he said maybe but regardless, this alll had to be done.

the photos show clearly the corrosion involved here.

so i guesss what i am looking for are 4 good keihin carb bodies, as i have pretty much all the parts and the kits for them.

if anyone has something and is looking to off load ...............flick me some pics on:

met9@optusnet.com.au

cheers
met





IMG_0235.jpgIMG_0234.jpgIMG_0237.jpg
 
I agree he should have said something the first time around. How can you not see the corrosion...he had the bowls off to put the need and seats in. I am not going to speculate any further though. Complete carbs are way cheap on ebay though. And plentiful.
 
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