Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: davepainter on September 13, 2011, 09:32:44 PM
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Well found out what the fault with the float bowls was, a snapped camshaft-OUCH??
carefull buying on e-bay to be honest the guy i bought this off probably thought he had cleaned thoroughly just goes to show
basically something blocked the oil feed to cylinder 3+4 camshaft.
i,ve yet to inspect the head and VALVES scary.
within 2 hours i found a mint cam carriers and followers for my 76 k6 inc postage from good old USA for
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::) 4 times in 30 years, yes been there done that and had the t-shirt.I'm surprised you did not hear the camshaft seizing up, they make one hell of a squeal before they give up the ghost (a sound i will never forget or want to hear again) mine snapped as i tried to get home i was only about two miles away so limped home on two cylinders,the problem in my case was stripped threads on the cam cap bolts and a bit of crap welded its self too the camshaft, ,everything under the rocker box had worked loose from previous owners rebuild it had run for six years so cant be to harsh on him ,the head has since been heli-coiled so better than before , you will learn from this ,so don't feel to peeved,i think it has happened to quite a few of us .i would say most likely bent valves aswell in my case anyway.just make sure them oiljets are clear and be ultra clean on the rebuild i always spin my engines over on the bench with the battery connected to the starter motor and check for plenty of oil on the camshaft, before installing the engine,this used to be a bit of p*ss years ago, with plenty of old chops about for spares and gasket sets only costing twenty quid ,just swap heads or even engines and off you go not any more lol ;)
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Yes it does seem to be a common problem, kind of makes you feel paranoid for a while after a rebuild and riding for the first 500 miles or so
The problem after something like this happening is all the ground up bits of metal that have managed to circulate around the rest of the engine, i would only be happy to run it after a full strip and clean out of entire engine after that
Pete
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Steff, the odd thing was that the side of the cam with no oil supply was for cylinders 3+4, which were the only 2 running! The cam snapped leaving the part above cylinders 1+2 not turning, but the oil supply to them looked okay! ???
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Well spotted,its going to be interesting ? to see which valves if any have been affected
my geuss would be that up untill the exact moment the cam snapped 3+4 were firing to allow bike to run?
however ground zero day at 160 miles on clock 12-9-2011 cylinders 1+2 on inspection as suggested by yourself were found to have NO play in tappet arms,---
The last time i rode the bike with all 4 cylinders running was 29-8-2011.at 160 miles,as you say odd that the undamaged looking side 1+2 was the side i,ve had constant stop start, wont run then ok then not.
when i had the bike delivered with 8 miles on the clock,from new engine build and start, it did,nt run on 1+2 the FIRST time i started her i had to ring the guy i bought the bike from and asked him what main jets he was running, i ASSUMED it needed bigger jets i ordered and tried 120s then 110s before putting the 105s back in which were the ones in the bike when it was delivered!
So mr watson for some reason all the time (160) miles that the cam damaged side 3+4 was disintergrating and turning into molten metal,
the bike covered at least 140 good miles running on all 4.
the 5 or 6 times i went out and rode 20-30 miles at a time good --then next time i went to use her she WOUld,NT fire on 1+2
Like you say its strange all the damage was being done to 3+4 .
prediction time i reckon when head comes off i,ve bent valves on 1+2--caused at the exact moment the cam snapped!!!!
=
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Sadly at least 1 bent valve is a certainty :( maybe more as the cam may have still moved at odd times due to the fact of the way it broke, leaving a rough edge which could occasionally move the cam a fraction and let other valves hit the pistons :'( One thing I will suggest though, when you have rebuilt the engine, as it's in a chop frame, leave the rocker cover off and leave the spark plugs out and wind it over on the starter several times to make certain that oil is reaching the top end as it should, then fit the rocker cover and spark plugs! (something we have to do OUT of the frame on a standard bike :()
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Used to see this semi regularly in the past when a person not used to Honda's would re-build an engine and not be carefull enough to claen out bits of gasket or use excess silicone.
Also 20/50 is too thick in the UK
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(http://i32.photobucket.com/albums/d38/ceofuller/cb750/1jacket008.jpg)
Here is the cam and carrier-i hope it shows pic? i bought on e-bay from the states
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Love the way they get "Sandcast" into every CB750 listing...
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CB750K cam cap bolts do work loose and this has happened way too many times, even with stock engines. This problem becomes worse when running aftermarket valve springs and cams. One solution is a modified cam cover which adds extra support to the cam caps to prevent them from coming loose and dropping oil pressure. Old modification from the 1970
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Nasty, saw a lot of them back in the olden days, snapped cams seemed to be very common, the cam and holders from the states look like a bargain as the last time I looked just one new cam holder from CMS was
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Hi Den thanks message ref spares prices, on that note i,ve just bought a very clean f2 to use as an engine doner shame to split a nice bike BUT i will run f2 for a while set timing balance carbs etc check she runs well then at least i can USE chop. with what i sell rolling chassis f2 for it will work out much cheaper than a ground up engine rebuild.If so then i can also sell whats left of k6 engine along with as new cam and carriers.
i will post pictures of f2 when she arrives fingers crossed!
as uk pete noticed f1 f2 prices are rising they dont make them anymore? look on e-bay at moment mint 1978 f2 in black gorgeous.
best wishes dave
ps some help needed advice as to fitting f2 lump to k6 chokes different etc electrics?
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With the problems F2's have with valve guides i would stick with the engine rebuild mate
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Use the F2 carbs with it, electrics are the same for your needs, the main problem you will have is the exhaust on your chop (K6 engine) wont fit straight on (different at the exhaust port mounting)
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At least you'll have enough room to get the head off easily to fix the guides!
Thought the K6 exhaust stubs may fit to F2 head in place of standard F2 studs and header combination but you'd have to check it through for yourself.
I'd say the K1-6 and F1 motors are much better for cruising than F2 if that affects your decision.
F2 runs quite lean as standard and gets going a bit as you open throttles (additional fuel pumped as you turn the throttle) but you may not like that feel if you just want to mooch around.
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just heard f2 should arrive thurs 30-9- will post pics, i saw a post ref alternate silencers to 4-1 standard plenty of after market sorts ie dunstall replicas gold star etc, does any one know size where silencer clamps on? provided downpipes clear forward controls.
at least garage -workshop looks tidier prep bench space for engine strip -k6- want to find cause of oil block, still cant figure out why she was firing- plugs 3-4 side of blockage yet no spark 1+2 that had plenty of oil. are all the engine covers same k6-f2 swop shiny bits.
when eventually fit f2 most riding done meon valley hampshire well known sunday run road steady 70-80 mph roads i joked about wobbling around on rigid frame seriously its not that bad .
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I think the size is 50mm but i will have a quick measure for you later, the engine covers will all swap over
Bit of luck and you should be back on the road soon maybee before the winter ice
Pete
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55mm where the silencer clamps on
Pete
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Well here she is arrived today better than i expected imo the 1978 f2 (possible) doner engine for the k6 chop, she,s got mot till April 2012 and seller sent old mots from 1994 at 3621miles with other mots from 2003 till now 7 all showing mileage rising to 5404 she is tidy NOT immaculate but most impressive suprisingly with much relief the engine solid tight and supersport fast when they rev and they do i thought my old gs750 sorry was fast NOT compared with the f2 this is seriously quick off the shelf as it were! i dont think it needs a tune up-so this has confused me even more! (it does,nt take much).
with the condition she,s in the possible mileage and the way she runs and rides and what i paid for her, when i went out today for a spin portsdown hill micks burger van theres allways bikes there-sunny days- old bikes allways bring out the i used to have one of them crowd i expect everyone on this forum gets the same when your out on the bike, 1/2 the people were,nt even born when the f2 came out!
i need to have a serious THINK before i do anything in a hurry ref engine out! also i could never fully use the engines potential in the chop?
head still not of the k6 to find cause of oil block to cam.
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more pics f2
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The F2 in that knick is geting rarer and it WILL need new exhaust valve guides plus associated work NO MATTER how well it runs now
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It looks fairly original from the pics and seems to have most of the stuff people look for when doing a restore of this model.
I look at it this way, it's the absolute end of the line of the original four cylinder japanese/world superbike and will only get more rare. A big shame to split it up when there are so many more K2-K6 range around that nobody would really miss.
I think the value of these will head much more towards K0 values in the coming years and will really come into it's own as a very valid historic, and as you've found they are really nice to ride.
My vote would be to keep it original (you'd probably be able to sell at a profit in future) and blue was the nicest colour they came in as well.
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http://s839.photobucket.com/albums/zz320/davehawk/honda%20750%20f2/ (http://s839.photobucket.com/albums/zz320/davehawk/honda%20750%20f2/)
hope this link works to my photobucket site more pics f2 appreciate points using engine as doner, and it does seem like f1 f2 prices are rising i,d love to keep her as she is but to tax mot and insure 2 bikes is pricy. its a quick fix to getting chop on road with a solid engine plus someone has a good basis- rolling chassis to put back on road.
i,m going to start a new post-engine swop- as problems arise-advice hopefully straightforward.
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I think that straightforward will NOT be the word for it as F2's have differentt inlet rubbers and carbs and do not take to any modifications in the inlet tract well (EG pods) and are a real swine to tune as well plus they are a bit "Peaky" for a chop and will wear the valve guides out quickly necessitating a top end strip and rebuild, for which a lot of parts like piston and rings (unique to the F2) are very difficult to find.
Without being silly you would be far better passing on the complete bike and getting a K engine to put in there