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Discussion Starter #1
...and now I have a nice loud mechanical knock somewhere. What usually goes on the 750s? Rod bearings, main bearings? It almost sounds like a very, very dead lifter, most of the noise comes from the upper end of the engine. It's almost too loud to be a lifter. I babied it for 5 miles back to my garage, and it didn't get any worse, so I am looking for opinions on what to do next. I have never had this or any other 750 engine apart before, so what usually lets go when something like this happens? I'm guessing a spun rod bearing.
Thanks in advance for any feedback. <img src=icon_smile_angry.gif border=0 align=middle>
 

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Yeah that's what I'd guess. Floated a valve and bent one. Might even be stuck in the guide and tapping on the piston.
JohnnyB
 

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Discussion Starter #4
Wow, didn't consider that. I have heard a bent, stuck valve on a older Ford f-350 diesel dump truck of all things. Timing chain slipped due to neglect, and after they replaced it (without checking anything else), it sounded like the end of the world. I could see how this could happen. I would much rather it be a valve, and only have to yank the head to fix it.
I will be pulling the head sometime this week, I will keep y'all posted.
THANKS!<img src=icon_smile_shy.gif border=0 align=middle>
 

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Check all your valve clearances...if one or two of them are very out of whack large...then you have a bent valve. The bend holds the valve off the seat so the clearance at the rocker will be way too large.
JohnnyB
 

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quote:
...and now I have a nice loud mechanical knock somewhere. What usually goes on the 750s? Rod bearings, main bearings? It almost sounds like a very, very dead lifter, most of the noise comes from the upper end of the engine. It's almost too loud to be a lifter. I babied it for 5 miles back to my garage, and it didn't get any worse, so I am looking for opinions on what to do next. I have never had this or any other 750 engine apart before, so what usually lets go when something like this happens? I'm guessing a spun rod bearing.
Thanks in advance for any feedback. <img src=icon_smile_angry.gif border=0 align=middle>
What cb750 do you have, what model and year? cb750 motor destruction is uncommon at 10K miles unless you were beating it like it owed you money or there was a problem to begin with. Most K bikes have a top end service life of 50K miles before you need to do a timing chain and possibly springs and cam - if you have been taking care of the bike. The 77-78 supersport has inlarged valves and has a top end service life of 30K before major failure will occur and then it is usually because the valvesprings are sagging (it has been known to happen earlier). The three top end things I always worry about with any cb750 engine is:

1) cam breaking in half - a stock cam is weak and if it walks (moves around unshimmed) or runs into a timing skip the cam will physically break in half. This is not common but it does happen more often than you would expect on a 60hp motor. This almost invaribly leads to a stuck valve and then a bent valve.

2) Timing chain adjuster failed - when the adjuster fails it is not hard for the chain to cause all sorts of problems in the top end (including breaking off a chunk of the block or causing the came to walk and break). If it skips then bent valve city

3) failed valvespring - stock components are crap, and the cb750 is no exception. Under normal use a valvespring sould lat you a long time...reving like it's apollo 11 on the moonshot will eventually cause them to wear down fast. Once they wear down then they don't get the valve back as fast as needed and the valave and the piston meet

BTW cb750 valves float at 500 rpm past the tach redline (I think it is 9000 rpm). Some K bikes are non interference motors, meaning you can float the valves and not worry about hitting a piston, some like the F bikes are interference motors and you must watch out for when you float the valves, or just stay away from the redline. BTW as a rule you cannot mix K and F heads.

remember when taking it apart you are not just looking for the problem you are looking for what caused the problem. cb750s have bad top end oiling so look for any scortching and such. Also check the engine's oil check valve - sometimes these stick and the oil tank empties into the crankcase, it is only harmful because it takes longer for the oil to get to the top end as the oil tank has to refil first (with the engine running the oil pump takes about 3 minutes to refil the tank)



Edited by - Geeto67 on Sep 11 2006 07:52:58 AM
 

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Discussion Starter #11
OK guys, I have found a good head with all valvetrain + cam that came from a 74 cb750K. I'm hoping to clean it up and swap it on. Just in case, can anyone suggest a good place to buy valve guides if I need to replace them? How about a better-than-stock clutch? I found a Barnett kevlar clutch kit on eBay, what do y'all think? Also looking for a rear sprocket thats 1-2 teeth larger than stock, all I can seem to find are stock replacements. A good jet kit as well, bike has header and pod filters right now, and could use more fuel. I will be posting pics of whatever I end up doing to keep everyone entertained.
Thanks! <img src=icon_smile_tongue.gif border=0 align=middle>

 

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quote:
OK guys, I have found a good head with all valvetrain + cam that came from a 74 cb750K. I'm hoping to clean it up and swap it on. Just in case, can anyone suggest a good place to buy valve guides if I need to replace them? How about a better-than-stock clutch? I found a Barnett kevlar clutch kit on eBay, what do y'all think? Also looking for a rear sprocket thats 1-2 teeth larger than stock, all I can seem to find are stock replacements. A good jet kit as well, bike has header and pod filters right now, and could use more fuel. I will be posting pics of whatever I end up doing to keep everyone entertained.
Thanks! <img src=icon_smile_tongue.gif border=0 align=middle>

sprocket specalists will hook you up with any size sprocket you need, even in aluminum and drilled for weight (totally worth it in looks alone if you ask me). Google them.

I have had barnett clutches in cb750s before, I happen to like them but I am also a drag racer at heart and ride like one. A lot of people I talk to find the whole barnett setup too heavy in the lever action for the street. The solution is to either replace the clutch lever with a modern adjustible one (check ebay for universial ones) or to run barnett springs with stock fiber plates. I have this setup in my daily rider and find it awsome for stop light to stop light use but find it makes an awful noise when cold and slips a little off the line when I am wanking on it.

Basic head rebuild parts should be available from honda. Cycle ex has a basic head work deal for $385 that I happen to think is a bargain:

http://www.cyclexchange.net/Engine Parts Page.htm

HOWN ABOVE IS ONE OF OUR STREET FIGHTER HEADS.

LABOR:
DISASSEMBLE HEAD
GLASS BEAD PORTS
PRESS OUT VALVE GUIDES
STREET PORT THE HEAD (DRAG RACERS WILL REQUIRE MORE WORK)
SURFACE BOTH SIDES OF THE HEAD
INSTALL NEW VALVE GUIDES
REAM GUIDES
GRIND VALVES (NEW OR USED)
MULTI ANGLE VALVE JOB
CLEAN AND REASSEMBLE

DEPENDING ON CARBS, PIPES ETC. PORTED HEADS CAN GIVE YOU UP TO 10% ADDITIONAL POWER. WE ALWAYS TRY TO HAVE HEADS IN STOCK ON A EXCHANGE BASIS. Labor Starting At $ 385.00 , Parts are Extra, Of Course


As long as you are doing the top end, spend the money for a cam (approx $200) they will even install it for you. I would definatly do their high perfromance valve springs and heavy duty chain ($60 for the springs, $40 for the chain). A new tensioner while you are in there is not a bad idea either ($60). But I could spend you money for you all day long.

check out cycle exchange's whole website tons of cb750 stuff, most of it chopper but a lot of it cool and can be used on a cafe bike.

http://www.cyclexchange.net/
 

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Discussion Starter #13
I'm going down to the garage tonight to pull the rocker cover, and then the head if I don't find the molested components. I will throw up some pics later.
One question, say I find a couple of bent valves, replace them or throw on a replacement head, do I absolutely have to get the head resurfaced before installation? I used to work in an automachnie shop for a couple of years, and I know how important it is with car + truck cylinder heads, but I don't know the specifics on a 750-4 head. Any advice welcome. I am only suggesting skipping the resurfacing because: the bike was not overheated, warping the head, this isn't a water-cooled bike, so there is no risk of oil/water passage leakage/seepage, and finally, both the head and the cylinder bank is aluminum. In car and truck engines with aluminum heads and a cast iron block, those two metals have different expansion/contraction rates from heat, therefore reinforcing the need for absolutey perfect mating surfaces to attain a good seal from the head gasket(s).

So yeah.
Should I have the head resurfaced if it comes off.
Sorry for the long, draw-outtedness.
 

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Discussion Starter #14
Well shit.
I didn't win the auction for the head.
kawakarate, wherever you are, you suck. I probably needed that head more than you did.
Oh well, there should be more to come.
 

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First off, I can't stress enough the importance of filling out your location in the bio. If you are close to one of us then maybe we can help you out. I know a shop local to me who has stockpiles of SOHC cb750 engines and head, whatever you need.

Second off, stop using the bike....just STOP!!!! if you know you have a major mechanical problem why make things worse? It is just setting yourself up for a trip to the junkyard or ebay with your beloved cb750. If you fuck something else up because you ran the engine when you should not have then you have no one to blame but yourself.

Ok, I think at this point it is safer for you to crack your bike's engine open (BTW you do know that on a SOHC you have to pull the engine to get to the top end...right?) and start looking. Who knows what other damage was caused by the 10,00 rpm moonshot or by subsequent running of the bike. Whatever broken bits you find, post pictures here as it helps us to help you.

Here is your next step: Pull the goddamn engine ut of the frame and put it on a bench. Take the valve cover off and spin the sucker over with the kick start - look for what is wrong or anything out of the ordinary. Get a manual and check all the top end tolerances and make sure it all checks out. Then if you need to go to the next level, pulling the head.

A loud mechanical knock can be anthing from rod bearings to main bearings to a cam breaking, too many thing to list. SOHC hondas don't have lifters BTW. Plus you could have cause the bearing to fail if whatever was causing problems for you in the firstplace could have droped metal into the crank case and ruined the crank for you.

pull her apart and let us know what you find.


edit: oh yeah and on decking the head - air cooled motors normally run hotter than water cooled motors, its one of the reasons air cooled bikes have looser tolerances overall. A warped head on a cb750 can cause loss in cylinder pressure and a horrendous oil leak which could fruther starve your top end. There is an easy way to tell if decking is needed, put a straight edge against the head and check for warpage.

Edited by - Geeto67 on Sep 17 2006 11:14:51 AM
 
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