Hitchcocks Forum

Hi all


 350 4 speed bullet 2007


I would appreciate some advise on a blown head gasket.  I pretty sure this is the problem.  Over the the last couple of weeks the engine noise has changed and become a little noisier.  On the way to work this morning audible pop heard and power was immediatly lost, however the engine did remain running, I limped the bike home.  After I got home from work did a bit more poking around and found when the kick start is depressed by hand, air was escaping from the front of the cylnder head.


Now I have read the service manual by Pete Snidel and especially top end overhaul. 


My question is do I need to take the rocker pillar blocks out and remove rockers and push rods prior to removing the head, or can I take it off as a whole assembly.


I also twisted the rocker oil feed pipes slightly when removing them - would this reduce the possible oil flow up to the rockers, I can blow down then no problem.


I will also be carrying out the dynamic timing checks following the rebuild as descibed in the manual because I am unsure as to why the head gasket may have blown.  There doesnt seem to be evidence of over heating on the exhaust pipe.


As always your assistance is appreciated.


Cheers Nick

Hi Navylark ----- did you do any time in the "mob"??   Does ROMFT have deep meaning for you ??  Now, your bike---before anything else do check that the de-compressor hasn't failed or is partially open. Remember that it has an auxilliary outlet into the main exhaust pipe (front f the cyl head ? ) and can sound quite alarming if operated when the engine is running normally. Make sure that the adjuster screw/lock-nut is not too tight and there is plenty of slack at the handlebar lever---typically about 45 degrees of free movement.  Give that a go first and tell us what you've found, OK?

Nick, I had similar symptoms with my 350 last year - turned out to be a burnt exhaust valve. If it's your head gasket I would have expected telltale signs of blowing (oil/carbon on fins) as you've managed to keep it running to get home. Only removing the head and examining everything will give you the correct diagnosis - yes,remove the rockers etc. before removing the head. Check the rocker feed pipes for splits now that you've twisted them - you won't have blocked them but they might leak when you refit them as they are fragile!

I would take the rockers off, you will want to check the condition of the bearing surfaces anyway while the head is off.


Twisting the oil pipe a bit should be OK. It won't have blocked unless you have actually kinked it. Just check for leaks when you get it running again.


These symptoms could just as easily be a faulty decomp valve or exhaust valve. A blowing head gasket (unless really bad) doesn't effect the running that much, it's just messy.


If it is the gasket, make sure you clean up the surfaces really well and rub the alloy head down flat on a bit of board (or plate glass if you have it) covered with wet & dry.


Mr H's composite gaskets are pretty good. 

Hello again--- Pwayman, yes I had my Classic 500 head off last year and fitted one of our hosts "oversize" exhaust valves having just caught it before failing. Do they do the same for the 350 ?? I once had a split exhaust on a BSA C15---- looked like a peeled bananna !!! Yesterday I went for an evening ride with the "lads"---youngest is 55---- coming off the Shropshire hills looking West into the setting sun...GLORIOUS !! Come-on Navylark, let's get you sorted and back on the Queens Highway---Heavy metal thunder-----searching for adventure-----then home in time for tea ??  Or as they say in Sweden --Bjorn 2 be vild!!!  HO-HO !  

Guys


 Thanks for the help so far, haven't had time yet to get the head off yet.  Might do a bit later.


Alan - yes time in the mob 26 years and still counting, I think ROMFT was a bit before my time.


 CHeers Nick

hi navylark ,i aggree with peter,the decomp valve can sometimes cause a chuffing noise but the engine will still run i had this problem once due toa trapped decomp cable opperating the valve in left hand lock,all the best paul.

I have just taken the head off and found it pretty simple considering it was the first time I have done it.  The rocker bearing look fine with signs of oil reaching them - no problem


Now having split the head I was surprised to find not too much damage to the head gasket,  However I think there is sufficient evidence to point to this:


1  Evidence of hot gases on the head bolt removed from the position nearest to the air leak founf during compression stroke.


2.  gasket split around this same bolt.


3.  Soot marks on the fins where the air leak was felt, which nearside 10 o clock position when looking down at the barrel.


If, like paul said, it was the decompression valve leaking would you be able to feel the air escaping on the otherside of the head at the above 10 o clock position.


Next point what is the best way and with what to clean up head , valves etc.


All the best Nick

OK, looks like it was the gasket.


You need to get the valves out so a valve spring compressor is necessary. With luck you should be able to grind these in without having to have the seats re-cut or replaced. You need to grind until all pitting is removed.

Check the play in the guides as well, this should be minimal.

Don't forget to grind in the decomp. valve as well while you have the paste out.


You can be quite agressive when cleaning the top of the barrel as it's cast iron and you are unlikely to damage it. It helps to take the studs out so you can really clean all the surface.


The head is alloy and a lot softer. I'd get some MDF board and stick some wet and dry on to it. This will give you a flat surface to rub the surface clean and flat. If you have a bit of plate glass this is even better.


Clean the combustion chamber and the top of the piston with a soft scraper. Scotchbrite pads are useful here (green scratchy)


Check the bore for wear and damage. There will be a slight lip at the top at the limit of the ring's movement. This is not a problem if it's not too big. Any significant damage to the bore will mean a rebore and new piston. (Actually it might be cheaper to replace the barrel as they are relatively cheap compared to a re-bore)


When rebuilding it's best to replace (or fit) the valve stem oil seals, new collets, better head washers, composite head gasket, oil feed banjo washers (Dowty type) All available for our hosts.


With the modern composite gaskets you don't need a sealant but you can use a smear of Weseal if you like.


THIS IS IMPORTANT

You need a torque wrench to tighten the head bolts (24ft.lbs.) Don't try to do without as you could easily strip the threads in the crankcase. The torque figure is for dry threads - if there is any oil on them the figure will be a lot less and difficult to determine. Tighten them a little at a time until you get them all to 24 ftlbs.


You are supposed to re-torque after running for a few miles but the composite gaskets don't really need it.

If its the head gasket why remove the valves? I'd pour some meths down the ports to see if they are leaking and if not put it back togethr with a new gasket and tighten it up. A torque wrench is good but I generally just use a ring spanner and do it up tight. Eveness is more important than exact poundage I believe but I generally take care to tighten up in an appropriate order and in four or five steps so as to pull it down nice and evenly. The decompressor vents into the exhaust port through a hole just below the valve seat so if you fill the exhaust port up with meths or some other low surface tension fluid you should soon see if its leaking.