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E: Engine Exhaust valve not right

evcomet

Well Known and Active Forum User
VOC Member
#1
Took an extended ride on the Comet today. All seemed to behave as one would expect.
I put the bike on the center stand put my gear away and got my Multimeter out to check battery charging.

Comet was off for 10 to15 minutes. When I attempted to kick the bike over, it behaved as if the compression release was being engaged.

I kicked a few more times without success. Got a cold drink, 15 minutes later the bike started immediately. But being a "the sky is falling" kind of guy, I
decide to put the bike on the lift and take a closer look.
I found the exhaust valve spring loose. I can grab the spring with my fingers and lift spring and valve 3/8".

Looking at the parts book, I am guessing that the valve guide has come adrift of its proper position.


Questions...

1) Is the easiest way to remove the head to remove the UFM?
2) Do you replace with an oversized guide?

Any advice would be appreciated.

Jerry
 

timetraveller

Well Known and Active Forum User
VOC Member
#4
Not realistically. The design of the Vincent is so clever that removing the front end is trivial apart from the fuel tank, which is a pain in the posterior. You need to support the engine/gearbox very well and then just get stuck into it.
 

vibrac

Well Known and Active Forum User
VOC Member
#5
I just had to do my comet head as a quick and dirty task two points:

!uckily I put extra length in my wireing to the headlamp I take out the front wheel (might as well check it) and the front end sits on a mat upright at the front on rear of mudguard and fork end the ufm does not unbalance it (at least mine was stable)
Be ready with a funnel and tin when taking off the rear oil supply at the back unless you know the stop valve works
 

greg brillus

Well Known and Active Forum User
VOC Member
#6
The real issue is that if the exhaust ones have broken then the inlet ones could let go at some point. The springs could be changed without head removal but it would be quite difficult. If the springs are failing then removal and inspection of the head/barrel and piston would be very wise. They are not difficult to carry this work out on, but there are a few important things that need to be brought to your attention to return it to reliable service. Keep us posted on what you decide, but there is plenty of help on this forum.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#7
Greg,

Can the springs be removed without removing the head?

Jerry
I have done it on a car, You will have to make a few tools, Get the piston to TDC, Make a rod to go into the spark plug hole to hold the valve up.
But if you can feel a lot of side play in the guide, The head will have to come off anyway.
You could take the Exhaust pipe off and see if you can see anything wrong with the lower guide.
The upper guide you could change once the spring is off.
I have known an engine work with just the top guide !!.
Good Luck. Bill.
 

timetraveller

Well Known and Active Forum User
VOC Member
#8
Sometimes with a problem it pays to walk away and sleep on it. You write that you can 'lift' the valve spring 3/8". Thinking about that and it seems to me that Greg was probably quickest to the problem. Even if the top guide had collapsed then the spring would still be supported at its lower end on the cylinder head casting. If the valve had stretched or the valve seat pulled through then the valve spring cover would be marked seriously. I've had a bike with the lower valve guide having disappeared totally and yet I did not hear or see any change until I took the bike apart for another reason. If you can lift the spring can you rotate it? If so then you might be able to see that both springs have turned into odd bits of steel. Either spring on its own exerts enough pressure to stop you moving things easily by hand. Good luck with it and let us know what you find.
 

Bob

Forum User
VOC Member
#10
You coud try the 'rope trick' to hold the valves in place. Remove the sparl plug and put a few feet of soft nylon rope into the cylinder. Turn the engine and the rope will compress against the valves. I chaned the valve seals on my V8 pickup this way and it saved a shed load of work!
 

greg brillus

Well Known and Active Forum User
VOC Member
#11
I've never seen a stock Vincent with broken springs, so I'd be worried there is more than just the springs that need attention, being an air cooled engine and all.
 

evcomet

Well Known and Active Forum User
VOC Member
#12
Well, it is a new day!
It is difficult to express the frustration with my current challenge. I also greatly appreciate the suggestions offered here.
Taking apart a perfectly beautiful Comet to find a fault that I do not understand was not what I intended to do today.
I will post more when I know more.

Jerry
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#13
With all these funny names, I had to look it up to see if it was our friend with the HOT Comet !.
I have said it before, With how old these Bikes are, We must be ready to do some work on them.
I feel sorry for people who pay a lot of money for a rebuilt Bike, Thinking they will have no trouble.
You will feel ten feet tall when you have fixed it. Cheers Bill.
 

davidd

Well Known and Active Forum User
VOC Member
#14
You can remove the spring, in situ. Something similar has been done on the side of the road. It is not easy and you might miss something doing it this way. Additionally, it took two other riders to help get it done.

That being said, you will likely need to replace the inlet spring. The springs do not last very long, but they are run very long...almost forever. I bought a bike once that had some power issues and I could move the springs by hand. The head came off and I replaced the springs.

It sounds like the same thing. If you did not break a spring, or it broke as it shut down, I suspect all is well. I also had a Vincent drop the valve guide and the ex stayed open just enough to do roughly the same thing. This tends to happen right as you are shutting down when the engine is hot.

The first time is always the worst, but racers tend to take the UFM and forks off in one piece, quite often and can do it quite fast. It is more difficult with a wiring harness. It is nice if you can do an overhead lift of some type just to steady the UFM and forks.

So, by the side of the road you put the piston at bottom dead center, or close. Cut one end off a bungee cord and slip the cord itself through the spark plug hole. The other end with the clip on it will stay outside the engine. turn the engine to get the piston toward TDC and the cord will close the valve. At this point you can do what you wish with the spring. In this case, the circlip on the valve flew off and it was reinstalled while others pried the spring and retainer down.

Best of luck,

David
 

evcomet

Well Known and Active Forum User
VOC Member
#15
I am just now ready to separate the engine from the UFM and Fork assy. (wire harness and cables are a pain)

Just needed to take a break and be certain I haven't missed anything.

What I do know is that no springs are broken. It appears that the upper guide seized on the valve stem.

A tap with a plastic mallet and everything went back into place.

Not sure why this happened, but complete removal and inspection of the head is required.

Jerry
 

evcomet

Well Known and Active Forum User
VOC Member
#17
Found that upper guide ET40 had galled to the valve.
Not sure if it was too tight a fit or lack of lubrication.

The alloy stuck to the valve stem was easily removed
Can I hone the hole in the ET40 and put it back together?

No other damage found. Piston and valve never made contact.

What do you suggest?

I prefer not to do this again.

Jerry

Exhaust valve and parts.jpg Exhaust Valve.jpg Comet ....Front.jpg Comet ... rear 2:3's.jpg
 

greg brillus

Well Known and Active Forum User
VOC Member
#19
Jerry it is a mixture of mist and splash as the oil passes down from the rocker feed bolts on the oil return pipes across the top of the head. I wonder how tight the cylinder hold down nuts were, when they are done up too tight this can distort the head and cause misalignment of the upper and lower guides, which may explain the top guide seizure. You should be able to refit the upper guide temporarily refit the valve and "Feel" some slight play sideways of the valve head in several directions. If the valve is very snug with no play, then the guides might need reaming slightly. Being air cooled they tend to need some running clearance as the engine temp climbs more so than a modern liquid cooled engine. You might be able to polish the valve stem, lubricate and assemble, and all will be fine. The cylinder head hold down nuts should be torqued to 30 to 32 Ft Lb's in an even opposite manner. Use either some oil, grease, or anti seize on the threads, and be careful to align the pushrod tubes as you lower the head back onto the barrel. Cheers and good luck..............Greg.
 

davidd

Well Known and Active Forum User
VOC Member
#20
I always give the upper valve guide more clearance just for this reason. You can do all the work on the lower valve guide, but there is no guarantee that it will put the stem exactly in the center of the top guide. I suspect there was a substantial misalignment to get the top guide to gall. As Greg said, check the lower guide for fit. Check to see that the button on the valve is not to worn. It probably is not too worn, but you can certainly see where it has been working. Get some extra circlips if you do not have a supply.

You can polish the stem. I keep new valves in the shop so I am not tempted to use old ones. I don't know what the free height of the stock springs should be. I have been using Gold Star springs instead.
Renwick v RD 01.jpg
The new spring is on the right. The Vincent spring is considerably longer and owners rarely replace them.

Good luck.

David
 
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