Getting Dads 750gt back on the road

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TomL
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Getting Dads 750gt back on the road

Post by TomL » Sat Feb 28, 2015 7:19 am

This is my first post although I've been lurking around as a guest for few years. What I've got is my late fathers 750gt that he had purchased new in 1975. After copious amounts of cleaning and rebuilding the carbs,oil changed I got it running after sitting for 35 years.I still need tires , throttle, brakes rebuild ext. My first query is that after I got it running it was smoking from the horizontal cylinder and that was why my dad quit riding it in the 80s. His original assessment was the valve guides which seems valid but later on he suspected maybe an overfilled sump. How feasible is it that an overfilled sumpcould cause this, are valve stem seals a valid possibility or should I just pull the head and do the guides? Tom

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Craig in France
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Re: Getting Dads 750gt back on the road

Post by Craig in France » Sat Feb 28, 2015 10:20 am

Hi Tom,

It's unlikely to be simply an overfilled sump - it's kinda difficult to overfill a bevel twin in the first place; but, if you do, excess oil tends to get spat out of the breather. But easy enuf to test - just see how full the sump is.

Guides? Yup, could well be - being a bit shorter than ideal, they are a bit vulnerable to wear. However, it could also just been torn, hardened or simply missing valve guide seals- just remove the valve covers and see if you can see if they're still in place. Steve sells new ones, btw.

Otherwise, it could be rings, of course - those in the front pot can suffer from corrosion if the bike is left standing. Do you have a simple compression tester? If so, take a reading on the front pot and then do the same again, but this time after adding a teaspoon or so of oil thru the plug hole. If the reading goes up, it's most likely rings, I'm afraid (the oil temporarily seals the leaky rings).

HTH

Craig

TomL
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Re: Getting Dads 750gt back on the road

Post by TomL » Sat Feb 28, 2015 2:43 pm

Thanks for the reply Craig, I think I was asking questions that I already knew the answer to. I was just hoping there was something I was unfamiliar with I and was missing. I pulled the rocker cover and glanced at the seal that was still in place. I feel pretty comfortable pulling the head but will I need to pull the barrel too if my compressor test comes up good? I'm hoping at this point to get it sorted out as a ridden "survivor" and not too nice to ride in the rain. I'll post some pictures once I get the swing of it.

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Craig in France
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Re: Getting Dads 750gt back on the road

Post by Craig in France » Sun Mar 01, 2015 4:38 am

TomL wrote:<snip> I feel pretty comfortable pulling the head, but will I need to pull the barrel too if my compressor test comes up good?
No, IMO, unless the compression test gives you reason to suspect the rings, don't disturb the barrel. You just run the risk of breaking rings, and all the hassle and grief that involves.

However, if you're happy taking the head off, then you can check the guides for wear - altho' be aware that measuring this isn't easy, even with a bore gauge/micrometer. If you do remove the head, then you'll be able to also check the condition of the rockers and the valves themselves. And you should reckon on changing the valve guide seals anyway.

Be aware, too, that valve guide replacement needs to be done by someone who knows what they're doing. The amount of support material in the head isn't too generous, and is longer on one side than the other. The result is that it's easy for the new guide to be installed out of line with the valve seat. Not good :shock: .

One solution is to remove the old guide and then ream the bore in the head into which the guide fits in order to ensure it is in line with the seat. And then install a new guide of a size to match the new bore (and also ream the guide itself to be 100% sure).

Another solution is to leave the worn guide in place and insert a valve guide liner - which you then also ream out.

But at this point I suggest you speak to Steve Allen ... :)

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Re: Getting Dads 750gt back on the road

Post by wdietz186 » Sun Mar 01, 2015 1:50 pm

Tom, Welcome to the forum and GT ownership. I like your idea of keeping it a rider and not a garage queen and please don't try to "improve" it by making it a Sport mash up. My crystal ball says it does need guides [most all of them do/did, it seems they were made of hard cheese] and if it has the 860 style screw type valve adjusters it will probably need rocker bushings too. Other than the possibility of rust, the pistons, rings, etc. are pretty long lived and should be ok. If the repair is too daunting for you I'll give you a $1000 for that old worn out obsolete piece of junk :lol: .

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Re: Getting Dads 750gt back on the road

Post by TomL » Sun Mar 01, 2015 5:12 pm

Welllll, I'll have to pass on your generous offer right now. I did do a compression test on it and ended up with 135psi on the front and 140psi on the vertical. I'm pretty sure it is the valve guides and it does have the screw style adjusters. So rocker bushings too eh. This bike has only 7600mi. on it and was well taken care of when he was riding it. The worst of it was time, not being ridden, and moving it around. Fortunately it spent a decent amount of time in parts of the country that have lower levels of humidity, Nebraska, Iowa and North Dakota.

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Craig in France
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Re: Getting Dads 750gt back on the road

Post by Craig in France » Mon Mar 02, 2015 3:53 am

TomL wrote:I did do a compression test on it and ended up with 135psi on the front and 140psi on the vertical. I'm pretty sure it is the valve guides ...
That's good news, Tom. Agree, it's almost definitely the guides - or even better, just the seals :-D .

(Btw, if you feel like passing it on, I'll raise Bill's offer to $1500 :lol: )

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Re: Getting Dads 750gt back on the road

Post by wdietz186 » Tue Mar 03, 2015 6:36 pm

OK 2K and I'll pick it up!

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Re: Getting Dads 750gt back on the road

Post by TomL » Wed Mar 04, 2015 8:23 pm

Now I haven't had a chance to look due to trying to kick a bug, but is it possible to do the seals by aligning the timing marks, pulling the rocker, supporting the valve via rope or air then pulling the spring?

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Craig in France
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Re: Getting Dads 750gt back on the road

Post by Craig in France » Thu Mar 05, 2015 6:02 am

Yes, people say it is, Tom, altho' it's not something I've done myself. And I can't now remember how you get the valve collets out. But some one here will know ... :)

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Re: Getting Dads 750gt back on the road

Post by TomL » Thu Mar 05, 2015 3:47 pm

That's what I was hoping to hear and kind of suspected. I imagine I can fab up a valve spring compressor pretty easily. I'm pretty excited to get it on the road . I've fired it up a few more times just to listen to it. I think this bike will be a little easier to sort out than his old '68 BSA 441 Shooting Star, but that's a topic for a different forum. Thanks

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Re: Getting Dads 750gt back on the road

Post by wdietz186 » Fri Mar 06, 2015 6:52 am

Carb, pipe, and four nuts, the head is off in your hands. Line up the timing marks first though. You'll be able to see how badly the guide is worn with the spring off. As a guess I'll say about 3mm. Mine were.

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Craig in France
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Re: Getting Dads 750gt back on the road

Post by Craig in France » Fri Mar 06, 2015 9:27 am

Good point , Bill - forgot this was just the front pot (3mm!?! Blimey, you must have smoked like a destroyer ... :shock: )

Tom, if you do decide to take the head off, don't forget to allow for changing the O-rings between the head and the barrel. Steve sells them as a set, btw:
https://store.bevelheaven.com/index.php ... &x=29&y=12

Ditto the wave washers under the head nuts. And me, I'd probably change the carb manifold rubbers too. But talk to Steve - he'll tell you what you need (he'll even sell you a tool for torquing the head down properly :-D )

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