XS750 to XS896 big bore

[XS750AU]

Reassembled the head today. New valve stem seals were the only new parts. All straight forward stuff, final timing of cams when the cam chain goes in later on.
Then I just continued to clean grim and muck from all the bolt heads and other crevices. In some ways the oil and dirt appear to have preserved the painted covers, as it is cleaning up pretty well.

 

[Tune-A-Fish]

Just wondering if you did a valve clearance feel before buttoning things up?

 

[XS750AU]

Hi Tune-A-Fish
After torquing the journals down the clearances are 0.16mm on the inlet and 0.22mm on the exhaust. So there is no need to play with shims at this point.
Thanks for reminding me.
Regards
Tim

 

[Tune-A-Fish]

Hmmm... Seems fat for the 896 bore, if you took notes for each valve, you might need to readjust if it's noisy or boggy off idle, that big bore will be hungry for air and tighter multiplies not just open more but longer.

I can tell you know all this crap, just offering some friendly thought

 

[XS750AU]

Hmmm... Seems fat for the 896 bore, if you took notes for each valve, you might need to readjust if it's noisy or boggy off idle, that big bore will be hungry for air and tighter multiplies not just open more but longer.

I can tell you know all this crap, just offering some friendly thought

Thanks for the info. The factory limit for the inlet is 0.16mm which is where we are now. The valves are going to grow as they heat up in operation and close the gap. So how far can you close it up with the shims before you have gone too far?
The exhaust valves will get hotter, hense their min clearance is 0.21 and we are currently at 0.22. The 896 is going to run hotter, higher compresion and the basic fact that more power = more heat.
Really interested to get you experience on the true minimum clearances. I will admit to being conservative.
Thanks
Tim

Sent from my GT-I9506 using Tapatalk

 

[Tune-A-Fish]

Unfortunately, I don't know what the pistons are and what relief if any is cut in them. I may well be wrong, just seems loose to me.

 

[datadavid]

My valves, new in a head with new guides and 3 axis seat grind, clearances actually grew slightly the first few hundred miles. Only reason i can come up with is carbon from ring seating collecting on the valve seats?

 

[datadavid]

Point is, run it for a while then the routine stuff, retorqueing head, camchain tension, valve clearance. After the first check they seldom change.

 

[XS750AU]

Most round slide (VM series carbs use Large Hex jets. Large hex jets are numbered to represent flow rates at 30cm of pressure IIRC. Large round jets are measured in mm of diameter, so a 150 is 1.5mm diameter.

In simple terms double the size of a hex jet from say 90 to 180 and it doubles the flow, but double the size of a round jet and flow increases by a factor of 4, so a 180 round flows 4 times as much as a 90 in a large round. There are tables on teh web that show a pretty close correlation for equivalent sized jets.

Thanks for the heads up. I found that comparison chart and as you said a huge difference in flows (refer attached). The 130 AMAL design that were in it are actually the equivalent to about a 105 round mikuni (reverse). So the PO actually leaned out the carbi significantly. The 160 AMAL that I bought are equivalent to a 120 round mikuni!!!!!! It is never a mistake, if you learn without doing damage - Thank you Teazer.
Cheers
Tim

 

[XS750AU]

Point is, run it for a while then the routine stuff, retorqueing head, camchain tension, valve clearance. After the first check they seldom change.

I have not done as much work on the head as you Dave, just a bit of basic maintenance. Totally agree with you though, run it a see how it goes. Should be OK at lower RPM and run out of puff towards redline.
The exhaust valves had a heap of carbon build up on the seats and valve crown, the piston gudgeon pins were distinctly blue which indicates to me that it has been running bloody hot. With the jetting being totally lean (per previous post) it looks like it was running bloody hot with no power - no wonder it did not see much speedway action!!
Cheers
Tim

 

[Tune-A-Fish]

My valves, new in a head with new guides and 3 axis seat grind, clearances actually grew slightly the first few hundred miles. Only reason i can come up with is carbon from ring seating collecting on the valve seats?

Not likely David. I think you just experienced valve train relax after rebuild. all the parts dont fully seat like keepers and spring base/retainers until a good hot run.

When my son switched from two stroke race bikes to 4 it took me a while to learn that you need to go a shim tight after a take down or it would surly be loose the first hot lap. But hey, we only got a few races out of a top end back then and now a head will last almost a season so :-\

Back to this tho... I'd run it and see how it goes, cant break it loose and half the fun of cycles is the wrench time me thinks :

Dig the jet cutaway will help splain it to others

 

[datadavid]

Not likely David. I think you just experienced valve train relax after rebuild. all the parts dont fully seat like keepers and spring base/retainers until a good hot run.

When my son switched from two stroke race bikes to 4 it took me a while to learn that you need to go a shim tight after a take down or it would surly be loose the first hot lap. But hey, we only got a few races out of a top end back then and now a head will last almost a season so :-\

Back to this tho... I'd run it and see how it goes, cant break it loose and half the fun of cycles is the wrench time me thinks :

Dig the jet cutaway will help splain it to others

That makes a lot more sense. Combustion chambers are still clean as a whistle..
Also its a mystery to me that people even dare to mess with the jetting, even putting amal jets in japanese carbs. Jeez.. cool to see that cross refence chart though!

 

[XS750AU]

Hmmm... Seems fat for the 896 bore, if you took notes for each valve, you might need to readjust if it's noisy or boggy off idle, that big bore will be hungry for air and tighter multiplies not just open more but longer.

I can tell you know all this crap, just offering some friendly thought

Hi Tune -A-Fish
I now understand where you are coming from. During lunch I looked up the factory spec for valve clearance on the GS1000 and Z900 (supposedly the GS engine was based on the Z engine design), both air cooled 2 valve Jap engines from the same vintage as the XS750. The Z & GS share the same spec of 0.05mm to 0.10mm, as you said a hell of a lot smaller clearance.
As the basic valve train design is essentially the same, it could be fair enough to say that at the cam end there wouldn't be a problem to cut the clearance down towards 0.05mm. But that would mean that the valve crown would project 0.15mm further into the combustion chamber, and stay open longer. Hence your comment about piston relief.
To check the valve to piston clearance is simple enough, using play doe and seeing what thickness it squishes down to after an exhaust cycle. Does anyone have a guide as to what clearance is RELIABLE and what is a minimum????
Cheers
Tim

 

[datadavid]

It would not be very good engine design if piston-valve clearance is less than .10mm, there should be a little extra in case of valve float i guess. Also people have skimmed 0.5-1.0mm off the cylinders without issues, so there should be some air left there.. of course playdoh is the way to go when in doubt.

 

[datadavid]

I go with .10 intake and .15 exhaust. No need to mess about with .01 up or down on these old things..

 

[Tune-A-Fish]

I was going to suggest squish, but it requires a tear down... I use silly puddy it releases with the least amount of distortion toys R us is the last place I could find it.

I also might suggest for anyone doing a big bore, you can do a 3 angle grind/cut just a hair deep in the seat and clearance port the head around the valve and seat, it will increase cc a tiny bit loosing some comp not much and not really a bad thing, many heads have a lot of material you can trim away and allow more air or better air flow into the valve... it's kinda like Draino for heads you get that hair out and the swirl comes back

 

[teazer]

I was going to suggest squish, but it requires a tear down... I use silly puddy it releases with the least amount of distortion toys R us is the last place I could find it.

I also might suggest for anyone doing a big bore, you can do a 3 angle grind/cut just a hair deep in the seat and clearance port the head around the valve and seat, it will increase cc a tiny bit loosing some comp not much and not really a bad thing, many heads have a lot of material you can trim away and allow more air or better air flow into the valve... it's kinda like Draino for heads you get that hair out and the swirl comes back

That is exactly right. On our Honda race motors we deck the head and block by "quite a lot" and there is never enough piston to valve clearance. I like to pull the valves back into the head slightly and create a nice radius blend. People will tell you that pockets the valve and hurts flow but that's not what we find.

Sometimes you have to deepen the pocket in the piston as well. All it takes is time and lots of build and strip and measure sessions to get it right. Can't afford to go too small on clearance there or else heads will toll - off the valves that is...

 

[Tune-A-Fish]

Term is "deck height" squish, essential for stroker rods and valve clearance, I wish I was better schooled but a guy I once drank too much with (makes a hellof uh 3 AM cake) told me some of his secrets... a 3 angle with a radius cut at the bottom is nice if you can get some air time on a Serdi and cut it all in one process with matching valve cut : John Force shit right there.

 

[datadavid]

Thats what my machining guy is for 30+ years of engine work and solid rep just half an hour away. He even took the vibes out of my crank. 896cc, vibes less than a stock 750.

 

[XS750AU]

Hmmm! This build originated with a wreck bought for AU$105 as a doner for my XS750.
Now I do have an excellent engine guy 10 minutes up the road who can do the headwork. But then if I do get the engine breathing fire and pumping out 80+ hp, then the chassis and brakes will not be up to it!!! Then we can start on them and....and ........and.

Or I could just buy the bike in the photo below! If your interested, try and find the article about the engine development. The centre line of the pistons is offset by 10mm to the crank, so that on the power stroke the piston is better aligned with the big end to reduce side load on the piston and resulting friction!!!!! No way of doing that sort of detail on this build.

All good advice, and gratefully received, but I have to temper it with reality and the budget. A porting job that I can do and new shims to cut down the clearance and extend the opening time, plus a extra base gasket to lift the valves away from the piston crowns is probably where it is going to end up. Sorry to dampen the party.

Picked up a block of ally 32mm X 150mm x 200mm to make up a sandwich plate for an oil cooler. I have been watching Ebay for 2 years and the factory XS850 sandwich plates just don't come up, so it is time to make one. Mill and rotary table work to come.
Cheers
Tim