New 750 piston dilema
#1
Hi Everyone.

I need an opinion.  

I am currently rebuilding the top end of my SR41.  I have sourced the first oversize piston and rings.  The valves have been replaced and my machine shop has don a stellar job with the valve seats and cleaning up the ports to standard specs.

The problem I have just noticed is that the original piston had pockets cut into the piston crown for valve clearance.

The new piston (Yes, it's the correct part number) has no pockets and is just a flat crown.

So the question from the machine shop is....."Do you want us to cut valve reliefs into the piston crown.....or NOT?"

What's the right way??  Should the flat crown be OK since the PN is correct?  

Or are the valve pockets necessary.  I'm running the standard "B" cam so no crazy valve lift or overlap is happening.

Thanks to everyone in advance.
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#2
Hi Ray, long time since we heard from you, hope you are well.
The later 800 used the flat top piston, and I`ve got a +1.00 piston with a flat top in one of my SR41s and had no issues.
It`s a direct supercession for the original pistons and Suzuki wouldn`t do that if there was an problem
I don`t think the valve lift on the cams was enough to merit cut outs really.
Finding new oversize pistons and rings is becoming really difficult, and they have not been available for years.
The supermono series in the UK, which ran for many years, meant all the oversize pistons over here went into race bikes.
Good luck with the rebuild.
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#3
Thanks for that. Real world experience is exactly what I wanted.

I was able to find the first oversize from an eBay seller in Lebanon (in all places). Seemed a bit risky but was the correct part and arrived in fairly quickly.

It was the flat top that worried me and I checked and re-checked the PN from various sources and all agreed it was the correct bit.

My machine shop has the ability to cut the pockets in and noticed the difference in pistons.

I did a rebuild a while back (all bearings and new transmission gears for the worn bits) but the bore was just a bit over the stated max spec in the manual. Since I couldn't source the piston I decided to run the engine with the out of spec bore. It ran OK but was unsing too much oil.

When I took the engine down this time I noticed that the combustion chamger had black flakey almost wet carbon in it. This carbon had been flaking off and had been pounded into the valve seats creating slight damage.

Other than that, I'm still enjoying the bike and it's still running well.

While I hate Facebook, I've found that there are a number of international riders there that still love the Bigs. Some of them are new to the breed and inexperienced about carbs, timing chains, etc.

If you can tolerate the rest of the site (and Facebooks interference in politics and other areas) I'm sure your expertise and the expertise of others here would be a great addition.

I've tried to refer others to this site also. Hope that's OK.
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#4
Hmmmm, I`m interested in your oil consumption being so high. 
Mostly because I`ve never experienced it myself, even on bikes with very high mileage (175k in one instance).
The one occasion I used fully synthetic it burnt off within a thousand miles and the level was low so I didn`t do that again.
For many years I used car 10/40 mineral oil, but as it`s becoming hard to find I`ve switched to car semi synthetic 10/40.
Never had any noticeable oil usage and certainly no clutch slip or any horrors that modern mythology would have you believe
I change every 3000m. Maybe I just ride like an old tart !
Your build up of carbon and its deleterious effect on the valve seats is interesting. 
Mostly as all my bikes are now suffering from black sooty carbon deposits on the plugs and in the exhausts and have been for the past couple of years.
I`ve checked and rechecked everything to see that it`s all within spec but to no avail. Everything is as stated in the Holy Book.
I`ve tried jetting down, even trying 110/112.5/115/117.5/ when it should be 120. Still doesn`t run right of course on any of them.
Also fuel consumption is down from 48-50 mpg to 40-42 mpg.
The only conclusion I can come to is that the quality of the fuel these days is utterly abysmal.
I use Esso super which is supposed to be ethanol free but it doesn`t really help.
Anyone else got this problem?
As for going on FB and trying to help out the newly converted................ don`t get me started!
I tried many years ago and just got so much abuse/spam/hate mail so gave up in disgust.
Legislators are trying to stop us riding old bikes and cars now  so I guess Bigs will be museum pieces soon enough.
I can`t ride into London because it doesn`t comply with some Euro emissions and can`t get breakdown cover due to it`s age, so maybe it`s time to move on to some boring modern bike.
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#5
I'm just rebuilding my SR43A. It was using lots of oil. I thought the piston and rings would be toast but they look fine and are in spec. Original suzuki judging by the markings. I might have found a leaking valve seal so will get this replaced and see if that helps.

I use Eurol Sport Touring 10w40 motorcycle mineral oil. Will be interesting if it uses oil when i next start it!

You can see some carbon on my piston below. My carbs were set up incorrectly and it ran really rich. Also one plug was not firing due to a faulty coil. All fixed now I hope!

[Image: TtllxiZ.jpg]
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#6
My piston was caked in black carbon like that before I rebuilt with a new one. My bike still uses a little oil, it will be low on the dipstick after 1000 miles or so and I have to keep and eye on it and top up.
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#7
Bob,

I'm wondering if your needle jets (emusion tubes) and needles are worn. This along with wear in the grey slide guides makes the carbs always run richer. Might be time to pull the carbs (ughh) and give the internals a look.

Jammin,

Thanks for the photo. I cleaned off my piston before my brain activated enough to think about a photo.

But my piston was entirely covered with what yours has right in the center. Thick, flakey black deposits. I'm using high quality non-synthetic oil in the Big.
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#8
Wierd! Mine used about a litre of oil per 1000km. One plug was not firing. Also running very rich. And my head looked like the photo below. Yours sounds worse! I live in the Netherlands and have only recently been forced to use E5 fuel.

Ladder - what do you mean by worn slides? See photo of mine below. Not sure how to measure?

[Image: XXdMGSR.jpg]

[Image: Vptk7aO.jpg]
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#9
I'm wondering if your needle jets (emusion tubes) and needles are worn. This along with wear in the grey slide guides makes the carbs always run richer. Might be time to pull the carbs (ughh) and give the internals a look.

It seems a fair comment, however, it doesnt explain why it`s happened to all my bikes at the same time.
I`ve also tried several sets of carbs as well with no success as they all still run rich.
My guess is that the fuel is just awful quality these days.
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#10
@BigBob, isn't all fuel these days, even super unleaded, at least E5?
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