stock bore size and a shell #1

ExtraRegularGuy

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Im sure it isnt a uncommon thing, i just cant find any video of people with a stock bore size XS and a shell#1. I see mostly 700s and 750s that are cammed. Im just trying to get a feel on how rowdy a stock bore XS with 250 /250 on 100 will act. Thoughts?
 
How rowdy? No offense meant, but that's kind of a hard metric to pin down, especially in the absence of any info on other modifications. But I'll take the question to ask if you'll get enough giddyup out of installing a Shell #1 cam in a 650 at stock displacement to justify the time and expense.

If the camshaft is the only thing you plan to change, then no, it's not worth it. For the cam to be effective, the motor has to be able to breathe, and that means relief at the intake and exhaust ends, and that entails mechanical carbies in place of the vacuum carbs, because vacuum carbs don't perform their best without a still air box and the air box on the XS650 isn't big enough. The biggest bang for the buck will be from Mikuni VM34 round slides from a reputable vendor, and that means 650 Central or Hoos Racing. Another thing: if you've been reading that godawful Minton article, do not believe him when he claims that stock valve springs will work with the Shell #1. (Well, they'll work, just not for very long before you wind up cupping a valve.)

Other needful things: either have the head ported by somebody who knows that you can't make power by hogging out ports that are marginally too big to start with, or just knock down the casting flaws yourself and call it good. And bump the static CR a bit; you should get to around 9:1 or a tad north of that by dropping the deck by around .020" with a thinner head gasket. If you don't know how to check valve to piston and piston to squish clearances, Google is your friend. Lonnie Visconti at Copper Gaskets Unlimited can help you with the head gasket; Google for contact info.

Of course all of that work will be wasted if you don't degree the cam in right, and the way the available units are ground, that's a bit of a challenge. What's worked best for me is to go for 45* exhaust opening. Other procedures have led to a lot of short shifting due to advanced valve timing.
 
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I've got uni pods currently and will be making a 1.5 inch exhaust for the bike. I plan on a set of RD springs also. So BS34 carbs are that awful? I haven't ridden a bike with the VM carbs so I have nothing to compare to. I figure with the adjustable needle I'd be able to tune out the lean spot. I definitely wouldn't mind a set of VM carbs though. When I have the top end off I will check out the ports and what not to see how bad the casting flash is. My biggest concern though is the regrind, makes me nervous. I don't want 4 seperate lobes on my cam haha. What can I expect for P to V clearence with stock gaskets? I have searched the Web and found conflicting data.
 
Good choice re. 1.5" single wall pipes. For clearance specs and much more, go to the 650 Central site and click the link to buy a download of Craig Weeks' performance modification manual (650 Performance)--best $39.50 you'll ever spend. You should have plenty of room on the piston-valve clearance if you drop the deck by around .020", but don't let the squish go much closer than .050"; Craig's squish clearance specs assume very spendy high performance rods. You'll probably be fine on the squish as well, but never assume--solder crushes are cheap and easy. A good way to test before spending money on custom gaskets is to mock up the motor without a base gasket.

Re. RD springs, the bucket that supports them will stop on the flange of the guide, not on the bottom of the well, and that's as it should be; fill the well with additional washers to support the bucket fully, then get a good assortment of washers with a range of thicknesses to dial in the installed valve spring height to the specs that come with the springs.

BS 34's aren't bad carburetors, they're just not a good choice for a performance build. Bear in mind that the throttle shaft and butterfly reduce the effective venturi diameter by around 2 mm., so you'd be running the equivalent of 32 mm. in an unobstructed carburetor. Also any slide control issues involved in using pod filters on vacuum carbs in place of a still air box will get worse the more you modify for higher output. I don't mean to give the impression that VM's are the best thing available; they're just far and away the biggest bang for the buck.

Don't worry about the regrind. I've been using a Shell #1 for some time, and it showed no wear when I inspected it at over 20K miles of use to move it to a fresh motor. I use 15/40 diesel oil with a little dose of ZDDP additive.

Good luck with the motor build--keep us informed!
 
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Great advice to ERG, Griz. I've never put together anything with a #1 cam that was less than a full 750 and 10+ - 1 c.r.. What ignition timing would you suggest to ERG as a starting point? I've always used 33 - 36 as a full advance setting for my engines
 
thanks for the advice on the RD springs and the carb info. Ill look into a pair of carbs, they can be found on amazon for relatively cheap. Problem being that they are are only right side carbs. They may not even be legit mikunis though... 650performance how much ring gap do you run on the 750s? if you dont mind divulging the information of course... Do you guys have any experience with the adjustable cam gear by chance? That seems like a pretty cool piece.
 
Craig, the motor I'm using the #1 in right now is a 700 running around 8.8:1 static CR; I got real tired of guessing doses of off-the-shelf octane booster on a 10:1 motor when I couldn't find premium gas in the boonies. The #1 works fine with the lower CR; working valve timing is very close to the old 256 stock cam, with a little more lift. Anyhoo, I set the advance at 38* BTDC based on reading the clean burn line on the spark plug electrodes; if it's wider than whisker thin (.010" or less), retard the spark a touch.

ERG, I've never used an adjustable sprocket mount, so I can't give you any advice on them. My only reason for not using one is that the good one (from Megacycle) costs an arm and a leg and I have access to a press and know how to use it without breaking a chunk out of the sprocket boss (and yes, I've seen that done). A search will turn up a fine thread by mrriggs on heating the sprocket to install without a press, but he's a better hand at heat control than I am; I'm afraid of screwing up and taking the temper out of the sprocket. So what I do is to chill the camshaft in a freezer, heat the sprocket to 500* F in an oven, get the sprocket in place with punch marks lined up, and finish in the press if need be.
 
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Yes indeed, that sprocket is expensive but looks as if it could make life easy when moving the cam around. If i get wild enough i just might hop on a mill and reference a jesel spider gear. I already caught a huge break on the build. through some ebay lurking i might have sourced a pair of VM34s off a seadoo. Better yet i may have a lead on TM34s, my heart is fluttering at the thought
 
The I ran a 650 with Shell #1 , 34mm vm's and 1 3/4" head pipes with open megs.. dyno was at 50-51 hp. Won a few races with it. I used just a Small arbor press with a old fork tube for extra leverage to remove the sprocket. The key is when you put pressure on the arm hit the top of the press with brass hammer, it breaks the seal loose then it press on and off ok. Probably have done over 400 that way. 0920161847.jpg
 
ERG, there are several variants of the VM34, and carbs from snowmobiles and jet skis are not going to work for you. Trust me on this, the carb bodies differ and will involve more issues than just jetting. The TM34 is fine, takes jetting very similar to the VM34.

Guys, when you look at Hooser's dyno number, bear in mind that it represents power actually delivered at the wheel; Yamaha's advertised power ratings are taken at the crank, and as we used to say back in the day, those Japanese horses were kinda small.
 
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ERG, Griz is right about the VM34s coming in lots of iterations. Racers in the '70s used to use VM34 carbs off Yamaha TD race engines for their XS750 flat track bikes and they were quite different internally. Check things out carefully before you buy. And, FYI, those TM34 carbs will handle up to 75 RWHP, so don't stray into the bigger-is-better thing. I've used an adjustable sprocket for years and really like them. With practice you can move cam timing in very small increments. And, though Gary has been successful with 1 3/4" pipes on his race 650, I'd recommend 1 1/2" for your engine. The rings you purchase will recommend the gaps ... just follow their instructions. Not all rings are the same (Total Seal, as one example) and the gaps can differ slightly.
 
after reading what others have to say about the TM series of carbs i think ill just pick up a used pair and refresh it with some gaskets and baseline jetting and go from there. The set of TMs im looking at is off a skidoo, would that render it not-so-desirable like the VMs?
 
No, the TM34 will work just fine, what Craig was saying was that you don't need more venturi than that. One thing about Mikuni flat slide carbies is that the slides tend to wear more quickly than other types. I'm not bashing the flat slide carbs; I use a pair of TM36/68 4-stroke pumpers (I'd have used the 34 mm. version, but the good people at Mikuni America don't think we need them, and on a street bike the performance difference between 34 and 36 mm. isn't something you'll feel in the seat of your pants). Anyway, be aware that refurbing and jetting a pair of used TMs may be more expensive than you expect. As to the jet ski carb, it's likely to be a TMX, which is a dedicated 2-stroke carburetor. Stay away from that stuff; even if the carb body is workable you'll be changing out every jet in the thing plus the slide (wrong cutaway). Do yourself a favor and don't go there! We're not talking about stuff that's not so desirable, we're talking about stuff that may not work no matter what you do and will wind up costing more than new carburetors if you can make it work.
 
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you sure have given me a lot to think/read about. I really like the idea of having TMs because they are just newer, but the VM series is pretty prevalent in this community. if you dont mind rewinding a bit back to the caveman BS34 carbs, would altering the diaphragm spring help with tuning when a non restricting intake and non restricting exhaust are present?
 
Actually the first generation of the BS series is a more recent design than the VM series, and the XS650 got the second generation of BS34's. The VM34 is the "caveman" carburetor: crude and effective. They're popular for three reasons--they're cheap, they work, and they satisfy the "period appropriate" rule in AHRMA competition.

Lightening the spring in the BS34 will in no way improve tunability. Some guys lighten the slide spring and enlarge the vacuum hole in the slide in an effort to get faster throttle response. IMO that's wasted effort, and lightening the spring is going to worsen the real trouble, which is--wait for it--slide flutter caused by unstable air pressure at the vacuum chamber vent due to lack of a still air box.

There are rules to follow to achieve a successful performance build. The first rule is, Define your goals. Second: Know what you need to achieve your goals. Third: Don't cheap out in getting what you need. Fourth: Don't throw money at it until you've thought it through. There are more of course, but those are the ones to keep in mind before you start buying stuff--especially #3.
 
If buying second hand carbs to avoid 2 stroke carbs with lots of complications I always try to buy Dellorto phf pumper carbs,
 
Michael Morse at 650 Central can sell you a pair of pre-jetted VM34s that will work well with your combination. Yes, it's a more expensive, but it's one solution that will solve your carb dilemma. He can also help you with follow up questions ... if, for example, you decide to make the engine a 700 or 750 and want to know how that will affect the jetting.
Another subject: I don't know your feelings about noise (or how the cops are around where you live), but speaking personally if I were going to run that engine on a street bike I'd start by opting for the 1 1/2" pipes and then route them down and under the engine just like the '70s TT race bikes. The pipe length I'd go for would be 29" measured from the exhaust valve to the end of each pipe and I would not use a muffler; just cut the pipe off. It should perform well and with 650cc and lower compression I think it would have a pleasing exhaust note and not be too loud.
 
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