Engine rebuild and big bore kit

Steve · Posted: April 21, 2006, 2:17 pm Post subject: Engine rebuild and big bore kit

Hi,
This is my first post on the site so hello to everyone.
I recently bought a partially stripped down 1980 XS650 US import (well, it was an import for me). There are a few parts missing, mainly electrical, points and so on but the engine seems in reasonable nick.
I've decided to strip the engine down completely and replace all the bearings and anything else that I find needs it. However, I have a question. I'm waiting on a big bore kit (80mm pistons take it to 744cc) to arrive from the US. I was thinking of replacing the cam with a performance version. Is this advisable given that I'll be putting in a big bore kit?
Thanks in advance for your help.
Steve

grizld1

Steve, if you have the 1980 XS650G, there were no points fitted to it, although some owners retrofitted contact breaker ignition as a cheaper alternative to replacing blown TCI ignition modules. You could also have a 1979 XS650F sold new in 1980 and so titled; if so it would have breaker point ignition. If there's a pickup coil (little black plastic ovoid fitting) on the stator and a single coil with two plug wires, you have a TCI-equipped bike.

As to installing a performance cam, that's a judgement call. Aftermarket cams need to be degreed, to work optimally; that is, the position of the cam sprocket on the shaft needs to be checked and, if necessary, altered, to achieve accurate valve timing. This becomes more critical as the state of tune of the engine is raised.

Also, an aftermarket cam needs to breathe, which calls for good port work on the head and good carburetion. There's no point installing a cam that'll move more fuel mix in and more exhaust out if the carbs and ports won't cooperate.

Have a look at the carbs. If the tops are chromed steel, you have the BS34's, which were stock on the G. They're marginal at best for the stock engine. If you have CV carbs with aluminum tops you have BS38's, which are decent carbs and would be tunable for a ported big-bore engine, although Mikuni VM36 or TM34 non-CV carbs would be preferable. For a ported and cammed big-bore, the freer-flowing non-CV carbs are pretty much a necessity.

Another thing to consider is service life. If you go for power at the high RPM range, be aware that the stock XS650 crank assembly doesn't like to be revved over the 7500 RPM redline. If that's done to it on a regular basis it expresses its displeasure by allowing the flywheels to separate, and sometimes throws a tantrum and pitches one out through the cases. That problem can be controlled by TIG-welding the crank pins to the flywheels. The second problem's not so easy. The small ends of the connecting rods tend to stretch when the engine's overrevved. Eventually the wristpins will develop enough rocking motion to shatter the pistons. This issue can be controlled somewhat with competition rods--Falicons, for example--but you'll still be dealing with race-engine service intervals if you wind the mill past factory redline regularly. And even with porting, well-tuned carbs and a stock cam, that engine will want to rev. If you decide to cam the engine and want effective power with a bit of service life, I'd recommend a mild cam that delivers power in the midrange, such as the Shell #1 grind.

Steve

Many thanks grizld, thats food for thought. I guess I'd best stick to the stock cam. Although I want the thump that a big 750 can give, I'm not altogether that interested in blasting it around, I have another bike for that, it's more a bit more torque and some sound I'm after and as I'm stripping the engine, just thought I'd blow a bit more cash in a big bore kit.
Are you saying that even if I don't put a performance cam in I could get the problem you wrote about due to the fact that I increased the bore & piston size?
BTW, thanks very much for the heads up on the points lol, thought I was going crazy or the guy that sold it to me had an aversion to electrical components. I'll check for the little black plastic pick up coil but I'm pretty sure your'e right. I'll also check the carbs but I think they are 34 Mikuni's.
Once again, thanks a million for the info, it has however raised a few more questions that it answered ;>)
Cheers
Steve

grizld1

Steve, the issues I brought up were in reference to operating the engine past its specified rev limit, and were meant as a caution against radical cams that develop power at high RPM's at the expense of low and midrange performance. Increasing displacement will boost power across the band, which is just what you want to do, and good port work will enhance it. I'd encourage both.

One of the world's great XS650 performance tuners, Tony Hall, is, sad to say, no longer with us. His shop would have been the ideal place for you to go for port work. I don't know of other UK tuners for the bike, but I'm sure you'll find someone.

Good luck--let us know how the build progresses!

Steve

Thanks again. Yes I theard about Tony and his untimely death. Unfortunately, the UK does not seem to have the same following that the US has for the XS but I'll try to hunt someone down.
any thanks

grizld1

Not so long ago there was quite a lot of very impressive XS650 tuning and building going on in the Netherlands--about as far from you as the state of Ohio, where I'll be sending some cylinders for boring and silicon carbide impregnation, is from me. There's a 650 club there with a very impressive site. You might check it.

mo650chopper

O/K grizld1 what is "silicon carbide impregnation"? What are the advantages of it for the XS motor, or any motor I assume? Also do you know of anyone doing quality Port / Polish headwork commercially?

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Steve...........When increasing an engines volumetric efficiency whether by increasing the engines displacement, installing a HP camshaft, carburetion,etc, the engines initial intake charge requires for fuel to balance out the fuel/air ratio and when this increased mixture is ignited you have increased the volume of burnt gas residue that must be expelled during the exhaust cycle to scavenge the cylinder more effectively for the next incoming intake charge. An unfortunately when dealing with the XS head,the exhaust ports are very restrictive causing more resistance rather than good flow,just the opposite of what your needing with the big bore kit. If you just addressed the Xs exhaust ports alone,you're still way ahead of your buddy in producing power. The head is where power is made regardless of how ya lok at it, period! Place a fan next to a window that's barely cracked,turn the fan wide open and feel for the volume and velocity of the air exiting,now keep cranking the window high and both elements start to icrease but the secret is keeping the air velocity higher not volume cuz velocity fills the cylinders not increased volume,sure you'll increase the power with increased volume but at the expense of stressing the motor out to get the piston speed up to enhance the loss port velocity. A properly ported XS head with the right intake valve( not larger than stock) and with a good all around intake port to start with, could have a flow increasement of 25% along with the same percentage for the exhaust.

If you can't locate someone to port your heads in the UK,drop me a line at xsjackheads @ yahoo.com and will work out a deal that's affortable for you..........Later Jack

And for those wanting commercial quality porting services,I would suggest you contact either Bob Bertaut or Michael Mercury Morse both are top notch in dealing with XS Wink

grizld1

Silicon carbide impregnation is a process used by Bore Tech, in which the pores in the cast iron liner are impregnated with silicon carbide, resulting in a harder bore with less friction. BTW, Steve, Jack does awesome work. He won't name-drop on his own behalf, but I can tell you that one of the best XS650 engine builders in this country sent a head to Jack for porting.

Phred

What kind of $$ are we talking about for a basic port & polish on a xs650 head? How fussy are these heads? I have done port clean-up, intake port matching, and cc'ing before (on cars), but I know doing a real port job requires knowledge of the specific engine.

Steve

Cheers Phred, you beat me to it.
Thanks very much grizld and Jack for the info. I will email you Jack.
Steve

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Phred...........Your absolutely right it takes knowledge but it also helps to know the right people who have been porting heads for aliving for the past 30 plus years and I 've been fortunate to hook up with a dear friend who has passed along some of his knowledge and tought me a few tricks or two but important it's a trial and erro business when dealing with porting to see if gains where a success and sometimes it's difficult to improve on a port which has already been well thought out by engineers.

It was'nt my intension to come here to solicit business but to offer an opinion on a subject that i've spent the last two years solid experimenting with where i've cut and welded ports trying to locate and extra gain here or there.This does'nt qualify me as an expert but it does give the upper hand on say someone like yourself on extracting a few more ponies on the XS,so my advice to you is grap a handful of heads and start grinding son and have them flow tested...................Jack

Steve

Jack mate, did you get my last email to you?

xs1961

Steve,I've built & run two 840's over the years and a Wiseco 750 for the last two.I think you'll find these conversions are more about extra torque than increased rev's & top speed,they enable me to drop 3 teeth on the rear sprocket & add 1 to the front to give approx 90mph at 5000rpm!.
As was mentioned previously in this dicussion the old XS doesnt like to rev high.
The only mods to the head Ive carried- out are to smooth out the inlet & exhaust ports with a 'dremel' type tool & 'open-out' the squish to match the barrel bore. I did once have a head gasflowed by Mez Porting (Newport Pagnell) although there was a difference I wouldnt say it was worth the £200+ nice unrestricted silencers & a pair of 34mm roundslides
had more effect !

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Question about the 7500 redline. My bike is an 82 heritage, and the tach says that my bike redlines at 9000. Do the "newer" bikes rev higher? I also wanted to know if anyone is using or has used the aluminum intake manifolds, and if they work any better than stock.

grizld1

The '82 is no different from other 447 engines ('74 on). There are two possibilities re. your tach: either you're reading the redline as the end of the red band rather than the beginning, or you have a tach from another model on your bike.

Billet carb mounts work great. If you're using them on CV carbs you'll need more support; an L-bracket from the carb bridge to a breather box bolt works great--be sure to rubber damp it. VM carbs are light enough to run unsupported.

xsjohn · Last edited by xsjohn on April 26, 2007, 10:46 am; edited 1 time in total

Better watch the cylinder temps they can rise above head temps.... You may build a real powerhouse but the cylinders do not dissipate the heat well because of the small fins.... This I know for sure.... Bet it won't last......Buy a laser thermometer and check out the cylinder temp even on a stock one after everything gets good and hot on a long enough ride to let everything get good and hot... Running in the summer the cylinders will get really hot and there is no real way to cool them without building larger fins.......You can oil cool the heads and oil cool the lower end but the cylinders are stuck in the middle without much help from all the oil coolers and get hot-hot-hot....Catch 22.......Will put a real big pinch on the fun---Might be fun to ride at 80 degrees for a while or a mad dash to the north pole........Raise the compression above 9-1 and it just will get worse....Put bigger pistons in it and it will just get worse....Lots of people will argue with this I am sure....How many miles did they get out of it.....Not a good subject....Buy a bigger bike it will be much cheaper and last a whole bunch longer and be faster........Wip the little guy for a while and he will just die.....Might water cool the cylinders and put a bank of coolers on them....If longevity is not a consideration then I am just a butt head............ Laughing

John Underwood

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Here is a picture of my tach, and the cluster. Looks to be original with only 5k on it. I do have the 36mm VM's. Was thinking about getting the manifolds to lean them out alittle. Will it help?

jayel

xsjohn wrote "I am just a butt head............"
not changing your online name are you Bevis?

xs content, finally got around to greasing my point cam/advance shaft seemed to smooth out the timing advance when looking at it with the light both sides firing at F mark(s) at idle and right up to advance line above 2 grand, I love it when a plan comes together jayel

grizld1

INXS, similar gauges were used on a lot of machines. Your tach's not original.

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

Looks like it may have come from an xs750 or 850 maybe? I am glad to know. I have been running her up to 9 grand regular like. Probably not too good to run her ragged like that.

INXS650 · 650Rider Supporter Joined: Mar 22, 2007 Posts: 645

'Nother question. If the tach is not from an xs650, would that mean it wouldn't read correctly? Is it set up like the speedo? Change the height of the wheel, and the speedo would be off correct?

grizld1

Tach gearing was pretty well standardized because (unlike wheel diameter) it could be.

Shannon60

Hello all I am new to the site I have been racing the 650's for a number of years both dirt track and ice racing here in wisconsin I also have about 15 years building racing engines from go-karts to outlaw sprint cars my specialty is cylinder heads and porting. I am setting up shop at my home near Appleton, Wisconsin and i would graciously welcome any opportunity to provide my services, if you would have any questions please feel free to contact me. I am feeling out the ideas of making this a full time operation and would also appreciate any input that you may have.
Thank You

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