XJ750RK Motor Rebuild Recommendations

Greetings Everyone

I've just pulled the motor from my SECA and am looking for a shop with Xj motor experience to do a complete rebuild. I live in Atlanta but am willing to travel to deliver and pick it up. Any recommendations? Much appreciated and regards to all.

 

It's not finding the shop that's going to be the biggest challenge;

it's the shop finding the parts.

How many miles are on the bike that you feel it needs a complete "rebuild?" What compression numbers are you getting, and are the valves in spec and all that good stuff?

These motors are so basically "bulletproof" and the bikes weren't popular as racebikes, so things like first or second-oversized pistons, rings, etc., are kind of hard to come by. And usually horribly expensive if and when you do find them.

You might want to research this idea a bit more; at least "diagnose" the motor so you know how far you need to go.

 

Thanks and I 'preciate ya bigfitz, I really do. Yes, you're right the motor is bullet-proof so here is some context for you.

The motor has 50,000 miles and the starter clutch is going south, and it's developed a small head gasket leak between the #3 & #4 cylinders in the front. I bought the bike in the late 80"s with less than 5,000 miles. It's really never been a day rider, not here in Atlanta, no fun.

My passion is camping and sport touring where I pack the soft luggage, throw it in the saddle bags, bungee my tent, sleeping bag, and camp chair to the passenger seat, up against the trunk, and head out to the north Georgia, Tennessee, Western North Carolina, Virginia, and West Virginia and "camp and play" in the mountains. Between May and October I'm gone anywhere from four to eleven days at at time. I can get out in some pretty remote spots.

Consistent with where and how I ride, in the early 90's I had the bike tricked out with a professional frame alignment and suspension set-up to minimize tortional forces on the frame/suspension. It's so sweet in the mountains; no wonky brakes, wonky suspension, and the front and rear ends arguing with each other about which wheel has the right track when entering the turns under hard trail braking, mid-apex adjustments if needed, or hard acceleration to bring the bike up out of the turn. Surgical carving of turns at speed is what I am passionate about.

One nice thing about the Yamaha Tour Pack for the 81-83 750 SECAs is that there is plenty of room in the hollow front fairing behind the headlight, and in front of and either side of the lockable fairing soft pockets. I carry a spare set of: pick-ups, regulator, TCI unit, a full set of relays, throttle cable, clutch cable, and clutch lever. I also carry a shop manual, tools and a multi-meter. Fortunate thing, in 24 years the bike has never let me down. (I also have another complete set of painted Tour Pack body work in case I have a bad wreck.)

But now, since I've taken the effort to remove the motor for the starter clutch and head gasket, I might as well get the motor gone through and set up for another 50,000 miles and 24 years of sheer pleasure. You can't find a 750 that is as light with hard and soft luggage and the storage and wind protection that the Tour Pack offers, not to mention the motor.

So, what am I going to do to the motor? Everything, head checked for true, milled if needed. Valve stem to valve guide clearance checked, and valve face and valve seat checked. Maybe new valve guides and valves, and valve seats reground. Camshaft to cam bearings clearances checked, as well as cam wear and lift. Maybe not new cams, because they can be replaced with the motor in the bike. Cylinders miked and piston skirt to cylinder wall clearance checked. Either a hone job or bore job depending. Perhaps 10 over pistons. New rings. Crank checked for true and check for in-spec main and rod bearing clearances. If not, crank regrinding and 10 over bearings, or new crank. New crank end bearings. Transmission gears and bearings checked/replaced. Anything that can not be serviced with the motor in the bike.

Also; Starter gone through. Armature checked for true, turned and mica recut. New brushes and bearings.

And, while they are off the motor, Carbs completely gone through; needles and jets checked, floats, float needles and seats checked, slides amd diaphragms checked, new throttle shaft seals, etc.

My goal is to put a new, tight motor back in the bike that will be hopefully trouble free for another 24 years.

Regards and thanks for all you do for the XJ bike website.

 

All well and good.

Except the following parts are either unobtainable or BEAUCOUP expensive:

-oversize pistons and rings-- maybe, good luck. If/when you do locate any, they won't be cheap.
-main bearings-- forget about it. Too many different sizes and no demand means they just aren't out there.
-new crank-- forget about it. "Newer used," maybe. But then you've got the bearing shell issue.

I would not rip into the BOTTOM end, "just because." Top end, sure. Cylinders, sure; bring $$$.

The cams run in the head; there are no camshaft bearings. If you get excessive wear there, the head gets replaced, and probably the cams.

The starter clutch may or may not be in need of repair; simply running the wrong oil can make you think it's "going bad." If the sprag clutch body is cracked, then it will need replacing; otherwise, "wearing out" is rare.

What you DO want to replace is the primary chain guide; the one that's known to disintegrate.

Good luck with this; just don't tear anything too far apart until you KNOW you have the pieces required to put it back together. And be sure you're working from a factory manual, so you can properly "decode" the main bearing size codes for your motor. I don't think the Haynes covers it.

 

This is just me, but if I were you, and I were willing to go that far with the motor, I'd PM chacal and see what he recommends, since he is THE parts guy, and he also lives down that way.

 

Greetings again and yes, Thank your to Bigfitz and tskazs

To tskaz, yes, Chacal was the first place I went. He did not have a recommendation for a motor rebuilder, but did say he had the "parts" needed. Of course, knowing this is a 29 year old motorcycle I took that as "upon advisement."

Thanks to you bigfitz on the advice on the primary chain guide. Again, I appreciate you and this website and everyone who is passionate about riding. With regards to the crank, rebuilding automotive engines I know there are people out there who will take a crank, and if the bearing surfaces are damaged, they will put them in a jig and weld them oversize and then grind and polish them down to spec.

However, I know that none of the manufacturers, Honda, Yamaha, Suzukiu, Kawasaki, etc., gin up all of their own parts. For example, the motor engineers know who the subcontractor; bearing makers are, who the piston casters/forgers are, who the bronze "valve guide" makers are, who the "valve makers" are, who the valve seat makers are, who the who the cast iron cylinder sleeve makers are, who the bearings and seal makers are, who the "O" ring makers are, etc. Because of economics of mass production, there are economic benefits of working with the subcontractors. The big four engineers work with these folks and contract to buy what's already been "engineered" and in production, and basically design their work around a subset of "standard" parts.

Yamaha, etc., may engineer up the motor case castings and the aluminum cylinder fin castings based on what these subcontractors already have in production for bearings, sleeve liners, pistons, etc,. but I know they do not put in the engineering time, machine set up time, tool and die work for castings time, etc,. to make "one off" parts for a particular run of motorcycles or engines. Not the big four. BiMota and others exotic builders might, but not your big four.

So the challenge is, to find a parts guy who can cross-reference the crank, or the piston, or the valve guide, or the valves, or the bearing sizes with some other motor, That's the challenge, not that the parts are "unavailable."

That's my story and I'm stickin' to it. I'll keep you'se guys posted on my adventure.

Regards

 

You have a point when it comes to rings, valve seats, and cylinder liners, but the XJ crank has got to be application-specific.

Speaking of crank mods, and unlimited budgets - -
Check out "worldsfastestXJ550" , search thru his posts.

http://www.xjbikes.com/Forums/viewtopic ... crank.html

For the cost of going thru your motor, you could get a 900 engine shipped to you. 98 HP and more torque.

 

It is. And it has a very entertaining method of numerically and color-coding the crank journals and bearing shells required. The rod journal and main journal numbers are stamped into the LH crank web. The main bearing codes are repeated on the top rear of the upper crankcase casting. "Decoding" them requires a factory manual, to turn the numbers into the "color" bearing shell required. (They don't bother to give you the numbers behind the colors.)

All those subcontractors and suppliers discussed DID exist, at least in 1980. In Japan. Good luck with that. I wouldn't bust into the bottom end of a solid motor, just to see how much money you can spend.

If you're gonna go this all-out, pick up a low mileage 900 mill and do the top end.

Don't get me wrong, I applaud your efforts. I myself gave up a few such ideas when faced with the reality of the situation (and in possession of a low-mileage XJ.) Quite simply put, a transplant from a low-mileage donor makes a lot more sense, dollar-wise, than trying to do a bottom-end rebuild. The decision is yours, but you may dig yourself into a money pit you can't spend your way out of; and end up doing a transplant anyway.

 

I'm about to assemble my 'spare' engine on the 750, and realistically it could have been left alone.
Slight ring land wear on #2 and #3 pistons, 2 main bearings with a slight mark and thats it.

The best thing about striping down a low milage engine is you can replace all the 30 year old rubber bits and the 3 chains.

I intend to do a lot of touring on my bike and its nice to have a 'known' engine.
Starting with a known engine and finding a big bore set of pistons for only $200 it still will cost over $1,000 for me to freshen up the motor without labor. I will still keep my old pistons in storage after finding out how hard it is to get replacements.

 

Greetings and Thanks 'Time to Ride"

With respect to bigfitz, I failed to acknowledge his point that in fact these machines were not raced . . . Reason was because of the significant loss of efficiency of a shaft drive. Counter shaft and rear sprocket drives were so much more efficient at transferring engine power to the rear wheel that no amount of engine modifications could overcome that. Anyone remember the BMW race series? Shaft drive verses . . . itself. (but not against anything else.) Why was that? Duh . . .

With respect to putting a 900 in the bike. No thanks. For me, with the frame and suspension I have, and where I ride, the 750 has all the power I need. I don't have pods on it or a 4 into one. All stock. Low maintenance, thumb the starter, occasional valve clearance check/adjustment, and carb synch is about right for me. Plenty of power for where and how I ride.

It's all relational: when I play with bigger bores in the mountains, if it's a rare, extraordinaterly long straightaway, then they main gain distance on me, maybe pull ahead. However, typically, when I'm behind some, and we all are at exceptional speed, and they start braking at their point for an upcoming turn, I'm still at speed, catching up, may be passing, and then late, late heavy trail braking to drop down, either just behind, or among them, or in front, and then typically they can't keep with me through the turn.

While I'm passing before the turn the big cc's are still upright and when they begin to brake hard and bank over into the entrance of the turn, they begin to encounter and wrestle with un-engineered torsional frame flex, where "arguments" quickly arise and quickly get transmitted through the forks (front wheel) and bike frame (rear wheel) as to which tire is on the right line. This undermines the confidence of the rider who then reacts by trying to "moderate the argument" by rolling off throttle, panic braking, and consequently taking non-assymetric and wobbly lines through the turns.

Also, you can tell they have the wrong sag settings for the weight of the bike and rider at speed under heavy braking, and non-complimentary compression and rebound dampening in the front forks where the front suspension begins packing, and not matched to the rear suspension so that the front and rear do not work in tandem to settle the chassis under stress. The bike is all over the place.

How do I know? I've just caught them at the entrance to the turn and I'm not in a position to pass. I watch the bikes wobble and squabble while I brake down hard and smooth and close up right on their rear. While I'm on my line, they are moving left and right of the optimum line throughout the turn.

Also, if I'm not in front, but find myself among others in a turn, I usually have to brake in the middle of the turn to not run into the bike in front of me, for all the same reasons as above. Wobble, erratic lines, and pre or post apex off throttle slowing. I try to pick a way around and I'm heavy on the gas, uprighting the bike, passing on the exit of the turn and pulling away.

I'm also careful if in front to not disappear on those behind me. How does this sport tourer with fairing and luggage go so fast? It's all in the suspension. Ever hear the expression "Ride your own bike." I had a bad experience where one rider tried to ride my bike rather than the one they were on. I spent two days sitting beside him in the hospital. He came out OK but his bike was toast. I like riding fast but I try to be conscious of my fellow riders if they are behind me.

As I said, it's all relational: the number of long straightaways to the number of short straightaways. Most of the mountain roads in north Georgia, eastern Tennessee, western Carolina, Virginia, and west Virginia work in my favor. For an '83 standard 750 with fairing and luggage . . .Bye, . . .I'm gone . . .

Thanks for the advice, but I'll pass on the 900 motor.

 

You missed my point in regard to "not raced."

A bike (like the Suzuki GSXR series, or the venerable RD 350, or the Yamaha TT/XT/SR500s) that was popular as a racebike generates a demand for "serious" motor parts. Things like oversized pistons, rings, big-bore kits, strengthened bottom ends, etc. The aftermarket responds to support the racers, because there's a market for the parts.

A bike that was never anything more than a semi-popular 3-year street bike is simply not going to generate a lot of demand for "deep" engine parts, plain and simple. And that's exactly the situation the XJ was and is in. Now that thirty years has gone by, it hasn't gotten any easier.

Like I said, just be sure you can actually get what you need before you do something you can't recover from. Money-wise, a low mileage 750 Seca motor would probably make more sense than attempting a bottom-end rebuild.

We've seen way too many folks appear here with overly ambitious plans who soon realize they've gone too far and end up with a dead project. Don't pull it down until you know you can get the exact parts you'll need.

 

Now that's dedication !!

 

this takes B S to a whole new level

 

I don't think he meant he's carrying the bodywork around with him...

 

Greetings Everyone

Yes, bigfitz, I have the factory shop manual and until you mentioned the multi-color crank bearing set, I had not looked closely at the engine chapter. Upon review, in light of your very helpful comments, I am astonished that Yamaha could not get someone to cast a crank and grind four consisten within-tolerance main bearing circumferences. My conclusion is that they had the ability to align bore the cases consistently, but to have to use four main bearing sizes to ensure tolerances, I'm at a loss for understanding that. But it is what it is.

Also not to deny your other points because it is obvious that you have much more experience with XJ motors than I, but the factory manual lists pistons in four .010 oversize increments. I'm hopeful that they may still be available. Although, You may be right.

However, all that being said bifgfitz, I had not considered it until I started this post but I may go with a low milage used engine. As a courtesy and out of respect and appreciation I'll keep y'all posted.

Oh, by the way, to "BS," in an earlier life I had a 1968 Harley Davidson XLCH (XL "Custom Hot) sportster for the street with a Ceriani Front Forks and HD rare gas shocks, and I campaigned a Bridgestone 175cc Road Racer in the amateur class ('68), and campaigned a TT Sportster ('69-72), and a Bultaco 250 MX ('69-72) in the expert classes. With respect to the TT sportster, Rod Pink of Reggie Pink Harley Davidson in White Plains, New York was my mentor.

Regards to all.