A question of two motors.

So, I've got my second xj750 motor ready to come out of its bent frame. Originally, I was planning on stripping this one down, but upon pulling the carbs off and finding them to be incredibly clean, I've gotten a bit hesitant.

So, I have two motors. One good frame. All the electrical stuff for 1.5 bikes.

How would you determine which motor to use, knowing little of the motors, but having them sitting on a bench? What would you look at to at least make an educated guess at which is going to be best?

What I do know:

The Seca motor(black and red painted one) needs a gasket set. It's mostly open, oil pan off, gears look good, shifts nicely. Broken intake boot screw stuck in the head, but that ought to be manageable. It's carbs are either wildly out of tune or there's something really sketchy, as theres a tremendous variance in butterfly positioning when you pull the throttle lever. They're not filthy, but obviously need a cleaning.

The Maxim motor - the one in the bent frame - has clearly been in an accident(as evidenced by a bent frame, forks, etc. The motor itself is intact, except for some damage where the ignition pickup cover connects. The cover can still be attached, and the damage could easily be covered with some metal putty to get the cover to seal again. It's carbs are immaculate, and set very close to each other. Either they've very recently been bench synced and not actually tuned to the engine, or things are very equal on the engine. This is what gave me pause, really. These carbs at least from outside inspection have been very well maintained.

I'll determine which set of carbs to use independently, as I'm rebuilding both sets. They seem to both be in good condition, though.

Related: how about bench testing motors? Anyone do this? I've got a full set of electronics on my worktable... Not sure about how to secure the motors though.

Thoughts?

 

Put two engines in one frame.

 

That takes levels of skill and insanity I (fortunately?) lack.

 

Compression test, use the best one.

 

I've never actually done this. Tester in place of spark plug, then just run the starter?

 

The engines have differnt mounting systems. The Maxim engine will slot into the Seca frame with some minor modifications to the rubber mounts (mainly just clearancing the steel collars on them).

Knowing that the weak points on the engine are internal, I'd go through the one that's already open. Rebuild the starter clutch, replace the alternator chain guide (even if it looks new), and maybe even undercut the 2nd gear dogs for a bit of insurance.

While you do that, put the Maxim engine into the Seca frame and get it running so you maybe can ride before the summer is over.

When the Seca engine is done, swap it back into the Seca frame and either sell the Maxim engine (video proof of it running will help. be sure to show the case numbers), or split the cases and take care of he weak points on it so you can have a good engine in reserve should the Seca engine have big trouble later on.

 

I've done a few sketches. It either ends up being a wide trike (side-by-side engines, mated via a central gearbox), or making a new case to build a V-8 out of the two. I thought about an inline coupling for the output shafts, but....so many gears and making cases, and.....no.

 

I'm riding my original Maxim now, so I'm not in a huge hurry to get the Seca going. The wrecked Maxim and dismantled Seca where new additions.

It'll take a while too, as getting the first bike into decent shape cost a lot, and I'm broke Doing things as I can afford em now.

But I'm thinking the Maxim engine was running pretty well, if the condition of the carbs is any indication. Will have to get a compression tester.

 

Compression Test XJ650

 

And happy day, compression testers are cheap!

As the Seca motor is open and dry, I'll hold off on that and test the Maxim motor before yanking it out of the frame. If the Maxim motor tests good, I'll just drop it right into the good frame. The second motor will be a little bit before I can get it oiled up again (need engine gasket kit), so I'll test it later. If good, I'll keep it as a spare motor, if not-so-good, I'll strip it for parts.

 

So, took a scenic ride around to Cambodian Tire, and bought the Super Style'n Equus Compression Tester for a whopping $35. Crammed a battery into where the battery box should be, connected the ground wire, got some oil for testing low readings, all ready to start my adventure into compression testing, when my phone's battery died. I want to take photos of the readings for posterity (if I write them down, I'll lose them. Take photos, and I have them forever), so now I have a 20 minute wait whist my phone gets juiced up. Did the first cylinder - not spectacular, 100psi/120psi after a bit of oil - but not unusably bad. No idea where the valves are as of yet, and that wasn't a huge jump in compression from the oil, so that's worth checking.

With that said, the motor sure turned over nice and smooth, no scary noises or labouring from the starter despite using an old battery that's been sitting on a shelf for quite a while. Good sign there, at least.

 

Off to a decent start anyways. The low compression just could be gummed up rings, or as you said, valves.

Sidenote, I'll be moving to Calgary this summer and will be able to lend a hand when I get there.

 

Ok, done. Who needs a battery box or electrical system anyways?

Final readings from the parts Maxim: 100/90/90/100. All increased substantially with the addition of oil, though (in retrospect obviously) the amount depended entirely on how much oil was added, so the final values with oil aren't really relevant. Pretty even across the board, though I'm disappointed at the 90's - was hoping for 100+.

That was pretty damn easy and fun, though. Glad I bought the tester.

Now I'm pretty twitchy - want to get a gasket kit so I can test the Seca's motor.

For this girl, I guess it's new rings at least (if it's better than the Seca's motor) or being torn down for parts. Rings would be an interesting adventure, as I've never tucked into a motor before.

 

Quite a few of us out here now. Where in Calgary are you moving to?

 

Not a clue yet. Will depend on when I can get out there and check places out, which depends on my July schedule, which I won't have till the 25th or so.

At least your compression numbers are consistent. Could just be gummed up rings from sitting for a while.

 

Bwahaha!

 

Yeah, it's definitely sat. The PO had it for a while, as he bought the two bikes intending to get one going, but then realized it was going to be a lot of time, effort, and learning and bailed on it. Who knows how long the guy before him had them. Both bikes where pretty filthy externally, and looked like they'd been sitting for some time.

I'm happy they're all pretty close together (at least I don't have a shot cylinder/piston in there), and that it sounds pretty good turning over. And as I noted above, the rest of it appeared well maintained with the carbs in particular being very clean.

What would you recommend with regards to the rings? Anything to try before cracking stuff open?

 

Two or three tablespoons of a 50/50 mix of automatic transmission fluid and acetone down each spark plug hole. Let it sit. That'll free up anything.

 

Will grab some tomorrow and give that a go.

 

Well, after a day of birthday stuff for the daughter, snuck out and fed the Maxim motor a 50/50 mix of acetone and ATF. Letting it sit overnight (and recharging the battery) and it'll be round 2 time tomorrow.

 

Give the ATF a week to work. The rings might free up in 24 hours, but longer is better.

 

Ok, not in any hurry there. I'll just retest next weekend.

 

Most times you'll also need to run it for a bit. The soak helps free up carbon deposits and sticky oil residue (from sitting), but the rings will need to be worked a bit for them to really free-up. This may be obvious, but do the startup and run outside; she'll be smokey until the penetrant burns off.

 

Won't be starting it anytime soon; no intake, exhaust, or electrical connected. Just trying to determine whether or not I'm going to mount it in the Seca frame or use it for parts.

Just hoping to get somewhat better compression numbers.

I'll run it via the starter for a bit before doing the test, though.

 

15 seconds at a time, with cooldown breaks and attention to the battery voltage.

 

Just realized I was reading the compression results wrong. I'm at 3500' ASL, this lends to a factor of 0.907. Thus, first read should have been roughly 110/100/100/110. I feel a bit better already.

 

Ok, after sitting a week with the ATF/Acetone mix, I ran the motor for two minutes on the starter, 15 seconds at a time with a full recharge on the battery after each run - also allowing for cooldown time as I just used my 2A maintainer to do the work. Did a visual check in the spark plug holes to ensure there's no excess oil in there, and to peek at the valves - all looked great.

Did another compression test - also recharging between cylinders, for reliability. Readings were 122, 118, 122, 122psi. Adjusted for 3500'asl, that's 134/130/134/134 - I couldn't be happier about that. Freaking fantastic.

 

The numbers may improve again once she's had some load on her while running.

 

It's good enough for this guy. Figure I'll check the valves, then drop this motor into the non-bent frame and go from there. If the other motor checks out too, I'll work well as a spare.

I'm still sketchy about the knocking on my Maxim - even if it's normal camshaft endplay - so having a spare handy makes me a bit more comfortable.