Swapping motors: XJ650 to XJ750

Turns out my Maxim 650 motor is on it's way out. It would cost me an arm and a leg to re-bore and get new pistons and so on. Would an XJ750 motor bolt right into the frame and be ready to run? Any mods needed for the carbs, etc?

 

Just out of interest, What were the compressions dry & wet?

And did you do it with engine warm, all plugs removed (and TCI unplugged!), full battery, crank till needle stops rising etc. How much oil for wet tests? (silly questions maybe, but I've heard of experienced people missing the simple stuff plenty times before!)

Even if it is "dead" you may not need a rebore, if the bores are in-spec then you could glaze bust them & fit new rings.

I believe the 750 motor bolts straight in yes, ecologito had/has a 750 engined 650. You would at the very least need to re-jet the carbs to suit the 750 (or it'd run lean and blow), fit the correct TCI unit (different advance curves, retarded ignition = overheating & flat performance) and possibly change the exhausts too although I'm not sure on that.

Apart from those 3 things I think it'd be good to go then, anyone else feel free to point out the glaring omission I'm bound to have made! :lol:

 

The mechanic I took it to (who also happens to be a friend of mine) Tested it about an hour after cool down. Dry was 170/120/170/90, wet was 170/160/170/100. That's about all I know.

 

I wouldn't trust those comp numbers as far as I could throw the gauge! (I'd throw it about 8 feet into the bin lol) The 170 are WAY high (IIRC maximum was 150 but not 100% on that, and normal would be around 130)

What made you take it for a comp test in the first place, using oil?

 

Like I said, my mechanic did most of it. I had no idea what he was doing. I wanted him to check things cause It was making excessive noise. I guess he used the oil to check the valves?

 

I guess the fact of the matter is not that the gauge read 170, but that the measurements were so far off from each other.

 

Not really, since the 170s show the gauge to likely be faulty, so I wouldn't trust it on the differences between cylinders either. in a "wet test" the oil (no more than 2 teaspoons worth) is used to check if the rings are sealing well or not, but what I meant by "using oil?" was if the engine was using any oil/smoking.

What sort of noise is it making, have you a video at all? valves in spec?

Not to say don't swap the engine, I'm just wondering to be honest if it really is anything major or not. :?

 

Gotcha. It doesn't burn oil. it does leak it though from the valve covers. No video. It just sounds like EVERY valve is chattering. More than likely need a valve job too. Mechanic also noted that the head gasket may be blown. I think that's why we tested compression. Again, I am a complete idiot when it comes to motor internals, so I could be very wrong for the reason.

 

Right, yes if the head gasket was blown a compression test can sometimes pick it up (though not always/all modes of failure).

I wonder from your description if it could be a loose camchain (failed/stuck auto tensioner) or dodgy chain guide. Or even out of whack valves, the service intervals sadly tend to get ignored... Did the noise just start one day, or was it progressive?

 

That was some of the options that we were throwing around too. The only thing that caused us to lean towards a rebuild was that one extremely low compression cylinder during the wet test. The wet test we ran got them all uniform (except the one)

 

When a "wet test" causes little or no change in the compression from a particular cylinder, it generally indicates a valve rather than ring issue.

Were the valve clearances put in spec before testing compression? If not, you're coming at it backwards...

 

They were not. It was just a quick 10 minute test. I suppose I should attack that first then.

 

Saying the gauge IS at least consistent if not completely accurate, the one that was 90 then 100 doesn't suggest bore wear to me, either valves (out of spec or burnt from being run out of spec too long) or head gasket

The one that would concern me was the 120 then 160 cylinder, 40psi is a big jump if the gauge was being true & that would suggest either bore or ring wear provided he only used 1-2 teaspoon of oil, too much oil would throw the figures up as well since it would fill up a lot of the combustion chamber.

Personally I'd at least try another set of compression tests with another gauge before condemning the motor, and I'd *definitely* check the valve clearances too (plus you can check camchain tension and guide condition to a degree with the cam cover off as well). EDIT-: Fitz beat me to it haha

 

The bike has an auto tensioner from what I'm told. And yeah, only a little bit was used (half a squirt from an oil can) I will do some minor work and see what I can do before I get into the bigger stuff!

 

Yep, hence

They can potentially get stuck or the ratchet fail, All we're doing is saying go at her methodically & make sure you know what's really going on with the engine before assuming it's worn out. You might save a lot of money this way, There's a member on here with 122,000 miles on a 550 that's never been stripped apart from to check/set valve clearances apart from that it's just had regular servicing, no rebores, no new rings or anything even a clutch. 8)

 

You need a competently performed compression test, with decent equipment.

Then, IF the compression test shows a problem it needs a leak down test to pinpoint the cause.

After that you'll have the information to make an informed decision.