650 to 750 engine swap question

I can hardly believe it but I've just finished the engine swap. With temps in the low 70s today it was perfect for a test ride. It started almost immediately, idled ok (haven't set mixture or synced the carbs yet), and ran great off idle. Geez, I'd forgotten how pokey that poor tired old 650 was (it had top end problems) until I cracked the throttle a little on this 750. Interstate speeds won't be a problem now.

Of course, things are only perfect in fairy tales.

Two questions: First: I don't know if this is normal for a 750 (from a 1982 XJ750J) but there is a noticeable whine in the engine. It kind of sounds like a supercharger on a big diesel. It goes up and down with engine RPMs. It's loudest when accelerating but not terribly loud. It starts the whine just as the clutch grabs. Is this normal?

Second: The oil sensor light stays on. I know there is enough oil in the engine so maybe I have a bad sensor. I looked at the part numbers for it on a 650 and a 750 and they are the same except for the first three characters (4H7 for the 650 and 5G2 for the 750) which I think only identify the bike model that the part goes on. Are they the same part?

Anyway, I'm going to drain the oil again so I'll put my old 650 sensor on the 750 and see if that fixes it.

By the way, I wouldn't have dreamed of doing this job if it wasn't for the expert help available on the forum. The neatest thing of all is that I actually enjoyed doing the work.

 

All XJ650 Maxim, XJ650 Midnight Maxim, and XJ650 Seca (non-turbo) oil pressure sending units are the same as the one for a 1983 XJ750 Maxim (that one is the 4H7 part number), while the 5G2 part number covers: all XJ750 Seca models, 650 Turbo models, and 1982 XJ750 Maxim. I don't know what the physically differences between the two units are (might be something as simple as a different length of wire leads between the two units) or it may be something significant (pressure trigger point or size/shape/fit, etc.). I do know that the 4H7 (XJ650 Maxim) version is about twice the price of the 5G2 version, so I suspect that difference is less than trivial........

Come to think about it, it seems that there is a different sending unit depending on whether the bike has an onboard computer or not, which makes me think that the output voltage from the sensor, when triggered, is different. I bet Robert would know better, but I'd guess that the output voltage from a computer-monitor sensor is quite a bit smaller (voltage level) than the one for a non-computer version.

 

I'll take a peek at mine this weekend, I'll get back to you on this one.

 

I think Alive has a video on you tube of his 900, and it has a whine that I thought was a turbo, but he says there isn't a suprise like that hidden. If you search for it, you could hear it there and compare to yours.

My 550 has a slight whine, but only when it's running.

Jon

 

You went from a non YICS engine to a YICS equipped engine. That's where the whine comes from.

650 sensor and 750 sensor work opposite to each other.

 

My XJ650K Maxim has a whine. I think it sounds cool. My local mechanic cut his teeth on these bikes, and LOVES hearing that whine. He said one time that he wishes new bikes could do that. He also said that if you stop hearing it, you got problems. Unless you can diagnose it as an actual clutch problem, I wouldn't worry about it, enjoy it.

dave

 

Run 20/50 Oil. If what you are hearing is noticeably softened in it's "Whine" ... the Higher Viscosity Oil has softened both the Valve Train Noise and that of the infamous Yamaha Shaft Drive Middle Gear "Whine" ... which at times, can be so loud as to almost drown-out the exhaust note.

You should be running a Racing Oil in that 750 Engine, anyway.
The Piston Skirts are Aluminum, the Camshafts Bearings are naked Aluminum Alloy without benefit of Bearing Half-shells and the Cam Lobes are always in contact with a hardened Shim that is Metal-on-Metal ... all the time the Engine is running.

Combined with the fact that the bike is dependent on a Middle Gear to change rotational forces from Horizontal to Vertical ... the Gears making the transition ... and "Whine" ... need all the Lubrication that good money can buy!

Brand of your choice.
20W/50 High Performance Oil

 

The middle gear whine is normal. She sings better when in the upper RPM's
you'll grow to expect it and lust for it's sound.

Damn, I miss these bikes.

 

That 750 Engine is noisy because your practically sitting onn the Head.

You hear the Valves opening and closing.
You hear the Power stroke happening.
You hear the Cams rotating
You hear the middle gear whining ...

and, ... if you get carried away ...

You hear a Whelp and see some flashing strobes in your mirror.

Tell him you are road testing the bike for unusual noises above 65 Mph.

Let him ride it ... and you follow him in the Cruiser.

You'll get a warning and a suggestion to up the viscosity of your oil to 20W/50 !!!

 

Is this a personal experience you've had?

 

Thanks, guys, for the replies.

Heck, now I feel proud of that whine. (it is a pretty cool sound)

I am using Yamalube 10-30, but at your suggestion, I will up it to 20-50. What brands do you recommend? Should I stick with Yamalube? I have a slight fear of switching oils due to an old experience with a BSA and the dissolving of the clutch discs.

Rick, you're right about the position of your head over the engine when riding a 650. The sound comes straight up under the helmet and like you said, you can hear every little tick, click, whirr, pop, and whine.

Another note: I swapped the oil sensor from my 650 to the 750 and now the oil light works right.

 

I was actually thinking of doing a similar swap, take the 650 out of my 81 XJ650H Maxim and put in a 750. There's nothing drastically wrong with the 650 but it's got almost 90,000kms (54,000 miles) on it and is pretty tired. Definitely doesn't have the punch it did 20,000 miles ago.
Maybe you can give me an idea of what you thought the hardest part of the doing the swap was.
thx,
baz

 

Hardest part: lifting the engines into and out of the frames!

Shifter covers and mechanisms are different.

No mechanical tach drive provision on a 750 enginbe, have to go to an electronic tach....which means either making a mini-hanress, or stealing a harness off an '82-4 XJ650.

Oil pressure sending unit needs to be retained off of the 650.

 

Does the 750 engine fit right into the 650 frame mounting points?
And do you need the whole electric tach gauge and harness or does a 750 harness somehow fit onto a 650 gauge?
thx,
baz

 

Chacal is right. The hardest part was removing and mounting the engines. (mounting points are identical) It wasn't as bad as I thought it would be, though. I used an electric hoist and did all by myself. Having the right tools sure makes things a lot easier.

The other stuff he said is right on, too. I would add swapping the ignition pickup assembly as the one on the 750 had a slightly different hookup.

Wiring the electronic tach was fairly simple. I just had to add one wire from the ignition coil to connectors behind the headlight. All the other needed attachment points are already in there; +12v, the ignition signal, and the two wires for the instrument light. I made a mini harness like chacal suggested so I could leave the tach plug stock. I can take some photos and give you the details if you like.

And another real important point: the 750 carbs are jetted differently from the 650 (except for the pilot fuel jet which stays at #40). The needle, main fuel, pilot and main air jets are different on 750 carbs. -- Special thanks to chacal for having all the necessary items in stock --

One other note: if you need to remove your airbox for any reason - do it when the engines out of the frame because you can't get that sucker out with the engine in place.

 

hey gitbox,
That massive storm out of the US Midwest is hitting up hard with something like 2 feet of snow so not much to do but tinker with the XJs or yack on the computer about them.
Anyway, thanks for all that good info and yeah, if it's not a hassle, photos would be great. I've already got a big winter project with the XJ900 but I got to have something to do after that's finished, right? The 900 was a basketcase I picked up at a wrecker's for next to nothing and I've been slowly getting it into shape. Not a factory perfect resto or anything. I'm just trying to see if I can get the thing all together and running well before I really go to town on the cosmetics.
As for doing an engine switch on the 81 XJ650H Maxim, I found an 83 XJ750 engine for $450. The place selling it says it's got 32,000kms on it (just over 19,000 miles) so it should be in pretty good shape. Before buying it I plan to take the valve cover off and check the cams, see if there's an pitting, scoring etc from oil starvation or lack of oil changes.
As for the carbs, the XJ650 has a K&N free-flow air filter and a 4-into-1 MAC exhaust system. That combination allows a lot more air into the carbs, so I upped the stock jetting from 110 main jets to 118. The bike runs much better like that.
But I guess I have to up the pilot jets as well from #40 to #41 if I switch to a 750 engine, right?
The Sudco website that sells Hitachi carb parts only lists 40 and 41 for pilot jets. Anything else I should plan on doing carb wise? I figure I might as well prepare now even though the project probably won't begin until the spring.
thx,
baz

 

The size of the MAIN FUEL JET determines how much Raw Fuel is **Available** to be drawn-up into the Intake Stream.

To actually HAVE that quantity be drawn-up ... it's also important to INCREASE the Size of the MAIN >AIR< JET too.

The Main Air Jet controls the Amt of Air surrounding the Emulsion Tube and facilitates the Flow of Upward Air drawing-in fuel. A slightly larger Main AIR Jet will allow the Pressure variant to allow more Main FUEL Jet metered Fuel to rise up the center of the Emulsion Tube thus making the Mixture slightly more Rich.

 

VERY good point Rick!

You know, I've always wondered about that also....everyone has a formula for increasing the size of the fuel jets, but totally ignores the air jets. I suppose they feel that the increased airflow created by pods, exhaust mods, etc. is enough "extra air", and don't consider how that air actually gets into the fuel mixture......

Although it seems counterintuitive that a LARGER main air jet would richen the mixture......

So what guidelines (if any) are there for incrementing air jets relative to fuel jets?

 

A set of guidelines would be great since, like everyone else, I used trial and error with a few different main jet sizes and yes, totally ignored the air jet but some ballpark figures would be real handy.

 

Search me!

I just want to impart some "Wisdom of the Ages" on the younger guys who are all swapping-over to Pods.

I imagine that it's almost Virgin Territory. Trial and many errors with your Number Drill Set and a few Main AIR Jets to ream.

I'm pretty sure that there's some Value that will allow the Main Fuel Source to get swept-up into the Intake Flow better than not altering the AIR Jet size at all.