Xj 550-ish?

Hi all! Been following this page for a long time and got myself an '81 550 so I think.. the previous owner told me a put a 750 head onto the 550 block and what I want to know is if that's even possible and if so, how would I be able to tell without needing to take the head off (easy task I know, but I'm trying to save most of the heavy work for this winter when I have down time).
It runs a bit rough when hot and I'm thinking they didn't change coils when or if a larger head was installed which I could assume would be needing more V for spark? She's a franken-XJ that I'm trying to give all the love I possibly can to make her beautiful again without insulting puritans thank you guys and gals and others!

 

I would suggest starting here:
https://www.xjbikes.com/forums/threads/tech-topic-model-id-and-vins.14577/

Once you look at the VIN on the frame and the engine block you will know what they started life as. Then we can work on sorting out what happened after that.

 

he lied to you

 

Fair enough lol. I'm mechanically inclined enough to take care of most things and have a friend who's some type of Yamaha certified who helps me with more things but he directed me here to get more expert opinions from people who work on these every day. I'm just trying to figure out what exactly it is that I have

 

I found it to be a 550 block and frame. The head is what I'm wondering about is if it is even possible to interchange the two

 

Upload a pic of the heads?

 

it is not possible. The bore centres are tighter on the 550/600 in comparison to the 650/700/750/900 which are all same bore centres

 

I suppose that with enough JB Weld anything is possible, but it seems unlikely that there are 750 heads on a 550 block.

I can't really help identify which head it actually is, but I can't wait to find out. The suspense is killing me.

 

The bore centres are tighter spaced on the 550.
It’s simply impossible. The previous owner is an idiot.

 

Somebody out there will accept this challenge.

 

Yes, yes they are. 99.9% of the time.

The only thing that seems to differ is how much of an idiot they were. And that translates into the length of the Period of Atonement, where thou shall atone for the sins of the Previous Owner lest there be much weeping and gnashing of teeth.

 

I will post pictures when I get home and see if anyone can tell by looks maybe?

 

I like a challenge sometimes and these bikes are worth the weeping lol

 

By bore centers, you mean like the crank is longer on 650+?

 

Looks to me like a regular xj550 head

 

It also looks like a maxim converted to a seca

 

Oh Christmas I gotta rerun the VIN to see

 

I should look closer I guess…

if the vin is 5k5 it’s a maxim, and if it’s 4G0 it’s a seca..

It’s definitely got maxim forks

but a seca tank and engine?

I have a maxim I rebuilt with a seca tank

 

The only interchangeable motor heads and sleeves are the xj550, Yamaha radian 600, and fz600.
There might be more but those are what I know.

but the 650+ Xj’s are entirely different

 

That was confusing lol. But it is a 1981 xj550 maxim

 

So you have a maxim with a seca motor, and a stock one at that.

 

Is there anything worth improving from stock? Not necessarily performance wise, but any specific issues or things I should be aware of with a goofy swap like how you say?

 

Honestly the more stock the better, they’re identical besides jets in the carbs I believe.. as far as the motor goes

I’m also not sure what other surprises lie within that build..

 

Camshafts are also different in the Seca motors.....higher lift, probably more duration.

 

Where was it determined that the engine is out of a Seca model? The only way to determine that is to look at the machined pad, on the upper engine case, that has the original bike's ID code and last 6 of its serial number.

 

So I could assume coil size, plug gap, cap resistance, and some other things would be different from maxim to seca?

 

Would that be this?

 

No, that is the cylinder jugs size indication, 550 engines actually displace only 528cc's. You can read more about the various component (and engine) ID at:

http://www.xj4ever.com/catalog/frame-engine-id.pdf

All of those items are the same between all XJ550 models.

 

Oh ok awesome thank you very much! I'll get that tomorrow and continue the quest from there

 

I definitely assumed, but only the secas came with the the factory black painted motor right?

 

no like left to right the spacing is wider on the 750 from center of bore to center of bore

 

Doesn't the 550 have a shorter stroke than the 600's?

 

it does

550
528.5 cc
57mm bore x 51.8mm stroke

600
598.5 cc
58.5mm bore x 55.7mm stroke