Do I have a XJ750 OR XJ700

Hello all, new to the forum and my first XJ bike, but I'm not sure what I have. Maybe someone here can help. So I was looking for a cheap cafe racer project and I picked up this bike for $600. It was listed as a 1985 Yamaha maxim 750 cafe racer project. The deal also came with a parts bike, a 1981 Yamaha seca 750, both bikes look like they have pretty much the same engine except the gear shifter setup on the 81 secs 750 is a little different and it looks like it has a cooler up front, maybe an oil cooler. The 85 doesn't have that. Also the title for the 85 maxim 750 cafe racer doesn't specify engine size. I'm really trying to figure out if its a 700 or 750.I will post a few pics. I will need to either rebuild the engine or buy a used engine in good condition because this ones toast I think. I tried starting the bike with a brand new fully charged battery and new plugs with starter fluid and nothing. I'm getting spark on all cylinders. So I checked compression today and got between 70 - 90 psi across all 4, which is really low, I also checked compression on the parts bike, the seca 750 and and got something like 40-90-80-60. So swapping engines is out of the question. I need a rebuild or a running motor, but what do I have here? A 750 or 700? Pics to follow, thanks in advance for your help. Ivan .

 

The displacement is cast between exhaust outlet 3 and 4.

also the serial number stamped on the engine will tell you

 

Dan, thanks a million sir! It's a 700.

 

I would check valve clearances first, as too tight can cause loss of compression. Also, compression should be checked with throttle fully open, alternatively carbs off. Finally, piston rings can stick if an engine has sat for a long time, causing low compression. This usually sort itself after the engine has run for a few hours. What I try to say is that unless you know there is something wrong with the engine internals, I personally would not rip into it at this stage.

 

As long as we're talking 1985 and air-cooled Maxim, it is a 700.
There is a long held fallacy that since Canada had a 750X we also had a 750N.
All the air-cooled Maxim's from 85 and 86 (and also 87 in Canada) are 700cc's.

 

Is it possible to drop the xj750 motor from the 81 Seca into the 85 maxim 700 frame? The mounting bolts/positions looks the same.

 

yes all the same

 

Thanks!