The XJ750i is back

Hi folks,

I've been away for a while (a year or so?). Basically, I got a second bike (an '83 GPz 750) and finally got it running well enough that I could do some more invasive work to the Seca. I had been frustrated in some of the tuning work on the fuel injection, and decided that a better intake manifold might help things, so I started off making a new one (and a new fuel rail - again). I was mostly done with that when I got a new old car, and that, since I could always ride the GPz, put the Seca project well onto the back burner.

Anyway, recently I got caught up enough on projects that I was able to get back to the Seca. Although the new manifold isn't night and day from the old one, it is better, and did allow me to make significant improvements versus the tuning issues (mainly idle and low speed behavior) that had been bugging me. The new setup is also, IMO, a more elegant fit - allowing straighter airflow from the air box to the intake ports, and leaving more room below the throttle body.

I still have some fine tuning to do, but the bike is quite ride-able now, and has been getting all my riding time over the past week or so. Cold start, warm start, and closed loop idle all are finally working quite well.

I posted a few shots of the recent work in my gallery.

Full Bike
Manifold in-work
Manifold installed


Cheers,
Paul

 

good to see you mate,

im a big fan of your awesome work. post every picture you have of it please!

 

Good to see you still working on improving the system. Pics are ALWAYS appreciated (as is the occasional update).
That manifold looks SO nice!

 

Very nice, love to see ingenuity at work.

 

This is another project that should be of interest to the Motorcycle Press.
This should be a Multi-Part Article with heavy illustrations and a back-story why you embarked on Custom Mod's.

There are some "Prize-winning" feature and projects, like this, that deserve to reach the Motorcycle-minded; not just the regular readers and contributors to our Forum.

The Manifold looks GREAT.

I hope you get everything work-out and running great.

 

Thanks for the appreciative comments.

I posted a couple of additional pictures of the work on my gallery.

The main differences from the previous setup are:

1. I got rid of the carb holders and built the manifold to bolt straight up to the head.
2. The old manifold had two 90 degree turns in the outside pipes but only two 45 degree turns in the inside pipes. It also had much longer pipes outside than inside. The new setup has much straighter and more even length pipes coming together into one main passage.
3. The injectors are aimed as much as possible into the intake ports. This required:
4. A new fuel rail which was inherently narrower and therefore allowed me to move the pressure regulator back to the end, but was REALLY a pain to build because getting of having to get all the angled injector ports lined up correctly without a good way to hold them in place during soldering.

Since the new setup is much more straight, it resulted in the intake side of the throttle body moving up about 1.5" and no-longer lining up well with the air box. The previous air box port wasn't in a perfect location anyway, so this gave me the excuse to pull the air box an make a new front for it where the port lines up almost directly with the intake of the throttle body.

Without changing any code or tune, the new manifold and fuel rail by themselves definitely improved the idle and mid-band response. Some new code helped more with idle control, and I then did some "street dyno" data logging with a wide band oxygen sensor, which helped me dial in most of the map pretty well. I didn't set things up originally for a wide band, so there were a lot of wires hanging off the bike during these rides. I've since removed it and reconnected the narrow band sensor. I still plan to do a few more sessions with the wide band; I may just tack it on again, or I may see whether I can permanently install it in place of the narrow band. For now, though, I'm just going to ride it for a while.

Cheers,
Paul

 

Welcome back!
Great work!

 

You know as I set here it makes me wonder just why it is so difficult putting a single carb on the XJ750 Seca because I had a 4 cylinder Farmall Cub tractor and it had a single updraft carb on it that was very economical to run and the engine would really turn out some power compared to others. Now, don't mis-understand me, I am talking about old technology vs new technology and I know that the way our guys are attempting this is extremely difficult but keeping in mind the simplicity of the old tractors and just how reliable they were working day after day in all kinds of weather and I will admit sometimes in the very cold winter I would enrichen the screw on the carb to give a richer fuel mixture it still ran great at -20 degrees! (RELIABILITY COMES FIRST FOR ME). Keep up the good work guys, I admire your determination to no end. If this could be achieved through this club I also agree that it could be a change that bikes of other breeds could also use to update the Vintage one's to good reliable machines. I also have to tip my hat to the expertise that I now see many in this group have. Heck, I am beginning to question if many of the members in here couldn't teach the engineers at GM, Ford, and Chrysler a few things, but then as I have learned over the years, working on cars SUCKS! Motorcycles are Inspiring to say the least. Watch the video, Worlds Fastest Indian and see what even an old man did! 81'XJ750 Seca in process of being brought back to life!!!! Long live the Vintage with HEART! GRUNT 007

 

I don't really know carburetors well enough to say for sure, but my guesses would be:

A tractor seems to me to be an engine that has a more limited power band and rev range than a motorcycle. On cars, two or four barrel carburetors are used so that you can have small bores for low RPM ranges and bigger bores for high RPM / high power ranges. Harleys get by with a single carb for two cylinders... are they more tractor-ish in their behavior?

Even on cars, high performance vehicles use individual throttle bodies for each cylinder, which allows but also adds the complexity of individual trimming. I think this is supposed to also provide quicker throttle response and better high end power, but, again, I'm not sure.

I went with a single throttle body because it was a relatively easy retrofit, provided a single point at which to measure manifold pressure, and allowed me to incorporate an idle air valve for closed loop idle control. All the other FI conversions I've seen use throttle body sets salvaged from newer FI bikes. These don't have idle air control. The one FI bike I looked closely at (a Kawasaki Z1000) instead has a "choke" to allow the rider to open the butterflies a bit extra. I don't know how well the idle control works on the conversion setups done this way.

As for reliability... while carbs don't have electricity or computers, and rarely use fuel pumps, I'm not too convinced they're more reliable. They work well enough when properly set up and used frequently, but they really have problems when left sitting for a while or in cold conditions. The biggest reason I prefer FI, though, is that, where I live, I could be riding at anything from 5000 feet to 14000 feet elevation within a two hour distance. Carbs, even CV carbs, do not handle that kind of ambient pressure change well.

 

i fuel injected my xj750 for a senior design project. i saw your build from a year ago for my inspiration. i used MS II and put an EDIS 4 ignition in it.

 

Cool! How did it turn out? And where can I see more about it?

 

http://xjbikes.com/Forums/viewtopic/t=29313.html