Dave's Yamaha XJ Page     |     home
XJ750-D2 Fuel Injection (Model 38X) 1985
This is my new baby but she is not too well behaved....

Since I got this bike it has suffered two breakdowns with the fuel injection. Firstly on the trip home from buying the bike it suddenly lost power and would not run over about 70kmh. I traced this fault after trailering the bike home to a seized linkage for the idle switch on the injector rack. The result of this was that the computer was still thinking the bike was at idle and had leaned out the mixture to compensate, however at open road speed there was not enough fuel to run with the resulting dramatic loss of power. Once I found the problem, one drop of oil on the seized linkage got it going again.

The second problem has proved a lot more elusive to trace. The bike started dying completely when throttled off. You could actually feel the power dropping down past idle and the engine go completely dead every time the throttle was closed back to idle. This made the bike almost impossible to ride in the city, it would come back to life when the throttle was opened about half way again but the resulting rush of power was a bit unnerving right behind a cage!. At idle the engine runs perfectly and under power seems almost normal, maybe slightly sluggish.

Well after stripping out the entire injection system for inspection I think I have found the problem. The fuel pressure regulator had fuel leaking from the vacuum line which means the diaphragm inside has a hole in it. This regulator is intended to keep the fuel pressure at 2.5kg above manifold pressure and with a holed diaphragm would not be regulating at all. I guess that the high vacuum created at throttle off would have meant the fuel pressure would have been lost at the injectors.

I have taken some pictures of the regulator which is obsolete now, I wonder if anyone recognizes it and may be able to help me find a replacement? The regulator is a Hitachi and has the number MPDB36-1 stamped into it. It also has the number 36D stamped on in ink.

The other part that is also obsolete that I need is the air filter. I would like to cut in some paper from another filter but am yet to find a suitable type. The pleats are 15mm tall. If I was to run a tape measure around the paper side of the filter its about 210mm long with the pleats layed out correctly. All up there are 55 pleats. The filter paper is 95mm wide and is glued into the frame down each side.

Below I have added some pics of the other XJ Injection parts for those who are interested.

Air Flow Sensor.
 I had found some dry joints on the circuit board inside and some dust build up on one of the hot wires, lets hope it still works when I get to try it out again! :-/

Injector Rack

This shot shows the idle switch linkage on the lower right, It had seized at the link with the E clip and just needed a clean and some oil.

This page is under construction....more to come....