dr750 wont go above 2k

[RORY]

hi guys, new here  i have a dr 750 ive had a few problems with it as you do with these bikes ,the problem i have at the moment is that it wont rev out over 2 grand anything above it want to back fire as though the timing is out ,but i have just done a engine rebuild, due to the cam chain slipping (new valves)  ,but this problem i have  now did this before the rebuild anyway ,ive done all the timing by the manual spec  and the bike stars straight away ticks over great but above 2 it wont have any of it ,also im running a 96 cdi on it as mine is a 89 model the only difference is the 96 model has a tps ,  which i havent got on mine ,,could this stop the bike revving higher , carbs seem ok cleaned them out ,wiring seems ok ,does it with filter in and out ,just wont rev ,any help woud be very greatful ,,cheers Rory

[002]

Yes the lack of Throttle Position Sensor will effect the running !

Jethro

[Dave#22]

Hi, you must have done some wiring mods to fit the TPS ignition unit, as the plug and sockets are completely different, as well as the unit being much larger.The other difference is in the stator/ignition pickup assy, are you using the 750 stator? Have you tried a std CDI? The 750 engine has a higher rev limit than the 800's, so I would assume the advance curve is different.
      Dave
ps,dont set the tappets too tight, use the 800 settings and the valves will last longer.

[RORY]

Hi Dave  thanks for your reply, Its a 89 dr 750 motor with a 90 dr 800 stator fitted with a  96 dr 800 cdi unit . The 89 >90 750/800 motors are the same apart from 6 mm on the stroke ,91 models 0nwards are different  ,fitting the 96 cdi was so easy it has just 2 wires more to incorparate the tps that i aint got ,I just took the connectors out of the blocks on the loom  and with the 2 diagrams placed them onto the new cdi , had to earth a green wire to let the decomp switch working with the cdi , then bingo it fired , Ive just been informed that the 96 motors had the key way altered on the flywheel aparently so that would mean that the timing would be to advanced or to retarded ,Ive yet to find this out ,I put a strobe on the bike today  and it was firing right on tdc and it was advancing after 2000  rpm but then all the bike did was struggle and backfire through the carbs  like i say 2000 rpm then it goes proper pants ,Not to sure about this but i might have a go at oveling the pickup coil holes and see iff it makes a difference on timing by moving it either way ,it seems to me that when it back fires through the carbs that the timing is too advanced thus explaining the keyway change on the later models hopefully to match the later cdi ,Ive done all the pickup and source coil ohm readings and they seem both to manual spec ,even visual they look ok.
        I have the standard cdi for the bike ,it turns over but it wont spark .and i have a friend who has the same bike and he cant get a spark on his , so i borrowed his cdi and thats a no go on mine either ,put the 96 cdi  back on mine and it fired and still wont rev out ,,,tell you the truth its proper headache

[MrFluffy]

As jethro says, it *NEEDS* the tps signal.
Whats happening when you open the throttle is, the computer sees the revs rise so it goes to look at its tps sensor to try and work out what you want it to do so it can match everything to rider input, as it cant *SEE* anything coming in from the tps it goes into a sort of default somethings really buggered mode and slows down the engine to stop it damaging itself.
2k is about the range off idle when the computer starts using tps to calculate injection event durations.

You either need to get the right cdi for your bike or a aftermarket one to suit, or fit the tps, be it on a homemade bracket somehow so it turns when you open the throttle, or use the whole carb from the 800 somehow... You could do a quick fudge test by plugging in the tps and putting it to midway position, and checking the bikes revs will rise. Theyre nothing magical, theyre just like a rotary volume control on a stereo, so the engine can see how loud you want it to be!

[RORY]

I tried the tps bodge up with a guitar pot wired off the diagram ,made no difference and i have tried a working cdi off a dr 750 the same as origonal one i have got that goes with the stator ,but that wont work either  it could be pickup coil even though the ohms readings are reading correct ,im really greatful for your or anybodys advice ,iff you think of other stuff to help out im all ears ,,,cheers Rory

[Dave #22]

The obvious thing to do is try a pre '96 CDI unit which doesn't have the TPS facility.
I hadn't heard about the revised keyway, as I briefly used a '95 motor in my '98 DR800,
which fealt pretty much the same.
   Dave

[bikeseamus]

Could be a defective coil, breaking down at higher RPMs. They can break down in a number of ways. Substitut a a known good one and let us know, so we all learn something. Does the mixture look rich on your spark plug? Suspect all components until proven to be functioning properly. They all have to work together in conjunction.

[RORY]

as far as the revised key way on the 96 onwards is just what i got told ,i thought it might of said in the manual but it doesnt
 so im not to sure myself on this
    
       Rory

[RORY]

Could be a defective coil, breaking down at higher RPMs. They can break down in a number of ways. Substitut a a known good one and let us know, so we all learn something. Does the mixture look rich on your spark plug? Suspect all components until proven to be functioning properly. They all have to work together in conjunction.

 It could be the source coil or the pickup coil ,but as far as the plug coils go, it has 2 to make a even fire across th large piston suface.
 Ive visual checked both intenal stator coils for damage but the look pretty good to me .
   

[bikeseamus]

OK
       Then try this.   Disconnect,then clean all your electrical connections that are related to your ignition. Watch especially the ignition coil wires. Assure that all your grounds are uncorroded and show no resistance, essentially. I would be interested in holding a timing light on the bike and seeing what it actually does when it breaks up above 2000 RPMs..... whether the timing goes really wonky or holds steady at an incorrect reading.   And you are absolutely certain that your airbox and intakes are nest and insect nest free and the carb has absolutely all clear and clean jets and the slide needle hasn't dropped into the nozzle?
                          It is not the Starship Enterprise.  Check those things with a timing light and a volt/ohmeter and clean your carb, and go from there. If you have access to a proper carb with the TPS sensor intact, you can just plug in the sensor and move the carb throttle rocker/puller and see if that allows the bike to rev properly. You must be absolutely certain that your carb is in proper working order to diagnose anything else.

     It's time to stop assuming things are good because they look good. Time for the timing light and the volt/ohmeter and a good carb cleaning, and a service manual that will help you diagnose defective stators, pickup coils. A ripped carb diaphragm will exhibit those symptoms too, if your bike has one.

  The beauty is in the logic of it all. It's a machine. We aren't dealing with a Human Being here.  It's logical.

   Keep us posted.

[Dave #22]

Hi Rory, another problem crossed my mind, I assume the 2000rpm is displayed on the tacho? When I fitted a '95 DR 800 motor, electrics and mechanical parts into a DR750 I found the tacho would not work with the signal from the CDI, so I ended up putting the 800 tacho mechanism into the 750 housing. Is your 750 tacho working from the 800 CDI?
ps which part of the country are you in?
             Dave

[RORY]

hi dave ,on the 96 cdi it has a separate wire B/G that comes from  the cdi to the tacho ,but that does not work on the 750 ,so i left the tacho wired to the w/bl wire that it normally goes to one of the ignition coil wires ,,,
im in Redcar , cleveland  Dave . I put a strobe light on the bike and the spark starts to go mad and  starts missing soon as the advance reaches its limmit so there is something that might be a clue to whats going on ,

this is what i did to get the  90 dr750 to run with the dr 800 96 cdi
 
  Rory

[Dave #22]

Hi Rory, How about removing all the unnecessary connections from the CDI, like removing the connectors for the decompression control, side stand switch, neutral switch and putting them straight to earth, disconnect the non original tacho supply and try again.
It may be an earthing fault?
Why are you trying to use a late model ignition system on a DR750, did you buy the bike with it fitted, or have you converted it yourself. I have an '88 DR750 a '95 DR800 and a '98 DR800 (TPS) all with different ignition systems and all run well with different characteristics.
       Dave

[RORY]

Hi Dave ,,cheers for posting back mate, when i got the bike it was a 90 dr800 that had no spark ,,,i thought it coul be the cdi so i tried to get another cdi for the bike but there like rocking horse s**t to get hold of ,and a new one was going to be £542 to by new,so i asked a guy on ebay who was selling a loom off a 96 model if he had a cdi for it ,he did and i bouight it off him for £60 ,the bike ran with that cdi but when the engine ran it sounded like a bag of nails the cam was well worn , so there was a 750 motor on ebay so i got that for £180 tried that with the old cdi ,again no go so i put the 96 cdi on it and it ran but would not go over 2000 rpm ,then i tried my friends  750 cdi on it and it wont fire the only thing it seems to start with is the 96 cdi.
        Dave is there any chance you can undo the connector off you tps and see if the bike revs out at all, then if it does i will probably the pickup coil that will be the cause,, or it will narrow the problem a little is there any chance of getting back to me on this tonight
    cheers Rory