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Thread: R90S Cold Start Problem

  1. #1
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    R90S Cold Start Problem

    My '75 R90S is difficult to start when cold. This condition is new, so I suspect it is a product of something I've done or overlooked.

    What I've done: took carbs apart and replaced o-rings and gaskets with kit by Dellorto. Sprayed carb cleaner through orifices and blew with compressed air, reassembled. Replaced cables with new ones form Motobins. Floats are good height per Dellorto spec of 17.5mm. Starter valves slide freely in starter body that is attached to carb. Heads torqued to 25 ft. lbs., valves set at .006" intake and .008" exhaust. New points at .016" and timing set with dynamic light.

    I read the Dellorto factory info on the starting circuit. It says if the plugs are wet after attempting to start, the mixture is too rich and starting jet needs to be changed to one with holes higher up on the emulsion tube. I haven't looked into replacing them because before my carb work it started OK with throttle slightly open and no choke.

    Once warmed up it starts, idles and runs well. Tried some choke but it will not fire unless the choke is fully "off", throttle is slightly open, then only after cranking awhile, seemingly to clear the rich mixture.
    Jets are stock, throttle and choke cables have appropriate slack.

    Any hints are appreciated.

  2. #2
    Registered User STEVENRANKIN's Avatar
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    Suggestion

    So it sounds like you have done some work on the carbs. One of the first things I always do if I have a problem with something I have done is recheck my work.

    You may have something assembled wrong or the choke cables are too tight, any number of things could be off. None of us are perfect and we all make mistakes from time to time. Sorry, nothing else comes to mind at this time. St.

  3. #3
    John D'oh
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    Dell’s are “pumpers” and after a thorough service need calibrated- as a rule.

    Try this next cold start- with choke on, turn on fuel taps. After fuel lines and float chambers fill, take the throttle in hand and turn it rapidly to fully open and close it immediately - one time. Ignition on, engage starter with throttle closed. If the ‘pump’ is set to deliver the correct volume of fuel it should start immediately.
    John D'oh

  4. #4
    Quote Originally Posted by migrant View Post
    My '75 R90S is difficult to start when cold. This condition is new, so I suspect it is a product of something I've done or overlooked.

    What I've done: took carbs apart and replaced o-rings and gaskets with kit by Dellorto. Sprayed carb cleaner through orifices and blew with compressed air, reassembled. Replaced cables with new ones form Motobins. Floats are good height per Dellorto spec of 17.5mm. Starter valves slide freely in starter body that is attached to carb. Heads torqued to 25 ft. lbs., valves set at .006" intake and .008" exhaust. New points at .016" and timing set with dynamic light.

    I read the Dellorto factory info on the starting circuit. It says if the plugs are wet after attempting to start, the mixture is too rich and starting jet needs to be changed to one with holes higher up on the emulsion tube. I haven't looked into replacing them because before my carb work it started OK with throttle slightly open and no choke.

    Once warmed up it starts, idles and runs well. Tried some choke but it will not fire unless the choke is fully "off", throttle is slightly open, then only after cranking awhile, seemingly to clear the rich mixture.
    Jets are stock, throttle and choke cables have appropriate slack.

    Any hints are appreciated.
    The Dellorto starting circuit opens a pathway that pulls in raw fuel from the float bowl on engine cranking. the throttle should be closed or only very slightly opened when cranking and with the chokes on.

    The starting circuit venturi is rather large so doubtful that it's blocked. Biggest problem I find when working on these, is the starter valve isn't lifting enough to open the pathway due to the cables not being installed correctly at the choke lever. Might be wise to unscrew both starter plungers (with cables attached) and verify they both move a significant and equal amount when the choke lever is engaged.
    Last edited by RPGR90s; 07-14-2021 at 07:58 PM.

  5. #5
    You might want double check your valve adjustment, tight valves can cause a cold start issue.

  6. #6
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    Appreciate the info on the starting circuit, I was wrongly assuming that I was flooding the bike by choking it. Pulled the starting valves and made sure they were retracting equally when the choke lever is moved.

    Fully depressed the choke lever, partial throttle opening, hit the button, bingo. If that didn't work, I was prepared to twist the throttle but it wasn't necessary in my case.

    I'm really liking this bike, and learning a lot in the process of maintaining it.

    Excellent info on carb setup: http://www.ducatimeccanica.com/dello.../dellorto.html
    Last edited by migrant; 07-15-2021 at 09:07 PM.

  7. #7
    Quote Originally Posted by migrant View Post
    Appreciate the info on the starting circuit, I was wrongly assuming that I was flooding the bike by choking it. Pulled the starting valves and made sure they were retracting equally when the choke lever is moved.

    Fully depressed the choke lever, partial throttle opening, hit the button, bingo. If that didn't work, I was prepared to twist the throttle but it wasn't necessary in my case.

    I'm really liking this bike, and learning a lot in the process of maintaining it.
    Glad you got it working.

    Dellorto's seem to get a bad rap but in my experience (140k miles), they've been nearly flawless and extremely simple to tune/repair.

  8. #8
    Registered User STEVENRANKIN's Avatar
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    Delortos

    I agree Delortos do get a bad rap, and I really don't know why, they are just as reliable as the Bing CV carbs and I bet if you dig deep enough into BMW's records, they were taken off the line more because of cost rather than reliability issues. Or there may have been a supply issue for them.

    I know more than a few people with extreme high mileage on Delorto carbs mounted on R90S bikes and other than a high mileage rebuild, they never had any more problems with them than I do with my Bings.

    They do need rebuilding at times, no shorter a period than Bings as far as I can see. They may be a bit more complex because of the accelerator pump part and perhaps the choke system but not terribly so.

    I am glad you found the problem without major fuss. St.

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