Start Up Benelli 750 Sei Experts Required
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Thread: Start Up Benelli 750 Sei Experts Required

  1. #1
    Senior Member tucandugu's Avatar
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    Start Up Benelli 750 Sei Experts Required

    Hi mates,
    my 750 Sei doesn´t wants to start up again. And this is really frustrating after all the work done on it.

    After changing plugs and checking coil wiring order it started up last week. At the beginning the engine run very smooth and balanced. I leave it on idle about 4 minutes in order to warm up for oil replacing. After this time lapse the idle begun to decrease and the engine appeared to run unsteady until it stalled.
    Once oil replaced I started up it again after several attempts. Ride it but after some hundreds of meters the engine appeared to lose the power until stalling again.

    Well, after this, had been impossible to start up the bike again.

    Checked the breakers and noticed that 2 of the 3 pairs apparently don´t give spark, or some times they give a very weak one -but they make correct contact-.
    Any body had experienced similar symptoms ?. Any suggestion/ procedure to discover the fault?.

    Thanks in advance!.

    Carles
    Benelli TNT 1130 Sport EVO 2008 - Benelli 750 Sei 1977 - Moto Guzzi 65cc 1957 - Vespa Primavera 125 1977

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    Hey Carles,

    Since none of the experts have answered yet, I'll throw out a few ideas, just remember mines not on the road yet so I don't have a lot of sei knowledge. Things to think about:

    Was the choke on all the time it was running? Could you have fouled the plugs?
    Stale fuel? Not sure what that would do but will find out soon.
    Breakers sparking? Are you checking for spark at the spark plug or do you mean sparking across the points (breakers)themself? If you have sparking across the breaker points then it means your condenser is faulty. If you want to check for spark at the cylinder, pull your sparkplug or put a timing light on that lead.

    Let me know where you're at and might think of something else.

    Steve
    76 Benelli Sei, 76 Guzzi Lemans, 79 Guzzi Lemans, 79 Kawasaki z650, 98 BMW F650, 08 Suzuki DL650

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    Senior Member tucandugu's Avatar
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    Quote Originally Posted by Doc59 View Post
    Hey Carles,

    Since none of the experts have answered yet, I'll throw out a few ideas, just remember mines not on the road yet so I don't have a lot of sei knowledge. Things to think about:

    Was the choke on all the time it was running? Could you have fouled the plugs?
    Stale fuel? Not sure what that would do but will find out soon.
    Breakers sparking? Are you checking for spark at the spark plug or do you mean sparking across the points (breakers)themself? If you have sparking across the breaker points then it means your condenser is faulty. If you want to check for spark at the cylinder, pull your sparkplug or put a timing light on that lead.

    Let me know where you're at and might think of something else.

    Steve
    Hi Steve, welcome the first help!.

    Fuel tank is totally renewed and fuel is new as also are fuel conductions, gas taps, etc.
    When engine started, the choke was on but I switched off in few seconds.
    Spark plugs and caps are new.

    After the fault I have checked two things:

    1- If there was electricity coming to each breaker, opening each one and looking for the spark.
    I discovered that only one point set gave spark, the other two gave no spark or a very weak one some times.

    2- If there was spark at the plugs.
    The plugs related to those breakers gave no spark or a weak one some times.

    What I planned to do now is to replace the coil wiring -still is the original one- and to check the wire connections related with the coils -like the attached image, which could be in wrong order-.

    Any ideas will be welcomed. Thanks for your help.

    Carles.

    Benelli TNT 1130 Sport EVO 2008 - Benelli 750 Sei 1977 - Moto Guzzi 65cc 1957 - Vespa Primavera 125 1977

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  5. #4
    Senior Member ianh's Avatar
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    Possibly coils breaking down? My old cb900 used to start fine but then start miss-firing as it got hotter.
    swapped the coils around to help identify the fault (ie miss-firing cylinders swapped about with the coils)

  6. #5
    Senior Member tucandugu's Avatar
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    Thanks ianh,
    I will replace the coils wiring (actually they are very old and quite loose). Hope this be the problem...

    Carles
    Benelli TNT 1130 Sport EVO 2008 - Benelli 750 Sei 1977 - Moto Guzzi 65cc 1957 - Vespa Primavera 125 1977

  7. #6
    Senior Member tucandugu's Avatar
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    Hi sirs,
    any other suggestion for the weekend?.

    Regards!.

    Carles
    Benelli TNT 1130 Sport EVO 2008 - Benelli 750 Sei 1977 - Moto Guzzi 65cc 1957 - Vespa Primavera 125 1977

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    If it sounded smooth and balanced, when it was running, I'd say you have the coil connections right. If you can't find anything obviously wrong try unplugging and cleaning every connection and earth in the ignition circuit. Remember you're playing with 30 yr old wiring and it doesn't take much corrosion/dirt to cause problems. I just went through similar with my Mk2 Guzzi, after riding for around 150 km it would suddenly lose all elec power and stop on me. I tried a few things and it stopped the same about 3 times before I cleaned every connection in the power circuit and its been fine ever since. Problem is I never actually saw anything that caused the problem.

    Another remotely possible cause could be your condensers, though I doubt 2 would fail at the same time, but they can fail and short to ground wich stops that coil from firing. Easy to check by disconnecting the condenser from the breaker and see if you get spark.

    Good luck and let us know how you go.
    76 Benelli Sei, 76 Guzzi Lemans, 79 Guzzi Lemans, 79 Kawasaki z650, 98 BMW F650, 08 Suzuki DL650

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    I have to agree with Doc. Look especially hard at the connections to the fuses and replace all fuses if they are the originals. Are you sure that it is not fuel?
    Ian

  10. #9
    Senior Member tucandugu's Avatar
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    Thanks guys,
    checked the wiring connections as you suggested, Steve. Very happy to discover that the blue wire, supose coil mass -see attached image- was loose because not tighten nut (my fault...).
    Also when checking the connections, one of the connectors unplugged without effort...
    This two things make me to be enough optimistic about the probabilities to start up the bike. Mounted the tank and at second intend with the quick start, the engine started!. Smooth running during some seconds but then the engine begins to slow down, take off the choke while maintaining 2000 rpm with the accelerator but engine finally stopped.
    Several attempts to start up again without success, no feeling of spark explosion. Quite fuel smell makes me to think about the possibility that is too rich. Left some minutes and tried again. In fact, when I tried to start up again, without choke, the engine gave some explosion feeling, just very close to start but the starting has not been achieved again.
    Those are similar symptoms that the other time and really confuses me a lot. How the engine can run smooth and after a while doesn´t starts again?.
    May be the carburation is too rich?.
    Tomorrow I will also check the fuses as you suggest Ian, but now, after the wiring inspection and this symptoms, I begin to suspect that something related to the carburation is wrong.

    Thanks and regards.

    Carles

    Benelli TNT 1130 Sport EVO 2008 - Benelli 750 Sei 1977 - Moto Guzzi 65cc 1957 - Vespa Primavera 125 1977

  11. #10
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    Smile

    I had similar problems with my Sei, all now cured by fitting Electronic Schse ignition which was simple to fit.
    Stripping the carbs, cleaning, new jets "o" rings and gaskets.
    Carbs balanced and now runs fantastic, hot or cold.

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