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  1. #1
    Kato johnw's Avatar
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    No Accelerator Pump

    Hi, my 1980 750c does not have an accelerator pump fitted to no. 2 carb and I have read on other threads that the European models did not have them fitted.
    It is really hard to start from cold and has been since I bought it last year.
    The choke mechanism is working OK and following previous advice I have re-set the valve gaps and cleaned the carbs out.
    I'll clean the carbs again.
    I also cleaned out the tank and fitted an inline filter. There is good flow of fuel to the carbs.
    I suspect the floats are sticking and not letting fuel in because as I turn the bike over there is no great smell of unburned fuel.
    After a few turn on the starter, it begins to rumble and then eventually will fire up if the battery lasts.
    Do you think the floats could be the issue?
    johnw

  2. #2
    CB750 Guru amc49's Avatar
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    Engine condition figures in big there, people never question it but the biggest determiner of quick cold starting there is.

    You're NOT supposed to smell raw fuel unless going way too far trying to light one that won't.

    The accel pump really doesn't help cold start as much as you think.

    Don't know if they add ethanol to your fuel there but if they do that can be a big part of it. It can stick floats sitting overnight.

  3. #3
    Kato johnw's Avatar
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    Thansk for getting back amc49.
    Yes we have ethanol too.
    Will go back through everything again.
    Cheers
    Johnw

  4. #4
    CB750 Member Pwnypower's Avatar
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    thats why i always fill up with 98 or more, we rarely have ethanol free gas here in the netherlands, i find it a bit unpleasing that 98 is also 5% ethanol. I always go to fuel stations that claim that their 98 is ethanol free. will make the bike much happier. I also put in some lead replacer just because the bikes hardened valves are usually not designed for these kind of lean gasses that are like sandpaper in comparison to the good old days. I have befriended a guy that specialises in fuels and additives and he also recommends sometype of lubricator because the gas nowadays is way more coarse(he calls it schraler, a dutch word for less lubricating)

  5. #5
    Kato johnw's Avatar
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    Thanks for the response Pwnypower.
    Can't get ethanol free fuel in the UK, at least not easliy.
    Will consider using additive.
    As an update, with help from amc49 I got a rebuild manual for the carbs and discovered that when closing the choke it should act on a seperate lever to open the throttle butterflies a little. I had mistakenly assumed this extra lever was somehow connected to the accelerator pump which is not fitted to my machine.
    The one on my carbs is bent out of shape and is not acting on the throttles.
    I've got them apart for cleaning now and will adjust the lever to work.
    Am hoping it helps my cold start issues.

  6. #6
    CB750 Member Pwnypower's Avatar
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    when you pull the carbs apart consider getting some rebuild kits for them, makes the job easier and also ensures that your carbs are back to spec(air cut off valves tend to be a bit stiff with a bike this old).

    I didn't do mine but checked them when they cleaned my carbs.

  7. #7
    Kato johnw's Avatar
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    Yes I will rebuild with new parts.
    I changed the air cut off last year and it made a big difference on the overrun - less popping / banging.
    My issue is very poor cold start.
    Will clean, rebuild and sort this choke mechanism and see if it improves things.

  8. #8
    CB750 Member Pwnypower's Avatar
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    good luck and keep us posted, really eager to know what your issue is.

  9. #9
    CB750 Guru amc49's Avatar
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    FYI Euro models do not have accelerator pump as they have a slightly richer idle/off idle setting than US ones do. US ones also had idle limiter caps to stop people from adjusting the idle mixture. The aircuts figure in that too, they lean the idle an additional amount and I tend to defeat them when I rework carbs. When you do that you open the mixture screws a bit more to make up for the extra high vacuum decel richness you lost when losing the aircut working.

    The aircut if working right only really does anything at high vacuum above idle like the overdraw situation you get when at deceleration with throttle closed. The engine runs lean enough at that time to not fire right, some of the mix then afterburns to pop in the exhaust pipe. With the aircut activated the idle mixture switches from two idle air jets to just one to richen the mix only when needed then when the high vacuum drops off it switches back to the super lean two idle air jets to lean out the normal idle for emissions reasons.

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