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  1. #1
    CB750 New Member BP92's Avatar
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    Cb750 1981 dohc build - issue with starting / firing

    Hey Guys,

    I'm in the middle of a rebuilding a CB750 1981;
    This is my first ever build and It's been a whirlwind of forums, google, youtube and a local motorcycle shop to get me on track to this point.

    However, I've hit a bit of a snag; the bike wont start / fire, and I wondered if any of you can help out with some advice...

    There's a few issues -

    - The fuel diaphragm valve doesn't appear to be working great, but in a bid to start the bike for now I bypassed this to fill the carbs with fuel and try turn the bike over. Once I know we're up and running properly I'm going to try get this sorted and ideally keep it working on the bike - I fear I'll forget to turn the Fuel Petcock OFF one day if not...

    - Following the bypass, the float bowls definitely have fuel, but after turning it over a few times I checked the spark plugs (which are brand new) and they were bone dry.

    - The spark plugs are sparking (tested against the engine casing), so I assume my wiring is correct to the coils and they are obviously working as they should be, right?!

    - Also do the CDI / Spark Unit Boxes have a particular order when they're wired up - there doesn't seem to be an outline for that online anywhere and the boxes aren't marked by number or anything.

    - I've removed the old air box and now have Pod Filters on the carbs - now I know there's usually issues on this front and people say avoid it in favour of the old Air Box, but what I'd read up on this seemed to really only affect the bike bogging down etc when riding, not actually issues with starting. Would it affect start up?

    - When I previously took the carbs apart I changed the Main Jet and Idle Jet in the middle of the carbs, plus the gaskets, but nothing else. Should I have adjusted my Pilot Screw or anything now that the jets have changed?
    (Overall, the carbs seemed pretty clean to me, but as a rookie I'm not sure what I'd be looking for otherwise or how to fix any issues I'd come across).

    - Lastly, any other ideas on what would be preventing fuel making it to my Spark Plugs?!

    Really hope you guys can help...

    Thanks,
    BP

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  2. #2
    CB750 Addict Wez_'s Avatar
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    Sexy looking bike. So what's the background? Did you or someone else rebuild the motor? Was it previously running before the overhaul? It looks pretty clean. The PODS will not effect start up. Not from my experience. I used PODS initially to get the bike going as it was a simple/cheap solution vs the stock box / rubber. It's more of a max RPM issue if you ask me. Also torque will be effected on pulls with PODS. Air box the way to go. Anyways, you are at the point where you need to check compression to ensure you do not have a major mechanical failure of the valvetrain, cylinder head and piston rings. Make sure you have the throttle WOT when cranking for compression numbers. If compression checks out good (~150PSI +/- <10% across all 4 cylinders) then you can relax. The intake cam sucks the air/fuel mixture into the head and the valves distribute the mixture into each cylinder. If you have the spark plug wires messed up, that could cause spark, but not fire. They are labeled from the factory 1-4, but they do not correspond to the cylinder. They are NOT connected 1-4 straight across the cylinders. Get yourself the Clymers manual and the firing order is outlined in detail there. The Clymers manual is a must for these bikes. Also, the two CDI packs do not have a polarity. Just make sure they are connected nicely. One final check you can perform with ease with those PODS is remove all 4 PODS and pull the throttle a few times while observing that fuel is squirting from ALL 4 accelerator pump nozzles directly into the center of the carbs. If not, then initial start up will be very difficult if not impossible.

  3. #3
    CB750 New Member BP92's Avatar
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    Quote Originally Posted by Wez_ View Post
    Sexy looking bike. So what's the background? Did you or someone else rebuild the motor? Was it previously running before the overhaul? It looks pretty clean. The PODS will not effect start up. Not from my experience. I used PODS initially to get the bike going as it was a simple/cheap solution vs the stock box / rubber. It's more of a max RPM issue if you ask me. Also torque will be effected on pulls with PODS. Air box the way to go. Anyways, you are at the point where you need to check compression to ensure you do not have a major mechanical failure of the valvetrain, cylinder head and piston rings. Make sure you have the throttle WOT when cranking for compression numbers. If compression checks out good (~150PSI +/- <10% across all 4 cylinders) then you can relax. The intake cam sucks the air/fuel mixture into the head and the valves distribute the mixture into each cylinder. If you have the spark plug wires messed up, that could cause spark, but not fire. They are labeled from the factory 1-4, but they do not correspond to the cylinder. They are NOT connected 1-4 straight across the cylinders. Get yourself the Clymers manual and the firing order is outlined in detail there. The Clymers manual is a must for these bikes. Also, the two CDI packs do not have a polarity. Just make sure they are connected nicely. One final check you can perform with ease with those PODS is remove all 4 PODS and pull the throttle a few times while observing that fuel is squirting from ALL 4 accelerator pump nozzles directly into the center of the carbs. If not, then initial start up will be very difficult if not impossible.
    Hi Wez,

    Thanks for posting.

    Background on the bike:
    This sounds crazy now it's come to not starting, but...
    I bought the bike via an import eBay shop that takes bikes from the USA and ships them to the UK.
    Most don't start and they advertise as such, so when purchasing I stated I'd only take something that starts and runs.
    They assured me it did (and I do trust that), but when it arrived to me via courier the battery had run to almost empty - so tried to start up and it took a second or two, popped and spluttered into action, seemed to rev really high and then drop low and cut out. Then the battery was straight empty.
    I didn't know why (rookie) and got caught up in my bike build dreams I started taking the thing apart (and there's the rookie mistake). I did spot when removing plastics however the Carbs were only half connected and half hanging loose - later also finding the petcock barely let any fuel through. So I'd sort of put it down to those two issues causing what went down. Now though (Carbs fully connected and a new petcock installed), I'm starting to think I'm a bit wrong on this one... The motor has never been opened up or rebuilt.

    Compression check is next for me and I'm waiting on the kit arriving.
    Once I've had a chance to run the test, I'll post the numbers up here so we can see I'm inline with requirements to get the engine started.

    Thanks for your help!
    BP
    HONDA CB750 1981

    Still learning the ropes...

  4. #4
    CB750 Addict Wez_'s Avatar
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    Who knows. They could have sprayed starter fluid in the carbs to get it to run for you. Lets assume it did run even though that's surprising for half disconnected carburetors. Then the compression check will definitely be a starting point. Was the bike in the current condition as it's imaged above? If so, then how was the price compared to the condition? A bike that has a modified electronics tray, painted frame, polished motor, café seat with all the integrated LED hoop should have been pricy to have imported. If you received the CB in it's as is condition for dirt cheap and it now wont start and it should, then you might want to make sure you have all your receipts and the agreement where it's supposed to run in writing. If you paid for a turd, and you restored it to that condition, then great. But the bottom line is, you get what you pay for. Motor needs air, fuel, spark and compression to start and stay running. That's the basic rule of thumb for EVERY engine. Have you ordered the Clymers repair manual also? Forums will only take you so far. You need the proper documentation. I also always recommend checking that the accelerator pump nozzles are squirting fuel directly into each carb body. Remove the PODS and blip the throttle to see that your carbs are functioning correctly. When starting for the first few times or "priming" the fuel system, you'll need extra fuel and likely need to twist the throttle for more fuel. If the AP circuit is clogged, then no good.

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