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Author Topic: Cold start trouble  (Read 440 times)

Offline CJM37

  • Posts: 11
Cold start trouble
« on: Mar 06, 2018, 11:39:12 »
Have a Honda 78 CB750 that starts great after it’s warm but is almost impossible to start cold. Carbs rebuilt and adjust. Top end rebuilt. Any ideas on what the problem might be. Any help is appreciated.


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Offline coyote13

  • Posts: 1197
Re: Cold start trouble
« Reply #1 on: Mar 06, 2018, 13:13:58 »
The '78 has an accelerator pump, I would put my money on either a pinhole in the accelerator pump diaphragm or something like a stuck choke.  Start there, but could also be that your mixture is off or possibly valves too tight?
Half the fun's in the get there...

Offline irk miller

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Re: Cold start trouble
« Reply #2 on: Mar 06, 2018, 16:11:29 »
That pump circuit notoriously clogs when they've sat for a while, and it's very difficult to clean.

Offline CJM37

  • Posts: 11
Re: Cold start trouble
« Reply #3 on: Mar 06, 2018, 17:25:17 »
The '78 has an accelerator pump, I would put my money on either a pinhole in the accelerator pump diaphragm or something like a stuck choke.  Start there, but could also be that your mixture is off or possibly valves too tight?
I bet you are right I rebuilt the pump but didn’t clean out that pinhole. Thanks a lot I will try that if I can figure out the best way to clean it


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Offline coyote13

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Re: Cold start trouble
« Reply #4 on: Mar 06, 2018, 17:31:08 »
If there's a pinhole in the rubber diaphragm that's your problem!  :o
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Offline irk miller

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Cold start trouble
« Reply #5 on: Mar 06, 2018, 17:50:30 »
Cleaning the pump won’t do you any good if the circuit is clogged. You should’ve checked for squirt on all four holes if you had them apart already. It’s very common for people to clean these PD carbs, but not get the pump circuit clean. Also, while they’re out did you check the timing of the squirt? Fortunately, these carbs will squirt through the choke plates.


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Offline CJM37

  • Posts: 11
Re: Cold start trouble
« Reply #6 on: Mar 12, 2018, 17:26:10 »
Cleaning the pump won’t do you any good if the circuit is clogged. You should’ve checked for squirt on all four holes if you had them apart already. It’s very common for people to clean these PD carbs, but not get the pump circuit clean. Also, while they’re out did you check the timing of the squirt? Fortunately, these carbs will squirt through the choke plates.


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Thanks a lot. Pulled the little spring and ball out of the accelerator pump cleaned that and cleaned the circuit. Problem solved.


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