Author Topic: CB750 F2 1978 PD41 Carb Vents  (Read 319 times)

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Offline Demi16

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CB750 F2 1978 PD41 Carb Vents
« on: April 12, 2020, 01:47:05 pm »
Hi all hope everyone’s well, been using the site lots whilst recommissioning my 78 750, and have been having trouble setting up the float heights on the carbs since rebuilding them with the famous keyster kits!
I bought the bike as a non runner a couple of years ago after it had been sitting for a few years. Did manage to get it running roughly with some fresh fuel etc so decided to overhaul the carbs, service the engine etc.
Anyway every time I tried to carry out the clear tube check I would undo the drain screw and fuel would shoot up the tube and then leak out of the main circuit tube at the rear of the carbs. It would run but the idle hangs up and just doesn’t sound crisp like the bikes I’ve watched online. I’ve had these on and off the bike more times than I can remember over the last two weeks, keep adjusting the floats, trying to clean them even though they are spotless. I’ve tried searching the site for where these carbs vent and the only vent I could see were through the drain tube, but I have at last today found that there is a very small vent hole on the side of each carb, they look like a brass blank but have in fact got a really small hole in them to vent into the carb body. I managed to clean them out with a piece of wire from a wire brush, they were completely blocked. With the carbs split this would be easy, bit fiddly with them together but not impossible. I have attached a picture for reference.
Refitted the carbs and for the first time managed to get an accurate reading of the fuel levels in the bowls :) have got to finally adjust to get them all spot on but I ran it and sounds the best it’s been so far.

 So just wanted to post about these vents as I’ve not been able to find any info online anywhere about these, hopefully help someone out. sorry for the long story but a bit of background sometimes helps.

Offline scunny

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Re: CB750 F2 1978 PD41 Carb Vents
« Reply #1 on: April 12, 2020, 02:07:38 pm »
Shouldn't those vents be joined ?
1&2 with a tee piece, 3&4 with a tee piece.
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Offline Demi16

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Re: CB750 F2 1978 PD41 Carb Vents
« Reply #2 on: April 12, 2020, 02:15:13 pm »
There weren’t any tubes connecting them up when I took them apart and when I was looking for a vent on the carbs I wasn’t able to get anything through the remaining posts, don’t look like they were drilled out from the factory. The only holes were these brass ones I found, as I said I’ve struggled to find out much on the vent side of things for these carbs, all the other pd types are quite well documented on here but I could be wrong, I’d be interested to learn more if anyone knows.

Offline Medyo Bastos

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Re: CB750 F2 1978 PD41 Carb Vents
« Reply #3 on: April 12, 2020, 02:30:00 pm »
See if this from the doggy at helps you

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Offline Demi16

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Re: CB750 F2 1978 PD41 Carb Vents
« Reply #4 on: April 12, 2020, 03:26:15 pm »
Thanks for sharing that, the bottom picture is similar to my carbs but mine don’t have the vent to atmosphere nipple, so if I joined the vents to the next carb I’d be effectively blocking them off I think.