Carburettor Vacuum Lines

[Tobias Funke]

Hey guys! I just bought a bicycle. It's the legendary Bros400 !! Yes !

Anyway I've not worked on bikes before. It has a problem where it only runs with choke on, and even then, as soon as any gas is given it cuts out.

I suspected main jet but took off the fuel tank, actually there's 4 small vacuum lines on the carburettor and another big vacuum line that are open to atmosphere!

1 = small passive vent, maybe just put a filter on it
2 = active vent pushing air out as engine runs
3 and 6 = strong vacuum
4 and 5 = weak vacuum

Where are these meant to plug in? Thanks..


20220824_072901 labelled.jpg

[Tobias Funke]

I just found this service manual (actually in English!)

Do hose 3 and hose 6 just accept air from the cleaner anyway? I thought vaccuum lines usually run to the crankcase..


AcroRd32_1CGbL7Hg0P.png

[Tobias Funke]

Possibly all of these hoses can go into a junction box behind another filter, actually. So the issues I'm seeing are more likely from a dirty carburettor? What do you guys think?


AcroRd32_A2BSipeAKU.png

[6]

They just stay open.

[Tobias Funke]

OK thanks. Next I check the boots from carburettor to the engine. Unless you think something else can be checked first?

[6]

Only runs on choke is probably slows. I'd start by cleaning the carbs. The choke is not a choke, it's an enrichener circuit. So it can help make up for iffy slow jets.

Give it a good old fashioned SOLID carb cleaning. Should be fun. If the I take boots are hard and old, replace them.

[Tobias Funke]

Thanks bro, 400. haha. The boots are old, I might see if some radiator hose can be adapted to suit.

I need to take time to read the manual for the correct carb cleaning procedure. You're 90% likely right that it's dirty.

Interestingly, if the vacuum ports 3 and 6 are blocked, the engine can be revved up to maybe 4000 before it cuts, but when not blocked it stops immediately that the throttle is turned..

[squirrelman]

due to some confusion, some tubes in the photo above are mislabeled. vent tubes open to the air mistakenly labeled vacuum lines.

using generic autozone hoses rather than oem isn't proper and will never look right.

[Tobias Funke]

Cheers. Do you mean number 4 and number 5 are open air? OK.

Number 3 and number 6 are definintely vacuum, I can confirm with fingers. I'm going to try putting them onto number 2, since it seems that the crankcase needs vaccum. At least, lots of 4-stroke engines are like that. Idle is smooth so unless the main jet is very plugged, I'm guessing most of the carburettor isn't too bad.

Number 1 has been removed, it once attached to something but it was just an empty pipe leading to the downdraught.

[Tobias Funke]

Here's the vaccum collector I fabricated:
20220824_153417.jpg

[squirrelman]

there are NO vacuum lines on the carbs unless they are California emissions carbs.

[Tobias Funke]

I must live in Cali then bro, coz those side ports suck like a hooker on friday night

[6]

I must live in Cali then bro, coz those side ports suck like a hooker on friday night

USA never got a 400, so it aint a cali model. What you have labeled as 3 and 6 for sure vent to open air. those are not vacuum.

[Tobias Funke]

Yeah I was kidding, I'm in Australia. But in another country it might be different.

Anyways the vac thing didn't fix the issue by itself, but I connected the lines to CCV since I don't like having crankcase open the air anyway.

I took apart the carby, spikes and jets, as you guys suggested. Everything was filthy and sticky so I cleaned her up and it's working 100 now. cheers!


20220824_181032.jpg

[6]

Anyways the vac thing didn't fix the issue by itself, but I connected the lines to CCV since I don't like having crankcase open the air anyway.

Please see the hawk wizards answers to common hawk questions...

Specifically#2

https://www.hawkgtforum.com/forum/ho...hawk-questions