Gravity fed carb, fit a pump?

Gravity fed carb, fit a pump?

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Discussion

Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
Yeah, did read that John, needs some thought to relieve the pressure.


Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
Would this work with the aforementioned pump John?

https://www.ebay.co.uk/itm/Universal-Adjustable-Gu...

Or still too much pressure?

oakdale

1,293 posts

170 months

Sunday 6th June
quotequote all
This type of gravity system worked well on motorbikes for years, I think under bonnet heat combined with poor flow is the problem here, extra heat when climbing hills (causing vapour bubbles) combined with extra fuel demand.

Two stroke mixture is quite viscous, so I'd ditch the funny little filter in the tank and use one of these where the return pipe is fitted to the carb supply pipe instead.
https://www.ebay.co.uk/itm/183113979336

I would also try to route the fuel lines away from heat as much as is possible.

ETA Not sure the separator includes a filter, so an inline filter may be required as well.

Edited by oakdale on Sunday 6th June 13:16

gazza285

7,612 posts

176 months

Sunday 6th June
quotequote all
How is the carb fixed to the engine? Could be excessive vibration causing frothing and float bounce?

Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
gazza285 said:
How is the carb fixed to the engine? Could be excessive vibration causing frothing and float bounce?
Yes, easily this. Bolted straight to the crankcase with nothing more than a thick gasket.

And heat may well be an issue.

Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
oakdale said:
This type of gravity system worked well on motorbikes for years, I think under bonnet heat combined with poor flow is the problem here, extra heat when climbing hills (causing vapour bubbles) combined with extra fuel demand.

Two stroke mixture is quite viscous, so I'd ditch the funny little filter in the tank and use one of these where the return pipe is fitted to the carb supply pipe instead.
https://www.ebay.co.uk/itm/183113979336

I would also try to route the fuel lines away from heat as much as is possible.

ETA Not sure the separator includes a filter, so an inline filter may be required as well.

Edited by oakdale on Sunday 6th June 13:16
Can't easily ditch the in tank filter, it's integral with the fuel tap. There is an inline filter too that I put in. The return pipe is already teed off the carb supply.

oakdale

1,293 posts

170 months

Sunday 6th June
quotequote all
Tyre Smoke said:
Can't easily ditch the in tank filter, it's integral with the fuel tap. There is an inline filter too that I put in. The return pipe is already teed off the carb supply.
Can't you just remove the tap and cut the filter off?

I realise the return pipe is on the supply pipe, that's why I suggested putting the vapour trap there.

If the return is for degassing the fuel which I think it will be, its orientation will be important, the return to the tank should be at the highest point.

Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
Why though, it's not restricted at all?

oakdale

1,293 posts

170 months

Sunday 6th June
quotequote all
Tyre Smoke said:
Why though, it's not restricted at all?
It's just that in the picture I saw of one it looks a bit small and restrictive and you've got another filter in the system anyway, but as you've seen it you will be a better position to judge. Also I'd got the impression you had a fuel flow problem. smile

Tyre Smoke

Original Poster:

17,862 posts

229 months

Sunday 6th June
quotequote all
Yes, granted. But it'll be easier to remove my in line filter than the DDR designed one in the tank.

I'm not convinced this is the issue though.

SamR380

632 posts

88 months

Sunday 6th June
quotequote all
A Mikuni diaphragm pump might be enough. They're designed to work with two-strokes and don't need any electricity (they use pressure pulses from the crank case).

Alternatively some carburettor motorcycles use electric pumps, that might be worth looking into.

Error_404_Username_not_found

337 posts

19 months

Sunday 6th June
quotequote all
You shouldn't need a pump. Trabitha was designed and developed (haha) to run without one on petrol not much better than paraffin and whatever oil could be scrounged or stolen, while enduring basic (if any) maintenance of the butter-knife-and-mallet school.
All you have done in recommisioning is replace new for old on like-for-like basis and I'm confident you haven't bksed the mixture or timing so she ought to be happy, and she mostly is it seems.
But clearly there is a starvation problem. I still suspect a breather problem.

If you do decide on a pump the scuttllebut among A series Spridgeteers is the Facet solid state pump is okay. It's available down to 2.5psi.
Hardi is another fave but a bit on the spendy side.
But you shouldn't need one. It wouldn't fix the problem, only hide it.

bmwmike

3,503 posts

76 months

Monday 7th June
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Would a bigger bore pipe help, as it would hold more fuel?

Tyre Smoke

Original Poster:

17,862 posts

229 months

Monday 7th June
quotequote all
bmwmike said:
Would a bigger bore pipe help, as it would hold more fuel?
Often the simple solutions are the most obvious!

Why didn't I think of that? laugh

I replaced all the fuel lines with what I thought was the right stuff, but of course it might be a narrower internal diameter!

Ambleton

5,295 posts

160 months

Tuesday 8th June
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I'd also remove the inline fuel filter you have fitted as there will be a pressure drop through that as well, even more so if its not vertical as it will trap some air too.

The fuel you'll be putting in now will be much more free from contaminates than Sergey filled up with in the Eastern block in the 70's, I wouldn't bother.

Tyre Smoke

Original Poster:

17,862 posts

229 months

Tuesday 8th June
quotequote all
Good point Ambleton. The East German handbook states that it must ONLY be run on good quality petrol of MINIMUM 88ron premixed with MINERAL two stroke oil.

She must think she's died and gone to heaven running on Tesco 95ron and two stroke oil from a garden machinery place (still mineral, but low ash and easy mix).

laugh

crankedup5

588 posts

3 months

Wednesday 9th June
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Whenever my vintage gravity fed petrol car was asked to work hard, like uphill work for example, it would start to splutter and finally conk out. Went trough all the usual suspects before finding that the fuel line was partially blocked. Fitted a new fuel line and all was fine.

tapkaJohnD

1,615 posts

172 months

Wednesday 9th June
quotequote all
Tyre Smoke said:
Would this work with the aforementioned pump John?

https://www.ebay.co.uk/itm/Universal-Adjustable-Gu...

Or still too much pressure?
TS,
You can read as well as I, TS, that this PRv can be adjusted between 1 and 5psi - 27 to 140 inches. If that bit of Chinese cr%p was reliable - I've found the opposite with other Chinese PRVs, which were badly designed and made.

Tyre Smoke

Original Poster:

17,862 posts

229 months

Wednesday 9th June
quotequote all
Ummm. Thanks I think.

I'm going with a bigger diameter fuel line, ditching the in line filter and see how that works.

I agree with others, a pump shouldn't be necessary.

Captain Answer

999 posts

155 months

Wednesday 9th June
quotequote all
Surely the only real answer is to go out and spit a bit more in manually...