Fuel lines and setup for dual idfs - problems

Hi guys, hope someone can offer some advice. I rebuilt my 1600 bay engine over winter and after a few cockups solved with help from Mike J and others have it running quite well. One problem i have now is the dual weber idf setup. When i rebuilt it the original fuel pump had to be replaced and i think this might be where the problem has come from. Originally i had the carbs running off the stock pump and that seemed fine, but i have a feeling the new pump is over pressuring the fuel lines as i have fuel coming out the passenger side carb dripping on the tinwork. I know that the webers need low fuel pressure so my question is what the best solution is- do i just fit a pressure regulator (where would u mount this) after the fuel pump and stay with the mech pump or go to all the hassle of electric. I am unclear why people go electric as it sounds like adding complexity and cost so any advice would be good. Also advice on how to convert from fuel pipe sizes would be good as i think the tank outlet is one size and electric (read about cb rotary) pumps something else? Ideally as i am sure lots of people have been in this situ before a full breakdown of what i need to get would be brilliant so i can get it right first time would be awesome!

 

I have a mechanical fuel pump, then malapassi filter king which regulates the fuel pressure to my dellortos.

Ill try and dig some pics out.

The malapsssi is best mounted somewhere that doesnt vibrate. So avoid mounting it on the fan housing!

Mine is on a bracket connected to a seam under the spare wheel well.

Hopefully some others will be along soon.

 

Or just buy a 3psi rotary electric fuel pump

 

Thanks - i would prefer to keep it mechanical so thats great. Did u need a fuel like size adapter or something to go on to it?

 

I changed to a CB rotary pump for reliability. No need for a regulator, simple installation and solid reliability – and no chance of a mechanical pump dumping fuel into the sump when it fails.

You can use a fuel filter to change pipe sizes if you need to.

 

Here is a picture of mine from a long tume ago, before wiring was sorted and fuel pipes changed and simplified.

It has a pipe from the mechanical fuel pump to the malapssi, then from the malapassi out to the carbs.

@77 Westy & @lhu1281 know what they are talking about far more than i do, so the electric rotary pump may well be a better idea.


Cant even remember why i ended up with a malapssi, except it was to regualte fuel pressure to my carbs. Probably recommended by engine builder or rolling rd. Don't know why they didnt suggest electric fuel pump.

 

You mention the filter dumping fuel into the sump, I wonder if thats what blew my engine- all was fine and then within the space of a mile it seized on a cool engine. When it went back together it took me a while to work i why i had no fuel getting through to the carbs on the original pump as thought it must have been ok as it was off the same engine. Is that a common thing with the pumps?

 

With ethanol enhanced fuel mechanical pump failures are becoming more common and when they fail fuel drips into the sump. And you only notice when the oil level increases but by then the engine has been lubricated by a mix of fuel and oil.

 

Original fuel hose size is 5.6mm bore or there about. You can get plastic or better still brass step up adapters off ebay if you want to go up to 8mm fuel hose. As for the pump choice is yours stay mechanical & fit a regulator or convert to electrical.

 

Paul thats a lovely looking noisey cupboard
Just needs a shiny 123 dizzy to finish it off

 

One day!

 

Thanks for all your advice. So if i go electric i will presumable need-
Fuel barb adaper to step up from 5.6 to 7.6mm from tank.
Rotary cb performance pump with filter built in 3.5psi
- do i just go 7.6mm fuel hose through the metalwork to a T adapter in the engine bay or should it be metal pipe?
Then theres the wiring - so i need an ignition live to the fuel pump i guess - does it need a relay ?

 

Keep your mechanical pump if you want to. It’s more than capable of keeping your carbs fed.

If pressure’s a concern, fit a Malpassi Petrol- or Filter King in the line to the carbs. Worth also checking that your float heights and float valves in the carbs are ok and working.

 

I think i like the idea of simple and the fact that mechanical means it will start with a bump on a low battery but the knowledge that the cheap mech fuel pumps can shag your engine is a worry after the headache of rebuilding it. On balance though i reckon i will go with the malpassi. My 40 idf carbs have i think 7.6mm hose. Will that diameter go on the malpassi that is listed at 8mm.

 

True 8mm will be fine, clamped up. The line from my pump to Malpassi is 6mm: can’t remember if the Mal has a stepped input. It’s fine, anyway. I’d recommend the Petrol King without the filter, which doesn’t really do much. Worth checking your carbs as well.

 

A cheap new mechanical pump may over pressurise the fuel system.

I am currently using the rebuildable mechanical SSP pump from Heritage that costs about £35 https://www.heritagepartscentre.com/uk/113127025f-fuel-pump-for-100mm-pushrod-alternator-type.html

This is cheap and needs less plumbing compared with the Malpassi or electric pumps. It also claims E10 compatibility. I shall test the pressure tonight.

If you go electric,the Carter rotary pumps have builtin regulators, which are based on a flow bypass ball bearing and spring rather than a demand valve in the Malpassi, which needs a pressure drop to work, which could end up with fuel starvation on a bigger engine if the pump input is already quite low pressure.

All needle valves in carburettors eventually wear with vibration from the engine. I suppose that as the Malpassi has a non replaceable needle valve, vibration will stop it working permanently.

 

Definitely worth checking the needle valves.

 

Thanks mike for your advice. Be very good to know the pressure off the ssp pump. So you dont have a regulator after the pump on yours with dual carbs- is that right. Youmention checking the pressure - how are you doing this- do you have one of those in line guages?

 

Reading the spec on the machine7 malpassi it says it has a stepped inlet/outlet so will take both size pipe. Only prob is that it doesnt have the guage connection on it to check what you have it set at which is only on the ones with 8mm inlet/outlet sadly.