Help with MQ carby

[TimMQ]

Evening all

Picked up a replacement carby for my 84 l28 patrol today. I believe it is from an earlier model than mine as it has a few differences.

The only difference that i cant work out is it has no return fuel line outlet like the previous carby.

Can i simply block off the the return fuel line on the car to use this carby?

My patrol runs an electric pump, would i have to convert to a mechanical pump or am i way off.

any help appreciated
Tim

ps. go easy on me im of the fuel injection era

[David_S]

The early MQ's with petrol engines all used return fuel lines according to my early Nissan Service Manual. The L28 used an electric pump and the P40 a manual one. Both engines used similar Hitachi carbs, model DAF342-44 on the L28 outside Europe and DAF342-52 on the P40 outside Europe. On my P40 carb there is no distinguishing mark other than a Hitachi symbol and a "spoked wheel" with a "1" in the middle, on the casting incorporating the fuel bowl.

I am not familiar with the operation of electric pumps and am definitely not a carb expert, but if you are using the original pump it would seem advisable to still use the return system. Can you use the inlet from your old carby? Remember that a positive pressure is maintained in the filter/float valve chamber by having the inlet hole larger than the outlet (return) hole. On mine they are approx 4mm and 1mm ID respectively. I once inadvertently connected the hoses incorrectly and it took me a long time to figure out why the truck suffered fuel starvation.

[TimMQ]

thanks for your reply David s. I can use the inlet from the old carb. However the person before me had stripped the bolt. I was hoping id get away without having to drill/dremel it out. By you suggesting i do that i assume the one inlet carby will support a return line? ie, theres nothing else different on the carby i have to change?

hope that makes sense

cheers
Tim

[TimMQ]

also

what does the anti dieseling solenoid valve do and do i need it?

cheers
Tim

[David_S]

As I said, Tim, I am not a carby expert, but am only applying common sense. If there are any carb experts out there please butt in!

What I think is that the purpose of the fuel line is to supply fuel to the float chamber. Once there the carb should work regardless of how the fuel got there.

With mechanical fuel pumps you can't turn them off. They are obviously designed to supply fuel at a rate in excess of the maximum demand of the carb so there has to be a return line. To avoid all the fuel heading off down the return line there has to be a restriction in the line at the carb inlet as I described in my last post to maintain pressure on the float needle. The pump has to provide enough pressure to pump the fuel around and operate the float needle.

Electric pumps seem to provide higher pressure than mechanical pumps. My mechanical pump is rated at 2.5 l/min at 1000rpm with a pressue of 3psi. The electrical pump on the L28 is 1.6 l/min according to the service manual but no pressure is given. (Incidentally I have just noted from the manual that some L28s had mechanical pumps).

Anyway unless there is some means of the electric pump switching itself on and off (a pressure operated switch say) there has to be a return line. So if using the original Nissan electric pump it would seem that you must use a return line. As I said before the carb should not notice the difference as the fuel in the float chamber is at atmospheric pressure. On my Hitachi there is a vent pipe from the float chamber to the air filter to ensure this.

One last point is that on my mechanical pump system there is a solenoid operated check valve downstream of the carb. This is not mentioned in the manual or shown on the wiring diagrams but presumably is there to prevent the fuel siphoning out of the system when the engine is turned off - "depriming" the pump. I don't know if electric pumps need these - probably not.

I hope this has not confused you! Any carb experts please correct me.

Finally I might mention that I have been having problems with my carb flooding when at a steep angle for long periods (i.e. winching up steep hills) so am looking at the possibility of converting to fuel injection! If I do so successfully I will post a "howto".

David

[David_S]

A correction to my last post. The P40 wiring diagram does show the fuel shut-down solenoid valve, but it is not shown on the L28 diagram.

My P40 carbs (I have 2 - one spare) do not have an anti-dieseling solenoid valve. The Service Manual shows the valves as "Europe only" and my later Parts Book shows them as being fitted generally from Dec 83 on the L28. Presumably they are to stop the engine "running on" or "dieseling" after you have switched off. This may be a characteristic of the L28.

If you don't have a problem with "dieseling" then you probably don't need it, but if you remove it I presume you will have to plug the hole to stop fuel leaks. The Service Manual merely says

"1. If engine does not stop when solenoid connector is disconnected, replace solenoid valve.
2. If click sound is not heard from solenoid valve when ignition switch is turned on-off replace it"

David

[bluemq]

I got a carby from an MK L28 if you want it.

May be easier in the long run to just use that.

[TimMQ]

once again thanks for the replies. Carby is on and car is running.

doesnt seem to be running on so wont worry about the anti dieseling shite

Although- wont hold a steady idle

[David_S]

Not idling properly? Check for air leaks at manifold.

[matsmad]

Hi Tim. Did u bolt straight on with manifold or did u use ur on manifold to bolt it onto? Either way, Id replace the gaskets.

[TimMQ]

Hi matt

I used my manifold. Didnt replace the head to manifold gasket i should tho. Did the carby to manifold gasket, new accelerator plunger, needle and seat.

[Heathx4]

My 82 L28 w/ elec fuel pump has a two pipe inlet to the float (although my manual pages show a one pipe inlet). One of them is blocked off (so the one pipe inlet would suit me fine). The other runs to a boss that is bolted to the manifold near cylinder 2 or so. That metal boss has an attachment at the bottom for the flexible hose that runs to the carby, plus two other outlets that run metal tubes back to the hoses that run to the fuel tank (one via the pump). Presumably it is this separate device that allows the excess to return while still getting fuel pressure to the float bowl.

Do you not have this two pipe one outlet adapter dooby?

And the anti-dieseling thing is pretty useless. Mine's there but I don't think it was ever plugged in. It stops the engine running on after you've switched the key off. If yours doesn't do that, don't worry about it.

Welcome to carby's mate. You'll soon be sick of them like the rest of us.

[Big Red Toy]

let me know when you want to put a V8 in i have a spare bellhousing

Yeah l28 carbs suck caused me grief for years and years, now gas