SU carbs inspection

[Buckshot]

I have a 1986 110 Landy with a 1998 V8 engine in it.

There's been an intermittant problem with it for ages now but has got alot worse recently.

Often, but not always, when I put my foot down she struggles to keep the same speed, not accelerating at all and sometimes slowing down. Remember this is a V8 not a diesel so it should move fairly quickly for a Landy when asked. :twisted:

I'm thinking the problem is either no fuel or no spark.
I've renewed the fuel filter and coil, HT leads, distributor cap, rotor arm and plugs. This has helped but hasn't cured it.

The next step is checking the carbs. The manual describes how to dismantle the entire carb and then inspect them but there must be a simpler way surely.
I've got SU carbs on this engine, does anyone have any experience of these ? I'm reluctant to take them to bits, because there a two of them I think that means I'll have to balance them when I put them back together - Is this correct.
This is getting to the technical limit of my tinkering ability so any help would be great.

Thanks

Mark

 

[Adi007]

I've got a V8 in a Stage 1 and have had a LOT of experience with SU carbs over the years (Minis, Maxis, Rover SD1s, Marinas, Landies ...), so I might be able to help you.

Have you changed the air filter and also made sure that the carb dampers are oiled properly because an over-rich mixture could cause that problem too (is there a lot of black around the exhaust (common when cold but not when hot)????? The slowing down could be a simple imbalance because of the dampers.

Also, before you rip the carbs apart, try cleaning them with a carb cleaning spray (you might need to take the pots off and clean any gum on there manually but the sprays will work - add some redex to the fuel too).

Here are a couple of useful links:

http://www.team.net/sol/tech/su-tune.html

http://groups.google.com/groups?hl=...anual&hl=en&lr=&ie=UTF-8&oe=UTF-8&sa=N&tab=wg

Also, did you change the points and condenser or is it EI system?

 

[Buckshot]

Wow, thanks for that Adi.

The air filter was changed about 1000 miles ago
Are the carb dampers the dip stick type things in the top of the carbs, if so they're full with oil (to within an inch or so)

Can you recommend a brand name for the carb cleaning pray.

I've heard of Redex but I thought that was an easy start spray - the engine starts very well indeed so I've not tried any, plus I thought it was for diesels.
I've not noticed excess amounts of black soot, they'res some but not huge amounts.

I didn't need to change the points as it's EI.

Thanks for the links I shall read that lot and try to inwardly digest...

Cheers

Mark

 

[MartiniDave]

I had a similar sounding problem on my series 3 which I couldn't decide if it was electrical of fuel related. While changing plugs, leads and coil I checked the fuel strainer just before the pump and found 1/4" of rust particles in the bottom. She goes like a dream now! (for a 2 1/4!!)

Dave

 

[Buckshot]

Dave,

My fuel pump is in the tank, I had the tank replaced about a year ago because the old one was leaking, since then I've changed the cartridge fuel filter 3 times (the last two didn't really make any difference but the first time was just after tank change so did help the preformance).
I don't want to look at the pump yet because it's such a messy job I thought I'd leave it as a last resort !

Thanks

Mark

 

[Adi007]

Might be worth giving the fuel line a blow out into the tank and checking it fr any kinks/crush marks just in case.

Redex is the red liquid available in pretty much every petrol station ... just add it to the tank. As to a carb cleaner, Wynnes Carb Cleaner is pretty good (Halfords).

 

[MartiniDave]

Mark,

I guess the pump set up must be different for the v8, might be worth checking to see if there is a strainer anywhere on the system, although I guess Adi would know that.

Dave

 

[Adi007]

If I remember right, there's a pre-filter in the tank but that's pretty massive and unlikely to clog up.

 

[Adi007]

Here's what I'd check ...

- That there's oil in the dampers
- That the pistons move freely in the pots (unto the pots and try them). Clean out the gun with WD40 or carb cleaner.
- Check that the needle is straight, clean and unworn

Chances are with SU carbs that the problem is a small one!

 

[Adi007]

Here are three of the best SU carb pages I could find on the web ... just in case!

http://www.mgdriversclub.com/tech/tuning.su.carburetors.html
http://www.moreg.org.au/carburettors.htm
http://www.zhome.com/ZCMnL/tech/su.html

 

[Buckshot]

Thanks everyone for your input, I was hoping there'd be some experts out there :-D

Is it possible to take the dome cover off the top of the carb in situ rather than take the whole carb off the engine. Can you see enough by doing that to check the piston and then the needle (if you remove the piston) or is there an inspection plate which is easier to remove ?

Cheers guys
:ekt:
Mark

 

[Adi007]

Thanks everyone for your input, I was hoping there'd be some experts out there :-D

Is it possible to take the dome cover off the top of the carb in situ rather than take the whole carb off the engine. Can you see enough by doing that to check the piston and then the needle (if you remove the piston) or is there an inspection plate which is easier to remove ?

Cheers guys
:ekt:
Mark

Just unscrew the (3?) screws and lift the pot (dome) off. The piston will then lift out. Check it in the pot for smoothness. That way you can check the needle too (don't drop it though!). No need to remove the carbs off the engine for that at all.

 

[Buckshot]

Brilliant, thanks Adi - I was hoping that was the case

:You_Rock_

Cheers

Mark

 

[bushwacker bob]

what a well timed thread this is. I have an almost identical problem with my beloved V8 range rover. Starts and run well when stationary but coughs and splutters when moving. I have the mechanical skill of a birch tree. so please let me know what the most likely cause is. :-?

 

[Buckshot]

I spent a very enjoyable :shock: Saturday morning and half the afternoon huddled under a tarpaulin taking the carbs off.

The result is my carbs are fine, the pistons are running free, there's no wear on the needle and eveything looks clean, fine and dandy.

Which still left me the problem of why the lack of power ?

So on sunday I tried to blow the fuel pipe back through to the tank.
I took the fuel filter out and found air was coming out from there. So I put the case back on and tried again but couldn't get a seal properly so the air escaped rather than went down the pipe. I'd had enough so put the fuel filter back in but the air leaking problem was obviously there because it took 4 goes to fit it properly. :yikes:

If I'm feeling brave I may have another go and see if I can confirm the fuel pipe is clear, after that I'm stumped.

Adi thanks for your advice on this, I wouldn't have known where to start otherwise

Cheers

Mark

 

[Adi007]

I wonder then if ... the electric fuel pump is not pumping well enough (V8s burn a lot of fuel at high speed :shock: )

When you have the key turned half way you should hear the rapid tick-tick-tick of the fule pump (initially 3 - 4 ticks per second). To check, disconnect the fuel pipe from the carb and tuen the key to position 2 and see if the delivery is fast.

Could also be muck in the needle in the float chamber or a low float meaning less fuel getting through ...

It's a process of elimination ...

 

[Buckshot]

Yesterday evening I was huddled under the tarp again trying to fix/ identify the problem.

I diconnected the fuel pipe from the carb, took out the fuel filter and replaced the case and then put a foot pump on the fuel pipe to make sure it was clear. I got to 30 psi several times but nothing moved.
Then I took the pipe apart about 2 feet from the tank and tried again, no resistance at all pumping from the engine, but the same resistance into the tank. But when I turn the ignition on fuel pumps out with no problem at all. So I don't think there's a problem with the fuel supply.

Because fuel is coming out so well I'd be suprised if the pump is at fault.

As you say Adi, it's a process of elimination, and damm annoying it is too

I shall continue on my quest ...

Cheers

Mark