Fuel starvation during driving - fuel return to tank

E

eggbert

New Member
I currently run a V8 (auto) with a 3.9 engine, SD1 unleaded heads and standard HIF carbs with an electric fuel pump (front mounted - puller).
During sustained 70 mph + or hard acceleration, the engine suffers fuel starvation and I need to back off or throw it into neutral. I have adjusted the carb float level but note there is a considerable amount of fuel being returned back to the tank via the return pipe. the pump supplies enough fuel for this engine.Can you suggest a means of restricting the return flow at the carbs or should I use a 'pusher pump' mounted at the tank.
Anty suggestions for this size of engine - thanks
 
Eggbert

I'm jealous, 3.9L I bet you rumble more than Ron ! When you get the car sorted you'l have post the 0-60 time (is it faster than Pete the editer's V8 ?).

You should be able to find out the rating of the pump (ie 100 to 180 BHP) from the supplier and be confident about that. If plenty of petrol is returning back to the tank, and the floats are not sticking shut (and there's no other blockages) then a surplus of fuel is being delivered to the carbs.
You do not say whether you've had the carbs re-calibrated. If not find a good rolling road with a tuner who knows about SU's (not some place who've never seen a carb, see if they tune Mini's !), the timings probably non-standard as well.

The pistons may be sticking, push them up and see if they fall steadily, the same speed both carbs, a good clean in carb cleaner may be all they need, (do'nt mix up the bits of both carbs though).

When you get in these conversations you never know whether you are teaching you Granny to suck eggs, hope this is usefull and good luck
 
Ron - thanks for the reply. I use BBW needles (not confirmed by rolling road though) which should be a suitable profile - the car spluters at speed and pulling the choke to en-richen the fuel mixture doesn't seem to help. It may be the floats but I have set them to alllow more fuel to enter the carbs. Can you confirm what the fuel delivery requirements should be for a V8 - the HP rating may be arbitrary if the car guzzles 18-20 mpg. I thought more fuel may be going back into the return fuel line rather than going into the carb as the two pipes share the same junction point. Although this is standard on all P6s.
Are puller pumps more preferable than pullers - the MGB V8 uses a pusher pump.
Eggbert :(
 
As Ron said, don't want to state the obvious but, what sort of colour are the plugs ??
If its spluttering it could actually be running rich rather than starveing.
Usual plug checks, if they're black its rich, if they're white its weak.

You may find the needles & jets are giving it too much fuel.

Could also be ignition related, a failing condenser can give similar effects.

Hope this helps

Richard
 
Thanks
The needles may be making the fuel a bit rich - difficult to judge the required spec given the SD1 heads (larger valves) and larger displacement engine without a rolling road. The plugs are grey though.
 
Ouch Mr Webmaster, ouch !. It could be the condenser first, but in this case.......... (it was a dig at me, was'nt it Richard ?).

MGB's and Midgets used to have there SU electrical pump at the back, so the BV8 would just use the same system if MG could get away with it. I don't know why it was at the back pushing, my Midget's pump was like that, they just got covered in muck, got wet and packed up. The same engine in a Morris Minor had the same pump under the bonnet, pulling. My Europa catalogue has Facet pumps rated by BHP, the one rated "up to 200bhp" (is that enough ?) pumps 27 gallons per hour.
I must admit I do not know enough about needles , I have always taken my engines to a rolling road. I still recommend this, it cheaper than a couple of valves and has always made a big difference on my engines. I mean, after spending all that money on the engine, why not make sure it really is all working at its best ?

An ancedote; I modified a Midget engine and it would run OK round town, but after a couple of miles cruising at 70 down the M1 it would start to splutter, and sometimes stopped. On this occasion it turned out that I had the wrong heat grade of plugs. After my modifications the standard plugs would overheat and short out. It was sorted on that rolling road.

Still want to know how fast it goes.
Ron
PS. I do'nt have any shares in rolling roads.
 
No Ron, not a dig at all, Just me looking for the simplest solution.

I'm always one for "could be this, could be that, could need a complete rebuild" only to find the problem is a dodgy conector !, so I thought I would throw in some simplicity.

I had a problem on my 2.2TC where it started loosing power at 75 ish, then 70, then 65... At which point I decided to take a look. Turned out the fuel pump had come loose on the engine and so wasn't being pumped fully. Bolted it back up and it was fine. I was thinking it was time to bin the engine....... LOL

Richard :D
 
I can't find out how fast it goes because it starts to splutter at 70-80 mph.
I restricted the return pipe so much once that the fuel just poured out of the vent pipe - despite adjusting the floats.

I thought the reserve tap may not be adjusted correctly but that seems ok - will probably remove it anyway and use the reserve fuel line.

The idea that the needles used are too rich is interesting. If the engine size is increased and head is different but the air filters and carbs are the same, then the needles may not need to be changed - does anyone know of a suitable rolling road in the Hertfordshire region ?


Eggbert
 
I've had a thought regarding the fuel flow in the return pipe.

I assume you can't actually see how much is flowing when your at speed, in which case its possible the there isn't enough flow still from the pump.

Another question.... At say 70-80 when it splutters, does it still splutter regardless of throttle, i.e. if you close the throttle so it just holds the speed, is it ok ?, and if so, if you hold the speed for a few seconds, and then floor it again, does it immediately splutter, or does it accelerate a bit first.

This is to try and eliminate fuel pump as a factor. Obviously the engine requires a lot less fuel at 70 with just enough throttle to hold that speed, than it does a 70 with the throttle wide open. And if you hold the speed for a few seconds, it should give the pump chance to fill the floats again. So if it immediately misfires then chances are its not pump related .....

Well thats my logic anyway.

Richard ???
 
Eggbert

Just following through what Richard was saying. If you think about it, the car will need the same power to cruise at 70 regardless of engine, ie the amount of power needed to push the car along has,nt changed. So all things being equal, about the same amount of petrol will be burnt at 70mph by your 3.9 angine as by the 3.5 engine. So if this problem happens when criusing at 70, I ca'nt see it being a problem getting fuel into the carbs. With the throttles wide open I should think it would take even a 3.9 engine quite a few seconds to suck a float chamber low enough to give probs (3.5 to 3.9 is not a huge increase % wise). Remember the fuel going into the engine is sucked by venturi effect from the float chamber, all the pump does is keep the float chamber at a reasnable level. When the float chamber is full, the float valve shuts and the excess goes back to the tank. Shame you cannot see whats going on whilst doing 70....you can on a rolling road! (sorry I won't mention them again)
I guess you know the usual prob with SU's is the valve NOT shutting and the chamber overflowing, giving a rich mixture if your lucky, or petrol all over the engine bay if your not (engine fires !!!). SU pumps worked differently to mechanical pumps by the way, they had a pressure sensore that stopped the pump pumping when the float valves shut, the problem was they often did'nt start again when the valve opened !, I hate old SU pumps by the way.

It [/I]could be something simple and unrelated to your mods, like a condenser, worth checking out, we all know what sods law is like.

Check out the V8 Gurus at http://rover-v8.community.everyone.net

There I've typed a lot again, here endeth the lecture, good job theres nothing on TV.
Good luck
Ron
PS Sorry I cant help with rolling roads in your part of the world.
 
Web Master , Ron - thanks for the reply. Having done a few trials last night (nothing on TV) I can confirm that the car will sustain 70mph continously. Accelerating to 80 mph causes the car to splutter after about 10 seconds. (Also hard acceleration form low speed to about 80 mph causes car to splutter once hitting 80ish). Backing off does not cure the problem immediatly, I need to reduce to moderate speed (50 mph for 10s or so) before I can run without mild spluttering. This suggests a fuel related concern. The car has electronic ignition and reving to 4000 rpm + has no issues with the spark creation.

The float level may still be out such that they shut off the fuel before the bowl is full or it may indeed be the pump.

The needles may be too rich (BBW ) rather than the std BAC - not sure if the 3500S had different needles than the 3500 given it had a larger bore exhaust. I have a SDI cylinder head and so the valves are larger which may necessitate the need for richer needles.
I have tried removing the fuel filter but this has no effect. Restricting or removing the return may be the only option.

:(
 
Eggbert

It does sound like some sort of fuel supply prob does'nt it.
As you might have guessed, I'm having difficulty believing that your upgraded pump is'nt capable of delivering enough fuel to the carb (or the pipes to thin), your mods are not that radical, but I stand to be corrected (as you probably will). When you get it fixed please post what it was won't you.

I also have an admission, Ron has just finished having SD1 heads and K&N filters fitted, but has not had anything else upgraded, so your problem may become very relevent !

Have fun
Ron

PS My new reslution is to not ramble on, typing aloud so to speak, apologies.
 
Ron

I'm booked into a rolling road at last - probably only to find out that the pump or needles are not right or something simpler !!


I put K&N filters on originally but thought they were a bit noisy! - I presume its an insurance declaration item. what needles are you proposing to use. I started using BBZ and than backed down to BBW. There's a good website with a SU neelde profile program which shows the next needle grade up or down from the one you've got now.

I retained the original air filter housing and just replaced the paper filters with K&N filters at each end inside - apperance unchanged!

Eggbert
 
Eggbert

You've found that you can get K&N's to fit inside the Filter housing too !. I wanted a quiter setup that looked more or less original, and got my motor shop to look up some K&N's that would fit in the original housing, and I thought I was the only person bothered enough to do this ! . I have also shortened the air intake to widen it to about 2.5". Demon Tweeks sell a "trumpet" (used in those boom boxes yoof fill their boots with) which has given the intake a nice big radiused ram pipe style end.

I do'nt know much about needles on the V8, I was just going to leave that to the RR man. Whats that web site ?, just to give me ideas (always a dangerous thing).
I am assuming the timing will be way out as well as I've got SD1 heads, HC pistons, a composite gasket and will be running on Super unleaded. What advance does that add up to ?!

Ron
 
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