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MG MGA - fuel starvation

I seem to have a problem with fuel starvation. engine will run strong thru the gears but after a mile or so leans out to point of missing. Backing off the throttle and coasting brings it right back only to repeat the same within a mile. In between it will rev freely past 5 thousand RPM. So far I have blown out the lines replaced the pump (new SU), fittings and the line to the tank, cleaned the carbs, dashpot pistons move freely, Tank is a few years old no rust is showing up when I tested the pump output, which was fine filling a pint in about 15 seconds free of bubbles except for the intial squirts. That points to anything but a fuel supply problem but what makes me belive its starvation is the way the motor weakens and pings just before it misses. I am thinking either the pump power is getting interupted when driving, I cannot hear the pump at speed too much noise, or a crack in the tank pick up tube allowing air in the line. Has anyone had such a problem? Chuck
Charles O'Brien

Charles - Remove the fuel line from the rear carburetor and direct it into a quart jar, marked at the 1 pint level. Turn the ignition on and time how long it takes to pump 1 pint of fuel - should be a maximum of 55 seconds. If you are not getting that amount of fuel pumped, or if the fuel stream is full of bubbles, see the articles Fuel Delivery Troubleshooting Guide and Air Leak Troubleshooting in the SU Fuel Pump Articles section of my Homepage at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
D W DuBois

Tank vent blocked?
Dave, he said it does 1 pint in 15 sec.
Art Pearse

Dave I have looked at your websight and did the test it passed taking only about 15 seconds.
Art, Tank Vent didn't think of that. I will check it out, Went for a long ride today after filling the tank I was able to go quite a few miles before it acted up again, almost thought I had it figured.
Charles O'Brien

Rear air filter box on upside down, blocking the rear piston air balancing holes?

Steve
Steve Gyles

"Dave, he said it does 1 pint in 15 sec." Oops, sorry, I missed that. Cheers - Dave
D W DuBois

I will go with Steve Giles on this one. If not it might be the needle valve of the inner carb getting stuck on and off. Also, check this needle valve for being screwed in place all the way up.

Frank
F. Camilleri

Usually a dying fuel pump or a clogged fuel filter. Check any filter on the car (look all the way down the tunnel back to the tank too) and also the gravel catchers inside the float bowl banjos.
Bill Spohn

Steve, Checked rear air filter box its right side up also checked fuel tank vent that is clear also. Just to cover all bases I took a look at the distributor and found a frayed terminal lead which I repaired to no avail. Tomorrow I'm hooking up a fuel pressure gauge to check pressure while driving. Any possibilaty a valve hanging up could cause this or am I just dreaming of the worst case?
Charles O'Brien

What Bill said about checking down the line, but also check the steel line for damage. When I tore my car down I discovered that the steel line had taken a significant rock hit, which put a kink in it. It's a wonder that the car ran at all.
D Rawlins

Del makes a valid point. Where did you measure your pint in 15 secs? Best checked at the delivery end at the carbs.

Steve
Steve Gyles

Steve, I used the fuel line going from heater shelf to carbs to check fuel volume.
Del, Fuel line is good down the line in fact I replaced the pump to tank line and fitttings when I installed the pump.
I ran it today after hooking a fuel pressure gauge into the line and it held a steady 3 psi no matter how it ran. I don't think its a fuel supply problem so that leaves the carbs which I love to tinker with but hate taking them off the car. Anyway spent the afternoon tearing them down found nothing obvious, floats are good, filter screens are clean, little to no dirt, no blockage anywhere and freely moving pistons. I'll have it together tomorrow and see what happens the Glen is coming up soon and I hate to end up there in a Saturn
Charles O'Brien

Throw some dry gas in the tank...it could be water. Just a little condensation in mine acts up. Just an idea and it only costs 2 bucks. You can rule out fuel when your done! :-)
Steven Devine

That is plenty of fuel pressure. In fact almost on the high side. For 12 years I was never able to tune my carbs lean enough. I had a facet pump pushing out 4.5 psi and a regulator reducing it to 3. After much discussion I lowered the pressure to 1.5 and immediately was able to tune the carbs to perfection. The high pressure manifested itself in my car with high fuel levels in the carb bowls. i.e. too much fuel was forcing itself through the float valves.

Stev
Steve Gyles

Charles

Following on from my lines of thought in the previous post, have you stopped and checked the colour of the plugs immediately you suffer this problem? In my cock-eyed way of thinking, if it is excessive pressure and over filling of the carb bowls this could manifest itself as similar to running the car with the choke out in the lower RPM band. When you hit the throttle (5000RPM you say) demand matches supply and the problem is not apparent.

I know I will get shouted down by everyone out there. Just trying to think of a few causes 'outside the box'.

Steve
Steve Gyles

Charles

A long shot but could it just be electrics rather than fuel as you don't seems to be making progress in finding a solution on the fuel side? After my experience from earlier in the summer I would never rule out the condenser for an intermittent fault. Long shot but....

Paul
Paul Dean

Steve I did think 3 psi was a little high but this problem started with the old pump and continues with new replacement. It is possible it could be loading up then clearing itself out it just don't feel that way. When I get the carbs back on I'll take along a plug wrench and do a plug check.
I To be clear it runs strong and will pull hard to 5000 rpm until it acts up. You first hear a change in exhaust tone along with what I took to be pinging though engine does not die there is loss of power leaving off the throttle and giving it a few seconds it recovers and all may be well for a few miles.
Paul I'll take any shot. I havent ruled anything out, in fact as I write this I'm convincing myself its a valve hanging up. I did pull the distributor for a good look over and found a very sloppy points plate after fixing that up it ran better(at least I told myself it did) but problem still came back. The condenser is on my list.
Charles O'Brien

Carbs together and back on, condenser replaced, engine timed, valve lash checked (they were tight) and it ran great, the longest is ever has, then it did it again. My daughter was with me and pointed out the exhaust sounded like a helicopter. I thought to compare sounds by killing the ignition at speed to replicate an ignition miss. Much different, leading me back to a valve hanging up. About 100 miles before this started I did have head work done in which a couple of bronze guides were replaced. Could they be tightening up? I did read Barneys site about using an old valve and lapping compound to loosen up tight guide, something the shop guys frowned upon. I've also been told bronze guides will wear in quick. I did get almost an hour drive at normal speeds with running hard the only way to make it happen
Charles O'Brien

I don't know whether you can still get the upper cylinder lubricant that was called Red-X over here. I don't think it would solve the problem but squirting it directly into the carbs, an old trick for sticking valves, would certainly temporarily change the situation hence would confirm your diagnosis. By the way do this away from others at it will generate a lot of smoke.

Paul
Paul Dean

Charles,

I had silicon bronze valve guides installed in my MGTC and the machinist didn't hone them enough to give correct clearance. When the engine started getting hot the guides would start binding on the valve stems.

Gene
Gene Gillam

Bronze guides do not wear in quickly. They can run for 100,000 miles with almost negligible wear. I am currently using a head with hard seats and bronze guides that is going on its third long mileage engine with the same guides.
Barney Gaylord

Well I'm at a loss. I took my engine builder buddy who guided me thru my rebuild and recent head work for a ride. He can't see it being a valve hanging up there's no backfire or popping thru the carbs also it takes a steady run at 4000 plus rpm to happen, good chance I would be touching the piston, yet once it straightens out it'll run like nothing ever happened, reving freely past 4000 till another steady high speed run. His thinking was more along the lines of ignition. Well so far I've replaced the pump, (new SU), tank to pump line and fittings, blew out the lines, checked fuel pressure , volume, dissasembled the carbs replaced the floats, needle and seat, replaced standard needle with RO rich, ran it with and without air filter boxes, loosened the valves, pulled dist, checked vacuum adv, replaced points, condenser, cap, rotor, fixed a very sloppy points plate, installed spare set of plugs. Coil is next.
I'm wondering is it possible that at speed something is causing the carb piston to drop in one of the SU's? They are an older Joe Curto rebuild which I have freshened a couple times. Shafts are still tight. Grasping straws now
Charles O'Brien

Charles,
Despite changing it, my money is on an intermittently defective rotor arm. As it shorts out it will give a similar feel to fuel starvation.

regards
Colin
Colin Manley

The valves do not hit pistons even if they do not return,
in the MGA engine.
Art Pearse

My money is on sticking valve guides. This is exactly the way they act, only stick with hot engine at high speed under heavy throttle. When the bronze guides get hot they expand. Being held in a stable water cooled iron head, they cannot expand outward, so they expand inward to constrict on the valve stems. When the valve hangs open the engine runs on three cylinders. Off throttle for just a few seconds, and it will cool down enough to run normally (until it gets hot again under heavy load). Been there, done that, got the T-shirt. The head has to come off to remove the valves to re-size the guides.
Barney Gaylord

Barney Thats what I keep coming back to as much as I try to talk my self out of it heads gotta come off.
Charles O'Brien

You say the dash pot moves freely, have you actually cleaned the pot and its piston properly. You could have a sticking damper piston.
In the "Carb damper oil use" thread here, today I wrote...

"If the piston and dash pot are wet with oil, any dust/grit which gets past the air filter will tend to stick in the oil film and can cause the piston to hang up in its bore and give a very lean mixture and a dreadful misfire. I couldn't even run at 40mph! Yes I've been there too.
I simply cleaned up the oily dash pot and the problem went away. Since then I hardly ever do any topping up. More recently I've also replaced the original oil soaked wire air filter with a felt type, it has be better..."

Have you done this? The car will run fine until a damper hangs up. After a bit of vibration / jiggling it can free itself and return at will. Good air filters should help. If you are still using the old wire type are they soaked in old oil? If not they should be - at least every year.
Pete
PeteT

Charles
Have you checked the rubber grommets on the banjo that connects the float chamber to the carbs. I had a similar problem and found one of the grommets was perished and causing one carb to misfunction causing hesitation on full throttle
Paul
P D Camp

Why not hook up a vacuum gauge to the inlet manifold. This will confirm if you have a sticking valve - compare vacuum reading when runing fine and when problem occurs.
Mike
Mike Ellsmore

Thanks for all the advice. Yes I have been thru the Carbs and took a careful look at the dashpot pistons and banjo grommets, all were good.
Mike, Thought of putting a vacuum port in manifold, should of done that when the carbs were off would of made it easier to clean manifold after drilling and tapping, still may do it for future use.
I haven't had the head off yet,decided on Watkins Glen for the vintage race this pass weekend. If I get a chance I will pull the head off this weekend. I'll post what I find.
Charles O'Brien

Charles, see photo of my Ryco vacuum and petrol pump gauge. See comments on identifying a sticking valve - maybe easier than removing the head as you are not sure is a sticking valve.
Mike


Mike Ellsmore

If you do consider a vacuum gauge there is no need to drill the manifold. I bought a couple of the thick spacers between the manifold and carbs that were already fitted with tubes for a vacuum gauge.

Steve
Steve Gyles

Steve

Are the thick spacers suitable for providing vacuum for a oil breather valve? Would also save me drilling the manifold.

Paul
Paul Dean

Charles, I'm having something of the same problem. I'm running H6 carbs.

I was racing this past weekend at Watkins Glen. At the top of the esses leading onto the back straight, I had what I believe is fuel starvation on several occasions, usually accelerating from about 4,500 rpm. One time I felt like I was coasting all the way to the Bus Stop before I was able to resume full power. (see 6:40 in this video https://www.youtube.com/watch?v=2MQtEn01Pr8) I did not have the problem at any other location on the track even though I took some corners in third well above 5,000 rpm. I did not have the problem on the main straight.

The last turn in the esses is a right turn, so centrifugal force would be left. My float bowl lid hinges are oriented front to rear. I have to double check, but I believe the hinge is forward on the front bowl lid, rear on the rear float bowl lid. I'm thinking that one of my floats is shutting off fuel on the uphill, under acceleration and with a sweeping right hand turn. I'm not sure how that would happen since it is counter intuitive, but that is my working theory.

I did notice the same problem at VIR in April, going up through the esses there, but attributed to the possibility that I was running too rich, a theory I have now abandoned.

So I'm very interested if and how you solve your problem.
Jeffrey Sienkiewicz

Paul

In Tuscany, Italy at the moment away from the car so cannot really make a judgement. l will measure the pipes at the weekend, but they are very small diameter. I have one blanked off and the other permanently connected to a vacuum gauge not dissimmilar to photo above. It gives a reasonable ongoing health check. It sits at 17 to 19 at idle and anywhere between about 5 and 15 ish in the cruise with a rapid response to throttle demand. Wide open about 0 and overrun about 25.

Steve
Steve Gyles

Pete

This is what I have: http://tinyurl.com/cn3tpsm

Steve
Steve Gyles

Charles
To me it sounds like your mixture is set too lean causing the valve stems to expand and drag in the guides
The pinging sound before the loss of power is a dead giveaway of this being the problem
What sort of fuel filter do you have and where is it fitted --after where you measured fuel flow ?
Some aftermarket filters have fittings for both 1/4'' and 5/16'' and I've lost count of how many of these I've seen fitted to 5/16'' lines without the 1/4'' ends cut off causing fuel starvation under load

You could try running your mixture a few flats richer and see if there is an improvement

Do you have stainless steel valves in the head - If so the guides need a tiddle extra clearance which is a head off, valves out and ream job
willy
William Revit

CORRECTION to above -- 1/4'' ends NOT cut off
William Revit

Jeffery Great video! Watched it all the way through hoping to see you pass. What a beautiful day to race. Yes that is how my car sounded allthough I wasn't chasing No.60 it was a diesel PU pulling a trailer. I would pass him on one of the long pulls that make up Rt 54 through the hills of PA only to have him pass me again when the car would act up. I should have an answer this weekend I decided one last check with a manifold vacuum gauge will tell for sure. Steve APT's manifold spacers are perfect thanks for the heads up. William I know I'm not lean I had gone up a needle size richer and plug checks are Ok, but I do have stainless valves.
Charles O'Brien

Charles
I'd still be tempted to try it a couple of flats richer just to see if it makes a difference
William Revit

ALSO with plug colours
the reading needs to be taken at the time of the event which means when it happens throw the clutch and head to the side of the road and pull em out and have a look at what you've got
Are the stainless valves a new addition
William Revit

William Thanks for the advice I had done much of what you said eariler on including a plug check when it happened. Its definitaly a valve hanging up vacuum gauge proved it. Although the stainless valves and bronze guides are not a new addition #2 exhaust valve and guide were just replaced due to carbon buildup on the valve pulling the guide down. Turns out guide was a too loose an interference fit. It took some machining but was able to use a bronze guide for an twin cam head they are slightly oversize. I bet thats the valve causing me trouble.
Charles O'Brien

A good lesson for all seeking a solution - give as much recent history as practical
Mike
Mike Ellsmore

Charles
I reckon you're on it - It would/will be interesting to compare the clearance of that valve stem in it's guide compared to another in the head when you get the head off
Just to explain where I was going with richening the mixture a bit for a try--------
Stainless steel valve stems are well known for needing more clearance as the stems expand more than normal stems when they get hot
By richening the mixture a few flats for a try I was sort of hoping it would be enough to keep the valve a little cooler and stop it dragging, proving the sticking valve
This would only work if the mixture was correct before richening it a bit
Are you real sure the mixture is heavy enough under load
If you can't get it on a rolling road you need to have a real good read of them plug colours especially if you're on ethanol added fuel

cheers willy

There are two ways of looking at this--
Firstly and most probable cause is,yes the guide is probably too tight for the ss valve stem
OR -although that guide might be a tiddle tighter than the others -a lean mixture could be causing the stem to expand enough to cause it to grab a bit
Where the mixture is is going to be the decider
William Revit

Stainless valves expand about the same as standard steel valves. It is the bronze guides that expand more than iron guides, and therefore need more running clearance. Don't be afraid to give the bronze guides some extra clearance, because they don't wear out like iron guides.
Barney Gaylord

This thread was discussed between 30/08/2014 and 22/09/2014

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