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MG MGB Technical - Intermittent fuel delivery re-visited

First of all, many thanks to those who made suggestions concerning my highway fuel delivery problem a few months ago.
I still have the problem on the 67' GT with new SU carbs (AUD 325 AF). These are the replacements Burlen sells for my car.
Problem: Around town the car starts & runs fine up to around 45-50 mph. At highway speeds it feels like the car is running out of gas, the engines looses power, then, the problem goes away for a minute then returns. It feels like the float bowls are running dry, then fill up again. The fuel pump is new, I have checked for air leaks in the lines, everything appears OK.
I am now checking the float adjustment in the SU's and here is the problem: What is the spec for the float height for these new carbs? They don't have the metal tab to bend on the float as the float is one piece plastic. It looks like you need to add washers under the needle/seat for adjustment.
When I invert the float assembly and depress the float to seat the needle, the float seems way to low, about 1/4" into the lid where you would normally want 1/8" between the machined lid and the float bottom (top?)
Help???
Steve
Steve Brooks

I had the same problem a few months ago...thought the fuel pump was going out or needed points, but after I got into it I discovered it was a loose wire connection under the hood! Take a look at your wire diagram and you should find the right color.
Bob Ekstrand

According to the Haynes carb manual the dimension from the lid to the float is .126 - .189 (3.2mm - 4.8mm) for plastic floats, and your right about using washers under the valve to set the height.
You might want to check the fuel delivery. The AUF 300 fuel pump should deliver 1 pint in 30 seconds, assuming you have a stock type pump.

Mike
'79B
Mike Janacek

This could be your ignition timing. I once had problems similar to yours when my timing was too far retarded (too close to TDC.)
I have also had problems with marginally bad fuel line connections - all appeared ok until I tore the lines out to replace them. Air was being siphoned back near the tank causing fuel starvation.
You may also want to check the points in your new fuel pump. New pumps can sometimes be out of adjustment.
Jeff.

Jeff Schlemmer

If pulling the choke helps it is not fuel delivery but some other problem with carbs or ignition.
Paul Hunt

Steve, Mike's info is well taken as is Paul's. Working with the newer all plastic floats is a pain in the ar*e as sometimes I have encountered situations where the float valve has no washer and still the damned float sits too high when inverted. Personally, I think the problem lies in the specs given vs. the newer carbs being made not bieng made quite as their predessors were and the dimensions of replacement float valves being correct for OE carbs but not for the new ones. I may be wrong, but if I had the units to compare side by side, I think I might be able to substantiate my musings. All I know is that replacing the float valves in my newer Burlen SUs has been a frustrating experience, with rough running being resolved only by use of Grose Jets and some slight machine work to the mounting area for the float valves to get them to move down sufficiently to accomodate them. They are the right grose jets for HS-4s, but probably for the earlier ones.

Now I am able to have the float heights come out on spec. BTW. use a drill rod to measure the clearance and rest it on the lips of the bowl cover while the whole is inverted. You should not need to force the float needle to seat, gravity should be sufficient. FWIW.
Bob Muenchausen

Update on this problem..Tonite I measured fuel rate at the flex line feeding the carbs as Mike suggested. I got a pint of fuel in about 10-15 seconds. Hmmm...
As Bob M. suggested, I am installing a pair of Grose Jets I had from another car. These have a much lower overall height than the needle type currently in my carbs and I have a selection of the small washers that will get my float to the 1/8" spec.
Update on this later
Thank you all and wish me luck!!
Steve
Steve Brooks

Thanks, Steve, for corroborating my experiences with these float valves. Sorry my previous post got garbled, but I hope folks understood what I meant to say. Comes of trying to beat the clock.

I think that the newly made Burlen SU HS-4 units are different in detail and some of the differences are not terribly well documented - either for us or for our suppliers. I am not trying to rag on Moss, VB or anyone else, but if Burlen has actually made the changes I think they have, then they need to supply that info to our suppliers so that they can offer the correct replacement parts for both the Old OE SUs and the newly made repros. If suppliers like these are making the same assumptions we do - that these parts are all interchangable, when in reality only some or most of them are - then we all need to sit down with Burlen, so to speak, and straighten this out.
Bob Muenchausen

Got the Grose jets installed and floats set to 1/8" clearance, carbs back on the car and carbs adjusted. Went out on the freeway and.....problem is still there, maybe a little worse. Pulling the choke out made the car want to die even more.
I am going back to step one and eliminate electrical and then fuel issues.
Does the above info give anyone any more clues?
Steve
Steve Brooks

Steve - I still keep thinking back to the fuel tank vent not doing it's job. You say that the pump delivers somewhere on the order of 4 pints per minute. I would like to suggest that you rig up a large catch tank (bucket or something on that order) and let the pump run for some time and see if the delivery rate starts to slow down after a bit. An alternative to this is try driving on the highway with the gas cap off. Jeff Schlemmer also has a good thing to consider, which would be checked by letting the pump run continously for a period of time (I let pumps that I have overhauled pump kerosene for 24 hours before I am satisfied with their operation). Good luck - Dave
David DuBois

Dave..I tried driving on the highway with the gas cap off, problem still there.
I can drive around all day without seeing the problem if I stay off of the freeway, as soon as I am cruising at 55/60 mph the problem re-appears.
Steve
Steve Brooks

Icing?
Paul Hunt

This is classic symptoms of debris in the fuel tank plugging the pickup tube under high fuel flow conditions . I'd drop the tank and see if there's crud in it .

Why not put a simple pressure gauge just before the carbies and see if the pressure doesn't drop at the same time the surging starts ? that should immediately pinpoint the problem .
-Nate
NSH

Friends....I researched the archives today for any more info I could get on my weird problem. Found some threads on fuel regulators.
So on a whim I purchased an adjustable unit by Purolator from the local Jensen-Healey parts place for $20.00.
I set the pressure to 1.5 lbs and installed the unit. Out of 3 fast highway runs the problem only reared it s ugly head once, and that was for just a second! Bear in mind that the problem would show up each and every time without fail. I added this regulator because of the volume of fuel I got when checking the fuel flow right at the carbs, which seemed excessive compared what you folks were telling me was normal.
Steve
Steve Brooks

Don't know if this is of any use or not. When I owned my Cooper, I hit a large rock one time, out on a logging road, and then after that the car would starve for fuel at speed on the freeway, or on heavy footing it. I found that the fuel line had been flattened by the rock for a one inch section. I cut it out and bridged the space with fuel hose, and no more trouble. I'd like to think your problem is as simple!
Ken R
Ken Rich

Steve - The volume of fuel being pumped that you observed will not be affected by a pressure regulator. The SU pump supplies only the volume needed up to the maximum amount that it will pump when pumping free (ie. when pumping into an open container). Your pump should produce a pressure of 1.5 - 3 lbs. The fact that you experienced the problem, even only once, after installing the regulator would indicate that the problem is still there, just being somewhat masked by the regulator. I would have to agree with Paul Hunt or NHS that you may be experiencing icing (in Phoenix???) or picking up some debris in the fuel tank or have a flap of rubber in a fuel line that is picking up and blocking the fuel when the flow is sufficient.

Looking back over this thread, I see some suggestions about ignition or carburetor adjustment. Both would be good to check out. You might also have a valve problem. Try doing a vacuum check and have the results analyzed by someone who knows what the various results mean. Finally, check for a loose wire (power or ground) to the fuel pump. The ground wire is attached under one of the mounting bolts for the rear license plate. Make sure that the area under the ground lug is clean and free of paint and corrosion for a good contact. It also needs to be firmly tightened so that there is no chance of vibration causing an intermittent loss of ground to the pump. Good luck - Dave

David DuBois

I missed Steve's comment that pulling the choke made it even worse. Because pulling the choke affects it at all then I don't see how the problem can be lack of fuel delivery to the carbs i.e. crushed fuel line, debris in the tank etc. But because it makes it worse it makes me think of an overly rich mixture for some reason. As well as icing possibly reducing air-flow and hence richening the mixture I have also heard of float valves being vibrated off their seats at certain rpms. You might be able to determine this by determining the 'normal' fuel level in the float chambers, then driving so as to bring on the problem, switching off immediately and coasting to a halt somewhere safe, then looking to see if the fuel level is higher in either carb. Pull the fuel line off the carbs first (watch out for a spurt of fuel) then remove the chamber lids. But as a first test you could try driving so as to cause the problem, then back off to 40mph or so and see if it clears and how long it takes.
Paul Hunt

Paul...Yes, as soon as I slow down the problem goes away in about 5 seconds, driving around town I have no problems, car runs very strong. I did change to a richer set of needles from Joe Curto last fall because of I was running lean with K&N filters and freeflow exhaust. My sparkplugs were telling me of the lean condition, no soot, no deposits at all. But, I had the this problem before then, I was hoping the problem was of a lean condition. This weekend I plan to start from troubleshooting from scratch, starting with the electrical side of things.
Question...I rested my hand on the Lucas Sport coil I have after the freeway runs the other day, it was almost too hot to touch. Is this normal?
Thanks everyone,
Steve
Steve Brooks

Sounds a bit hot to me, although I don't think it is anything to do with your main problem, 5 secs isn't enough for it to cool down and correct anything, I'd say. Are you running it off 12v and what is its primary resistance? A typical Lucas Sport is about 2.4 ohms.

As for the other I'd try the fuel level thing, but first if you have the older style 2-wire in-series timing light you could try rigging that up where you can see it and see if there is any visible change in flashing between when the problem is evident and when it isn't.
Paul Hunt

This thread was discussed between 17/02/2002 and 22/02/2002

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