MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - gas starved carb

I am trying to get my '75 started (bought it not running). It has a weber on it and a facet fuel pump in the trunk. I have confirmed fuel flow to the carb but it seems to stop flowing for some reason. If I disconnect the fuel line at the pump and reconnect it does fine for a few minutes then stops flowing and continues to tick enlessly. When I disconnnect the fuel line at the carb it doesn't flow until I disconnect & reconnect the fuel line at the pump, then it pumps enthusiastically. One last comment - it backfires through the weber (flame included) if I get it started and try to goose the accelerator to keep it running but runs great for a minute or so occasionally on its own. I'm open to suggestion.

Randy Myers
'59 roadster, coupe, '75 B
Randy Myers

The OE fuel pump should stop ticking once it has reached it's peak pressure. If yours continues to tick and you are not cranking, then your pump has a hole in it's diaphram (I believe).

Since you can remove the hose and the pump comes back to life after reattaching, I'd assume that's because likely a lot of fuel came out of the line while disconnected so it now is only "compressing" air in the line. So perhaps that tear in the diaphram is small enough for the pump to work, but once pressurized, the fuel pushes through the diaphram instead of building a constant pressure.

I can't really help on your backfire through the carb issue as I'm not a carb guy... but have you confirmed your timing?
Jeff Grant

Randy - The Facet pump normally runs all the time (this is due to it's using a bypass valve internally to establish output pressure). I suspect that the pump is dying from your description of the symptoms. Facet pumps are relatively inexpensive and available from local auto parts stores. See the article, Backup Fuel Pump in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ for part numbers of the Facet and Carter pumps. Cheers - Dave
David DuBois

Sorry, I was thinking the stock OE pump, not a facet. I agree with Dave's comments above and in fact recently installed a $40 carb fuel pump from the local parts store to replace my OE pump that had likely gummed up points.
Jeff Grant

Weak mixture can cause spit back in carbs so this should vanish once you get the fuel supply problem fixed. If changing the pump does not do it try the venting of the fuel tank or possible build up of debris in the tank.
Stan Best

Monday morning update-

This weekend I replaced all the rubber fuel lines when I found one spraying gasoline all over the trunk. I also bypassed (experimentally) the round flat canister in the trunk. I had disconnected it and blown it out and replaced it and the car ran for 10 minutes until it stopped. Upon bypassing it the car runs without failure now. The engine runs really rough and at a high idle (2000+rpms)I suspect the Weber is in need of a kit. I unplugged a vacuum hose that came from the vacuum advance and the idle dropped about 500 rpms only to rise again when I reconnected it.

The timing is the next thing I'm going to look at. I did pickup a new facet pump (one that I'll carry as a spare in the MGA) but saw little difference between the two. It seemed like airlock was the biggest problem combined with air sucking gas lines.

Thanks for your responces thus far,

Randy Myers
Randy Myers

I have always found blowing some carb cleaner through a fixed jet carb is worth a try.
Stan Best

The tubes and canister in the trunk are for the tank ventilation system, strictly speaking there shouldn't be any fuel in there. The canister is to prevent fuel being pumped into the charcoal canister in the engine compartment if the tank is overfilled and then expands, it should drain back into the tank as the level lowers, which is why it is higher than the tank. A blockage in that system will cause a vacuum to develop in the tank which will starve the engine of fuel, which reveals itself as a large 'gasp' when you remove the fuel filler cap.

The drop in idle speed as you disconnect the vacuum advance pipe is normal, it occurs on all cars to some extent, but is more noticeable on manifold vacuum cars and can stall the engine.

The valve clearances, plug gaps, points gap/dwell, timing, operatinon of the centrifugal and vacuum advance mechanisms and the condition of all components in the HT system should be addressed before fiddling with carbs - "90% of SU carb problems are due to Lucas electrics" (yes I know you have a Weber but the principle still holds true).
Paul Hunt

Stan & Paul,

Thanks for your comments. Taking Paul's advice the next step was timing. I found it at about 20 degrees and moved it back to 12-15. Turned off the engine - turned it back on and it was at TDC. Reset it to 12-15 engine off / on and it was at 20+/- again. Even when correctly set the mark moves in about a 5 degree swing. While I was at it, I moved the pickup to each wire to see if I was getting power to each plug and noticed that when on #4 the timing was the same as on #1 plug. I'm not enough of a mechanic to know if that is OK or means something.

Thanks for your help,

Randy
Randy Myers

Sounds like your distributor or timing gear is badly worn. Explain in great detail how you're doing the timing though so we can see if you're doing everything by the book or whether you're going wrong somewhere.
Ross Kelly

I have seen this once on 1725cc Sunbeam Rapier, that was eventually traced to a broken dizzi drive dog on the camshaft. Hopefully your problem will be less dramatic than that. Have you taken the dizzi cap off and looked at the condition of the centrifigal advance and retard? It may have some rust causing it to move sluggishly and stick. You should be able to hold the rotor arm and move it clockwise(advance) and see it spring back when you let go. My old 25D4 dizzi had about 20 thou of wear in the top bearing but the car still ran I replaced it to get the advance repeatable for modern low octane fuels . The vacumn advance should either work or not, testing this has been covered before.
Stan Best

No. 1 and No. 4 *should* flash at the same point on the crank pulley because the crank pulley turns twice as fast as the distributor rotor (and camshaft), hence is in the same position when both are firing.

I'm not sure whether you are saying the timing is varying from 12-50 to 20 when measured dynamically both times, or when you set it statically to 12-15 it shows up as 20 when tested dynamically. If the latter, then on a US 75 this would be normal as full vacuum advance is applied at idle. Dynamic timing on *manifold* vacuum cars must be done with the vacuum disconnected and plugged at the manifold. It doesn't make any difference on carb vacuum cars as there is no vacuum advance at idle.
Paul Hunt

When timing it I hooked up the timing light (pickup on #1 plug), loosened the dist. locking plate and pointed the light at the pulley and comb and tried to alighn the pulley mark with the comb between the second and third knotch from the top +/-. After I turned off and then restarted the engine I checked the timing (after tightening the locking plate nut and being careful that that didn't move the dist.) and found that it was no longer ar 12-15 degrees but up at the top knotch (TDC). Readjust, turn engine off/on and now it reads 25 degree +/-, readjust, etc.

Can I take a distributor out of a spare motor ('64 1800) and swap it out to test for a bad dist.?

Thanks for your endless help,

Randy Myers
Randy Myers

For test purposes the dizzi will do fine. All sorts of advance curves were run over the years but to get it going OK as opposed to squeezing the last bit of go out you will be fine.
Stan Best

I agree that it sounds like the advance sticking. Shouldn't be too hard to sort out.
Tom

This thread was discussed between 14/06/2008 and 20/06/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.