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MG MGB Technical - Weber Carb Woes

Greetings;
My darling 1970 MGB GT (wire wheels & chrome bumpers of course!) had been modified before purchase with a 4-into-2 header, some cylinder head work, and a Weber DGV and intake manifold swap. I have been attempting to get the little bugger to stop dumping raw fuel into the intake at an ungodly rate with no luck. The carb has been completely rebuilt with an OE kit including new float, needle assembly, and power valve - I believe the fuel pump to be aftermarket although OEM style - no markings in evidence. I have heard that the DGV is fuel pressure finicky, but haven't been able to get an accurate reading - any suggestions?

Thanks in advance!
MRP Porter

What model DGV? I am assuming that if you have a "4 into 2" header, you have a cross flow head?

In any event, the recommmended fuel pressure for the DGV is 3.5 psi max. They tend to run fine at the slightly lower pressures supplied by the SU pumps. I would assume that either your needle valve is not closing tightly, the float level is set incorrectly or you have too high a fuel pressure. A vacuum/fuel pressure gauge costs less than $20. Pat Braden's book, "Weber Carburetors" costs about the same and gives the fuel pressure specs, the rebuild procedure and the initial set up procedure. Do not know how you could have "rebuilt" the carb without these specifications. Les
Les Bengtson

Thanks for the advise - I've ordered the book AND the pressure gauge, although the output of the pump doesn't seem excessive - it's a 32/36 DGV, and I believe it is a cross-flow head... hopefully, I'll be able to get her running again! Thanks again!
Matt Porter

Matt- I blew the new $4k engine on the 77 MGB last year due to 4 corner scuffing after an open jet caused raw fuel to wash the cylinder walls. You say your Weber is dumping raw fuel, I say stop the motor, change the oil, and don't run that puppy till you've solved the problem. Otherwise I fear my plight may vist you and I'll be reading your next terrible-story posting. PS- A x-flow carburates from the left side when you are looking at the engine bay and my gurus say the HS6 or Web siders are the recommended aspirators over the 32/36.... although I'm hearing great things about the 38 mm Web "Outlaw" ( currently orfferred at Brit-Tek, for one) as equiv to the Web sider
vem myers

Having problems with webers on two cars, how can I get
a copy of Pat Bradens book. I have a Haynes manual on
the webers, but how should I say this, well it stinks.
Thanks
Tracy

Tracy, go to Amazon:-

http://www.amazon.com/exec/obidos/tg/detail/-/0895863774/qid=1061126195/sr=8-1/ref=sr_8_1/002-2238293-6679208?v=glance&s=books&n=507846
Derek Nicholson

Oil has been changed and all settings verified with Braden's book - (Border's $17.95) Still won't fire. The carb checks out for proper needle valve operation with Braden's method, and it seems to be functional. Fuel pressure is an even 3.25 PSI on the flow/pressure meter. We're moving on to the ignition system now to see if she's jumped time or something else equally hideous - thanks for you help!
MRP Porter

Although I haven't had a Weber dump raw gas I've had SUs and a Holley do the same. In each case it was the float needle valve HOWEVER a high float will do the same.
M Landskroner

Perhaps I should back up a bit for a better history of the situation - car was purchased last year for a rock-bottom price in POORLY running condition. There was a large amount of fuel being dumped into the motor - so bad that the oil smelled of gasoline after only about 150 miles of driving. (Was changed IMMEDIATELY) With the problems, we parked the car. It sat, only being started about once a month or so, fo an entire year. Circumstances lead me to decide to get it running, so I purchased a rebuild kit with power valve and a float. I've installed these and set them according to Braden's recommendations on an engine that WAS running before the installation. Now, the only thing the car will do is backfire through the carb repeatedly. The timing and ignition system has not been touched as of yet, although I suspect there may have been some drastic changes made to the timing by the previous owner to make the car run with the fuel-dumping carb. (Needle and seat VERY visibly worn) Any further suggestions?

Thanks again...
MRP Porter

Backfiring through the carb indicates a lean problem. I would check the timing and do a complete tune-up, as well as retorque the head and adjust the valves. Valve adjustment will change the mixture.
M Landskroner

Thanks for the heads up - going to order a new timing chain set tomorrow, check the valve train, and looking REAL hard at a Crane 3000 electronic iginition for her - points - YUCK.....
MRP Porter

MRP. Carb tuning is the last thing that is done as a part of the tuning process. The reason is that if the ignition system is not in top order, carb tuning cannot be accomplished in an accurate manner. My Weber DGVs have always started and run well when using Braden's initial set up procedure. Hence, since the engine never ran properly since you have owned it, I would expand my field of investigation checking the ignition system, setting the timing, adjusting the valves, then going on to the carb. The NGK BP6ES spark plugs are an excellent choice. Bad spark plug wires are also a possible problem. Go to my website at www.custompistols.com/ and click on the MG section, then articles. There are articles on checking out the ignition system and checking out the spark plug wires. That would be a starting point for further discussion.

By the way, adjusting the valves does not affect the mixture. Les
Les Bengtson

Thanks Les - we're going to go through setting the initial timing today, finding TDC and etc. I've read through this section on your website; thanks for the excellent info! Also looking into valve timing with an eye towards the timing chain and whether or not she's jumped time...

Thanks!
MRP Porter

MRP. I have never seen an MG engine "jump time" they way some other engines are reputed to. By this, I mean the timing chain has never, to my knowledge, jumped a tooth on either the crankshaft or the camshaft sprocket altering the correct relationship between the two. If the distributor is loose, it can move a little and alter the timing. But this is relatively rare. Setting the timing, using a dynamic timing light, is always a good idea. Make sure you do so while the vacuum advance line is disconnected from the vacuum advance can and the line is plugged. Use a dwell/tach to determine the engine rpm required when timing. The car tach is not as accurate as the test instrument. Les
Les Bengtson

Thanks Les - nasty, annoying things (spelled w-o-r-k) kept me from completing the timing check today, but I'll be sure to use your advise. I've always used an old timing light I got from my father, but I've wanted to replace it anyways! We'll be sure to use a dwll/tach as well, as the factory tach is non-functional since purchase. Haven't tracked that little failure down either yet.

Thanks again!

Matt
MRP Porter

I'm still confused about the 4 into 2 header. I've never seen a B-series engine with 4 exhaust ports crossflow or no. Maybe a twin-cam?
Leland Bradley

Although I'm joining this thread a little late, I am also having a similar problem with too rich a mixture and rough idling. I converted to the Weber 32/36 DGV about a year ago on my 77 MGB and added a Pacesetter exhaust system at the same time. After setting-up the carb to Redline Weber specs and getting a "lean best idle" at 1 3/4 turns on the speed screw and 2 turns from the stop on the mixture screw and idle just above 500 rpm on the tach, I checked the timing. It was 40 degrees advanced, so I checked the timing by removing the cover and setting No. 1 to TDC on the compression stroke. The timing marks were 180 degrees off. I returned the marks to their opposite position as described in my manual and adjusted the valves. When I rotated around to get No. 1 at TDC on the compression stroke the marks were again 180 degrees off. Is this the correct timing position?

Doug Cook
D. Cook

The header has two outlets - the pair join downstream just past the driver's seat. It's a cast iron unit; could be the stock exhaust for all I know; all my toys have been big block Chevys or my favorite, my little GLH-S that went the way of the divorce.... (sigh) Loved that little rocket ship...

Matt
MRP Porter

Doug,
Not sure which timing marks you're talking about but if mean the marks on the timing chain sprokets; when they are lined up and next to each other, number four is firing.
Leland Bradley

Matt,
That is the stock configuration. Number 2 and 3 share an exhaust port and feed one runner in the cast iron manifold and 1 and 4 have seperate runners that 'Y' together to feed the other outlet. Don't be in a hurry to replace that iron manifold it does a better job than most of the aftermarket headers.
Leland Bradley

GLH- "Goes Like Hell"
GLH-S "Goes Like Hell-Somemore"
Terry

GLH-S - isn't that the Carroll Shelby invention that showed the world what "dangerous torque steer" really meant?

Regarding the MGB, I'm surprised that no one has suggested a compression check. Granted, a carb who's pouring gas down the manifold needs fixing. But as long as you have an poorly running engine whose history you don't know, I would test the compression.

While backfiring out the manifold is commonly caused by over-advanced timing, it can also point to a burned or sticking valve. In addition, if the Weber's been pouring gas down the cylinders, the rings may be history.

A compression test will at least give you the an indication of what you've got to work with, or the peace of mind to know that all's well with the rings and valves.
Matt Kulka

This thread was discussed between 14/08/2003 and 20/08/2003

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