• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB stumped by a 74 MGB

Share this page

B

backyardmechanic

Freshman Member
Offline
Ok I replaced a nonfunctioning distributor with a new dist. with electronic ignition on the 74 MGB. It has a new weber downdraft with new manifold that was installed by the owner. The car has a backfire through the carb when accelerating. It idles ok but a little rough still. I have gone through the carb tuning process which did not seem to improve the situation much. From all the information that I have read the backfire is a sign of too lean of a mix. I have backed out the mixture a significant amount to get it to run as well as I can. I think this might also be a sign that the jets are too lean although when I pull the plugs they are a black powdery looking which would indicate a rich mix. This stumps me? I also have the timing sharks teeth on the upper side of the crank cover unlike the two books that I have that show these to be on the underside.There are five sharks teeth indicators. By watching the #1 cyl. I have determined the TDC to be the top tooth in the array. Am I to assume each of these teeth represents 5 deg. of timing? If so, the only way that I can get the car to run well is setting the timing to about 15-20 deg. BTDC at idle. With the new dist. I have plenty of advance now and it advances to well Before TDC now. Is my timing a problem as well? Thanks for any help/insight you might provide. Backyard Mech.
 
Just out of curiosity, how many years has this car been standing unused?
 
And ~yes~ the "big" pointer is th' TDC one.

If you can get the timing to hit at 8* to 10* BTDC, you haven't a timing issue. The real problem will lie with that carb. Jetting issues. Webers are a combination of plug cuts and voodoo. I ain't tryin' t' scare ya' off'n it but it's mainly got to do with idle jets and acelerator ones from what ya describe.

HIGHLY fat idle and not ideal fuel to start with.

Get some fresh fuel in it before ya go further.
 
Then go look at the teglerizer site. You should find a goodly amount of info on jetting and adjusting Weber carburetors for the MG line..
 
Tony, I think the car stood for years unused. I thought the owner said he changed the fuel out but by looking at the inline filter I suspect not I think that might be a good place to start to fix the carb problem. Do the tanks have a drain? If not what is the best way to drain the tank?
thanks backyard
 
Ahh yes,.. another Mg owner with the Weber conversion on it. I just put the conversion on a week ago and guess what. She's farten and spitten through the carb just like you outlined. This carb is known to run on the lean side on the MG's and the answer to mine at least is the jets. Its runnen heavy on the "lean" side and the mixture cant be adjusted enough to get it where it needs to be. After some serious reading I found that anything beyond 2 1/2 turn out of the mixture screw is starting to require different jets. Its not a difficult thing to change for a standard mechanic that knows regular carbs so I would consider it to relieve a lot of future issues. Good luck my friend.
 
Ron, thanks for the tip on the web site. It is very informative. For starters I think I need to get the old fuel drained and the tank cleaned out the best I can. Do you have some tip in this process?
backyard
 
great pumpkin, After thinking about this a little more I am definitely convinced that I have the timing too far advanced. before I replaced the dist. the car would turn over smoothly and easily while cranking over with the starter. Now when cranking with the starter it seems slow and difficult for the starter to turn the engine over. This to me would indicate that the timing is too far advanced and firing too soon making it difficult for the starter to turn the engine over.
This forum is just great. With all experience and input it really gets me to thinking about things.
Thanks for the info.
 
backyardmechanic said:
Tony, I think the car stood for years unused. I thought the owner said he changed the fuel out but by looking at the inline filter I suspect not I think that might be a good place to start to fix the carb problem. Do the tanks have a drain? If not what is the best way to drain the tank?
thanks backyard
Click to expand...
I really think you need to print https://www.theautoist.com/CHECKLIST.HTM out & follow it...it's time consuming but when you're finished, you have a safe/roadworthy car....getting the motor running is the least of your problems - you just haven't found them yet!
And you need to but the proper Bentley manual.
 
Tony, thanks for the tip on the check list. It really is extensive and I think it would be worth trying to get it all done if it were my car. I would probably take the time to do it, but since the car belongs to a friend of mine I'm not sure how far and how much time I'm willing to spend on it. There are a couple of things that are very helpful that probably wouldn't take that much time and would be extremely valuable doing. For me I think at this point the most important issue is to clean the gas tank out and make sure it has good clean gas in it, reset the timing and then retune the carb.
If the Irish owner wants me to do more than that it just might cost him more than a couple of Guinness' at the local pub!
 
Back
Top