MGExp

MGB & GT Forum

77 MGB engine trouble WHAT THE HECK is happening???

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

HardlySlept Avatar
HardlySlept Chris Gatti
New Berlin, New York, USA   USA
1980 MG MGB "Blythe"
Yes, absolutely confirm firing order. If that's correct, confirm camshaft timing is good.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
As noted, stick the timing light on plug wire 3 and 4, it will be obvious if their not firing or firing inconsistently at idle.

ernest Ernest Fallowfield
Windermere, CUMBRIA, UK   GBR
If they are not su carbs there is a warm up thermocouple in yours and this can give the vibrating symptoms. Does it improve or change with choke?

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
1. Firing order correct

2. installed of set of SU carbs all cylinders fire when switching back and forth with starting fluid but does not run on gas from carbs. They are 20+ years old so that's not surprising. since we now can separately feed starting fluid between 1&2 and 3&4, it appears the Weber carb for some reason will not support combustion of the rear two cylinders. Suggestions??

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Lose the Weber... winking smiley

Dick



In reply to # 3608870 by timatron 1. Firing order correct

2. installed of set of SU carbs all cylinders fire when switching back and forth with starting fluid but does not run on gas from carbs. They are 20+ years old so that's not surprising. since we now can separately feed starting fluid between 1&2 and 3&4, it appears the Weber carb for some reason will not support combustion of the rear two cylinders. Suggestions??



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
What model of SU carbs have you installed, HIFs or HS4s?

Also, what fuel pump are you running? Have you verified its output pressure?

oleanderjoe Avatar
oleanderjoe Silver Member Joseph Baba
Fresno, Ca, USA   USA
You said Weber "TWO" Barrel ????? Side draft "Two Barrel" 45 DCOE or "TWO Barrel" Downdraft, 32/36. ???? Fuel delivery on a Down draft would effect All cylinders. Side Draft would effect front barrel cyl 1&2 Back barrel 3&4 Might be the issue. I had a lower "Grub" screw on my Race car Weber 45DCOE come loose, and the Venturi tube shifted, and it ran like crap BUT." PERFECT AT IDLE " ??? Something to check if it is a 45 DCOE Weber Cheers

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
OK here is the latest. Had my tired old 74 MGB engine under the bench so we pulled the twin SU's and manifold off. cleaned up the carb pots and center needles and they move freely. installed and attempted to start engine. I should have checked the float and needle an seats because the engine would not run on gas from the tank but front and rear cylinders would run on starting fluid. Confirms ignition is good on all cylinders even though compression is low. how difficult is it to rebuild the SU's and tune them. Shop here in Buffalo charges $700.00. Definitely not in the budget.

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
SUs from a 74, I'll assume these are HIFs?

Before digging to much further into the carbs, I think you need to verify fuel flow from the pump. Disconnect the fuel hose at the first carb and direct it into a suitable container. Turn ignition switch to ON, you should get a smooth clear stream without many bubbles and get roughly a pint in 1/2 minute.

We're also assuming you started this project with fresh gas in the tank and the fuel filter is new.

HIFs are not know to wear shafts quickly. If the dashpots are free, the carbs probably just need a good cleaning and setting. The tuning process is not difficult but the exact steps prescribed in the SU manuals needs to be followed if you're not familiar.

Any chance you're a member of a local club that might offer some assistance?

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
rony12 Ron Roberts
Avoca, PA, USA   USA
The SU Carbs are relatively simple to rebuild as long as you get the correct kit. Even though they are over 40 years old most of the time they can be saved. I have rebuilt them for around $350 - 375 for customers.
Also, if you have a local club I'm sure at least one of the members know where to take them locally, or have a clinic members sometimes get together to experience "Hands On " repairs of various areas of their cars by experienced mechanics.

If you're interested, you can PM me and I'd be glad to help you out if you wanted to ship them to me.
Good Luck.
Ron

timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
Update, pulled Weber carb and manifold and installed SU carb setup from 74 MGB found sitting in field for 20+ years. Cleaned air chambers, pistons and jet needles. freed up throttle shafts and choke shafts. After thought I should have checked needle and seat but overlooked in rush to test. Cranked and engine would not fire on fuel but would run on starting fluid sprayed in either carb. SO, now cylinders 3 & 4 does support combustion. Unsure now why 3 & 4 would not run on a new Weber carb setup. Cost to rebuild SUs 700.00 locally not in the budget. Suggestion???

Sorry guys, didn't see page 2, thought my update didn't post.



Edited 1 time(s). Last edit at 2017-10-13 09:56 AM by timatron.

timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
Ding Ding Ding you nailed it on the fuel pump issue! we barely had 2 ounces in 30 seconds. We replaced the fuel pump and had great flow. We reinstalled the Weber thinking the fuel pump was our problem. Now that didn't fix the problem totally. It still ran very poorly. Stumped again I only saw one difference between the left side and right side of the manifold. There was a fitting we had running to a T for the Brake booster and the vapor canister. I plugged off that fitting and YEP runs great! I believe that means the configuration I had was leaning out the mixture for cylinders 3 & 4. So I don't know what is supposed to plug into that fitting. I looked on line at pictures ands the Brake Booster is on several installs. I didn't see any instruction from Weber (See photo and you'll see the fitting on the manifold going to 3 4). Comments?
Oh yes, THANK YOU EVRYONE for your help. This is a great resource. You guys ROCK!!!


Attachments:
fitting.jpg    38.3 KB
fitting.jpg

GlennMGB Avatar
GlennMGB Glenn G
Fort Worth, Texas, USA   USA
1967 MG MGB GT "Rose"
Yes, a manifold vacuum leak will cause problems.

BumbleB74 Avatar
BumbleB74 William Milholen
Tidewater, Tidewater VA, USA   USA
Glad you figured it out!....winner was post #7.



1974-1/2 Roadster, "Bumble Bee", Corvette Yellow - in shambles, wire wheels
1976 Roadster, "Virus", Sandglow - "driver" condition (stock + 32/36 Weber DGEV, cast iron header, 25D distributor), bolt on wheels, ON the road!

lewk Avatar
lewk Silver Member Keith Lewis
Cambridge, Ontario, Canada   CAN
OP, you need to show more detail as to your hose set up. Post a picture of how your hoses are fitted.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

To add your reply, or post your own questions

Members Sign In   or   Create an Account

Registration is FREE and takes less than a minute!


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster