MGB & GT Forum

I'm getting a bit sick of my SU carbs on my 1976 MGB having problems and the engine randomly having poor power, backfiring under load and generally being stressful to drive. I've spend the best part of $2000 with mechanic now with rebuilds, fixes and tuning and the car runs terribly. Maybe its something else he has missed but I was considering replacing the twin SU carbs with the Weber conversion kit which costs about AUD$700 https://www.ebay.com.au/itm/Weber-conversion-kit-w-Electric-choke-Weber-carb-fits-MG-MGB-1962-1980/273889389985?epid=22034234940&hash=item3fc5146da1:g:zNsAAOSwEzxYQXgK&frcectupt=true OR https://www.weberperformance.com.au/product_info.php?products_id=369
John from university motors says the that SU carbs provide "better driveability" whatever that means
Has anyone else done the conversion?
What was the smoothness and power like afterwards?
How hard/many hours was it to install?
Any particular components to make sure are included, and the correct model of carbs or alternative suggestions?

Don't do it. Find a new mechanic. I doubt the problem is the carbs.

---

Si Vis pacem, para bellum

Read this https://www.mgexp.com/forum/mgb-and-gt-forum.1/his-carbs-vs-weber-conversion-personal.4218108
He will have your conversion for sale cheap he says.
I went from dismal Weber to HS4's and never looked back.

Edit: I just watched your John Twist video - maybe I missed something but he clearly says Weber’s are great for grills but are dumb and stupid for cars and further says SU carbs are a better option. I agree with a previous comment to find a new mechanic as you've spent $2k with no solution. I also suspect your poor performance issue lies somewhere other than in your carbs. Best of luck.

---

Buz

“Every problem, has a gift in its hands for you”



Edited 2 time(s). Last edit at 2020-09-22 06:35 AM by naturbar.

DITTO!

If the mechanic can’t make SU carbs work. How do you expect him to make a Weber conversion work?
The Webers take work to make right.



Edited 1 time(s). Last edit at 2020-09-22 06:40 AM by Rick Fawthrop.

" poor power and backfiring under load " that tells me your timing is out, get a new mechanic, su are far superior than weber.



Edited 1 time(s). Last edit at 2020-09-22 06:49 AM by bev sleet.

Welcome to the MGB forum - Lots of Aussies on here.

Bottom line is that it's your car and do what you want but... You are crazy if you put the Weber on.

I am sure there are Sydney-siders that can make a mechanic recommendation if you are not the type to do the work yourself but if you mechanic can't get twin SUs to work he ain't worth a snot. This is a butt simple engine to work on and there is no mystery to getting it tuned correctly.

In a nutshell -

1 - Compression check
2 - Valve clearance
3 - Distributor mechanical indexing and initial timing @10*BTDC
4 - Carb float height correct and bridges set at about 0.070-0.080 - check fuel height in fuel jet
5 - Fuel delivery sufficient
6 - Initial throttle rig, dampers have oil and damper drop check
7 - Start, screw idle screws to 3200 RPM and set dynamic timing @32&BTDC with vacuum removed and plugged
8 - Return to idle ~1,000 RPM
9 - Disconnect butterfly interconnect and balance carbs - setting idle to 850-900RPM
10 - Rig enrichment (choke) properly

Drive it.

You don't have to understand all the above steps but if your mechanic doesn't understand anything above then he is the wrong guy...

---

The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

Sounds like webers aren't the answer and I'll have a crack at tuning the carbs myself or find another mechanic to work on them. Any recommendations in Sydney?
I was going to try http://peninsulasportscars.com/ as they're close and look to specialise in MG.

I suspected timing causing the blow backs and adjusted it back and forward but still have the issue. I put in a new battery as the old one was low on CCAs, condensor, spark plugs, distributor, cleaned all the contacts from the battery, through to the starter and up to the ignition thinking maybe it was an electrical or grounding problem with poor/intermittent sparking.
I found today the valve on the bottom of the left SU carb was "sticky" so pulling the choke was dropping the right one but not the left, which may help explain why it was so rough when starting up and would often cut out. I have WD40'd in and got it moving but it is still intermittently sticking so it looks like needs some work, disappointing after already paying for a rebuild on them.

Just checking I would have the HIF4 carbs https://www.mgsales.com.au/pages/mgb-c-v8-fuel-systems-induction-carburettors-carburettors-su-hif4-mgb-1972-80
Notes on this say: Original HIF4 type of carburettor. All original types are now replaced with FZX1229T
Is there numbering on the carbs to indicate the model?
I have not been able to find much info on FZX1229T. Are these some improvement over the HIF4?

Should be a metal tag on one of the float chamber screw to indicate the carburetor type.

You show HS Type with no air cleaner.

Just to let you know there is a 22 page? tome to sorting this aftermarket Weber downdraught.
Have aread to talk yourself out of fitting one.

https://www.mgexp.com/forum/mgb-and-gt-forum.1/skeletons-in-the-closet-a-weber-dgev-photo.2372702/

Any carburetor that can be installed 180 degrees to itself raises a question in my mind.

Sort what you have and fifteen minutes of tuning will see you good to go.

B

---

Life's most persistent and urgent question is, "What are you doing for others?"

Someone stole your steering column - LOL...

Hang in there - you can get this tuned.

Tip - At some point you should plumb the carb vents to each other and then plumb the overflow across the back of the engine to the right side and down past the starter. A stuck float in current condition could spray fuel on the exhaust and that would be a bad day...

---

The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

interesting to see a anti run on valve installed.

B

---

Life's most persistent and urgent question is, "What are you doing for others?"

Thanks for the great list.


1 - Compression check - done and good
2 - Valve clearance - Mechanic said he checked valve clearances. I think he said he adjusted them.
3 - Distributor mechanical indexing and initial timing @10*BTDC - its been set to a bit more forward at 15-20, seems idle really poorly when at 10, but maybe I just need to increase the idle to help it run better. I suspect it has a sports cam installed as it really lurches at 10C and low RPM. Any idea on how to tell what camshaft is installed?
4 - Carb float height correct and bridges set at about 0.070-0.080 - check fuel height in fuel jet - Will check this, suspect the mechanic did but I'll measure myself
5 - Fuel delivery sufficient - How to tell? The fuel filter has fuel sitting in it when empty, the fuel pump sounds fine but when running I don't see any fuel in the filter. Plenty seems to come back out the carbs when it blows back though!
6 - Initial throttle rig, dampers have oil and damper drop check - mechanic did this, I'll check it.
7 - Start, screw idle screws to 3200 RPM and set dynamic timing @32&BTDC with vacuum removed and plugged - haven't done dynamic timing @ 3200 - I think the timing marks are at 5 degree increments and only run to 20 or 30? How to you measure 32?
8 - Return to idle ~1,000 RPM - OK, as above seems to prefer idling at about 1200rpm, maybe due to camshaft?
9 - Disconnect butterfly interconnect and balance carbs - setting idle to 850-900RPM - will try this
10 - Rig enrichment (choke) properly - any tips on this

One other query is what spark plug gap to have. The manual and other places seem to give varying gaps to use



Edited 1 time(s). Last edit at 2020-09-22 08:06 AM by BrendanK.

Brendan open the page with all the recent posts and scroll down to the post by Msapanara . He discusses pretty well his experience with the answer to your question.
My 1980 LE came with a weber conversion and it worked ok, the previous owner knew enough and took the time to dial it in. I didn't like the look so rebuilt a set of HS4s and sold the weber to a guy for his volvo. The car runs better now ...but at the same time the distributor was replaced with one recalibrated by Jeff Schlemmer.
Its probably going to be expensive to set the car up and keep it tuned unless you do it yourself.

Do you have points or electronic?

Regular backfiring and poor driveability often make me think of an issue with the points-gap too tight, carboned up, or some other issue. If you do have points, it might not hurt to file them(there are files for this purpose) and then set the gap to 16 thou(in a pinch, a business card is often close enough, but I'd suggest a feeler gauge if you have one). You can also go straight to gapping without cleaning and just see if that improves things.

Remember, always, ignition before carbs.

Have you checked the vacuum reading? The carbs could have worn throttle shafts/bushings.
Also is there oil in the carbs.