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MG MGB Technical - THIS IS GETTING ANNOYING

Now that the ignition problem is sorted out. I still have the original issue. I still get a backfire through the carbuetor under load as well as a "lag in acceleration"- sometimes. This is what I am working with:

79 MGB 18 V engine. Rebuilt 10k ago.

Bored .030 over
New Alloy head (orig. cracked)
Crane Cam #342-0107
New distr. #41427 converted to Lumitronics
Weber downdraft carb DGV with PECO header
Timing set at 12-13 BTDC @1500 RPM(vacuum disconnected)

Yes the new ignition items helped and the backfiring is much improved. I did clean the carb PRIOR to the new plugs.

The old plugs were a brownish-grey before the carb clean. So mixture seems fine.

The idle is set @ 1025-1050 RPM. The exhaust will "skip" during idle. Idle is not as consistant as it used to be.

Checked for vacuum leaks by spraying carb cleaner around the base of the carb as well as vacuum hoses - seems OK.

Before I tear into a carburetor rebuild, I would like to know if I have it all set up right as decribed above or do I need to do something else.

Any thoughts on this.

cheers

Gary
79 MGB
gnhansen

Did you dial in the cam, or at least confirm the cam
position after installing it?
Daniel Wong

As a test, try advanceing the timing a little to see if these problems disappear.
Peter Murray

Backfiring through the *carb* sound more like a valve timing or sticking problem than anything else.
Paul Hunt

Well since you state that this has always been the problem it leads me to suspect that the carb is not jetted right. Have you ever had it setup and running the way you wanted ? I would start with jetting the Weber. Making sure you have the correct fuel air mixture throughout the powerband.

James

Did you install bronze valve guides? It is possible that one or more is slightly binding. Thia would give you all the symptons you have.
Sandy
conrad sanders

This seems to be a reoccurring problems with many of us. I have the same issues and have narrowed it down to the amplifier on my dizzy. Others have found it to be a fuel issue. Why is this happening to 79's? Could there be a connection between the original OPUS ignition and the replacement CEI?

P.S I also have the WEBER downdraft and a CEI dizzy.
Bob Stonestreet

The backfire through the carburetor is a relatively new problem. It started about 4 weeks ago. That is when I replaced the ignition items. Prior to that, the car ran fine.

I may try Peter's avise and advance the timing 2-3 degrees.

Also, the temperatures here are falling to the 20-40's F range ( zero and below for the Centigrade Folks).

More later

cheers

Gary
gnhansen

Gary, by chance when you put the new ignition in did you take the distributer out? If so, I would recheck that it is set correctly. One tooth off could be the reason for the backfire.
Bruce-C

Gary,

I am watching this post because I have a similar problem with my new 77 roadster. It has the Weber dual choke downdraft, and an Opus ignition converted to an Allison.

Mine only pops back through the carb prior to fully warming up, and only if I stick my foot in it, but the hesitation remains after warming up. I am suspecting the accelerator pump.

When I remove the air cleaner operate the throttle, I don't see any fuel spray. Am I right that these Webers should squirt raw fuel on acceleration? Have you looked at that on yours?

I also suspect an intake gasket leak on mine, and have bought a replacement that I have not yet installed.

Maybe we can get to the bottom of this problem. I might add that the popping back did not occur until the temps dropped recently.

Charley
C R Huff

When I converted my stock 25D Lucas distributor to a pertronix ignitor, I found that I had to alter the position of the distributor body greatly relative to it's prior position. It may be strictly a timing issue, and your ignition timing may be WAY off.
Chris '66 B roadster
Chris

Gary
Sounds like a shortage of fuel to me. These little Weber carbs have very fine accelerator pump jets and it doesn't take a real lot of rubish to block or partially block the acc. pump squirter. I'd suggest you have a look and make sure you are getting a good squirt every time the throttle is opened. If it's blocked at all it will backfire through the carb when trying to accelerate specially at lower revs and just won't take the throttle properly till the revs build up a bit. Willy
WilliamRevit

Daniel W; I have no way of checking the camshaft positioning. I just trust the rebuilder knows the proper settings and did so.

I did advance the timing to 15 degrees BTDC @ 1500 RPM
and I did get a noticeable improvement. The prior setting was @ 12-13.

The idle was sputtery and inconsistant. Set idle around 1000.

Went for a ride on an Up hill Down dale road where I floored it going up hill to 60-70 MPH and let off going down dale to 40 MPH. I repeated this process for a most of the ride which went for approx 15 miles. Returned home and idle was around 1300. Reset idle again to around 1000. I did notice a smoother idle, but noticed some smoke from the exhaust. Perhaps the engine is not used to revving over 3500-3700 RPM.. I guess I made it "sweat" for the first time.

Willy: I shall look down the carb to see how the fuel is sqirting into the venturi.

Charley Huff: You seemed to hit on something also. The temps here in the Adirondacks have dropped into the 20's range and the car does not operate at the same engine temps according to the temp gauge. This is also when I started having problems. Your statement: "Mine only pops back through the carb prior to fully warming up, and only if I stick my foot in it, but the hesitation remains after warming up. I am suspecting the accelerator pump."

Same here, Charley


We'll get through this. Again, thanks to all for the help.

Cheers

Gary
gnhansen

Gary,

I was doing a search in the archives and found a thread on drilling out the accelerator pump jet with a number 68 drill. Some people swear by it. I got it back open, but the thread name doesn't show up after it's open. The link is below. I hope it turns out to be a hyperlink.

I checked mine again this evening, and this time the accelerator pump is working, but maybe not enough.

Charley

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=71&subjectar=71&thread=2001041707510418112
C R Huff

Took the car out for a good 160 mile drive today. Some on the Interstate, main roads, hills etc.

The car is currently running great. Good idle around 1000 RPM, exhaust has a good steady tone and beat.

So, the timing set to 15 degrees BTDC @ 1500 RPM was a great help. I believe the ignition is about as good as it gets.

The tramping on the accelerator over the hilly ride of yesterday probably cleared some debris out of the carburetor.

After my ride today, the engine is smooth, steady and backfired through the carburetor only when it was cold.

Perhaps my winter project will be to put a rebuild kit in the Weber DGV.

Any one have any comments on the Weber DGV rebuild kits???
Is there one kit better than any other?
Is one jet size more advantageous given the mods to my engine?


Again, thank you all for your input

cheers

Gary

P.S. Charley, let me know how your Weber mod works out. T.I.A.
gnhansen

How about a pair of SUs for a winter project?
Leland Bradley

Gary,

Glad to hear things are getting better for you. Mine popped back today after it was warmed up. That had not happened before. If I remember right, I set my timing to 10 BTDC at 1500. Whatever I set it at was straight out of the book for my engine model. I've seen a good bit in the archives that it should be advanced to about twice that with a downdraft Weber, but I haven't tried it yet.

I've got a few other things to set right before I try drilling pump orifice, but if they don't fix it I will give it a try.

Leland,

SUs are my favorite carb, but I've never had a dual choke downdraft Weber before. So, I want to see what the competition has to offer. It seems to me that the progressive barrel set up with that little primary might offer good cruise economy. If it doesn't work out I've got a few SUs around that I could build. In fact, I was rummaging through an old box of parts the other day and noticed that I still have a set of 1 & 3/4s from my last Volvo Amazon.

Charley
C R Huff

"Dollars-to-donuts" - I'll bet that the camshaft maker had retarded
the TDC valve timing by 5º (or more) to increase power at the higher end (a very common and totally legit practice). This
does not account for any manufacturing variances in the
camshaft, timing chain length, etc. You'd be surprised at
the sort of things you find with a good micrometer.

Increasing your ignition timing by 5º compensated for this.

As an experiment (= road tuning), I would further advance
the ignition timing, bit-by-bit, until it pings under load
- and then back off by a few degrees.
Daniel Wong

Gary, Remove the cap and inspect the terminals. Look closely at the wear marks. They should be at the center of the terminals and not at the ends. Check the rotor as well. If the wear marks are not centered, the ignition pickup is not centered on the point plate causing the coil to have a longer distance to jump the spark and failing. This is called rotor phasing and is common with the electronic conversions. It burned a hole in the cap of my 45d. The mounting holes must be enlarged to allow for fine adjustments to the pickup location. RAY
RAY

Daniel: I believe the Haynes manual Ihave for the car indicates 12-13 BTDC @1500 RPM setting for timing. Now that this engine is no longer built at factory specs, I regard these settings as a refernce point from which to "experiment" Your point about the camshaft timing is very interesting and makes sense. I now set the timimg to 15 BTDC @ 1500 RPM and got a significant difference in acceleration as well as idle.

Ray: I did look at the old dist. cap and the wear marks were pretty much on center so I did not get into any fine "tweaking" other than a timing adjustment.

Charlie: Your car seems to be a "modified version" as well. So some timing adjustment may be a good thing for you as well.

The major culprit seems to be the carburetor. Yes some ignition timing and component replacement was a help.

As it has snowed here this past week, I have not taken the car out of the garage. We will see what the next few days brings weather wise.

Cheers

Gary
gnhansen

This thread was discussed between 18/11/2008 and 27/11/2008

MG MGB Technical index

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