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MG MGB Technical - dgv backfire

Finally think i nailed the lose of power to a backfire through the top of carb not not the tailpipe. On this weber dgv what is the procedure assuming everything is tight.? I would think its too lean but its been a while since i played with a carb. I appreciate any info from the group. thanks , Doug staten island ny
dd doug46chief

Doug. I have had three cars equipped with the Weber DGV carb and have never had the problem of back firing through the carb.

More information might be useful if we are to assist you. First, what is the year of your car? My experience is that the DGV is an significant upgrade over the Z-S carb on a rubber bumper car. However, I would never think of replacing my SU carbs with a DGV. Hence, knowing what model year you have, and any engine modifications you may have had accomplished, might be of use.

Of additional use would be what jets you currently have in your Weber, both on the primary (32mm) and secondary (36mm) sides. Hard to consider mixture ratios without knowing what jet sizes are installed or having some form of exhaust gas readings.

To find out what jet sizes you have, the top of the carb must be removed. Then, the fuel in the reservoir is drained and the jets are unscrewed from their mountings using a common screwdriver. Each jet should have a number marked on it. With that information, we can compare what our jet sizes might be, how our engines are performing, and how that compares to your experience with your jet sizes. But, in my experience (three rubber bumper models) the jets supplied are adequate for normal running. In fact, I have had to go to a slightly smaller jet, on the primary venturi (32mm) to meet local emissions requirements. But, no problem with back firing through the carb.

Have you run a compression check? Lean mixture, in my experience, causes back firing through the exhaust system due to improperly burning fuel/air mixture. Back firing through the carb I have only seen due to either a leaking intake valve (giving access to the intake manifold and carb) or improper ignition timing (igniting the mixture before the intake valve is fully closed).

More information would be of use to us in trying to determine what may be the problem. I would suggest adjusting the ignition timing to factory specification, setting proper valve clearances, then a compression check, would assist us in helping your determine what your problem is. And, knowing your current jet sizes so that those of us who are running the DGV can compare your jet sizes to the ones we are successfully running.

Les
Les Bengtson

ok Les, thanks. The timing was going to be done this weekend by myself and a friend. the yr is 79 and as far as i know its stock except for the carb. I just bought it and have never done work on a weber , only su and that was 40 yrs ago. I also am having a problem with the linkage. Whoever installed it didnt have the proper fittings and the carb wont open all the way. Can u recommend components from the carb back to the accelerator cable? Thanks, Doug
dd doug46chief

Doug: backfire through the carb is usually an ignition problem. Check the points gap and contact faces. Next, try a different condensor. Condensors can be fickle.

Let us know how it turns out

cheers
Gary
79 mgb
gary hansen

thanks Gary will do
dd doug46chief

Doug. Your car, as shipped from the factory, did not have a points type distributor. Rather, it had a Lucas 45DE4 distributor which is one of the "electronic" distributors. In 80, they used the Lucas 45DM4 or CEI distributor. A photo of your distributor and your coil (CEI type has a black box under the coil) would be useful in determining what you currently have in the vehicle.

In a similar vein, a photo the the carb showing the current linkage would be of use in determining what is happening to prevent the throttle from being fully opened. Rather important that it be able to do so on a DGV because it is a progressive linkage with the secondary (larger) venturi only being used when the throttle is more than half way open. Thus, depending on how much the throttle is kept from being fully open, the secondary venturi might well be only opening half way--perhaps even less.

It would also be interesting to know how you made your determination that the engine is backfiring through the carb.

Les
Les Bengtson

les i fixed the throtle by fabricating a new bracket for base on carb and reconfigured the plate on side of carb to make a finer adjustment to fully open. the backfire was a pop through top of carb.the compression from front to back with throtle open and chock open as well as all plugs out was from front to back. 152,155,155,160. done cold with gas off. the plugs were black so maybe its too rich. I believe i have electronic ignition, the valves were in spec and i am performing a timing soon. I thank you 4 your help. doug
dd doug46chief

This thread was discussed between 25/01/2012 and 27/01/2012

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