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Midget 1275 and Weber DGV Jetting

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Festus

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Ola. First post here, and on what appears to be a hotly debated topic: Weber DGV on the 1275 engine. It's an interesting subject to me in as much as I'm surprised at how little technical information appears to be available on the topic. Searching this and other forums for tried and tested jet specs turns up almost nothing. Before spridgetmania.com evaporated (apparently they went bankrupt) I found a post on their site with suggested specs. Beyond that I've found a single post with suggested specs on The MG Experience. That's it. Otherwise most of what I find is opinion expressing how badly the DGV works on the 1275. And yet, I've also found posts from owners saying how great the DGV works on their 1275, but with no supporting technical info.

Does anyone out there have a DGV on their 1275, and if so, what jets are you running? Does it run well? The SUs on my Midget were driving me crazy with a constantly fluctating idle and generally very poor performance. I've gone through the steps to synchronize and set fuel/air multiple times. The result has been the same every time; a hard-starting and generally unevenly performing engine. Compression is excellent, valves are adjusted, timing is currently set at 10 degrees BTDC.

I already had the DGV, new and in the box from another project car (Volvo PV544) someone gave me (now gone), and decided to source a manifold and test the waters. At certain levels it's brilliant. It starts on the button and idles well. Cold starting on the SUs was terrible. Not so with the DGV. But, and of course there's a but, I'm experiencing the stumble on initial throttle reported by many. You can drive around it, but it's pretty obnoxious. I'm going to keep playing with jets, but I'd love to hear from anyone who's happy with their DGV conversion and most importantly what jets you're running. For the record, following are the two jet recommendations I've found and the jets currently fitted to the 1275.

Suggested Jetting SpridgetMania.com:
Primary Idle Jet: 50
Secondary Idle Jet: 55
Primary Main Jet: 135
Secondary Main Jet: 140
Primary Air Corrector Jet: 165
Secondary Air Corrector Jet: 160

Suggested Jetting MGExperience.com:
Primary Idle Jet: 45
Secondary Idle Jet: 40
Primary Main Jet: 135
Secondary Main Jet: 130
Primary Air Corrector Jet: 165
Secondary Air Corrector Jet: 160

Current Jetting:
Primary Idle Jet: 45
Secondary Idle Jet: 40
Primary Main Jet: 135
Secondary Main Jet: 140
Primary Air Corrector Jet: 165
Secondary Air Corrector Jet: 160

I went with spridgetmania's jet suggestions as a starting point. A first trial showed idle circuits too rich (fuel/air needle screwed out less than 1 turn), so I swapped the idle jets for the MG Experience suggestions. That improved the fuel/air setting (now at 1.5-.1.75 or so out), and seemed to improve throttle tip-in, but the stumble is still pronounced. The DGV is apparently sensitive to fuel pressure, which I've yet to confirm. That's up next, and I'll report what I find. In the meantime, I'd love to hear from anyone with verified results.
 
I have a DGV on my 1275. I have no idea of the jetting as it was set up on a dyno by someone knowledgeable who had access to the necessary jets.

I know it can be a PITA trying to do it yourself as each jet costs $$$ and you could be going thru a bunch to get it right.

The problem(s) are trying to get rid of "off idle stumble" while also getting a decent idle. My car doesn't have this. The one I have as an electric choke, and it's cold blooded. At the time of installation I was told that it would die at the first STOP sign, then run fine and that's proven to be true. If I let it warm up a couple of minutes that doesn't happen. The only time it hesitates is if I STAB the gas pedal. That is suddenly mash it to the floor. Then it will hesitate for a 1/2 second, then take off. The idle is stable and consistent unlike the SUs that idled differently depending upon ambient temperature. I can easily get 35 to 38 mpg as long as I don't get into the secondaries, going down the freeway, with a Dastun 5 Speed and 3.9 rear end.

I'll never go back to SUs.
 
If you "suddenly mash it (the pedal) to the floor," you are opening that big secondary barrel to very few revs. The secondary linkage is mechanical rather than vacuum or more properly air velocity. That small displacement just can't create that much depresion at that speed. I have owned several Fiats in years past and the all used that same DGV perfectly, never any flat spots or stumbling. the difference, 1800 cc rather than 1275.
 
Pythias said:
I have a DGV on my 1275. I have no idea of the jetting as it was set up on a dyno by someone knowledgeable who had access to the necessary jets.

I know it can be a PITA trying to do it yourself as each jet costs $$$ and you could be going thru a bunch to get it right.

The problem(s) are trying to get rid of "off idle stumble" while also getting a decent idle. My car doesn't have this. The one I have as an electric choke, and it's cold blooded. At the time of installation I was told that it would die at the first STOP sign, then run fine and that's proven to be true. If I let it warm up a couple of minutes that doesn't happen. The only time it hesitates is if I STAB the gas pedal. That is suddenly mash it to the floor. Then it will hesitate for a 1/2 second, then take off. The idle is stable and consistent unlike the SUs that idled differently depending upon ambient temperature. I can easily get 35 to 38 mpg as long as I don't get into the secondaries, going down the freeway, with a Dastun 5 Speed and 3.9 rear end.

I'll never go back to SUs.
Click to expand...
It's an easy carb to work on, so that's not a deterrent. I just feel like I'm stabbing away blindly jet-wise.
 
I think I'm done with jetting. The Weber is now fabulous. Excellent startup, zero off-idle stumble, zero mid-range stumble, solid idle, revs cleanly, just lovely. It will pull cleanly in 2nd gear from 2-5mph. I appreciate the naysayers who are loyal to the original SU setup, but mine were knackered, as the saying goes. Throttle shafts pulling air, etc..., wouldn't run the same way two times in a row. Synching didn't help for the obvious reason of worn throttle shafts.

The expense was much lower than the cost of a comprehensive rebuild of the SUs as I already had a new Weber 32/36 DGV that came with a different project. Total cost including intake manifold, jets, 1-7/8" tall air filter - necessary for hood clearance - and misc. was $250. I used the original throttle and choke cables (shortened as required). Final* jetting:

Primary Idle Jet: 50
Secondary Idle Jet: 55
Primary Main Jet: 145
Secondary Main Jet: 140
Primary Air Corrector Jet: 190
Secondary Air Corrector Jet: 180
Drilled 1mm hole in primary throttle plate to enhance vacuum signal for idle.

I learned a lot about Weber jetting, perhaps most importantly that contrary to popular belief, you can jet a 32/36 DGV to work very, very well on a 1275 Midget. A plus for me is throttle effort is greatly reduced, something that drove me crazy with the SUs and their three return springs.

*At some juncture I might swap in a 45 primary idle jet to see if I can get the fuel/air screw at 1.5 turns. It's currently at just under 1.25 turns. Don't have a 45 on hand and it may not matter, but would be interesting to see if that has any impact whatsoever.
 
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