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MG MGB Technical - 45 DCOE Backfire

I recently picked up a '76 MGB with a 45 DCOE 152. Runs terribly. When I got it, it was basically running on 2 cylinders, and would idle forever like this until it shook itself apart. I did a compression check and I had 140 out of #2 and 3, but just 95 out of 1 and 4. I did put oil in each and then got the same readings. Must be top end. I went completely through the carb and cleaed it out. It had a little sediment in there, bt was basically in good shape. I did the usual tune-up proceedures. Nothing was really far off, but all was out a bit. I put in some fresh gas with gumout, and it ran a bit better. It runs much better with the choke on, even when warm, but still not good. After running it a while as I was working on it, I checked the compression again and got 148 in out of 2 and 3, and 110 out of 1 and 4. Again, the same with oil. I have to turn the idle mix screws out to 3 - 4 turns to idle almost smooth, but it still wants 1/2 choke to run better. Basically showing tendancies to be both rich and lean at the same time. Needs choke, but leaves black stain on the car sitting behind the exhaust. Sounds to me like a vacumm leak, but I haven't been able to find one. I have not checked vacumm level yet. On the list. I would expect that the poor compression would cause low power, but I would think I'm getting enough to at least run fairly smoothly. Maybe I'm wrong on this. Maybe this big carb need full compression. When driving, and I get the revs up and throttle on, it seems to have good power, but it hates to cruise at a steady speed. The wife nick-named it "Jiffy-pop". Nice.

Any suggestions would be apprreciated.
Mark R.
Mark Rotsky

A vacuum leak wouldn't cause low compression. You say 'must be top-end', and I'd tend to agree, but don't say what your investigation of the valves, piston tops, head gasket etc. was. Not much point in fiddling with carburation until you have reasonably balanced compressions, I'd say.
Paul Hunt 2


I agree with Paul. You could check compression again using another gage and making sure the car's up to temp and throttles full open but the trend looks pretty clear.

While you're planning the head work you could do a few more things though. The DCOE's can have a vacuum leak around the shaft outlets. You can check this with a small bottle of propane. Get it running and carefully put some propane around the shaft exits. If it cleans up the engine that's where the leak is.Check also the manifold connections and the security of the set screw and nut for the aux venturi. You'll have two to look at. As you look into the carb (inlet side) they will be at the lower right and lower left position. A set screw with a jam nut each. They should be tight.

Re-bushing a DCOE is an involved job requiring specialty tools-best to farm out this work if it's required.

3 to 4 turns out is way too much normally. Even on the older models and the DCOM's the most you should see is about 2-2 1/2 turns out.This with your choke comment points to a way too small idle jet. You can find several references for jetting setups on a DCOE on the web., Pierce manifold and Redline come to mind. Just google them and check what you have against what they show as baselines.

Another check prior to headwork would be to throw in some new plugs, drive it and then read the plugs. You might be able to tell a little more that way.

Have a friend with a set of good SU's and manifold they would lend you? Swap it out and see how it does.

I'd also recommend highly the Braden book on webers, it's great.

hope this helps, let us know what you find.

Tom M
Tom M

Ditto Paul's comments. The "Choke circuit" would be better called a cold start enrichment circuit in that it adds additional fuel. Your wet tailpipe is probably unburnt fuel from the two low compression cylinders.

If your engine were otherwise in good nick, I would say either your idle or main jets were clogged, your float level is too low, fuel pump is dying, or fuel lines or filter were otherwise clogged.

Tuning the sidedraft is a tricky business. First, you are assuming the prior owner got it right. Don't. Since the head is likely coming off anyway...not really that hard a job, get someone knowledgable to tell you if your head is stock, ported or sports oversized valves. Also clean the top of the pistons and look closely for any markings like .040 or anything else. You want to know what you are starting with in terms of the engine displacement, CR and tune to take a rough guess at the starting point for tuning the weber. There is an excellent book on tuning webers, I think it is part of the "How to power tune..." series but I can't swear to it.

Tune one part at a time first selct the right venturi inserts - main chockes I think they are called, then idle, then fast idle, transition, cruise and WOT. You will also have to modify your timing advance curve to compensate for the lack of vaccum advance. Many swap to the twin point Mallory distributor for this reason, others might play with the stock distributor and alter the springs, weights and the total amount of advance available from idle to full advance; still others do nothing at all leaving the stock distributor as is.

I would suggest get the compression problem sorgted first and then plan on pulling and rebuilding the weber taking careful notes of what is inside: Idles jets, main hets, emulsion tubes, chokes...I forget the rest of the bits... The beauty is, once it is cleaned up and reinstalled you can alter damn near everything without removing it. I see a tackle box of weber bits in your future.


Brian C.
Brian Corrigan

I agree with getting the head sorted, however once you know what it is like you should be able to just copy someone elses carb set up, Webers may be a black art to set up but they are so repeatable that to all intents and purposes they are identical.
Stan Best

Hi Mark,
Although the compressions are not too great, I've seen worse and the engine should run, albeit being a bit short on power. I would suggest that you check the valve timing (inlet valve full open 108 degrees ATDC on the std cam), ignition timing on a new set of plugs, leads, points and condensor ( say 10 degrees BTDC fuel dependent), valve clearances (15 thou hot)and a decent fuel delivery rate (at least 9 Imp galls /hr) before blaming the Weber.Disconnect and plug the vacuum advance so as to check its influence on reconnection.
Having said that, the calibration could be out.The following will be very close to what you need.

Float Level 5mm
Emulsion Tube F16
Main Jets 165
Air Jets 180
Idle jets 60F8
Pump Jets 60
Pump bleed 1.00
Pump stroke Long (62mm)
Pump Spring Heavy
Main Chokes 36
Aux Venturi 5.0
Ram Tube Length 63mm( if no cleaners are fitted)
The carb should be inclined no more than 5 degrees from the vertical.

These settings are for a modified 18v engine (770 cam,1860 cc, 9.8:1 CR) A more standard motor could use 34mm chokes, 155 main jets, 55F8 idle jets, 55 pump jets and a 4.5 aux venturi.

I have used a 45DCOE on my "B" engined MGA for many years and can only report good starting( when cold, not so great when hot), great throttle response, and stay in tune reliability. Huge improvement over SU's.

Best Regards,

All, Thanks for your comments. I've been out ot town so I'm only responding now. I went after the Weber on this car only after doing a standard tune up. The car came with a new weber book, and I'm pretty familier with carbs in general. This was pretty straight forward to rebuild. I feel pretty good that everything is where it should be, although I did not replace anything with a rebuild kit, and did not have all the data on the right component sizes. (thanks for that info Tony). My wife's 77B runs great with a DGV on it and I was thinking of a tempoary swap to prove it out. But I still think that the problem is beyond the carb only. I don't know what is in the engine or how the valves are timed. If this helps tell you about the car, the PO didn't know the car had OD, and he had the windshield washer hose stuck on the distributer vacuum pipe instead of the washer pump!!! So anything could be going on in there. I've played with the ignition timing, and even swapped the cap, rotor, wires and plugs with the other car. No difference. All in all, I just can't justify the low speed backfiring through the carb unless the jets are just so far out that ti's a total mess. Maybe a temporary carb swap is the way to go, and failing any improvement, a full head and cam job. I rebuilt my wife's engine a few years ago and it has been rock solid ever since. (Knock on wood) That would take away any doubt in that area, but I hate to have to do that if the real problem is something simple or obvious. Any other suggestions would be very welcome.
Mark Rotsky

Well I kept thinking about it and I just couldn't reason the carb being the only problem with the backfiring. So I took a big hunk of distributor and gave it a twist. Turns out that (according to my simple timing light) it likes to run at about 50 degrees BTDC. No vacuum advance (it doesn't have a source). I "fine tuned" it a bit more, but it was still around the same area, and everything just smoothed out. I cleaned the plugs, took it for a ride, and it behaved nicely. Just a very minor miss once in a while, but that could also be attributed to older gas. But it ran. Now either there is something wrong with the pully installation, or something else weird is going on in there, but I even pulled the plugs out again after the ride and they are normal for the first time. However, the carb still wants to run with the idle screws out about 3 1/2 turns.

Tony, I took a look at the jets, and they are different. I left the actual numbers home today, but I think only the emulsion tube is the same as your standard engine numbers. The rest are lower numbers. Maybe it would help to get some bigger jets to better match what you see and she would run even better.

All this just in time to put it into storage for the winter. I even got all the lights and instruments working. (yeah, this was a mess when I got it) At least I have something else to look forward to next spring. Now to make room for two cars in that storage unit.

Mark R.
Mark Rotsky

And here's another piece of the puzzle. While getting the engine to run better by advancing the timing so much, I had pulled the brake booster hose to get a vacuum reading, and it was reading almost 25 lbs. And super smooth. That's a lot of vacuum for what I consider a weaker engine (remember that 2 cylinders are only putiing out 110 psi compression). Does this sound weird to anyone, or is it only me?
Mark R.
Mark Rotsky

This thread was discussed between 30/10/2006 and 15/11/2006

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