MGB & GT Forum

Just as I think I have one problem solved, another crops up.

For background, my 1970 has twin HS-4s. I'm running a Jeff Schlemmer points distributor set(now) at 12º idle(no vac). I'm also using a Winterburn CDI module, although the problems I'm having are present running both Kettering and CDI so I don't think it has any relevance. Plugs are coming out nice and light tan.

I've been fighting two separate issues, though. The first is that the car seems to "stutter" between 1500 and 2000 rpms. Again, this is independent of the ignition type. I've run the base timing as low as 10º and as high as 15º, and it seems to make no difference(Jeff specified 12-15º with the distributor when it was returned).

The second-and more concerning thing-is that the front carb seems to spit gas out of it on heavy acceleration. I have my dash pots properly topped up with 20W-50. I have new black floats set at the correct height along with new viton needle valves.

Even more disconcerting, when I was out driving around this evening I had two separate backfires(through the carbs). Both of them happened in 3rd gear when I was accelerating at around 2000 rpms.

Does anyone have any thoughts on what might be causing any or all of the above?

Change the condenser...

Dick

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Lol! I was guessing the same thing. Sounds ignition related...Also, ensure everything inside the dizzy is making good contact including making sure the points themselves are imprinting a nice "bullseye" pattern and not mis-aligned.

Why are you running a CDI on that car??

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

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73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

Alright, I spent 45 minutes on the phone with the honorable Mr. Moritz a little while ago and we tossed this around.

I'll change the condenser and reset the point gap for good measure when I get home from work this evening.

I've been rather pleased with the CDI overall.

What issue did it cure or what aspect has it improved over stock?

I'm asking because many old snowmobiles run them and people avoid them like the plague...

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

I just resolved a simular intermittent problem. In checking things over I found a small spot rubbed through the insulation of the white black wire that goes to the dist. It was rubbing against the block. I taped it up and wound a plastic guard around the wire. No more problem.

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Ben, Just saw this topic now, and if it weren't for the fuel spillage, I would have picked ignition timing as the issue. Are your plug leads in the right order, and if they appear to be, could they all be 180 degrees out? I did this on my Morgan a few years ago when I put the dizzy back together 180 out of phase. Amazingly the engine ran and had quite good acceleration but it did backfire and stumble at certain speeds. (I know there are some here that will say it is impossible for an engine to run with the plugs firing 180 degrees out of phase. I would have thought so too, except it happened to me). However, fuel spilling out the front carb makes it look like a carb issue. Too much fuel pressure perhaps? I'm a fan of Grose Jet brand float valves as they seem less sensitive to crud, but on the other hand they won't tolerate high fuel pressure without leaking. I've found the rubber tipped variety seal better against pressure but are prone to leaking with tiny amounts of dirt. Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
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Fred - Would you please elaborate on your comments on Grose Jet brand float valves. Are you talking about traditional needle and seat as opposed to "grose jets" of yore which are now in disfavor. The "rubber tipped" reference is the reason I ask.

Ben, When you wrote that it was backfiring through the carbs, was it an actual backfire, or a 'spitting' sound? I've found a spitting sound is from an overly lean carb. To have a backfire through one requires the ignition timing to be way off, such as would happen with a couple of ignition leads out of order. Food for thought, Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
'You can't shake hands with a snake'

I had similar issues. Also was idling rough, almost like it was running on 3 cylinders. I had reason to suspect head gasket, but that didn't help, although there was a leak in the old one. So I did a plug test, removing 3&4 had no change in idle. My timing was perfect, so I figured fuel. Dismantled carbs, the front carb jet bearing was stuck to the jet, fused might be a better term. I have no clue as to what event could cause such traumatic force as that. I replaced everything and she purrs like a kitten.....Oh and on a previous carb cleaning a few weeks prior, I didn't realize what the temp reg. Was, and I also felt that improper tensioning could be the problem, as it would run pour in the come, improving after warming. But after seeing the bearing, that was my culprit. Easy to check, if you've looked everywhere but fuel delivery, worth the gasket set. Hope this helps.

Fred,

Thanks for the response.

I've had my distributor 180º before and it didn't run at all. I've also had the leads off, and at best it will cough a few times and not run. I'm sure the order is correct(1-3-4-2 ccw).

I'm running an SU pump(new electronic) and new viton needles and seats along with black floats.

As for what's happening-most of the time it's simply spitting gas. I can see it when I rev the engine, and can smell it if I'm driving without the air cleaners(I don't make a habit of that, but it's useful for diagnostics).

I had two of what I'm calling backfires-they were distinct "booms" coming from under the hood in the vicinity of the carb. I've had my fair share of afterfire(backfire out the tailpipe) that I tracked to misfires, but this was the same sort of sound out the carburetor.

I'll add that with work and other obligations, I haven't had time to work on it-I am going to try the honorable Mr. Moritz's suggestion to change the condenser.



Edited 1 time(s). Last edit at 2017-02-25 06:52 PM by benhutcherson.

Ben, Almost seems like a slipped timing chain then, but if you want to check the condenser it's easy. Just disconnect it and run the CDI in CD mode. The CDI doesn't need the condenser. If the condenser is shorting, it will affect both CDI and Kettering, but if the condenser is going open circuit, it will only affect Kettering. Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
'You can't shake hands with a snake'



Edited 1 time(s). Last edit at 2017-02-25 07:04 PM by Fred Winterburn.

If the only symptom is gas spitting out the front carb intake and periodic backfire, I would be thinking float height, even though you said it was checked good.

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