Moss Motors, Ltd

got a 69 MGB. have owned car for 4 years and have always had trouble cold starting, especially in winter. engine and car run fine, actually real well, in summer.  but in cold, and fully choked, it will not start at all unless I spray carbs with starter fluid--then it kicks in immediately, and runs well, smooth idling and acceleration, no misfires.... Decided to work on ignition and carbs in parallel. 1) Ignition - installed a Petronix electronic ignition and new Lucas coil. Don't have fancy equipment but used the static timing procedure.  The Petronix is rough, with apparent misfiring, does not run well.  When I simply swap it out with the old 25D4 distributor, the engine runs fine again.  I've done this swap a few times with the same results; Petronix doesn't run well and the old distributor seems fine.  I really want to keep the Petronix but can't make it work.  any suggestions?  2) carbs are twin SU HIFs.  except for the cold start, engine runs fine.  Got to use starter fluid.  Maybe need a new choke cable?  Any suggestions?  Thanks, Joe

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Joe P.

Joe: About a year ago I also installed a Pertronic "Flame Thrower" distributer in my '80 MGB. I had a lot of trouble getting the car to run well until I recognized that the Pertronic centrifugal advance was very different than the MGB 45DE4 distributer that I was replacing. It had a much steeper advance curve below 1000 RPM. Thus if I set the advance at idle I ended up with much to much advance at operating speeds. My solution was to set the advance I wanted at 1500 RPM. I am now very happy with the performance.

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BobK

Joe, I'm not sure about the workings of the HIF but it sounds like your choke is not working right. Have someone operate the choke while you watch at the carbs. Be sure you are getting full movement. If that doesn't help then you will need more info on how the HIF choke works. Your symptom could be from some other cause, such as a weak coil, but I would still bet on the choke. Also, there's no need to change the cable unless it is actually broken. The problem will be at the carbs instead.

As for the dist, did you replace the whole unit with the Pertronix? If so then Bob's advice sounds right. If you just replace the points then that would not change the advance curve.

 

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Bayless
Never express yourself more clearly than you can think
'48 Prefect
'67 Sprite (project)

Bob, thanks. would you guess at an equivalent static setting like x degrees BTDC that I can try instead of setting the advance at 1500rpm?  I'm using 5BTDC now.  I don't use a strobe light.  We just got snowed in so it might be a while before I get back under the MGB. But I do want to try to make the Petronix work.

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Joe P.

Thanks.  I've since been reading up on the SU HIF carbs and it does appear to be a choke (not cable) problem.  I'll check it more next time I'm under the car.

Distributor is entire unit, not just points.  But, as you probably know, it is very easy to replace the entire unit.

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Joe P.

Joe, I suspect that you have two problems the first one is inherent with HIF carbs and second one is the due to the difference between early 25D distributors and later distributors, 45D and derivatives.

HIF carbs have a temperature compensator  [bimetallic element] that alters the mixture as  the engine/fuel warms up. Setting the carbs is a bit of a pain in the butt as you have to keep blipping the throttle to ensure that you are getting a fresh load of cooler fuel as you are adjusting the mixture .If you ignore this and do set things up for tick-over  with warm engine/fuel then the mix will be too weak at tick-over on a cold day/ engine. This might be so pronounced that  full choke is not  quite sufficient to git ya goin on a really cold day! I got fed up and made up some new non bimetal elements!

The second problem could be to do with  the vacuum control on your distributor, early 25Ds used ported-vacuum advance whereas later distributors use manifold-vacuum advance. Make sure that you set up your new distributor with the vacuum connected correctly and static/low tick over position set up correctly. Manifold vacuum is high at tick-over whereas port vacuum is low, at larger throttle openings they are the same. Get it wrong and tick-over will always be a problem even if you get things right by suckandsee methods at higher revs If your new distributor does not have a vacuum advance then again set up static/tick-over timing correctly and [importantly] always make sure that you have plugged the old vacuum lines properly!!!  Sorry about the long essay but hope some of it may be usefull. Cheers   - - steve

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To a man equipped with only a hammer, most problems look like nails

To help with Steve's good points the Pertronix wants the ported-vacuum. I have mine piped to my down-draft Weber. As for timing I measured the advance on my 45DE4 distributor at 1500 RPM and used that value on the Pertronix. I choose 1500 RPM because that was above what I measured to be a break-point in the Pertronix advance curve. I do not know how you would map that to a static advance.

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BobK

Joe

I am not familiar with the 69 MGB but can relate to the SU Carbs.  Living in a cold climate with temperatures well below freezing in the winter months I have seen the pistons in the suction chambers of the carbs freeze after a cold soak such as overnight.  Next morning no start.  Removing the crankcase ventilation hose from the rocker cover to the air filter solves this cold weather start problem as this is the source of the condensation and subsequent freezing of the piston(s). At one time BMC fitted a band heater around the suction chambers and also incorporated a coil type heater in the airstream to assist cold weather starting.  It was fitted into a spacer between the carb and manifold.  Neither worked very well and removing the hose worked better.  Thought this was worthy of mention seeing you too are in a cold climate.

 

John  

Thanks for all the info.  An update on my specific car.

There is a single vacuum hose running from the distributor to the intake manifold. 

There are only 2 external hose connections to each of the SU HIF4 carbs; one for fuel, and the other from the oil separator.

The choke cable indeed works fine and turns the cam as far as it will go.

That's it.  It's very simple.  I believe it's a "hybrid" of the elementary 68-ish evaporative control, fitted with the 74-ish SU HIF4 carbs.  When this car was re-stored (before me) there was obviously a hodgepodge of parts used.  It's a rebuilt engine and I was told pre-68 block.  But the HIF carbs weren't used until after 72 maybe?

Next chance I get, I'm gonna check out the full choke mixture and air settings.

thanks.

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Joe P.

Joe, There will be a nipple coming off the back of the rear of one carb  which would normally be connected to 25D distributor [on a 73 B anyway]  in your set-up check that this nipple is blanked off [e.g. bit of rubber tube with something bunged in it!] This is vital!!

Some 25D distributors did indeed have manifold vacuum connection but you would need to look at numbers on the distributor. It sounds like you may have a mixture of components here. I'm not sure I would take your current settings as any guide for a new distributor. I would set up as Pertonix instructions with the manifold vacuum line as recommended.

Cheers    - -   steve

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To a man equipped with only a hammer, most problems look like nails

more thanks.  And more car info.  As the advice pours in, I have more things to look at and discover.

Each HIF4 carb does have an unconnected nipple.  Appears to be used in emissions for a 72-ish model.  But my car has a pre-68 block, with little emissions tubing.  Doesn't even have the charcoal cannister.  These "open" nipples are right next to the fuel lines.  I do have the connections to the oil separator. 

Should I block/plug these open nipples?

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Joe P.

Absolutely, unless someones already bunged  em up with Bondo!! If you can start the car try putting your finger over a nipple at fast tick-over, if you can feel the suck then block em off with the tube and plug as described earlier. This does  however mean that the carbs will have been set very rich to compensate for this air intake, you will have to re-set the mixture again, properly , ticklish job as I mentioned earlier.  [Incidentally the usual HIF setup was only one of the carbs having the vacuum nipple so somebody must have a pair of HIFs with no vacuum nipples somewhere!]. Sounds like you've got a handle on this one Joe. Cheers    - -steve

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To a man equipped with only a hammer, most problems look like nails

Joe, I've just reread your last posting and realise that your reference to nipples is in fact to the vac connections normally used to connect via a Y piece to each other and go on to the crankcase.  [Whereas he distributor vacuum nipple is behind the carb between the carb and the manifold/header.]

Having said that I am surprised that the car will  actually run with crankcase vacuum lines wide open, they must be sucking air by the newtime. These should be connected up as above. Your Rocker case should  then have an airtight oil-filler plug and the Rocker-case should have a stub in which there is a very fine hole to allow a very small amount of air back into the crankcase . This stub would then have been linked back to your carbon canister. Running without crankcase vac connection usually means oil leaks everywhere! Sounds like previous owner may have created you as few problems?  Cheers   - - -  steve

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To a man equipped with only a hammer, most problems look like nails

Steve, thanks. I've been diddling with the B today. Maybe let's start over. My 69 MGB has a pre-68 engine block and a pair of 72-ish SU HIF4 carbs, and a 25D4 distributor.  So I have a mix of parts. There is no port on the valve rocker cover; no hole. There is no carbon cannister. The only emissions tubing is from the engine block (maybe oil separator) into a Y-adapter, going into rear of each carb before the intake manifold. The only other vacuum tubing is the distributor to the intake manifold. That's it; very simple-ain't no more And it all works very well. There is no excessive use of oil. There is no oil blow-out anywhere. The B doesn't consume oil at all. Certainly there's the typical oil drips and drabs on the garage floor, but nothing to be concerned about.  Each carb does have an open nipple near the gas lines, and I think that's part of the evaporative control system  on a 72-74 B. I tried plugging these nipples and can not get the car to run at all. So, that's not a good path.

My initial post was for cold starting; can only be done with a shot of starter fluid in each carb. Then vroommm; everythings good.  So, that's where I still am.

Oh, and my original post mentioned the Petronix electronic ignition. I still want to make that work. But right now the old distributor works fine. I'll keep that on while I solve the cold start problem. One thing at a time.

thanks

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Joe P.

Joe, if you have open vacuum leaks (those nipples), you'll likely never get it to work right under all conditions. Sounds like the carbs have been adjusted to compensate for the leaks at some condition. At any other condition, the mixture will be wrong. There should never be any air allowed into the engine that does not come throught the designed metering system, the carb throat in this case. I still think you need to close those and readjust the carbs properly. With just a little luck, that just might even fix your cold start problem too.

 

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Bayless
Never express yourself more clearly than you can think
'48 Prefect
'67 Sprite (project)