1972 HC 1256 skipping uneven

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1972 HC 1256 skipping uneven

Postby Viva Man » Wed Apr 22, 2020 11:25 pm

Hello All,

I'm seeking any advice on my engine running uneven. I have been trying to solve this problem for a long time
When its running it's like a it's missing a heart beat. The engine kicks like it's missing but it's not. The power is there.
Things I have done to find this problem:-
Changed plug leads
New spark plugs
New coil.
Refurbished the carb . Fuel pump and petrol tank.
Reset the tappits
Check the compression which ranges from front to back in PSI 105 , 105 , 107, 109 . Which is low end for 1256 hc.
The thing I have noticed when I took the plugs out to do the compression test is number 1 and 4 cylinder plugs were basically black, the other 2 middle ones looked correct colour.

Apologies for the long explanation but this is driving me crazy (no pun intended)

Any ideas would be very welcome.

Many thanks
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Re: 1972 HC 1256 skipping uneven

Postby thomas » Thu Apr 23, 2020 1:11 am

Just thinking aloud here, don't really know what it could be, there's lots of possibilities.

I'm assuming being a 1256, it's the Stromberg side-draught carb, rather than the down-draught Zenith?

In either case 1 and 4 being richer is strange as owing to the longer intake length, they're usually weaker when the middle pair's mixture are correct, and to get the outermost pair mixture right, the middle two end up unavoidably a little rich.

Clues point to fuel/carb/intake, but don't rule out electrical causes.

An intermittent misfire affecting one and two leading to unburned mixture hanging around is possible, you might though along with that get an occasional pop or explosion of unburned mixture igniting in the exhaust, if bad enough such can blow out of the manifold to downpipe joint gasket/sealing ring.

Wear in the offset-drive Delco distributors can lead to inconsistent points gaps depending on which lobe of distributor shaft is activating the points. It could be too that the mechanical or vacuum advance is sticking, try some thin oil (3 in 1) on the felt cap under the rotor arm, and through the hole in the baseplate marked 'Oil' and between fixed and moving plates of the distributor.

On the Delco distributor the points spring may be riding up off its pivot.

Could be the distributor cap or rotor-arm to blame, twist the rotor arm in the direction of its normal rotation (anti-clockwise looking down at it) and see that it springs back round.

Or mistimed enough that it's too early or late firing. The rotor arm might be too far from the appropriate contact in the cap, the cap or rotor or both worn or carbon-fouled. A new rotor arm might be worth trying, but cap and rotor are best replaced together. The kick could be arcing inside the cap either to ground causing misfire or even leading to the wrong plug firing at the wrong time.

I've probably missed many other possibilites, it's late ... Good night!

:goodluck:
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Re: 1972 HC 1256 skipping uneven

Postby 1972nail » Thu Apr 23, 2020 9:59 am

I may be silly in suggesting this but I made a mistake many years ago on our Minor that caused similar symptoms. I got plug leads 1 and 4 mixed up. It started and ran but misfired.
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Re: 1972 HC 1256 skipping uneven

Postby Viva Man » Thu Apr 23, 2020 2:03 pm

Hello Thomas and Nial,

Thank you for the information ,

Thomas it's great in depth thoughts.

Just to add , when I run the engine and if I hold it at high or mid rev , the engine twitches like its coughing of missing. This is why I thought it was the valves or pressure. The carb is a zenith 301z. The the distributor cap and rotor arm is Bosch old electronic ignition which is strapped to the coil with flashing light when running.

Thanks again, should any more ideas come to mind please share.
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Re: 1972 HC 1256 skipping uneven

Postby thomas » Thu Apr 23, 2020 3:01 pm

It's very similar to my own (an HB) in that I have a late (1984) low-mileage Chevette 1256 engine, but with the Zenith 30IZ carb, plus Bosch Dizzy too, and electronic igntion (Powerspark) too.

I'm Clutching at straws really and without experiencing it hands-on, at a disadvantage.

These are not the most precisely engineered engines, and anything from worn camlobes to weight differences in pistons and other moving parts could cause inherent imbalances manifesting as rougher running. Is there any variation in behaviour as engine temperature changes? Tappets are best a bit on the wider side rather than too tight, even if it means it's a little bit noisier while warming up.

The only outright failure I had with this setup, once the carb was attended to with new diaphragms, float-valve etc., and keeping the accelerator pump linkage setting on the summer setting all the time, never winter as it was too rich, was with the electronic ignition unit's wires inside the dizzy having the insulation worn away by contact with either the shaft lobes, underside of the rotor, or the trigger ring (most likely) causing a misfire when near flat out climbing a hill due I surmise to movement of the advance mechanism eventually causing one or other of the units exposed wires to contact ground. These wires too through flexing and the internal strands failing, maybe through getting pinched could intermittently break their connection especially as the advance mechanisms are in constant motion one way or another sometimes quite violently, as revs and throttle opening change.

You might try moving the ignition unit's trigger rotor round one quarter turn to the next lobe, as it could be gone faulty from friction against something, check too that there's no swarf, iron filings or anything sticking to it. It might move the problem to the other pair of cylinders, but would need a good run starting with clean plugs to determine any change. I've assumed the blackened plugs were sooty, rather than oil-fouled.

It might just need an 'Italian Tune-Up'. A damned good fast-blast run and sustained wide-open throttle on a long-straight, after getting it fully-up to operating temperature. I never really got satisfactorily smooth running until I started using Tetra-Boost tetra-ethyl-lead additive, just a little, less than recommended to create true four-star which really got it back on song. It would be making it more rich still, but can compensated for with carb tweaks and running near enough (just half a degree less) original specification static ignition timing.

I'm out of ideas, hope someone else can chip-in too. Have to run, do keep us posted!

:respekt:
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Re: 1972 HC 1256 skipping uneven

Postby Johnboy HB » Thu Apr 23, 2020 9:14 pm

Could be fuel related as you suggest, mine run better on super unleaded
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Re: 1972 HC 1256 skipping uneven

Postby jpsmit » Thu Apr 23, 2020 11:14 pm

unless it is old fuel (ie. last years) I suspect it is more likely something else.

My Midget was cutting out like that and a new coil made all the difference in the world. I would say that new parts doesn't mean they work - especially these days. :roll: I don't know if you have known good parts to try swapping out one at a time.

Is the misfire happening all the time or only when the engine is warm?
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Re: 1972 HC 1256 skipping uneven

Postby Viva Man » Thu Apr 23, 2020 11:48 pm

Hi

Well I think I have made a real mess of things.
I was looking at the timing and for some reason the old bosch electric ignition still worked off points as well, confusing I know. Anyway the gap was not standard. So I took off the points to clean, when replacing them Ithe small plastic on the spring part cam off and fell into the vacuum housing connected to distributor. Took out the distributor and during finding the plastic piece and putting back in the distributor core had twisted 180°. I did not notice and placed back in. It made horrendous back fire with a few bangs. Scared the hell. I reversed the problem and is started with other bangs very roughly with spitting out sut and water via exhaust. After lining the timing distributor it run better but rocking even more.
I think the head gasket has blown. All for not paying attention.
Anyway I will return tomorrow to assess and hope my fears are not true.

Thanks.
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Re: 1972 HC 1256 skipping uneven

Postby Fred Dukes » Fri Apr 24, 2020 9:07 am

As David said have you checked the plug leads are in the correct order. Also recheck that the dizzy offset is correctly lined with the top of oil pump drive shaft (if it is offset that is and not centre drive).
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Re: 1972 HC 1256 skipping uneven

Postby thomas » Fri Apr 24, 2020 5:23 pm

Fred Dukes wrote:As David said have you checked the plug leads are in the correct order. Also recheck that the dizzy offset is correctly lined with the top of oil pump drive shaft (if it is offset that is and not centre drive).


I think it's the Bosch Dizzy, Fred, thus centre-drive.

Viva Man wrote:Anyway I will return tomorrow to assess and hope my fears are not true.

Dearie me, I hope not too.

It's unlikely that the head gasket could go simply from having the distributor 180degrees out. If it has it must have been already failing. And might have been the underlying problem, all along. Though it's rarely as bad as things seem!

Most modern electronic ignition kits do away with the points (and condensor) altogether, instead using a Hall sensor and a plastic ring with four embedded small ferrous metal pieces, both sensor and ring completely contained within the distributor body. What you appear to have and describe still uses the points, which has the advantage of reducing burning and pitting of the points themselves though they'll still drift as the plastic heel part which contacts the distributor shaft lobes wears. I can't recommend the newer type enough, they're simplicity itself and deliver juicy sparks.

This picture shows the Bosch Dizzy.
Image
Fom the front of the car the vacuum-module sits roughly at about the five o' clock position.

If the engine is turned (spanner on the crankshaft pulley, turned clockwise) until cylinder number one is approaching crank-pulley TDC mark (verify that on this rotation it's not number 4 approaching TDC, by looking through the oil filler orifice to see that both of number one's valves are closed), then the rotor should be pointing towards the vac advance unit, plug lead 1 should be there too, then anti-clockwise round the cap from there are plug leads 3, 4 and 2.

:goodluck:
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