Can’t get it to accelerate !

[TimdlF]

Hi,
I’m a new A7 owner and new to the forums. I have a problem and wonder if anybody out there has any suggestions ? I bought a 1937 Ruby in running condition which quickly blew a head gasket. When I opened the engine up for the simple gasket replacement I found a horror story. 2 years later the only part of the original engine remaining is the con rods. The carb, distributor, dynamo and fuel pump have been rebuilt. I have rewired the car and recored the radiator. The car now turns over easily and ticks over nicely. However, when I press the accelerator the revs will just not build so it is not drivable. I have checked the timing which seems right, I have checked that fuel is getting through, which it is, I have cleaned through the rebuilt carb which seems clear of debris but all to no avail. 

Any advice gratefully received but I do have a particular question. I have had the throat of the intake manifold bored out to take an SU carb which I have yet to fit. I thought I would get it working with the original Zenith 26VA carb first to prove that it does actually drive before I start modifying it. Would boring out the throat of the intake manifold alter the manifold pressure in such a way as to prevent the fuel being sucked in when the throttle is opened ? What do people think ? Grateful for any suggestions.

Tim

PS: Before I messed with it it did drive normally

["Slack Alice" Simon]

Try putting another condensor across the points - I usually do it by getting any condensor, with at least one wire connection, from my local car spares place, clamping one end under a cylinder head bolt, and fixing the wire to the existing connection on the outside of the distributor.

[Stephencrisp360]

Welcome. 

No I had a similar experience some years ago while I replaced my head gasket on a hill in Scotland somewhere at midnight with ice forming on the sheep all around me. Anyway I'm digressing.  After i have done all that, the poo car would not drive.. After calling out the AA for recovery they said "have you checked the points".  I grumbled to myself, "I've not even touched them in this repair"

But, i checked then and sure enough you could drive a bus though the gap. 

My point is , check your points and plugs gaps.

Always room to adjust the advance and retard of the timing too.

Steve

[Nick Salmon]

If this needle valve is not correctly adjusted, the engine will not pick up cleanly.


  carb.JPG (Size: 35.07 KB / Downloads: 324)

[Peter Naulls]

When I first built my special many years ago I inadvertently fitted a Ruby type manifold which had been bored out for a larger SU. With the 26VA fitted it would drive ok but had a dreadful flat spot just above idle. Pulling away from rest was difficult but on the go it felt perfectly ok.

[John Cornforth]

Hi

That's puzzling behaviour. I'm assuming that you are testing the engine in neutral, i.e. revving up off load ? Even at quite high sustained revs the engine airflow and fuel requirements are still only a fraction of what's needed when out on the road, and the throttle will still be maybe only 5 to 10% open once the flywheel is up to speed. If it won't rev under these conditions I can think of two main possibilities:

A very restricted fuel supply such that the mixture leans out as soon as the airflow increases above idle. This will start with a lean misfire and spitting back.

A very weak ignition such that increased combustion chamber pressure as you open the throttle is enough to kill the spark. This is also usually accompanied by a progressively worsening misfire.

I can only suggest you methodically go through the ignition system (especially the points and condenser) and carburettor. Even with a rebuilt carb, its not impossible for there to be debris in the internal passages which may move along and block fine jets. A good blast with an airline and carb cleaner is recommended. Borrowing a known working Zenith might be an idea if you know a nearby Sevenner.

The 26VA has a tiny progression jet which is used to overcome a lean flat spot just above idle. The jet is under a brass cover screw on the underside. Don't try unscrewing the jet itself, you will only damage it. It can be blown through once the cover is removed.

Under these conditions, I cannot think of any detrimental effect from a bored out manifold eye. Manifold pressure is determined principally by the pressure drop across the throttle butterfly. Any pressure drop across the eye restriction would be small, and only significant when the airflow is quite high i.e. full throttle and maximum RPM simultaneously.

[TimdlF]

Thanks for all the suggestions chaps. I’m glad that I joined the forum. Sorry for the tardy reply. I imagined that I would be emailed when I got a reply to my question. I’ll have to check the forum settings. I will have a look at all those things that were suggested and report back.
Regards

Tim

[bob46320]

Just add tappet clearance to the list.

[John Mason]

Also check that there are no massive air leaks at both manifold to block and manifold to carb.
L
John Mason.