Timing? Carburation?

[Dad]

Here’s the problem. Starts fine. Carbs seem really well-tuned. I recently stripped and serviced them. Shiny!

But when driving, all is well until I try to accelerate firmly, at which point, upon climbing much over 1,500, the car lurches, judders and bunny hops.

Now this may be really obvious to many, but is this likely to be advance in timing? I did work through setting timing up properly although maybe I need to go back and do it again?

I should mention, I recently replaced the distributor with an electronic one; https://mossmotors.com/d171600-flame-th ... -pertronix

I may have assumed too much when the product marketing says “ Tailored advance curve for optimal performance and drivability.”

[Geoff Allam]

Did you have the problem before you replaced the distributor? Or before you rebuilt the carbs?

[Dad]

Can’t be sure. I know that’s not very helpful, but it’s been a long time. It’s not been driven much in the past decade-plus.

[Dad]

The distributor manual says;

“ Locate the vacuum hose that was previously attached to the vacuum advance canister. This hose should originate at a ported vacuum source. Some applications have vacuum advance hoses attached to a manifold vacuum source, due to the performance advance curve, we recommend that you relocate this hose to a ported vacuum source. After setting initial timing the hose will be unplugged and attached to the vacuum advance on the distributor.”

What’s the difference between a ported vacuum source and a manifold vacuum source? My car has a manifold vacuum source. Is that supposed to be connected to the distributor? As it was with the original distributor? Is it because I haven’t yet relocated this hose to a ported vacuum source (whatever that is)? And if so, how do I do that?

[cactusman]

The e type vacuum advance is taken from the front carburettor (assuming you have triple SU set up) via a thin tube. I guess this could be blocked off and a tap taken of the vaccum hose that goes to the brake servo but I can't imagine it would make any difference. Try running the car with the vacuum advance removed. It should run OK but will lack power as revs rise...if it still lurches and hops it probably isn't the vaccum advance...
Lurching and jumping sounds to me like possibly a loose connection somewhere that is exacerbated by vibration. Or else it may be interference from the HT affecting the pertonix...have you used old copper HT leads or replaced with new carbon core leads as these generate less interference? Could even be a loose HT lead? Hopefully someone who has fitted the flamethrower distributor will have some ideas...

[caveman]

I would try a new set of NGK spark plugs (can present the same as you describe if plugs are breaking down) and a decent Red rotor arm:
https://www.distributordoctor.com/red-rotor-arms.html
Then a new set of HT leads, particularly if it’s not been run for some time.
https://www.sngbarratt.com/English/#/UK ... 20leads%60
My Pertronix has been faultless, it either works or it doesn’t. Simple. It was the internal distributor type though, not the flamethrower.

[lowact]

‘Ported’ vacuum is when the vacuum is taken from the carburettor, for vacuum advance it is above and just outside the throttle plate. There are two benefits compared to manifold vacuum:

Manifold vacuum drops off as the throttle is opened. Ported vacuum, because of the relatively high air velocities in the ‘port’, persists for a lot longer, helping with acceleration and economy, if the distributor is designed to take advantage of this.

The 2nd benefit, the throttle plate serves to turn the vacuum signal on and off. When the car is idling, the throttle plate is closed, the tapping point is on the outside (air cleaner side) of the throttle plate, there is no vacuum. When the throttle plate opens is moves out and over the tapping point which is then on the inside (manifold side) of the throttle plate, where strong vacuum exists.

[mtnjag]

Are you timing the car with the Vacuum Disconnected?
Whether Manifold (vacuum at idle) or Ported (no vacuum at idle) the Initial timing should always take place Without Vacuum connected and the Vacuum Port Plugged so the car doesn't have a vacuum leak.

With Ported Vacuum connecting the Vacuum after your Initial timing setup at idle should not change the timing since there is no vacuum and no advance due to no vacuum at the distributor.

With Manifold Vacuum connecting the Vacuum after your Initial timing setup at idle with Increase your Advance since you have introduced Vacuum to your Vacuum advance at the distributor, and it will raise the idle speed. You then adjust your idle speed back to the desired level. Your Advance will remain at a higher level than set with no vacuum.

Ported and Manifold vacuum have small differences in road behavior but are very similar once you are actually driving.

I believe the Vacuum Port on the front carb of my 62 is before the throttle plate and is therefore Ported Vacuum.
If taken after the throttle plate it is Manifold vacuum.
The General rule is do not share a vacuum port with your Brake Vacuum Booster.
And the brake booster always draws Manifold vacuum since you want boost at idle.

Some like Manifold Vacuum, some like Ported.

[Simonpfhc]

Definitely sounds like not enough advance. All the above advice is quite correct, but an easy test is to just manually advance the distributor (turn clockwise) to see if that improves the performance.

If it does, then the distributor is not providing enough mechanical advance. You could leave it like that, but you are then running at high advance at tickover. Not a massive problem but could mean harder to crank when cold, lumpy tickover along with the engine generally running under a bit more stress at low rpm. I’ve worked on engines that have been running like that for years.

Not sure how your distributor handles the lack of vacuum advance, but vacuum advance provides better throttle response, better fuel economy and a generally more efficient running engine.

[Jack the lad]

Old petrol would be my first check. If it has been stood with the tank, pump, lines and filter full for many years the petrol will be useless. Drain and replenish with new.

[lowact]

because it sounds exactly like a blocked fuel filter, that there's only enough getting through to support gentle driving below 1500 rpm. When there's not enough fuel combustion will initially be sporadic = "bunny hopping".

On the subject of vacuum signal source; imo the most responsible advice would be to use the source that the distributor (advance curves) are designed for, preference would only be if you don't know what this is. In this case its ported so ...