Twin plug weak spark after dizzy ?

[tctnd]

As noted, phasing is not affected by rotating the distributor, but it does change as the advance increases. The rotor and cap terminals are deliberately made quite wide to insure that the gap between them remains small throughout the advance range. When fully retarded the leading edge of the rotor should overlap the "trailing"(for want of a better word) edge of the cap terminal some small amount and when fully advanced the trailing edge of the rotor should still overlap the same cap terminal. This insures that the gap between them (and the voltage needed to bridge it) remains constant.

regards,
Phil

[chris_seven]

Originally Posted by haasad

Yep 120 degrees. Bang on. And as I understand it if I have the spark occurring at 6 BTDC 900 rpm and at 30 BTDC 3000 rpm. the rotor must be staying in "contact " with the dizzy pin. There is a clear grey track on each rotor tip and each dizzy pin there is no evidence of tracking in the cap.

Marelli Twin Plug Rotors are ususally 150 degree.

[haasad]

Sorry Chris... yes total brain fade its a 150 degree rotor. I'm just waiting on Accuspark to let me know what out put tells the CDI unit that the correct firing point has been reached then I can mock it up on te bench and check my phasing.

Good news is that the bonnet is fixed....

[haasad]

Thank you to all who have helped me understand rotor phasing its been a usefull learning experience and still something I have to do.. However made excellent progress today and now have a healthy sounding 911.

I discovered after some input from friends that I had not proven the idle circuit on the LHS carb and in particular No3 cylinder properly. It was blocked solid on the fuel suction side from the bottom of the emulsion tube.

I drilled out the lead plugs cleared the blockage re plugged the holes with some lead beads that fishermen use and ran it up. Instantly better and pulls dead clean to 4500rpm when it has a little stutter which I'm sure a rolling road session will sort.

Beer o'clock me thinks

[LJ851]

Originally Posted by tctnd

As noted, phasing is not affected by rotating the distributor, but it does change as the advance increases. The rotor and cap terminals are deliberately made quite wide to insure that the gap between them remains small throughout the advance range. When fully retarded the leading edge of the rotor should overlap the "trailing"(for want of a better word) edge of the cap terminal some small amount and when fully advanced the trailing edge of the rotor should still overlap the same cap terminal. This insures that the gap between them (and the voltage needed to bridge it) remains constant.

regards,
Phil

Are you sure about this? When i put a timing light on my car with the hole in the distributor cap and rev the engine up, the rotor stays exactly in the same place.

[haasad]

No expert here as you will note but I would imagine the rotor tip would have to move relative to the dizzy post as the timing advances with rpm otherwise there is no evidence of advance. I think the response given is therefore correct.

Andy

[LJ851]

Originally Posted by haasad

No expert here as you will note but I would imagine the rotor tip would have to move relative to the dizzy post as the timing advances with rpm otherwise there is no evidence of advance. I think the response given is therefore correct.

Andy

The advance happens between the two halves of the dist shaft and thus changes where the trigger is in relation to the crankshaft. The trigger never changes it's position relative to the rotor, it cannot.

I have shot a timing light and seen this happen in real life, i get the feeling those arguing against this have not. Maybe i should shoot a quick video...

[tctnd]

"The advance happens between the two halves of the dist shaft and thus changes where the trigger is in relation to the crankshaft. The trigger never changes it's position relative to the rotor, it cannot."

This is true. If the rotor were fixed to the lower dist shaft then its position relative to the cap would be fixed, but it is not. The sensor is fixed to the dist body as is the cap. As the advance mechanism works, the trigger (on the upper dist shaft) moves in relation to the sensor and dist body as does the rotor.

regards,
Phil

[LJ851]

Originally Posted by tctnd

"The advance happens between the two halves of the dist shaft and thus changes where the trigger is in relation to the crankshaft. The trigger never changes it's position relative to the rotor, it cannot."

This is true. If the rotor were fixed to the lower dist shaft then its position relative to the cap would be fixed, but it is not. The sensor is fixed to the dist body as is the cap. As the advance mechanism works, the trigger (on the upper dist shaft) moves in relation to the sensor and dist body as does the rotor.

regards,
Phil

Think about it some more, you are incorrect.

It is the same reason the rotor phasing does not change when you adjust your timing by rotating the distributor body (and pickup).

The only time the rotor phasing would change is if you had vacuum advance (which we are not talking about here) and the vacuum unit rotates the pickup in the distributor relative to the housing and the trigger.

[tctnd]

You are correct. Sorry if I've led anyone astray with my brain fart. Indeed, long term exposure to vac advance distributors clouded my mind.

regards,
Phil

[haasad]

I love this stuff!

So I should be able to be confident in rotor phasing as long as the rotor to dizzy relationship stays static and aligned when checked with a holey dizzy cap and a timing light. This function is independent of the mechanical advance which changes the point at which the spark is generated relative to crankshaft position ?

If this is true I will stop fussing about with distributor phase checks and concentrate on sorting my fuelling ( It now has a big fat flat spot above 3900 rpm in any gear)

[haasad]

Hi Guys, thought I'd bring this up to date and report findings.

So I had a 3 hour rolling road session this morning, we dropped air correctors one size and things improved mixture wise but there was still a misfire loaded only at 3800 rpm onwards.

We went back to the dizzy to look at phasing. What we discovered was that once the total advance was "in" the rotor tips were clear ie past the relevant dizzy pin by some margin ( as discussed previously).

We did some measuring and re-positioned the rotor relative to the shaft to be sure that at idle the rotor tips were just starting to close in on the dizzy pins and as the full advance comes in they are still in good proximity.

Ran it up misfire nearly gone just a little high end (5900-6000) miss and pulling like a train below. We are sure the NGK6ES plugs which I've been running it on are a bit soft so will swap out to some harder hotter ones tommorrow. One other thing to do is make a little insulator for the modified jag rotor end since their is a lot of exposed conductor there which may be tracking under heavy load.

We didn't do a full power run but we know we have a minimum of 180hp at the wheels. This dyno is known to be conservative in its figures and the best bit is how torquey it is pulling some real load easily.

I'll get some figures once its got a few more miles on it.

Did have a scare when we noticed a loose rocker shaft bolt....... but all fixed now.

[LJ851]

Originally Posted by haasad

We went back to the dizzy to look at phasing. What we discovered was that once the total advance was "in" the rotor tips were clear ie past the relevant dizzy pin by some margin ( as discussed previously).

We did some measuring and re-positioned the rotor relative to the shaft to be sure that at idle the rotor tips were just starting to close in on the dizzy pins and as the full advance comes in they are still in good proximity.

How did you check your rotor phasing?

It sounds like you did not use a timing light to view the phasing while running the engine because you would have seen that the rotor phasing (alignment) does not change when the timing advances.

You really need to check this, especially since you are modifying the phasing.

[haasad]

ok I need to be clearer. We have physically refixed the rotor on its adaptor . Remember I designed this dizzy so built in adjustability.

The net result is that when the spark occurs the rotor tip is now properly aligned to the dizzy pins . Previously the rotor was misalinged. I agree we did not use a strobe with a cutaway cap but as long as one knows from the trigger signal when the spark is occurring it is a simple matter to check rotor to pin alignment and get very close.

We believe their was crossfiring as the spark took an easier path due to misalignment.

Now in my mind this is a phasing adjustment and the dyno and diagnostics all tell us we've nailed it as does the seat of my pants.

So time will tell if there are any sub issues. PS I'm rebelling and running a different plug than the norm too... NGK B7ECS...they have been recommended by the Boss at the dyno and he certainly knows his stuff..I couldn't resist fitting them and blatting up the road...it ran and revved perfectly

[LJ851]

Originally Posted by haasad

... as long as one knows from the trigger signal when the spark is occurring it is a simple matter to check rotor to pin alignment and get very close.

This is true and should be close enough.

What made me question your method was that you said the rotor was moving away from the terminal as the distributor advanced which is not possible.