Tuning out a flat-spot - Weber 32/36

[dogoncrazy]

Re: Tuning out a flat-spot - Weber 32/36

Mike
Dti- Dial test indicator
By taking the step of 'removing 'any ignition advance by the vacuuum unit appears to have brought the advance curve ie the point at which the spark occurs, to somewhere near where it needs to be, at least as far as just off idle is concerned. However it's unlikely that it will be correct for the remainder of the rev range which is why I suggested that dizzy advance is probably incorrect for the cam that you have installed, which is what your friend has suggested. When Ford put the engine together it was to give a reasonable power output combined with good economy and a steady idle, so the dizzy was engineered to provide the spark in the right place throughout the required rev range.
Installing a 'performance' cam alters those parameters, so the advance curve is rarely right [something the cam manufacturers omit to mention when they provide claims of an increase in bhp]
Running the engine without a vacuum advance is NOT recommended for anything other than full race applications for anumber of reasons, again the least being that it affects part throttle economy.
Firms such as Aldon Automotive have years of experience with many engine types and numerous cams fitted, all tested on rolling road. They will almost certainly have the data required to provide a dizzy that works with your cam!

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

Hi Alan
Thanks for the info. Very interesting. I'll try Aldon Automotive and go from there. Amazing that this saga originally started a couple of months ago by breaking down on the M25, changing the guzzling carb from a Granada to the correct Weber, leading to the gradual changing of various components.

Cheers
Mike

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

Just an up-date on events: Contacted Aldon Automotive, and while trying to be helpful, they recommended that I book a Rolling Road tune-up session. Problem is that they are 110 miles away!

By blocking up the vacuum pipe, the Aldon's guy didn't think the problem was with the ignition timing. I am not convinced. At the moment, I am running the car with a blocked-up vacuum pipe and my car goes very well like this.

As far as I know, the dizzy on my engine also has centrifugally operated weights that changes the advance/retard settings as well. It would help me to understand what I am actually doing by running without the vacuum working.

Can anyone tell me how the vacuum and the weights interacts with the dizzy?

Cheers
Mike

[greyV8pete]

Re: Tuning out a flat-spot - Weber 32/36

Mike. On a completely different track. I had a very similar problem with a 1500GT Cortina (Weber Carb) some years ago. Eventually found that in the past the dizzy had been fitted with the wrong cap! Result was that the four H.T pick up contacts were timed incorrectly (about 10deg of dizzy shaft)at the firing point. The effect was that when accelerating the drop in vac / advance caused the rotor arm to dizzy contacts to suddenly become very large, with associated flat spot and jerky throttle. Correct cap fitted and problem sorted! Long shot but worth a check? Cheap & quick too! Peter.

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

Hi Peter
I replaced the cap as part of a search for a solution to the poor running, plus leads and rotor. The original main HT lead where it fitted into the top of the cap was burnt, so that didn't help things too much!

The guy in the motor shop was positive that I had the correct cap, and even knew the Cortina part numbers from memory.

To conclude, the engine now runs fine throughout most of the rev range, from tickover up to 3000rpm+. It is just at 1500rpm that there is (or was) a distinct jerking action flat-spot. A temporary cure is to block up the vacuum pipe. Flat-spot has virtually disappeared. Performance reasonable.

Unless someone says something suggesting that my engine will blow up, or mpg will be down to 20mpg, I will try the blocked vac pipe for a while longer. With the Grannie carb my mpg was often 22mpg anyway!

In the meantime, I would still like to know from someone what my blocking of the vac pipe actual does, and how the original set-up interacted with the centrifugal weights in the dizzy

Cheers
Mike


Cheers
Mike

[h_m_cumming]

Re: Tuning out a flat-spot - Weber 32/36

Hi Mike,
I’ve been following your saga and maybe can throw some light on it.
When you set up your static ignition this is with no advance from the centrifugal bob weights and no advance due to vacuum. The bob weights don’t start to cause advance until above about 8oorpm. As revs increase the bob weights fly out with centrifugal force and advance the firing point to enable the mixture to start to burn sooner so that the explosive effort is effectively applied to the piston at the same point of its rotation despite the increase in rotational speed. When the engine is lightly loaded, especially on the over-run, the ignition is further advanced by the increased vacuum to enable better burning without causing detonation as would happen if it was so far advanced under load. Remember the vac. all but disappears as soon as you open the throttle wide
If you set up your timing with a strobe you can watch the ignition advance as the revs. go up. With vac connected you can see the ign advance as you close the throttle due to the increase in vac then decrease as the revs drop.
If you look at the ign timing with a strobe with the vac connected at tick over it is further advanced than when you set it up statically since at tick over you have maximum vac and therefore max vac advance.
You should always set the static advance with a strobe with the vac disconnected.
My Fiat engine has no vac. advance and was set up on the rolling road under load at ¾ revs. to give the best power, so I don’t think you’ll do any damage by leaving the vac blocked off, but it would be worth getting the ign adjusted on a rolling road to find its best point for your engine.
As for finding TDC, get an old sparkplug and knock out the core. Find a bolt that fits and bolt it into it so that when screwed into the head it stops the piston from going over TDC when the piston is about 1” down the bore. Make sure you screw it in when the piston is on the firing stroke then both valves are closed and there’s no danger of it fouling a valve head. If you fit your timing disc and rotate it forward and mark the angle then rotate it backwards and mark the angle, then true TDC is at the midpoint.
Hope this has been of some help.

[h_m_cumming]

Re: Tuning out a flat-spot - Weber 32/36

Bt the way, centrifugal advance is usually about 30deg

[greyV8pete]

Re: Tuning out a flat-spot - Weber 32/36

Hi Mike. I am sure that your man knows the part numbers of the dizzy cap but mine said so too! The quick check that I did with mine was to set the engine at the static advance BTDC on #1 cylinder and note the position of the rotor arm with a mark on the dizzy body. Then I checked that the contacts in the cap were exactly in line when the cap was in place. The wrong cap was well out of line, the correct one lined up exactly.

My old Classic car workshop manual says “the vacuum advance unit gives improved efficiency and economy at part throttle opening”.

When cruising or on overrun (high vacuum) the vacuum unit advances the ignition further than the mechanical centrifugal (engine speed dependant) system. As soon as you press the throttle pedal the manifold vacuum drops and the advance returns immediately to whatever the mechanical advance is at that speed. If you run with the vac advance disabled you loose that benefit.

I don’t think that the diameter of the vacuum pipe is too critical. My classic has a fine metal capilliary tube whereas the Cortina and my Rover V8 engines both use 5mm bore rubber tubing. The important thing is that if rubber is used the wall thickness is sufficient to prevent collapse at max vacuum. Peter.

P.S. Typed this up in draft before I saw Hugh’s comments! PL.

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

Thanks guys
That's been a big help. I've got a few more things to sort out now.

While it is nice to achieve things on one's own, there may come a point where I might have to bite the bullet and contact the experts. Could be tempted to go for a Rolling Road, if I can find one in Herts. Will look in Yellow Pages.

Cheers
Mike

[GaryW]

Re: Tuning out a flat-spot - Weber 32/36

Hi Mike,
If you can't find a rolling road closer to home, then try Northampton Motorsport (http://www.northamptonmotorsport.com/), it would be about 40 miles for you. I took my Berli there and they're pretty good. As it happens I'd got the timing about right and they only changed a carb jet.
Good Luck!
Cheers,
Gary

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

Thanks Gary
That's very helpful. Looking in "Yell.com" I suppose I was not altogether surprised that all of the tune-up shops only did a glorified Sun-tune. Obviously, the likes of us needs a bit more than that.

I'm waiting for a timing disc to come in the post from Kent Cams, with the idea of accurately checking the dwell angle on piston No.1 and hence an accurate TDC. If the latter is much different from that scribed on the crankshaft pulley, that may answer some of the questions as to why the engine has not been behaving as hoped.

If this procedure fails, I think I may have exhausted all of the possibilities that an amateur DIY guy can do with limited kwow-how and tools, and I will certainly give Northampton Motorsport a try. I couldn't check out their web-site at work, because the firm's policy seems to block certain companies. I'll check them out tonight.

Cheers
Mike

[Dane_Rescueman]

Re: Tuning out a flat-spot - Weber 32/36

I had a rover once that was impossible to tune properly. I eventually traced the problem to the timing belt being one tooth out between Crankshaft and Camshaft sprockets.
Checking the TDC and valve timing will show this up.
Even if the old camshaft sprocket is used with the new camshaft, the timing marks (if it has any) do not tally with the new cam timing. Hopefully the methods described previously will highlight any such problem.

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

OK Dane
I'll let you know the outcome once I've received the disc and checked it all out.

Cheers
Mike

[b_caswell]

Re: Tuning out a flat-spot - Weber 32/36

Must be too cold in the garage!!

[gouldsborough7]

Re: Tuning out a flat-spot - Weber 32/36

No, I've got a butane heater, so not a problem in cold weather! As promised, I have summarised below the finale of my problem.

I fitted the new timing disc to crackshaft pulley, such that the TDC mark on the pulley lined up with the one on the disc. I thought at this stage that any other arrangement didn't make sense.

Per Hugh's suggestion, I cannabilised an old sparking plug and fitted it with a nut&bolt. With rocker-box cover off so that I could judge when No.1 piston was on the firing stroke, I rotated the engine towards TDC with the modified spark-plug fitted. I had to experiment with the nut&bolt so that it stopped the piston at 30 degrees before TDC as shown on the timing disc. This figure seemed reasonable to me.

So in theory, on rotating the engine backwards on the other side of compression/TDC, I should also find that the piston stopped at 30 degrees. Bingo it did - absolutely spot-on!

I was pleased to find the TDC marks lined up with the marker on the engine. Also, at TDC the inlet & exhaust cam-lobes formed an equal very flat V. So nothing was apparently wrong anywhere!

With the engine running, and the vac pipe blocked, I strobed the timing marks at around 750rpm, and found that the ign timing was 10 degress before TDC. Leaving things alone there, I fitted the vac pipe with a home-made valve having just a 1mm bore as an experiment. I didn't get a chance to drive the car much after that, but on a local journey of a few miles, the engine behaved very well. No flat-spot, good tickover, good starting. So what more could I wish for? I'll have to run it on a more serious journey, but so far so good.

Cheers
Mike