Tuning out a flat-spot - Weber 32/36

[gouldsborough7]

Guys
You may have read some of the messages I have started on ignition & Weber problems. Thought I would start this one as a separate issue to be resolved.

My engine is a 2 litre Pinto, and so far I've replaced the carb with a new standard manual choke Weber 32/36, new HT leads, rotor & dizzy cap. Ignition timing was too retarded, so that's now at 8 degrees btdc with vacuum tube clamped flat. The camshaft I identified as a Piper 270 - so a bit hotter than the Ford original, but not too quirky!

I've experimented to some extent with idle (low running) primary jets, and ended up with size 50 on both chokes. The tickover & general running is now quite good, but I cannot get rid of a small flat-spot at around 1000-1500 rpm. It's better than it was, but not cured. Could be expected on an old carb, but not a brand new one!

On searching the Internet, I came across an American Weber trouble-shooting site, where a guy said that the vacuum pipe should NOT be connected to the inlet manifold. He said that this would produce a flat-spot as soon as the butterfly was opened, as the vacuum would drastically alter for a moment. Sounds like my problem, but where else would you connect up the vacuum pipe?

Also, is the bore size of the vac pipe important? Mine is near to 5mm, whereas a local motorshop sold me one in a bubble pack with a tiny bore of 1-2mm, with bits of rubber tube at both ends to connect up to carb & vacuum. I've not fitted this yet. Might worth an experiment.

It doesn't help my problem that the transmission is a bit on the slack side, so it winds back & forth if the revs are allowed to drop in any gear to less than 1500rpm when the flat-spot will recur. It doen's help that I normally drive a front-wheel drive Nissan with fuel injection, so I naturally make comparisons with that.

Any hints would be appreciated.
Cheers
Mike

[dogoncrazy]

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

Mike
Are you refering to a vac pipe from carb to manifold?
Presume you have vac pipe from dizzy- where is that going to at present
Back in the days when I had Midget/MGB I'm sure the vac pipe from dizzy went to either a take off on the manifold or a take on on one of the twin SU's [downstream of the throttle butterfly] Maybe same applies here ?

[Dane_Rescueman]

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

Mike, your nice cam may need to be nearer 10°BTDC but that would not in itself cause a flat spot.
Despite the size of the vacuum tube not much actual air travels to the dizzy vac unit (assuming no leaks!). It is less than a teaspoonfiul whereas a 2 litre engine sucks around one litre of air every rev so there should be no effect. The vac outlet at the base of the Weber is designed to have little effect on the carb (which is where my vacuum take off is). The one on the manifold was for the brake servo on my old engine.

Assuming all gaskets, pipes, float settings, dwell and timing are OK then try a few things. The problem may be a variety of things but I would firstly check the amount of fuel expended by the throttle enrichment pump. The weber needs this to be in perfect working order to avoid leaning out.
If the problem happens upon flooring the throttle then you may need a slightly larger secondary jet,
Check that the fuel level in the new carb is correct. This is easily adjusted.
I fitted a pancake filter which was wrong for my engine. Apparently there must be a gap of at least 1½ times the width of the choke to the lowest part of the filter housing. This can cause strange airflow effects, so they say!

On alighter note, a pal had an old 1275 midget which had a super flat spot/roughness on accelerating. I traced it to a broken engine mount and stretching/relaxing of the poor connections on the dizzy cable! This won't be your problem but it did case grief until found out.!

Good luck with it - you are nearly there!

[h_m_cumming]

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

If you blank off the vac pipe does this make any difference to the flat spot? If it improves it try keeping the large bore pipe but fit a restricter with a tiny hole at the manifold end. This will slow down the build up of vac at the dist. and may be a cure while still allowing the vac to advance the timing on light throttle.

[gouldsborough7]

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

Thanks guys, I'll try out your ideas. By buying a new carb, I had hoped that I would not get all of this trouble with jetting or whatever.

I did try 10°BTDC but the engine appeared to be pinking under load, so put it back to 8°BTDC. Before I changed this, the previous owner had left the setting on around 2°BTDC. That's why I had a lot of running-on when turning off the engine. Mostly disappeared now using 8°BTDC

The original vac pipe (say around 4-5mm bore) is connected from a little metal pipe on the flange of the carb to the dizzy in the normal way. I don't know what the guy in the article I read meant, other than perhaps wrongly tapping the vacuum from the larger openings on the manifold, serving the brake servo etc.

Strangely, when I was improving my Robin Hood with 1600cc Pinto and similar Weber, I had the Weber refurbished by one of the 'experts' in the trade, and it worked a treat without any adjustments at all! Not sure that I even had to alter the tickover setting.

I bought a K&N filter with that re-furb, which was the flattest model available, so that I could close the bonnet.
Interesting idea about the air-filter,Dane. However, my very flat K&N did not appear to cause any trouble. The one I am using at present is a no-name variety, but is of modern design with an metallic-looking element. Perhaps as an experiment, I should try a few laps around my village with it off, and see what difference that makes to the flat-spot.

I did quiz Fast Road Cars who supplied the new Weber, and while very helpful the rep could not resolve the flat-spot. I did suggest that the accelerator pump was either squirting too much or too little, but he suggested to find another candidate for the problem before dabbling here. He said that he had a couple of customers who were boosting up fuel delivery something wicked before realising they had an air-leak somewhere. I don't believe I have an air-leak, but I wouldn't rule this out.



I'll let you know how I get on.
Cheers
Mike

[dogoncrazy]

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

Advance curve not matching the cam? just a thought!

[b_caswell]

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

Is the camshaft correctly timed up?? Sometimes a reground cam may be off the mark. Just because the points on the pulleys line up isnt good enough. You need to establish true TDC of the piston no1 mid way in the dwell position and also the full depression mid way point point on no1 inlet valve. If these points are out you may need either an adjustable pulley or offset dowels.

[gouldsborough7]

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

Wow guys! Now you are getting really technical. Ben, have you any tips on how to get a true TDC, and mid-way in Dwell position?

To be able to rotate the engine with a degree of control, (rather than struggle with a spanner on the crankshaft nut, bashing knuckles etc) my normal procedure is to take out all 4 sparking plugs, jack up offside rear wheel, select top gear, and use the wheel to rotate the engine.

I suppose I could carefully insert a metal rod down No.1 plug hole, and with a friend's help to turn the rear wheel find TDC fairly accurately. In terms of the crankshaft pulley-markings, I guess TDC would be midway between the 2 points where the piston stops/starts.

Is there a tool to put down the plug-hole, to more accurately measure what is going on with the piston?
Cheers
Mike

[dogoncrazy]

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

Have to agree with Ben on the fact that you can't rely on timing 'dots' especially with reprofiled cams. The process of grinding a new profile within what is already there can not only alter the cam timing and the lobe centres but result in a smaller base circle. This not only affects the valve geometry but also the amount of lift on the valve itself, and the geometry on the pinto engine is a lot more complicated than you would at first think. The only way that i know of of ascertaining true T.D.C would be with the head off and the use of a Dial test indicator and magnetic stand, same goes for determining point of full lift.
Reprofiling also removes any 'case hardening' which regretably is not always reinstated resulting in the cam adopting a completely 'new' profile within a very short space of time.

[Dane_Rescueman]

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

There is an insert available that can be placed in the spark plug orifice that, together with a dial test indicator will allow TDC to be found.
Once assertained, ideally a timing disc should be fixed to the crank pulley and alligned with TDC.
With the cam cover off, the dial test indicator should be positioned with a finger on N°1 inlet valve. Rotate the engine until max lift can be found. Check the reading off the timing disc and adjust the camshaft as neccessary. In my case my cam needs to be max lift at TDC -110°. I have a vernier pulley to allow fine adjustment.
It is not as hard as it sounds - just needs patience!

[gouldsborough7]

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

Hugh
Just tried out you idea of blanking off the vacuum pipe, i.e. disconnected the pipe at the dizzy end, and plugged the pipe with a suitable bolt, so no air is getting into the manifold by this route.

Bingo, flat-spot has gone without trace!!!

So without the vacuum, the ignition timing is being altered only with the revs, affecting the mechanical advance & retard within the dizzy. So, what does this mean? Is the dizzy duff in some way?

When I first checked the timing (as inheritted from a previous owner),I found the timing at tickover without vacuum to be only a few degrees before TDC. At this setting, the engine was inclined to run-on when switched off, and I had the flat-spot. Setting the timing to 8 degrees before TDC, cured the run-on, but the flat-spot remained in varying amounts depending on the carb settings, in particular the mixture screw.

Alan & Ben suggested that the timing marks might not be in the correct positions, and that I ought to do an accurate 'piston at TDC measure' to verify this.

So where do I go from here? I would be grateful for further suggestions.

Cheers
Mike

[b_caswell]

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

Best bet is get a copy of BURTON catalogue (www.burtonpower.com ) all is explained.
Using a dti you need to mark a point about 5 deg before TDC and note the reading on the dti then go past TDC and stop on the same dti reading after TDC. Then measure the angle between these two points on the pulley. This will give you true TDC.You will need a rod through the plug hole to transfer the piston action.. Then you need to do the same with the cam.

[danny_nelson]

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

Mike,you did do all your setting up with the vac pipe disconnected?
Regards
Danny.

[dogoncrazy]

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

Mike
Sounds as if the dizzy requires a different advance curve to match the characteristics of the cam, running without the vac pipe connected will pull down the mpg at best.I would talk to Aldon Automotive or put a standard cam back in!

[gouldsborough7]

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

Thanks Guys
Ben, what's a dti? I'll follow up your Burton info idea.Looks useful.

Danny, I originally set the ignition timing with a modern probe light, blocking-off the vacuum take-off pipe at the bottom of the Weber, as I thought this was the routine method.

Alan, I've no idea about the model of dizzy on my car. Is there some way of identifying what it is?

It was interesting to discover that, by disconnecting the vacuum pipe and blocking of the vacuum - the flat-spot totally disappeared! Is this a sign that something inside the dizzy is worn or slack? The flat-spot occurs at around 1500rpm, and if the revs are held at this, the car see-saws back & forth on the slack in the transmission. Above or below these revs, and the see-sawing stops.

A friend is convinced that I should carefully check the relationship between the camshaft timing and that of the dizzy, and also take the dizzy off the engine, & check its general condition.

So, lots of things to sort out. I don't really want to spend big bucks on getting a tune-up shop to sort it out - not what kit-cars are all about! So I'll check out all of the settings, and see where that gets me.

Cheers
Mike