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I'm currently having trouble setting up my engine which I finished building last year.
The trouble is the car gives good power with the throttle open but under light throttle, at cruising speed it feels as if it's towing a huge weight.
The car has an over bored 998cc engine with a 12g295 head, 3 branch exhaust, RC40, twin 1.25" strombergs, piper 270 cam and an Aldon yellow distributor with not vacuum advance.

I initially had the engine set up on a rolling road so I'm happy with the carb set up but the dizzy had to be returned to Aldon. My initial set up had the static timing around the 15-16 degrees area, with this the car ran slightly hot and was difficult to start. I've since returned it to 12 degrees static and it runs cooler but the light throttle problem is worse. I've never heard pinking on this engine.

I have it in my mind that adding a vacuum advance to my distributor would help this problem, what do you think?

For the driving situation you are describing a vacuum advance should help as it is specifically aimed at providing more advance under part-throttle cruising. I believe the intent of the vacuum advance was more towards improved fuel economy but the advance should also give a bit more power at the same time. However, when you depress the gas pedal the vacuum will drop and that advance will go away... so it's not going to help with acceleration from one speed to another.

You are running twin Strombergs? There are lots of twin SU parts out there and the wealth of knowledge on needle selections for the A-series. What made you decide to use twin Strombergs and how did you determine which needles were right for your application? Among my other cars I have a '72 Triumph Spitfire which I took the other direction, replacing its single 150CD Stromberg carb with a single SU HS4.

That was my thought but when I contacted Aldon, they didn't seem convinced.

I simply saw the strombergs for sale on an Alexander manifold and fancied trying something different. I agree parts are a bit of a nightmare for the small earlier CD carbs. I had to use an imp specialist to get hold of a rebuild kit. An old copy of vizzard's book recommended the needles but on the rolling road these we're found to be a bit lean so I fitted the needles that came with the carbs which luckily seem to be fine.

I can't see vac advance helping. If the curve for the dizzy was something like correct then this problem should go away.

When you had it rollered, did they do anything to the dizzy? What did Aldon suggest?

Vac advance "MAY" help a bit but it is more a case of masking an underlying issue than fixing the problem.

The distributor was set up by Aldon to the spec of the engine is supplied.
The rolling road operator found excessive play the distributor cam so he set it up as best as he could and I returned the dizzy to Aldon. Since then it's been timed statically be me.

Aldon also thought it may help but thought it could be caused by something else and could not give any guarantee. I'm not sure what else it could be. The mixture was check on the RR, I've checked the cylinder compression, there's no pinking or smoke. I'm a bit lost.

TBH I'm not surprised you are a bit lost.

If the Dizzy had been sorted by Aldon, why did it have play in it? It certainly shouldn't have if it had been reconditioned, and if it had play why did Aldon modify it without pointing this out?

If I was you, I'd send it back to them and ask what happened.

I have been driving around in my 1330cc Mini-Marcos today, having
recently enabled a vacuum advance system.

What I can report is that I have just managed to better my previous
best tank range by at least 20 miles and that the car is much nicer
to drive at part throttle.

I am adding a maximum of 14 degrees of vacuum advance...

I can turn the vacuum on or off at the flick of a switch and it is quite
obvious to me that the vacuum is beneficial for this engine.

Conclusion: If a vacuum system can be easily installed, it's probably
worth a try...

Ian

Just re-reading through the posts here...

dklawson wrote:However, when you depress the gas pedal the vacuum
will drop and that advance will go away
... so it's not going to help with acceleration from one speed to another.

My experience here is that it will help... This is because the vacuum
advance is not "all-or-nothing", it varies dependent on the weight of your boot!

Consider this made up example:
At 5000 RPM an engine will take 36 degrees of advance at Wide Open Throttle.
At 5000 RPM at "three-quarters-throttle" the engine will take 40 degrees.
At 5000 RPM at "half-throttle" the engine will take 44 degrees.
At 5000 RPM at "one-quarter-throttle" the engine will take 50 degrees.

So, if you had no vacuum advance, at 1/4 throttle the engine would be
effectively running with 14 degrees retard compared to what it wants
ie (50 minus 36). This is because the base/centrifugal curve is always
set to allow Wide Open Throttle to be obtained without pinking.

If you can give the engine as much advance as it needs at each RPM for any
given amount of gas peddle, it should produce better results. It should
perform better, produce better economy and just generally feel better.

When I switch off the advance on my engines, they both become more
hesitant at cruise, almost feeling like the mixture is too lean.

Ministrone, the requirement for vacuum advance can be tested...

Strobe check your timing, at say 4000 RPM with the "Yellow".
Replace the "Yellow" with a distributor with a vacuum advance.
Strobe the Vac Dizzy to give the same advance at 4000 RPM.
Get a long piece of flexible vacuum pipe, long enough to reach
from the distributor to inside the car. Some windscreen washer
hose would probably do for this test.

Carefully drive the car up to 4000 RPM. The timing may be
completely wrong up to this point, so NO WIDE OPEN THROTTLE.
The timing will be OK at 4000 RPM as it should be the same as
"Yellow" was...

OK, here goes... You're now doing 4000 RPM at cruise...
Stick the pipe in your mouth and SUCK! Or stick the pipe in
someone else's mouth and tell them to suck!!!


Doing it this way enables you to have a definite off/on or
without/with scenario for the system.

Unless your vacuum can has a hole in it, you/they should feel
suction resistance when the device reaches its maximum. If it
does have a hole in it, it will be totally useless...

If the engine feels better, or accelerates, you have just
proven that your engine would benefit from a more permanent
arrangement. I am confident that this will be the case.

I am sure that in days gone by, you could get a "Yellow" with
a vacuum from Aldon? I am fairly sure that you could just
replace the back-plate with a vacuum type one anyway.

Obviously this assumes that "Yellow" is correct anyway,
but that's another matter...

Ian

Check the bob weights in the aldon dizzy, mine where stuck up out of the box. To do this remove the dizzy, take of the points etc and remove the baseplate so you can see the bob weights and springs. Stick the rotor arm back on, firmly hold the drive end and turn the rotor arm. You should see the weights swing out until they hit the advance stop pin. If they don't, get some sort of solvent and clean out all the old grease until they move freely. Once you've got em moving re grease the weights and springs and reassemble.

is the compression ratio correct for the camshaft?

While adding Vac advance may help, I feel that it will be masking an under-lying problem.

You should be able to get it to run perfect without it, then add it by all means.

Thanks for your advice chaps.
I've been playing with the timing over the weekend and it's much improved and pulls cleanly from 1000 revs. However, I still think it could be improved with a vac advance.
I think I'll leave it alone now and see how it feels on a good drive out, I hear Blyton is nice this time of year.