JowettTalk

Thanks David,
Yes the 40897a is a good match for the original distributor. BUT the reason I started all this was because of my experiences in Europe and in this country with pinking (pinging) and slight overheating sometimes . The distributor in the SA is a little worn but I have checked its advance curve on the distributor tester and it is not far out. Note I have a variable pitch mechanical fan and a thermostatic electric fan on the Jup with an aluminium rad so overheating is not actually a problem. But I also have temp gauges on each cylinder head, in the header tank and on the oil coming out of the oil pump so I know what is happening throughout the engine. Thus I notice very quickly if something changes.

I also have a temp gun that I aim at various places after giving the car a spirited run.

My contention is that the original curve does not give the best power or economy on today's petrol. It also may not be the best for fast motorway driving, let alone any competition work.

I do agree that fitting one of these Chinese or Indian dizzies would improve on a worn one but I also think there is an extra 10bhp or a few more lbs-ft available if we get this right across the rev range. 1 degree difference in the timing curve can make a vast difference to power.

In my last post I highlighted two distributors for the same engine but with one using normal octane the other low octane. I also highlighted two where one had been given a higher compression ratio.

Note that on my dyno test we were able to get another 3 deg of advance when we took the air filters off (presumably because the improved breathing (more air) and leaner mixture on full chat), and some more power and torque. After this I fitted hi-flow foam filters in place of the standard AC wire (yes they were clean!)for rallying and used a hyperbolic tube for racing. Whilst on the subject of breathing, do not forget that I found that the airbox on the Javelin caused a great loss of power over 70mph caused by the standing wave set up between the carbs through it. Solved by fitting paper air cleaners for each bank of cylinders inside the air box after removing the oil bath.

Points really are a pain, they soon go out of adjustment particularly when new, capacitors break down and wires break and screws fall out. My electronic Lumenition system has given no trouble and has been given no attention since it was fitted in the late 80s. It probably blew when I was trying to start the car with a spare battery. I suspect these Chinese cheap versions have a few missing components such as a Zener protection diode. Indeed in my discussion with Accuspark that is probably the case and an updated version is planned. Note the Lumenition was sited in a cool place whereas the Accuspark is inside the distributor.

There is ANOTHER reason I am doing this. Peter has had an engine rebuilt in Germany and it will need a distributor to match its performance. As yet I have no idea how it will perform; but we need to get the best out of it.

Been a buysy day , back
Couple comments above

So the question is what has changed since then? Petrol certainly, wear on engine and I must try to remember what I have done to it since then!

If u go back to my orginal post I think...
Compression will change timing... a so called "worn " engine is likely to have not wear as a factor but carbon deposits that increase compression..
Fuel type...not just octane... eg if one runs 105 octane LPG it has a very different curve to 105 octane petrol.
And the same applies to petrol, 2 petrols may have the saame octane but made up of different solvents/ hydrocarbons

Then throw another curly in...2 fuels may have the same octane but very diferent specific gravity....potentual energry per kg and volume...which then means the carb requires re jetting, both on power and cruise circuits... idle mixture screws usually only require minor adjustment.
So if the pump fuel has gone from leaded ( higher SG) to unleaded (lower sg) one is going to have to get bigger jets een with the same octane....THEN once the AFRs have been put back into ball park, the timing set up.
What ball park to start with?
I went over this further back..
OK get the AFRs right .. with out that ball park correct , anything one does is just a total waste of time.
Start with total (intial+cent) all in about 3K about 34/ 36 degs...best way is lock the dizzy with rubberbands...start at 36 , do 3 runs, above 3000 rpms.. retard 2 degs ... 3 runs retard 2 degs 3 runs and find the piont where the time suddenly increases.. torque/ hp drops off , about 2 to 3 degrees above this will be your total.
The total is what u work around for everything else...
The difference between the total and 40/ 42 degrees is what u want in the VA
Now set the dizzy still locked , no VA and find the lowest advance at your choosen idle rpms best stable idle...
This will be your idle advance (intial+VA)
So if u take your VA degs (40/42 minus your established total), off your established idle advance.. that will be your intial.
Then if u take your intial off the total (intial+cent) that is how many degrees u need to put in the cent.
NOW u can set tup the springs so they come off about 200 rpms above idle rpms... the concept here is that there is NO cent movenment at idle rpms....I have my LPG camaro coming off 125 above idle rpms.. my stable idle in drive is 425 rpms rpms...yeah thats low.. that is the nature of LPG as a fuel ok... the piont is no cent movement at idle rpms.
But more important is have the cent all in about 3000 rpms...which depends on car weight and rear end ratios...general rule of thumb tends to be a little over 3/4 ofd the open road rpms... 60 mph.
If there is detonation, usualy if going to will be at part throttle, 3/4 of the way up in the cent curve...increae the weight of one of the springs slightly
If any detonation at cruise, take a couple degrees out of the VA.
The VA .. needs to be all in at idle . general rule of thumb, depending on cam profiles etc start about 5 to 7" all in about 12".

you can find/establish the above specs, , with any dizzy, in good repair, AFRs correct, and HT leads etc in good working order....THEN take a spare dizzy and start playing, or send off to your favourite Dizzy man to config to those specs.

Little of subject.. there is mention of electonic ign....I assume u mean HEI.
If coverting a pionts to HEI use the Chev 4 pin module.. this is very robust reliable and highly underrated by chev guys, praised by the Dodge, Ford and the high rpms motor bike guys...also it has a function, if u leave the ign on it will cut the power to the coil for u... I run HEI in the Chev.. because by nature LPG is more ssensitive to spark quality.
But talk to any experianced long time Chev guy who knows hands on what they are talking about, they will stick with the stock pionts dizzy, unless hitting very high V8 rpms in the 7000 to 8000 rpm range... which basically is dedicated track car.
The 1st production of the chev 4 pin modules back in 72 to 74 where crap, unreliable, which even today they still get a bad rap by those not in the know...fortunately I have not seen one of these 'orginals' for at least 25years.
How reliable... well I replaced a dizzy a few months ago.. which I got 2nd had of a early 80s chev...since had it I wore out the shaft and bushes and rebuilt, replaced a cap.. still orginal rotor.. so basically this modulr has seen 2 dizzy life times...and still going....we are talking well over 175000 miles plus whatever the orginal car had done.
I have put the module to one side and when ghet time look at putting it into the Bradford some day.

Thanks Keith A..

As some may not know all the TLMs and abbreviations that you use I will try to summarize.

AFR or air to fuel ratio as delivered by a Zenith 30VM is hardly likely to be spot on for today's petrol or today's motorway driving. You might be able to re-jet say to a Jupiter spec or take it even further by trial on a rolling road or by laborious journeys up your favourite hill. Hence my use of the DelLorto but that was jetted many years ago and probably not very scientifically. So perhaps another visit to the rolling road is called for. BUT what is the point of doing that before I rebuild the engine?

That is not going to happen yet on the SA, but we might do it on Peter's car.

I think I can live with a bit of carbon, although modern petrol does burn clean. I can shove my endoscope down the spark plug hole to see if I need to decoke!

I think the 25 degree max advance found on the dyno in '96 probably does need retesting. But I am not convinced a road test will be accurate or reproducible enough. Finding a suitable road close to home which is safe and without traffic is not easy. What you need is a straight constant gradient hill about a mile long where you can hold full throttle the whole way and where the engine revs is always above 2500 and preferably constant. That means about a 1 in 8 hill otherwise you will have to change gear or break the speed limit. You also need a couple of cones to act as timing points. You enter the first timing point doing exactly 2500 rpm , then stamp on the throttle and reach the next timing point without changing gear or relaxing the throttle. You could use a Tapley meter or some electronics similar to that I had fitted to the GTB Subaru that calculated bhp, g and a whole host of other stats.

Are there any rolling roads available to the public in the UK? There are a few in Western Canada.

What free?
There is one a couple of hundred yards from me but it costs a lot per hour!
There are also mobile ones that come to car shows to entice the foolish to blow their engines up.

Free would be nice. I had my Corvette on rolling road Dynos twice out here, and it didn't seem to be too expensive. The advantage is that you are operating under comtroled conditions, and it is easier to make changes and see the results. I am aware of Clubs out here, renting a dyno for an evening and having several club members cars tested.

Rolling roads are not especially expensive - about £50 for a session and they asre great to establish just how well an engine is running. Quite a few will do Club days which are much cheaper per car and if you have several cars with a similar engine it makes it easy to establish a benchmark... I know of one in Fareham near Portsmouth who have just RR sessions for the MGCC and the Citroen BX Forum...

If timing on a road u dont need great distance... a steep hill 100 yrs or so in a lower gear, and a passenger with atop watch.. just hit that 1st spot at the same speed/ rpms each time... an ave of 3 runs per setting gets one damn near spot on
As to chassis dynos... 99% of shop dynos havnt been re calibrated in years, all they are good for is comparison tuning, actual readingsa will be inaccrurate.
And 99% of operators are either imcompentent or jusat bloody lazy... anything from have no idea about timing curve/ mixtures AFGRs etc, and /or too damn lazy to enter all the parameters into the computer (or dont know how) Ambiant tems, engine temps, Air pressure , car weight , air resistance, final gear ratio, altitude etc.
I have yet to have a dyno engine that hasnt needed minor changes in the real world of tarmac.
Dyno time is expensive, disling an engine in... u can right off a good 4 to 6 hours re jetting carbs, re setting dizzies etc.
For slightly less money one can pick up an INNovate data logger, O2 sensor, rpm sensor and jerry rig throttle /and or imap sensors... and still have the data logger in your tool kit, permently, for othewr projects or after a rebuild
Taking matter further, it is not uncommon for a serious chassis dyno shop to also run data loggers or pico scopes along side the dyno.
Basically having a data loger boils down to a chassis dyno in the real world on tarmac.

I bought one of these yesterday with points and an electronic conversion kit. I also bought a kit of parts to make up ignition leads, although normally they make them up for you to length and choice of terminator. Straight or right angle, spark and distributor terminators are available. I chose straight for spark and bent for diziie . I thought straight would not foul the head cover fitted to my heads.

I put the dizzie on the tester.
All following are dizzie degrees and should be doubled for crank degrees. Anti clockwise rotation.
Dwell consistent on all four lobes at 45deg.
Readings with no vacuum .
approx10 rpm - 0 deg
500 rpm 3 deg
1000 - 5
1500 - 11
2000 - 16
2500 - 18
3000 - 20
3500 - 22
>4000 max 24

Vacuum advance 5 deg at 0.2 bar

Remember from another post in this topic that this is a generic MGB dizzie.

As David Morris posted in this topic, the advance curve of his 40897A for the MGB starts very similar to the Jowett and this Accuspark one. Thus the Accuspark should be good up to 1800 rpm but after that there may be too much advance. The possible problem with the Accuspark is that it continues advancing after 1500 rpm when it is at the Jowett max advance of 22 deg at crank. This may be desirable on the modern Jowett engine.

Note the change from static to 500 rpm is 3 degrees. This might be testing machine error but I do not think so. It might be slackness in the weaker spring allowing the weight to advance at low rpm. Trevor experienced this on his worn old dizzie.
It does not seem correct to me that the advance continues up to 4000 rpm. As Trevor experienced , pinking occurs.
Thus we are planning to restrict this by half by welding onto the stop. This should give a max mechanical advance of 24 degrees at the crank.
However too much advance may come in too early as the weights and springs reach this currently at about 1800 rpm. This might be compensated by retarding idle advance to zero or perhaps -2 deg on the crank thus putting the curve back 4 degrees (3+1) on the dizzie.
This might make starting difficult. We would also have to file some of the weld off the stop to allow us to reach our desired max of 22 deg at the crank.
This is probably not going to be optimum so some adjustment of weights and springs will be required.
The saga will continue when I find time to make all the adjustments and road tests.

Once the dizzie has been set up with the points, the two minute conversion to electronic can take place.

Why do all this? The consistent dwell because there is no wear makes for good consistent spark timing.
I also have the programmable box of tricks that gets rid of the springs and weights so I can tap in any curve I want.

u should really dial the curve in rather than simply estimate the values you require as tuning specs....Forget any factory settings... our engines have been re bored, different compressions and the fuels are very different from the old British pool fuels...very different time from spark to max cyclinder pressure.
Simple way to do this is keep the all up initial +cent+VA under 42 degs when dialing in without a knock sensor.
Lock the counterweights with a good rubber band, fire up and estblish the lowest advance for a stable idle /mixture/required rpms.
say 500 rpms at 12 degs
now take the 5 deg (thats in the VA) off the 12 =7 deg and reset the engine/ mixtures to that and measure the engine vacuum... u will need that number later say 6"
Now set the locked counter weight dizzy to 36 degs... will have to change idle adjustments .....and do full running runs, preferably up a steep hill.. hitting the 1st mark at reasonable speed, and lower gear can help.
And then repeat at 2 deg increments till there is a power drop off....2 to 3 degs above this will be your total advance
Say 32 degs
these are non EGR engines so run manifold vacuum

Vacuum advance 5 deg at 0.2 bar

That should be a vaccuum not a presure??? so lets assume 5 deg at say 7"
So an idle of 500rpms@ 12 degs 12minus 5 in the VA = 7 degs
32 degs total minus 7 degs= 25 degs required in the cent advance and generally on modern fuels with good compressions all in around the 2800 top 3200 rpms...ball park is coincidentally usually around the rpms at 60 mph.
The idle is 500 so needs to start to come off at least 200 rpms above idle.
The VA needs to be all in at no more than 1" above the VA idling at the non VA idle noted above....6" which is good

So the dial in specs will go something like this from the above example
intial 7 degs... easy start low loads on armitures etc
idle (intial+VA ) 7 degs +5 degs= 12 degs
The cent of 25 degs coming off 200 rpms above idle rpms and all in at 32 degs at around the rpms at 60 mph giving a total (intial+cent) of 32 degs
And a all up(initial+cent+ VA) safe under the 42 margin ...7 degs+25 degs+5 degs =37 degs.
So your tuning specs will now be established and build into the dizzie by changing springs...stops on the cent...which is usually done by changing the shape of the counter weight tails .....and the VA deg range and range of vaccuum it works in by changing .... stops mounted to the VA mounting screws or extending the distance the arm at the rear works in.

Jump-starting the vehicle:
Simonbbc recommend using caution when jump-starting the vehicle fitted with our electronic kit. Connect the battery's + terminal to the other vehicle's battery + terminal. Connect the negative (black) part of the jumper cable to engine ground points, such as a bolt on the engine block, on both vehicles.
Do not turn the ignition switch to the ON position while the vehicle at the other end of the jumper cables is running. Charge your vehicle battery with the other vehicle or with a battery charger and then remove the jumper cables or charger before turning on the ignition switch and starting your vehicle. If the ignition switch is in the ON position while both vehicles are running, the electrical surge resulting from both vehicles' charging systems being connected together with jumper cables could be enough to destroy the ignition module and/or other any electronic components.

Having played about with and built my own electronic ignitions over about 50 years, I have given them up! For me, it's back to straight-forward points.

Put that into the context of the obsoleting of the points system for HEI by manufactures around the world
it was NOT an upgrade made in the early to mid 70s
We had the 1st world oil crisis, economy became an issue along with governments starting in on emissions targets... and the introduction of EGR...recycled exhaust gases which created a lean burn.
Lean mixtures require a far larger voltage/amps spark due to the resistance of the mixture between the electrodes of the spark plug... One can get the bigger spark with points, BUT it also means a far shorter lifetime of the points.
This shorter lifetime relates directly back to manufactures reputation as the the reliability of their product....so along came the HEI module.
Do not think a hei will improve performance over points...if the points system is in good repair...
the great falicy of "improvement"/ "improved performance" is due to in 99% of cases that the orginal points system was due for a rebuild replacement any way.