Really flippin' strange ignition and RPM anomalies!

The Original Tom · Post by **The Original Tom** » Thu Jul 05, 2007 9:04 pm

Had a good go at setting up the static timing and idle today. (for my ain problem, see the LPG section)
It's impossible to get an idle below 1400rpm without the engine stalling, on gas, or 1500 on petrol, with or without the vac.
If I measure the revs usin a 'Gunson Testune' borrowed from a neighbour, it shows 1400rpm idle, fine.
The rpm range can be switched between 4, 6, and 8 cyls, so I got that right. It increases the revs shown proportionally when I move the switch, so this works fine (e.g if it reads 1400rpm on the 8cyl setting, it reads 2800rpm on the 4 cyl setting). To be honest it sounds like a fast idle, but then it always has. But now the really odd bit - When I rev the engine to 5k, it sounds not that fast. When I revved the engine to what I frequently rev to (on sound) it was way over the max on the scale, which is 6k. It's probably doing 7k and doesn't sound like its struggling at all, and it revs there easily. I'm using a torquey grind cam (similar specs to the 'max-torque' one RPI sell), but other than that the engine's standard (but totally rebuilt).
I'm not running solid lifters or anything and it's not been ported in any way, and still has standard Rangey headers.
Now I'm no expert, but 7k shouldn't be possible on a standard engine should it, let alone easy?!

And another thing to add: When I was doing the timing, I held the strobe up to my eye at idle. It was slow enough that I coul see the individual flashes at idle. Occasionally it misses a flash (engien misses a spark) and sometimes it doulbe-flashes. This is about every 10 seconds.
I'm sure this is to do with my cap and rotor-arm so they're getting replaced tomorrow.
Could the double-sparking, if it increases proportionally to engine rpm, account for the extra rpm I seem to be reading, and could the double spark also account for the back and front-firing with the vac unit? It just seems strange that it doesn't still do it without the vac.

Sorry for the long post, but there's a lot to consider!!

Tom.

The Original Tom · Post by **The Original Tom** » Thu Jul 05, 2007 9:29 pm

Oh, and another thing to mention.
If you crunch the numbers for my drivetrain (5th gear = 0.8:1, transfer box = 1.41:1, diffs = 3.54:1, on 31" tyres)
Then 3000rpm in 5th gear gives you appx 69.7mph. However my old rpm gauge (before it broke) when set to 8 cyls, always showed 3000rpm when I was doing 55mph!

Wierd or what? Discuss...

The Original Tom · Post by **The Original Tom** » Thu Jul 05, 2007 10:19 pm

Had the same results with 3 rpm gauges. Mine before it died, the gunson testune, and a mates RPM gauge who I saw today.
Freaky s**t.

Now, I've been thinking outside the box (cue collective sigh Laughing )
Bear with me, this could well be crap (or really good stuff!)

When my engine was first built, I was advised, amongst other things, by many (including RPI) that exceeding 3000rpm wasn't a great idea within the first 500 miles.
So, Using my rpm gauge, set at 8 cyls, I increased my speed in 5th gear to 55mph, where the gauge read 3000rpm.
When flicking through Difflock.com a while later (months ago now) I found a thread where 'fender 90 gear ratios were being discussed. It was stated that at 60mph, the engine should be doing appx 2600rpm. I piped up "oh no it's not" and gave my figures. I then went away and crunched the numbers, and discovered the gauge was wrong. I then went back to uni and forgot about it, and the gauge was broken when I got back.

Now:
The mph at the SHOWN 3krpm, was appx 55. The ACTUAL mph at 3krpm is 69.7 (can't argue with cogs!)
( 100 / 55 ) * 69.7 = 126.73. e.g the shown speed was 26.73% more than was true.

This is where it gets really wierd.
If you play with the other numbers I'm getting:

1400 (idle) / 1.2673 = 1104. appx 1100rpm is a much more believable idle for an engine with a slightly more aggressive than standard cam grind.

5500 (suggested redline for std RV8) * 1.2673 = 6970rpm. Nearly 7k which is what it was shown to struggle at.

And again:
1/8 = 0.125. 1/4 = 0.25. Now, 0.25 (25%) is nearly 26.73%.
I am seeing double-sparks occasionally.
Is it not entirely possible that 2 of the contacts inside the dizzy cap don't make proper contact with the rotor, allowing the spark to arc across the gap? (all terminals look black and pitted, but a couple more than others). Because of the arcing, 2 sparks could be transmitted with one pass of the rotor.
This would account for the back-firing, the pitted terminals, the approximately 25% extra rpm that's being shown, the fact that my old coil conked 1k miles after fitting the new engine, and the unfeasably high idle and redlines.

Extreme coincidence or not, it should be cured by the new cap and rotor I'm getting tomorrow...

*Prepares to get laughed out of the forum*

Post by **katanaman** » Thu Jul 05, 2007 11:17 pm

cant knock your maths but your double spark in the cap is wrong. A double spark wouldn't change your rev readings unless it was double triggered as that's where the meter is sensed from. You could be getting double sparks but its the ignition module that's causing it and the cap is just a symptom. Only way to be sure what's going on is to put a a scope on your ignition and it will stick out like a sore thumb. haven't heard of the rover setup doing double sparks before but dropping sparks is common double wouldn't surprise me though.

The Original Tom · Post by **The Original Tom** » Thu Jul 05, 2007 11:34 pm

Ok...

The ignition module is an A&R power amp from RPI. I've done all the tests they list and the unit seems fine.
I've had the dizzy apart and the 8-pointed star under the rotor arm on the dizzy shaft, that triggers the ignition module, isn't an irregular shape and looks fine. (35DM8 dizzy)

Would it be better if I got someone else to try and time it for me, or better if I just bought a new dizzy?
I'd rather not spend £150 on a new one if I don't have to!

Post by **katanaman** » Thu Jul 05, 2007 11:45 pm

I have heard of them fail regularly but I don't have any experience with the AR unit so don't know if its true or not but it could also be the pickup sensor that's at fault. A lot of garages have scopes in their testing equipment. I would find one and get them to test it. You will then know what your dealing with for sure and you can then look at changing parts if needed.

r2d2hp · Post by **r2d2hp** » Fri Jul 06, 2007 6:43 am

Try taking the AR unit off and see if it makes any difference.

When I had my car setup on rolling road I had to ditch the Mallory stuff as it was somehow upsetting the ECU and it was over fueling.

If there was tracking in the dissy cap then I would think it could be possible to double count the pulses and get a false reading.

The Original Tom · Post by **The Original Tom** » Fri Jul 06, 2007 8:08 am

I can't remove the A&R unit because it's also the ignition amplifier, so the car won't fire at all without it.
The car is carbed so there's no way it can interfere with the ECU as it doesn't have one. Also even if it were efi, it has no bearing on anything but timing so it couldn't cause over-fuelling.

Cheers, Tom.

The Original Tom · Post by **The Original Tom** » Sat Jul 07, 2007 7:55 pm

Ok, my dad and I wasted a whole new day on it today.
We left the new vac unit in, but blanked it off and the vac pipe at the carb to stop air leaks, as it was even more tempremental when connected.
Job one - stand around and scratch our thick heads as to why it won't start.
Job two - dismantle the dizzy, strip the insides down, reposition the base-plate to put the magnetic trigger as close as possible to the sensor without touching. You can get a green rizzla in there but not much more

Job three - Trace back through the entire ignition system, checking for sparks and getting bitten many times by the HT

Job four - stand around and scratch heads (thick) some more, mutter under breath, swear a bit when it still won't start.
Job five - remove all (very charred-black) spark plugs, clean with white spirit and check all gaps, refit.
Job six - strip down a spare SU in the garage to discover how it should function. Re-tune carbs according to book. Will attempt to fire but won't actually start without about 50degs of advance and a can of 'power start', and then blow more soot than a steam-train.
Just like at the start of the day
Job seven - do this

Then we'd had enough. My dad was about to throw in the towel and I said: "Well, for sh*ts and giggles, lets put the old (siezed solid) vac unit back on, it used to work like that and that's where the problems started" He laughed at me, and went to feed the dog.
Job eight - change the vac unit for the old one again. (a true 5 minute job when you've done it as many times as me recently!)
Job nine - swear loudly in a mixture of surprise, indignation, joy and despair when it fires and runs like a dream on the first crank of the engine.
Flat spot at the bottom is still there because there's no vacuum, but it runs.

AAAAAAAAAARRRRRRRGGGGGGGGHHHHHHHH SHOOT ME NOW WHAT THE FUDGE IS WRONG WITH IT!!!

ChrisJC · Post by **ChrisJC** » Mon Jul 09, 2007 8:16 am

Whilst it's running, I suggest you get the carbs set up properly, idle mixture etc, so you know that the fuelling is spot-on.

Then you can come back to the distributor knowing that everything else is OK.

Have you got a complete spare ignition system to swap in?

Chris.

The Original Tom · Post by **The Original Tom** » Mon Jul 09, 2007 9:03 am

Yip, I've done the carbs and the gas system now. But I've also checked and re-checked the entire ignition system and the only thing that seems to cause any problems is the sodding vac unit.
Maybe I should take the plunge and let RPi charge me £42 for one. At least that way if it still doesn't work I can send it back to them and then be really confused!

sowen · Post by **sowen** » Mon Jul 09, 2007 5:58 pm

Have you considered going backwards and trying a points/condensor dizzy in there? A cheap 2nd hand unit should do you fine to see what it does, and would be easy to wire in.

If I were having trouble like you are, thats what I'd do. It would eliminate the entire electronic ignition system and let you start from scratch, and might save you some money aswell.

Simon

r2d2hp · Post by **r2d2hp** » Mon Jul 09, 2007 8:03 pm

If we go back to your first post you mentioned double counting sparks. This does sort of make sense and the revs seen on the gauges may not be proportional to actual engine speed.

If it is being cause by pickup of adjacent cyl spark within the cap then due to vac and advance this may only occur at certain revs or driving conditions.

Could it be the dissy is work, internals of cog.

Is the Rota Arm and Cap matched - think Ian had some issues with the wrong Rota Arm being supplied for the cap he was using.

Is the dissy aligned correctly, could it be that you have adjusted the AR unit to compensate for dissy setup.

I assume the gauges you are using are counting the sparks going to plugs, have you tried on all plugs and are the results the same.

Just some ideas

The Original Tom · Post by **The Original Tom** » Mon Jul 09, 2007 8:25 pm

I'll try and locate a points dizzy for few quid (or from someone local that I can just borrow) to try and find the problem.

Rotor arm and cap matched? Who knows. Both are new and seem to work fine. I inspected the inside of the cap after a few miles and the rotor is picking up some blue on the end from the plastic around the terminals, there is no evidence of arcing (early days I know) and it's not missing any sparks to no1 cylinder at least (tested by listening to the strobe at idle when each spark is audible and definable from the rest)
I'll re-test the sparks to the other cylinders and see if any of them are missing.

Dissy aligned? Yep. I've never adjusted the A&R so it's not taking up the slack. It provides an overall retardation of xdegrees so whatever it's set at it won't affect the timing differently at different RPM's.

And finally, the gauge I'm using measures the collapse of the LT circuit from the -ve of the coil. Therefore it takes every spark into account and divides the number seen by 8 to get engine RPM.

Anyone on here local that wouldn't mind lending me a working dizzy? I'll travel or even post it back to you and pay costs (obviously). I can transfer a deposit via paypal if you don't trust me

.

Post by **katanaman** » Mon Jul 09, 2007 10:57 pm

Points dizzy isn't so simple as they don't fit without cutting the shaft and fitting the later drive gear. This obviously cant be reversed so if you borrow one make sure they know they cant use it as is again. There may or may not also be issues with the coil you have on a points dizzy.