Tachometer and ignition

piggdekk · 11-09-2010, 12:52 PM

Hello
on my 911 71T we're trying to troubleshoot an ignition issue. At idle, and only when the car is hot, the idle from time to time bumps just for a second. As I was running out of ideas I took it to a workshop and they've swapped the CDI with a newly refurbished one, then they fixed some loose cables, and checked the dizzy (marelli, but working fine with timing spot on). It looks like the car still has the same problem (therefore now is back to the orignal CSI), and the mechanic suggested it could be induced by the tacho or by its wiring. Now we've disconnnected it and tomorrow I'll give it a try, but do You think it is possible that the tacho sends some strange signals (grounding) back to the CDI? As usual with electrical problems it is hard to tell, because they look quite random.
Thanks for Your help
luca

wwest · wwest's Garage

Engine idle actually bumps or does the tach indicate so without your actually hearing the engine idle up...?

Tach "bump" alone would usually indicate that the batteyr is being overcharged.

T77911S · 11-09-2010, 07:08 PM

carbs? one could be leaking.

i just pulled my dizzy and cleaned it, all the corrosion that is. the dizzy plate that holds the points has to make good contact with the dizzy body, and that has to make good contact with the engine. i first caught on to this whenn setting the timing. the engine would "bump" or drop out very quickly when i would touch the dizzy. my screws that hold the clamps where very rusty. i cleaned the dizzy body on the grinder with a wire wheel. but i think your problem is carbs.

piggdekk · 11-09-2010, 09:46 PM

Thanks for the answers.
The engine also idles up, it is not only the tach reading. In fact the idle bumps up by 2K rpm and than goes back as quickly as it went up. Say the whole cycle doesn't take more than 2 seconds to complete.
The fact that it is random makes me think that it is not a carb issue. Moreover, it looks really too fast to be a carb issue, but of course I may be wrong. Also, from time to time the engine misfires at higher RPM. It is only a single misfire, and the mechanic thought it could be due to the same cause.
T77911S, what do You exactly mean with carburaotrs leaking? the level in the carbs is set just fine, and we' re running at 4.8% CO. We'll be having a second look at the dizzy as well, thanks
luca

T77911S · 11-10-2010, 06:54 AM

i remember the carbs on my brothers T would drip fuel from the nozzle inside the carb. the one for the accelerator pump i think. it was a long time ago, but i think you could look in there and every now and then a drop of fuel would come out. sounds like the same thing your car is doing.

extra fuel would be the only thing i can think off that would increase rpms, except maybe timing. maybe the advance is messed up or if you have a vacuum retard, maybe it is messing up causing the timing to jumping around. i am leaning towards fuel.

piggdekk · 11-10-2010, 07:08 AM

well that would explain why it happens more when the engine is hot. Maybe the fuel tends to boil and drips into the carbs. Zenith's have a very long vent, so the boiling fuel has to go a long way before it exits. However I thought that extra fuel would lower RPM (too rich when it is already reach...), not increase the RPM.
Timing (mechanical) is ok and doens't jump around, but the car now and them misfires also at higher RPM, and that should be more ignition reltaed. I'm going to the workshop in one hour or so, and test the car. I'll let You know later, thanks
luca

piggdekk · 11-10-2010, 12:23 PM

Guess what? He was right!!!
He has disconnected the tacho from the coil and the idle bump is gone completely. I want to test the car further, but it looks to me like problem solved, or at least troubleshooted. When I'm sure that disconnecting the tacho solves the bump I'll need to figure out where the problem is. Could be the wiring, or the small box close to the CDI or the tacho itself. At least now my idle stays still.
luca

twin plugged targa · twin plugged targa's Garage

Hi Luca, I have exactly the same problem!
I am running Zenith carbs and Marelli distributor.
Car runs fine cool, but tacho does bounce when i decel down to 2K revs when it bounces, however my engine revs are not stable after this happens like 'Kangaroo petrol' as my 4yr old will say!
Once the car has sat parked for a while, I have a missfire like I am missing a cylinder or even 2 occaisionally.
It does not happen if I do not let the car sit warm or if I continue driving - not stopping.
I have put all new plug wires in, but this problem continues
I have just purchased a Bosch distributor to rule out the Marelli (plus there are fewer and fewer 008's around and so luckily got one from a kind person on this site)
I have not installed new distributor yet but will try the tacho detachment.
Which wire did you detach?
Many thanks
Ben

piggdekk · 11-28-2010, 05:48 AM

The wire from the coil, I believe the color is whitish, or light brown.
BTW I'm in Barcelona, we may not be that far away.
luca

twin plugged targa · twin plugged targa's Garage

Hi Luca, was in Port Vell, Barcelona this year on the yacht I 'skipper'.
I am based nr Nice, France.
Did you resolve all of the ignition issues with the unplugging of the tacho cable or is it unresolved?

thanks
Ben

piggdekk · 11-29-2010, 01:27 PM

Hi Ben,
yes, looks like ignition problems are all gone. I wish I could say the same about the carburators, which are still spitting now and then. But the idle is absolutely stable and I don't have any misfire. Haven't had the time to reconnect the cable to check again what it does, though.
luca

piggdekk · 11-29-2010, 01:35 PM

Hi Ben,
yes, looks like ignition problems are all gone. I wish I could say the same about the carburators, which are still spitting now and then. But the idle is absolutely stable and I don't have any misfire. Haven't had the time to reconnect the cable to check again what it does, though.
luca

twin plugged targa · twin plugged targa's Garage

In have disconnected my tacho cable and now await good weather to try the car as it is currently snowing in South of France!
I will let you know if this fixes the symptoms.
If it does we should keep each other posted to try and find a permanent solution.
Thanks
Ben

piggdekk · 11-30-2010, 02:08 PM

It could be everything! My next move will be connecting it again and see if I can figure out any issue with connections, grounds and so on. If wiring is fine then it gets tricky because it is either the tacho itself or the box in the middle, and it can be solved only by getting a proved working box. Not easy since I believe they've used it only for a couple of years. I'll keep You posted, anyway.