Undriveable under load

[rekka]

HI, I'm new to all this. First post first vehicle without a distributor.
I've built myself a 2276cc naturally aspirated VW motor (Empi HPMX carbs) for my bus. Megajolt with map installed. It is undriveable under load. I found a map that is close to a stock VW distributor as my starting point, but I cant get over 2000 RPM in second gear, without it missing, cutting out and a massive backfire through exhaust. Ive tried a few different maps, with plus or minus 5 degrees at that point in the grid, but no difference, on a gente downhill run it passes through that point and pulls well. I just happen to live at the top of a 15km hill. I have checked my voltages and grounds and that all checks out. Coil is new from Dubshop.net. I'm confident my static timing is OK. I've run out of ideas, but this is all new to me so I might be missing something obvious. Any ideas greatly appreciated.

The point it goes bad, is 2000RPM, 90Kpa and anywhere from 15 to 25 degrees advance. NO change.

[rekka]

I unplugged the megajolt and it runs much smoother on edis alone. So I made a map with all points at 10 degrees (limp mode map) hooked the megajolt back up and it runs much rougher with lots of misfiring. So it has to be megajolt or wiring. Correct?

[brentp]

Thanks for the note.

One thing to check is by using a timing light on the crank to see what the actual timing is under varying conditions - EDIS only, or with Megajolt plugged in. Are you able to perform this test with an actual timing light? You should be able to get some useful information just running the car through the rev range while observing the timing.

Thanks, will watch for updates.

[rekka]

Thanks BrentP. I made some progress this morning and made a really conservative map with the same advance for all load conditions. Dramatic improvement. Not massive performance but smooth, and no backfiring. Definitely have a usable starting point now. I'll double check with my timing light to verify that my indicated advance matches the actual, before I start tweaking the map.

[brentp]

Great, keep us posted!

[rekka]

Well it turns out I have two problems at the same time. A bad coil (brand new aftermarket) and it only goes wrong once the engine is warm and too small main jets in my carbs. Switched out the coil for a ford one one and much better. Still got to deal with the carbs.

[brentp]

Good detective work. Thanks for the update!

[rekka]

It turns out my mechanic buddy's diagnosis is bad too. I switched out the coil for a brand new one, new plug wires too, changed my jetting slightly (richer mains, leaner idles) and it hasn't got any better. I've added advance, taken it away and I cant get rid of the problem. It happens at about 2500 rpm now. Once again, it only happens once the engine is up to temp. When cold it is fine. My nearest rolling road is 90 minutes away and they don't deal with carbs, I don't know where to go from here. If anyone has a map for a torque orientated 2276 with 40 IDF's id be very grateful.
The only good thing to come out of this is that SOHC ford focus plug wires are the right length for a VW type 1 motor.

[brentp]

Hm.

One other issue to consider is if you observe missing or hesitation at a specific RPM range, then the bracket for the VR sensor might be vibrating due to a harmonic resonance and could be affecting the signal to the EDIS module, causing it to lose sync with the trigger wheel.

Are you seeing this problem with the Megajolt unplugged?

What does the timing light on the crank show when you rev it to that RPM range?

[rekka]

I just tried it with the megajolt unplugged and it still happened, but I had to push it harder before it did. The bracket is one I bought from mario at the dubshop. It is very short and there isnt a lot of room for vibration but ill double check. It only happens under load, not with the vehicle stationary so i cant check it with a light. Im starting to think fuel again.

[brentp]

keep us posted. As before, if you can verify the ignition timing is stable when the engine is stumbling, then that could be a further indicator it's fueling related.

[rekka]

Just a two year follow up to say we found the culprit. (I've been running with distributor for a long time now) so I was happy with the fuel side so it was time to have another crack.
The issue was too much crank float which caused the sensor to touch the trigger wheel at just a certain rpm and load combination. Once the flywheel shims were corrected the interference stopped.

Now to dial in the map. My local dyno said 6-8 hours dyno time to create an ignition map. (Still running carbs so no fuel tuning going to happen). Does that seem excessive or is that about right?

[brentp]

Glad you got it figured out.

If they're doing a comprehensive tune, then that sounds pretty correct.

[rekka]

Bringing this post back from the dead. After 5000Km last year with no issues at all, the problem has returned, on the first two drives of the year. I managed to get some runtime data while it occurred this time, two instances on one session.
I've attached a screen shot. The red RPM trace flatlines for a few seconds, the ignition cuts out followed by a loud backfire once it sparks again. The white load trace goes crazy, but that is just my foot blipping the gas to see if its kicked back in. It seems to be worse when the engine is hotter but happens at the same points on the hill leading to my house. The steepest parts. I also noticed my tachometer quits. I haven't seen the tach kick back in as I'm trying to drive, but it isn't at the same time the ignition starts working.

Is this a symptom of a bad sensor wire or connection?

[brentp]

This could be a symptom of the sensor losing sync with the trigger wheel. Make sure the distance is correct (~1mm) and the bracket is very rigid to reduce risk of harmonic vibration. Also, check all wiring.

If you unplug the megajolt and run the engine at the default 10 degree BTDC, you can see if the problem persists. If so, then you'll know it's around the EDIS installation.

Hope this helps; let us know what you find out.