Backfire at 4900rpm

Turbo B · Post by **Turbo B** » Thu Mar 01, 2007 4:05 am

Not sure if this is the place for this but this is where I thought it fit best.

Megajolt has been running on my car for just about 4k miles. Within the last 200 miles the car has developed a misfire at around 4900rpms which produces a substantial backfire.

I have swapped ignition modules- still backfires
Run the car with MJLJ disconnected- still backfires
Checked the plug wires for leaking spark- none found
Cleaned ground wire connections- still backfires

So I know this problem is not the module or the MJLJ. Everytime I try to datalog this event the car runs perfectly.

The backfire occurs at the same RPM everytime it occurs. I get a single "bang" then it clears up and will pull smoothly to redline. RPM readout is steady, doesn't look like I have any electronic interference. Coil and pickup are all I have left to swap out. Any suggestions?

The car:
1969 MGB
bored, camed, ported head, N/A, stock HS4 carbs with reprofiled needles

Post by **brentp** » Fri Mar 02, 2007 11:43 pm

You've done some excellent troubleshooting work so far. One questions:

1. does it happen under the same load conditions? i.e. revving in neutral, or under load?

Possible things to investigate:

- the bracket holding your crank sensor might be harmonically vibrating at that RPM, causing the signal to be lost. I've seen this happen before.
- check for sensor alignment and gap
- Did your air/fuel ratio change for that RPM range? are you going lean? clogged fuel filter?

Report back,

Turbo B · Post by **Turbo B** » Sat Mar 03, 2007 4:22 am

I think I have solved the problem but I will know for sure after the weekend as the car is probably going to travel over a thousand miles.

My thinking initialy was that the engine was setting up a harmonic vibration at 4800rpm and causing the sensor to loose sight of the reluctor.
When I got to inspecting the sensor I found it to be more than 1mm away from the reluctor. So with a feeler gauge I set it to 0.2mm away from the reluctor. For now this seems to have cured the problem. I'll report back on Tuesday or Wednesday and let you know if all is still well.

Thanks

Post by **brentp** » Sat Mar 03, 2007 9:27 am

There we go! Every time I hear about problems in the 4000 RPM range I think about sensor vibration / bracket rigidity.

Another way to determine if your sensor bracket is rigid enough is to see if you have any teeth strikes on the sensor head. If you do, you are experiencing some motion in the bracket.

Turbo B · Post by **Turbo B** » Sun Mar 11, 2007 4:04 am

Well, I ended up covering somewhere just shy of 700 miles of winding mountain road over the weekend. The car ran nearly perfectly. I had a few ignition related stalls but when I needed the engine to make power it ran pefectly. I'll have to either make a new VRS bracket or reinforce my existing one as I now have a low end stumble when cold - I know that is ignition related as the tach quits when the stumble occurs.