High Load Missfire on Fiat T/C

[robert kenney]

Guy,
From early on the new engine I built for my sisters Spyder has had a small issue with low RPM, high relative throttle missing.

If you need a engine spec refresher I'll be happy to post.

Here are the symptoms in 3rd or 4th gear @ around 3400 rpm, and with high throttle settings like up hills the engine suffers from an miss fire (goes away with less pedal) . A down shift and the miss is gone and runs extremely strong. This is at sea level and did not improve after reaching 9000+ feet altitude. To me that kind of rules out detonation with the low effective compression. I had my sister do a hard main jet run (4-5 minutes 1/2 or more throttle @ 4000 + rpm) plug check and she got a good tan to off white reading at 9000 feet. Around town the plugs can look dark brown.

When the engine was first test driven I had 180 air correctors installed but it had a fairly pronounced transitional flat spot so I went up to 190 A/C's. After that the flat spot was much better but the miss surfaced. I don't know for sure if the miss was there before the AC change but I had not experienced it to that point. Could it still be lean? I figured that if it was a lean condition the increase in altitude would be an improvement. I believe the effectiveness of the air corrector jet may be diminished at higher density altitudes by imparting less emulsion motivation on the fuel thus a later effective emulsion creation.

Would appreciate having your professional opinion here.

Robert

[Guy Croft]

Hi Rob

misfire is very unlikely to be jetting. Do a comp test and if OK look for blocked idle jet (if you're on IDF - very common) or ignition pack/cabling breakdown. If the tacho flickers when the misfire occurs it's low tension ignition circuit failure, which the Marelli electronic amp unit is prone to after about 60,000miles (or in old age, 10 years plus).

Could just be a plug too I guess but they are a lot more reliable than they used to be. Worth mentioning that if the idle mixtures are out of true a plug can foul after prolonged low speed driving and only clear after several attempts at full throttle. Sometimes they never clean off.

Hope this helps in the 1st instance.

GC

[robert kenney]

Guy,
I leaked down the motor several times duging the break-in period and after say about 1200 miles and always less than 1% hot or cold.

The ignition is a covverted Marelli dual point distributer. Converted to Crane CDI box with a optical/shutter pick-up on the breaker plate and MSD high output coil. I verified the rotor/shutter phasing and it looked fine. I will recheck. I really don't like the mickey mouse shutter wheel mounting and may make a custom shutter to replace the points cam completely.

The plug wires are wire wound 10 mm (IIRC) and new.

I have gone through a couple of idle jet changes during tuning and not found any blockage.

Funny you bring up plugs. Some times the most simple item is over looked and the cause.

Thanks for the reply....Robert

[Guy Croft]

Thanks Rob

Check out that tacho behaviour I mentioned.

Idle jets, OK. I assume the pump jets are functioning perfectly - not dribbling or failing to inject a good laminar spray on throttle opening..

G

[robert kenney]

Guy,
I will check the tach function also.

Pump's work flawlessly.

Thanks Again

Robert

[robert kenney]

I wanted to update this post with my findings. I finaly got the Spder back to my house from the paint shop and had a chance to look it over. The tach function was good. I had upgraded the ignition with a Crane CDI and optical trigger, all plug wires and other ignition components were new with the new engine.

I suspected the trigger timing to the rotor/cap alignment was a possible cause. After removing the cap and viewing the post /rotor tip errosion it was clear that at the moment of spark that the rotor was before the post as the errosion was located on the leading edge of both. A larger spark gap and high cylinder pressures are a bad mix.

So after going through the rotor phasing regiment again and retiming the engine to 36 degrees total timing it seems to run much better, starts better, idles better and will take an instant full throttle snap of the pedal without even a wimper. No sign of missfire can be picked up. More driving will be required to know for sure but so far so good.

The car is back with its owner being reassembled. so I will wait and see what the feed back is.

Thanks for the help,
Robert

[hollypenos]

OK, I am the owner of Little Putt. My car is back from the painter and all the exterior parts are on...I think the the misfire problem is solved, but I have a heater core leak that needs fixing before I will know for sure. (Good thing the whole interior is out of the car.) I guess this is the best time to fix this problem. I am fortunate that the person who designed the upgrades for my car is going to be in my city at 8000FT in the middle of July. We will be able to adjust the Twin 40IDFs properly as well as the ignition misfire problem. I just need to install the interior so we have seats to sit in to test the car by then.
I will keep you posted.
Holly Kenney

[tmvolumex]

You misfire sounds familiar. I bought a Fiat 131and it had a misfire after 4,500 RPM. The previous owner of my 131 said he had tried, but could not locate the cause of the misfire. After buying the car I worked at eliminating the misfire. I changed the plug wires and plugs and that helped, but did not cure the misfire. I noticed that the previous owner had installed a Crane optical trigger ignition module and by looking at the optical shutter wheel, inside the distributor, I discovered the problem. The Crane optical shutter wheel is held onto the distributor cam buy a four sided, leaf spring arrangement. By using this design, the shutter is supposed to fit several different models of distributor cams. The problem is the design does not accurately center the shutter wheel, thus the wheel is not concentric with the distributor shaft. Because of the eccentricity of the shutter, the ignition timing is off on all cylinders except the one you used to time the engine (usually #1). I checked my ignition timing on #1 and it was correct but the timing on # 4 was off by 8 degrees. By touching the shutter wheel, the timing changed again. I could not position the shutter so that it was concentric and perpendicular to the distributor shaft. I removed the entire Crane optical trigger system, installed the original ignition point system and a capacitive discharge ignition. This change solved the misfire problem. The CD only runs a few milliamps through the points so they last 40,000 miles easily. The rubbing block wears out not the points themselves.
Tom

[robert kenney]

Tom,
Thanks for the hint.
I noticed that deficiency when I installed it and designed a replacement to be made from aluminum (flat black anodized) that would lock down with 2 set screws. I also saw that the shutter wheel is molded off center. Can't believe they could ship such a poor component. Maybe another reason thay are out of business (opinion not fact).

I have the old points if I need to use them. If I need to redo the ignition I am going to a MSD unit which I am brining with me when I visit. A bit bigger but I have used many in my career and have always been happy.

Robert

[robert kenney]

want to update all on this issue. I went up for a visit to my sisters home with the intent of debugging this issue along with helping here get other parts on the car.

After driving I wanted to do some final jetting (145 mains F9 ET and 190 air correctors. I believe the pilots are 50's at 9000 feet) and adjustments which all improved drivability much. Unfortunately the miss was still there(like I figured it would). Interesting thing is that it would pull hard right past 7500 in 2nd and 3rd causing me to question major ignition defects.

After some looking I noticed the radio noise suppressor condenser was still installed. That coupled with the CD electronic ignition may be causing primary voltage shunting to ground. I removed it and the miss was gone. A stupid mistake that at the time was rationalized as being necessary. Lesson learned- build a race prepped engine keep the niceties in the trash can.

I really can't believe this engine has the cam duration it does because it drives so well even driving through town at 2000. Still has plenty of torque to accelerate and will take full throttle at that RPM.

Thanks everyone for a great form and support..........

Robert Kenney

[Guy Croft]

After some looking I noticed the radio noise suppressor condenser was still installed. That coupled with the CD electronic ignition may be causing primary voltage shunting to ground. I removed it and the miss was gone. A stupid mistake that at the time was rationalized as being necessary. Lesson learned- build a race prepped engine keep the niceties in the trash can.

A very useful post Rob, thanks!

G

PS - thanks for the kind words about our forum!