Still chasing cold weather misfire

Road-race engines and ancillaries - general discussion
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turbofiat
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Still chasing cold weather misfire

Post by turbofiat » Sun Dec 25, 2011 1:18 am

I'm about to give up on this odd cold misfire that comes and goes and just deal with it until spring gets here. Who knows this phenomenon maybe related to blow through turbo setups which may not occur otherwise.

I've still got a list of things to check/replace that have been suggested. By that time I do all of them it will be spring and the problem will go away won't occur until next fall when the temperature drops. I do believe this misfiring and backfiring are related. I'm just not sure if the misfiring it related to the ignition system itself or I have a lean fuel mixture that is causing the engine to misfire. I'm now leaning toward the carb because I've just about checked/replaced everything on the ignition system and it seems the misfiring occurs when I am slowly coming off the idle circuit below 3K rpms.

Since advancing the static timing seems to eliminate the misfiring, I can only advance the timing so far until the engine pings under a heavy load/boost. I replaced the 100K mile distributor with one with 13K miles on it. Thinking the centrifical advance was not working properly because this happens below 3K rpms. On that particular day it was in the 60s (17C) and the engine ran smooth as if it was fuel injected. This problem comes and goes as the temperature swings. And humidity seems to play a big part as well. Such as if it's raining.

I checked the plugs and plug wires. The plugs looked OK and no cross acring in the dark and I even misted the plug wires with water and the engine idles fine. One thing I did think was odd was I was testing for spark with one of those in-line spark testers and it looked as if the light bulb was rather dim. I used another tool and still a dim light. As if the coil was producing not enough voltage.

I can't remember why but a few years ago I replaced the original Bosch coil with an Accel yellow coil. I noticed afterwards oil was leaking from the main lead since the coil is mounted horizontally. Perhaps most of the oil has leaked out causing a low voltage drop? I picked up an MSD Blaster 2 coil today and will take a video of the light using the spark tester and compare to see if there is any difference. I'm just thinking that perhaps the coil is producing a low voltage spark and under certain conditions can't fire the mixture properly. Also Accel makes terrible plug wires (at least the 7mm yellow wires).

Here's a few more questions.

1) Could advancing the static timing mask a problem with the carb?
2) if the main primary jet was too lean, would the engine run fine at 10C or above but below that the dense colder air creates a leaner mixture which causes drivabilty issues?
3) What size main jet would be typical for a 1500cc Fiat engine?

As of right now I have a 100 main primary jet and a 150 secondary jet in a 32/32 DFTA. So far I can't find any evidence that a 1500 Fiat engine should be using a 100 primary jet. I even found some information on this forum that suggests a 1500cc engine should have at least a 120 primary main jet.

4) Is jetting related to engine size or the size of the carb? This carb was originally jetted for a 55 BHP 1100cc engine. In stock form this 1500 engine produces 75 BHP. I haven't messed with the primary main circuit, just the secondary.

So the other thought could be I am riding on the brink of a lean primary main jet and the colder air is leaner out the fuel mixture as the temperature drops.
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parrish
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Re: Still chasing cold weather misfire

Post by parrish » Sun Dec 25, 2011 2:51 am

I know this is obvious and probably been looked at before but have you checked the distributor cap for cracks?
Cracks cannot always be seen easily, because of the relative cheap cost of a replacement i would fit a new cap.
The problem with these little gremlins is finding them, keep on looking im sure you will find it and when you do no doubt it will be something so simple you will kick yourself.
Regards
Steve
Parrish 366

turbofiat
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Re: Still chasing cold weather misfire

Post by turbofiat » Sun Dec 25, 2011 7:32 am

parrish wrote:I know this is obvious and probably been looked at before but have you checked the distributor cap for cracks?
Cracks cannot always be seen easily, because of the relative cheap cost of a replacement i would fit a new cap.
The problem with these little gremlins is finding them, keep on looking im sure you will find it and when you do no doubt it will be something so simple you will kick yourself.
Regards
Steve
Actually a guy I work with (but who is into V8 Chevrolets) said from what I described the cap is cracked. Mainly because the condition gets really worse when it's raining.

This is about the last thing on my "ignition" list to swap out. The only reason I have never suspected this part was faulty is because the misfire does not occur at all times. Just off cruise as the primary is opening. Looks like a cracked cap would cause the engine to misfire under various conditions especially during power mode.

This cap and rotor button has only 7,000 miles (11,0000 killometers) on it. Of course that doesn't mean it's good. I suppose if it were any actual Bosch distributor cap it could assume it is good. Most of the autopart chains here in the US get's their parts from Mexico.

I will get a try a new cap and see what happens.
124 Spider, Yugo,131

Guy Croft
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Re: Still chasing cold weather misfire

Post by Guy Croft » Thu Dec 29, 2011 2:23 pm

Is it on contact breakers or electronic ignition?

G

turbofiat
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Re: Still chasing cold weather misfire

Post by turbofiat » Fri Dec 30, 2011 6:06 pm

parrish wrote:I know this is obvious and probably been looked at before but have you checked the distributor cap for cracks?
Cracks cannot always be seen easily, because of the relative cheap cost of a replacement i would fit a new cap.
The problem with these little gremlins is finding them, keep on looking im sure you will find it and when you do no doubt it will be something so simple you will kick yourself.
Regards
Steve
Swapped out distributor cap and rotor button to no avail.
124 Spider, Yugo,131

turbofiat
Posts: 67
Joined: Thu Nov 30, 2006 10:09 pm
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Re: Still chasing cold weather misfire

Post by turbofiat » Fri Dec 30, 2011 6:10 pm

Guy Croft wrote:Is it on contact breakers or electronic ignition?

G
Electronic.

What I don't understand is how over advancing the static timing seems to correct the problem. Of course you can only advance the timing so far until the engine pings either under a heavy load or boost.

I've been wondering if advancing the timing is somehow masking a lean condition or it's increasing the temperature in the combustion chamber and counteracting the effects of cold weather.

For what it's worth, I'm checking the timing using the #1 plug wire. That should be the correct plug wire since the rotor points to #1 cylinder at top dead center?
124 Spider, Yugo,131

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