More on cold weather tuning and the GM HEI module

Road-race engines and ancillaries - general discussion
Post Reply
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA

More on cold weather tuning and the GM HEI module

Post by turbofiat » December 13th, 2011, 7:04 am

I wired in that 5 PIN GM HEI control module to my Bosch distributor on my Yugo and it works great. It backs the crank timing off by 5 degrees when the small pin is grounded through a pressure switch. It's a rather low tech approach but it works if you need a minimal amount of ignition retard in a low boost application. If there is any interest in this or you want to see photos and how to wire it up, I'll eleborate more about it.

After wiring in this GM module and determining the dual purpose Golf mk1/Rabbit vacuum advance was faulty, I'm able to run more advance and the car runs so much better now. It seems there are several factors involved. The main one being humidity, followed by ambient air temperature, RPMS, and ignition timing. In that order.

Seems this bucking issue crops up at it's the worse when it's raining at almost any temperature. I thought it was because the air was more humid and it was causing my plug wires to cross arc but otherwise the car idles fine, runs fine under cruise and accelerates fine under boost.

This bucking can occur while trying to accelerate off cruise in top gear (4th), sometimes in 3rd but not often under these following conditions:

1) RPMS are less than 3000
2) Humidity is high (rain). In this case if the temperature is above 60F/15C, the bucking can occur.
3) Temperature is <60F/15C
4) Boost guage shows engine vacuum is between 10" mercury and atmospheric pressure. Probably the point where the vacuum advance is not doing anything. Sorry I don't know how vacuum is measured outside the United States since we are still using the Imperial measurement system. It's roughly -5 lbs so I suppose that's something like .7 BAR maybe?

When this bucking occurs, my air/fuel meter goes from rich to full lean (no bars). I was told this was a missfire but couldn't this also indicate that fuel is not being drawn up through the jets properly?

Now if I advance the static timing from 10 BTDC to 15 BTDC the condition seems to goes away completely but the engine pings under boost so I can't have that. Could advancing the ignition timing/running more advance be a band aid solution to an issue with the carb?

This condition is probably occuring when making the transition from the idle circuit to the main circuit.

When the problem does not occur:

1) RPMS is above 3000 (any gear)
2) Temperature is above 60F/15C
3) Static timing is bumped from 10 degrees to 15 degrees

If the ambeint temperature is above 60F/15C and it's not raining, I cannot simulate this bucking condition under any condition. I've tried lugging the engine by pulling out in 1st and going directly to 4th to drop the RPMs down as low as 1000 and the engine accelerates smoothly. Weird.

One trick I found is if I am tooling along at 40 mph/65 KPH in 4th gear and want to accelerate, if the engine starts to buck simply downshift into 3rd gear and bump the RPMs to at least 3000 then accelerate. If I am doing 55 mph/88 KPH and I am in 4th gear, I'm turning more 3K rpms anyway so the problem doesn't occur.

It seems that the condition is somehow related to RPMs and timing advance. On the other hand, the high humidity might be messing with the fuel mixture which could mean I am running too lean a primary main jet.

Another weird thing is I drove this car home from work the other morning and it was 23F/-5C . BBRRR!!! But the engine seemed to run better than it did at 45F/7C in the pouring rain. I have also noted this condition can occur in the summer at 75F/24C but only if it's raining.

In comparison, before making my ignition improvements, at 45F, humid but no rain, the engine ran so poorly, the engine backfired several times. So just by sorting of my timing advance issues, the engine runs so much smoother. But the condition still exists to a certain extent.

I just don't get it. Why is rain playing a major factor here? Looks to me if I had some issue with leaking plug wires, the engine would missfire all the time. Plus my Magnecore plug wires I am using are only 6 years old. In comparison the Magnecors on my Fiat Spider are 13 years old.
124 Spider, Yugo,131

Posts: 459
Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: More on cold weather tuning and the GM HEI module

Post by WhizzMan » December 22nd, 2011, 7:57 am

I could be wrong, but this sounds plausible to me. At 100% humidity, your fuel will be competing with water for energy to vaporize. There will be lots of little water droplets in the air already, making the formation of a homogenous fuel mixture a lot harder. Wall wetting with fuel will not be the same if the wall of your inlet tract is already wet with water. Maybe that is what is happening? 15C sounds too warm for me to have ice formation inside your carb, but maybe I'm wrong about that?
Book #348

Post Reply

Who is online

Users browsing this forum: No registered users and 4 guests