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webinar #205, cold start,

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hi guys,

what i'm experiencing is what sounds like knock on a hot restart but only if it sits for a minute or so, i don't get this if i restart immediately only it sits before restart like after fuelling up or going into a shop. could this be over fueling engine does have 220psi static compression if that helps

You may find that the timing is too advanced during cranking which can result in the engine kicking back. Alternatively if the ECU is getting a poor or erratic signal from the ref and sync sensors at cranking speed this can result in an errant spark occurring at the wrong time. I'd make sure first that the ECU is properly syncing during hot restart. Beyond this try retarding the timing during cranking a little. 5-10 deg BTDC should be sufficient for most applications.

Question on an unusual arrangement. Running an EightStack injection system on a Ford Big Block (FE). The injector bodies are modeled after a Weber 48IDA carburetors, the injectors themselves are mounted inside the float chambers and spray on top of (not below) the throttle plates. Probably not the most efficient or best performance but sacrifices are made for realism or 'period correct' appearance.

For the most part, the system runs fine. The issue is cold start. With the injector prime pulse, it sprays on the backside of the throttle plates and then leaks into the runner area. With warm startup, it works ok, there's enough vacuum to suck the fumes in and fire. But with a very cold start 35F or lower, the fuel doesn't vaporize as well and it becomes difficult to start.

A work around is to hold the throttle plates open to let the fuel spray/drip/pass into the runner system. But the amount of open is tricky and the minute the engine fires, you are then sitting there with a throttle held open to a degree you wish it wasn't

Looking for any suggestions or ideas on how to deal with cold start when the injectors are above the throttle plates.



Hi Paul, I tune a jet sprint boat that uses ITB's with the injectors mounted above the throttles. We get reasonable cold start performance by using a prime pulse based off throttle position. The ECU (m800) will provide a prime pulse above 80% TPS when the engine isn't running. Two of these coupled with slightly cracking the throttle gets the engine running pretty quickly but it's obviously never going to give the results of conventional injector placement.


not sure if an old post will kick to the top or not, but I'll try.

We've done another car with a similar setup and back to fighting the cold-start issues. I understand the comment about tyeing a prime pulse to a throttle position but curious how that works with temperature. Do you just have to remember to not 'push the pedal' if the temperature is above XXdegrees or is the ECU smart enough to sense temperature and adjust the squirt accordingly?

I am digging thru the forum posts now as I recall you mentioned a particular ECU that had a 'priming pulse' tied to throttle position. My ECU maker has provided that function for me but they can't offer any suggestions on how to use it.

Looking for any ideas or links to 'the other brand's' user manual to gain some knowledge on this.



Is the issue how you determine the proper amount of priming pulse? If so I suggest you start with about 2x the normal cold idle pulsewidth.

You will need to keep good notes, as you can only do a few cold starts per day at best. I would try as early in the morning as possible. Note the air temp, coolant temp, pulse width used, and behavior from doing one pulse then cranking. Make sure the battery is fully charged so cranking speed isn't the issue.

I suggest press the pedal to the floor to get one pulse into each intake. Then with the pedal at idle, crank the engine. If it does not start (or cough) in a few cranks, try adding 25% to your priming pulse. I wouldn't hesitate to do that a few times. If you are sure you are getting spark, then check that the plugs aren't fouled, if so clean or replace before trying again.

If you get a cough, but it won't stay running and more priming pulse doesn't help, perhaps you just need to open the throttle a bit more, and/or adjust coolant enrichment.

Good Luck, Cold start can take forever to tune sometimes, but when you get in the right zone it seems like magic.

Thanks for your comments, David. The basis behind the question is twofold

1) how much, which you have provided very helpful input


2) The question then arises, if your starting sequence at 20F is to press the pedal once, then turn the key at 70F or with a warm engine, it is likely you won't need the pedal press. Any suggestions on how to learn/understand at what point will the pedal press be required and when should you not do it

Maybe overthinking this but this cold start stuff is not easy to do


I don't know the answer, but that's where taking good notes and coming up with a process will work. If your ECU let's you have a table that controls the pulse width with temperature (Motec M800 would do that), then you could have it be automatic. Always one or two throttle presses to get the right amount of priming fuel into the intake. Or maybe the prime is the same at all temps (just get plenty of fuel in there while the throttle blade is open), but the coolant temp compensation is all that needs to be tuned for different starting temps. Make changes -- there are only three results -- better, worse or the same -- take good notes and undo anything that is not "better".

I worked with a Bonneville Streamliner running CNG (Compressed Natural Gas). Sometimes it would fire right up, and other times it was 20 minutes of cranking and throttle teasing to get it going. Turned out that the CNG injectors would leak when the system was off, and the fuel (which was a vapor) would flood the intake manifold and that was the reason it wouldn't start until we could clear the manifold (turbo charged too). So the solution was about turning the car off -- we cut the "fuel pumps" (actually solenoids that controlled flow to the regulator), and let the engine run out of fuel to stop it. On startup, you just cranked, then flipped the fuel on and it fired everytime.

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