TheSamba.com :: View topic - Dual Carbs and Cold Starts

Mon Nov 02, 2020 6:24 pm

I recently did a carb swap on one of my toys and what happened next is a bit of a head scratcher for me...

I have a 1915 engine that I had been running Italian Weber 40 IDF's. The car is driven on weekends only, and sometimes only once a month. Whenever I'd start it up after sitting it took a while to start and a couple of attempts to keep it running smoothly. After a minute it would clear itself out and run like a top.

For that initial start up and first minute, here's what I would do and experience:
1. Turn the ignition on, let the electric fuel pump fill the carb bowls (you can hear it pump to a point and then the pump changes pitch telling me the carbs are full)
2. Pump a few times, and turn the key. A quick initial start and then it promptly dies. A couple more pumps on the gas and repeat. Might take a couple more times. Then it fires up.
3. Engine sounds like my carbs are out of sync or the engine isn't firing on all 4. Then it smooths out and I hold the throttle at about 1300-1500 rpms.
4. For that next minute the carbs let off a little pop up through the tops (for those of you who've owned webers will know what these little pops sound like). Then after a few seconds the engine clears up and it purrs.

At this point I can take by foot off the throttle and the engine will come down and idle on its own, very smooth.

Now, something about me... I can't leave my sh!t alone, and recently installed a pair of IDA's on the engine. All of the above continued, and have actually gotten worse. When I get the engine fired it sounds like I have a plug wire off, for about 15-20 seconds, and then the pops up through the carbs start. And they're more frequent and last longer. Eventually the engine clears itself and like before with the IDF's I can left off the throttle and the engine will idle fine and rev fine.

When I begin to drive the car, the IDA's pop/cough a bit and it seems to take a little bit to drive normal. With the IDF's I pretty much could drive off without issue.

The temps around here have dropped, but were still in the 50's when I experienced this recently. So I don't think the weather is the issue.

So I've been trying to figure this out. I don't believe this is carb tuning as I've messed with webers for a couple of decades and can tune carbs. I originally thought the IDF's were just adverse to cold starts (all my previous builds ran IDA's or DCNF's; first experience with IDF's). But when the symptoms worsened with the IDA's, it caused me to think that there must be other things causing this.

About the engine -
As mentioned, its a 1915. It has a C45 cam (my first experience with Scat cams as well), ported 40x35.5 heads, and low compression (only 8 to 1). I recently upgraded from the old school Bosch 009 and Blue Coil to the CB Performance Magna Spark ignition, and there was no change to my start up procedure but the overall performance improved nicely. Plugs are NGK 5's (equivalent to OE Bosch 145's) set to stock specs (because I had been running the 009).

Its a relatively new build. Probably has about 1000 miles on it, at most.

Could my low compression build be causing poor starting characteristics?

This has been how the engine starts since day one. I'm thinking the IDA's have accentuated the problem. And I don't believe this is a lean jetting or tuning issue. When the engine is warmed up the thing rips. Runs smooth at cruising speeds and coming off speed, and idles like a champ.

The only thing I can think of is that my compression is the issue. Interested in peoples thoughts.

Why did I go with such low compression? It wasn't my original plan. The place that machined my case didn't face the cylinder face surface when they bored for the 94's. I have nearly .100" deck height, and didn't want to delay my build another few weeks to have it machined again. The math showed 7.96 to 1 compression and I decided to give it a shot. Kind of an experiment - I had heard that engines with high deck height are soggy engines (too much burn outside the chamber/squish area). The engine turned out to be a nice, peppy and cool running engine. I was going to tear it apart over this winter to machine the tops of the cylinders to net me .050" deck. We'll see...

Thanks all for any insights into what I'm experiencing with this engine.

Mon Nov 02, 2020 6:28 pm

Sounds like what to expect with no chokes when it's cold, baybysit it for a few minutes before driving, or try running it a minute and letting it sit a couple to let the heat soak in.

Mon Nov 02, 2020 6:30 pm

larger carbs with all else equal = lower air speed and weaker vacuum signal. both of these things contribute to the fuel not atomizing well and falling out of suspension on its way to the combustion chamber...popping and running rough is actually lean. The raw fuel is not burning and is it just flying out of the exhaust wasted.

the symptoms of the IDFs are completely normal for dual carbs in cold weather, but you just exacerbated the problems by sticking big carbs on an engine that doesn't need them. (the low compression sure doesn't help)

also, bumping up the compression to an appropriate level and tightening the deck height is going to make a big difference :D

Mon Nov 02, 2020 6:31 pm

Bigger float bowls.
There is just MORE stale fuel in the bowls to burn, before you get fresh fuel.

Turn off the fuel pump and let it run out of fuel when you park it.

Mon Nov 02, 2020 6:43 pm

sled wrote: larger carbs with all else equal = lower air speed and weaker vacuum signal. both of these things contribute to the fuel not atomizing well and falling out of suspension on its way to the combustion chamber...popping and running rough is actually lean. The raw fuel is not burning and is it just flying out of the exhaust wasted.

the symptoms of the IDFs are completely normal for dual carbs in cold weather, but you just exacerbated the problems by sticking big carbs on an engine that doesn't need them. (the low compression sure doesn't help)

also, bumping up the compression to an appropriate level and tightening the deck height is going to make a big difference :D

Thanks. I did up my idle jets before parking it this past weekend. As the engine had been run and warmed up I couldn't experience the cold start issues described. Will check this coming weekend.

But I'm thinking bumping the compression is in order.

Mon Nov 02, 2020 6:45 pm

modok wrote: Bigger float bowls.
There is just MORE stale fuel in the bowls to burn, before you get fresh fuel.

Turn off the fuel pump and let it run out of fuel when you park it.

Interesting thought...
I don't run ethanol fuel so I'd expect the fuel to not go stale that fast.

Mon Nov 02, 2020 10:12 pm

That't been a normal start routine for me since having dual carbs-way back in the 90's.

Always sounds like a cylinder is not firing, but hold the gas down to around 1500-2000 RPMs for a few seconds and things even out and calm down.

Don't try to adjust for cold starting with them, the carbs are just that...cold and get colder in the first few minutes due to the atomizing of the fuel.

Run it for about 10 minutes, shut it off and let the manifolds and carbs warm up from the head heat.

Tue Nov 03, 2020 4:45 am

What you described is perfectly normal behavior for any internal combustion engine ever produced that does not have a cold start device to richen the mixture. It has absolutely nothing to do with your compression ratio what so ever. In fact if you lived in a cold enough climate you would find that as it got colder your "pump - start - stall - pump - start - stall" cycle would continue until you ran out of battery. Quite simply, all engines need a richer than normal mixture to start cold, yes even at 70*F!

This is why every carburetor installed by every car manufacturer since time began has a choke. Yes the Ford Model T had one too!

There are 3 ways to approach this problem in my view. #1 put up with doing exactly what you are doing, #2 jet the idle circuit rich enough to mostly mask this problem, #3 install carbs with an appropriate cold start device.

#2 I believe is what the vast majority of guys running dual carbs do resulting in less than ideal fuel economy.

#3 is rarely done but yields the best results. Many years ago all Dells and Weber carbs had cold start devices which the 1/4 mile guys removed and disabled. Like sheep all the street guys did the same. Now-a-days this practice is so deeply engrained in the VW world that you cannot buy reproduction carbs with these circuits any more and original unmolested carbs are very hard to come by.

Anyone that denies the need for these devices uses method #1 or #2.

I was very lucky to find a set of original Webers with the cold start devices still intact, in excellent condition and they work a treat! My start-up procedure goes like this: Pump the gas peddle once (this step can be eliminated but it will crank just a couple more turns before it starts), pull the chokes on, without touching the gas peddle - start the car. It will start and immediately go to a fast idle, after 3 to 10 seconds (depending on how cold the day is) it will start to lope a bit so move the choke to the 1/2 open position and let it warm up. A couple of miles down the road the choke can be returned to the off position. With these devices there in no Olympic scale footwork required to start your car saving a lot of wear and tear on the throttle shafts and accelerator pumps! No popping back up the carbs either that can at times blow the seal on your air cleaner.

I have a 8.4/1 CR and run regular gas with 10% ethanol if that helps clear things up.

I hope this helps.

Tue Nov 03, 2020 5:09 am

When a motor is cold there is zero fuel atomization. You need a lot of fuel droplets in the combustion chamber so that a few of them will burn [ poorly ] and keep the motor going. Most of the fuel goes out the ex.

Once the motor is hot the droplets flash to vapor when they enter the hot combustion chamber. Vapor burns vigorously and little escapes. Motor runs great.

Tue Nov 03, 2020 5:13 am

^^^Yes!^^^

Tue Nov 03, 2020 5:14 am

wardvwracer wrote: I I was going to tear it apart over this winter to machine the tops of the cylinders to net me .050" deck. We'll see...

I'd love to hear the results of the change, more squish should help, but who knows :?

Tue Nov 03, 2020 7:36 am

oprn wrote: What you described is perfectly normal behavior for any internal combustion engine ever produced that does not have a cold start device to richen the mixture. It has absolutely nothing to do with your compression ratio what so ever.

I hope this helps.

It does, thank you.

Tue Nov 03, 2020 8:15 am

jet a tad rich on the idle...mains too will help run cooler.no not over rich ,just on the rich side or right. I never had any issues driving mine when cold other than my butt being cold and fingers too. never pumped the throttell, just got in and hit the key. after about 8~10 seconds drive off.( usualy the time it takes to put on seat belt& release the brake) do your IDA's have the extra progression holes?? what sparking bolts? I will only run platinums or irdiums, but platinums take more heat than irdums so thats what I like best.there is a beter higher heat plug out for turbo usage but Ive never used them and dont recall what they are called. you may need to replug it....and or add the progression holes...and or...do some tuning.

Tue Nov 03, 2020 8:24 am

bottom line...anything you try to do to richen the mixture for cold startup will result in the car running too rich when it warms up

Tue Nov 03, 2020 8:29 am

^^^Yes!^^^

Tue Nov 03, 2020 8:36 am

I run 20 degree of timing from 600 to 900 rpm then drop it down to 15 at 1000 where I want it to idle. MSD power grid lets me do this. Acts like a choke is on and never had a Vw start and idle better when cold. The Dyno tech at westech taught me this. Might cost you a new ignition but worth every penny.

Tue Nov 03, 2020 10:50 am

I tried that with my crank trigger system but the result was an unpredictable idle.

Tue Nov 03, 2020 11:18 am

I didn't think it would work, was always having to warm up the car for 10 min, or have the Idle so high once it warmed up it was idling at 1300 due to the push pull linkage. My car likes to be at 15 degree at idle, 32 total. The engine dosent care if it is at 15 or 30 at idle, it just needs to be at 15 when taking off with a load on it.

Look way back on my build thread and I was locked out at 32, once warmed up it pinged like a diesel off idle. John at Aircooled.net told me i had to have it down around 15 so It was streetable. Drag cars don' cruise around ad that how they get away with the locked out timing..

Tue Nov 03, 2020 11:27 am

The timing trick probably helps more with a plenum intake.
I do retard it to start and then run extra advance until it's warm, but it only helps a little bit.

The lighter fractions which evaporate easily will go first, so the the more the carbs get hot at shutdown, and the longer it sits, the heavier the fuel will be. just like simmering a stew. The longer you simmer it the thicker it gets.

Tho some engines are a lot more sensitive to that than others, and it hard to say why.

Does have to do with compression ratio, squish, wet flow of the intake manifold, how much vac it pulls during the intake stoke, and how well the the carbs atomize the fuel.

Why it's worse sitting 48 hours vs 12 I really don't know, but sure seems like it is. Maybe some kind of chemistry thing.

Tue Nov 03, 2020 6:38 pm

I don't let mine idle at all on a cold start up. I get in, tap the throttle a couple times, hold it partially open, and turn the key. As soon as it fires, I hold the RPM at 2000-2500 until the EGT's get to 1100 degrees F, and then away I go.

I don't get any pops through the carbs at all when I follow that routine, even on cold winter days.

I also use an electric oil pre-heater, which I'm sure helps some.

You might try opening your spark plug gaps to take better advantage of your ignition system. Hotter spark is good for lighting lean A/F ratios.