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DCO/DCOE Carburetor and choke / venturi sizing on large displacement engines

T

TurboDutch

Active member
Joined
Aug 5, 2019
Location
Nieuw en Sint Joosland
Starting a new thread because I am experiencing difficulties finding anything useful about the subject.

As you might probably know I am busy with a 3 liter stroker redblock.
I went down the Weber road coz I had some 48's wondering around in my barn.

I build the engine and tossed them 48 dcoe's on and the struggle began.
I tried like a million things but didn't manage to get the darn thing properly running.

Main issues where, proper idle adjustment seemed impossible, carb was hardly reacting on any adjustments of the idle mixture screws.
Especially cyl 1 was having a hard time. Also seemed to misfire, working only a small amount of the time.
Spark plug was black as tar.
Then, I can't say what issue was worse, it more or less drove quite ok. But cruising was impossible, AFR gauge was always showing 10's.
So the liquid gasoline was just coming from the exhaust, sometimes it even just stopped running because of the rich mixture, immediately ruining the spark plugs.

I tried a lot of things, smaller idle jets made it hesitate during slight accelerations and didn't help shit with the rich cruising.
At a certain moment it seemed to more or less run right, but idle was shit, that was the moment when cyl1 idle jet was blocked.
So I kept f*cking about without much luck, even tried to replace the cam by a milder one, for better vacuum signal, but for sure that didnt improve things as well. Might have even possible worsened it.

Then I have been reading a lot about them webers and sizing of chokes etc, but none of the graphs even go to 750cc single cylinder size.
I can obviously extrapolate a bit and I ended up with 42mm chokes since this is the max for the 48dcoe's.
It seems run right in higher revs with 180's main and 200's corrector. With F7 tubes.

Then, I got it running I mean more or less driveable, so I went to this guy with a dyno. He is almost 70 years old and been adjusting carbs for his whole life.
We saw 160whp without corrections so say closed to 200crank hp. But Cyl 1 was giving difficulties, sometimes completely misfiring.
Then even with the smallest idle jets 40f2 it was still running darn rich during cruising.
The dyno dude told me that cyl1 spark plug was oily and that the ignition parts should be replaced.
So I went home, after spending a huge amount of money, but still with the same issues. Yes it was idling a bit better but that was it.
Obviously replaced the ignition parts, cables, leads, distributor cap and rotor.
Using stock B230E distributor ign.

So I decided to take the head of and see if he was right, and to toss the carbs in the river.
The engine looked fine, just the usual slight oil traces around the exhaust guides.
So I have had the had shaved more, like total abt 4mm and did more porting, since I didn't like the 160whp.
Only the intake valve of cyl1 was just soaking wet with fuel. Checked the carbs and could easily see pas the throttle plates.
Cyl 1 was the worst! So that was probably causing the idle issues?

Sorry for the endless story, so I decided to order new carbs, CFline OY sells FAJS which are checked before shipping for like 300 euro's a piece.
Decided to go with 48's, then 55's then changed again to 50's and the head is still spinning.
But eventually they send me 50's. They come with 46mm chokes standard.
So guessing, larger chokes and larger aux venturi would probably hopefully solve the extremely rich cruising?
Then the idle issue would probably be solved with new carbs and properly closing throttle plates?!

I am back at sea now, and overthinking the thing, also looking for evidence that the engine will work with 46mm chokes.
But all the info I could find is about 40's or max 45 dcoe's on "small" engines like 2 to 2.3 liters.
If I extrapolate the weber graphs for choke sizes I get somewhere around 45mm chokes, and when I use online calculator for jetting and carb size I keep getting between 44 and 47mm size chokes. https://www.carbparts.eu/calculator

For complete info, some specs;
- 3 liter bottom end
- 46/38 valves 531 home ported head
- 13.1mm lift cam, 275 degrees duration at 1.25mm lift (kyrk ky126)
- 2nd cam 280 / 12.1 advertised
- compression 11.9 to 1
- chambers 35cc and 25 dished pistons
- facet carb pump and King filter and press regulator. Running just above 3 psi

Who can shed some light on the evil carb thoughts in my head?
 
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Nice build you got there, good to see a running stroker build here. At what RPM do you have the rich-cruise issue? And does it clear up once you cruise at a higher RPM, let's say past 3k-ish RPM? Also curious about the dyno chart of that ~160whp run, to see the shape of the torque/power curve.

I've posted earlier on about this, and i'm not posting this to say "i told you so"....
But i told you so. :roll:

You've overcammed the engine and the additional displacement won't save it from the negative effects at idle/cruise. Especially with those carbs. I don't want to be the salty prick here but you've got yourself a tricky combo and it will be difficult to get some good low-rpm drivability out of it. I'm definitely not a carb-expert, not even a facebook expert i'll say. But i do know a bit about how carbs and engine operate and I've seen some first-hand experience with a comparable setup that had the same issues. 275 degrees at 1.25mm lift is serious stuff on carbs. I'll try to explain what i think is happening here.

The first reason why these setups are hard to tune is the lack of vacuum/intake pulse strength variation. With this much cam, the engine won't pull that much vacuum at idle and cruise. And with carbs, the mixture is partially regulated by the amount of vacuum. Having only a small change in vacuum between idle/cruise and wot means you've got a very small tuning/operating window. Now with DCOE carbs you won't really have a steady vacuum due to the lack of a plenum, it's more about the intake pulse strength. But it's the same effect really, for the sake of simplicity I'll keep it at steady kPa (vacuum) numbers for now.

As an example, a stock cam gives about 40kpa idle, 60kpa cruise and 100kpa wot. (NA) This makes for a 20kpa operating difference between idle and cruise, and an additional 40kpa between cruise and wot. The carb is experiencing a broad 60kpa operating range and will be easy to tune. With a large cam things are a bit different. It may idle at 70kpa, cruise at 80kpa and have 100kpa at wot. This makes for a smaller 10kpa operating window between idle and cruise, and only 20kpa between cruise and wot. The carb still has to make the same-ish mixture adjustments but now in a far smaller operating range. This makes it harder to tune.

The second reason is intake reversion which is a bit of bigger issue since it isn't really fixable with a fiddly finger-perfect tune-up. These long cam durations don't behave that well at low-rpm/low-load. The intake valve simply opens too soon for optimal low-rpm operation. It opens at a point where the exhaust cycle isn't quite finished yet, which means the cylinder pressure is still higher than the intake-manifold pressure at this point. This will cause a brief reversion of flow from the cylinder into the intake manifold and carbs. Once the cylinder pressure starts to drop below the intake manifold pressure, the engine will start to suck air again through the intake and carbs.
reversionillustration.jpg
Intake reversion in itself isn't that big of a problem, but on most carbs it's nasty because it messes with the mixture as you've noticed. The reversed flow pushes air out of the intake manifold/runners. The air that was sitting in the runners and is pushed out now has already been through the carb once and is mixed up with fuel. Once the engine starts sucking air, the same air that was pushed out is pulled through the carb again and thus will be mixed up with fuel for a second time. This double-mixed air will be pig rich as it enters the engine. This double-mixing causes the afr to (slowly) drop at low-rpm cruise until it reaches an excessively rich mixture. This can only be fixed by increasing the load and/or RPM to stop this intake reversion.

There are two practical ways to fix this i think. The easiest solution would be to swap in a milder cam and go for area under the curve / driveability instead of peak power numbers. Grab a V, K or maybe even a H cam and your cruise-afr will most likely be far more stable and easier to tune. The compression ratio is a bit on the high side for a V cam probably, but the higher knock resistance of LPG could save you here.

Another fix would be to ditch the carburettors and go for sequential fuel injection. This way you can control the amount of fuel that the engine gets regardless of flow direction and you can time the injection pulse such that it starts injecting after the intake reversion which greatly improves afr stability at low-rpm cruise.
 
Morning mate,

Thanks a lot for taking the time to make an extensive reply first of all.

So as I tried to explain in my initial message, after struggling a while, I also thought the issues where caused by the cam.
So I had the cam replaced by another, much milder cam from Tinus tuning, it was comparable to the FS280 folkrace cam.
Specs are abt 280 adv duration and 12.1 lift. So it's just a tad away from the H cam.
But also that didn't change anything, maybe even made it worse.
Then I do have a vacuum gauge in the car and it showed good idle vacuum with both cams. Not much different than with A cam.

The good vacuum is probably caused by the long stroke/large displacement, so my conclusion was that this was causing the rich cruising.
Then, about the cruising I also found this, about the aux venturi " Aux venturi affects mainly higher RPM and set when main phase will start. If you need to make transition phase leaner and start main circuit later, put bigger size."
So my professional guestimation was, because of the stock aux venturi's (4,5) and the needed ones acc to the calculator (6) this probably is causing the rich cruising coz it's taking fuel already from the mains to early. But ofcouse, you won't find any size 6 aux venturi's. The largest ones to get is size 5 and they come in the DCO/SP 48, 50, 55 only. So that's how I came up with ordering them. And keeping some hope it would work.
So as you might be able to read from my story, I am actually thinking opposite, I am thinking that chokes/carbs are to small, causing
" too much vacuum " making it very hard to adjust, making the cruising and idle very rich, and making the slightest adjustment of jet a huge difference. At the dyno we made small changes to main jet and it had a huge impact in afr. Don't forget a 750cc single cylinder size takes a huge amount of air. But of course, it's just a guess/gut feeling.

So after replacing the cam for a milder one and the the failed dyno session. I took the head of and the carbs and started finding other reasons then only the sharp cam.

I had a conversation with a carbie geek and he told me to check the throttle plates, that if they weren't closing properly you'd be taking already gas from the mains. So that was probably the case, causing the issues with the idling. The throttle plates of the no.4 and 2 cyl where closing the best, they where also possible to adjust at idle. No1 was leaking the most and that was the one going to rich at idle. Also no.3 cyl was not doing that while, and also there you could quite easily see past the plates.

Your question about the cruising, I didn't use the car that much, so cant say for sure, but have been trying to cruise until somewhere 3000 rpm. But it kept getting too rich.

For the LPG part, didn't even got that properly running, idle was easy, but anything away from idle it wouldn't work.

So, I mean, probably your right and all, but I will give it a try anyway with the new carbs.
If that doesn't lead to anything drivable, I still have DBilas TB's wondering around with standalone management and LPG injection. So that might be a solution as well, but I am quite determined to get the darn carbs to work to be honest.

Then the question remains, will I get away with 46mm chokes?

edit 1: I don't have the dyno figures with me, I was too embarrassed to take a picture of it. But it made good low down torque and HP reduced just before 6000rpm, so what you would expect from a H ish cam.

edit2: I even went so far, that I thought I should close one of the progression holes, and I did, this was improving the cruise rich condition a lot. But I didn't look into that any further any more and opened the whole again because I thought the float level was the problem and carb was overflowing. But this was also not the case. Obviously you try a lot of different things when your trouble shooting. Which still leaves us in the dark a bit.
Dyno guy even told me I should install 40mm dcoe's. Which I highly doubt.
 
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@TurboDutch I agree with your carb sizing, 40s would be too small for your displacement. DCOE 50s or 55s may even be the correct choice.

I’ve used 43-48mm carburetors on 450-550cc single motorcycles.

Set them all up correctly. They really need to have the throttle plates all adjusted the same and be balanced with airflow before any real tuning takes place.
 
woodrowstar said:
I am a confused side-draft carb guy. The dyno guy clearly tells you to go smaller, but you go bigger? I would tell you the same even after seeing the throttle plate gaps. Bigger is not always the answer. Where can I see photos of this setup?
Click to expand...

3.0litre stroker crank set, k-jet fueled?

Morning gents, Found the 3L stroker crank set in the NL. Was thinking to use it in my pistonslappy B230E, so it is using K-jet. Using LPG as main fueling, looking into getting larger size venturi mixer but that shouldnt be to difficult. But actual question will be will my K-jet dig this shit...
turbobricks.com turbobricks.com

Above link shows the build topic.
I am not like completely new into tuning, only I come from EFI + Turbo, ( I am doing the software of the half the volvo 850 forum in the NL) only carbs are quite new for me, as well as N/A tuning. So looking for proper advise here.
I just filter out what seems right, if you run the weber calculators you also get up to 55mm dco.
So in this case I am not listening to the 40dcoe advise, the rest of his advises I took, I replaced all ignition parts and took of the head to see for an oil leak. Even though I was convinced there wasn't any oil leak, and I was right. Then same about the ignition parts, since replacing them didn't help as well.
culberro said:
@TurboDutch I agree with your carb sizing, 40s would be too small for your displacement. DCOE 50s or 55s may even be the correct choice.

I’ve used 43-48mm carburetors on 450-550cc single motorcycles.

Set them all up correctly. They really need to have the throttle plates all adjusted the same and be balanced with airflow before any real tuning takes place.
Click to expand...

Very difficult (for me) since I hardly have any comparison material. I have but it's all racing material, should be able to do some proper cruising as well :-P. So don't want to go too big on the carbs, preserving drivability, but 42mm chokes seemed to small looking at how sensitive the engine reacted.
Thanks a lot!

Still hoping for someone to pop up with abt the same setup :p
 
TurboDutch said:

3.0litre stroker crank set, k-jet fueled?

Morning gents, Found the 3L stroker crank set in the NL. Was thinking to use it in my pistonslappy B230E, so it is using K-jet. Using LPG as main fueling, looking into getting larger size venturi mixer but that shouldnt be to difficult. But actual question will be will my K-jet dig this shit...
turbobricks.com turbobricks.com

Above link shows the build topic.
I am not like completely new into tuning, only I come from EFI + Turbo, ( I am doing the software of the half the volvo 850 forum in the NL) only carbs are quite new for me, as well as N/A tuning. So looking for proper advise here.
I just filter out what seems right, if you run the weber calculators you also get up to 55mm dco.
So in this case I am not listening to the 40dcoe advise, the rest of his advises I took, I replaced all ignition parts and took of the head to see for an oil leak. Even though I was convinced there wasn't any oil leak, and I was right. Then same about the ignition parts, since replacing them didn't help as well.


Very difficult (for me) since I hardly have any comparison material. I have but it's all racing material, should be able to do some proper cruising as well :-P. So don't want to go too big on the carbs, preserving drivability, but 42mm chokes seemed to small looking at how sensitive the engine reacted.
Thanks a lot!

Still hoping for someone to pop up with abt the same setup :p
Click to expand...
The best advice I can give is to find a shop that tunes webers. Maybe a race rally shop?
I know a few people stateside who tune webers, maybe they need a trip to go see you :)

They also use DCOE 45s on 1.6l engines, so 40s on the 3l is not correct at all 🤣
 
On my used set of 40's, I had to gently(!!!) twist the throttle shafts to get both sides of the car to close the same amount. Impossible to get a smooth idle otherwise with one cylinder pulling more air than the one next to it. I used a Unisyn and just gently held one side of the throttle shaft while twisting the other until I got similar readings on both sides.
 
JohnMc said:
On my used set of 40's, I had to gently(!!!) twist the throttle shafts to get both sides of the car to close the same amount. Impossible to get a smooth idle otherwise with one cylinder pulling more air than the one next to it. I used a Unisyn and just gently held one side of the throttle shaft while twisting the other until I got similar readings on both sides.
Click to expand...
The FAJS carb and ITBs also have really strong return springs and a flexy center link.

So there’s very noticeable deflection in the throttle blades when they’re connected as a unit. In the bench a single unit will look fine.

@TurboDutch does your throttle linkage attach to the center of the carbs, or at one end? If in the center, you might be able to remove the end springs (if installed).

If your throttle linkage attaches to the end (front or rear of engine) you will never get the deflection out unless you remove all the springs on the throttle shafts.
 
Yeah, I reconfigured mine so the throttle linkages and return springs were all on the same side, not acting through the carb. I figured lots of use like that prior had twisted them.

That shouldn't really make cyl #1 run rich, though.
 
I have the control in the middle yes. Thanks for the replies :)

I live in the south of NL not much to find here like weber specialist, need to become it myself and show the dyno guy how it's done I guess.
 
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woodrowstar said:
Those carbs on 1.6l are either heavily choked down and running unique venturi or the engine is race spec with gearing to match. 40s on a 3L is correct for case use until you require larger than a 36mm choke.
Click to expand...

That specific 1.6l is a race engine, but it runs just fine at low revs, and absolutely screams at higher revs. I’ve driven a 1.6 street engine (stock CR and a mild cam) with DCOE 40s on it. Drove wonderfully cold or hot.

Anything under 38mm (of note, this should be the choke diameter) is choking an b21-b230 (let alone this 3 LITER ENGINE!) just based on the valve size. Add in the taper needed for most engines and you easily end up with a 45mm+ choke.

The only single cylinder engine that comes with a carb sizing that you’re suggesting (carb Venturi diameter vs cylinder displacement) is something like a klr650. So extremely choked up they can barely get out of their own way. You can put a 44-48mm carb on one of those and increase power and MPGs! This 3l engine is 15% bigger per cylinder vs the klr650 example.
 
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You are spitting nonsense.
Yes, 40 are daily drivable on a 1.6 4 cylinder- duh. 45s as a daily with same usewould be to.....with a tiny choke. At that point, 40s is all you need.
 
woodrowstar said:
You are spitting nonsense.
Yes, 40 are daily drivable on a 1.6 4 cylinder- duh. 45s as a daily with same usewould be to.....with a tiny choke. At that point, 40s is all you need.
Click to expand...
If by nonsense you mean direct experience with carburetor sizing, then yes.

I’m trying to show some examples of real world carburetor sizing. If I’m unclear on anything, I apologize. I’ll gladly clarify anything I’ve said.

Suggesting smaller chokes on a much larger engine is the real complete nonsense.
 
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Today did a little bit of research, found some on savar turbo website. They use dco55/sp on a 2.3 with a timos 13.0 cam. Quite similar to ky126.
So I think the 50's with 46mm chokes will be fine.

@woodrowstar check out savarturbo website.
Oh and do also please check the weber jetting calculator website.

I think I ran a 28mm carb on my 70cc zundapp. So yeah... Enough said maybe
 
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