Stumble at Throttle Tip In - Dual Carbs

[Bmartin]

I am having trouble solving an issue on my new engine,maybe 100 miles on it.  I have a 230 with a milled head, split manifolds with true duals, and an Edmunds dual carb intake with two factory B&B's. 

I have a stumble at throttle tip-in and have to feather throttle to get it to take off smoothly.  I've tried the middle position on the accelerator pump as well as the winter (richest) setting.  Currently at the richest setting.  Only real difference I noticed was needing more return spring to keep the throttle from hanging. 

Seems to improve some with higher engine temps, but I have trouble getting it fully heat soaked since there are no hood or fenders right now.  The weather is also cool.  Temp is sitting at 160-180 typically. 

Ignition:  I'm running a twelve volt system with resistor wires, Autolite 295's gapped at .025, and twelve volt coil with internal resistor.  Plugs have been reading lean to good.

Timing:  I set piston 6 at TDC, the rotor pointed to cylinder #1, and timing indicator was right at the center mark on the pulley.  I have tried 0 advance, about 4 degrees advance, and also about 12 degrees advance - amazingly I never detected any pinging. 

Gears:  I'm a little under geared right now with 3.55's in the rear and 29" tall tires.  So it bogs a little at take off.  Still stumbles when moving if the revs drop low enough <~1200 rpm.  Factory 3 speed with O/D but thats locked out right now. 

 

I'm looking for some ideas to try and especially any first hand experience.  For those with a similar setup, do you have any stumble or does it roll away from a stop as smooth as stock?  Do you have to feather the throttle at all?  Right now I'm wondering if I have to go with Tom Langdons two barrel carbs to get rid of this.  Any help is appreciated. 

 

 

 

[Plymouthy Adams]

I will have to ask now that you seemed to have checked the basic tune up aspects, what device did you use to sync the carbs and did you sync at idle and then balance the linkage at speed?

[Don Coatney]

This tool might or might not help.

[Bmartin]

The only syncing I have done is to match the throttle linkages to be equal, and only visually.  Same visually for the idle mixture.  I set the idle mixture for max vaccum, measured at the manifold output for the wipers.   If there is a process for syncing them, I'd love to read about it.  First time with carbs for me. 

 

Don:  Do you have a name for that contraption so I can read up on it?

Edited January 6, 2017 by Bmartin

[Plymouthy Adams]

that contraption is a Uni-sync...very much available on the internet, E-bay for the best prices...will come with instructions...else, they can be read in advance by internet search..this is a very handy tool for dual carbs...have used this on many different carbs and yes on B&B's on a Plymouth dual intake...

for those times you do not have a vacuum style sync tool you can set the base line by use of a wire style feeler gauge for low end (idle) and then set the upper end with a larger drill bit...just set up a Ninja 900 this way and it screams bloody murder on top end and smooth as a goose at idle..excellent transitions...their carbs were reverse manifold set up and thus could not use the Uni-sync

 

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR12.TRC2.A0.H0.Xuni-sync.TRS0&_nkw=uni-sync&_sacat=0

 

 

Edited January 6, 2017 by Plymouthy Adams

[Bmartin]

OK, I have one on order.  Hope to get it by Sunday. 

 

If there is anything else I should try in the meantime, let me know.  Especially looking for any first hand experience with this setup. 

[Plymouthy Adams]

1 minute ago, Bmartin said:

OK, I have one on order.  Hope to get it by Sunday. 

 

If there is anything else I should try in the meantime, let me know.  Especially looking for any first hand experience with this setup. 

you will like that tool, one of our owners drove over with a Plymouth and dual carbs and remarked how powerful and smooth it was...I rode in the car detected some hesitation and loading...asked if we could tweak a bit with the sync...his remark afterwards...close is just not close enough compared to balance...

till it gets here and you wish to try to improve the balance in the mean time....follow advice above in paragraph two...

[Don Coatney]

The tool is used to balance the linkage only. Has nothing to do with mixture. I used it to adjust my linkage with the engine running before I installed the engine in my car. Forty thousand miles later I have not had to use it again. Adjust it once and forget it.

[Plymouthy Adams]

as an added note on adjust once and drive...if you are ever out under the hood tweaking and pulling on the linkage...be sure to always use the common pull point when revving the engine...

[sser2]

Two carb setup is for increased performance at wide open throttle, at the expense of throttle response and performance at partially open throttle. 

Wider carburettor bore = lower air speed = worse fuel pulverization = worse response and worse performance at lower engine speeds. However, as engine speed increases, air speed increases, and performance improves.

Narrower carburettor bore = higher air speed at low rpm = better pulverizatiobn = better response and better performance at low er engine speeds. However, as engine speed increases, narrow bore chokes the engine, and performance degrades.

An optimal carburettor design is a compromise between these two opposing tendencies.

Two-barrel carburettors offer optimal performance at both low and high engine speed. The second barrel remains closed at lower speeds, so that engine sees a carb with narrow bore. At high speeds, second barrell opens, making an equivalent of wider carburettor bore.

(Flame suit on)

[suntennis]

Since the problem is reduced with higher engine temperatures,  have you tried choking the carbs some to see if the problem is reduced?

[timkingsbury]

13 hours ago, sser2 said:

Two carb setup is for increased performance at wide open throttle, at the expense of throttle response and performance at partially open throttle.

Wider carburettor bore = lower air speed = worse fuel pulverization = worse response and worse performance at lower engine speeds. However, as engine speed increases, air speed increases, and performance improves.

Narrower carburettor bore = higher air speed at low rpm = better pulverizatiobn = better response and better performance at low er engine speeds. However, as engine speed increases, narrow bore chokes the engine, and performance degrades.

An optimal carburettor design is a compromise between these two opposing tendencies.

Two-barrel carburettors offer optimal performance at both low and high engine speed. The second barrel remains closed at lower speeds, so that engine sees a carb with narrow bore. At high speeds, second barrell opens, making an equivalent of wider carburettor bore.

(Flame suit on)

Why would you need your flame suit on ?    It appears to be pretty much directly plagiarised section from a well know carb manufacturer.  From a pure engineering and scientific standpoint their statement is correct. In fact in pure carb functioning making 6 barrels would be better, and on and on until you get to the point that you get to the fact that a carb is obsolete technology.

That being said your piece, well it wasn't written for an engine with 3 Siamese intake ports feeding a 6 cylinder engine where its clear that the separation of fuel distribution trumps a single multiple barrel device!

[knuckleharley]

14 hours ago, sser2 said:

Two carb setup is for increased performance at wide open throttle, at the expense of throttle response and performance at partially open throttle. 

Wider carburettor bore = lower air speed = worse fuel pulverization = worse response and worse performance at lower engine speeds. However, as engine speed increases, air speed increases, and performance improves.

Narrower carburettor bore = higher air speed at low rpm = better pulverizatiobn = better response and better performance at low er engine speeds. However, as engine speed increases, narrow bore chokes the engine, and performance degrades.

An optimal carburettor design is a compromise between these two opposing tendencies.

Two-barrel carburettors offer optimal performance at both low and high engine speed. The second barrel remains closed at lower speeds, so that engine sees a carb with narrow bore. At high speeds, second barrell opens, making an equivalent of wider carburettor bore.

(Flame suit on)

I pretty much agree with one stipulation. The only way you are going to benefit from using 2 brl carbs is if you have a 2 brl intake and the engine is built to be used at higher RPM's than normal street driving.

I also happen to think that the ideal street setup for a inline 6 flathead is a 3 x 1 intake with 3 small 1 brl carbs,but since I haven't ran a flat 6 on the street or anywhere else with either a 2 x 1 or a 3 x 1 setup yet,this remains a theory.

When I do end up running it,it will be with a 3 X 1 Tattersfield or Edmunds intake (I have both) using either 1904 Ford/Holley carbs or B&B's,a higher compression aluminum head,a 3/4 cam,and split exhaust on a 251 Chrysler engine,so what I discover probably won't be very useful for guys running stock 218 or 230 engines with dual carbs.

I have seen some 2 x 2 intakes for 218-230 Mopars though,and wondered how they worked on street cars. I can see how they would work good on race cars,but would think a 2 x 1 intake and carbs would work better on a stock engine.

Edited January 7, 2017 by knuckleharley

[knuckleharley]

1 hour ago, timkingsbury said:

Why would you need your flame suit on ?    It appears to be pretty much directly plagiarised section from a well know carb manufacturer.  From a pure engineering and scientific standpoint their statement is correct. In fact in pure carb functioning making 6 barrels would be better, and on and on until you get to the point that you get to the fact that a carb is obsolete technology.

That being said your piece, well it wasn't written for an engine with 3 Siamese intake ports feeding a 6 cylinder engine where its clear that the separation of fuel distribution trumps a single multiple barrel device!

 

 

I've often wondered how a 6 port inline engine would work with 6 individual manifolds using small 1 brl carbs like used on some motorcycles or small furrin cars.

Why not? I used to run a  SU carb off of a 4 cylinder Volvo on my bored and stroked cammed up knucklehead Harley,and that thing was a LOT faster than most modern buzz-bike owners would believe.

 

[sser2]

4 hours ago, timkingsbury said:

It appears to be pretty much directly plagiarised...

 

 

Saying that 2x2=4 is not plagiarism of elementary school textbook. I just wanted to point that with twin carb setup, degradation of throttle response, as described in the original post, is to be expected.

I am thinking of replacing a Carter carb currently on my 201 engine with the two-barrel Hitachi DFP 384, which was used on Nissan Z24 2.4L engine. The flow of this carb is near perfect match to 201, and fuel metering at various speeds is superior to medival Carter's. As a bonus, it has automatic choke and idle fuel cutoff valve.

[DJ194950]

Seems nobody that I noticed have on this  post mentioned your  0.025 plugs gap as stated. Should be at least 0.32 for your car with points and standard ignition setup. Does make a difference. Your problem may or may not be affected by this. Kind of doubt a big change but thought I would mention this.

Others have stated things most likely. A call to George Ash or a PM to  member Tim Kingsbury  about dual carb jetting might be worth your time.

Electronic ign. setups are recommended at least 0.035 plug gaps or more on some with the same plugs!

DJ

Edited January 7, 2017 by DJ194950

[55 Fargo]

1 hour ago, sser2 said:

Saying that 2x2=4 is not plagiarism of elementary school textbook. I just wanted to point that with twin carb setup, degradation of throttle response, as described in the original post, is to be expected.

I am thinking of replacing a Carter carb currently on my 201 engine with the two-barrel Hitachi DFP 384, which was used on Nissan Z24 2.4L engine. The flow of this carb is near perfect match to 201, and fuel metering at various speeds is superior to medival Carter's. As a bonus, it has automatic choke and idle fuel cutoff valve.

Interesting, and I wish you luck and look forward to your documented data.

There is no way on gods green earth that any single carb, whether it be a 1bbl/2bbl/4bbl, on this engine and 3 siamesed intake ports will have better fuel distribution, fuel atomization or fuel economy and performance than a multi carb intake, actually 3 matched carbs, each delivering fuel to each siamese port,no single carb design will be better than this set-up.

Hey but if the 1 carb from a Nissan works for you great, not putting your intent down, just it will not compete with a properly matched and tune ,multi carb set-up on these engines.

Keep us posted.

[sser2]

I will, although I cannot promise quick progress. I have full-time job and many other commitments.

As to advantages of multi-carb intake on a flat six engine, I fully agree, but on one condition: these advantages are only realized at wide open throttle and high engine speed, when you are trying to squeeze out maximum power. At partial throttle and low engine speed, a single small-bore carb beats a bigger carb, or multiple carbs, in response and economy.

As an everyday driver, I use partial throttle most of the time, and full throttle high rpm only occasionally. Accordingly. I am much more concerned with partial throttle performance.

My goal is 30 mpg at steady 60 mph highway driving, not winning races.

[knuckleharley]

24 minutes ago, sser2 said:

 

My goal is 30 mpg at steady 60 mph highway driving, not winning races.

Everybody needs a dream.

[sser2]

Aren't we all dreamers here?

My '93 Nissan truck is capable of 30 mpg, with careful driving. '37 Plymouth has same curb weight, but better aerodynamics. With higher compression, better fuel system, and stronger spark, the engine might be capable of sustaining 60 mph, while staying in the economy range.

[greg g]

What does your vacuum gauge tell you?  Wiling to bet you have a vacuum leak some where south of the carb bases, you have doubled the possibility....

 

Also you need to be sure your enrichments jets are getting their proper vacuum signal.  They are fed through the base of the verb by a slot in the carb to intake gasket.  If the gasket slot and carb port are not aligned they get no vacuum and default position is full open so what seems like a stumble is actually an over rich condition at low rpm.

Edited January 8, 2017 by greg g

[Matt Wilson]

On 1/6/2017 at 6:19 PM, Don Coatney said:

The tool is used to balance the linkage only. Has nothing to do with mixture. I used it to adjust my linkage with the engine running before I installed the engine in my car. Forty thousand miles later I have not had to use it again. Adjust it once and forget it.

Don,

You mention that the Uni-sync doesn't do anything to help with fuel mixture.  Maybe this is a dumb question (although maybe it will help the original poster), but how does one go about ensuring the fuel mixtures are equivalent from both carbs?  I'm contemplating a dual-carb setup and want to know what I'm getting into first.  Also, if a single two-barrel carb is better for less than full-throttle operation, as indicated by some folks, I'd like to know that as well.  Feel free to PM me on this if you don't want to get into it here.

Thanks,

Matt

Edited January 8, 2017 by Matt Wilson

[knuckleharley]

4 hours ago, sser2 said:

Aren't we all dreamers here?

My '93 Nissan truck is capable of 30 mpg, with careful driving. '37 Plymouth has same curb weight, but better aerodynamics. With higher compression, better fuel system, and stronger spark, the engine might be capable of sustaining 60 mph, while staying in the economy range.

Your 93 Nissan truck has a computer running your engine,and while it may look less aerodynamic,it has a smaller frontal area and blocks less wind. It also has a MUCH more efficient OHC engine.

[Sharps40]

I drove a 37 dodge daily for 15 years.  Flat 6 217.8.   Single carb.   Then dual/split/bored/shaved etc.    

I can render an opinion of confidence grounded in real no crap experience....its highly unlikely anyone'll ever see 30 mpg in a 37 flattie on carbs with no OD and a 4.10 outback.   60 mph is dooable all day long with a stock engine though. 

[MarkAubuchon]

I have run dual carbs on my car since I put them on the road with the same set up as yours.  My sedan runs flawlessly my coupe does have a slight hesitation ( i rebuilt the carb myself). 

check for any leaks on the carbs using starter fluid, reconfirm the linkage set up.  Who built the carbs? I do know that when George Asche builds them he makes sure they are the same casting/part numbers, float adjustment is huge and he does alter the jet sizes, also labeling #1 front & #2 rear . I also agree with the other tips, points, plug gap timing are important.  MY money is on the carbs not being built the same.  I'd also call george, send him the carbs and they will be ready for Spring.