SSD off-idle transition stumble

[tekenaar]

Was asked about this SSD carb problem some time ago by a fellow user here and was told the problem remains, even after trying numerous tuning (jetting) suggestions by myself and others in the group. This problem appears to be somewhat unique to SSD installations, so I've given it some more thought and also discussed it with some racers and Harley owners I know. Here's their consensus:

If you're sure that your idle (transition) jets are correctly sized, the only cure remaining is to enlarge the transition holes (not including the lowest idle hole) at the throttle plates. Both racers and Harley owners told me, and it makes perfect sense, that these holes (fixed orifices) may be too small for the transition mixture VOLUME requirement (i.e. correct mixture, but too little volume) of multi-cylinder-per-throat applications like our 4-cylinder SSD engines.

A lot of Harley owners use DHLA40s on hopped-up bikes and have had to enlarge their transition ports to get rid of off-idle stumble. If you think about it, each barrel has to feed ~700cc or more on a Harley and even more on our SSD multi-cylinder applications. ALL the other jets, etc. are variable, but the transition fuel/air mixture VOLUME is controlled by the size of fixed orifices (holes) and can only be changed by enlarging them.

Remember, these carbs were specifically designed for one-throat-per-cylinder, high RPM, limited displacement engines. Ideally, carb bore size is typically determined by and matched to an engine's single cylinder displacement flow requirement and are offered in a wide range of sizes for that reason. Though ideal and easily tuneable for one-throat-per-cylinder applications, they were never really designed for multi-cylinder-per-throat applications like ours.

Within their design specifications, they work beautifully even in their transition phase. In a multi-cylinder-per-throat, high displacement application, this design becomes somewhat compromised. In effect, each throat must now pass twice the mixture volume for each engine cycle (two revolutions). All the areas of the carb can be adjusted to tune for this except one, the transition phase, where maximum mixture VOLUME is controlled by fixed orifices (holes).

So, if you have an off-idle transition stumble in a SSD setup, you may want to think about slightly increasing the size of the transition holes . . . making sure that both the size changes and the order of them are identical in both barrels, of course.

In case you're curious about my 2.15 SSD carb, I bought my DHLA48 from a racer who had already done this. He basically just sold me his "spare" (only 48 I could find at the time) with my specified jetting in it, but without the shorty, 15mm trumpets (air horns) I'm using inside my fabricated cold air box.

[opeldean]

otto:
you have a wide variety of engine configurations. i would be curious if you had a synopsis of each car and how it performs relative to the other. hp,acceleration,revabilility,torque ,even gas mileage

I would also like to drive each one a bit and decide for myself (but i guess that may never happen! )I think my rebuild will be closest to your 1907cc with fuel injection.

how does each of your cars compare to stock?

thanks

[N61WP]

Okay, since we have started a new thread here and others may benefit from my trials:

Otto, That is something that I surely have not thought of. As I told you before, I have tried several different jettings, yours, suggestions from Gabrielle and others, and nothing has worked to get rid of it. I tried the 34 chokes as Nobody recommended. My current settings are: 34 chokes (don't think this made any difference--will go back to the 36's next time the carb is off), 165 main, 190 air, 45 pumps, 50 F8 idles. This has been the best set up so far. Gabrielle recommended trying the F9 idles, but he didn't have them in those sizes. I have used a color-tune to play with the different idle jets and just last night installed a fuel/air ratio gauge. I have it just slightly richer then stoichimetric at idle. I have not driven the car with the meter yet. It will be interesting to see where the meter reads when it stumbles. I give a report sometime this weekend, if the weather permits. Other then that, it idles well and pulls harder then I have ever seen before. I drove it to an autoX last weekend and had some fun with four other autoXers I met on the way. All four of us nailed it on a stretch of interstate and I was able to take a Miata and a Toyota Celica quickly. Looked down and I was passing 110 when I lifted! The M coupe and Audi A4 pulled me...quickly.

Seems like your idea has lots of merit.

Thanks,
James

[N61WP]

Here's the latest....

I spent last Sat. and today playing with jets and watching my new air/fuel meter. It seems to work fine and reacts quickly. Without taking up too much space here, just let me say I have tried every combination of jets I have and many different idle needle settings. So, fat or lean...nothing has gotten rid of the stumble. I have even tried retarding the timing advance some, just to see if it made any difference. None. The A/F meter does not change when it stumbles and bucks. Today while playing with it, I realized that (sitting still) if I ease into the throttle, I feel a roughness at 1300-1500 rpm, no matter what jetting I use.

So, i guess I'll try Otto's suggestion about drilling out the ports next. Otto, are both the holes (2 per barrel) enlarged? Do you have an idea what size? On my old style DCOE, there is a brass plug above the ports so they are easy to measure with a drill bit. Mine are slightly larger then a 1/32" bit.

The saga continues....
James

[Paul]

James,

Tell us more about your AF Meter... brand and type of O2 sensor, etc..

Thanks

[N61WP]

I got the meter, the oxygen sensor and a weld in bung from Summit racing. The meter has 10 LED's. It is their brand, not a high priced name brand. It seems to work fine and react quickly. While playing with it, I'd pull the vacuum line off and it would immediately show lean. Then if I cranked the idle adjustment screws open, it would go rich.
Jc

[davegt27]

Which SSD manifold do you have?

there are about two or three different ones for the Opel and some don't work to well


what type of Carb do you have.
my SSD has a Weber but i like Mikuni better

I could dig up my single side draft setup if you like and get the Jet sizes.

the only thing i can remember off the top of my head was it had a 200 main jet

yeah that big

Damn car would fuel starve on a WOT accel from a dead stop.

If I could ease into the throttle man was that sucker quick.


Dave

[N61WP]

I have a Mangoletsi SSD manifold with a 45 DCOE9.

[davegt27]

Sorry I could not find my Carb setup its around some where.

it had a Weber 40 so i don't think the Jet sizes would help on your 45 setup

First thing I did when I got a Weber Carb was get a Weber carb book.

Hp books "Weber Carburetors" by Pat Braden is real good.

Sorry for jumping in at the middle but go for the easy fix first.


Davegt27

[madhatterpdc]

Originally Posted by tekenaar 1972 GT, 2.4 (2410cc) roller FI, Getrag 240, BMW vented disks, 3.44 Quaife

slightly off-topic, but is this motor full roller or roller rockers?

[tekenaar]

Originally Posted by madhatterpdc slightly off-topic, but is this motor full roller or roller rockers?

Right now still roller rockers, but soon full roller . . . have the parts, just not installed yet.

[N61WP]

For the last 3 months I have continued experimenting with my SSD 45, trying to remove the stumble while operating on the transition ports at low speeds. I read 2 Weber books, many web sites and talked to numerous Weber experts, users, tuners and sellers. No one’s advice worked completely. Jetted rich or lean, the stumble remained. I tried every jetting combination imaginable. I couldn’t even begin to list them all here. I have almost bought Gabriel out of parts! So I began to experiment deeper. I put the 36 chokes back in. Ran better then the 34s. So I started thinking that I needed more air in the mix, not less. But it likes lots of fuel too since it ran worst on smaller jets. So I started trying different E-tubes, just to see if I could do anything to change the stumble, be it better or worst. I had F-16s, I tried a friend’s F2s which made it worst. Then I found some F9s and finally used some F-11s. They have more air holes in them to emulsify the fuel. This made the largest improvement. I then began experimenting with the idles. I took a set of 50s and drilled straight through the air hole so now there were 2 air holes. Again, much better. The plug’s color was text book perfect. Just tried the same trick with a set of 55s. It likes it. The stumble is still there, but not nearly as bad. It’s now drivable at low speed. It pulls very hard at the top and is stable on my A/F ratio meter, even wound tight. The plugs are now darker then ideal, but not sooty and the meter is showing 1-2 bars over Stoichic. This engine seems to run better slightly rich.

Here are my current settings:
36 chokes
55 pumps
F-11 E-tubes
175 Mains
220 airs
55 idles w/extra holes

James

[tekenaar]

Originally Posted by N61WP For the last 3 months I have continued experimenting with my SSD 45, trying to remove the stumble while operating on the transition ports at low speeds. . . . . . . This engine seems to run better slightly rich. Here are my current settings: 36 chokes 55 pumps F-11 E-tubes 175 Mains 220 airs 55 idles w/extra holes James

Don't know WHY I didn't think of this before, what size aux venturis (AVs mount above the chokes) are you using?! Weber has AVs from 3.5 to 5.0, stepped in .5 increments. What size are yours? I went to the largest available in my 40 SSD 1.9 because my carb seemed to lag slightly when I romped on it, not hesitation really, just seemed to struggle a bit keeping up with the cam.

The AVs control the overall VOLUME of mixture available in the RPM range from transition to full throttle and what you described in your post above indicates you dont have enough mixture volume just as you're starting to come into full use of the main circuits.

The mixture itself may be just fine, there's just not enough volume available . . . restricted by too small AV passages perhaps?! This is OFTEN overlooked in SSDs because traditional side-draft carb usage assumes twin carbs, not a single.

[tekenaar]

Originally Posted by davegt27 Sorry I could not find my Carb setup its around some where. it had a Weber 40 so i don't think the Jet sizes would help on your 45 setup First thing I did when I got a Weber Carb was get a Weber carb book. Hp books "Weber Carburetors" by Pat Braden is real good. Sorry for jumping in at the middle but go for the easy fix first. Davegt27

The best "Tuning" book for Weber and Dell'Orto SD carbs I've found is:

How to Build & Power Tune Weber DCOE
& Dellorto DHLA Carburetors
- Revised & updated edition! -

By Des Hammill

SPECIFICATION
Softback. 250 x 207mm (portrait). 112 pages.
Over 140 mono photographs/line illustrations.
Published July 1999 · Price £14.99.

ISBN 1-901295-64-8/UPC 36847-00164-3

Bough mine from Veloce Publishing in UK.

[N61WP]

A friend ran across a set of 185 mains the other day, so I figured why not? Low speed driving is as bad as ever. Plugs and A/F meter are about the same, but it pulls even harder after about 3500 rpm. Man, this engine sure loves the fuel. But when is enough, enough?

Someone had a post saying he ran 200 mains in his SSD. I can't seem to find it. I am finally starting to believe that the only way to get this thing to run perfectly would be with a set of dual 45's. (Or EFI, of course)

I've got to get this thing on a dyno this summer and see what's really going on.

Jc

[RallyBob]

Originally Posted by N61WP A friend ran across a set of 185 mains the other day, so I figured why not? Low speed driving is as bad as ever. Plugs and A/F meter are about the same, but it pulls even harder after about 3500 rpm. Man, this engine sure loves the fuel. But when is enough, enough?

Agreed, Opels seem to like a fairly rich mixture....4.5 to 5.5% CO at full throttle makes good power.

Someone had a post saying he ran 200 mains in his SSD. I can't seem to find it. I am finally starting to believe that the only way to get this thing to run perfectly would be with a set of dual 45's. (Or EFI, of course)

I believe that was DaveGT27 that ran the large mains in his SSD.

I've got to get this thing on a dyno this summer and see what's really going on.

It's the only way! It's worth the extra money to have it run with a wideband O2 sensor for some real-time data-logging. It will tell you more in 5 minutes than a week of street driving.

[opeldean]

i watched a part of the downdraft carb rebuild..i think this is a dellorta...maybe its a weber..anyway..at the end i was asking him about common problems and where the most likely place to get in trouble...anyway he said ( as i understood it) ..that a common area to clog is between my fingertips...between the idle mixture and idle jet..right where you MAY be having trouble on dcoe40's..

1-"he said you this is the only place you can blow air WITHOUT a dissembly" the rest of the carb needs pulled apart before you pressurize it...maybe you got loose crud that you cant see

2- has the carb been stripped and profesionally soaked in and automatic washer...xylol is a common solvent i believe that the pro use..i dont know if over the counter stuff is NASty and toxic enough

3-also..are your ball bearing or stuffing ball floats in the proper position? these are commonly lost during a rebulid..i know i lost mine and had to reorder from gabrielle

the mechanic doing the demo worked for intermechanica or their competitor,

if you all didnt get a chance to see the porche replica cars..they were sweet..26,000 and looked like a porsce speedster...carbureted!

[GTJIM]

I recollect reading about this (on a BMW site - he shamed-facedly admits...).
The problem there was reversion and fuel stand-off outside the trumpets at certain revs between 2,000 and 3,500rpm. The discussion centred about the internal porting of the SSD manifold with the improvements being most apparent when 1&3 and 2&4 cylinders were connected instead of the way most Opel SSD manifolds have 1&2 plus 3&4 connected plus a "balance" tube between the two chokes.
There is another fruitful source of tunning - the size of the balance tube - much used in the TR3 and TR4 plus Mini SU manifolds ........
Just a few more varriables to get the "rats-in-the-brain" whirling onthe treadmill.

[davegt27]

Give us a recap where are you now?

when you say low speed still sucks do you mean on a WOT accel from a dead stop

or as you slowly pull away

my SSD was strange the main jet was 200. air jet was smaller 185 maybe

still haven't found that thing

Why a 55 idle jet?

davegt27

[N61WP]

Dave:
Here are my current settings:
36 chokes
55 pumps
F-11 E-tubes
185 Mains
220 airs
55 idles w/extra holes

It idles well, pulls well (no flat spots), and screams on the upper end when you are in the mains. It just doesn't like running on the progression ports. It ran worst with smaller idle jets. It likes the 55's with the extra air holes in the idle jets. I think it just needs more volume (as Otto says). Plug color is OK, just slightly darker then ideal, but not sooty.

I haven't pulled the carb back off yet to check the A/V as Otto suggested, but I will. I may even try another 45 I am rebuilding just to be sure there isn't something wrong with this one. After that, I'll be looking for some short dual manifolds and use both my 45's.
Jc

[davegt27]

What Weber book are you using?


I went back and started reading the old posts.

So your using a 2.0 with a ported head?

not sure if your game for any more help since so much has been put out already

my experience is mostly with Mikuni's the previous owner of my race GT had major problems with an off idle stumble/bog.
i started reading his diary/notes of all the stuff he had tried. (the problem was on quick throttle application the car would bog. this was a dual sidedraft setup)

Long story short I found it to be no big deal I could make the stumble come or go depending on how I set up the Carbs. (I am sure he had fixed most of the problems but gave up at the 98% mark you know Murphy's law.)

let me know
Davegt27

[davegt27]

Hey JC i found my SSD setup it was in the lower cabinet next to the oven ha ha
never thought to look in the kitchen.

here is the setup

DCOE 40

34MM Chokes

45 pump jets

F16 E-tubes

200 main jets

120 Air Jets (yep tinny)

55F8 idles

sounds crazy but it worked great

HTH
Davegt27

[Betta Carotene]

what motor was this carb on dave?

[davegt27]

it was on my street GT i had back in 1996 1.9 low comp

that motor did a 17.3 in the 1/4 with one of Bobs prototype intakes and a 32/36 (rejetted)

i pulled the down draft off and put the SSD on, it was way way faster

i had a big grin on my face just saying this sucker is fast, 30 sec after i said that to myself some lady wrecked my GT

Davegt27

ps oh i keep forgetting Bob did the Head and Cam

[PROPEL]

Originally Posted by davegt27 it was on my street GT i had back in 1996 1.9 low comp ...ps oh i keep forgetting Bob did the Head and Cam

So the top-end was not stock, Dave?
Thanks for posting your settings as I'm getting ready to add ssd to a lo-comp stock 1.9L. Between these numbers and
Otto's, I should be in the ballpark without spending a fortune on jets.