Dual BXOV-2's

[35joe]

About one year ago I installed dual BXOV-2's on a fenton intake. My car is a 35 Chevy with the original drive train. Except the dual exhaust as well. I can't find anyone here to troubleshoot my issues. With the car that is.
The car runs well. But when I pull the spark plugs they are jet black with carbon. When I ask someone to do an adjustment it comes back three steps backwards.
My carbs have adjustable jets and venturi restrictors. Please help.

[zulu]

Nice looking donk
I have heard that adjustable jets can be problematic

I had my dual strommies rebuilt prior to fitting them with no venturi restrictors and they have been relativley trouble free
But maybe a little thirsty

The chap who rebuilt mine has long since moved, to Qld I think

[35joe]

Thank you.

[Harv]

G'day Joe,

Mmmm....Chev . Is this the Standard 6 (206ci or 181ci), or the Master (206ci) stovebolt 6?

There is some good info here if you want to understand how your Strommies work. It is written to be Holden-centric, but is just as applicable to the Chebbie:
FB/EK Holden Stromberg Carburetion Enthusiasts Guide

Whearabouts are you located? If you are in Sydney, I'll tune it (as long as you take me for a ride in it  ).

It sounds like the vehicle is waaaaaay over-jetted. I am assuming here that other than the venturi restrictors and adjustable jets that the rest is stock standard. I'm also assuming that it soots up the plugs when driving normally (ie mostly cruise, not mostly at idle or mostly under load). The good thing is that over-jetted is safe (won't hurt the engine), but expensive on fuel.

It should be running something like a pair of 0.048 main metering jets. The main metering jets are a bit finicky to tune - easier if you have an exhaust analyser. Without an analyser, and as a rough guide, they should be 2.5 turns open from a (gentle!) seating. The drama is that the adjustable needle valves are very touchy between two and three turns -

Opening   Equivalent Main Metering Jet (”)
Two turns   0.043
Three turns   0.061
Four Turns   0.067
Wide, wide open   0.073

Here's how I'd do it:
a) Clean the plugs.
b) Undo the locknut on both main metering jets, taking care not to move the needle. Gently screw the needles in to a (gentle!) seat. Record how many turns open they were. This is your "Ooops, the process is not working, put it back to how it was" setting  .
c) From your Oooops setting, screw both needles in 1/4 turn and lock the locknuts.
d) Go for a good drive. Get it out on the freeway somewhere and cruise it along at 100km/h, listening for any sounds of a slight miss (this is pinging, and means the main metering jets are screwed too far in).
e) Stop and check the plugs (do not let it idle too long or stop after a big long run under load as they will change the plug colours). Pull the plugs and check for colour. If they are still sooty, clean them then unlock, screw the adjustable jets in another 1/4 of a turn, lock them and drive again.

Gotta be careful - you are leaning up the car. Too lean and it will ping and burn out piston crowns... I'm really not a fan of adjustable jets. I've got plenty of standard jets here if you want to return back to the "normal" way of tuning them (can even lend you a tuning kit: For Loan: Stromberg tuning tools).

Cheers,
Harv (appreciator of Chebbies).

[35joe]

Harv (appreciator of Chebbies),
Thank you so much for the info! It's just what I needed. I actually live on the other side of the pond. So as much as I would love to give you a ride, it probably won't happen soon. My car is a Standard 207. You guys are the only ones that know anything about Strombergs. I bought my venturi restrictors from a fellow (Mr. Whitby) he is on your side. He was extremely helpful when I originally put them on my car. Well this weekend I will give your advise a try. Thanks again.
Joe

The first one is the 35. Second 1956 Chebby, last is a 1935 Ford.

[hsv-001]

Just keep in mind that you must draw enough air [air speed] to atomize  the fuel otherwise jets will make little difference .Cheers Haydn

[KFH]

Love the 35 Chebbie.

Keith

[35joe]

Compliments appreciated...

[Harv]

I actually live on the other side of the pond. So as much as I would love to give you a ride, it probably won't happen soon

Damn.... I was itching for a ride  .

I bought my venturi restrictors from a fellow (Mr. Whitby) he is on your side. He was extremely helpful when I originally put them on my car.

Frank is a great bloke, and his venturi restrictors are good quality.

Well this weekend I will give your advise a try. Thanks again.

Let us know how you go. If you ever need parts for the Stommies, give me a yell.

Cheers,
Harv

[35joe]

Thanks will do.

[35joe]

Hi Harv,
Well this is how it went today. There were not enough o's in ooop's. The first adjustable jet I loosened the nut which was tighter than I thought. Sooo as I loosened it the knob slid out of my finger tips and I lost count.
I went to the second one being alot more cautious I counted 3 full turns out. Sooo assuming they are both the same I turned the both of them in to a gentle seat and backed them out 2 turns. Drove her a while, came back and pulled the plugs and noted their condition. Cleaned them up. The car ran smoother. Did the quarter turn test and checked the plugs four times in all.
She seams best at 2 full turns but I did not use the Uni-syn yet. The plugs and I are now on a first name basis as all 6 have individual names. Kind of like seven dwarfs. I am dopey...

[Harv]

Good news.

At three full turns out, you had (roughly) a pair of 61 jets. As a rough guide, our local GM big 6 (the Holden 202ci engine) ran a single 65. A pair of 61's would have been waaaay overfueled on your Chebbie.

Remember that you are playing with the mixture, and leaning up the engine. If you lean it up too much, there is a risk of significant engine damage. Keep a sharp eye out for the warning signs - plugs that look slightly burnt or eroded, a slight miss under cruise conditions, the engine running hot, or pinging (knocking). If you see these signs, richen it up a bit.

If you want to get real fancy, get a mechanic to run an exhaust analyser over the vehicle. This will let you lean up the idle (either by adjusting the idle needle valve or swapping idle tubes), fine-tune the cruise mixture (by playing more with the adjustable main jets), and set the power mixture (by changing power bypass jets). The idle tubes are hard to get (I may be able to help depending on what you already have), though the power bypass jets are available new from the Stromberg Carburettor Co (your BXOV power bypass jets are identical to Stromberg 97's, which are common on your side of the pond). Having said that, if it was mine I'd be just as happy not running an exhaust analyser, and tuning it the way you have... there are several hundred thousand of those carbs in Australia that never saw an analyser and run fine.

Cheers,
Harv

[35joe]

Will keep a close ear and eye on her! Thanks...

[35joe]

Hello all,
Sunday the weather here (New York) was abnormally warm so I took the 35 out to a "cruise in). It was at least a 45 minute ride at 50 mph each way. Thats revving up there in the rpm's. She has 4:11 gears. The car ran great!
I have not touched the adj. jets. I just pulled the plugs and 5 of 6 are clean. The one closest to the firewall #4 cylinder was sooted up a bit. Whatta ya think?
Rgds, Joe

[Harv]

Good news - sounds like you've fixed the problem.

Cheers,
Harv

[Dr_Terry]

One thing often overlooked with carby mixtures & not just with Strombergs either, is float level.

Float level is critical & probably even more so with dual carbs. The fuel level in the float chamber has a much greater effect on mid-range running mixture than does main jet sizing. The size of the main jet really only matters at high speed or WOT. I can't see oversize jets causing the plugs to go sooty/black with normal driving.

Back in the 70s, every 2nd car that went thru my workshop for tuning had a Stromberg fitted, I used to pull the air horn & check & adjust the float on every one. I found that over 60% were out, usually on the high (rich) side. On a car with factory spec carbs, the only really tuning available is float level, idle speed, idle mixture & ignition timing. Beyond that you need to resort to changing jets & recurving the dissy.

Whenever I came across a car with one of those adjustable main jets, it always went straight to the bin. We were often asked how to adjust one & our stock answer was to turn it inwards 1/4 turn at a time until you burn an exhaust valve & then back off 1/4 turn.

Dr Terry

[Harv]

G'day Dr Terry,

Agree about the adjustable mains... not a fan at all.

Not so sure about the effect of fuel bowl float level though, as the fuel level moves around a heap with the car in motion... assuming of course that the level is not so high that the carb is puking. The hydrostatic difference between a full and empty Strommie bowl is only about 1% of the cruise vacuum... given that flow is proportional to the square root of pressure drop, the difference between and bone-dry and chocka block full bowl is only about 0.5% of fuel flow. The orifice (jet) sizing has a bigger effect, with flow proportional to the square of diameter - a change of some 10 jet sizes will increase flow by 50% at the same vacuum. This is rough science, but not bad for order of magnitude.

Cheers,
Harv

[Dr_Terry]

I will have to respectfully disagree, in part anyhow.

My experience is very much 'hands on' & over many 100s (perhaps 1000s) of cars.

While a low float level doesn't have a huge effect, i.e. it doesn't go drastically lean, a high float/fuel level does !!

With the fuel level not that far below the discharge nozzle's outlet, an overly high fuel level will increase the fuel output significantly.

I've proved this many times, with a float level tweak resulting in noticeable fuel economy improvement, with no other adjustments at all. In extreme cases, after dropping the level back to stock, i've had to lean off the idle mixture & drop the idle speed because of the resultant change in overall mixture.

Dr Terry

[Harv]

It's interesting that it takes so little for the carb to puke and start to siphon from the main discharge jet. I can see why low fuel level is less problematic - there is at least an inch of fuel height before the carb starts to lose suction.



Cheers,
Harv

[DJ]

Great discussion, of real value to the novice like me.