Running rich , the story continues

bcatcmc

bcatcmc

Member
There comes a time when you realise that self help is just not gonna cut it, that time is here. With regards to my failure to overcome a rich idle with my 1940 Merc engine rebuild there are a couple of options, go cap in hand to Mr Stromberg Support or throw it at the Barn door and see what you fellas think.

Some background info , this is a long term project ,final assembly this year engine bored 060” over , Offy heads, new valve gear and springs new Offy intake with 2 new Stromberg 97 carbs. Electronic ign. home grown headers ect.

I will post up a series of numbered photos with descriptions here , most of this info gathered today. Did a compression test (hot ). 135 psi all round , I am 2400 feet ASL so a bit over 2” of Vac to be added to gage readings, the ignition set at 6 Deg BTDC, the spark looks puny but jumps 1/2 “ lead to plug, new NGK B-4L plugs gapped at .025 “ ( swapped over from B-6L ), all 4 mixture screws seated, the fuel bowl levels are both right. Engine starts up within 2-3 revs most times.

Photo 1 shows general set up.

Photo 2 shows some of test gear 3” Anemometer for balancing airflow, fuel bowl level indicators , spark plug tester.

Photo 3. Shows vac gage jumping around a little at 15 “ ( 17 “ ) 465 rpm popping a bit at exhaust, idle not smooth.

Photo 4 Shows 1 “ gain on vac gage ( now steady ) still popping a bit , still not smooth, 680 RPM

Photo 5 Shows Offenhauser exhaust smoke indicator after a 1 sec. blip on throttle off idle .

Photo 6. Shows fuel bowl level indicator, basically eliminating fuel pump pressure concerns.

Photo 7 Shows matt black painted ? Spark plugs sooty but not wet. ( usually after 3-4 min. Run time

A couple of other things, if I deliberately run it out of fuel the last 20 seconds before it conks out , the idle gets quite smooth, the revs increase slightly and the vac gage goes up about 2 “ . I have tried lifting 1 carb at a time , removing linkage and sliding a blanking shim under , still idles rich. Adjusting idle mix screws in or out doesn`t effect vac gage or sound much at all. Painted around carb. bases with oil looking for vac leak only made a mess. No fuel visible down throttle bores. Still runs rich with gravity feed , 20 “ head, fuel bowl level dropped to 5/8”

The only good thing to come of all this , I have fumigated most spiders and bugs in the workshop and my reproductive health is of no concern anymore . Any help would be appreciated.
 

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The Stromberg 97 carbs will flood if the fuel pressure is too high. Like anything over 3.0 psi. Having either float level set too high can be a cause. From the picture, the float level on the front carb looks okay. How about the rear carb. I had a float develop a leak and sink a bit. That let the fuel level get too high and caused exactly your problem.

Having either float valve leaking will cause the problem. Maybe a bit of crud on the float valve. To clear that, disconnect the fuel line at each carb and blast the valve with carb cleaner.
 
The rear carb checks out ok , I had managed to get fuel pressure down to approx. 3 PSI. but made up the indicaters as a means to confirm whats going on . Also mentioned gravity feed on the fuel , the problem still existed.
 

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Not being there makes it hard to diagnose. I can think of three things that might be at play here. First and for most. This is a 80 year old engine design, they are not efficient or clean burning by any standard. Second, what's you engine temp? Third, it sounds like your idling on your main circuit not the idle circuit. Try backing out all four of your idle mixture screws two turns. Start it and turn the idle stop screws out until it wants to die. Now your idle mixture screws will make a difference. One last thing, there is no load on this engine so it never gets a chance to clear its throat and bring the combustion chamber temp up. I've set up more multiple carb set ups then I can remember . I wish you were closer ,I'd come over and help. Tim
 
I may have missed it but I don’t know if you have air filters installed. There are some air filters on the market that simply cause a motor to run rich and there is nothing that can be done about it except to replace them.
 
Had the same problem with almost the same set-up. Two 97s on a high rise Tattersfield manifold. I lowered the fuel pressure to less than 2 psi, lowered the fuel level almost 1/4". All in the right direction but not enough, still rich smell and black plugs. So I took the idle jets out and soldered them closed and redrilled them - problem solved.
 
A couple of photos to clarify things

1 The handy/ dandy fuel pump pressure problem eliminator.

2 Air cleaner setup.

For Vincent, Idle jets ???
 

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I may only suggest that decreasing jet or orifice size without soldering or changing, I use different size wires . Usually I bend them in half and insert the bent portion . Experiment with different restrictions without any permanent modifications.
I will add , check your throttle plates for bends oor not completely closing. I agree to open your idle screws, at least with the primary carb , and close your throttle plates on all carbs .
 
I am trying to find this information online but so far, no luck. Do all the Stromberg’s (81, 97, 48, etc) use the same idle jet?
 
It is very common to solder and drill the jets in Zenith carbs as used on a Model A, there are even kits with the correct drill bits for the task. In addition, the jets should be flow tested because most of them do not actually flow what they should. Does any of this go on with Stromberg jets?
 
Have you inspected your idle adjustment screws to be sure they do not have any indentation or ridging in the cone shaped tip. If not perfect, replace them becaused damaged idle adjust screws can make it impossible to smoothly regulate fuel passage in that circuit.


If the idle adjust needles are good, I have a question: If you run the idle adjust screws all the way in/closed (just finger tight), can you get the engine to run at a slow idle (700 rpm , or under), or only at a higher rpm?
 
Seems like a lot of confusion here… the two screws being referred to as “Idle Adjustment” have no control over fuel delivery. They are air adjustments. Typically they should be 1.5 to 2 turns out from closed and are only effective at idle. They will affect idle smoothness which might resemble speeding up.
Best set using a vacuum gauge and adjusting to the highest and most steady needle reading. Typically 15” to 19” of vacuum while idling.

Idle speed is controlled by the screw on the bell crank where the accelerator rod connects to the carburetor.

Fuel control is by jetting and power valve.

Sounds like too much fuel pump pressure and or possibly the float valve not shutting off.
 
Regards post 17, new carbs but will check adjustment screws, the engine starts well, idles around 450 rpm or lower with adjustment screws seated , rich , popping slightly, not smooth

regards post 18, please refer post 3
 
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