Stomberg mixture puzzle

[steve lyle]

A couple of years ago I had the Strombergs on my '72 Sprint rebuilt by Joe Curto to home market/Euro specs, since all of the Federal emissions gear had already been removed by the PO.

That, along with recurving the distributor to Euro specs transformed the car. Very quick, no flat spots. Joyful.

But the plugs were dark. And lots of smoke on acceleration. After about a year, I utilized Joe's needle exchange to go to leaner needles.

I set those at the leanest setting. Car still ran fine. But the plugs are still dark -see pics. And still smokes when accelerating.

So in preparation to call Joe for even leaner needles, I went through the Stromberg mixture setting process.

When I lift the pistons the 1/32" inch (or so), the idle goes from 900 to 1000, then down to 850 or so, very slowly dropping from there.

Which is my understanding of what should happen with a mixture that's slightly lean, not rich.

So if my mixture is lean, why are my plugs black?

Signed,
Confused in Tulsa.

[sprintsoft]

Hi Steve, are you confident you are not burning oil? That can also blacken your plugs.

A leak down test is probably the best way to check the state of your rings and piston/bore clearances if you are not sure.

Iain

[steve lyle]

Hi Steve, are you confident you are not burning oil? That can also blacken your plugs.

A leak down test is probably the best way to check the state of your rings and piston/bore clearances if you are not sure.

Iain

"Confident" may be too strong a word. But the compression is good, power is good, the smoke isn't blue, which I've always hears was a sign of oil burning, and the plugs aren't wet, or even fouled, which again is what I've heard for oil issues. I suppose oil could be coming through the valve guides, but it's a little hard to believe it would be that much.

That said, the engine does lose some oil, but I haven't measured the rate - I'll start a record to monitor that more closely.

Thanks for the thought.
Steve

[sprintsoft]

Steve, Ok, yes the black smoke normally indicates a rich mixture. I wonder if this is only a problem when you accelerate, there is no accelerator pump on the Stromberg so that’s not the issue.

The oil damper on the piston governs the mixture during acceleration so you can play with the oil grade you are using in there to see if any difference, also as you are swapping needles check with Joe he has given you the right springs.

Iain

[69S4]

Your oil damper O rings are not leaking are they? There's not much oil in there but you could end up oil burning without it being engine oil. Easy to check though - fill them up, go for a drive and see if its gone at the end. If the carbs have been professionally rebuilt it's unlikely the choke mechanisms are incorrectly assembled which could cause rich running.

If you're producing enough smoke that you can see it in your mirrors it's unlikely to be a 'normal' rich Stromberg mixture - in my experience anyway. I've had mine so rich the car needed no choke when cold and spluttering / won't hot start when warmed up and it didn't produce significant visible smoke. At idle on a still air day I could see it but not out on the road.

[elangtv2000]

Your plugs indicate an overly rich mixture. Plugs can be wet from fuel as well. Blue smoke = oil, black smoke = gas, white smoke = water. Oil and excess fuel smoke will generally look more black. Can be useful to have someone else drive while you follow behind, or sit in the passenger seat and look back at the exhaust.

If you get lots of smoke, mostly blue, that could be worn valve guides. Check for vacuum leaks at the intake manifold/intermediate manifold - that could create lazy idle.

I use B1BT needles for my ~138bhp motor and mixture seems great across the powerband. My motor doesn't smoke unless I'm really gunning it up a steep hill, and then only slightly with oil smoke.

Damper oil can affect transition from idle to WOT. Try 30wt., 10-40wt., and 20-50wt. and see if you notice a difference. Thoroughly clean your plugs after each run and check. Even an Italian tuneup won't clean up fouled plugs without really extended high-speed runs.

[RogerFrench]

I don't see anyone asking the most obvious question - what plugs are you using? Maybe you just need a hotter plug?

[elangtv2000]

They look like BPR6ES, which should be a good starting point unless he's got pretty high compression, etc., no?

[RogerFrench]

They look like BPR6ES, which should be a good starting point unless he's got pretty high compression, etc., no?

Yes, indeed. I hadn't looked hard on my phone, sorry.

[derek uk]

What sort of jet adjustment do you have on your carbs? Emissions Stroms aren't easy to adjust but you can get a kit which converts them to outside adjustable jets.

[steve lyle]

They look like BPR6ES, which should be a good starting point unless he's got pretty high compression, etc., no?

Wow, good eyes! Yes, they are NGKs, BPR6ES.

What sort of jet adjustment do you have on your carbs? Emissions Stroms aren't easy to adjust but you can get a kit which converts them to outside adjustable jets.

Not sure what the different kinds are, but to adjust I remove the piston dampers, and insert a special tool that prevents the diaphragm from rotating while you turn an allen key to adjust the needle. About 2 turns total range. Left is lean, right is rich. With carbs disconnected, balanced, and idle set, lift the piston 1/32" (ago, lift it enough but not too much, and hold it there), if revs rise and keep rising, too rich. If revs rise and then drop and engine dies or wants to, too lean. If revs rise and then return to normal or close, just right.

Re: smoke - I honestly haven't noticed any when driving, but then I haven't focused there either. When I've noticed is when revving the engine by hand while doing carb adjustments. I'll take a look next time I'm out.

At this point I'm thinking it's oil. I'll monitor that more closely, and also fuel mileage, for the next few hundred miles and see what that tells me. Will report back.

I'll also call Joe and get his thoughts.

[steve lyle]

Your oil damper O rings are not leaking are they? There's not much oil in there but you could end up oil burning without it being engine oil. Easy to check though - fill them up, go for a drive and see if its gone at the end. If the carbs have been professionally rebuilt it's unlikely the choke mechanisms are incorrectly assembled which could cause rich running.

If you're producing enough smoke that you can see it in your mirrors it's unlikely to be a 'normal' rich Stromberg mixture - in my experience anyway. I've had mine so rich the car needed no choke when cold and spluttering / won't hot start when warmed up and it didn't produce significant visible smoke. At idle on a still air day I could see it but not out on the road.

Dampers appear to be holding oil.

Generally I don't need to choke when starting cold, and it can be tough to start when hot, which supports the "rich mixture" theory. Not that I understand why a rich mixture would make it hard to start when hot.

[SENC]

If you can't get enough needle adjustment (it is limited), you may need to reseat your jets (they aren't "adjustable" but they can be adjusted manually). How far below flush with the bottom of the bowl is the top of the jet? Deeper is richer for a given needle, shallower is leaner. See the attached Lotus Service Bulletin on the topic.

[steve lyle]

If you can't get enough needle adjustment (it is limited), you may need to reseat your jets (they aren't "adjustable" but they can be adjusted manually). How far below flush with the bottom of the bowl is the top of the jet? Deeper is richer for a given needle, shallower is leaner. See the attached Lotus Service Bulletin on the topic.

Thanks, Senc, I'll take a look.

I went for a drive today, did a couple of drag strip starts and looked back through the rear view mirror - couldn't see any smoke. So while it's noticeable in the garage, it isn't from the driver's perspective, fwiw.

[69S4]

Never seen that service bulletin before. Wish I had the info it gives when I was rebuilding my carbs some years back as the jet height info was something I had to estimate based on the old ones. Anyone got one of those tools that sit in the jet and you bash with the drift? If you have you wouldn’t care to measure it and post the dimensions so I can try to make one (I’m assuming they’re not commercially available).

Re hot starting, it’s flooding with excess fuel, a bit like trying to start with the choke pulled out.