Stromberg adjustment

[gary1725]

Hi Folks, I am fettling my engine after a complete rebuild and am setting up the Stromberg carbs (SV) and have followed the basic set up - checked float level, starting from jets flush , centered, have nice clink on dropping the piston, backed off 2 1/4 turns, still nice clink..

So how many more turns do people see before the mixture is correct? I seem to be backing them off a lot more to get the engine to "rise in speed and then level" when the pin is lifted!

The carbs were rebuilt with new spindles, butterfly's, jet's and metering needles and was running well, if a little tired, before the engine rebuild and have since been refreshed with new gaskets, float needles and diaphragms only because the rebuild has taken over 4 years and it seemed like a wise thing to do.

thanks
Gary

 

[RootesRacer]

Did the rebuilder change the needles or the main jets?
If the parts are correct, nominal adjustment position should be very close to the 2 1/4 turns rich.
Otherwise, you may have a leak somewhere you are trying to compensate for with the mixture screw.

 

[gary1725]

Thanks, everything in the carbs is stock from Sunbeam Specialties and I put it together myself. I will look for leaks and start again.. the engine was opened up a little per your original spec for pistons, also strocked a little.. should I have compensated for any of that with the carbs ?

 

[RootesRacer]

Thanks, everything in the carbs is stock from Sunbeam Specialties and I put it together myself. I will look for leaks and start again.. the engine was opened up a little per your original spec for pistons, also strocked a little.. should I have compensated for any of that with the carbs ?

Mild engine mods wont have a huge impact on the way the stroms work, the question though is that if you have the carbs reworked (new spindles ECT), did the rebuilder use your old needles and jets or supply their own?

If you have vacuum leaks, the jets will need to be lowered to provide more fuel to offset the added air (from the leaks). This could explain why the adjustment doesnt match the factory nominal position.

When you have a vacuum leak or if the throttle spindles are heavily worn, the carbs wont provide a functional idle and proper driving mixture at the same jet positions. This is the main reason for needing new spindles.

In the end though, if the same main adjustment position works well for idle and drives well (and produces a nice brown color on the plugs), it doesnt much matter what the position of the mains are becuase they seem to be working.

 

[hartmandm]

How do you check for vacuum leaks with the Strombergs?

Thanks,
Mike

 

[RootesRacer]

How do you check for vacuum leaks with the Strombergs?

Thanks,
Mike

If its not already, set the carbs to run slightly lean then spray carb cleaner (with the straw attached) judiciously around places where there could be leaks.
The intake can leak at the head, the carbs can leak at the intake, the PCV valve could be stuck open and pulling in excessive air from the crankcase and the spindles should again be tested for leakage.

A lean running engine will pick up some RPMs or at least smooth out when given extra fuel through leakages.
A rich running engine will usually drop a little RPM when exposed to even more fuel, but this depends on "how rich" and "how much" extra fuel its getting. That's to say its not very reliable.

The lean method is a far better indicator.

 

[Tom H]

Gary, I , and probably others would be interested to know about the mods you did to your engine. How did you achieve more stroke? Chevy rods? Custom pistons? Did you do a Vizard mod? I also sent you an e-mail

Tom

 

[oLD lIMEY]

Gary, I , and probably others would be interested to know about the mods you did to your engine. How did you achieve more stroke? Chevy rods? Custom pistons? Did you do a Vizard mod? I also sent you an e-mail

Tom

As far as I know you can only change the stroke by changing the crank throw, like a different crank.

 

[RootesRacer]

As far as I know you can only change the stroke by changing the crank throw, like a different crank.

Its much simpler then that. When you have your crank journals reground, they put the crank in a chuck offset to the journal centerlines. An offset of .050" will increase (or decrease) the stroke by .100".

Toms question probably relates to trying to understand the cleverness of dealing with the not so obvious problems you have to solve by offset grinding the crank which left on its own results in the pistons sticking out the bores.
Possible solutions are shorter rods or custom pistons with altered compression height.
Other not so obvious issues are where do you get your bearings now that you have shifted outside the available factory sizes with the large journal cuts.

 

[gary1725]

Hi Folks,
If I quote numbers they are from memory and so may be a little suspect! the principles however are correct!

I followed the well documented modification to use the Chevy Rods - they have a smaller big end journal allowing the crank to be offset ground thus adding to the stroke (my crank had already been reground 10 thou so I did not get everything I could)

The head received the following

Reworked for better flow and the compression area opened up from 39.2 to 42cc (I did provide the machine shop the Vizard details but I think he somewhat did his own thing – not to start a debate because I do not know enough but he said, that info was somewhat dated in its thinking! I have the flow numbers if anyone has Vizard numbers to compare!)
New valve guides machined to accept VW valve seals
Hardened valve seats and new valves – the inlets were custom made and slightly larger than stock
New valve springs from an H120 Rapier all balanced to the same seat pressure (90lb if I recall correctly)
Valve train and followers were sent to Delta cam who cleaned up the rocker faces and re-profiled the followers
Cam also received their KB grind

Block
All usual cleaning and testing, new bearings for the Cam
Bored to 3.248 (this number was given as the best to find “good ringsâ€)
Crank offset ground as mentioned
Crank journals opened up to accommodate the Chevy rods (this allowed for some wear to be removed and tolerances to be restored since the old rods are slightly narrower than the Chevy rods)
Flywheel received a new ring gear and was lightened by 1lb

To compensate for the head work I opted for custom pistons from Venola rather than using the possible modifications to other Rootes piston options – this approach allowed me to specifiy the position of the wrist pin to maintain compression height (Chevy rods are slightly longer than stock) and specify the piston Dish to set the compression I wanted

I left stock Carbs but fitted a tubular header from Sunbeam Specialties (I can share an ignorant story about assuming, and reworking and fitting, early headers only to find the Alternator will not go back on! I really got as far as bolting them up before noticing the front pipe was awfully close to the Alternator bracket )

I found reconditioned and weight matched 6 cylinder Chevy rods on eBay pretty easily

Overall the rods worked out 7.5 oz lighter per rod but the pistons only 1oz lighter.

So I figure I removed over 3lds from the rotating assembly.

I am still playing with setting things up so have not driven the car properly and so I have no idea what difference any of this made but since the engine was pretty tiered before I hope the rebuild alone will have helped!

The folks on this forum helped answer a ton of questioned, particularly RootsRacer and TomH who provided the XLS engine calculator that makes doing the math a relative breeze!

Without this support and the well documented procedure I would never have undertaken this work!

Thanks

Gary

 

[Tom H]

Gary, thanks for posting this. Great to see you did just about all the mods I did and more. Interesting comments from your machine shop about the Vizard mod. I hope someone can look at your flow number and compare. That's beyond my expertise. Do you know how much stroke you added? You mention that the previous 10 thou regrind of the crank as limiting what you could do. Not really. Going from Alpine rods to Chevy rods requires narrowing the crank journals from 2.375" to 2.000" dia. That means you could (theoretically) shift the centers by .187" , adding .375" stroke. so having a 10 thou reground would have no effect on actually adding 0.100" stroke, like I did, or even 0.120 or more. I could never figure out what maximum made sense, but thought 0.100" was significant. It might be neat to get all your details along with mine and maybe publish here a drawing or specs for the pistons to help others accomplish the same result.

Rootes, I think custom pistons is a simple solution. Note that Gary is at least the second guy (after me) that followed your idea to go with a 3.248" bore to match common 82.5 mm rings.

Old Limey, fairly simple conversion using Chevy Rods and regrinding the stock crank.

Tom

 

[RootesRacer]

Gary, thanks for posting this. Great to see you did just about all the mods I did and more. Interesting comments from your machine shop about the Vizard mod. I hope someone can look at your flow number and compare. That's beyond my expertise. Do you know how much stroke you added? You mention that the previous 10 thou regrind of the crank as limiting what you could do. Not really. Going from Alpine rods to Chevy rods requires narrowing the crank journals from 2.375" to 2.000" dia. That means you could (theoretically) shift the centers by .187" , adding .375" stroke. so having a 10 thou reground would have no effect on actually adding 0.100" stroke, like I did, or even 0.120 or more. I could never figure out what maximum made sense, but thought 0.100" was significant. It might be neat to get all your details along with mine and maybe publish here a drawing or specs for the pistons to help others accomplish the same result.

Rootes, I think custom pistons is a simple solution. Note that Gary is at least the second guy (after me) that followed your idea to go with a 3.248" bore to match common 82.5 mm rings.

Old Limey, fairly simple conversion using Chevy Rods and regrinding the stock crank.

Tom

Stock SV rod journals are 2.125" std (not 2.375"). You can therefore get a .250 stroke increase by going to std SBC bearings, and bit more with undersized SBC bearings.
The mains are 2.375" BTW but you cant offset grind them (and expect the crank to still turn )

 

[Tom H]

Duh! I knew that seemed odd. But I think my point remains, that a 10 thou earlier grind would not prevent any "reasonable" added stroke. Or maybe Gary did add close to 0.250 stroke??!! Hmmmm. Maybe he did go with much larger stroke increase. As I said I did not go with the max increase the rods and regrind would allow, as not wanting to go too far into an "unknown" area with my limited understanding of any issues involved. I do have an understanding that increased stroke increases piston velocity and may limit max revs.

In fact Jan de-stroked his Tedder racing engine by 0.150" specifically to limit piston velocity above 6500 RPM.

Tom

 

[RootesRacer]

Duh! I knew that seemed odd. But I think my point remains, that a 10 thou earlier grind would not prevent any "reasonable" added stroke. Or maybe Gary did add close to 0.250 stroke??!! Hmmmm. Maybe he did go with much larger stroke increase. As I said I did not go with the max increase the rods and regrind would allow, as not wanting to go too far into an "unknown" area with my limited understanding of any issues involved. I do have an understanding that increased stroke increases piston velocity and may limit max revs.

In fact Jan de-stroked his Tedder racing engine by 0.150" specifically to limit piston velocity above 6500 RPM.

Tom

Piston velocity can become a concern but not at the piston speeds most of us are seeing (whats .25 inch in 3.25 gonna get you on a 6 grand motor, 500 added rpm?). IIRC the piston forces on the rods goes up by the square of the strokes radius and when you consider our little ports and tiny valves dont really like high RPMs without a lot of work, destroking an engine can make sense in some cases.