Solex Carb Tuning

[pls01]

I am having trouble tuning the Solexs on the Bristol engine.  The engine won't run smoothly at an idle.  It randomly spits back when crusing at steady throttle.  It accelerates well but deceleration has occasional small pops from the exhaust.
   I syncronized the carbs using a Syncrotune tool (measures air flow through each carb).  The volume control screw is a half a turn.  Main jet was upped from 125 to 130, but all other jets are standard size.
   Plugs look good (slightly dark brown).  Compression is good.  Ignition points are new and set.  Valves are all set correctly.
   
   Any suggestions?
   
   Peter (BEX 375)

[BE774]

Peter, I had a period of rough running, especially at idle, last year and the problem was a needle valve in one carburettor float chamber intermittently sticking open. Also, in my limited experience, some exhaust popping on the overrun is normal, caused by the camshaft profile having a lot of overlap allowing some unburnt fuel to escape to the exhaust.
   
   But no doubt there are more experienced readers out there who can put us both right!
   
   Best wishes, Graham.

[pls01]

Graham,
   Thanks for the tip.  I had that happen on the #1 carb last year, clue was gas pouring out!  Maybe I try replacing all the float valves.
   I opened the volume control screws to 1-1/2 turns and upped the main jet to 140.  It runs smoother, but still spits occasionally.
   Plugs are very dark.  Maybe a hotter heat range is required.  Current plugs are NGK C6HSA.  I could go to C5HSA
   Idle won't go below 1000 rpm.  The #3 carb idle screw is back all the way out.
   
   Thanks for the input
   Peter Stanwicks

[pls01]

Here's an update on the carb tuning.
   
   I went as high as 150 on the main jets but the plugs were really black and the engine didn't pull very well.
   
   Went back to 140 and the spitting went away but there was surging at small throttle openings.  The car would not run smoothly cruising at 2000-2500 rpm (about 50 mph).
   
   For this condition, the Bristol shop manual suggests leaning out #1 and #3 carbs while keeping #2 slightly rich.  This tuning technique doesn't make sense to me, but I gave it a try.
   
   I set #1 and #3 with 130 main jet and the VCS at 1 turn.  The #2 carb got a 135 main and 1-1/2 turns on the VCS.  I only got a short drive before thunder storms moved in, but the acceleration was good, it cruised well, just a couple of spits and plugs are light brown.  I'll give it a longer test later in the week.
   
   Anyone know why this technique should work?  Anyone else use it?

[Bernie]

Peter, I've been the same situation as you for over a year. My copy of Road&Track's 1957 ACE Britol road test mentions the same spittback as being somewhat normal. The biggest improvement I found was converting the ign points to electronic (Pertronix), as my distributor shaft was approx .005" sloppy. Thats half of the point gap setting. A noticeable improvement was immediate. I'm running the following jet sizes:  Main is #130, Pilot is #50, Air Corr # 180, and I've had to use two fiber sealing washers under the needle valve seats to get the float levels where I want them. It normally idles at 700-800 at full ign advance, and just over 500 in full retard,BUT its not passenger car smooth and thats the wild camshaft profile. My engine starts immediately either hot or cold, spark plug color is normal.
   
   Regards, Bernie Havel
   BEX494     864D2

[pls01]

Bernie,
   Thanks for the information on engine settings.  Here's where I'm at now and it's getting very close.
   
   The engine is freshly rebuilt with 9.5:1 compression ratio.  It has new pistons, rings, liners, valves, seats and bearings.  The cam is a 1962 vintage grind from Isky with 0.236" lift and 230° duration @ 0.050 which is slightly more radical than the Bristol Sport cam.  It's timed at 108° which is slightly more advanced.
   
   I set the ignition timing dynamically using a timing light at maximum advance.  I run the engine up until full centrifugal advance.  The timing is 33-35 BTDC at this point.  I degreed the flywheel when the engine was out.  Counting ring gear teeth could do the same.  This is more accurate and repeatable than idle or static timing since all backlash and free play is removed.
   
   Some years ago, the timing was jumping all around.  As you found, the distributor was worn.  It was rebuilt and now timing is perfectly stable.  This made a world of difference in idle quality and running smoothness.  I have a Pertronix on order just for better spark.
   
   The manual recommendations didn't work very well and they are now close to your setting.  Enriching the middle carb didn't help and they are now all the same again.  The settings are Main #130, Air correction #190, Pilot Air #55, VCS 1-1/4 turn, Part Throttle enrichment rod last hole at end. It idles and runs very well, but still spits occasionally.
   
   I'm going to try the pilot air at #50, but wait on the Air Correction since that corrects for over rich condition at large throttle openings.
   
   Thanks again for all the information
   Peter Stanwicks
   BEX 375 / 100D719

[Bernie]

To; Peter Stanwick; I read your reply, glad to see you have the improvements you want. I've got another thought, and its totally by accident. I rebuilt the diaphram fuel pump last year, and its been fine,BUT, I decided to switch over to an SU electric pump(MGB style) just so I wouldnt have to hand prime. This change has nearly eliminated any engine "spitting", even at cold temp. Its not running rich, and hot starts are immediate. I'm not going to reinstall the old pump just to do a pressure or capacity test, but I have an inkling that the diaphram pump design is probably marginal at best. Yup, I had to reverse the diode's leads in the pump so it runs on + gnd.  Food for thought,,,,,,,,,,,,,,
   
   Bernie Havel
   BEX494  864D2

[pls01]

Bernie
   
   Thanks for the info on the fuel pump.  I have a vintage AC (as in AC Delco) pump in the trunk.  It was installed during the car's racing days.  It's wired to a switch on the dash.  I don't use it much because it tends to over pressure the carbs and blow the float needles open.  I use it mostly just for priming.
   
   During the recent installation of the Pertronix, I found another problem.  The centrifugal advance in the distributor was not working.  It was always fully advanced.  I got some springs.  It now idles great, the low to mid range torque is much better.  I haven't gone beyond 3500 rpm because the engine only has 400 miles.
   
   The spitting is almost gone.  It'll go about 20 miles without a "phspit".  I'll start another thread on the centrifugal advance so it'll have the right heading
   
   Thanks again
   Peter Stanwicks
   BEX 375  100D719

[administrator]

Keith Lessiter has kindly dug out some period instuctions on Solex tuning.  I will scan these and put them up on the Members' Section - time permitting, at the next update.

[Bernie]

Peter, Did you ever install the Pertronix ign?, it made quite an improvement in controlling the timing(no spitting). Check the fuel pump pressure, although my diaphram pump was nearly new, it wouldnt stay up with the demand, the engine would tend to run lean at steady speed. I solved this with an SU electric.

[Bernie]

Peter, After rereading your posting(my brain is in the retirement mode) there's another thought. Your volume control screws should be out somewhere around 1-1 1/4 turns, yours sound very lean at idle. This shouldnt affect the midrange running at all. Are you sure the carbs are balenced, ie, all butterfly's nearly the same, are the springs onthe 3  throttle operating rods set to the same tension/compression when opening the throttles, if not, the carbs will not stay open the way you set them at idle especially at highway speeds. This is rather tough to do, its by feel and checking the length of each spring with vernier calipers-pretty subjective guesswork.

[Bernie]

Peter, Sorry, I missed that you already installed the electronic ign, I just reread ALL the postings, I must be getting brain dead since retirement. Think I'll go out to the garage and chain whip my MGA for punishment.

[pls01]

Bernie,
   No need to kick the MG.  Thanks for all the help and suggestions.
   
   For a long time, I thought this was an old unruly ex-race car prone to rough idle, low and mid range flat spots, loved the upper rev's and just generally temperamental.
   
   Now, the spitting is gone with carb tuning, electronic ignition and working centrifugal advance.  It idles smoothly and quietly, pulls like a freight train and is a general joy to drive.  I never drove a new one, but I now know what they were really like.  I wrote down all the setting and data.  I can check the settings if it ever runs poorly.
   
   If you are ever in the Connecticut area look me up
   Thanks
   Peter
   BEX 375 100D719

[Bernie]

Peter, Nice to here yours is finally running properly.Tell me what jet sizes you ended up with, I'm just comparing notes here. Where did you get the various jets you used?, my supplier guy has retired and closed up shop here in Detroit, he was tired of rebuilding carbs for 35 years and went to Florida. Yup, they'll pull like crazy once you stuff the throttle into the kitchen and give that long stroke bastard some rpm's. I dont drive mine hard, but I think its going to get a new clutch this winter, its the original disc.  Regards, Bernie

[pls01]

Bernie
   
   I've have the following build and set-up for this engine
   
   Pistons – 9.5 Compression ratio, two compression rings, one 4-piece oil control
   
   Cam – Iskiderian 1962 vintage grind B-65-HR
      0.236" lift
      230 degrees duration at 0.050" lift
      108 ATDC timing
   
   Valve Springs – Custom
            72 lbs seating load
            140 lbs valve open load
   
   Valves – Standard Bristol 100D
   
   Distributor – Lucas DXH6A with Pertronix LU-168P12 electronic ignition
         Mechanical centrifugal advance
         ~8 degrees advance at 850 rpm
         ~38 degrees full advance
   
   Carburetors – Solex 32PBI-6
         130 main jet
         190 air correction
         55 pilot air
         1-1/4 turn Volume control screw
   
   The engine runs great, but it is still in the break-in period.  It has not been run over 3200 rpm so tuning is not quite complete.  I don't expect the carb or timing setting to change much.
   
   Most of my jets are from the big pile of spares that came with the car.  My father raced in the late 50's and early 60's.  The Ace was raced hard and this is the third rebuild in only 23,000 miles.  There were many internal non-standard modifications.  I went back to more standard parts because a fast road engine was the goal.
   
   If you are looking for jets, the first person to call is Ron Leonard.  Great to work with and usually has the part or can point you in the right direction.
   
   Peter
   BEX 375   100D719