Setting up Solex 35 PDSITs

[Under Construction]

Installing the Scat Solex 35PDSIT dual 1 bbl carb kit

I wrote this page up as help to those installing the kit.  The things I figured out about installing it my help others. 

Note:  There are a lot of problems with this kit, in terms of fitment.  Not just one, but several.  If those problems do not exist, it's a solid carb setup.

I started out with a set of Weber 36 IDFs and wanted to get rid of the carburetors I had, so I purchased a set of Scat Solex 35PDSIT carbs, a product sold by Scat and its resellers.


First things first.  Disconnecting the linkage from the carburetor throttle arms, disconnecting the fuel line, and removing the manifold nuts allows me to pull out the carburetor, intake, and air cleaner all at once.


Picture of the 3/4 side with the carburetor and manifold removed.  I took the opportunity to change the spark plugs while I was at it.


This is the 1/2 side with the carb and manifold removed as well.


At this point, I moved my coil from the center mount spot back to the shroud, and started to install the centerpull linkage base.  In order to do it, you either need a longer stud, or you have to use the supplied bolt.  Here you can see me removing the stud from the case, preparing to install the base and the bolt. I just double nutted the stud and pulled it out.


Now, the base is supposed to sit against the case stud boss, and if you look at the bolt they supplied, it's too long, so I will have to add a washer to it.  I happen to have a few thick head stud washers around, so I used one to space the bolt a little.  Depending on your engine case, this may not be an issue, but watch for it.


The base is supposed to be secured using the bolt AND this bracket which goes to the lower left alternator backing plate hole.  Since the bracket wouldn't line up correctly with the base right against the case boss, I moved my extra washer in between the base and the case stud boss.  Now the bracket lines up with the hole pretty well, BUT to get the base to be correct and vertical the bracket sits away from the backing plate.  As you can see from my wrench slipped behind the bracket, there is substantial space to be taken up.  The use of a 13/8mm nut between the backing plate and the bracket got things very close to vertical.  Again, this may depend on your application and is an easy fix.


After all parts were in place, I secured the bracket using the supplied allen head bolt (fortunately it was long enough) to the backing plate, and then tightened up the bolts.  It came out pretty straight.


After installing the center base, I installed the carburetors to the intakes and then installed the 1/2 side first.  It's easier if you install the carbs to the intakes, and then the entire assembly to the engine.


Using a universal and an 11mm socket, I tightened the manifold down.  Note that Scat supplied steel 11mm nuts but I like brass ones better.  They don't freeze on or rust.  There were some linkage interference issues that took me almost an hour to work out with some handyness built into the Harney blood.


Now to the right side.  The carburetor wanted to foul the shroud.  After some struggle, I gave up on clearing the shroud for it, and had to do some machine work to get it to clear.  Apparently Scat has no intention of having you install this kit on an OEM shroud!  Know why?  They don't sell OEMs, they sell their cheap ass 36 HP shroud which is no real problem.  Their solution would be to replace your CORRECT shroud with a Scat 36 HP shroud.

Now it clears, but another hour wasted.  This is taking me a LOT more time that it should have.  I should have been done by now.


With the carburetors installed I finished installing the linkage, and added a return spring by using an old spring from a stock PICT carburetor and drilling a 1/8" hole in the alternator ring to attach it.  It's a good thing I had something, because guess what?  Its not included in the kit.  They assume you won't need any additional return springs.  Silly them.  This one's strong enough to make sure the linkage returns without requiring He-Man to press the gas pedal.


When installing the linkage, the 1/2 rod wanted to foul on the bottom of the generator strap.  Go figure.  Keeps getting better with these guys.  No big deal though.  Intially I had the supplied washer for the linkage hinge stud between the linkage and the base.  I moved it above the linkage and you can see it on the other picture above now between the nylock nut and the linkage center (positioned just above that return spring I added). That gave me a gnat's ass worth of space between the linkage rod and the generator strap.  Any more and I would have had to either modify the base, to lower it, or mill a little off the linkage center piece to drop it down some.  Whew.

In retrospect, this might have been my fault.  But it's not like the instructions were clear or anything.


After moving my coil back into place, reconnecting all my wiring, and making some other little changes, including adding a pressure regulator, I have it all together, and ready to start up.

Note on regulators: See the one I have on there (dial type)?  That is the perfect example of what NOT to get.  But you should use one with any electric pump and these carbs.  They do not like too much pressure.

With a mechanical fuel pump, you can just buy gaskets and stack them as spacers until you get the right pressure.  No need to use a regulator on a mechanical fuel pump.



Well, now.. here we are.. all bolted up and ready to go.  I even have a cheap little pressure regulator.  Yeah, yeah, I know.. I replaced it with a good one later.  It worked for the time being... well, sort of.  They function more like restrictors than regulators, which in the full range of operation renders them pretty worthless.

These carbs need a low fuel pressure.. about 2 lbs is recommended.  I had previously bought a Carter rotary pump that regulates at 3.5 lbs of pressure, but that's too much.  At 2 lbs, I was overflowing fuel like mad.  I ended at about 1 psi, with no starvation problems, AFTER I did some mods to the carbs to get the floats to stop sticking!  Sheesh.

Note: I've been asked on a few occasions to provide instructions on tuning these carbs, and really it's no different from tuning IDFs or other dual carbs, except you only have one barrel per side to deal with.  The use of a balance tube will require that you pinch off the tube to set the flows. The idle speed will rise, but you just need to even the flows and then release the tube, set the mixtures, and then pinch the tube to re-check the flows and raise/drop them if necessary to get the speed about right when you release the tube. The flows must be set with a good flow meter like a snail synchrometer.  They MUST be equal, and around 7-9 KG/hr is about the norm, per side.  If you don't have a way to balance the flows, you will not get it right, and you will be frustrated.  Get the snail, pinch the tube if you have one installed, set the flows, release the tube, and then set the mixtures to lean best idle.  Then go back to the flows, and start over and you will be close to where you need it after tuning the mixtures for the last time.

Once those are all set, then adjust your linkage when the engine is warm to get it to fit the throttle arms as they are positioned, and so that they actuate both arms at the same time.  This should ensure smooth throttle action, and the balance tube setup can help the lower RPM ranges quite a bit.. more on that down the article.


Initial tuning

First things first.  We must pre-set the carbs.  Now I got these carbs out of the box and saw that the idle speed screws had paint on them, and assumed that meant someone had pre-set them to something acceptable.  So happily I set the idle mix screws to the recommended 1-1/4 turns out, set the pressure regulator, turned on the ignition to let the pump do its job (nice thing about electric pumps.. they fill the bowls for you).  I noticed that at 2 lbs setting on the regulator, the fuel wasn't filling the carbs up, and I couldn't get any squirt from the accelerator pumps.  So I turned the regulator up another "half pound".  That did it and I could now after a few pumps get some gas out of the accelerator pump nozzles.  With the linkage set up and all else ready, I cranked it and it started right away, no problem.  It ran rough, as expected, but I got it warmed up and it smoothed out a little once the circuits were fully charged with fuel. 

After warmup, I started to try to adjust things out.  The idle was high, like 1200 RPM, and the mixture screws needed to be turned a lot to get any kind of response either way.  This seemed wierd to me, but I didn't pursue it too much.  I just tuned it close and started out with high idle.  There should be NO vacuum advance at idle, but the vacuum line was pulling a vacuum even at idle, so rather than mess with that for now, I wanted to drive it, so I disconnected the vacuum line and took it for a drive.

The response was a little weak with the vacuum line disconnected, and my timing was set at about 8 and 28 or so with my old SVDA.  Cool for now.  Around the block I went.  I got about half way around the block and the right side carb went dry.  I could see the head temps going up as the left side started to labor to keep the engine pulling the bus, and so I limped it home, rapidly moving to about 350 degrees.  I pulled up in the drive and figured that crappy regulator was causing the trouble.  After turning the knob another "half pound" (LOL) that was solved and no spillover at idle.  I'm thinking a new regulator (a REAL one) is in order quickly.

Around the block I went again, again waving at all the neighbors that see me do this sometimes, and watched my air fuel meter show about 11-12:1 AFR, which is WAY rich.  So now I have got 55 idle jets coming.  They should get me closer to 13-14:1.

The mains are still a mystery, and will be, until I get the idle circuit finished, so I know exactly what I am looking at for transition and such.


Vacuum Advance

Now it was time to tackle the vacuum signal.  There should be no vacuum in the vacuum advance line at idle.  I figured that maybe these advance ports were a little aggressive, and that maybe they were designed to run a distributor with a vacuum can and no advance.  I called Scat (pity you that have already tried this) and got the secretary, then Ron, then Tommy, who was apparently the guy in the barrel today, who never answered his phone and I got dumped to voice mail. I left a message in hopes of getting a call back.  Nothing like that happened, so I played a little more and decided I would try to get the throttle plates closed as much as possible and see what happened.

Remember the part where I said that the white paint on the idle speed screws made me think that they were pre-set?  Well, I got news for you.  They weren't even CLOSE.  I ended up backing the ALL the way out until the screws no longer touched the stops, and going back in about a turn.  By the way, for any of you with a full OEM doghouse shroud, another annoyance about these carbs is that you cannot adjust the idle speed screw on the 1/2 side unless you open the throttle about 1/3 and use a screwdriver (a short one) to adjust it, so forget doing it while the engine is running.  No big deal.  Once I got it set for about a turn in or so on each side, I started it and it was RICH as hell.  Good sign.  So I readjusted the mixture screws which were MUCH more responsive now, and got my idle AFR about 13, and my idle set to about 800 RPM.  Lo and behold, there was no vacuum signal at idle.  Yay.  At some point during all this mess, actually first thing this morning, I installed my new SVDA distributor from after moving my Compufire over to it, and got it timed and such, to about 10 degrees at idle, and 30 degrees full advance.  Now with my vacuum advance connected, the idle timing stayed at 10, which again is a good sign.

So, with vacuum advance set to initial settings, I took it for a test drive and WOW what a difference, like night and day.  Lots of power and lots of response.  These carbs really lend themselves to vacuum advance.  Both have ports, but I only have one of them connected and it is killer.

To re-cap: out of the box, they were adjusted too high on the idle screws.  The way they were set up, they were into the progression ports, and if you look at these carbs, they have a couple vacuum ports too, so that also figured into the problem.  Backing them out until I got a decently low idle was the solution.



The funky thing about using these carbs on these engines is that the engine idles on only two cylinders, for the most part.  These engines fire sequentially on each bank (2, then 1, and then 4, then 3).  While I have talked to people about this the only thing I have really gleaned is that it's normal for this to happen, but exactly WHAT is happening is still a tough one for me.  There's no real solution to it from a tuning standpoint, so it is what it is.. but it's funny to watch my #1 and #3 cylinders drop to about 200 degrees at idle.  This requires more thought, in the direction of vacuum pulses and fuel delay, for me to reconcile it completely.  I mean, both are getting a vacuum signal, and the only thing I can come up with that makes sense is a situation where, say, #4 pulls, and gets fuel, and the flux in air speed at the shared intake plenum causes a delay of fuel, and the fuel makes it out to the intake, but gets pulled by #4 the next time.  By the time #3 pulls air, #4 has already gotten the fuel charge, and all that is left is a vacuum while the carburetors idle circuit tries to catch up and shoot fuel to the intake.

But it doesn't matter, because without a fancy solution, this can't really be fixed.  That's the way it is.  For any of you that have this kind of setup (dual 1bbl carbs) try pulling your #1 or #3 plug wire and your idle will stay the same.  Now pull a #2 or #4 and you will notice a BIG change.  That'll prove it to you.  Adding a balance tube will help, but it won't completely solve it unless you use a big one, and that has some problems associated with it as well.

But beyond idle, once you give it a little gas, it cleans up nicely, and takes off.  So with that in mind, idle is no big deal.  Consider it an economy boost at idle.



The kit as Scat supplies it can be a giant pain in the ass.  It wouldn't be so bad if:

1. they returned calls
2. they offered what they said they did in terms of vents and such
3. they didn't jet the carbs for Death Valley!
4. they gave good instructions, instead of "how great these are"

The technology used is great.  These are quite linearized carbs, and tune EASILY.  The linkage leaves some to be desired.  Push/Pull linkage is subject to synch variations due to expansion.  This could be improved on and I intend to do just that and update the article when I do.  I'm a linkage nazi.  Most linkage needs some improvement IMO.


The good news:

AIRCOOLED.NET offers their own version of the kit from Scat.  They make all the modifications to make it fit correctly, now that they know what kind of things are problematic.  They jet for you too.  If you look at their product listing, you can get them jetted the way you need it.  They have an excellent jetting program and will make sure you have the right ones.  What took me 5 freaking hours to do should have taken a LOT less, like maybe two hours.'s kit should be a much easier kit to deal with and issues (if any) should be minimal.  You will have to adjust them still, as they cannot do it for you.  Each engine requires its own adjustment, so you will have to set the throttle positions, synch the carbs up, adjust the idle mixtures, and then adjust the linkage, no matter what.  In fact, you will have to run through it a couple times no matter where you get them.  NO ONE can do it until it goes on your engine.  The jetting you get from will be right, you will just have to do the tweaks when you bolt them on.  In the past, I've seen that also takes customer feedback into consideration, which you won't find to be true with about anyone else.  So given the history of these kits, they've done their homework on the problems with them.  In other words, when they when ACN finds out about a problem previously unknown, they do what they can to reconcile it for future customers.

I hope this article helps you - it sure made ME feel better.  What could have been a great kit requires a lot of engineering and at least one company now does something about it.


Updates (in order)

* I got the 55 idles installed and it runs better, and gets better air-fuel numbers.  The numbers are closer to 14:1 AFR until you give it any amount of gas, and under load, it's rich.  The 150 mains are too big.

* I got some 140 and 145 mains and installed the 140s, which were a little leaner, showing me the upper end of the 14's range on the main circuit.  I got the 145's in there and it was at 13.5 most of the time.   A 147-ish would have been the option for power, but the limitation here is really the vents.

* The supplied 27mm vents are a little small for my engine.  They have now been replaced with some billet 30mm vents from  You can get them here.  The vents made a big difference in the performance of these carbs, and even with jetting adjustments (went to 160 mains) to get the same AFRs, I noticed a drop in cylinder head temps!  The head temps under load dropped almost 25 degrees across the board!

* Jetting - with the 30 vents, so far what's worked out best for me is 55 idle, 157 main (made some.. they're not available in that size), and 110 air jets.  These air jets work differently from IDF carbs, so don't go over there and read that and think you are going to tune these with the same thinking.  Also, accelerator pumps have been backed WAY off.  They squirt tons of gas out the way they come out of the box.  I have watched my air-fuel meter jump down to the 10's for a second or longer with them adjusted the way they come.  Sheesh.

* I installed a balance tube.  It did make some difference, less rough idle and low speed operation.  Mileage went up too. 

* Current mileage as of 9/3/06 - about 25 mpg!

* A note about air-fuel ratios.. I got away from these carbs and sold them before I got into lean AFR tuning.  I can tell you that tuning on the leaner side of stoich (higher than 14.7) can have benefits in temp and economy if you can tune it right.  Don't assume that you can't go leaner than 14's AFR.  You can tune into the 15's and 16's.  It's a matter of getting it to remain in those numbers and not spike leaner much.  I run 15's and 16's consistently on my bus with other carbs, and I get great mileage and low head temps.

02/??/07 The carbs have been sold off with all the extras.  Here are additional pictures of how I installed the balance tubes.




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Last modified: 12/27/07