SU Carb Throttle Plate Early vs. Late

First Published Oct 2017, R. Kwas 

Throttle Overrun Valve 


This tech note is intended to make rebuilders of the SU Carbs aware of a subtle difference which exists in the Throttle Plates between Early and Late production HS6 carbs fitted to Volvo vehicles.  It's not an issue if the same parts are being refitted at rebuild time...but when replacing or changing Throttle Plates, extra care is called for...Beware:  All SU Throttle Plates are NOT the same!....and if one is not looking for the differences, they can easily be missed, which can (no...WILL) lead to adjustment and running problems later on, when carbs are back on the engine.  The overall shape is different, and edge of the Throttle Plates are cut at different angles as a result, so it's important that likes only be replaced with likes!

The author was first made aware of this issue by a thread in the Volvoniacs Forum, and when consulting SU specialist Tom Bryant about it, who  pointed out all the problems which will occur when a rebuilder gets it wrong...  Problems arise with vacuum port (if present), and can result in a real mess...see Reference Material below. 

Early Throttle Plate.  Note relative angle to plane of manifold mounting surface. 

Late Throttle Plate (This applies also to the HIF version.).  Relative angle to manifold mounting surface is significantly less. 


A close-up with angular difference of edge highlighted:


Comparing the edge-bevel of the two styles, shows the angular as well as the height difference. 
This is the comparison which should be made between new and old when changing Throttle Plates during a rebuild. 
Clearly, NO difference should exist!  

 Reiterating and the Bottom Line:  When changing SU Throttle Plates, likes must be replaced with likes! 



Volvoniacs Thread:


Reference Material: 

Excerpt of an e-mail exchange with explanation by Tom Bryant, [SU rebuild expert of Maine.  See:  ] of which throttle Plates were installed in which Carb Housings, and all the things which will go wrong when the incorrect Throttle Plates are installed:  Republished with thanks, and his permission.  (Highlights and Clarifications added)


"...I'm well aware of the difference in throttle plates.  I frequently rebuild carburetors where the previous rebuilder screwed up and installed incorrect throttle plates, and I have to replace them with the correct ones.

Basically, the difference is the angle at which the throttle plate sits in the bore of the carburetor.  The later carbs have a nearly round throttle plate that sits closer to the perpendicular than the throttle plates in the earlier carbs, which have a more oval throttle plate.  The difference in angles of the edge is just a reflection of the difference in the angle at which the throttle plate sits.

AFAIK, *all* "two-hole" Volvo SUs 2 hole vs 3 hole carbs (that refers to the number of Air Filter holes) use the more oval throttle plate, that is, unless someone changed them, and I have encountered some that had been changed.  In fact, I just mailed a two-hole pair back on Monday where one of the pair had the proper throttle plate, and one had the later (more nearly round) throttle plate.  Of course, I had to replace that throttle plate before the carb could ever be expected to work properly.

In addition to the difference in throttle plates, the throttle shaft hardware (the thing with the fast idle screw in it, and which rests against the idle screw in the carburetor body) is also different to accommodate the differing angles of the throttle plates.  If you put an earlier throttle plate in a later carb, the throttle shaft will be turned more toward the open position, and the idle screw will not be long enough to work properly.  The fast idle screw will also be too short to work properly.

If, OTOH, you put a later type throttle plate in an earlier carb, the throttle plate cannot close completely because the throttle shaft hardware will fetch up on the carburetor body, right beneath the idle screw, so it will be impossible to get the idle speed down to where it belongs.

In addition, the early two-hole carbs, the ones that have a vacuum advance tap on the rear carburetor, cannot work properly with the later type throttle plate installed, even if the throttle shaft hardware is also changed.  That's because the throttle plate will be too far away from the hole for the vacuum advance tap, which is supposed to be just *upstream* of the throttle plate at idle.

The "three-hole" carbs come in both types; earlier ones have the more oval throttle plates, and later ones have the nearly round ones.  Typically, if the throttle shaft hardware includes a lever for the auxiliary throttle plates (in the intake manifold), then the throttle plates will be the later type, and 3-holers without the levers will have the earlier type throttle plates.  But, this does not seem to be a hard-and-fast rule.  Also, earlier type throttle plates generally do not have the poppet valves, whereas the later type throttle plates often do.  Once again, however, this is not a hard-and-fast rule.

As for why SU (or Mother Volvo) chose to go from the more oval throttle plates to the more round ones, I don't know.  But, I can speculate:  Gasoline engines tend to have a "nasty" throttle response at low throttles, that is, a slight movement of the throttle plate off the idle position will often result in neck-snapping acceleration, or unpleasantness, at least.  I personally find this quite annoying when switching from driving my Diesel cars to driving one of my gassers, or someone else's gasser. Now, just think about the implications of having a more nearly round throttle plate..... Slight movement of that throttle plate off the idle position will result in less of an opening, and therefore less air flow, than the same angular movement of the more oval throttle plate.  The result will inevitably be a smoother throttle response when coming off idle.  So, that's my theory, and I'm sticking to it.




WOW!...I guess you don't just know about this issue...I guess you know just about everything there is to know about this issue!...

Regarding the reasons for the interesting thought about the "touchiness" of the gasoline engine just off-idle...speaking strictly geometrically...I note that the closer to 90degrees it mechanically stops at, the more in the non-linear (and less angular change related to gas pedal) region of travel (Cosine Curve!) the plate action is ...maybe this is's clearly a number of non-linear actions playing together...

The first 90 Deg (π/2) of a Cosine Curve, shown here, describe the non-linear relationship of "Gas Pedal" to actual Throttle Plate opening. 



"...The closer that angle is to 90 degrees, the smaller the cosine of the angle.  Thus, the variation of throttle opening with idle screw rotation is smaller with the more vertical throttle plate.  That should make it easier to fine tune the idle speed, and the idle speed should also be less sensitive to slight variations in throttle shaft rotation which will naturally occur every time the throttle is opened and closed.  (The throttle doesn't necessarily return to exactly the same position when closed.)  So, the later throttle plate design should result in a more stable idle speed that is easier to adjust and to fine tune..."




Throttle Overrun Valve (TOV):  

While on the subject of Throttle Plates, the subject of these spring loaded pop-off valves should be addressed.  Their purpose is to open on downshifts and engine braking (and only during periods of maximum Intake manifold vacuum) to allow the intake of a small amount of mixture.  They are not essential to the proper operation of the carbs, but they decrease emissions and exhaust burble and backfiring. 


Note TOV is only open under maximum manifold vacuum condition.

Failure of the TOV: 

The typical failure mode of the TOV is to stick open, with the Sealing Plate failing to return correctly into place along its axis, because of the miniature hurricane winds it is exposed to, resulting in a raised idle RPM...the author has troubleshot an otherwise uncorrectable elevated idle, down to the TOV of one of the carbs being completely missing(!)...leaving a hole in that Throttle Plate (and pretty much explaining the high-idle!)...the valve having "self-disassembled" from the Throttle Plate or having been blown into tomorrow by maybe a backfire (and after that, likely eaten by the engine... not so good!). 

The spring-end of the valve can be seen for the purpose of inspection, with Air-Filters removed and Dashpot lifted, looking down the throat of the carb (to allow an even better inspection, Dashpot may be removed entirely)..

This is what it might look like, looking down the throat of a TOV equipped SU HS6 (Picture of a grungy parts carb!): 

Looking past Venturi, TOV is seen in-situ. 

TOV Action: 

TOV viewed from above (through Dashpot opening) so that TOV spring is well visible. TOV depressed with an soft wooden tool.  Movement is only about 0.10" and should be along axis of spring (not askew, in which case, it may jam open! **)

**  If a high idle suspected of being a result of a jammed TOV, gently manipulate the valve with a soft tool along its axis, to assure it seats properly, closing off the opening. Force required against the Return Spring is significant, but should be definitely noticeable when manipulating.  

Disabling the TOVs:   

The TOVs are one of those anti-pollution complexities which are fine in theory, but in practice, less than reliable, and risky in their mere presence!  They can be simply disabled by soldering the brass components permanently in place onto the Throttle Plate, from the manifold side (as even recommended by Volvo in an Internal Shop Bulletin). Carbs do need to be removed from the manifold to gain access from that side, but the Throttle Plate does not need to be removed from the housing for this, increasing the scope of the job and adding further complexity.  The springs and their cage can remain in-place vestigially on the upstream side...their presence wont bother the mixture flow significantly...concerns of disrupted or diminished flow are highly exaggerated in the opinion of the author (the weight increase of the car because of the added solder probably makes more of a performance difference...!).   

"...when the butterflies are wide open they interfere with the smooth flow of air into the engine..."  copied from Facebook page "Practical Classics".... from an otherwise very good write-up on the SUs...(...honestly, how often are your Throttle Plates at WOT?)


TOV permanently disabled by unifying Sealing Plate and Throttle Plate by soldering.  . 


Excerpt from Volvo Internal Notice For B30E motors, so this specifically applies to Injected 6 cylinder motors...however, it is substantially similar and applicable to any motor with a carb fitted with a TOV also, and makes the point that even Volvo recommended removing or disabling (by soldering) them!  (Translations:  R. Kwas) 


Hoher Kraftstoffverbrauch bei Fahrbetrieb mit abwechselndem Anhalten und Anfahren
Increased fuel consumption with many Starts and Stops.


Um den Kraftstoffverbrauch zu senken, sind die negative Unterdruckverstellung des Zündverteilers sowie das Überströmventil in der Drosselklappe beim Motor B 30 E ab Fahrgestell-Nr. 94818 aus der Produktion genommen worden.
In order to reduce the fuel consumption, the Timing Retardation of the Distributor as well as the Throttle Overrun Valve were dropped from production after chassis No. 94818.

Bei älteren Fahrzeugen können diese Funktionsträger nachträglich als Wartungsmassnahme entfernt werden. Dazu die nachstehende Anleitung.
Selbstverständlich wird vorausgesetzt, daß der Motor im Übrigen in jeder Beziehung normal funktioniert und den technischen Daten entspricht.
On older vehicles, these function can be retrofitted.  Procedure follows.  Naturally, the motor should function normally and according to technical specifications. 


Configuration change posibility

6. Ventilteller des Überströmventils sorgfältig an der Klappe festlöten oder diese gegen eine entsprechende ohne Überströmventil, Ersatzteil-Nr. 461410-3, auswechseln.
Carefully solder Sealing Plate of the Throttle Overrun Valve to the Throttle Plate, otherwise, replace with a Throttle Plate not having such a valve.  Such as PN 461410-3.


External material sources are attributed.  Otherwise, this article is Copyright © 2017.  Ronald Kwas.   The terms Volvo and SU Carb are used for reference only.  I have no affiliation with either company other than to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with possibly a wise-a** comment. 


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