Throttle shafts in Webers by Carter

[1QuickS]

Surprise, surprise, surprise!

I just disassembled a set of Webers built by Carter with the expectation that the loose throttle shafts were a classic wear-out of the Teflon bushings with resultant shaft wear. There were no Teflon bushings (brass sleeves were installed in their place) and the long shafts were 5/16" diameter (0.3125") while the short shafts were 8mm. When I slipped one of my new, 8mm shafts into the long shaft bores the fit was very good.

I can tell from the set screw divots from the shaft couplers, the quality of machine work and of the black oxiding of the long shafts and from the staking on the throttle plate binding screws that the shafts were installed this way from new!

Most Carter built Webers have been discarded due to porosity issues or machining inaccuracies (progression hole locations and emulsion tube counter-bore depth variations, etc.) but the use of what I assume to be Italian made, 8mm short shafts (with the staked on end plates) and American made 5/16” diameter shafts in the same throttle body with 8mm throttle shaft bores is news to me.

[304065]

Wow. Brand new carbs with 7.9375mm shafts instead of 8mm. What is the precise measurement of your NOS one?

The opportunity to take this thread in the direction of the sophomoric and salacious was almost irresistible with that last sentence but I've managed to resist for the moment.

[1QuickS]

Thank you for your restraint from going sophomoric and the opportunity for me to discourse more upon this topic.

Typical diameters for OEM 8mm shafts are 0.3135" to 0.3140" and my shafts stock currently measures 0.3150" plus 0; minus 0.0005".

Throttle shaft clearance and the degradation of this fit is what provided me an opportunity to employ myself in this niche business. Of the approximately 400+ throttle shafts I have measured (for Weber three barrel carbs) I have found only two that were not originally OEM "8mm" in diameter (unless replaced with 8.5mm throttle shafts) and these two were the long shafts installed into throttle bodies using 8mm diameter short shafts. The boring of the throttle bodies was for 8mm shafts and was presumably an 8mm diameter bore. The resulting initial clearance between the 8mm bore and the 5/16" diameter shafts provided a larger amount of area for ambient air to be drawn into the high vacuum area below the closed or partially closed throttle plates. The % increase in area for ambient air to be drawn into this low pressure volume is 168% larger than for the OEM 0.3135" diameter shafts. This percentage sounds large and the diametrical clearances seem small and when new there wasn't much of a running or tuning issue.

After around 90k miles this shaft/bushing wear issue starts becoming an issue with maintaining a smooth idling and early progression performance on our Webers, especially apparent on higher performance engines. When you start with shafts that are undersized you will achieve the wear-out syndrome earlier than for correctly sized throttle shafts; this is the nature of my surprise. Most Carter made Webers have had their QA issues documented in previous posts but I though this one rather unique and not discovered without shaft removal and measurement.

Typical shaft wear begins at the end of the long throttle shaft where the throttle lever arms attach. The sum of masses of the lever arm, drop link and of the lever arm attached to the cross bar act like a hammer which is activated by the pulsing of the throttle shaft. This pulsing is due to the closed throttle vacuum pressure acting on the throttle plate and forces the throttle shaft to act like a cantilevered beam which vibrates vertically through the journal clearance gap. Also on the end of the long shaft is the applied forces of the external throttle return spring, the concentric throttle return spring and the reacting force of the throttle stop screw, all of which are reacted by the journal in closest proximity to all these forces while the next journal inboard only sees 1/2 of the vacuum force applied to the throttle valve...a simplification of the forces in play. The larger the bearing clearance the larger the dynamic loading since all these masses have the time to accelerate through a longer space resulting in higher kinetic energy which the bearing/throttle shaft interface must react before the next pulse repeats the process. Throw into this mix ambient dust which acts like lapping compound and Teflon "shaft bearings" that don't support the throttle shaft and you have the classic Weber throttle shaft wear-out syndrome. If the wear is unchecked then the next bearing inboard from the outer one begins to wear and this one is more difficult to repair without consideration of shaft replacement. I typically recommend shaft replacement when the shaft diameter at the second journal reaches 0.3120" diameter, otherwise the erratic spitting at idle will not be arrested without overly rich idle settings.

I show a picture of this shaft wear and of the Teflon "bearing" on my web site under the topic of "Throttle Shaft Rebushing" if interested: http://www.performanceoriented.com/t...#ThrottleShaft

I have discovered a couple of interesting side notes in that the Solex carbs (which were suffering throttle shaft issues within 20k miles if not earlier - like 4k which I have read) used Teflon bearings between brass bearings in each end of each throttle shaft and that the Zenith carbs used brass bushings through the entire length of the available throttle body length for each throttle shaft journal. Most Zenith projects I see do not need shaft bushing repair.

The choice by Carter to mix shaft diameters was a cost cutting decision (my guess) and was probably made by non-engineers or by engineers who abdicated engineering principles for corporate benefits.

I doubt variations of similar magnitude in the journal diameters on a crankshaft would be dismissed as trivial.