FYI, I just went ahead and ordered the three guage set from British Tool Works late Sunday night. Got an email back at 1 am from him saying it was packed and shipping the next day. Must never sleep!
Anyway tracking shows it will be here Thursday, so I’ll try it this weekend and report on the functionality. Basically a set of runout guages that thread into the top of the dashpot.
Larry This type of synchronizer is or was available via a set of 90 degree bent wires (3) that were inserted through a screw-in-piece, on the top of the damper. The idea was to align the tips of the wires with each other to ensure the pistons were of equal height. Unfortunately this method is based on a misunderstanding of how the SU carb works. The piston’s only purpose is to regulate mixture, it does not regulate air flow. Air flow is regulated by the butterfly at speed and the bypass screw at idle, and that is measured generally by a device that measures air flow directly, such as the Uni-Syn. The piston regulates mixture by regulating the position of the needle in the jet in accordance with the vacuum in the throat of the carb. Fine adjustment of this is made by the mixture screw, and this eliminates the need for all three pistons to be at exactly the same height. Having all three at exactly the same height is not desirable or obtainable anyway as individual clearances in the assembly, spring variations, weight variations, and more come into play. On the jet side we need to take into account wear of the needle and jet, height of the fuel level as regulated by the float etc. Finally not all cylinders want the same mixture - spark plug placement, compression differences, temperature differences, and finally shapes of the intake manifold, and unequal vacuum at the carb entrance from air intake systems, all play factors. Equal piston height is a goal perhaps but only if everything else is also equal, and that’s an impossibility.
Terry,
I remember those wires way back in the 60s. And I agree they serve the same purpose as the gauges that I bought. But if memory serves me right, the air passing through the butterfly creates the vacuum that raises the piston. If that is correct, then the height of the piston os based on the volume of air. In the video of the gauge usage, you adjust the position of the butterflies to equalize the piston height which would indicate a similar vacuum being pulled and if the geometry is the same on each carb, would indicate equal airflow. Or am I all bonkers?
Regards,
Larry Trom
804-677-5090
Angel Fire, NM
No you’re definitely not bonkers, but we are not using terms in the same sense. Synchronizing carbs, by the usual definition, means setting multiple carbs to flow the same amount of air. It is usually done at idle – the object being equal flow throughout the rpm range. HD8 carbs are different from many carbs in that idle speed is set by a bypass screw, as opposed to small openings in the butterfly. HD8’s are meant to operate with the butterfly completely shut at idle. That is done with an adjustment of the levers on the butterfly shafts, so synchonization as you open the throttle is purely mechanical. I take it that when you talk about different positions of the butterfly you are actually talking about opening and closing the bypass screw. If you open one of the bypass screws at idle, for whatever reason, including raising a piston, airflow (volume of air) increases in that carb. That increase can easily be seen on a Uni-Syn. I believe that one of the major misunderstandings about SU’s concerns the effect the piston has on air flow. It has none – at any
throttle opening. It’s purpose is to regulate the speed of the air flow over the bridge and keep it constant regardless of the throttle opening. It does that by making the opening over the bridge bigger or smaller. It does not regulate the volume of the air flow – that’s strictly the bypass screw or the butterfly. By keeping the speed constant the depression over the bridge remains constant permitting the needle to regulate fuel flow with it’s taper. That’s why these carbs are called CD carbs or constant depression carbs. Matching minor differences in piston height at idle is completely unnecessary – it won’t hurt but it adds nothing. Yes, if you don’t do it minor air speed differences over the bridge will lead to minor changes in the depression and it will suck, or not, more fuel – but you compensate for that by adjusting the jet position, which you have to do anyway even if you even the pistons out…
From:
Larry Trom via Jag-lovers Forums
The primary value of the little wire thingies is to confirm that you get approximately the same piston lift on all carbs at higher than idle air flows. It is much easier to eyeball whether the wires are at the same height, than whether the pistons themselves are. This is mostly useful to confirm the throttle linkages are properly adjusted and synchronized.
Regards,
Ray L.
Terry, thanks for the information. I’ve also been reviewing the SU publication on the HD, now I’ve got to sit down with them to make it make further sense. I do have a uni-syn and will start there. The engine actually runs very well after needle replacement with UDs and setting them up the other day. Just trying to make sure they are as good as ca be gotten.
Alright, After letting this percolate in my mind I agree with Terry that I had a mischoice of words when saying “synchronizing”. It is the airflow equalization that I was after as the butterfly/ throttle synconization was done a long time ago when I restored the car, and I was smart enought not to fiddle with that afterwards. That said, after trying, my Uni-syn is pretty much worthless after 30 years. The guages arrived yesterday and I figured I’d give them a try. The mixture had been set I screwed the guages in and adjusted them to read the same, with as little downforce on the pistons as I thought would work. Started the beast and all three guages read fairly differently. Now we’re talking several thousandths, let the rear carb stay as it was, increased the airflow on the middle, and decreased it pretty significantly on the front. All matched at the end
Increased the throttle a bit to watch at higher RPMs and they stayed in good agreement. Wish I had a good Uni-syn to check it against, but I feel comfortable about what these provided and it runs quite well. Proof will be in a couple months when the snow disappears and I can open it up with load