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Basic bench carb balance

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McCabe-Thiele (Ted):
I found this video helpfull for assembly.
I actually had the laptop video in freeze frame as I did it.

I found an Ultrasound bath cleared my jets with a single strand of wire brush bristle to clear jets where needed.


https://www.youtube.com/watch?v=JFTOfYl5BSc

davidcumbria:
Bryan’s opening post is the best description I found of this process and there are various other posts and videos out there with different approaches so I thought I’d add my tuppence worth having just spent ages doing this. These ideas are intended to fine tune the basic approach to get the bench sync as near perfect as possible.

One of the resources is the motormac series of 6 videos in German where he meticulously rebuilds a set of carbs. Though it is laughably poor you can use auto translate for subtitles but the camera work is really good  so you can see what’s going on anyway
https://youtu.be/ZXuTxsCIi-c?si=A1-qHQQLy1XbhLbw

1. Motormac starts by trying to set the linkage initially at the middle of its adjustment range which.makes sense as a starting point. To do this he sets the individual adjusting screws 2.5 turns clockwise from the point when the step in the screw is exactly flush with the top of the bore that it screws into. You do end up moving well off these 2.5 turns but it starts off the linkage across the carbs  at equal and reasonable angular positions. This is done before thick washer and 1-2 , 3-4 linkage plates are put in place.

2. When the carbs are assembled to the linkage loosen the bolts which tightens the operating arms to the main chrome shaft so they are free to slide axially on their woodruff keys. This way the arms can be precisely located so the rest of the linkage is square and equidistant between the carb pairs. Otherwise rotating the linkage will move different carbs by very slightly different  amounts as they move through their range. Once set retighten before continuing the sync.

3. Motormac suggests setting the bench sync position of the slides only at the top of their travel with the main spring off. I tried this but then noticed that the sync was clearly out from closed so disregarded it. Important in any case to have the spring on so the linkage is kept tight as it is when finally in use.

4. The classic octane video shows the sync with a 1/8 drill through the back whereas Bryan and others suggest using a 1mm drill through the front. Subsequent to writing this it has been pointed out by Sesman that this amounts to the same slide height. Although I used a 1 mm drill in the front  with the benefit of hindsight I reckon the 1/8 drill from  the back would be easier to hold and  manipulate to get a good measurement with.

5. Before using idle screw  and 1mm drill I wanted to set each piston so it just bottomed with the idle screw out of contact. Care needed as one carb bottoming can then start to lift the other three. By going along, screwing the adjuster and pressing on the brass screws you can just feel and see when the pressing just stops moving the slide. At this point all the slack in the linkage has been eliminated, all the slides are fully bottomed and one carb is not holding up the others. But because of tolerances in the slides / carb bodies the slides  might not actually be  the same height when raised off the fully closed position.

6. Next step is to set the idle stop to raise the reference carb ( first one to bottom )to the 1mm drill position. Then set the other three to the same height using the drill. Again I found that pressing the the brass nuts down onto the drill to get the same amount of nip as I adjusted the screw gave  the most consistent results. With such a small drill it’s hard to judge the friction otherwise and you can see the slide actually moving down on its linkage springs after the drill is removed. Just like tappets the adjustment changes once the lock nut is tightened so I just played around moving the adjuster by 1/8 turn from its prelocknut correct setting to anticipate the stretch until it was spot on.

7. Another thing  i think is wrong in the motormac video is that he says that with the throttle fully closed there should be a 1-2 mm gap between the eccentrically adjusted pin and the shaft pulley so it’s is not actually working as  a stop. I think that stop us there to prevent all the pressure from closing to be on the delicate carb linkage and   Slide bottom if the idle stop screw  is not in play.  By setting the stop  so that it just contacts the pulley when the carb just bottom it prevents overstrains so that’s what I did. Idle will be off this stop as  set by he idle screw. Maybe I’m missing something here.

Hope that wasn’t too long winded. I spent ages trying to get bench sync spot on so it seems worthwhile to share that effort here. Obviously a vacuum sync might change these settings( and I’ll certainly check that )  but with new piston rings and valves relapped  I’m hoping for equal compressions so if it runs well I’ll be confident the bench sync is as good as possible.

Sesman:
Thanks for the link. Regarding point no4, I believe CO actually uses a 1/8th (3mm) drill via the choke inlets, not a 10mm as you suggest? This translates to a 1mm drill if you approach from the port side due to the shape of the slide. In short, it’s approximately the same setting.

Nice vid👍

davidcumbria:
Thanks yes you are right he does use 1/8 inch drill not 10 mm. Have corrected my post.

McCabe-Thiele (Ted):
When I did my initial set up on the 500 I tried something slighly different - I will not know if it has worked until next year when I start her up.

On my 400 I used the drill bit method, it has worked well. This time as a result of a tip from a member I have gone for a slightly different approach. I have set the carb sliders so that visually they are just at the point of just rising off the carb body as you turn the link adjuster screw. I started with Carb 1 then continued to do the same with sliders 2,3 & 4. My thinking is that this should reduce any error as the gap is effectively the smallest it can be on all four sliders to start with.

Then when adjusting the throttle stop screw I checked  for a 1/8" gap using a drill shaft on carb 1, then likewise on 2,3 & 4 to confirm they are the same. I'm hoping this will give me a better starting point before I finally use a set of vacuum gauges after the engine has done a few miles & settled down.

It might not work my logic being that an error on a minute gap is less than an error with a drill bit. (I have my helmet on ready for incoming fire) ;D ;D ;D

 

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