Carb tuning continues

I've posted a few times on trying to get my 73 850 to idle consistently when hot. I needed to baseline the carbs and make sure that all the settings were correct, here's what I've done.

1) Set the floats to 2mm below the top edge of the bowl with the bowl (right side up, not upside down). They were both low.

2) Cleared the pilot jets with a #78 drill bit. The right carb may have had a little blockage in the jet, but it's really hard to tell because the jet is so tiny.

3) Changed the balance tube and replaced the o-rings on the throttle stop and pilot jet screws.

4) Set the pilot jet screws at 1.5 turns out from full stop.

5) Used a 3/16" drill bit to set the thottle slide height with the throttle stop screws. I thought I had a throttle cable problem because one slide on the left carb would barely touch the 3/16" bit with the cable at full adjustment and the right slide would not touch the 3/16" bit at all. I took the slides out of the carbs, removed the springs and needles and put the slides back in the carb bodies and found that the slide heights were a function of the way the carbs were manufactured - there was no way that the right carb would ever get to to the desired 3/16 clearance, and there was no blockage or interference. The slide moved easily through the entire range. These slides are a 3.5 cutout, but I have a set of 3.0 and when I put in the 3.0s, I could get the 3/16 clearance. So I installed the 3.0s and set the slide height at 3/16. This may have been the wrong thing to do.

6) Syncronized the movement of the throttle slides using two 3/16 bits. I got them as close as the my eye can see.

The bike fires up on the first kick like always but would die at idle when cold. I kept some pressure on the throttle and let it warm up, then went out for a 20 minute ride. When fully warm it still would not idle and would die immediately as soon as the throttle was closed. Also, I noticed that the bike seemed to be slightly surging at constant throttle. Maybe that has something to do with using the 3.0 slides.

I plan to reinstall the 3.5 slides and set the slide height with the throttle stop screws as evenly as possible then try it again. I plan to get a Morgan Carbtune to fine tune the carb setup, but until I can get it to run consistently and get a consistent idle, there's no sense in any fine tuning. From what I've read and heard, the above setups that I did should have the bike idling a little high, but with these settings it doesn't idle at all.

Any suggestions are appreciated. Also, does anyone know what the difference is between the 3.0 and 3.5 slide and why the 3.5 is better suited for the 850?

Thanks,

Mike

mgrant said:

4) Set the pilot jet screws at 1.5 turns out from full stop.

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That is just a basic setting to start from though, and the bike being set up on the Bushmans info is a Triumph Bonneville, so the initial setting may not be quite right for a Commando anyway? . Once the engine is fully warmed up the pilot screws should be turned to obtain the best idle running. Are you doing that, or just leaving them at 1.5 turns as you did not mention anything about re-adjusting the pilot screws?

mgrant said:

5) - there was no way that the right carb would ever get to to the desired 3/16 clearance,


These slides are a 3.5 cutout, but I have a set of 3.0 and when I put in the 3.0s, I could get the 3/16 clearance.

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I guess you are using the Bushman's info, which says, Quote: " 4. Insert two 3/16" dia drill bits (or 4" long peice of round bar) into each carb (diameter size should be adjusted to suit the throttle cut away)", so you may require larger diameter rods for checking with the 3.5 slides?

mgrant said:

Also, does anyone know what the difference is between the 3.0 and 3.5 slide and why the 3.5 is better suited for the 850?

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A throttle slide with a larger cutaway (and higher number) causes a leaner mixture than one with a smaller cutaway, so a 3.5 is leaner than a 3, if you compare them you should see the difference?

Quote from the Hitchcock's Amal tuning pages:

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"THROTTLE VALVE CUTAWAY. The atmospheric side of the throttle is cut away to influence the depression on the main fuel supply and thus gives a means of tuning between the pilot and needle jet range of throttle opening. The amount of cutaway is recorded by a number marked on the throttle, eg: 622/3 means throttle type 600 with number 3 cutaway; Larger cutaways, say 4 and 5, give weaker mixtures, and 2 would give a richer mixture."
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For more Amal tuning info see the Hitchcock's website: http://www.hitchcocksmotorcycles.com/am ... nc_ht.html

Thanks for the information. Yes, I'm using the Bushman's guide and also inputs from Norbsa. I have not yet gone back and readjusted the pilot screws. I'll do that when I can get it to idle without requiring me to have a hand on the throttle.

I think you're right about needing a reference larger in diameter than 3/16. It will idle okay if I keep slight pressure on the throttle, which tells me that I need to raise the slides a little with the throttle stop screws to get it to idle on it's own. I'll try the same procedure with a 7/32 bit and see how that works, after I change back to the 3.5 slides. Putting the 3.0 slides in was kind of a bone-headed move on my part in retrospect, but now that I know why the 3.5 is used, I won't make that mistake again.

mgrant said:

I think you're right about needing a reference larger in diameter than 3/16. It will idle okay if I keep slight pressure on the throttle, which tells me that I need to raise the slides a little with the throttle stop screws to get it to idle on it's own. I'll try the same procedure with a 7/32 bit and see how that works, after I change back to the 3.5 slides.

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Ok, but I think you might have misinterpreted what the rods are used for?

The purpose of the rods is to synchronise the slides, so they both lift together, setting the slides to 3/16 (or 7/32) rod diameter is unlikely to give you the correct idle speed (throttle stop) setting. So, once the slides are synchronised using the rods you will then need to adjust both throttle stop screws by the same amount to set the correct idle speed, and the then re-set the pilot screws (which could mean re-adjusting the stop screws again?)

If you were to use a vacuum balancer (like the Morgan Carbtune) you could do all of this while the engine is running as you can then see the result of each adjustment to synchronise the slides with the cable adjusters and synchronise the throttle stops as you do them, which is much easier in my opinion, and more accurate.

Yes, you may have taken the advice too well. Nothing wrong with moving both screws a 1/4 turn up of the 3/16 setting. It is also a good practice to find the highest idle hot with the air screws and turn them in about a 1/8 of a turn towards rich to smooth out any stumble when coming off idle. When using the vacuum gages just be sure to make small adjustments always keeping in mind the reference points you established when you did the cables and slides. You should get ready now for plug reading. So mark a spot on the throttle for off 1/4 1/2 and full, use some tape that can be pulled off easy. Almost all your riding will be done on the needle jets when you write in about a problem it is best to refer to the throttle position.

Okay, thanks. I was trying to hit the 3/16 as a sort of "magic number" set point for the slides, when the goal of using the drill bits is simply to get each slide the same. If I would have gone another ~1/4 turn on the throttle stops I probably could have gotten a stable hot idle. Then I would've adjusted the pilot screws.

As strange as it may sound, I'm really enjoying working through this. I didn't know anything about Amals before I started this troubleshooting and I've learned a lot over the past few weekends.