Truckle slide

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David Taylor
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Truckle slide

Post by David Taylor » Fri Oct 19, 2018 8:42 pm

I have a '71 250 Trail Blazer which you can read about in "Members bikes".
I have it running really well but had difficulty getting good pickup from idle (called "truckle" by Odane, my Jamaican friend).
I tried various different slides from 2 1/2 to 3 1/2 and although I got great performance, pickup from idle was always poor unless I speed up the idle then reduced it again by richening the air screw.

After some thought I tried filing a micro carb in the slide as shown in the photo which I thought might create good conditions for a perfect truckle and pickup.
It didn't work well so I filed the micro carb out until the slide matched the radius of the venturi.

What a difference! Great performance and faultless pickup from idle. Bimbling along at low throttle was smooth and the truckle seems reliable.

Has anyone else tried this in the past?
Screenshot_20181019-194146~3.png
Unsuccessful micro carb

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Re: Truckle slide

Post by beat » Fri Oct 19, 2018 9:11 pm

My Mikuni Flatslide was definitely to rich around 15 to 25 % trottle.
I coured it in a same way as this:
BSA1007.jpg
BSA1005.jpg
bypassing some air to the Needle Jet.

I guess you have had the problem as " to lean " at pick up.

beat <212

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Re: Truckle slide

Post by David Taylor » Fri Oct 19, 2018 10:30 pm

That's a complex shape Beat. Impressive lateral thinking to come up with that solution. <200

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Re: Truckle slide

Post by beat » Fri Oct 19, 2018 11:00 pm

Well David, it was not a straight go to this solution, - no.
first I made a couple of copies of the original one, each with a different cut away.
BSA907.jpg
BSA954.jpg
but as there was no reaction in this trottle range with all this slides tested, I came to the shown solution.
DSCN9152.jpg
beat, very happy now with the <069

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Re: Truckle slide

Post by David Taylor » Sat Oct 20, 2018 9:48 am

Such satisfaction when an afternoon's thoughtful tuning brings good results. <205

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Re: Truckle slide

Post by David Taylor » Wed Nov 07, 2018 6:00 pm

Here's an update. Amal concentric.
If you have poor throttling just off the idle with fluffing and stalling and fussy low rpm behaviour then it can be improved very simply by filing a depression in the bottom of the slide with the same radius as the carb venturi. The optimum depth is somewhere around 1.75 mm. Idle will be more stable and pickup from idle will not fluff or stall.
The riding experience is greatly improved. Before I made this change I was thrashing the bike to the red line and riding in low gears to keep the revs up because low throttle low rpm was so wheezy and fussy. Now I can truckle around at 30mph in top gear with a nice soft feel to the throttle. The bike feels more powerful right through the rev range.
Starting is easier because it can run reliably with the throttle just slightly open.
If you try this mod please report your findings.

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Re: Truckle slide

Post by JB » Wed Nov 07, 2018 6:29 pm

Sounds like you have got a good result from your experiments there Dave.
it would be interesting at some point to compare how yours runs to mine although I've not really had the issues you've described from low down after I'd thrown away the old worn 106 needle jet and eventually ended up with a .105.

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Re: Truckle slide

Post by David Taylor » Wed Nov 07, 2018 6:44 pm

Yes. We'll get together sometime.
My brother has a 1979 Bonneville with concentric MK2s. He has the same symptoms. Difficult starting and stalling at traffic lights when opening the throttle. I will modify his slides someday to cure that. I had a 1980 Bonneville with exactly the same problems. I fitted JRC carbs which cured it. I have a JRC on my Tiger 90 and Mikunis on my Trident. If only I'd sussed this out decades ago...

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Re: Truckle slide

Post by BobM » Sun Apr 28, 2019 11:03 pm

Hi David
Iv'e always had British bikes and the only bike I had which didn't seem to suffer this issue of stalling from the lights or roundabouts was an A65 (single carb) but all the rest of them have which I could never master a fix. At the moment I have a C15 which I've rebuilt as a trial/green-laner which does stall and fluff when opening the throttle quicker than normal. Is it really as simple as just filing a small gap in the slide. I have both the monobloc and premier concentric carbs (new carbs)which both fluff and really want to try this out.

Bob M

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Re: Truckle slide

Post by minetymenace » Sun Apr 28, 2019 11:15 pm

<135 to the forum BobM.
Would you update your profile and let everyone know roughly where you are (I Blighty?) please.

...and the answer to your question is: Yes, maybe....
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Re: Truckle slide

Post by David Taylor » Mon Apr 29, 2019 2:04 am

Yes. It is that simple. I have a Chinese 50cc that fluffed so I did the same mod and it fixed it.
There's no risk because in the unlikely event it makes the bike worse just file the skirt down to remove the micro carb or buy a new slide from Amal.
My advice is to file small amounts at a time then test ride it. If you file too much off you will get a fast idle that you can't slow down.
Even if that happens you can file the skirt down until you have a good truckle.
My slide gives a 1000 rpm default truckle (ie. with the idle screw not touching the slide). When I rode it through London recently I turned the screw up to increase to 1200 ish for safety. I had idle and stalling problems due to overheating but that wasn't the truckle slide's fault.
Please let us know how you get on.

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Re: Truckle slide

Post by Velofello » Mon Apr 29, 2019 7:39 pm

If I understand correctly, when the hole you filed in the slide didn't workout, you re-filed the slide to match the original curve. When you do so, you are also lowering the needle by the amount you filed the slide, which leans the mixture.

Ed from NJ

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Re: Truckle slide

Post by David Taylor » Thu May 02, 2019 1:00 am

The filed part is just about 1mm to 1.5mm deep and should follow the radius of the carb throat. The running mixture is not affected at all other than to improve pickup and low rpm running. You might find a slightly different sweet spot for the idle air screw though.
If you don't like the results and want to return the slide to a flat skirt then yes you would have to raise the needle and possibly file a little off the cutaway. It won't come to that though. I expect you will be amazed and delighted by the result. Just file small amounts off at a time then test ride.
Before I did this mod I could not ride my 250 at 30mph 3000rpm in top gear. It was awful but now I can bimble along happily at really low rpm. It's a different bike and I find I am riding it more slowly and enjoying it more.
You've got nothing to lose because you can buy a new slide from Amal if you make a hash of it.

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Re: Truckle slide

Post by hhh » Thu May 02, 2019 1:14 am

David Taylor wrote:
Thu May 02, 2019 1:00 am
The filed part is just about 1mm to 1.5mm deep and should follow the radius of the carb throat.
I don't understand this part. It looks like a tiny "mouse hole" in the bottom of the port side of the slide. Where does the radius of the carb throat come into it? Thanks.

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Re: Truckle slide

Post by kommando » Thu May 02, 2019 1:30 pm

It looks like a tiny "mouse hole" in the bottom of the port side of the slide. Where does the radius of the carb throat come into it? Thanks.
First attempt was the mousehole but dropped instead for hole with radius of carb throat.

On my 250 and 441 I do not get this small opening issue, wonder what is the cause that effects some bikes and not others.

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Re: Truckle slide

Post by hhh » Thu May 02, 2019 2:54 pm

So the radius is upside down, i.e., it parallels the the shape of the top of the throat?

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Re: Truckle slide

Post by minetymenace » Thu May 02, 2019 2:55 pm

hhh wrote:I don't understand this part. It looks like a tiny "mouse hole" in the bottom of the port side of the slide. Where does the radius of the carb throat come into it?
throat.png
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Re: Truckle slide

Post by hhh » Thu May 02, 2019 7:10 pm

Thanks for the clarification. I think what's being achieved is a more linear opening rate, i.e., more gradual from idle to small opening.

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Re: Truckle slide

Post by David Taylor » Fri May 03, 2019 5:16 pm

Thanks for the diagram minetymenace.
Remember to only file to a depth of 1 to 1.5mm max. Any more than that and you'll get a fast idle that you can't reduce.
I'm really looking forward to hearing your results because you seem like someone who is thorough in your approach.

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Re: Truckle slide

Post by BSA_WM20 » Sat May 04, 2019 6:05 am

I would bend a slightly different way
The two pilot circuit holes are very small and in the middle of the bottom of the mixing chamber
Thus only the air rushing across the exact centre of the throat will be causing the venturi effect to suck the idle mix out of the tiny idle ports.
Concentrating the air flow to the region of these holes will cause a stronger venturi thus preventing a too lean situation just off idle.
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Re: Truckle slide

Post by David Taylor » Sat May 04, 2019 8:20 am

Yes that's exactly what I thought so my first effort was to form that mouse hole tiny little micro carb. It didn't work. I can't remember whether it was worse or just no different. As a long shot I blended it out to match the venturi radius and hey presto!

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