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Hi all,

I am at a loss with trying to sort out the idling revs on my 1968 T120. Whatever I do, I cannot get them below 2000 RPM.

For background, the bike was off the road for the winter whilst I fixed (replaced) the wiring loom. I had some dodgy wiring, including the Boyer EI being wired in reverse and then butchered to try to get the bike to run (previous owner). I have replaced the butchered part and built a custom loom that only has what is needed for the Boyer MKIII EI and Podtronics Reg/Rec. As well as sorting the wiring, I removed both carbs and gave them a good clean and blow through with compressed air, plus new seals / gaskets. To be honest they looked pretty clean anyway. Finally, I have checked / adjusted the valve timing.

The bike now starts first kick with no choke, which is a vast improvement over last year when I would often wear myself out trying to start it. Also, last year the bike, once running, would tick over ok (1000 rpm maybe).

So, to the idling problem. I started with the throttle cable(s) and checked that both sliders lift simultaneously - they do. I then set the throttle stops to 1/4 turn in after the point they first touching the slider. Then 1 1/2 turns out on the air pilot screw. Set like this the idle is about 3000 rpm!

Next I tried the approach suggested in the workshop manual for synchronizing the carbs. Removed the left plug cap, started the bike and tried to get it to run smoothly. With no throttle stop at all and 2 turns out on the air pilot screw (RH carb) I could get it to run smoothly at about 1200 rpm.
Repeating this with the LH carb. The best I could get on this side was 2000 rpm with no throttle stop and the air pilot screw wound out about 4 turn (I don't think anything after 3 turns is supposed to make any further adjustment?)

I have checked for an air leak by spraying the inlet manifold side of the carb with Easy Start spray. This has no impact on the revs.

I have replaced the spark plugs. Having just checked them again, they are slightly "oily / shiny" looking I would say, but certainly not sooty and not looking like the engine it running lean either. Difficult to read too much into this though, I think, as they have only just been fitted and the bike has only been idled for a couple of minutes whilst fiddling with stuff!

If anyone can suggest where I need to start looking next I would be very grateful.

Many thanks
Peter


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Check that the slides are not being hung up by the cables, ie there is slack in the cables when the throttles are closed shut fully. Did you fit new O rings to the throttle stop screws and the idle mixture screws. Did you probe the pilot jet bushes with a #78 or 16 thou drill, compressed air does not clean the pilot circuit nor does carb cleaner.

When only running on one plug and having EI the other plug needs to be earthed to the head and sparking.

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Hi Kommando,

Thanks for the quick response.

The slides definitely drop fully. A solid click when the throttle is shut.

I did fit new o rings to the air pilot screws and the throttle stop screws.

I didn't probe the pilot jet bushes. The carbs looked very clean though (not sure if they are original or replacements). I can do this. Will need a couple of service kits again though, so will be a few days before I can retest.

I didn't know about earthing the other plug lead. Hopefully no damage done as both cylinders still fire when both leads are replaced.

Thanks
Peter


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( as a lower order of potential problem )
have you checked the timing at idle .
the Boyer may not be a retarding or retarding enough .

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Another possibility is an air leak into the intake between carburettor and cylinder head.

Spray the joints with WD40 while it’s idling. If the engine reacts, there is a leak.


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Originally Posted by quinten
( as a lower order of potential problem )
have you checked the timing at idle .
the boyer may not be a retarding or retarding enough .

Even if stuck at full advance, you can get a reasonably low idling rpm.


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Originally Posted by quinten
( as a lower order of potential problem )
have you checked the timing at idle .....
+1 on this suggestion.
I was going to say but Quinten beat me to it.
Advanced timing will speed up the idle. The twin carbs make the engine fussier in this respect.

Second comment once timing has been strobe checked. How worn are your carbs? Well worn carbs will internally leak air around the slide and speed up the idle. This is also fussier on a twin carb engine and can't be checked for easily using TT's suggestion. (Which BTW, is also a good one.)

The balance pipe is still on there isn't it?

Hope all this helps.

Last edited by Stuart Kirk; 05/09/22 8:06 pm. Reason: Thought of something else.
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Thanks all for the suggestions.

Originally Posted by quinten
have you checked the timing at idle .
the boyer may not be a retarding or retarding enough .

I think this is ok. The bike starts really easily, so I guess the timing is where it should be at start up (fully retarded). Obviously I can't see what it is at low revs, due to the idling problem, but the strobe shows it advancing from (at a guess) 20 degs at 2000 rpm through to 38 deg as I open the throttle up to about 5000 rpm.

Originally Posted by triton thrasher
Spray the joints with WD40 while it’s idling. If the engine reacts, there is a leak.
I used Easy Start (cold start spray). Sprayed pretty much the whole carb (lhs and rhs) and there was no noticeable change in revs.


Originally Posted by Stuart Kirk
How worn are your carbs? Well worn carbs will internally leak air around the slide and speed up the idle. This is also fussier on a twin carb engine and can't be checked for easily using TT's suggestion. (Which BTW, is also a good one.)

The balance pipe is still on there isn't it?
Balance pipe is there.
Not sure how I would know how worn the sliders / bodies are? Visibly they look ok and the slides slide "nicely" (don't rattle in the body).

I was thinking about whether I should bite the bullet and buy new carbs, but at £360 a pair that is quite a big speculative purchase. I'd like to be sure I have covered all other options first.

Will try having a poke about in the pilot jet bushes as kommando suggests before committing the cash to new carbs.

Thanks again
Peter


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You have told us that before the winter layup the engine would tickover ok, though hard to get started in the first place.

So I don’t buy it that those same carbs are now too worn to idle at less than 3K rpm.

Blocked pilot jets on Concentrics is very common, and will cause difficult starting (and usually poor idling), but I doubt this is the cause of the present issue.

I’m not sure cold start spray is a helpful diagnostic tool for detecting inlet air leaks, as it will be drawn through the leak and encourage the engine to rev.
You should use a non-flammable liquid that temporarily blocks the leak, even ordinary oil will do.
So I don’t believe your test has eliminated the possibility of intake air leaks at all (which I think is the second likeliest cause of your problem).

I still think the most likely cause is the slides not dropping to the bottom of their travel.
That you can hear the clack could just be the slides hitting the throttle stop screws (not the bottom of the body).
With splayed carbs it’s easy to check, air filters off, have an actual look at what's happening.

Then take the stop screws out and see what happens then, do the slides go down further?

It’s best to begin the sync process with all the cable adjusters fully wound in (ie maximum slack), and tighten/adjust from there.

Best of

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I find it difficult to set mixtures on twin carbs if the balance pipe is connected, plug one end of the pipe and cap the stub.
With the pipe connected the RH pot can run off the LH mix, and visa versa.
Do use the drill kommando mentions, its essential, no point in proceEding unless the pilot jet is clear.


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I have a small pair of vice grips just for pinching off the balance tube.


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Once, after having no problems previously, I thought I would go on vacation, and suddenly found the same problem you describe. Throttle cables completely slack, idle stops turned out far enough so to of be no use, still wouldn't idle below 2000. The slides were just plain worn out. They had been replaced, as part of a rebuild, but the bodies, it turned out, were warped. It happened so suddenly, I couldn't believe it was true. The previous owner hadn't used the proper nuts, o-rings, and the cupped washers over them, and over tightened. I had replaced that mess with the right parts, but suddenly, there i was. Go figure. I now have JRC's, by the way.

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Hi Peter, It sounds like slides are fully down.


Try this. Back slide stop screws way out. Give lots of throttle & let it snap back.


Now grab the throttle cable on left carb. Lift throttle cable. It must have play. It souled lift a 1/32-1/16 inch or more. Check right side.

If you don’t have slack, start by pulley carb tops. Verify tip of inner wire is fully seated in slide. Verify needle clip is fully seated in desired groove & clip is & spring are fully seated in the recess.

Then as needed adjust cable adjusters shorter.
You should be able to adjust slide low enough that motor will barely idle its so slow.

Even though you hear click it’s sounding like cable(s) is holding slide up. Remove air filters & you can see slide position.

Don’t worry about carb sync until the idle come below normal. Then start the carb sync process.

That is process in its own right. Get the idle down first.
Don.


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Thanks again everyone for more suggestions.
Originally Posted by koan58
I’m not sure cold start spray is a helpful diagnostic tool for detecting inlet air leaks, as it will be drawn through the leak and encourage the engine to rev.
You should use a non-flammable liquid that temporarily blocks the leak, even ordinary oil will do.
So I don’t believe your test has eliminated the possibility of intake air leaks at all (which I think is the second likeliest cause of your problem).
I had misunderstood the purpose of this and thought the idea was to draw through a flammable liquid / vapour to change the revs. I will repeat this test with some WD40 and report back.

Originally Posted by koan58
I still think the most likely cause is the slides not dropping to the bottom of their travel.
That you can hear the clack could just be the slides hitting the throttle stop screws (not the bottom of the body).
With splayed carbs it’s easy to check, air filters off, have an actual look at what's happening.
Originally Posted by TR7RVMan
Try this. Back slide stop screws way out. Give lots of throttle & let it snap back.

Now grab the throttle cable on left carb. Lift throttle cable. It must have play. It souled lift a 1/32-1/16 inch or more. Check right side.
I am pretty sure they have dropped fully. I have a 2 into 1 throttle cable and have slackened it off to the point that there is slack at the twist grip. I have the air filters off and can see the sliders drop and come to a positive stop (with the throttle stop screws wound out). When I twist the throttle there is a pause before the sliders move (for testing that there is actually some slack)

Originally Posted by Notdoc
The previous owner hadn't used the proper nuts, o-rings, and the cupped washers over them, and over tightened. I had replaced that mess with the right parts, but suddenly, there i was. Go figure. I now have JRC's, by the way.
I am not sure I have all the right parts. I do have a new o rings and gaskets on the manifold. I have sealing washers but not cupped washers. I guess it is possible that I have damaged the flanges when refitting, though I have been careful not to use too much force when tightening them up. Hopefully the WD40 test above will highlight if this is the case.

I'll leave the sync'ing until I have the idle revs down to where they should be.

Many thanks
Peter


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Peter- I had some trouble with my '67 tr6 carb after removing it recently . The revs wouldn't drop down properly to idle but would stay up at about the rpm you are talking about. I started off out alright but it felt suddenly dangerous when the revs wouldn't drop. I loosened the manifold nuts slightly and the problem went away. I normally only do the nuts up until the spring washers have compressed. They don't need to be very tight. It is probably caused by the carb body being warped. The slide did seem to drop down with a click when the engine was off.

Dave

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Thanks Dave. Seems like these carbs can have a mind of their own! :-)

I think I will try removing both again, doing all the bits suggested above and put back together again, paying attentions to every little aspect, especially the bolting back onto the manifolds.


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" I have a 2 into 1 throttle cable and have slackened it off to the point that there is slack at the twist grip." Not good enough, there must be slack on the two cables from splitter to carb. Simple test , grab cable above carb top , pull outer up, if the slide moves directly there is not enough slack in the section from splitter to carb.


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Thanks Gavin...that makes sense. I had no adjustment at the twist grip originally (the screw adjuster was at it's limit), so I wound out the two adjusters at the carb end a bit to give me back some at the twist grip. I assumed (incorrectly it seems) that the splitter was sort of floating between the single cable and the twin cables and that the adjustment would sort of "flow through".

I will check as you suggest (and as Don suggested previously).

As an aside, does anyone know the part numbers for the 3 cables (2 from the carbs and 1 to the grip). I only have an export models parts list and they don't appear to have had the 2 into 1 setup.


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Thanks Gavin...that makes sense. I had no adjustment at the twist grip originally (the screw adjuster was at it's limit), so I wound out the two adjusters at the carb end a bit to give me back some at the twist grip. I assumed (incorrectly it seems) that the splitter was sort of floating between the single cable and the twin cables and that the adjustment would sort of "flow through".

I will check as you suggest (and as Don suggested previously).

As an aside, does anyone know the part numbers for the 3 cables (2 from the carbs and 1 to the grip). I only have an export models parts list and they don't appear to have had the 2 into 1 setup.


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So, somewhat embarrassingly, it looks like I had taken up too much slack on the carb end of the throttle cables and, as Gavin and Don suggested (and others), this seems to have been at the heart of my problems.

However, having readjusted and then tried to refit the tank, I am noticing that the cables are binding / catching once the tank is in place. There is not a lot of room under the tank (UK model with big capacity tank). I am finding that the sliders do not always return home fully when the throttle is released. With the tank off they return correctly and smoothly. I have been trying to find images / videos that show the correct routing for the cables.

There is not a lot of headroom above the carbs (couple of inches at most), so the cables have to bend back fairly quickly. The adjusters and the splitters have to find home somewhere under the tanks too. Does anyone have an image or video (or link to these) that clearly shows where the throttle and choke cables should be routed?

Thanks all for your help above. Probably should have mentioned that I had adjusted both ends of the throttle cables but that was lost amongst everything else I had done.


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Flanders makes 90 degree metal cable stops with adjuster. You could add those to the cables. They are 30mm high, 60mm long including adjuster. Some Japanese bikes used these.

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This problem is common , the best fix is to make your own cables with a swept bend from the carb top, this will delete the carb top adjuster so in line adjusters are fitted close to the swept bend, this will make synching the carbs difficult, a temp fuel supply will be needed fro access to the cables.
Venhills cable parts suppliers are very useful, they used to list a flexible spilt swept bend which might help you but the website has been changed and I cant find it.
https://www.venhill.co.uk/cables-an...-bends/unthreaded-ferrule-bends.html?p=2


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The late Unit Bonnevilles I’ve seen have no room under the tank, for cable adjusters in the carb tops,


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Don't think routing this the problem , there isnt much room to work with .

sounds like the cable adjusters are kinking when the tank is added
( is the tank properly mounted , rubber shims and all )

when the carb cable Adjusters are near the carbs , where the cable must
make that tight radius bend
... all the cable bend must happen before the Adjusters .

because of the segmented nature of the Adjusters , they want to kink more than make a smooth radius bend
if the Adjusters kink ( the cable sheath effectively becomes longer )
and the cable inside becomes tighter , losing that small amount of necessary slack .

once adjusted , slip a piece of fuel line over the Adjusters , or something like this ,
To prevent this assembly from kinking . ( this reinforcement also allows the adjuster to be part of the radius , without kinking )

( better to move the adjustment away from the radius at the carburetor , to the splitter side , where there is no radius happening )
dont know if they make cables like this , you might have to make your own .

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Hi Peter,

The lack of headroom between tank and carb tops is an issue common to many Brit bikes.

It causes the cable to exit the carb top at a less than ideal forward angle.

In an ideal world, the cable would exit vertically, then gently curve to horizontal.
This isn’t gonna happen with the standard cable set, so you have to make the best of a bad job.

First of all, hopefully you don’t have adjusters on the top of the carbs. This only makes the headspace situation much worse.

Secondly, if the splitter-to-carb cables have their adjusters nearer to one end than the other, put the adjuster nearer the carb. This makes it easier to access with the tank on (useful for reasons later explained) and also allows a little more angular accommodation of the cable as it nears its difficult approach to the carb.

The ferrules and outers at the carb ends of the cable tend to be eroded by the inner wire after long use (because of the angular entry) which will impede free movement.
This also produces wear of the inner, resulting in fraying (you’ll see the shiny evidence on the inner). If so, replace the cables.

Avoid clamping the cables to the frame, just use loose cable ties if necessary.
The junction block can be usefully tied to the underside of the spine, with the cable tie just ahead of the fat middle section. You can then tighten the tie around the spine such that the cables to the carbs have some reasonable arc into the carbs. You can only do so much here, by trial and error fitting of the tank. This can be tightened firmly, but not so as to distort the junction body (assuming it is plastic, if the junction is metal then clamp it harder).

As long as the cables have been adequately lubed, that is about the best you can do with the standard setup.

When it comes to synching the carbs, I’ve always found that it is delightfully easy to do with the tank off.
However I have found that when the tank is installed (pressing on the cables) it changes the relationship between the carbs.
That is why I suggest being able to get to the adjusters after the tank is replaced, for final precision.

There are many different opinions on synching, but I’ve done fine for 40+ years with 1 finger against one slide, an eye on the other slide, other hand on the throttle.
Start with the throttle stops out and all the adjusters slackened for free play, so you’re getting a genuine baseline to start from, with both slides starting to lift simultaneously.

Now expand the 2 junction-to-carb adjusters by exactly equal amounts, until there is only ~1-2mm freedom in those cables.
Now adjust the throttle cable to leave just a little free movement in the throttle cable.
Only now put the tank back on, and check the synch. You may be lucky and it has stayed as it was, but make any fine adjustments as needed with the tank in place.
If this is not possible, you will have to do it by trial and error, removing and refitting the tank.
It can be tedious, but worthwhile for good running.

Only then can you re-install the stop screws, note when they start to lift the slides, go 2 further turns.
Ensure the idle jets are clear and set the air screws to 1.5 turns out.

Start the bike and adjust the idle and air screws as needed.

Best of…

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