No way to have the engine running at idle

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laurentdom

laurentdom

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Hi All,

My problems started with an ignition issue, fixed with a new coil / new sparkplugs.

But the bad news is that it's not possible to have the engine running at idle.

The carb is a single 32mm Amal Premier with a # 19 pilot jet, 3 cutaway slide, 106 needle (middle groove), 240 main jet and has been working very well in that configuration for more than 40 000 kilometers until now.

To start the engine, I need (as usual) to tickle the carb until the fuel overflows and to close the choke, and the engine starts easily.

But as soon as I release a bit the throttle (say below about 1500 rpm) or I start releasing the choke, the engine stops immediately and systematically.

As I suspected an idle circuit issue, I've cleaned and re-cleaned / checked and re-checked the carb, and the idle circuit (including the two tiny by-pass holes) looks OK.

I've even tried another brand new 32 mm Amal Premier with no improvement (it was even a bit worse, but probably because of different slide cutaway, pilot jet ...).

I've verified that there is no air leak between the carb and the cylinder head.

What else would you suspect and check?

Thanks for your help!

Laurent
 
Have you drilled out the pilot jet with a 18 thou drill, blowing air and/or using carb cleaner does not work. The Premier makes it easier to get to the pilot jet but it still has to be drilled out to be properly cleaned. If re-tickling the carb after the engine is running improves the engine that tells you the pilot jet is blocked.
 
Well, it certainly SOUNDS carb-related as Kommando noted but the fact that it began immediately after changing the coil/sparkplugs would make me look suspiciously at whatever work you did at that time as the first likely suspect. ;)

Re Carb - Could you have done something that affected the slide stop screw or throttle cable? IOW, is the slide now dropping to fully closed instead of at the appropriate idle position?
 
Thanks for your prompt and clever replies Gents.

Kommando: I have not re-drilled the pilot jet but I have carefully verified that it is perfectly clean and that the main hole and the 2 side holes are not obstructed by any debris. Furthermore, the problem persists with other (#15 and #17) new and/or clean jets.

Mike: It started as a coil problem cured by installing a new one. I took the opportunity of that change to verify all the electrical connections and to verify also the fuel / carb system which had'nt been cleaned for a long time :(.
I cleaned it thoroughly from the tank / petcock / fuel line to the carb and air cleaner, removing the jets, air and idle screws etc. and installing new o-rings and gaskets. But I'm not sure that the slide is not dropping now to fully closed as you say. I'll keep you updated about it very soon.
L.
 
You mentioned 40K klicks-have you ever renewed the slide or checked the slid-to-bore clearance? When the slides and bores are worn, the carbs seem to go lean, which is especially effective at preventing idle. I think the effect is to reduce low-rpm vacuum in the intakes to the point that fuel is not drawn into the idle circuit.
 
Hi Danno,

No, I didn't check the slide-to-bore clearance neither install a new slide. I don't think it's the reason for my current problem, as it happened really suddenly. I imagine that if the slide and bore wear was the cause, the symptoms would have appeared slowly.

Thanks to Mike suggestion, I noticed that the slide was fully closed. After having re-adjusted that, the engine is now able to run without the choke ON. It is still not possible to have an idle below 1500ish rpm, but of course I will have to tune the slide stop crew and pilot air screw once the engine is properly warmed up.
Thks again Mike!
 
Laurentdom - glad one of my lucky guesses worked! Just goes to show that luck is always better than skill! :)
 
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