ENGINE WILL NOT DIE off the THROTTLE - CLUTCH? CARBS?

5twins, I have just checked on the Carb Guide for the settings for 78/79 BS38s, 27.5 and 135, but with my pods and through pipes, the 37.5s and 140s I have here, with a needle raised to #2 should be within range.

Anlaf
 
Get rid of those shitty emgo pods as well for some foam unifilters while you're fiddling with the carbs.
 
37.5 pilots are rather large. You can try them but it's more common to use 30 or 32.5s in the '78-'79 carb set (one or two sizes over stock). Even 35s are usually too big.
 
Thanks, 5twins - yes, they were too large, and although the bike started it struggled and gave up. I will order some 30s and 32.5s,

The needle set-up surprised me - it was not like the 76/77, with needle and 5 slots, the 78/79s were housed in a plastic needle-shaped tower and held on the rising disc by a large circlip.. So, no jet needle adjustment possible on this set.

Anlaf
 
The Boyer ignition will fail to retard if voltage is insufficient, and this can cause the idle to spike. If you're running a 2-phase Sparx ignition with a capacitor, this could well be the case. Unstable idle can also result if the pilot mixture is too rich or too lean; if the idle settles down when you blip the throttle, that's the rich signal. Diaphragms have nothing whatever to do with idle function. We gave you a test for vacuum leaks in the Carb Guide; it's best performed with the throttle stop turned down to low idle. If idle is unchanged when you spray carb cleaner, etc. where the throttle shafts enter the carburetor, you don't have a seal leak.

The worst mistake you can make in trouble shooting is to try to logic your way through the problem like most rookies insist on doing. Trouble shooting is a process of testing and elimination. Don't skip steps.
 
Grizld1, I have been doing this set-up using the battery only to make sure of the voltage.

The idle seems to be fine, if anything just a tad rich according to the plugs - it is the intermittent hanging throttle that I want to resolve. I have done the Carb Guide diaphragm test (workign fine on bioth 76/77 and 78/79 carbs.

On the bike at the moment are the 76/77 carbs. I will do the carb cleaner test to check for a possible leak at the shaft seal.

I have gone through systematically, but I take your point, and I will return to the beginning of the Carb Guide. Your help much appreciated.

Anlaf
 
ANLAF said:
Grizld1, I have been doing this set-up using the battery only to make sure of the voltage.

The idle seems to be fine, if anything just a tad rich according to the plugs - it is the intermittent hanging throttle that I want to resolve. I have done the Carb Guide diaphragm test (workign fine on bioth 76/77 and 78/79 carbs.

On the bike at the moment are the 76/77 carbs. I will do the carb cleaner test to check for a possible leak at the shaft seal.

I have gone through systematically, but I take your point, and I will return to the beginning of the Carb Guide. Your help much appreciated.

Anlaf
Click to expand...

Yes, a hanging throttle can be very annoying. I had that problem for a few years, and this is what cured it.......................replace the needle jet 0-ring. The 1970 to 79 carbs have the 0-ring that sits in the groove.

If your ATU is working, throttle cable not binding, butterflys centred in the carb bores, carbs have been synced, and mixture screw is set correctly (dead cylinder method); then I suspect new 0-rings will solve the hanging idle.
 

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Thanks, retiredgentleman, there is just one thing to add - the bike is running rich (rider behind can smell unburnt fuel coming through, black powder deposit on exhaust pipe ends). The jets are as recommended by 5twins (27.5 and 135), the mixture screw is out as per instructions (1.5 turns), and the jet needle position is leaned to #2.

Result is the plugs are becoming black, too.

Hmmm! - hanging throttle and running rich.

Anlaf
 
Retiredgentleman, apologies, I forgot to mention, thanks for the O-ring tip.

I swapped over to my spare carbs this afternoon, and the hanging is still there - perhaps both banks have need o fnew O-rings - just my luck!

Anlaf
 
Anlaf, I don't know how many times we have to write this stuff, but hotdog already told you something you'd better pay attention to. The Emgo/Emgo clone cone filters you're using have internal stops that obstruct the pilot air jet and main air jet--you know, those little holes in the intake bells. When those are obstructed the mixture turns richer than Bill Gates. The stop also obstructs the vent that keeps the lower diaphragm chamber at atmospheric pressure. You can cut reliefs in the stop to let the air jets and vent breathe, but you'll still have poor filtration and poor performance; vacuum carbs work best with a still air box, and very poorly with small-volume pods.
 
Thanks, grizld1 - I cut away anything that might do that when I first got the bike. There is nothing at all that will obstruct the intake vents.

I have been trying to locate UNIs to fit the XS here in Britain - no luck. The nearest is TC Bros, who want $51.85 for shipping from USA to UK, then there is import duty and shipping from the port to her. It's a lot to pay, but if I can't find them here, looks like I have no choice.

Anlaf
 

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