[Chasfield]

My Bullet 500 engine loves to quit at traffic lights when they turn green and I open the throttle. I was thinking it was purely down to Murphy's law but the technical diagnosis is weak fueling in the transition from idle circuit to main jet running. Does it worse when the motor is hot - which I don't understand because a weak mixture is supposed to be more of a problem with cold engines.

I'm running a 27.5 pilot jet and the pilot air screw definitely likes to be only 3/4 of a turn out, which indicates that ordering a bigger pilot jet would help - a turn or a turn and a half out is more normal.

However, in the mean time I checked the carb's float height, which was set at 24mm, and which I believe is in spec. I tweaked it up to 23mm, figuring it wouldn't hurt. I also cleaned a small amount of reddish sludge particles from the float bowl. It is only 1800 kms ago that I did this and the petcock has a built-in fuel filter, so where that stuff comes from is a mystery.

The other thing I did was make a proper packing ferrule to seal the throttle cable into its receptacle on the top of the carb. The standard fit is very loose and I'd tried to bodge this with tape previously. The last thing you need are air leaks that short circuit the venturi path. I made this part out of a small piece of stainless steel sleeve from a ballpoint pen. It might be going too far to say never throw stuff out in case it proves useful, but this does apply to anything of tubular form!

What I really need for clean throttle transition is a carb slide with a richer cutaway. These seem hard to come by in the UK at the moment.

The outcome of all this fettling has been some improvement. I can feel that the fueling hole has been filled in somewhat and the engine doesn't get left behind when the throttle is blipped. The other thing is that the motor now runs sweeter - and starts more eagerly - with a couple of degrees less ignition advance than was set before. This, I believe, is a good sign as weak fueling likes more advance.

Does anybody else have cough 'n' stall anecdotes to share?

Chas

[bob bezin]

my re dos'nt cough, but when coming to my driveway and slowing down from 60mph to idle sometimes the engine just dies if i am coasting in nutural. i suppose if i kept it in gear it would be ok . i tend to have my idle set low.

[mrunderhill1975a]

Mine has similar behaviour, but it varies with the ambient air temp and humidity.  The mikarb needs to be adjusted for the time of year and weather conditions.  One way to help is to increase idle screw when the weather causes problems.

[baird4444]

Chas- I thought a carb cough was a sign of rich and spitting back out the carb could be a sign of lean or not enuff advance. Have your points closed up causing a little retard in the timing? what air filter and silencer are you running? I ran a 27.5 pilot with a K&N air filter and drilled silencer.
If you are stock; I'm thinking you are rich; especially since you are only
3/4 out on the air screw. 
                         - Mike

[Chasfield]

The points are gapped correctly.

Silencer is a Brituro classic style.  Air filter is standard.  Carb needle is set on bottom groove.

I'm thinking I have a bit of crossover on the mixture front. Maybe it's a bit rich on the pilot jet but still slightly weak in transition to main jet running. Unfortunately I need the influence of the pilot jet for that transition until a richer carb slide comes my way.

It is now better than before I started tinkering (always a bonus) and on-throttle performance has  been great all along  - I only have problems when dealing with hold ups at junctions, when the carb gives me the run around.

I was getting some kick backs until I retarded the ignition, particularly on a hot start when some throttle is required. The motor actually blew the carb off its rubbers at one point

 


so I will keep her slightly retarded for now.

[Leonard]

On my bike the choke (enricher) sometimes doesn't close down completely when the big lever is pulled up (off) and I have to reach down and give the plunger a push with my thumb.  When this happens it will run fine until I come to a stop then it spits, sputters, coughs and dies.  Took me quite awhile to figure out what was happening, now I just automatically push the plunger in after lifting up the lever.

[baird4444]

 
    "so I will keep her slightly retarded for now."

Keep an eye on your header pipe. Too much can cause excessive bluing.
                                          - Mike

[Vince]

     Most engines, particularly a long stroke single, do not take quick throttle openings  off idle well at all. That is why cars with carburetors have accelerator pumps. Yanking the throttle open at idle lets in too much air without the velocity to lift fuel out of the float bowl. It will run lean, cough, and die no matter how rich you jet it.
     I suggest trying this: Roll the throttle on gently from a stop. If that seems to help then re-jet for best over all running. None of my customers are running the jetting combination you are running.
     Other issues are timing and valve adjustment. These bikes can be surprisingly finicky for these tuning points.

[Bath Bullet]

Up until today I had similar problems to yours, although annoying, not as bad.
Start up was reasonably easy, choke needed depending on the ambient air temp, but it pulled at lower revs with some slight strain, and on run down from higher revs using the engine breaking then pulling away again I could feel a 'gap' as it went from main jet to low, then back up again.
Never was able to fix it to a permanent throttle stop tick over, it always altered from too slow to too fast.
Today I changed the slow jet from 25 to 27.5, and the main from 110 to 120.
WOW!!!!! Its a new bike!!
it started so easy both from stone cold and hot, ans runs so much smoother, and it pulls like a train!
Bearing in mind its still running in too, I have to throttle back, its totally different!
Looking forward to see what happens when the warmer, drier weather arrives back!

[Chasfield]

Vince

It was never an issue of yanking the throttle. The hiccup would occur in gentle use, just as I was picking the engine up from idle prior to engaging first.

Does seem much better now. The engine is right there with me when I transition from pilot to main jet running - throttle feels more directly connected.

Regarding float height on a VM28: I went for 23mm but I can't find any hard published official numbers other than some weird ones for the original Mikuni instrument, which I am guessing pertain to a different (plastic rather than brass) float assembly.

[DaveG297]

You gotta remember that these engines were  setup lean to begin with.    I use a shorty muffler and rejeted to 115 on the main jet.   Haven't had to fool with the pilot jet yet.  I cough and quit now and then but the bike runs just fine.   I wonder how much gas quality makes in how the bike runs.   I have had some really stinking gas and felt the effects.   Clean , fresh good grade gas helps a lot.    I notice a lot of guys like to really idle the engines down slow.   It doesn't make for smooth acceleratiion.    Happens on a lot of small engines if the idle is too low.............d

[Chasfield]

The quest to eliminate cough 'n' stall continues.

My richer carb. slide was delivered yesterday, together with a new throttle cable - might as well change it since the carb. was going to come off anyway.

Here's how my "45 minute" job went.

Taking out the old 3.0 slide and building the cable retention bits into the new 2.5 one was a few minutes work. Then I installed the new cable. With just a carb. rubber left to coax into place, I noticed that the throttle was stuck one third open. The new cable inner seemed to be too short, even with the adjuster wound all the way in.  I took everything apart again and compared new cable to old. They seemed to be dimensionally identical, with the same amount of exposed inner available. So I put the new cable back in. Still no good. So I put the seemingly identical old one back and normal operation was restored.

Now, to test the new slide. I turned on the petcock and petrol spewed all over the place. The fuel pipe had split. Time for supper.

Next day, I drive to the car accessory shop for some fuel pipe. I have to guess the size because the old piece of pipe has disappeared to a parallel universe (where a lot of my stuff seems to go) even though I definitely left it on the bike's seat.  The new pipe goes on in 5 minutes.

After a TEST (Total Elapsed Shed Time) of two and a half hours, I am finally ready for a test ride.

Previously, I had been compensating for weak mixture on transition from idle to main by setting the pilot jet air screw rich. This improved the cough 'n' stall a bit but I was dead certain the engine wasn't getting enough air on a closed throttle and this was making it awkward to start. With the new richer slide in place I confidently set the air screw to one and a quarter turns out.

She fired up on the first kick after a week in a cold shed. This was encouraging. I can report that the cough 'n' stall situation seems much improved though I need to do some more test miles to be certain.

[500KsGerry]

What I really need for clean throttle transition is a carb slide with a richer cutaway. These seem hard to come by in the UK at the moment

If its  the Mikarb get  the  richer slide online    http://store.royalenfieldusa.com/aftermarket-parts-accessories/performance/richer-throttle-slide  Previous owner of  my bike  did this one.  No stall or  cough after  warmup any  wheather 29degrees to 80    Gerry

[VMAX]

Gerry: Thanks for the site    Todd

[Chasfield]

Cough 'n' stall continues to be much better than before I intervened but the engine still hiccups and dies once in a while just to keep me on my toes, most often when I come to a halt smartly.

My current theory (since I have done more than enough carburettor jet changes and the machine pulls like a train at all throttle settings) is that my auto advance unit springs are tired and they let the engine intermittently move off the static advance setting at cranking and idle rpms. This would explain my bike's mischievous tendency to sometimes kick back on hot starts. I am sure my static timing is spot on and I have measured the auto advance increment to be 12 degrees at the distributor, which corresponds to 24 degrees of crankshaft rotation, giving 34 degrees total max advance.

So, I have ordered up a new pair of springs and the auto advance unit extractor tool.

I will report back when they have been fitted.

[Chasfield]

Got the new auto advance springs and extractor tool. The auto unit came off in a few moments and shortly thereafter it was dismantled on the kitchen table. Guess what, the points cam carrier plate was bent all over the place (not by me) so that the slide pins wouldn't slide cleanly and worse, the one that hits the stop to control the degree of max advance couldn't make it all the way home, so the advance range was compressed a bit.

I gently bent the assembly back into a better shape (the metal is v. soft)and installed the new springs, which are noticeably more snappy than the old ones.

When I reinstall the unit tomorrow, I will be able to set a conservative static advance, to allow kick back free starting, without stifling performance at normal engine rpms.

I think another poster discovered that their auto advance had been mangled at some stage by a previous owner. I think this happens if somebody tries to lever the thing of without using the proper extractor tool.

[Chasfield]

I put the auto-advance unit back in, complete with snappier springs. It is interesting just how an engine's characteristics can change upon a slight alteration of a minor but critical part.

Yup, those new springs were too snappy. They drove the auto advance actuation threshold up the rev range, making the bike retarded and intractable at low engine speeds. Oh, and the engine still hiccuped and faltered once in a while, as before.

So I put the old springs back.

Unbending the auto advance plate, as per my last post, did slightly extend the range of auto advance available and my engine is now definitely a bit sweeter at all speeds. I had previously been vexed a bit by ignition timing issues: a static advance setting for easy starting stifled performance at higher revs.

BTW, if you want to check your auto advance mechanism but don't have the extractor tool, you can, with care, dismantle all the movable bits off the back plate without pulling it off the distributor shaft. If the central retaining allen bolt is removed, the movable bits are only then being held in place by the advance control springs. These can be unhooked and re-hooked with care.