What is with the POP STALL

[blackbetty]

OK. Can someone explain what's going on during a pop stall?

I seem to have had this issue since the day I got my bike, even with the motor in stock configuration. I put on a 700 kit and a shell 1 cam, and the problem persists. I am working with a factory deck height with both head and base gaskets.

I was working with my 76-77 BS38s trying to get the bike to run, but unless I ran a huge pilot, (35+), I couldn't get a decent idle.

I have since switched to VM34s, and while the mid and high circuits feel strong, the idle is just so finnicky. My VM34s are from MMM, 210 MJ, 30 PJ, 6F9 in 3rd position, P-6 NJ. I have my mix screws out at 1.5 turns. She wants to pop stall randomly at lights (annoying), and I have also found that when I'm coasting to a stop the engine speed will drop and sometimes the motor will die. I assume this may be a clutch drag issue pulling down RPMs, and that's leading it into a stall.

Plugs have been making me feel like things are on the rich side, I attempted to drop the needle and fatten the mix with no change. The video is with the clip back in the middle slot.

Manometer shows each cylinder within a half inch of each other. The levels will oscillate from side to side during the idle ab an inch but will usually level themselves out after a few seconds.

Here's a video, first "pop" at :11, 2nd one kills the motor.

I also found this video of this stocker doing the same thing:

 

[yamaman]

What rpm you going for at idle? Double check float height? Fuel flow out of tank? Fuel filter?

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[cleoncleon]

Sounds like it could be a small air/vacuum leak at the Carb intake Boot. With the engine running, spray a little WD-40 around there and see if your RPM climbs. I never really liked this method to test so instead, I would just cup my hand around the intake boot and wait for the pop, you should feel some air forced back at your hand around the seams. The boots are rubber, and the bikes are pretty old, pretty much all the manifolds on my old Honda's needed to be restored/ jerry rigged/ or replaced.

 

[jd750ace]

I just recently heard of people using an incense stick to find air leaks. Obviously, glows with increased flow, be it suction leakage or pressure leakage. Works like a champ, on the video I seen anyway, guy does both. Don't have a link, but I'll revisit. Bonus: keeps you from slathering your motor in the name of troubleshooting!

 

[blackbetty]

All good suggestions. Idle speed is around 1200 rpm. Float height was checked by MMM before he shipped out at 25mm. I wonder if maybe they're off given the drop in RPM under braking?

I keep thinking air leak, I even went so far as to pull my pipes to make sure I didn't have any leaks at the collars, though I know this would contribute more towards a backfire. I have replaced everything rubber so if an airleak is still happening, it's right under my nose.

The carb holders are new units from MMM, and I've got them on there pretty tight. I suspect it would be coming from my left cylinder, because I recall it being much more finnicky to set an idle using dead cylinder.

What's strange is that even the most incremental changes in either mix or sync will affect the frequency of the pop stall.

 

[yamaman]

I've no idea with the shell cam, maybe a higher idle is in order? I'd start with a float check. Maybe bring idle up till synch stabilises? Of course you could always give MMM a call. Ignition and advance system all check out?

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[Ironsled77]

mine does the same F-in thing its always fun at a red light in the city : )

 

[ippytattoo]

What ignition are you running?

 

[blackbetty]

It's a boyer micro digital, I have the timing set spot on, tested full advance at 3500rpm with a timing light.

I can conclude the issue is not an ignition or compression issue. For a while I wondered if perhaps my shell 1 cam was off a couple degrees, but it's a hoos cam so it was degreed in a jig and I am running stock deck height, so that's probably not the case. I do recall xsjohn saying that a cam off even just 2* can put a motor in a false lean state. I didn't bother degreeing the cam seeing as the engine is healthy across the power range aside from the goddam pop stall.

It seems as if this problem is common on these machines, as I even noticed my buddy's bone stock 83 doing it from time to time. What's strange is that the only instance I've ever seen of "pop stall" being mentioned is in the VM34 carb guide under the slide cutaway section:

The slide cutaway is the "accelerator pump" on these, and greatly affects throttle response and low throttle open mixture.
If the slide is too small, response will be great but you'll be trying smaller and smaller needle jets to cure the "black plug 15 mpg syndrome". If the slide is too large, response will be poor, and you'll be trying larger pilots etc. to
prevent that "pop-stall" problem.

I noticed on my BS38s that I could resolve the problem by incrementally upping my pilots. By the time I got to a 37.5, the engine finally started and idled on cleanly its own, but of course a 37.5 in a 76-77 BS38 is way too big.

My VM34s are currently set up with 2.5 slide cutaways, spec for XS650s. I should also mention that the chokes on my VM34s are good for prime kicks only. No way the motor will start with both or even one choke pulled, but I can flick on a choke only after its running and only if I hold the RPMs up.

 

[xjwmx]

WD40 good for nothing. Except maybe a squeaky door hinge and it wouldn't be the best thing even for something like that.

To look for air leaks use carb cleaner. The idle will go down if it finds a leak. Cheap. Evaporates leaving no mess. Won't explode. Side benefit of cleaning what you're working on. Doesn't smell like shit.

 

[pregrid]

I agree with giving Mike a call. He has always been helpful.

 

[blackbetty]

Indeed, I've tried a few times but keep missing him. Regardless, I wanted to see if anyone else has dealt with this.

 

[pregrid]

I have not and I'm runnin' the same carbs (from Mike) but I'm curious to know what's causing it.

 

[grizld1]

1. The generic slides (2.5) in your carbs are fine.
2. I don't know where you're running that thing, but unless you're a few hundred feet below sea level, you're jetted way too fat on the pilots and probably on the mains as well. Refusal of the motor to fire on the chokes on cold starts should be making that very clear to you.
3. John Underwood had to ask me what to do about his cam timing (he never mentioned that in his posts, but hey, he never gave credit to anyone else whose advice he sought either). His claim that 2* of advance made the engine fall on its face made no sense at all, especially in regard to the slow-ramp stock camshaft he was using. In racing, the Shell #1 is commonly run 2 or 3 degrees either side of straight-up timing; depends on what sort of event's being tuned for. If you have any doubts, check for yourself with a degree wheel and dial indicator; but you have a lot of leeway there, and I'd be very much surprised if cam timing was your problem.

 

[kopcicle]

Damn , I'm about to agree with Dick , again , as much as it hurts . Cam timing 3 to 4 degrees either way will have little to do with idle quality .
There are a couple of over looked possibilities . Please , what is "stock" deck height ? To little and there is no quench or squish effect . Too much and the piston is nearly into the head . Here I'm referring to "assembled" height lest the uninitiated think I have it backwards . So the first issue is overlap quench forcing unburned combustion products into the intake tract because the previous exhaust negative wave has not yet made it to the exhaust port . So in this case we have a little bile being urp'd into the intake tract and poisoning the next charge . This results in a soft shot and possible stall .
The next issue is too much clearance . In this case there is no assistance from the quench area in scavenging the cylinder and with an overly rich idle mixture the waste spark from a crank triggered ignition actually has something to fire . Compound this with idle reversion from the lumpy cam and the condition only gets worse . Single fire you say ? When fired from the cam all this mess goes around to the next shot and you have another way of producing the same symptoms as in the first case for entirely different reasons .
Both of these scenarios can be exaggerated by short primary exhaust length , velocity stacks , and slide type carburetors . Where a butterfly carburetor exposes the idle discharge port to vacuum the slide type carburetor exposes the idle discharge port to velocity via the slide cutaway . Both accomplish the same thing but the butterfly has the advantage of exposing the idle discharge port directly to vacuum rather than indirectly via velocity . With slide type carburetors you have a comparatively larger discharge area exposed to flow and relatively more sensitivity to reversion .
Let's revisit reversion for a moment . When mixture is expelled and then drawn back through the venturi it is significantly more dense than surrounding air and as such affects the general metering to pick up even more fuel . You only have to look at a open velocity stack anywhere below resonant RPM to see the effect .
Not knowing the complete dimensions and tuning of the several engines in previous posts I'll not even try to supply a cure . I'd rather offer general good practices that will help .
Primary exhaust lengths around 32 to 34" , slash cut if an open pipe .
Zero slop in the advance mechanism .
As much initial advance as it will tolerate without making it a beast to start and still limit overall advance to something short of windows in the pistons . This will allow smaller throttle bore openings at idle and more accurate metering due to higher carburetor venturi velocity .
Don't bang the head with the piston but get it close . Due keep in mind that these are roller bearing assemblies and exhibit quite a bit of growth with heat , centrifugal force and inertia . How much will have to be determined by experiment on a case by case basis .
CV carbs as the BS38 require air boxes . End of story .
All slide carburetors can benefit from air box volume but require significant air filter volume to contain standoff , reversion and allow for a brief straightening of the air flow on the way to the venturi .
If all else fails and Dick and I are fatuous bags of wind you can increase valve lash and artificially shorten duration . You can close intake lash and open exhaust lash to artificially advance a cam or the reverse to retard it . Do keep in mind that these changes will have to be done with a degree wheel and a dial indicator at best or at the very least some low pressure air in the cylinder to verify valve opening . The usual caution to not increase the lash so that the valve train beats Itself do death or decrease it to the point where a valve is held open applies .

Now go do that voodoo that you do sooooo welllllll....

~kop

 

[blackbetty]

Griz;

Thanks for the response and for checking out the thread.
I wasn't too concerned with the slide cutaway, but your comment on my jetting is interesting. I thought a 210 MJ and a 30 PJ was a bit high, but I figured since MMM is tuning these carbs all day long, his assessment would be correct, especially since the baselines on the carb guide are for stock applications.

I have a set of Mikuni 27.5 pilots I'll try tonight. I'll also order some 200 and 205 mains and some 25 pilots to be safe. I'm in Dallas, TX btw. Also, thanks for clarifying the 2-3* scenario on the camshaft. That's peace of mind.

KOP;

Your explanation kicks ass, I finally have a grasp on what's physically happening in the cylinder. Regarding deck height, stock gaskets are .020" base gasket and .048" head gasket, uncompressed. They'll lose about .002" when torqued.

Because I wanted to run close to stock clearances, I ordered copper gaskets with a .021 base gasket, and a .042 head gasket with a 78mm bore. The copper gaskets only compress about a thousandth of an inch when torqued.

The copper gaskets come in the following thicknesses:
.016 .021 .026 .032 .042 .050 .063 .092 .125

I could swap out one of the gaskets for something slightly thinner if you think that the condition I'm experiencing is caused by the high comp pistons with a nearly factory squish. What do you think?

The pipes are both longer than 32" and straight cut. Completely open, no baffles:

Running stage 2 unis on the VMs.

Ignition is a Boyer, so no mechanical advance. I have the timing set with enough advance to start easily. Any more and the thing is just to hard to fire.

I'm reluctant to add artificial duration to the cam in an effort to avoid too much wear and tear. Again, this is a street motor so I feel like this may be kind of a bandaid fix?

 

[pregrid]

I'm curious about your statement on the jetting. Is what you have what came with the carbs from Mike? I ask because, as I have stated, I bought the same carbs from Mike and the settings I got were as follows:

Main jet- 180
pilot- 25
needle/clip- 6F9/3
needle jet- P6
slide- 2.5
float level- 25 (checked and setting is correct)


Mine was running too rich (black plugs and poor-bout 20mpg-mileage)
made these changes:

dropped the needle by raising the clip to the #2 slot
changed the pilot to 30

everything improved including starting and mileage-bout 40mpg

idle wanted to stay way high at warm idle until I moved the air bleed from 2 turns out to 1. Fixed that.


I'm at sea level running Omar's exaust and velocity stacks. Bike starts easy hot or cold, is smooth throughout the driving (riding) range, does not backfire on decel, idles smooth. No complaints, really. Don't know if this will help or fix your problem but you never know.

By the way bore is first over stock, top end is fresh. Also Pamco ignition.

 

[kopcicle]

wax rod

Check piston to head clearance . it was just the first assortment I found . I don't recommend solder or lead of any kind as the top of the piston is easily deformed by gorilla handed idiots that don't stop when resistance is felt at tdc . At least with wax if you leave some in the hole it disappears with first ignition and there is no chance of piston deformation or a bent valve .

~kop

 

[pregrid]

So, Blackbetty, you did state that you had this problem even in the stock form did you not? If so, ya gotta start with the things that are the same between the two configurations. did you have the Boyer before the change? If so, was the initial timing set the same. You got a big bore kit, changed the carbs and intake manifolds, but if the problem existed before those changes you can, initially at least rule those out. You said with the BS38's you could solve the problem by upping the pilot jets, but that the cure was way out of whack for what you should have been runnin'. I'm pretty sure a weak spark will fire a rich mixture easier than a lean one (I could be wrong). If so fattening the mixture may give you a false positive and masking the real problem. Just swinging for the fence here, looking for possibles.

 

[osteoderm]

I'll be following this thread closely... It's good for all of us to share settings as we stumble towards VM34 nirvana.

My VM34 set-up:

Stock engine internals
PAMCO ignition
Gordon Scott 2-into-1 1-1/2" exhaust, minimal muffler
5" UNI filters, no pre-filter sleeves.
Main jet- 185
pilot- 27.5
needle/clip- 6F9/3
needle jet- 159-P6
slide- 2.5
float level- 25
mixture - 1.5 turns out
sea-level...

Had some popping, closed-throttle backfires, and off-idle surging with 25 pilots. Mixture screw position suggests 27.5 is rich enough, and indeed they cured most of the issues. With 25 pilots, the "choke" would improve running; with 27.5 pilots, the choke worsens running, and is generally unnecessary for starting. Changed 180 to 185 mains, but haven't hit the highway yet!

I'm still having a hard time tuning for an even idle through the temperature range: either I can get a nice warm idle, or a nice cold one, but not both... happy with a lopey cold idle now, as it starts on the first kick regardless.

Have you tried retarding the timing a bit to see the effect on off-idle running (probably the rpm range where you get the pop-stalls)? I think most people (including myself) get caught thinking that advance mechanisms advance the timing for high(er) rpm running, when in proper use they actually retard the timing for idle, off-idle, and starting.