What is with the POP STALL

[blackbetty]

So here's where I'm at.

Kop tells me I need to measure piston/head clearance at TDC and presented possible scenarios. I'll take his advice and take a wax measurement through the plug hole. I know I have no piston/valve clearance issues, we're trying to figure out if the squish is adequate. He also told me to fix my too long exhaust by cutting at 32" or inserting a reversion cone of some sort at that point of length.

I also called Roger at Hoos, who did my machine work. He tells me adamantly that piston/head clearance on my setup is probably somewhere around .008 to .01, which is plenty adequate and wouldn't be contributing to my issue given his experience building 700s and 750s. I did discover that his jetting for his 750 race bike is exactly the same as my 9.5:1 700cc street bike. Obviously, per griz's suggestion, I'm almost certainly overjetted. To put the piston/head clearances into perspective, Roger tells me his 750 race motor is at .005 and really maxed out in terms of what he can do before harmful piston/valve interference occurs.

So then I spoke to MMM. He sad to pull the air screw out to 4 turns to check on the performance. I'll give that a shot; in the meantime I ordered 25 pilots and 200 and 205 mains to have on hand. He also told me that running the base gasket and keeping at 9.5:1 isn't really letting me take full advantage of my Shell 1 cam, and that idle on these will naturally need to be raised given the cam specs. Makes me wonder if I should have gone higher on compression. He also told me to increase my plug gap.

So, I'll order some wax and do a squish measurement under kop's suggestion, and while I wait for the wax I'm going to play with the carbs and open the gaps a bit.

 

[pregrid]

 

[Geers]

Mikuni vm 34s on a hoos built engine. Running p5 jet 22.5 pilot 6f9 needles with the clip one step up from the middle with a 180 main. I am pretynmuch at sea level. Runs a liitle on the richer side but better than lean! I too have a idle issue after running and coming to a stop. Usually not enough to stall it but it has happened. I'm with ya man can't seem to figure it out. Thinking about ditching the mikunis and running something else.

 

[blackbetty]

I forgot to update this, thanks for the reminder.

I took Kop's knowledge regarding pipe length, exhaust reversion, and baffles. I did the lollipop mod. It made a night and day difference even with my jetting unchanged (210MJ, 30PJ, needle middle position, and 1.5 turns out). The bike will still give me the occasional hiccup, but it's not enough to knock out the motor. I've gone down from stalling 12 times a ride to ab 1-2.

I have new jets to install, I'll be dropping to a 205MJ and a 27.5PJ to see how it runs. Plugs look ok, but there's a bunch of carbon on my piston crowns and I don't have more than 500 miles on it since rebuild.

I'll report back after a jet change if it completely solves my problem.

 

[pregrid]

What's a lollipop mod?

 

[Halloweenie]

What's a lollipop mod?

I'm thinking something like a homemade Snuff-or-Not.

 

[pregrid]

OK.......................what's Snuff-or-Not??

 

[blackbetty]

http://www.bobhenneman.info/dragpipe.htm

I've been experimenting with the angle of the washer; even small changes make huge differences in the way the motor runs. Decel sounds a bit flappy, but i can live w it.

 

[pregrid]

wow, learn something new every day! Thanks.

 

[Geers]

Can a tight exhaust valve cause pop stall

 

[ReycleBill]

Learned a lot here, signing on to learn more even though I'm not suffering this malady.

 

[ReycleBill]

OK.......................what's Snuff-or-Not??

A Snuff-or-Not was basically a lollipop baffle (think butterfly valve) that mounted inside a straight pipe and could be switched on and off by turning a small knob on the outside of the pipe. Back in the day we use to love to close one or two of them of a buddy's 750 Four and watch him try and figure it out.

Haven't seen them in years.

 

[Halloweenie]

Haven't seen them in years.

You can find them every now and then on eBay, but you'll be spending buku bucks for them.

 

[blackbetty]

I dropped to a 27.5 pilot and a 205 main and gave the bike a few miles.

It definitely rides cleaner and doesn't fell as blubbery, especially at WOT. Still won't take choke though and died a few times at lights, but it wasn't the classic pop stall I've been getting.

I've been leaving the mix screws at 1.5 turns initial for each jet change so I have some sort of baseline.

Anyway, after looking at what other people have jetted their 700cc motors at, I decided to drop the jetting even more to a 25 pilot and a 200 main. I haven't ridden yet like this, but I'll adjust the mixture and play w my baffling a bit more to see if the problem completely resolves.

 

[pregrid]

Cool, keep us posted. You're about to become an expert!!

 

[Geers]

What are your valves set at. I found out that the if the exhaust valves are a little tight it will effect the stalling at lights after longer runs. For instance Gary @ HOOS told me to start with .005 all around. I have been playing with the valves a good amount and I'm currently running .006 intake and .012 exhaust. My engine is at it's best right now.

 

[blackbetty]

Hey Geers,

I'm running .006" intake and .012" exhaust, spec for the Shell #1 cam. I'm about to go out back and dial the mix with the new jetting. Stay tuned.

 

[timbeck]

subscribed

 

[blackbetty]

So I didn't get a chance to get around to the bike until last night.

I put the manometer on the carbs and set the mix, I'm at about 1.25 turns in.

With the 25 pilot and the 200 main the bike pulls cleanly and once warm, it didn't give me a single pop stall in traffic this morning on my way to work. Awesome.

Here's the drawback. It's a real bitch to start now; it still won't take choke, so I have to catch it with throttle to warm it enough to keep it going; if I turn the idle up, it ends up spinning too fast once the motor's warm. So, that's telling me my pilot may still be too big, what with the lack of choke and all.

I'm getting there, feel like I need to drop one more size on the pilot and give it another go. I don't think I need to go lower on the mains, it seemed to pull very cleanly when I was rappin it at WOT.

I'll see if I can inch my way towards great success with another drop in jetting.

One more update coming.

 

[osteoderm]

Ah, these marvelous Mikuni VM's...

I still haven't had a chance to test into the main jet range, but for off idle and around town, the combination of 27.5 pilot, 185 main, 6F9 in middle position has been working well for me. The only deviation from the standard tuning scheme was to set the slides to 11/64" instead of 5/32" as often recommended.

Following along with your tuning, I'm curious if your larger main /smaller pilot approach is closing in on the same sweet spot as my larger pilot/smaller main. Have you been playing with the needle position any?