[makenzie71]

Alright I need some VM34 help. I've had this set of VM34's in my shop for ages that had originally been set up for an EX500 race bike. After screwing around with jetting I got a mix that worked really well with the CX500 until a couple weeks ago. Started up and revved through the whole range without any problem...and then all of a sudden she's like NOPE! The bike currently starts right up no problem with the choke on, but the very instant any throttle is given it stalls. I can cup my hand over the intake to restrict flow and get it to rev up but it's certainly underpowered. If I take the choke off it stalls. If I mess around with it and get RPM up above 5k where it's drawing out of the main it'll pull the rest of the way pretty hard and smooth but once it falls back down it's a struggle to save it before it stalls.

Ideas? My first thought was the most simple and obvious...fuel starvation...but the jets are pristine. I've had them both apart multiple times and can't find any issues. I even tried swapping from the 145 mains to a NEW set of 150's thinking maybe the colder air lately was just making it ask for more fuel and there was no change.

 

[Randall-in-Mpls]

Air leak at the head? Another thread on a bike with similar behavior is leaning that way.

 

[makenzie71]

I would think an air leak would just cause it to rev up, not stall completely with throttle. Got a link to the thread? I'll see if I can find it....

 

[Randall-in-Mpls]

Now, I can't seem to find it.
In the last few days, someone described needing full choke for normal running and the engine racing with the choke closed. It might have been tacked onto an old thread.

 

[makenzie71]

145 main, 40 or 45 pilot. It was running fine, then suddenly it decided it would not run unless choked and would die as soon as you gave it any throttle. If I keep the RPM up it does okay, but once it gets low it struggles.

I moved the needle up one position and I have the air screws at 1.25 turns out and this is as good as I could get it and this was after 10 minutes of messing with stuff so the engine is good and warm here. It ran fine this summer...only difference now is it's about 30 degrees colder. I know that means denser air so it needs more fuel. I'm going to go get some bigger pilot jets in the morning and see. It's been suggested a few times to check CDI and stator...the low rpm coil resistance is a little low at 325ohms but every other test is textbook...and I was getting 325ohms back in 2020 when I put the bike together.
I did test for air leaks around the manifolds and boots and found nothing.
Any other suggestions would be appreciated.

 

[jlbutler81]

Air leak at the head? Another thread on a bike with similar behavior is leaning that way.

This is just an idea and I hate commenting on this as I know little about your carbs, but running with choke on, or hand choking it (blocking the intake) to get it to run right, to me indicates the same kind of vacuum/compression loss. In chainsaw stuff I'm more familiar with, it'd be an indicator that there's a compression issue, like rings are bad leading to no vacuum through the carb and you're opening that butterfly up and there's now less vacuum being created through the carb and it dies. Your bikes behavior is identical to my last weedeater that I had to rebuild with a new piston and ring set. Compression or leakdown test maybe if it's not terribly inconvenient?

A sudden change means something suddenly changed; I don't imagine this was a simple jet size problem. Although I could see trash in the carb doing it too.

 

[CXPHREAK]

Why don't you test the spark strength?

What do the plugs look like after running? Black and sooty?

It does sound like it could be a low speed coil issue.

Your jetting sounds normal enough based on my limited experience with these.

 

[makenzie71]

Why don't you test the spark strength?

What do the plugs look like after running? Black and sooty?

It does sound like it could be a low speed coil issue.

Your jetting sounds normal enough based on my limited experience with these.

I haven't pulled a plug out...I already know with all my messing around they're gonna look awful. I do know that the coils are kicking out bright blue spark clean spark (through a spare plug I have on hand).

I'm certainly not discounting the low speed coil, it's just that the low speed coil reading hasn't changed since I put the bike together back in 2020.

 

[CXPHREAK]

Scratch that on reading the plug. If you've been running on 'choke' {not actually a choke} they'll be sooty regardless.

Perhaps the plugs themselves have failed and I would at least try another set though.

These may help with the miks.

Mikuni VM Carburetor Tuning Manual

In order to look at the manual, please click on the image below. Mikuni VM round slide carburetor tuning manual If you would like to take a look at our tech tips page, click here.

backwaterperformance.com

http://www.mikuni.com/pdf/vmmanual.pdf

Not being a smartarse but you do know the mik mixture screw works backwards? In for rich, out to lean as it works on an air circuit not fuel.?

 

[makenzie71]

I just went down the list again and all stuff checks out except that low speed coil, but it still measures the same as it did back in 2020. I have 120VAC at 2000rpm on the blue wire and 90VAC on the white wire. By 4000rpm both have dropped off to near 30VAC but by that time the engine is pulling hard past it.

If I have to give up on it being a fueling issue then I'll probably look at the Rae San hall kit...my understanding is that it eliminates the need for the low speed coil: is that correct?

 

[makenzie71]

Not being a smartarse but you do know the mik mixture screw works backwards? In for rich, out to lean as it works on an air circuit not fuel.?

I don't mind lol my experience tuning VM carburetors is pretty limited. I understand the idea, just haven't had much opportunity to put tools to them in the past. I do understand the air screw is backward and turning it all the way in seemed to make things "a little" better. I'll try a #50 or #65 pilot and see if it improves things.

 

[CXPHREAK]

The raesan hall replaces the whole lot. Given that you already have coilovers none of the original ignition system will remain in use. The hall replaces the pickups, the ignition head unit takes over the advance functions.

It was previously running OK with those coilovers?

 

[makenzie71]

yeah I've ran those R6 coils since September of 2020. This issue didn't come up until November...I put the carburetors on in September of this year.

 

[CXPHREAK]

If you have the factory coils it would be 20 minutes work to refit them ... just to see ... and remove them again if the coils aren't the issue. You don't even need to mount them, just ground them to the bike with jumper leads.

I don't expect they will be but if it were me I'd do it just to strike it off of the list of possible issues. I've had a couple of coil failures through incompatibility with regards to resistance.

 

[makenzie71]

I'm 99% sure they're the coils that came off the bike and same results

 

[Randall-in-Mpls]

Do the compression test, just to eliminate it.

 

[Sidecar Bob]

If it was working with those carbs and then suddenly wasn't changing the jets is probably a waste of time unless one of the ones you removed was blocked.

If it isn't making funny mechanical sounds and it has good spark I'd check the simple things like valve clearances & maybe compression but only to procrastinate about taking the carbs out & apart to find out what is blocked.

 

[makenzie71]

Probably but it's the easy thing to eliminate.

Compression test was great, but I doubted compression to begin with...I have never had a low compression situation start beautifully...and the carburetors are spotless. I haven't checked valve clearances yet, though, probably should...

The slow speed coil has been a little out of spec since I got the bike...I hate saying "I'm sure that's the problem" but really it's the only thing not in spec. I'm going to drop the motor and swap the stator out...or buy the ray san kit, I'm not sure yet. I got another stator sitting here that's fully in spec but I really don't want to drop the engine haha