Carb Vacume at Idle

[rdrguru]

Hey everyone, wrapping up my mods to my 1982 v50iii. Before my engineer anal retentivness kicks in, it is running great, nice and smooth and accelerating nicely. I think the timing is a slightly retarded as I fell it should be a little snapier than it currently is, but I don't have a timing light. I am running a Dyna Electronic ignition with Dyna coil coils. I was syncing the carbs on Friday and was pulling about 42 mmHg, and I can't find the spec, does that value sound about right?

Thanks!

 

[V700Steve]

I haven't seen info on how much it is supposed to be, just equal. Borrow a light for timing, you need to sync it also at full advance.

 

[rdrguru]

Thanks, they are equal, I am looking to base line it currently to try and determine if I'm having any valve or piston ring leak. On acceleration the vacuumed needles reactly, as well as my eyes can tell, simultaneously, so they do appear to be balanced as well. I do need to borrow a timing light though, that is no doubt! Thanks again!

 

[john zibell]

Thanks, they are equal, I am looking to base line it currently to try and determine if I'm having any valve or piston ring leak. On acceleration the vacuumed needles reactly, as well as my eyes can tell, simultaneously, so they do appear to be balanced as well. I do need to borrow a timing light though, that is no doubt! Thanks again!

The vacuum drawn won't answer that question. A leak down test is what will tell you if you have ring or valve leaking. Timing lights aren't expensive, just buy one. I have inductive pickup one I've been using for over 40 years.

 

[Paul Brice]

As John says, the vacuum draw isn't going to tell you anything about valves/rings etc & you need compression & leak testing to check that. Just for info, my V50 3 (when balanced & set for stable lowinsh idle) will pull a vacuum of -4inch Hg or -101mmHg. I have video but they are too big, so here are screen grabs...

 

[rdrguru]

Yep, mine is not that high, and again I'm almost sure the timing is retarded. I'm going to check that next, set it to spec and check it again. When I took it for a test ride, I couldn't get the rpms any higher than about 6000. I know my needle is set correctly and I have full range on my slide. I pulled my plugs after the ride and they look good, so not too lean or rich.

 

[Paul Brice]

Definitely get a strobe & do your timing & check the advance bobbins are working (marks moving on rev rise)....lots of carburation & ignition problems have the same symptoms

I've gone up approx 20% on main & pilot jets to compensate for lack of airbox. Before that the engine would also struggle at higher revs as too lean - plug colour didn't really help as much of the time I was also just bumbling around town at low revs or on idle !

 

[dbarale]

Did you time it statically?

 

[rdrguru]

No, like an idiot I trusted that the guy who owned before me had that all set up correctly...Well that would have been the only thing because so far just about everything has had issues.

 

[dbarale]

Just for reference as I was recently dealing with the same thing and I had the picture in my phone.
When the right and left timing marks on the flywheel are lined up, the magnet on the rotor (I added the sharpie lines because I can’t see crap anymore) should be lined up with the pickup sensors (little square bumps on the pickup modules).

Ignore the melting charging wiring at the bottom. That will be addressed next.

 

[Paul Brice]

If you have electronic ignition, don't forget previous owner may have kindly moved the pickups on the plate - I vaguely remember having to do same when I moved Dynatek unit from V50 to V65.

 

[john zibell]

Ignore the melting charging wiring at the bottom. That will be addressed next.

I had that same issue a long time ago on my Mk V LeMans. I removed the connector and went with three individual spade connectors. Also tin the contacts with solder so you won't get the corrosion issue.

 

[dbarale]

If you have electronic ignition, don't forget previous owner may have kindly moved the pickups on the plate - I vaguely remember having to do same when I moved Dynatek unit from V50 to V65.

Yes, left pickup is adjusted by rotating the plate, right pickup is slotted and is adjusted by moving the pickup on the plate.

 

[rdrguru]

All great info, once this polar vortex slides off a little to the north, I'll dig back into it. I think I have a combo of 2 issues, I am sure I have my slide needle slightly lean, I need to lower the c-clip one slot, and I think it should be more snappy into mid-range. It doesn't bog, but it feels very restrained. It's hard for me to remember that this is just a little 500, use to riding my KTM 1190 Adventure with way too much power!

 

[rdrguru]

New question, but same vein, has anyone ditched the 28mm carbs and gone with a 32 or 34mm carb. In doing some research, it looks like this engine could greatly use a larger carb...thoughts or experiences?

 

[john zibell]

New question, but same vein, has anyone ditched the 28mm carbs and gone with a 32 or 34mm carb. In doing some research, it looks like this engine could greatly use a larger carb...thoughts or experiences?

There is an excel spread sheet in the downloads to help determining optimum carb size. Enter bore, stroke, and carb diameter and it will calculate the optimum RPM. The larger the diameter of the carb, the higher the rpm for optimum performance. Unless you are making it a drag bike, I recommend staying with the stock size.

 

[V700Steve]

What John says!!!! I have installed 30mm from a V65 on a V50 but not larger. You need to check size of intake manifold & head if you swap.

 

[rdrguru]

Ok, doing a regroup and am abandoning chaging the carbs. Using a velocity stack with a dynatek electronic ignition (that's the way I bought it) the previous owner has a 40 idle, 125 main, and the original 50 slide. Knowing that a 125 is way too lean for that set-up, I have opted to go to a 135 main, a 42 idle, and swap the original 50 slide for a 40 slide. From what I have read this slide/idle jet setup smooths the midrange and then the bike should have always had a 135 main the the velocity stack, so it won't be too lean going forward. I think this will give me the performance that it should have.

 

[MotoGuzziFan]

"V50iii" and "performance" have never ever been used in the same sentence.

My advice is to realize this going forward.

The engine is small, low horsepower (48 brand new under optimum conditions, more realistic on yours is maybe 40) and maximum torque of 32ftlb, and nothing is going to significantly change this. It is what it is.

More power comes from displacement. Hence the displacement size of the main 90 degree twins.

The sub 750cc motorcycles were an afterthought, built as reliable transportation for the masses. Due to the limited price, (and much lower profit) they are constructed differently (less expensive components) than the larger 90 degree twins.

They will not take the hard running you can put on a 750cc or greater V-twin Moto Guzzi.

Whichever way you go in your desire, I wish you great success!

It's nice to see people keep these rare ones going.

 

[rdrguru]

By performance, I simple was referring to evening out the mid-throttle lag, and then the engine starving for fuel at wide open. I fully understand that this bike/engine is not a testosterone infused wild beast that will rip my arms off with a twist of the throttle! I've been tinkering with bikes for 40 some years, and just want to clean up the low end and mid range.