Plug Chop Questions

[Phaseman]

OK, I seem to be getting pretty close to having my carbs all dialed in. I have some questions regarding the plug chop method of tuning the carbs. I have asked these questions a few different ways and I'm still having trouble getting it down, thanks for your patience.

Low RPM test:
So I do my laps around the neighborhood and cut the engine... When I get my color results on the plugs for this test, is it the screws on the very bottom of the carbs that go up through the float bowl that I adjust? I have these currently set at 1 turn out (all 4). The advice I have seen suggests starting at 1and1/4 to 3/4 of a turn out. is this low rpm plug chop test to fine tune these screws?

Mid RPM test:
My understanding is that when I get the plugs colored for this second test, I will fine tune with the screws on the side of the carbs, am I correct?

High RPM test:
So now I check the plugs after a full bore run through the gears. Is the fine tuning done for this test through the tops of the carbs? As in the same screws/nuts adjusted for the carb synch?

I would appreciate it if someone could look through this for me and telling me what I have right or wrong about each step, Thanks!

[BassCliff]

Hi,

Only at idle and low throttle settings do the air and fuel screws help. At medium throttle settings you adjust the needle jet and jet needle. At full throttle you'll adjust the main jet. Here are a couple of references.

http://www.650motorcycles.com/CarbTuning.html

http://geocities.com/mraxl/carb/mikuniindex.html

There is also a VM manual on my website.

Thank you for your indulgence,

BassCliff

[posplayr]

Carbs

Maybe Bill will be here after a while, but you need to get the mains set first, then needles then the air screws.

Bike needs to pull hard to redline or it is not right no matter where the needles or air screws are.

Then adjust needle given main is correct

Final tuning for idle comes last.

Have not done a plug chop but it should not alter this order of carb setup.

Jim

[BassCliff]

Hey Jim,

He's got VM carbs. Don't you start low and work your way up to the mains with those?


Thank you for your indulgence,

BassCliff

[posplayr]

Cliff,
OK I did not notice that. I was refering to CV's as per my recent mountain top tuning of the 36mm 1150 carbs.

Never mind. .

Jim

[Phaseman]

The plot thickens..

[BassCliff]

Hi Mr. Phaseman,

Doh! I forgot I had saved this information on my website...

Plug Chops-Highest RPM Methods
(by Mr. tkent02 and Mr. psyguy)

If you're running lean, install larger jets. If you find your running rich, install smaller jets.


Thank you for your indulgence,

BassCliff

[hampshirehog]

For plug chops it's the amount of throttle opening that you adjust, not the revs.
For your pilot circuit just let the bike tickover for a few minutes (don't let it overheat!). There are 2 adjustment screws on VMs - the simplest method is to set the screws underneath the carbs (the fuel screws) 1 turn out from lightly seated (careful with seating them - the tips break off really easily) and then adjust the airscrews on the sides of the carbs. If a Colourtune is in your budget buy one - I've had mine for donkey's years and it's never lied to me yet!

You can follow the slide / needle circuit and mains tuning guide on Basscliff's site or you can again use the Colourtune. If you use the Colourtune you have to pull the other plug leads off or the bike will rev its head off - tricky to do the first time you try it but once you've sussed it you'll find it easy.

[srsupertrap]

+1: To check the VM carb circuits the only sure way to ensure your on the pilot, needle or main circuit etc. is based on throttle/slide opening not RPM. The needle circuit actually operates from 1/4 - 3/4 throttle opening, it can operate as low as 1/8 throttle opening. Stating one can check the needle at 1/4 to 1/3 throttle is a bit low on the operating scale. I would suggest checking the needle circuit by doing your plug chop at 1/2 throttle. The VM Main circuit operates from 3/4 throttle to WFO and the circuits are cummalative (sic) in operation.

Phaseman, if you index your throttle twist grip against the throttle slide postion and do some test riding I think you will quickly determine that during 95% of the time your carbs are only operating on the pilot & needle circuit. One word of caution when checking the main circuit leave lots of room out on the open road.