Chop Test Results

[2T5]

Hi all,

Hoping for some tuning advice regarding an air-cooled, 500cc, single cylinder 2 stroke.

I have been attempting to jet a Keihin PWK 39 carb and am having some issues.

A recent chop test (see attached pic) shows a lean condition, yet under acceleration (higher rpm) the bike is not revving out cleanly.

The main jet used is a #210.

I have recently changed to a 1/3 VP C10 race fuel mix to contain detonation.

I am wondering if the race fuel changes the spark plug reading and also if retarded ignition timing would cause poor running at higher rpm.

[m in sc]

possibly.. but id throw more fuel at it.

[RDnuTZ]

what bike and what carb was on there stock? That main jet seems awfully small.

[SoCal250]

I am wondering if the race fuel changes the spark plug reading and also if retarded ignition timing would cause poor running at higher rpm.

Yes and Yes. You should tune the bike with the fuel you will be using. Race fuel will change the required jetting slightly. Timing that is too retarded can cause a lack of power and performance issues.
Lean jetting will also cause lack of power, the feeling that it is running "flat", and will make it difficult to rev out. I would follow Mark's suggestion and increase your jetting.

[Jung]

Is that from a single pass? If so, the pic on the left looks plenty rich to me. I've always aimed for a consistent, milk-chocolate brown line a couple of mill up the insulator.

When I'm not sure on jetting (which is regularly :)), I go up a BUNCH of sizes until it chokes up noticeably on the pipe, then come down one at a time until I'm happy.

[2T5]

what bike and what carb was on there stock? That main jet seems awfully small.

Original carb was Mikuni VM38.

#210 is pretty big, there's only a few sizes bigger available. It's also much larger than what I've seen other 500's list as suitable.

I am wondering if the race fuel changes the spark plug reading and also if retarded ignition timing would cause poor running at higher rpm.

Yes and Yes. You should tune the bike with the fuel you will be using. Race fuel will change the required jetting slightly. Timing that is too retarded can cause a lack of power and performance issues.
Lean jetting will also cause lack of power, the feeling that it is running "flat", and will make it difficult to rev out. I would follow Mark's suggestion and increase your jetting.

Have had to increase fuel with the addition of race fuel.

Timing was fine before I introduced the race fuel but am now wondering if it needs to be advanced to suit.

It has been running 'flat' up top since I introduced race fuel, and it also splutters a bit. It feels so uncomfortable at the top of the rev range that I now don't push it to rev right out.

Is that from a single pass? If so, the pic on the left looks plenty rich to me. I've always aimed for a consistent, milk-chocolate brown line a couple of mill up the insulator.

That is a single, short pass. It's just short of two millimetres up the insulator.



Will richen the main and give that a try.

Also, is this diagram an accurate guide?

[2T5]

An update:

I halved the amount of race fuel I was mixing with the bowser/pump fuel and did another chop test with the same main jet.

The bike runs much cleaner now but still shows slightly lean. A good result!

I'll go to the next main jet size and try it again.

Here's the plug from the latest test:

[RDnuTZ]

RE: Big bore AC 2T singles: ALL my '75-79 Yamaha 400cc MX and YZ w/stock VM38s run at least 350 main jets. If memory serves, same for a couple YZ465 and even YZ490s I owned years ago.

I have no idea how that correlates (if at all) to PWK39 jet sizing, so just offering FWIW.

[Jung]

PWK jet sizes don't correlate to Mikuni. 210 is a whopper for Keihin.

2T5 - that looks OK to me mate, if that was mine, I'd probably try one size leaner. Going richer first can't hurt though.

[m in sc]

thats my point.  go richer 1st, if it gets worse you then safely know which way to go

[2T5]

RE: Big bore AC 2T singles: ALL my '75-79 Yamaha 400cc MX and YZ w/stock VM38s run at least 350 main jets. If memory serves, same for a couple YZ465 and even YZ490s I owned years ago.

I have no idea how that correlates (if at all) to PWK39 jet sizing, so just offering FWIW.

Looking at some jets I have from the old Mikuni carb the jets are #450, 460 and 470. That would be from a time when the bike was unmodified. My bike is a '92 WR500, same motor as '87-'90 YZ490.

I read somewhere that number-wise, Mikuni jet sizes are about 2.5 times the Keihin jets, with the Keihin measured by diameter, the Mikuni by flow rate. I have no idea whether that's true.

210 is a whopper for Keihin.

Yep, the PWK 39 is specified to come with a #160 main, and I have seen other 500cc bikes listed as using #160-170. My bike has a few mods and is set up for road use only. It can be under a bit of pressure at times, but I'm surprised such extreme jetting is required.

thats my point.  go richer 1st, if it gets worse you then safely know which way to go

I will go up two sizes to 220 and if it's rich I'll go back one, or, depending on how rich, I may leave it.

Will hopefully get to test again tomorrow.

[m in sc]

trying to compare the keihin/pwk sizes doesn't matter. ones orifice, the other is flow related. air jet, jet tube series and even needle will play a part as well.    throw as much fuel at it as it can take, then just too much. thats the upper limit of range. then, go down 1 or 2 sizes & probably be perfect.. and safe. then over time using it will let you know where it likes to be dialed in, needle, etc. . 

[rodneya]

Some race fuels leave no or little color on the plug making it hard to read, but at just 1/3 it should not make a difference to the reading.
Retarded timing will lower power throughout the rev range.

If the plug chops were done correctly (Wide open through the gears and then about 10sec kill motor and pull clutch in and stop) then both look rich.
When you say it is not revving out cleanly at the top is it spluttering or bogging (just getting a bwaaaaaaaaaaa type sound)

[2T5]

then both look rich.

Why do you say that?

When you say it is not revving out cleanly at the top is it spluttering or bogging (just getting a bwaaaaaaaaaaa type sound)

It was spluttering but less race fuel solved that issue and it's now revving cleanly.


Another test today and I'm baffled.

I went from 210 to 220 main and used no race fuel in the mix. If the diagram above can be relied on, it's now showing leaner.

I don't know what to do now. Leak down test? Restrict the air intake?
Have just ordered a 230 main jet.
It shouldn't be this hard.

[m in sc]

how does it pull wide open power wise? if its too rich it will lay down and get boggy. if its too lean will cut out.

theres a point where it can be -too- rich and 'wash' the plug clean because you cant burn all the fuel.