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Author Topic: RD350 VM28's on a CL350  (Read 1101 times)

Offline scott_jangers

  • Posts: 68
RD350 VM28's on a CL350
« on: Sep 24, 2017, 18:49:49 »
Hi there,

Yesterday I got hold of a CL350 which had been given a half-arsed cafe makeover but was being sold as non-runner due to tuning issues. It has a pair of Mikuni VM28 round slide carbs which look very similar to those found on an two-stroke RD350. I was told it did run when the PO bought it, but it was pretty rough. He messed about with it changing main jets, etc, but had no joy.

The plan is to check the valve clearances and ignition timing and then see if I can get the engine to tick over, but the carbs will need some love before that happens.

I've opened up the carbs to find out what's in them and to put them through the ultra sonic cleaner and this is what I know....

Main jets 280
Pilot jets 30
Slide 2.0
Needle 5L1 (richest setting)

So I understand that none of this is stock (apart from the slide maybe?) so someone's fiddled about with it. The main jets seem way out. I'll inevitably reduce the main jets to around the 160-180 mark, but first I am focusing on getting the engine to idle. The biggest question I have is regarding the air jet, as it appears to be intentionally blocked. A stock VM28 should have man air jet size of 0.5. I'm uncertain as to why this blocked and can only assume that is due to it being used on a two-stroke. I'm wondering if I can drop the pilot, needle size, etc to compensate, or drill the blockage out and tap for a 4mm threaded air jet?

Any advice would be great as I would like to at least attempt to get these carbs working before I fork out my hard cash on a set of 'pre-jetted' VM30's.

Many thanks!


« Last Edit: Sep 24, 2017, 18:52:05 by scott_jangers »

Offline cxman

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Re: RD350 VM28's on a CL350
« Reply #1 on: Sep 24, 2017, 18:53:22 »
those carbs have a few proprietary mods

including to the enricheners

you might be better off selling them as working rd carbs and putting them towards a set of

new carbs ie baby 28s or 30 mikunis

 
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline scott_jangers

  • Posts: 68
Re: RD350 VM28's on a CL350
« Reply #2 on: Sep 24, 2017, 18:59:06 »
 
those carbs have a few proprietary mods

including to the enricheners

you might be better off selling them as working rd carbs and putting them towards a set of

new carbs ie baby 28s or 30 mikunis

 

Hi cxman. Yeah, I went to show today and spoke to guy who works for a Yamaha specialist who restore RD's and he said exactly the same thing.

Offline raptormeat

  • Posts: 400
Re: RD350 VM28's on a CL350
« Reply #3 on: Oct 24, 2017, 05:05:42 »
if you go with mikunis, id try 30 pilots, .5 or .8 air jet is fine, its really only for just off idle IMO, which can be adjusted somewhat with the needle and needle jet. for the main jet, jet lean and see what doesnt lean out from a 1/2 to wide open throttle snap. probably 160 to start. as for needle and needle jet, the best needles I have found so far are 5F3's (really lean midrange, but short) and 6DH7's, which are longer and meant for vm30's, but work well in vm28's, they help lean out the midrange too. Needle jets would be O or P, maybe try one of each.
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Offline scott_jangers

  • Posts: 68
Re: RD350 VM28's on a CL350
« Reply #4 on: Oct 25, 2017, 20:30:01 »
if you go with mikunis, id try 30 pilots, .5 or .8 air jet is fine, its really only for just off idle IMO, which can be adjusted somewhat with the needle and needle jet. for the main jet, jet lean and see what doesnt lean out from a 1/2 to wide open throttle snap. probably 160 to start. as for needle and needle jet, the best needles I have found so far are 5F3's (really lean midrange, but short) and 6DH7's, which are longer and meant for vm30's, but work well in vm28's, they help lean out the midrange too. Needle jets would be O or P, maybe try one of each.

Thanks raptormeat. Thatís useful info and reassuring. Iíve had the bike running on the VM28ís for a couple of weeks now. Hereís where iím at...

1 - pilot went from a 30, 25 to 22.5 with the mixture screw a couple of turns out. The colortune 500 and plug colour all point to a nice mixture.

2 - the main at WOT is too lean. Started at 160, then 170, now 180 and still too lean according to a plug chop. Will try 190 and 200 next.

3 - the needle is in the richest setting. Not happy with its steel taper so may try an alternative one. If still too lean I may change the jet to an O-6.

The bike had some shorty pipes installed and a pair of velocity stacks so the air flows quite freely and because of this it empties the tank. Because of this I may try and change the setup before I get involved more heavily. Plus I just bloody love jetting and whipping carbs off on dark cold lay-bys.

Iím considering ditching the stacks in favour of the stock air filters by extending the hose to join them to the 28ís.

Iíd also like to get rid of the straight shorty pipes in return for a pair of 17Ē reverse cone megas. Would this be better for the engine regarding fuel efficiency? How will it affect performance?
« Last Edit: Oct 25, 2017, 20:35:06 by scott_jangers »

Offline teazer

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Re: RD350 VM28's on a CL350
« Reply #5 on: Oct 25, 2017, 20:56:32 »
How are you reading the plugs - with a WOT plug chop under load or after riding home?  That would make a huge difference to how the plugs look. 


here's a good article:  http://www.strappe.com/plugs.html

Offline scott_jangers

  • Posts: 68
Re: RD350 VM28's on a CL350
« Reply #6 on: Oct 29, 2017, 19:35:18 »
How are you reading the plugs - with a WOT plug chop under load or after riding home?  That would make a huge difference to how the plugs look. 


here's a good article:  http://www.strappe.com/plugs.html

Thanks Teazer. That's a really interesting and useful article. It's definitely made me question what I'd previously assumed was correct. I am definitely 'the amateur tuner' he refers to. The paragraphs discussing ignition advance and then plug heat range were enlightening.Not sure I'd recognise what fuel film would look like, but I may go back to the timing and play about.

In answer to your question, I have been looking at the plugs generally to see what colour they are under normal riding conditions, but when concerned with the main jet and operating under WOT, I have been riding along a straight section of road for a minute or two, cutting the engine, and then removing the threaded part of the plug to check the electrode colour.

The Cycle Magazine article has me now questioning how to read the results. I had always thought you wanted a nice brown section along the bottom, but it states that anything over a mm is too rich? I've included a photo of a plug chop I did on a 125. I had assumed the #92.5 plug was the right colour (it also ran the nicest) but the article points to this being too rich and the #90 being better? Have i misunderstood this?

Or are the readings on these plugs difficult to read because there's too much soot caused by the ignition advance/heat range not being dialled in first?
« Last Edit: Oct 29, 2017, 19:37:08 by scott_jangers »

Offline teazer

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Re: RD350 VM28's on a CL350
« Reply #7 on: Oct 30, 2017, 00:44:17 »
That's a really good question.  Plug color reflects the heat that the plug reached and so if we have a plug that runs too hot, it will look "lean" because it's running hot.  Likewise a plug that runs a little cold will look as if it's running rich because it doesn't get hot enough to burn itself clean.  Plugs are designed to run under load in the self cleaning range of temperatures.

The tricky part is that a plug/timing/jetting set up that runs hot enough at mid range may be too hot at high load and vice versa.  That's why it's important to start with the right plug heat range and then get ignition timing spot on and then set main jetting.

And after we get that right, we can start to fine tune mid range and transitional jetting.  In a production facility they have teams of engineers checking plug temperatures with sophisticated test plugs and they can run almost any load scenario on the dyno and then on the test track and street to get the jetting spot on.  Most of us lack those resources so we do what we can.

What I like to do is to set idle jetting first and then the main jet and then off idle transition - go up or down one jet size from ideal and see how it feels when I roll on the throttle from a dead stop. That off idle response is more important than having a perfect idle and the bike stumbles as I start to move. We live in the real world and that's not what we are looking for.

After that I dial in mid range and transition by feel.  On a bike with a new combination of carbs and exhaust and state of tune, I will typically mark the throttle so I can determine what it does at different revs and at different throttle positions. As a rule, the actual Air: fuel ratio is not so important as long as it's
s withing generally accepted limits. At peak revs under load, I need to know what power it makes, what the A:F looks like and then I can examine the plugs and see what they look like when that combo makes best power and then I can jet it up one size or two to be safe.

I don't do a lot of street miles so economy is not really an issue for me, but smooth response is critical and so is slightly safe jetting.

Offline scott_jangers

  • Posts: 68
Re: RD350 VM28's on a CL350
« Reply #8 on: Oct 30, 2017, 12:08:07 »
Thatís massively helpful Teazer. I appreciate you taking the time to run through this stuff with me.

That's a really good question.  Plug color reflects the heat that the plug reached and so if we have a plug that runs too hot, it will look "lean" because it's running hot.  Likewise a plug that runs a little cold will look as if it's running rich because it doesn't get hot enough to burn itself clean.  Plugs are designed to run under load in the self cleaning range of temperatures.

If I take my setup as a hypothetical scenario, you have a stock bike that now runs a more free-flowing air filter/exhaust setup (in my case itís velocity stacks and shorty megaphones) could I correctly assume that...

A - more air flow needs more fuel added so increases the amount of mixture the cylinder can pull in.

B - the increased volume of air/fuel puts a greater demand on the stock plug, which has had its heat range selected for a stock setup.

C - a hotter plug would have a better ability to ignite the increased volume of air/fuel mixture

Iím currently using the recommended NGK B8ES but would wonder what the benefits of using a B7ES might be? I imagine the risk is when the engine is flat out and the temperature is at its hottest? Iím currently using the bike to get to work, which means a lot of low revs and stopping and starting, but soon iíll get back on my commuter and this bike will be used less for city rides.

Hereís a photo fresh out of the cylinder after some rush hour traffic. Would you call this carbon fouling? From what you said it could be different after I ran the bike up to temp and rode flat out for a period, in that it would run hotter and burn away any deposits?
« Last Edit: Oct 31, 2017, 03:22:06 by scott_jangers »