VM18 issue

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Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
I think that filter could be getting oil saturated from the blow back which is normal on piston port engines and it can throw off your mixture enough to have you going nuts to tune it since the blowback sends the fuel and air mix out the carb inlet then it gets sucked back in on the next intake stroke making the mixture extra rich momentarily, then on the next down stroke it sends some of that charge out the inlet where the filter can catch part of it again. You should notice the engine behaves better without the filter if the foam in the filter is holding the blowback then it's getting sucked in on the next intake stroke.
You may need to make some kind of a runner tube between the filter and the carb inlet to put more distance between the 2 to help prevent the filter from getting too wet with the fuel and oil since it can throw out a good tune or make tuning very difficult if not impossible... maybe try a larger cotton gauze screen type filter like the K&N or its clones. And if you ride in a very clean and dust free environment you can get a cheap K&N clone filter and carefully burn the cotton gauze out so it's just a screen, this will stop bigger debris but not the small dust particles so it can't trap the fuel and oil mix during blowback. This will make tuning easier but you definitely can't ride in a dusty environment. Another solution would be to use an oversized K&N type filter and use a longer intake duct to prevent filter saturation.
That foam filter you got most likely works great on a reed valve engine where blowback isn't a problem but may be what's keeping you from dialing your engine in since these are piston port type. The easiest way to prevent saturation from blowback is to use the biggest filter you can fit on the bike, even if you need to fabricate an adapter or longer wider intake duct. Luckily pvc piping is cheap and plentiful for this, and it can be painted to hide what it really is.
One of my bikes uses an oversized K&N clone filter with a pw80 inlet duct clamped to the carb so the filter is larger but also about 8" away from the carb inlet.
You can also use an oversized K&N clone with the cotton gauze burned out so it's just the pleated screen then stretch a nylon stocking over it which will still give more than enough surface area, very little restriction, but much safer if in a dusty area. I use this method on my Harley but I use an Outerwears rain sock over the filter to keep dust and water from getting in... it's truly the next best thing to running no filter at all but it protects better than a freshly oiled k&n...
 

fischer550

Member
Mar 24, 2014
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Fort Collins, CO
I dont think blowback is the issue, cause I was only testing it for like 5 minutes and the filter had just been oiled. I don't ride in dusty areas. Is it a big deal if I don't use oil on the foam part of the filter?
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
Just make sure the air is nice and clean before riding without a filter or with less oil in the filter, you can try cleaning the filter and just letting the blow back oil do the oiling for you and see if you see any improvement.
Another thing you can try is to take the foam out and stretch a nylon stocking over the filter housing to see if you get improvement, if this helps then it's definitely the filter holding too much of the blowback.
Hopefully just taking most the filter oil off the foam will help, but there are other ways to see if the filter is what's preventing it from tuning out easily... tuning a bigger carb can become quite tedious and this is part of the reason most people won't mess with an oversized carb, but once you get it all dialed in it'll be worth it.
The basic rule of thumb is the bigger the carb, the tougher it will be to tune, but the 18mm mikuni is still small enough to be fairly easy to tune... trying to tune a 21mm Dellorto can become a real challenge, and trying to tune a 26mm racing carb for every day street use would be near impossible... but the power they make on the track sure is impressive...
 

fischer550

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Mar 24, 2014
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Fort Collins, CO
So I swapped the 70 main for a 65. When I did this, the 0 to full throttle hesitation came back. So I swapped the 22.5 pilot back to the 25. This worked. As of now, the whole throttle range is smooth, just some 4 stroking with very little throttle. I have the air screw 1 and 1/2 turns out, I will try turning it out a little to get rid of the 4 stroking. If that doesn't work, I'll set the needle back to the leanest setting and hope it does not cause the 0 to full throttle hesitation.

All this testing is with no air filter. Hopefully, I won't have to compensate much once the k&n gets here.

I am very close :)
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
Yup... those things are fun to tune... but worth it once you get it there... Glad to hear it's coming around for you... Hopefully the K&N will fit in and stay invisible to the carb once you get it...
BTW, if you need a good looking intake duct to put some distance between your filter and the carb, the stock Yamaha PW80 intake duct fits the VM18 almost perfectly... For some reason imagesack is down so I can't post a pic of mine with the VM18 and the pw80 intake duct with a k&n clone but I'll post it when the site comes back up.
 

fischer550

Member
Mar 24, 2014
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Fort Collins, CO
Now I am getting frustrated. It 4 strokes heavily at very little throttle, but if I lean the air screw or the needle the throttle cracks from 0 to full! Running great 1/4-full throttle, but at minimal throttle it sounds very bad.
 

fischer550

Member
Mar 24, 2014
234
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16
Fort Collins, CO
Getting closer. Set the needle to leanest and it really helped things. Gonna turn out the ir screw more to see if that helps.

Also, as for the pipe, I had a silencer for an sbp pipe, so I hammered it in lol. I'll see what it does
 

JethroBodine

New Member
Sep 23, 2015
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Pontiac.Mi
Sounds like you have yours set up like mine.I have been working so much I have not had time to do much else with mine.Did you use the extra silencer sbp sells?
 

fischer550

Member
Mar 24, 2014
234
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16
Fort Collins, CO
I did use that silencer. It works amazing! It cut down the noise dramatically, and doesnt seem to affect performance too much. I have finally gotten it tuned to where it rides nice, went on a 10 mile ride this morning. Hit 42 on the flats (36 tooth), could have maybe hit 44-45 but didn't want to push it.
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
Sounds great... yeah the bigger carbs can be a pain to tune but once they're dialed in they can make a good power difference. Top speed will go up some but the acceleration difference is really nice since it adds more power pretty much everywhere. Your bike should be able to hit 45 mph or faster pretty easy now
 

fischer550

Member
Mar 24, 2014
234
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Fort Collins, CO
Reaching 45 is a little stressful. Keep in mind I live at 5,000 feet so I lose roughly 15-20 percent horsepower. I run a 5cc fred head, to compensate for compression lost up here I'm sure at sea level it'd go faster, with a 6cc head
 

fischer550

Member
Mar 24, 2014
234
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16
Fort Collins, CO
Thanks. Here is the set up:

Dax F80 Bottom end
Fred ported cylinder with special intake (he no longer makes this cylinder, wish he still did)
Fred head 5cc
MZ MIAMI "best" exhaust
HD Lightning CDI
36 tooth srocket
41 chain
Mikuni vm18 carburetor
32:1 amsoil dominator
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
That's a really nice combo for a really mean engine...
My setups are
Dax F80 bottom end
MZMiami 40mm cylinders, ported by me to increase intake and exhaust duration, exhaust port widened to 29mm wide, intake port widened by 4mm total, transfer roofs cleaned up. Cylinders are set on spacers to set port timing, decks shaved down and gaskets chosen to give a .5 to .6mm squish clearance and 12.7:1 static compression ratio.
Fred 6cc head
Fred's Firebreather exhaust adapter connected to modified kx65 pipe, other engine is using a Pro Circuit CR80 pipe, both RIP
wide intake from Mzmiami
Mikuni VM18 carb, other engine is using a Dellorto PHBG clone 21mm carb.
 
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exokinetic

New Member
Mar 18, 2016
108
4
0
Lake Forest, CA
On the filter issue, although it sounds like you already have it sorted...

A LOT of people, MoPeds included report a loss in performance with the Malossi E8 Filter:




Many of those same people report a performance gain over stock, and over no air filter at all with the Malossi E12 Filter:




I run this Malossi E12 filter on my race engines, and I noticed a slight performance increase in bottom end torque VS. an UNI filter. It looks like its too small and impossible to flow well, but its shape creates a type of velocity stack effect that makes it flow deceptively well.

Race Engine:

-40mm Stroke Crank

-HCH 6202-ZZ C3 Crank Bearings

-Jakes "Diamond" Head (spark plug hole corrected - 5.5ish CC's)

-Squish Gap @ 0.024" (Results in 230-240psi with spark plug corrected Jake Head)

-NGK BP7HS Spark Plug (Gapped at 0.032")

-Custom Ported Wide Intake Port Cylinder (No "boost port".... yet. Decked to set Squish Gap. Intake port relieved for reed valve petal. Widen Exhaust Port. Basic Port Matching, De-Burring and Chamfering)

-Custom Machined Cylinder Base Spacer (Set Exhaust/Transfer Duration)

-Custom Machined Stock Low Wrist Pin Piston (Window Cut in Piston, see Pic)

-Stock Wrist Pin Bushing and Bearing (I get great longevity considering the stress I put on the engine)

-HD Lightning CDI (Retards Ignition at high RPM's. Allows use of higher compression ratios)

-RSE Reed Valve (single petal version)

-Custom 3D Printed Intake Manifold for RSE Reed Valve (Info Coming Soon, Still in R&D Phase)

-Mikuni VM18 Carburetor (Modified Slide, Google U.F.O - Ultimate Flow Optimizer)

-Malossi E12 Filter (Replaced Filter Foam with Polini Blue Foam)

-Fred CR Machine Firebreather Exhaust Header (Literally, not only one of the coolest parts I have ever bought for my MB, also the highest quality machining, as well as HUGE performance increase. I cannot say enough good things about Fred and CR Machine)

-Pro Circuit KX80 Motorcross Pipe (Awesome pipe, exactly what I wanted from the power-band. Great low end grunt, plenty of top end run-out. Seriously, try one)

-16:1 Oil Mix Ratio (Maxima 927 Castor. Before you flame me, read the compression number again. As long as I run oxygenated fuel things stay incredibly clean in the cylinder and exhaust. If I don't, 16:1 Castor results in thick dry carbon deposits literally EVERYWHERE.... such a pain)

-VP U4.4 Racing Fuel (If you have never tried it, I cannot recommend it enough. BUY SOME)


Manufacturing Draft (lol) for Windowed Piston:




If you have any questions, don't hesitate to ask!





Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/
 

Dobby

New Member
Aug 14, 2016
13
0
1
Forks, Washington
So I got my vm18 and my pipe from mzmiami. I live at 5,000 feet elevation. I have a 60 main jet, and 25 pilot. The air screw is turned out one turn only. Needle clip is on leanest setting.

It 4 strokes a tiny bit at very little throttle. I runs great throughout the power band, just one issue. When I open the throttle from 0 to full quickly, it hesitates, like 2 little pops. Then engine doesnt die, but it hesitates just for a second.

Any idea on what I should do?
I have the same carb and filter setup. Try an 75 or 80 main jet. You should notice a significant change in the sputtering /4 stroking.

I'm having somewhat of the same issue with my vm18. I get air bubbles and gaps of air in my fuel line. I also have no throttle response whatsoever from a pedal start. Pulling the throttle with the clutch engaged as the bike is moving actually almost cuts my engine off. With the clutch disengaged, I get normal throttle response and revs. It's really weird. I'll then disassemble the float bowl and pour the fuel out and reassemble and then it works as it should and then a short while late it goes backs to its same bullshit. So if their is anyone here who can advise me on this, I'd greatly appreciate it.
 
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