Damaged Spark plug - Page 3

ChrisWNY · 07-24-2018, 08:07 AM

What size pilot jet comes stock in a Mikuni VM26 carb? I haven't changed mine out but have the main jet at #120. Is the 25 pilot jet a "sweet spot" when running a #120 main jet?

blanc12 · 07-24-2018, 08:40 AM

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

tomrom · 07-24-2018, 09:06 AM

Quote:

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

My ride takes me from 200 ft to 1872 ft. Going home is mostly downhill. Where it failed was on the flats closer to home. Almost wide freaking open but not all the way.

Vermont ain't flat... :-)

ChrisWNY · 07-24-2018, 09:40 AM

Quote:

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

Out here in WNY 500-600 ft. elevation is about average...it's mostly flat where I ride. I did order a #25 pilot jet (genuine Mikuni) online just in case I find that I need it. I also modified the air box, installed the performance exhaust, and jetted with a #120 main jet. Seems to run well in all throttle ranges, at least with the current hot weather we're having (mid 80s and humid daily). I rode the Hawk a few miles on the #110 (after the pipe upgrade) and it ran fine but upon inspecting the spark plug it was definitely running a little lean. Doubt it was lean enough to melt the engine down but better to err on the rich side than too lean.

I'll be avoiding those Wingsmoto main jets in the future and will stick to Mikuni brand - the Wingsmoto jets don't thread in so well.

ben2go · 07-24-2018, 01:37 PM

Quote:

Originally Posted by blanc12

I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.

Please see diagram. Main jet (120) is above 3/4 throttle.

blanc12 · 07-25-2018, 03:07 PM

Quote:

Originally Posted by ben2go

Please see diagram. Main jet (120) is above 3/4 throttle.

Where do you think the fuel comes from when using the throttle valve and needle jet? I think it comes from both the pilot jet and the main jet. Both need to be sized correctly.

ben2go · 07-26-2018, 12:38 PM

Yes. The needle meters the flow from the main through the carb. A thinner needle or raising the needle equals more fuel from the main sooner. It is the transition from the pilot jet to the main. Yes, they both need to be sized correctly.

Megadan · 07-26-2018, 12:55 PM

The main jet does not play a direct role in fueling until right around the 3/4 throtte mark. You would be surprised at how big of a difference in main jet you can run and not be aware of the difference below that point. The main jet is the total fuel level that passes through the needle jet (emulsion tube), but the needle restricts flow through the needle jet as low as 1/8th throttle through 5/8 throttle, thus rendering the main jet size itself more or less inert until the passage restriction of the needle has opened up an area at least equal to the area of the main jet at the emulsion holes.

You could put a 110 and a 120 main jet in the same carb, on the same bike, and never know the diffrence until you go beyond 5/8 throttle, and definitely once you reach 3/4.

blanc12 · 07-26-2018, 02:41 PM

Quote:

Originally Posted by Megadan

The main jet does not play a direct role in fueling until right around the 3/4 throtte mark. You would be surprised at how big of a difference in main jet you can run and not be aware of the difference below that point. The main jet is the total fuel level that passes through the needle jet (emulsion tube), but the needle restricts flow through the needle jet as low as 1/8th throttle through 5/8 throttle, thus rendering the main jet size itself more or less inert until the passage restriction of the needle has opened up an area at least equal to the area of the main jet at the emulsion holes.

You could put a 110 and a 120 main jet in the same carb, on the same bike, and never know the diffrence until you go beyond 5/8 throttle, and definitely once you reach 3/4.

Thanks for the info Dan.

Although you don't feel the difference, fuel is drawn through the main jet as the needle valve is raised. That is what I was getting at.

Megadan · 07-26-2018, 06:31 PM

Quote:

Originally Posted by blanc12

Thanks for the info Dan.

Although you don't feel the difference, fuel is drawn through the main jet as the needle valve is raised. That is what I was getting at.

It does, but since the needle restricts that flow, the size of the main jet orifice itself is a very small factor. That was what I was getting at.

OP it is nice that they are actually working with you to fix the problem. Hopefully the process goes smoothly.

ChrisWNY · 07-26-2018, 01:00 PM

I noticed a big difference when going down a notch on the needle inside the Mikuni carb. Definitely made the Hawk run a lot richer, so rich at all throttle positions (combined with a #120 main jet).

07-26-2018, 12:55 PM	#8
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,100	The main jet does not play a direct role in fueling until right around the 3/4 throtte mark. You would be surprised at how big of a difference in main jet you can run and not be aware of the difference below that point. The main jet is the total fuel level that passes through the needle jet (emulsion tube), but the needle restricts flow through the needle jet as low as 1/8th throttle through 5/8 throttle, thus rendering the main jet size itself more or less inert until the passage restriction of the needle has opened up an area at least equal to the area of the main jet at the emulsion holes. You could put a 110 and a 120 main jet in the same carb, on the same bike, and never know the diffrence until you go beyond 5/8 throttle, and definitely once you reach 3/4. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
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07-24-2018, 08:07 AM	#1
ChrisWNY Join Date: Jul 2018 Posts: 212	What size pilot jet comes stock in a Mikuni VM26 carb? I haven't changed mine out but have the main jet at #120. Is the 25 pilot jet a "sweet spot" when running a #120 main jet? __________________ 2018 RPS Hawk 250cc Mods so far: 1. Mikuni VM26 Carburetor w/#115 main jet 2. 17T/45T JT sprockets 3. JT 428 X-ring chain 4. Air box mod 5. Hawk Digital Cluster 6. Aftermarket IMS shift lever 7. Performance Aftermarket Exhaust 8. 295mm Nitrogen Gas Monoshock, 20W oil front forks
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07-24-2018, 08:40 AM	#2
blanc12 Join Date: Mar 2018 Location: Northern Maine Posts: 55	I am not sure what comes in the mikuni. I think 20. but again not sure. The main jet would the one to worry about here because at wide open throttle that is the one delivering the gas. If your 120 works and up to 1/4 throttle is good. you should be all set. I am running a 27.5 and a 115 in a mikuni. I have the exhaust and modified the air box. I am at 400 feet. Different places need different jets. Elevation and air temp all play in to the mixture.
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07-26-2018, 12:38 PM	#7
ben2go Join Date: Jun 2011 Posts: 1,335	Yes. The needle meters the flow from the main through the carb. A thinner needle or raising the needle equals more fuel from the main sooner. It is the transition from the pilot jet to the main. Yes, they both need to be sized correctly.
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07-26-2018, 01:00 PM	#11
ChrisWNY Join Date: Jul 2018 Posts: 212	I noticed a big difference when going down a notch on the needle inside the Mikuni carb. Definitely made the Hawk run a lot richer, so rich at all throttle positions (combined with a #120 main jet). __________________ 2018 RPS Hawk 250cc Mods so far: 1. Mikuni VM26 Carburetor w/#115 main jet 2. 17T/45T JT sprockets 3. JT 428 X-ring chain 4. Air box mod 5. Hawk Digital Cluster 6. Aftermarket IMS shift lever 7. Performance Aftermarket Exhaust 8. 295mm Nitrogen Gas Monoshock, 20W oil front forks
	0