Jetting a V twin

[Williamjr55]

Plug readings on my 23HP V twin indicate it's lean..have a after market intake and Air Filter and header..looking at the carb I notice that the cylinders are jetted with different size jets..is that because the manufactures assume that the engine will be stationary? has anyone rejetted a Honda GX670?

 

[Bass Hole]

There is a few posts out here about drilling or changing jet sizes. If I remember correctly its about .010 bigger jet with a header.

 

[Williamjr55]

I have checked with the local Honda dealer and they can order quite a few larger jets ..so drilling a jet ( done that) is not the problem I was curious if anyone has tried using the same size jets in both cylinders ..or is there a REASON why Honda staggers the jet size.

 

[EZ-GOnewild]

Nubbins???

Your presence is requested...

 

[HotRodCarts]

I'm not sure why Honda staggers the jets. Probably because the rear cylinder will run hotter. Maybe it's the design of the head or intake. When you take a plug reading do the plugs look even?

 

[Nubs]

the 670 has a two barrel carb, one jet for primary the other for the secondary. the 670 came in two version's, industrial and consumer and are completely different although look similar. either way an upgrade to a mikuni is the only way to dial it in

 

[Williamjr55]

This motor is a VAB2 (industrial I believe) 2 barrel carb one for each cylinder..and that is why I'm questioning the staggered jetting and yes both cylinders appear to be running lean..have had the theory that the rear cylinder runs hotter thus the richer jet..not sure I agree..really want to stay with the factory carb if possible...I can't believe I have the only Honda GX670 out here with the Stretch Plastic cold air intake and stainless header...am I the only one with a lean mixture..and this is at 25-2600 ft above here in Tucson..could be a problem when I go to the sand dunes at 300 ft above

 

[qwik]

if theres primary and secondary jets they both will supply gas to both cyclinders. the primary would be low speed and the secondary would be high speed.

 

[Williamjr55]

My understanding of the carb..and it's probably wrong is that it has a low speed pilot jet (#38) and main jet 1 each for each cylinder the factory main is front cyl #98 rear cyl #102..the pilot are plastic from what I see and the air mixture jets are not adjustable (not supposed to be adjustable) haven't tried but assume the little metal anti adjusters could be removed...at this point in tuning I'm going to increase the size of the main jets and try a non staggered pair..they are only $4.00-6.00 ( not cheap but what is for a Honda

 

[dirtysouth]

Best I can figure, the rear cylinder always runs a larger jet to compensate for lean condition do to more heat.

I'd be tickled for someone to tell me different.

 

[TouringArtist]

It sounds like the rear cylinder runs a little hotter than the front. At the cylinder gets hotter, it runs leaner. Using a larger jet in that cylinder will both richen the mixture and also help cool the cylinder. A hot cylinder runs lean and a lean cylinder runs hot, so a richer jet always helps.