Carb Tuning/Jetting Cynicism - An Investigation

I spoke to a bloke whose shop sells Mikunis today. He told me that any shop stating that they jet Mikuni carbs for your bike type before shipping them off to you is lying and just trying to make a sale, as they all come pre-jetted from the factory.

I found it an interesting and rather cynical thing to say, so I thought I'd investigate. Anyone found this to be the case? Surely lying about pre-jetting your carbs isn't much of a deal breaker, so what would be the point in it?

It's a slow day at work and I'm bored.

Wouldn't "pre-jetting from the factory" mean that the factory knew which bike and with what air filter and exhaust set up the buyer had, before they sold them to the shop to sell to the buyer? Doesn't make any sense.

Maybe he means they come with nothing done to them and that's how shops also send them out...

Beats me.

I sell Mikunis.

They do come with jets already in them from the factory, but those jets may or may not work for your bike. Almost definitely the latter.

el barto said:

I spoke to a bloke whose shop sells Mikunis today. He told me that any shop stating that they jet Mikuni carbs for your bike type before shipping them off to you is lying and just trying to make a sale, as they all come pre-jetted from the factory.

I found it an interesting and rather cynical thing to say, so I thought I'd investigate. Anyone found this to be the case? Surely lying about pre-jetting your carbs isn't much of a deal breaker, so what would be the point in it?

It's a slow day at work and I'm bored.

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ok mr bored lol . . his statement is overe geeneralized . . some shops may claim they rejetted a carb for a certain application which they may have but who knows how accurate that jetting is . . dinee city claims to have a carb jetted specifically for a 450 honda i think so you could simply call them and ask but as mentioned its impossible to accurately pre jet a carb due to too many variables one of which is elevation.

barnett468 said:

ok mr bored lol . . his statement is overe geeneralized . . some shops may claim they rejetted a carb for a certain application which they may have but who knows how accurate that jetting is . . dinee city claims to have a carb jetted specifically for a 450 honda i think so you could simply call them and ask but as mentioned its impossible to accurately pre jet a carb due to too many variables one of which is elevation.

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Mostly right, but not entirely true.

It's probably splitting hairs, but I suppose a lot of what you're saying depends on the precision you're looking for in your state of tune.

Obviously most bikes, from the factory, came with the same sets of carbs. Dealerships would sometimes replace a jet or two depending on their geography and climate, but the majority of bikes came with the same jets.

This can be done by carb manufacturers and dealers, too. The factory doesn't have a monopoly on setups.

The real question is, "How good is good enough?"

for all intents and purposes it can not be done its that simple . . if a shop took a box stock bike and installed a mikuni and jetted it at sea level up to arond 1000 feet in around 75 degree temperature in fairly dry climate then they could claim the arb was jetted for all this criteria and if your bike and location etc were similar to those the carb was jetted for it would in fact work just fine but how many are going too meet this specific criteria . . if you took the samne carb and used all the same criteria with the exception that the exhaust was changed from stock to a high flow type the bike would in fact be very lean and would most likely turn the chrome on the header blue at the head . . this is the same if only the air box was changed or just the cam was changedf etc . . now that being said just how many people only change one thing on their bike . . the idea that a carb can be pre jetted is ridiculous.

It's not that simple at all.

Many people do sell pre-jetted carbs that work in a wide array of climates and situations. Depending on the vendor they may not work well, but most do work.

You think factory bikes came with a warning sticker that cautioned riders to not use their machines when temps got over 100°F or if the elevations exceeded 2000ft? Factory bikes came with pre-jetted carbs, no?

Yeah... if you're wanting perfect, pre-jetted kits are not likely to satisfy your desire unless you're working with someone with a lot of knowledge and skills (e.g. Murray's Carbs), but if you're wanting your bike to run and ride, pre-jetted kits will often get the job done.

well since i actually worked at a motorcycle factory and personally jetted all the models i was responsible for for production i know more than anyoneb how they jetted bikes . . i also worked at the biggest honda kawasaki motorcycycle dealership in the us where we were the test fitters for basani exhaust pipes and can guarantee with 100 percent certainty that every single bike we put headers on wa in fact leand and needed to be jetted up . . also every service manual does in fact include jetting specs for certain elevations. which is typically above 4500 feet . . i personally would not want to modify my bike so it had more performance then spend a lot of money on high performance only to have my bike jetted to less than the optimumn level but obviously some other people dont care.

This is very interesting.

barnett468 said:

well since i actually worked at a motorcycle factory and personally jetted all the models i was responsible

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Cool. So you know what I'm talking about.

Sonreir said:

The real question is, "How good is good enough?"

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Indeed. Go back to before the mid eighties when every car had carburetors. Drive anything from Key West to Denver and you would not get many complaints. No doubt a huge difference in altitude and humidity, but not like many cars ever suffered any real problems. The performance level of motorcycle engines on the average is greatly higher than most old cars which make a big difference in their sensitivity but "prejetting" a carb and getting it in the ballpark for typical and reasonable applications is not unreasonable at all. I think it is pretty reasonable also to think that anyone making changes to their vehicle has some clue that there are variables involved. If you go buy an OEM carb for your unmodified 1980 Chevy pick up for instance, you expect it will bolt on and be flawless. But if you opt for a 4 bbl Holley (can you still buy those?) and a new intake and headers you expect some adjustments after the fact, but if that Holley carb is in the range suggested by the manufacturer, it won't be that far away from acceptable. You can slap it on and drive it around and only need some minor adjustments. But no - it won't be close for an old 600hp stock block Champ car engine, but guys that build those know that.

yes yyou can still by holleys lol howver a couple ex hollley engineers started a carb company called quick fuel and have ttaken a big share of holleys market because they supply more features at a much lower cot.

if someone buys a prejetted caeb and has to do even one jet change they pretty mnuch just wasted the extra 20.00 or 25.00 dollars they spent for a pre jetted carb and even if they spen 20.00 or so on jets they will have a better running bike in most cases than if they ran a pre jetted carb its that simple. . . i for one would be pretty pissed off if i paid extra for a pre jetted carb and it caused the chrome on my brand new 600.00 headers to turn nlue.

barnett468 said:

if someone buys a prejetted caeb and has to do even one jet change they pretty mnuch just wasted the extra 20.00 or 25.00 dollars they spent for a pre jetted carb and even if they spen 20.00 or so on jets they will have a better running bike in most cases than if they ran a pre jetted carb its that simple.

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Saying it's simple doesn't make it so.

If I bought a pre-jetted kit for an extra 25 bucks and had to change only one jet, I'd be OK with that. Obviously, the preference would be that everything just worked out of the box.

But seriously... $25 and I only have to change one jet instead of four? That seems like a bargain. My time isn't free.

Sonreir said:

Cool. So you know what I'm talking about.

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yes but that is using the assumption that the vehicle is perrfectly jetted to the criteria i mentioned . . if it is jetted perfectly many vehicles will in fact be so rich at around 6000 feet or more that they will start to stumble slightly . of course they will still run and get you to the too of the mountain but they would be far better of if yhey were properly jetted for this altitude if this is where they were oerated most the time . .. also remving the airr box or installing headers can have an effect that is similar to a 7000 fot change in elevation mmeaning that the effect would be huge and big enough to warrant changing the jets

Good. So we agree then. Awesome.

Pre-jetting makes plenty of sense when you are ordering a new carb. YMMV depending on whom you order from but the magic trick is to tell them your bike, elevation, and intake/exhaust modifications, etc and they'll sell you "pre-jetted" carbs. It should be very close to what you need, maybe not perfect but unless you are doing plug chops and testing on a Dyno, it can't be. Everybody has to start with an educated guess.

For instance, Dad ordered a new set of carbs for his 81' GPZ1100 from Sudco. He gave them what they needed to know and when the carbs showed up, boom, nice running bike. Not "Perfect" but damn close.

canyoncarver said:

Pre-jetting makes plenty of sense when you are ordering a new carb. YMMV depending on whom you order from but the magic trick is to tell them your bike, elevation, and intake/exhaust modifications, etc and they'll sell you "pre-jetted" carbs. It should be very close to what you need, maybe not perfect but unless you are doing plug chops and testing on a Dyno, it can't be. Everybody has to start with an educated guess.

For instance, Dad ordered a new set of carbs for his 81' GPZ1100 from Sudco. He gave them what they needed to know and when the carbs showed up, boom, nice running bike. Not "Perfect" but damn close.

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Just to clarify, i was not referring to custom jetted carbs . . I was referring to carbs like the one that the dime store sells . . they say it is jetted for a 460 Honda but do not say if that is for one with headers or big cam or velocity stacks or bored cylinders etc.

barnett468 said:

Just to clarify, i was not referring to custom jetted carbs . . I was referring to carbs like the one that the dime store sells . . they say it is jetted for a 460 Honda but do not say if that is for one with headers or big cam or velocity stacks or bored cylinders etc.

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Right. They probably setup for pods and stock exhaust but I'll bet a phone call can figure it out pretty quick. Personally I wouldn't order a brand new carb without talking on the phone to somebody where you're ordering it.

It's just splitting hairs in the difference between "pre jetted" or "custom jetted".