How exactly are jets categorized?

[DooD]

I'm looking at nabbing an '86 GS750 with an 813 kit in it, pods, and full exhaust.....but the owner says it needs to be jetted properly, 'cause it doesn't run very well.

How might I go about finding jets for it? Do I look up a certain brand/model of carb, and hope to find jets for it? Or are they pretty straightforward, and each manufacturer will have jets available for basically all of their carbs?

I'm just worried that I'll end up buying a bike with good carbs - but won't have the necessary parts to make 'em run right!

Any help much appreciated! Thanks!

[tmk7c9]

The best place to get carb jets will either be in a jet kit (but the factory suggested sizes will be off because of your big bore kit) or just by going to your local motorcycle dealership and talking to the parts guys. where I work we have entire trays just full of jets for different brands of carbs, and different styles of jets within the same brand.

Be sure to take out one of the main jets and bring it with you...many of them look identical but they are not. When you get home its frustrating to find that you bought jets that don't fit quite right.

You will probably need to go up a couple of sizes on the main jets, AND probably shim your needles just a bit to get some midrange back as well.

Given the trial and error you need to go through to get it right you may end up bringing home several sizes of jets, and you need 4 each time, at around $2-$4 a piece....it might actually be worth your money to take it to your local shop and have a mechanic tune it for you. If you find a nice shop, they will probably even have a dyno and exhaust gas analyzer to make sure they get it right the first time.

[DooD]

Hrm...I don' think this'll end up being a trial and error job. I'll probably just take the 1216 and 813 in at the same time, and have them both jetted/tuned on the dyno.

I don't REALLY care what it costs - I'm just worried about the shop being able to get jets for carbs that MAY be 20 years old!

[The CyberPoet]

Each manufacturer of jets uses a slightly different design scheme, so while numbers may match, the actual sizes do not (i.e. - a 120 DynoJet jet is not the same flow as a 120 Mikuni jet because of the taper shape of the flow-area of the jet).

On an '86 with an 813cc overbore & full aftermarket headers, my advice is to contact either Marc at FactoryPro (FactoryPro.com) or the techs at DynoJet (DynoJet.com) and get them to recommend a specific kit/combo directly out of their inventory & knowledge base. Both companies will have records of what should work well with that modified engine configuration and those particular carbs/filters/exhaust (make sure you mention everything).

Once you have the pieces, install them & leave the fairings off, and book time on a dyno (dynometer) that has an exhaust gas monitor built in (some older units won't have this option, so ask before booking the appointment and wasting your $$). In most areas, a full tuning cycle should run between $30 (already spot-on when you roll up) to $150 (if they have to swap jets, tweak, and keep re-running it), although for particularly difficult-to-match set-ups an entire afternoon and bills in the $450 range are not unheard of (which shouldn't happen if the FactoryPro or DynoJet guys give you the recommendation in advance). Typical cost to tune is around $100 in my area.

Cheers,
=-= The CyberPoet

[DooD]

Thanks!

I was kinda planning on having a "dyno day" just before spring, with the Kat and GS, and get both done at once...I just want to make sure I have a good selection of parts for each bike before I go about booking time. I suppose I'll need to figure out what model of carb the GS has before I can contact anyone about a kit, right? I'm not sure if they're stock carbs, but they sure don't look like it.

[The CyberPoet]

Yes, I would highly suggest figuring out if they are the stock carbs or something else, otherwise any advice from a jet-manufacturer would be useless.

Start with the simplest part and work backwards: what's the name on the carbs, and then is there an identifiable series number on each carb thats the same?

Cheers,
=-= The CyberPoet

[DooD]

Cool...Thanks again for the info....

BTW...is there any actual way to test the final displacement of a motor? I seem to be having problems deciding which kit is actually in the bike..... Wiseco lists a 3mm kit for the GS750 up to 1984.....which makes it an 816cc...but I have yet to find any bore kits for the 1986 year! It'd be nice to know what the heck is in this thing!

[The CyberPoet]

There are ways of calculating it by ripping the head down and checking the size of the bore and the length of the stroke.

In theory, you could figure it out simply by measuring the compression ratio (the wiseco kits normally have different compression ratios based on the size of the overbore kit -- e.g. the 771 & 813 kits for the 88-97 Kats are 13:1 and 12.5:1 compression ratio, although I can't recall which was which off the top of my head this morning).

Cheers,
=-= The CyberPoet