Re-jet issues

**J-Adix** · 07-10-2010, 01:55 AM

So I had some issues with my carb after I put on a k&n pod filter and shorty mufflers on my '85 250. It was running too lean and it would sputter in the first 1/4 throttle and the last 3/4 throttle. So I went up two sizes from the stock jets, (#35 slow and #135 main), to a #40 slow and #140 main, and it stopped it's sputtering until I hit 3/4 throttle topping me out at about 60mph. So I figured I need to get a bigger main jet, and went up again and put on a #145 main jet and kept with the #40 slow. I took it out for a ride and it looks like I'll have to get a bigger main jet again because it's still sputtering after 3/4 throttle. Not only that but it's now doing this thing where it has a very slight sputter in the first 1/4 throttle again, nothing major but it's annoying. Why would the slow jet run so beautifully, and then start acting up again? I will definitely put in a bigger main jet, probably a #150, but should I change the slow jet again? It's already a little on the rich side and I don't want to be "wealthy"

, but I do want throttle response. Should I just continue plaing around with jets sizes?

**GearJammer** · 07-10-2010, 03:22 AM

When I had the '85 250, was running a foam Uni-Filter and EMGO 12" Shorties, but lucked out and didn't have to re-jet. On the 450, with the stock airbox and EMGO's, still didn't have to re-jet, but only had to turn up the idle a bit.

Am thinking that you may be jetting up too much on the main jet. If the #40 slow jet gives you proper idle, which it sounds like it does, then you might try sticking with the #40 slow and going back to the stock #135 main and see if that helps.

I haven't really had to mess with the carbs on either Reb, but I know that on the stock 40mm Keihin on Sportsters, the slow (pilot) jet keeps feeding fuel to some degree even beyond idle, so the #40 slow jet might give you enough fuel to keep the whole range happy.

Usually, one dials in the idle, then mid-range, then main jet. Sounds like the #40 is working good for idle and mid-range, but it's possible that the #140/#145 is putting the main jet into the too rich condition that you've mentioned. Please keep in mind that overly rich can also cause sputtering and bog-down.......

**GearJammer** · 07-10-2010, 11:49 AM

I'm just going by the numbers that you provided and it's possible that I'm absolutely wrong.....not an expert here, I keep learning as I go and am fine with that as it keeps the trip exciting. There's no shortage of modified 250's out there and if others chime in with their experiences on this subject, that'd be great.......

You're free to try what I've suggested in the previous post and it'll either work or not, each bike being different and re-jetting being pretty much trial 'n error. If it had been me, I might have started with a #38 slow jet (if they're available ?) and continued from there.....

Just a thought for the Mods: A sticky thread might be valuable for something like this as there are a lot of 250 Rebs that have been modified and dialed in. Maybe others can offer the:

Year of Rebel
Type of air cleaner
Type of exhaust slip-ons
Re-jetting #'s
Idle screw # of turns out from seated, etc.

This might provide at least some reference points for others to consider to help lessen the amount of trial 'n error that has to be gone thru when others modify their 250's.......again, it's just a thought, but such a data base could be pretty helpful......

JunkyardDog · 07-10-2010, 10:01 PM

The K&N pods and shorty mufflers have drastically increased the airflow to the engine, possibly to the point where the carb throat (venturi) is now the limiting factor. But you shouldn't need main jets THAT big.

There are 3 different things to change. The main jet, the slow jet, and the idle air/fuel (pilot) screw. Since tour modifications changed airflow at all speeds, you will need to change all 3. Go back and start over again. I am not sure what jet sizes are available for the Rebel carb, but you said you went up two sizes. I would start with the next size up on the main and slow jets, and open the pilot screw about 1/2 turn more. Evaluate the performance with that setup, install new plugs, and check the plug color after about 100 miles. IMO, a 150 main jet is simply too big.

When motorcycle emissions testing goes away here, I have a like new non CV flat slide Mikuni carb from an XR250 that I intend to try on the Rebel. I have hated CV carbs since I first saw one. EPA meddling again. Jerry.

**J-Adix** · 07-12-2010, 04:44 PM

Those bastards have been selling me the wrong jets! I'll have to go through the entire re-jetting process again from the beginning. The place I've been ordering my jets through have been selling me Mikuni jets, instead of Keihin! These are not the same! All of that wasted time and energy... oh well, at least I've discovered the truth and can now move forward.

My research shows me that while at first glance, Keihin and Mikuni jets are practically identical in appearance, they have some pretty big differences.

For example, if you were to place a Keihin #140 small round main jet, standing up next to a Mikuni #140 small round main jet, the Mikuni jet would be about 1mm taller.

Secondly if you look at the markings on the jet itself, the Mikuni logo, (which is a box with another smaller box within it), will be seen on the top of the jet by the slot followed by the jet size number; while the Keihin logo (which is a slanted K) will be found on the side of the jet underneath the slot and also followed by the jet size number. Those are the only visual differences between a Keihin and Mikuni jets.

The most important difference between these two jets, is that the holes are not the same size, and DO NOT provide the same flow.

1. A Mikuni fuel jet size represents how much liquid gasoline will pass through the hole when gravity fed for a period of 1 minute. For example: a Mikuni #145 = 145 cc's of gas in 1 minute.

2. A Keihin fuel jet size represents the exact measurement of the hole's diameter in millimeters, and has nothing to do with flow rate. For example: a Keihin #145 = 1.45 millimeters exactly.

Both systems are scientifically correct, but you need to be aware of their differences because the guy at the parts counter probably isn't.

So I'll be starting over, this time with Keihin jets, from a different place. I'll start with a #38 slow & #138 main, and go up from there.

JunkyardDog · 07-12-2010, 05:16 PM

Best of luck. Yes, there are significant differences between Keihin and Mikuni carbs. I much prefer the Mikuni, and always have. But for now my stock (engine wise) Rebel is running ok with the Keihin, and I will leave it that way for the time being. Jerry.

**GearJammer** · 07-13-2010, 12:09 AM

Good grief! The parts guy made a HUGE mistake......sorry that you had to go thru that frustration, but glad to hear that you're back on the right track. Thanx also for the info on the difference between Mikuni & Keihin jet sizing. That's valuable info and interesting as well.

You might find this interesting.......I run a 42mm Mik on my Harley. The standard pilot jet size is a #25 as opposed to the #40 that you received in error. The standard main jet size is a #160......not far away from the #140/#145 that you tried.

Those guys had you running jets with fuel flow rates within an "approximate" range for a 1200cc motor on a 250cc motor!!! That'd be like having your own BP disaster going on inside your Keihin! (Not joking here as that BP mess is a downright tragic situation)

Good luck with it & please keep us up-dated on your progress and eventual success! You'll make it happen......

**cagie** · 07-13-2010, 01:55 PM

IMO, I would take that error up with upper level management. If there is any engine damage (now or in the future caused by this error) I would make them liable for it since you as customer are placing your trust and faith in them. Perhaps as a settlement you could have them do a complete bike maintenance on the engine, including a carb rebuild, head rebuild (checking valves in particular), etc. You might not get anywhere, but they would know that you could bring them to court if anything did happen in a reasonable amount of time for damages.

JunkyardDog · 07-13-2010, 04:09 PM

Fortunately you had it way rich instead of too lean, which is a lot less likely to cause engine damage. I'm surprised it even ran that way. Jerry.

**J-Adix** · 07-13-2010, 06:27 PM

Yeah, I'm glad it was rich instead of lean. It also explains why I've only been getting about 150 miles before I hit reserve, and why my engine oil got so dark so quickly. My plugs have been telling me it was rich, but I think because I've been using about 4oz of seafoam in the last two tanks the plugs didn't look as black as they should considering how rich it has been running.

I'm frustrated about the fact that Mountain Motorsports has been selling me the wrong jets but I don't think there has been any serious damage done. I've only ridden my bike about 400 miles since I've started the re-jet process. I'll probably go in there this friday to speak to a manager about this, but only to let them know what I've found out, and to educate their staff so this doesn't continue to happen. Of course if they wanted to redeem themselves in some way, I wouldn't be opposed. I think this problem is pretty wide-spread, because even Chaparral was trying to sell me Mikuni jets at first until I flipped through their parts catalog and noticed how similar the two jets are and started asking questions. In fact they didn't know the differences between the two either. I ended up buying two Keihin jets from them, going home and doing my own research online, and that's when I learned about it. I think there's a pretty good chance that many shops don't know the differences, and just go by the numbers and thread patterns. It's an honest mistake, but a big one none the less. I just wish the shops provided better training for their staff.