what jet size?

deecha · Posted: March 18, 2008, 1:37 am Post subject: what jet size?

I just put on a set of custom 1 3/4 inch pipes on my bike and when I let off the gas the bike pops. I have been told this is a lean misfire which I would think that I would need to put a bigger pilot or slow jet in the bike is this true? Can someone please help to avoid a lot of wasted time. thanks

xsjohn · Posted: March 18, 2008, 4:32 am Post subject: Re: what jet size?

Could try turning the mixture screw out a bit more.....if you havn't already....

xsjohn

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

What kind of carbs have you got?

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

Yes, the pilot jet is the usual cause but you have several other factors working against you. First, from your photo, it appears you're running straight pipes with no mufflers? Those can be very hard to jet for and really aren't the best choice for an exhaust. Second, 1 3/4" diameter is really too big for street use. Large diameter pipes move the power band up to near or above redline. Sure, racers run them, but they want their power up high in the revs. To use them on the street, you may need to add reducing inserts at the head.

That being said, you can try larger pilots and see if they help. Go up one size to start. Re-set your mix screws when you change the jets. Ideal setting will be different with a bigger jet than before. That should help but if it's still popping, go up another size. 2 to 3 sizes up is about the most that these bikes usually need, one size up is the norm. If those sizes won't cure yours then, as I said, it may be related to your exhaust style and diameter choice.

deecha · Posted: March 18, 2008, 12:57 pm Post subject: Re: what jet size?

Thanks a lot guys I really appreciate it. The pipes that are on the bike in the picture are not the ones I just had made. I have stock carbs but I will mess with mixture screws and also pilot jet once again thanks a lot guys I will let you know how it works out.

royfisk · Posted: March 18, 2008, 5:16 pm Post subject: Re: what jet size?

better gotten some cans on those pipes or you may need a valve job next theres only 325 cc in those cylinders.

NateDogg · Posted: March 18, 2008, 8:10 pm Post subject: Re: what jet size?

ok, just out of curiousity as I am learning, what would prevent one from running just there head pipes and going up a couple sizes on their main and pilot? I guess I'mm just not comprehending why it matters if the hole where the smoke comes out is 30inch farther back or right near your feet. I've seen some of these bikes with just a short turn out bolted to the head pipe.

royfisk · Posted: March 18, 2008, 10:16 pm Post subject: Re: what jet size?

nate
good breathing is essential to make any engine run. Over breathing causes other problams such as engine operating temperatures. without sufficiant back pressure you will burn valves and possibly pistons. I have seen bikes with short drag pipes myself. You may be able to get it to run, the question is how long will it last? Most of the bikes I have seen were either all out full race bikes which were torn down and rebuilt on a regular basis anyhow, or choppers that got ridden very little in the past 30 years. which reminds me most of them the pipes are blue from the exhaust port all the way down to the middle of the crankcase. HOT. The flames out the pipe are cool looking though. If it were me I would use 1.5" pipes and a washer baffle, now you got your looks, sound, and something to keep in a tad of back pressure. 13/4 is way to much pipe in my opinion for a cylinder of this size. Mayby you can use a bigger washer?? I dont know.

deecha · Posted: March 19, 2008, 1:27 am Post subject: Re: what jet size?

well I went up on the pilot and she is running good right now. I have a digital thermometer that I can use to check the exhaust temps and the bike will not be ridden much and I am looking to get some mufflers for the bike thanks for the info guys.

NateDogg · Posted: March 19, 2008, 8:07 am Post subject: Re: what jet size?

Thanks for explaining that, my stock pipes are actually starting to grow on me in combination with the other changes I've made to the bike's appearance....just wish they were a tad healthier sounding though

deecha · Posted: March 19, 2008, 10:37 am Post subject: Re: what jet size?

I like the pipes I have on my bike right now they are simple. I like to be a little loud so people can actually here me coming. Loud pipes save lives.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

You might consider the shorty megaphones from Mike's (#07-0014). They'll give you a short style exhaust, add some needed back pressure, and are sized to fit a 1 3/4" pipe without the need for reducers (a good thing for sealing). My buddy has a set on one of his 650s and I can attest that they are quite loud Smile

royfisk · Posted: March 19, 2008, 12:22 pm Post subject: Re: what jet size?

Ive run them too and find them very obnoctious. parden my spelling. loud they are.

deecha · Posted: March 19, 2008, 8:00 pm Post subject: Re: what jet size?

Thanks for the advice I was just looking at the megaphone mufflers and I would get them but I don't like the look of the megaphone style. I found some shorty mufflers from 650 central that are straight and if anyone has tried those or has a friend that has some let me know how they worked out thanks.

NateDogg · Posted: March 19, 2008, 10:20 pm Post subject: Re: what jet size?

I've always considered those but never knew of anyone that had them or how they sounded. Would you recommend re-jetting with those?

Cooltouch · Posted: March 20, 2008, 12:07 am Post subject: Re: what jet size?

Anybody running these? (07-0014) These have been my first choice for my cafe project. I'm curious if the baffle is removable, and if it is, if it is possible to pack the muffler with glass to quieten it somewhat.

Best,

ichael

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

You should always re-jet when you do intake and/or exhaust mods. If the bike didn't need it, you can always revert back to how it was, but if it did need it, you'd be running lean without the change. If it did need it, you'll also get increased performance and more power with the larger jets. If you don't try them, you'll never know.

You don't have to go nuts with a bunch of sizes. Just get the next size or 2 above what you're running now and take it from there. These bikes really don't need huge increases in main jet size, usually one to 3 or 4 steps up at most. For just an exhaust change, sometimes only one size up is all that's needed. But here's one of the first rules of jetting - when you think you're good, try the size above and below that to make sure.

deecha · Posted: March 20, 2008, 6:41 am Post subject: Re: what jet size?

Well I ordered the shorty mufflers from 650central and should be here on monday so when I get them put on I will let all know how it went. Right now I went up from 42.5 on pilot to a 45 and it seems to be running ok but I need to make more fine adjustments and she will be running good.

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