re-jetting

[ekoehler]

i have stock 1978 bs38cv carbs on my xs, that has been bored, pod filters and with the stock exhaust, running leans, backfires when engine breaking and at idle, what would anyone recomend for some jets?

[grizld1]

See www.amckayltd.com/carbguide.pdf

[putty]

Hi all, just looking for a little advice on jetting.  I discovered carbs were missing pilot jets and had stock 127.5 main jets. I have K&N pods with 32 33 inch headers and taper tips. I put in 130 pilots with 150 mains which i had in my parts collection, this solved my deceleration popping i get the occasional pop if i gear down revving high there also seems to be a hesitation or surge (hard to tell which as it is minimal) when the engine is revving around 5-6,000 rpm's. my conundrum is do i go lower or higher on either or both jets.

Thanks
John

[richard]

You can check you plugs to see if they are running lean or rich then jet accordingly, and If it misses at idle when it's hot you can check your valves to make sure they are not set too tight.

[putty]

thanks Richard boy do i feel dumb should have thought of checking plugs duh!

[putty]

okay checked the plugs they are toasty tan so still not sure whether i should go up one more on both or down one more on both or just one as it only seems to do it after 5,000 rpm i know they both affect fuel flow at that high an rpm.

[richard]

Why don't you post a picture of your plugs. Also if they are tan in color you are probably ok on the jetting. It's better to be a little rich than too lean. Any jetting changes you make to your carburetors should always the same for both carburetors on both cylinders.

If you find that one cylinder is burning differently than the other you missed something, ether in your valve lash, your timing or your ignition if you have points.

It could be something as simple as the connections on your plug wires. In the case of back firing on deceleration it could be a pin hole in your exhaust if you are running a little on the rich side.

If you have a bike that has been running fine with no recent changes and it suddenly starts back firing on deceleration that could be an exhaust valve starting to burn or it may be that your valves have just become too tight and need adjusting.

In cases where the valve have become too tight it is usually right after a recent re-torquing of the head which compresses the top of the head just a hair, which is enough to effect the valve clearance. I had to re-torque the top of my head on the 71 four times and reset the valves every time before I got it right.

[putty]

Thanks Richard, i have decided since it only happens at higher rpm's than i usually ride at, then i will play around with it when i put a new exhaust on it which I'm currently saving for as i will probably have to re-jet then anyway.

[grizld1]

John, your posts don't specify which of the many BS38 carb sets you're using, but the 76-77 set was the only one that specified 122.5 as the stock MJ size. In any case I can tell you right now that your #150 mains are too fat. Trying to dial in jetting by looking at the color of insulator deposits will get you nowhere, you need to tune by function. 5twins did a good job on tuning for mods in the XS650 Garage Carb Guide. I'm not going to play "Write it again just for me." Read it, do it, or take the consequences in delay and confusion.

[XS Rod]

Agreed with Griz on reading plugs... Even if you start it, run thru the gears once and shut her down you've gone thru idle, mid, and main ranges in the carb 5 times. All you will get is an indication of something... not a dial in.