jetting

[Shockman54]

ok, so i've read about a thousand posts on jetting, but i wanted to be more specific with my bike. i have mikes xs oval pod filters, and stock headers with harley sportster muffs. right now its stock jetting, and it runs decent on the higher rpms, but still needs some more gas. lower rpms it doesn't have much power under load, and just revving it in neutral, it takes a bit to get up to the higher rpms. opinions? questions? thanks again guys, last thing before i get her on the road for good

[BA80]

Take a plug reading and see what they look like. You are pretty close to stock so you may not improve anything with jets. Jets don't magically cure problems in fact messing with them causes problems more often than not.

That 79 engine has some pretty long duration cams in it and don't have much power under 3000 RPMs but when they hit about 4500-5000 you had better be hanging on

These engines have a solid steel automotive type crankshaft and do rev slower that a crotch rocket, they have much more torque though.


IMHO

[TADracer]

+1 on what Greg said. These motors do not wind up quickly when compared to newer inline 4's. Everyone always goes straight for the carbs when everything is not honky-dorey but that is usually a mistake. Check your initial timing, vacuum advance and machanical advance mechanisms, plug heat range, compression, valve clearances, and the quality of your fuel first (if it has been sitting along time or is high in ethanol). Then sync and adjust your idle mixture and get everything right before you start with re-jetting.

If you decide to re-jet, it is a progression of adjustments. I recommend you jet from the top down like in this link:

http://www.factorypro.com/tech/carbtun.html

It works.

[foreda]

Infrared heat detector

I have found using an infrared thermometer is a useful tool for tuning and trouble shooting. You can get one from Radio Shack for pretty cheap.
Dale

[sparks]

I am ignorant about motorcycles. That said, I used to build automotive race engines and did a bit of road racing. It used to amuse me to no end when folk would jump on a Holley 750 double pumper when thier car could only breath less than half that.. There are a lot of things you have to do to give the engine a 750 cfm apetite efectively.. If I were to give advice (and I often did) I would say leave the carb for last and then get one that matches
your cars apetite..

Lee

[sparks]

I used a thermometer for many years.. Saved a lot of pistons that way finding lean mixtures before they got costly.. This was before the fancy ones available today.. That and a vac guage are pretty good evaluation tools.

Lee

I have found using an infrared thermometer is a useful tool for tuning and trouble shooting. You can get one from Radio Shack for pretty cheap.
Dale

[BA80]

I am ignorant about motorcycles. That said, I used to build automotive race engines and did a bit of road racing. It used to amuse me to no end when folk would jump on a Holley 750 double pumper when thier car could only breath less than half that.. There are a lot of things you have to do to give the engine a 750 cfm apetite efectively.. If I were to give advice (and I often did) I would say leave the carb for last and then get one that matches
your cars apetite..

Lee

+1 sparks, it's called volumetric efficiency.

I used to be chief mechanic for a buddy dirt tracking back in the day. He had a nice alky setup for the race engine but with the wrong cam and heads it was a dog.

[Shockman54]

btw, i tried it with the airbox and stock exhaust, and i could barely get it started. dunno if that tells you anything. and could someone walk me thru the methods you just described? how do i do a plug reading? how do i use the thermometer to tell whats lean and whats rich?
i also personally checked the initial timing, vacuum advance and mechanical advance mechanisms, valve clearances (just reshimmed and then ran it and rode it a bit, then checked again) as far as quality of fuel, it is 89 octane oxy and ethanol free. i also had a mechanic who has 35 years of experience do a quick check of the timing and advances, and the wires going to them. also checked the coils (put 3 ohm ones from an 81 special and removed the ballast resistor and plugged the wires in correctly) and then tested them for correct voltage etc. i just feel stuck with the carbs and tuning.

[BA80]

A plug reading is done after riding it for some distance.

You can do a throttle chop by running it hard up through the gears and then cutting the ignition and pulling the plugs to check color but you have to start with good plugs.

That way only shows how its running when your on it hard anyway.

The plugs under normal riding conditions should be light to dark tan. White is lean and black is rich.

To check the pipes with a thermometer the bike has to be ridden a ways also. A lean cylinder will have a hotter pipe and a rich cylinder a cooler one.

But, if you check with a themometer at idle all pipes should be hot if they are firing.

Clear as mud?

[Shockman54]

thanks greg, much clearer.

if i decide to re jet, do the ones on mikes xs fit? they look like they do, but i want to be sure...

[Shockman54]

ok, did a plug check after some riding with new plugs, and the #1 and #3 plugs are really dark and the #2 and #4 are very light, not quite white, but getting there...