Dialing in the carbs

[Greasey Fingers]

Heeeelp!!!! Just in the process of trying to dial in some carbs off a 1978 XS400.

So carbs have been cleaned inside and out, vapour blasted & compressed air through passages.

Running pods and shorty type free flowing mufflers. Pods are attached to the stock plastic pipes that the original air filters go in to, so they are not directly on the carbs.

Carbs synced with manometer.

Valve clearances, point gaps and timing done.

Plus don't look lean.

New mixture screws, turned out 2 turns.

Needle clips raised one notch towards pointy end.

Main jets were: 142.5, now running 150

Pilot jets were: 42.5, now running 47.5

Idles well maybe a tad high, around 1800 rpm, I would prefer around 1500rpm. Theres no hestitation on take off at all.

It runs really good up until around the higher rpm where it just doesn't rev any higher.

In 3rd gear it revs out to 8000 rpm real nice then no higher. In 4th gets to about 7500 rpm and in 5th gear it gets to about 7200 rpm. So as you go through the gears the point where it revs up to progressively gets lower.

Other than the higher rpm's as mentioned its running great. Not sure if my main jet is too big or too small?

Fuel seems to be flowing ok. Correct sprockets for that year are on. The back sprocket is a 38.

Could coils be faulty? Sorry I know this topic has been thrashed to death nut I did reads heaps and heaps of threads first

 

[16VGTIDave]

Sounds to me like the main jets are too large. And the needles might be too high.

You need to get the engine to operating temp by riding briskly for 20 min. Then test the main jets for WOT above 7500 RPM in 3rd or higher gear. What ever it take for the bike to pull strong. Once the main jets are dialled in, then do the needles, WOT from 4000 to 7500 RPM, 3rd or above.

This can be an "all weekend" tuning event, but the results will be worth the effort. Have patience!

Dave

 

[Greasey Fingers]

Cheers for the help dave, I was hoping you would drop by Should I put the needle clip back one position and maybe put back the stock main jets just to see where its at from there? Does the float height effect the higher rev range at all that's one thing I didn't check.

 

[16VGTIDave]

For initial tuning, set the floats at stock height. Float height can be used to fine tune the pilot jets, but unless it is way off, won't have much effect on the needle or main jet tuning.

I'd suggest you take notes of how the bike is operating and what the spark plugs and exhaust outlets look like. Then make 1 change, ONLY! Test ride again, take notes, and then consider your next change.

Again, it is imperative that you get the main jets correct before you can make any correct adjustments of the needles, etc. I have posted a link (sorry, don't have it on my mobile, but I think it is factorypro.com or something similar) a number of times that has detailed instructions. I'm just parroting their procedure, as it makes sense and has worked well for me.