Carb Question

[audijunky]

A liitle run down first. 79 standard, 45 pilot jets, 147.5 mains, Pod filters w/TC's velocity stacks, Open header w/washer inside pipes for back pressure, clean carbs, pretty steady acceleration from idle to redline, definetly great acceleration from 3000-seems to never end. I have drilled out the pilot/idle screw holes in carbs 1&2 and they are set a 3/4 turn out( replaced w/new screws). Carbs 3&4 are set at 1 1/2 turns out. Plugs look good. I have only done a presync on the carbs. My problem is when cruising in any gear around 2-2.5k rpms my carbs start popping, and I can feel it in the speed.(bike kinda slows down for a split second when it happens). Any Ideas as to what may be causing this?
Thanks for the help fellas.

[TomB]

Popping in your carbs can be weak mixture, next time it does it hold the revs the same and try putting on the choke slightly, if the engine picks up a bit and the popping stops weakness is confirmed.

If weak start checking things like carb manifolds first.

[skids]

Back-firing through the carbs is pretty serious (preignition?). Back firing through the pipes while decelerating is no big deal.

[Ken Talbot]

It will probably run much better with a proper full sync. The bread-tie method is quick and easy, but it just doesn't do the job of the real thing. Also, how are your float levels?

[missinglink]

FWIW. I had an issue where I had a hesitation around 2.5K to 3K rpm. I chased it around until I found that the two caps on the vacuum ports on cylinders 1 and 4 had hardened to the point that they weren't completely sealing the ports. I replaced those caps and the problem went away...

I also second the carb sync suggestions. I rebuilt my carbs and used the tiewrap method to presync and though it was ok until I did a sync using the mercury stix.....Big improvement.

[audijunky]

Thanks for the info fellas, but does anyone know what causes this?
Is it fuel/air mixture or something else?

[goldcoastguy001]

Just a quick note, check your advance curve....had similar prob, and found my advance not working properly..

Cheers

[audijunky]

That requires a timing light no?

[DiverRay]

SET the SYNC on the carbs with a mercury stick or vacum gauge!! You CANNOT find ANY carb problems until the carbs are synced. Once done, you can then ride it , and see if the problem is still there. I think the problem will be gone.
These bikes WILL NOT run properly unless the carbs are synced.

[audijunky]

Thank you DiverRay,
I will do that before anything else

[goldcoastguy001]

yep i agree, sorry was rather late when i read the thread i thought you'd sync'd the carbs already....on reading again i saw the word pre-sync....lol.

I had sync'd my carbs and placed everything to stock specs and had cruising speed pulse like hesitations with spitting sounds in the air box/carb areas....on inspection i found my vacuum advance was leaking causing no#1 to run lean and giving me an anoying varying advance curve.

Cheers!

[John]

#1? That sux

Did you say that the vacuum advance is connected to The #1 cylinder? That is incorrect/ The advance should be connected to the small nipple on the body of the #2 carb. Connecting to one of vacuum nipples on the carb boots can really mess things up by applying wildly fluctuating vacuum pulses, and the advance will be chattering like crazy, constantly advancing and retarding. Check the hose connection point and if it is on the carb boot nipple, correct it as soon as you can.

[goldcoastguy001]

John.

yep hose is on the carb, not on the boot....as you said that would give too much vacuum. I only have one carb with a vacuum nipple and thats no#1. I Have found my XS to have plenty of wierd stuff on it when i found....maybe you could help identify exactly what model it is...frame says 7/82 XS1100R. 5n5 model. but it has all the stuff that the midnight special has.....ie; gold airbox-side covers etc blacken'ed engine forks and black speedos(round)H/light etc....also has front to rear brake line valve. but it has the large tank of the standard.....damn thing was so black i lost it in the dark so i stripped it down and polished most of the engine and gave it a nice Maroon/gold pearl paint job with gold flames...


look forward to any ideas guys.....

Cheers!

[mro]

7/82 XS1100R. 5n5 model. but it has all the stuff that the midnight special

XJ MNS???, did Yama put the YICKS on the yurop models?

goldcoastguy001
Post a pic, would like to see your ride.


mro

[goldcoastguy001]

Not sure, I know the bike is defintely an XS......but has so much more than the standard and all the mns stuff....really confuses me when i order parts from yamaha.....to say the least yamaha are not sure 100% either...they said it looks like a run out model that used alot of the old with the new type deal.....???????

damn thing runs like a rocket on two wheels and has been dyno'd at 125hp at the wheel......