First Bike, GS450 high idle/backfire issues

[Guest]

Hey guys, i bought an old non running gs450 for 200 bucks after reading this site and figuring the problem with the bike was the cdi (it was). I replaced the reg and cdi, checked the coils and it started but ran really rough. I let it sit through the winter (unheated garage) and have since done the following to fix her up: full carb dip and rebuild with cycleorings stuff, new intake and carb boots, the aforementioned cdi/reg, valve adjustment and new gaskets, new spark plugs/caps, stator check and new crank gasket, new petcock (although i fear it might be chinese and of poor quality but not sure. Its heavy...) oil/ filter change, some new wiring and connections, new battery, new exhaust gaskets. I have yet to: adjust idle mixture screw, and tune the carbs.
So my problems are: a relatively high idle with choke off, a racing idle with choke on, seems to run pretty hot (its my first bike though, so maybe i am wrong.) It also has white smoke coming from the left side crossover tube, I ran it for like 5 minutes and it has mostly cleared out, maybe old oil/grease burning off? If i dont have the petcock on prime it will die (i have an upward bend in my gas line, think that might be this culprit.. gas line is new). It also backfires when i turn it off, from the same exhaust side as the smoke.
Any thoughts? I put alot of work into it and want to get her up and running well.

[Gorminrider]

That's a lot of stuff!
White smoke is usually steam.

petcock:It comes down to which one you bought and from who. But if it's not allowing gas without prime on , maybe it's not getting vacuum?
An upward bend in your gas line makes no diff unless it's above the fuel level and even then acts like a siphon to carbs if there's no big air bubbles in it.

Hi revs and backfires might mean the opposite: too MUCH gas. Too much gas in ONE carb perhaps.....at least the"choke" is having some effect so it's tube isn't plugged.
Did you bench-synch the carb throttles? They need to open together just the the tiniest amount. Or, it might be your throttle cable routed wrong and not letting the throttle close... not so uncommon. A good test overall is to move your handlebars..engine should not speed up!
smoking white?...I guess you mean the exhaust crossover tube? so it's leaking...has rust and holes at the clamp

[Kellencmiller]

yeah, i wanted to be thorough, so i followed all the advice in the stickies. and i like working on stuff.
I did bench sync the carbs when i rebuilt them, although tjat was months ago.. I vaguely remember that process involving using a needle or something to get the opening about right?
the crossover tube and where it connects has got some decent dents, thats why it leaks there. Im wondering if the backfiring is because the petcock is allowing fuel down the vacuum line and that could also maybe explain the white smoke? i still have the old petcock, maybe i will try it again too... ill also try the handlebar test.

[Gorminrider]

Im wondering if the backfiring is because the petcock is allowing fuel down the vacuum line

Maybe. Backfiring as a symptom...that can be several things...Lean,Rich,timing,ignition...you can look that up on google as "backfiring" ... overall unburned Gas vapour explodes down in the exhaust....on a decently tuned old bike, it can be alleviated by richening up the idlemix, but some makers put special devices to keep the idle lean for antipollution while opening a special valve just when the manifold vacuum was very high when decelerating...

gas down the vacuum line is a common problem if the petcock's diaphragm gets a leak...but in a new petcock? anyways it's easy to find out. Disconnect it. Is it dripping?
also, an airleak in the vacuum line can be a problem..it'd make that carb and cylinder run very "lean"
to see if either is happening, just disconnect the vacuum line at the carb and plug the place it connects to on the carb...turn your tap to Prime...

White smoke? The only place I see it except early mornings on cold days..
Otherwise, think two-strokes. THEY smoke whitish..where the oil is intended to vapourise ... On a four stroke,where it's not intended, oil is getting into the pistons suggests valves and seals....too much oil to burn in a cold cylinder or the cylinder is not firing anyways...Id guess your spark plugs would indicate...

You can look up "compression testing", "Leakdown tests" and "vacuum gauges" -any of these can indicate issues of how the motor is functioning as a "pump" with "good seals" and might set your mind at ease.
Or at least save you trying to fix serious issues by tweaking carbs. It can't be done.

[Kellencmiller]

Thanks, ill look into all of that. What kind of readings am i looking for in a compression test? And ill look at the plugs when i get home and compare them to the pics in the clymer manual.

[Gorminrider]

from the Clymer manual, 130-170psi is compression.

PS...you can make a"leakdown tester" from an old sparkplug remove the ceramic and glue in a tire valve (jbweld epoxy)...

You can do without all the " instruments" ie: train your thumb to plug the spark hole! and see which side blows it off harder but really, it's easier to pick these up as you find them-second hand if you can. Be careful when buying a compression tester it has adapters that WILL fit Suzuki plug threads.

[Kellencmiller]

compression tester shows above 130 on both cylinders. i really think it might be the vacuum leak and an idle mixture screws that needs to be reset.

[Gorminrider]

An airleak at the manifold rubber collars won't affect compression. But leaks at the valves or rings will.

130 psi is at the LOWEST permissible limit. "above 130" is not reassuring from here -not like "above 150" would be, given the the bike left the factory with 160-170...? so from here, I fear a worn-out bike ...
OR maybe,...(you probably read the directions but some people don't)...Did you have the throttle open WIDE? You need it wide open, preferably, on a warm bike.

But if you are happy with the compression...an important thing is that both cylinders are very nearly the same...
When my gsx400 showed 130 on one cylinder, I took it apart...It was still running "ok" but really, I figure was riding the equivalent of a 250.

[Kellencmiller]

sorry, didnt mean to scare you, i just meant they were both above 130 psi. actual measurements were 154 and 152.

[Gorminrider]

yes, That's a relief. Since you bought it "non-running", it's good to make sure the problems aren't deep ones.