"backfire" & chain questions

[Anonymous]

OK folks, with no shop near me that will touch "a bike that old", I turn to the trusty GS list for help with two questions regarding my ongoing effort to resurrect a long-stored (how long--who knows) but little used (5,000 miles, one-owner before me) 1978 GS-400. Two questions (for today):

1) I took the bike out for the longest ride yet and noticed a muffled backfire (a popping sound out of the pipes) when backing off the throttle when headed down a substantial grade. What do I do to correct this?Not sure if any of the following is relevant or not but: I have timed the bike with a timing light but have not synched the carbs yet (synching tool on order. . .). The mufflers appear to be the originals and are a bit corroded at the after ends.

2) When I got the bike, the chain was a rusty, dusty mess. I tried cleaning the chain (in place, it does not have a master link) followed by repeated lubing with some Suzuki chain lube. After about a 30 minute ride today I checked the chain tension and have about 1" of deflection for part of the chain and about 1/4" in another position (deflection measured mid-way between the sprockets on the bottom portion of the chain). With the bike up on the centerstand I can spin the rear wheel and it will spin until it binds up at the tight spot on the chain. Time for a new chain? With 5,000 miles is it really time for new sprockets too?

Thanks,

Seth

 

[Anonymous]

my $.02

my $.02

When riding along you are supplying a steady metered rate of fuel and air. When you back off the throttle, the vacuum of the engine is still sucking the same amount of air but your carbs are only allowing fuel through at the rate of an idle so you are then running lean(there's your backfire). I would check for cracks around the intake boots and air box boots anyway. It doesn't take muck extra air to mess things up when the proper ratio is something like 14.7air/1fuel. A good color on the pipes is a light straw color. As for the chain, spend the money. It takes one binding spot to ruin a perfectly good chain case and it also rides smoother. Bob

 

[Jeff Tate]

I can't understand a shop not wanting to work on an "old" bike", sounds like easy money to me. I guess they would have to actually figure out a problem instead of plugging in a computer.
Back to your problems. "OLD" bikes like ours need to have the carbs cleaned out real good now and then especially if you weren't sure how long the bike was stored. Start with what "cabinover"suggested about the intake boots. Mine got very small cracks in them over time and made it run terrible plus the popping and banging. Also check to see if the exhaust system is tight at the head, loose header pipe(s) will cause that too. And see if there might be any holes in the exhaust at all. The engine basically needs to be sealed tight so to speak from intake thru to the exhaust.And as stated , put on a new chain, and sprockets too at the same time if the teeth are pointed and do not have a flat tips on them.

 

[Anonymous]

Thanks Jeff and Cabinover. When I got the bike, I took the carbs off and down to a place that put them through an acid cleaning tank procedure. When I put them back on, I couldn't get new gaskets that go between the heads and the insulating block(s) so reused the old ones. Perhaps I have a small leak there although I have tried spraying WD-40 at that joint while the bike is running and haven't noticed any increase in RPMs. It's been a while since I made that test though so I'll repeat the procedure.

New chain will be on order as soon as I get that old one off.

Thanks again,

Seth