New Mikuni Carbs on my 1983 Yamaha SX650, Still backfires on the right side out the exhaust

[MickFerreira]

So I picked this bike up off a kid and started by putting new handlebars on it, ordered a seat. It is a classic bobber just like the one I had back in 1984. It ran but had a backfire on the right-hand side only and out the exhaust, not the carb. Everything on it has been thoroughly molested but seems to all work. When I sprayed starting ether in the right-hand carburetor, the backfire went away.

Could it be the right side coil, is there a separate condensor for each side? The plug wire? I am baffled. It starts after about four kicks and with starting ether but runs great once started except for the loud popping noise the backfire makes.

I have read through several threads addressing this but non quite hit it on the head. I know the points are not adjustable on the year bike I have, could the valve need to be adjusted? Could it be a bent valve

?

 

[5twins]

Thoroughly molested is an apt description, lol. Not much they didn't fool with. Your rubber intake manifolds are supposed to have a sheet metal shroud on them. This adds needed support, especially when using pods and without the added support of the original airbox. The manifolds can split and crack pretty quickly and that may be the case with your right side one.

 

[5twins]

Also, originally the carbs had a cable operated choke for cold starts. You can see it and it's attached cable in my pic above. Looks like yours isn't hooked up anymore and that might explain the need for starter fluid for cold starts. Without a choke, these bikes are very hard to start cold.

 

[MickFerreira]

Sorry, old photos. I since put on a sweet pair of new Mikunis from TC Bros. I have been able to get it to start on one kick warm but the aggressive popping and back firing took over. Then, I pushed down on the choke lever on the right side only and the backfire stopped and I was able to ride her for the first time. She idled but has a little out of sync until I hit the power band and then she really took off. I almost forgot I had removed the front brake and the rear was a little dicey so I kept my speed in check. Just like old times. My only concern is running in such a rich environment on the right side will give me problems later on. I was able to start her on one kick when I got back to the garage several times. I need to figure out another way to end the backfire without choking the shit out of her.

 

[grizld1]

Very likely an intake leak, as 5twins suggested. A few searches will turn up some simple tests. Intake leaks can be devious and stubborn. I once fought one on a new acquisition for longer than I like to admit before discovering that the PO had cranked down on the fasteners securing a carb boot so hard that the edges of the holes were extruded. How that can happen before the threads strip is a mystery, but a few others have found the same thing.Tip: clicking on the Tech button will open a lot of good information to you. Welcome, and good luck.

 

[Jan_P]

Is that a 2 into one exhaust system
If so is it certain that it is the right cylinder ? Ok i se that choke adjustment makes a difference
But if 2 new carburetors are installed one must assume identical jetting.
Also a change with tools has been done
Rarely one installs new carburetors without new gaskets and inspection .
The same fault more or less with two sets of carburetor
You have prior experience .. since the early eighties
I would inspect the exhaust systems connections for leaks checking that bolts a tight
It can happen that a rich mixture ignites in the exhaust because more air comes in through a leak

 

[MickFerreira]

OK, So I did notice I have an exhaust leak that sounds like sip sip sip sip on the left side so I know to fix that. I tried to put on a new two into one stainless setup from Delkevic but it is designed for a stock cycle and interfered with my forward controls so I took it off. Sad because it was such high quality. It luckily has two separate exhaust pipes so I sourced the backfire to the right side.I can back out the right side carburetor and smear some silicon around it to see if there are any voids. Better yet I should order new rubbers.
I will check the intake as I am the rare one who installed new carbs without new gaskets and inspection. If it is that simple I can move on to front brakes and some items to make it legal so I can register it. Or not. (To the registration.)

 

[grizld1]

A number of leak tests can be used to localize flaws--unlit propane torch, starter fluid, carb cleaner, water--and you'll find them if you look. Done here.

 

[MickFerreira]

I will go to a few tests and track down this gremlin. Was ready to take it to a shop.

 

[Team Junk]

Nice bike. You have to wonder about this sort of thread. Did it get fixed etc. Hope his absence has nothing to do with no front brake.