Carb sync and tune guide

[Ccanady]

I need some expertise in tuning my carbs. Here’s the issue I’m having: my compression is at 150 on both cylinders which is great but when I crank my bike, exhaust from right cylinder vs left cylinder is like night and day. I’m assuming it’s in need of a good tuning. I’ve already torn them down and done a rebuild but when it comes to tuning them, im lost.

My question is...does anyone have a write up or video with easy instructions on tuning bs34’s?

 

[highboy_coupe]

There is a carb guide in the tech section.

Someone who knows way more than me will probably give you better advice, but as you mention the exhaust; is one side cold / warm and the other side red hot? Have you check your plugs / coil / HT leads to see if there's an ignition problem first before you start with the carbs.

If you have and the problem is at idle, have you cleaned the pilot circuit?

It's a start!

 

[5twins]

If you still have the carbs off, do a "bench sync". That will get the sync very close, maybe even right on. Other than sync, the only other setting you have is the mix screws. Being so E.P.A. strangled, the BS34s need the screws open more than the earlier carb sets, usually somewhere in the 3 to 3.5 turns out range. Start out by setting both screws at 3 turns out.

 

[peanut]

do you mean 'crank my bike ' or do you mean 'run the engine ? there is a big difference. One has the engine turning over (cranking but not firing) and the other has the engine running (firing)

If you are noticing a difference in compression at the tail pipes whilst 'cranking' the engine then I'd suggest you revisit your valve adjustments. Don't forget that you adjust the valves on the cylinder that is at TDC on its compression stoke . Then you turn the engine over 360 degrees and adjust the valves on the other cylinder.

When syncing your carbs on the bench I find the easiest and most accurate way is to put your mouth over the butterfly end of the carb and blow into the carb then do the same with the other carb and adjust the sync screw until the pressure feels about the same

 

[Ccanady]

If you still have the carbs off, do a "bench sync". That will get the sync very close, maybe even right on. Other than sync, the only other setting you have is the mix screws. Being so E.P.A. strangled, the BS34s need the screws open more than the earlier carb sets, usually somewhere in the 3 to 3.5 turns out range. Start out by setting both screws at 3 turns out.

I have put them back on the bike but could take them off later if I can’t get this issue fixed. When I rebuilt the carbs I pretty much put them back the way I found them.

 

[Ccanady]

do you mean 'crank my bike ' or do you mean 'run the engine ? there is a big difference. One has the engine turning over (cranking but not firing) and the other has the engine running (firing)

If you are noticing a difference in compression at the tail pipes whilst 'cranking' the engine then I'd suggest you revisit your valve adjustments. Don't forget that you adjust the valves on the cylinder that is at TDC on its compression stoke . Then you turn the engine over 360 degrees and adjust the valves on the other cylinder.

When syncing your carbs on the bench I find the easiest and most accurate way is to put your mouth over the butterfly end of the carb and blow into the carb then do the same with the other carb and adjust the sync screw until the pressure feels about the same

I mean “engine running” when I start it and let it idol, I obviously get compression from the exhaust on both the right and left side but not nearly as much on the right as the left. And it seemed to be back firing (to the point where I could see flames popping inside my right pod filter causing it to smoke) I did my valve adjustment correctly with it being tdc on compression stroke too.

 

[peanut]

until you have checked everything systematically you cannot rule out this fault being due to an electrical or ignition issue .

A simple test to eliminate this being an ignition fault would be to unplug both the spark plug caps and swop them over . If the fault transfers over to the other cylinder then there is clearly a problem with the ignition supply to the weak cylinder .............(possibly in addition to a fuel/air delivery issue/s .)

If there is no change then you can concentrate on the fuel air delivery without being concerned that the ignition and electrical system could be at fault.

Spark plugs are a valuable diagnostic tool. .................if you replace the spark plugs with brand new and gap them accurately then run your engine for a few minutes to warm the engine up your plugs should give you an idea whether the mixture is lean or rich in either cylinder . it can also tell you if you have a slight oil leak into the effected cylinder from a leaking head gasket perhaps

 

[highboy_coupe]

^^^ I agree; it was a bad spark plug for me, the misfire/backfiring followed it when I swapped the plugs round.
It's hard to chase carb problems if your ignition is also giving you grief.

 

[grizld1]

I don't know why it is, but when a motor misbehaves we seem to have a tendency to blame the carburetors right off.That's a mistake.The combustion tripod stand on three legs: compression, ignition, and fuel. The first two can be directly inspected with the aid of simple instruments and easy swaps. But you can't directly examine the air/fuel mix without some very pricey gear, and even with that you need to be sure that ignition is healthy in order to get readings that mean anything. Bottom line: in the absense of compelling evidence to the contrary, troubleshoot carburetion last.

Ccanady, I wrote a 26 page carburetor manual for the Yamaha 650 Society and included detailed adjustment, repair and troubleshooting steps. It's available from the Society director, desmoman900, for $17 a copy mailed.

 

[5twins]

Carbs are usually synced on the bike using some sort of gauges (vacuum or a manometer). But if you have them off, a "bench sync" is easy to do and a good way to check the sync. I wouldn't pull the carbs off just to do a bench sync though.

 

[Ccanady]

So I swapped plugs and it runs fine now...weird. Sounds like I may need a new set of plugs. Anyway I still have a backfire issue coming back through my pod filters causing them to smoke (i.e. burn). I think I’m going to pick up a new set of plugs and see if that may help fix the issue.

 

[grizld1]

Backfire, eh? Time to break out the strobe!

 

[peanut]

You can click on the thumbs up symbol if you wish to acknowledge and thank someone for their help and advice if it is useful most folk appreciate this.

I would suggest you check your valve settings before you do anything else .
If you hadn't even thought to replace your spark plugs before asking for help then I suggest you need to do a proper full service and check of everything. There are plenty of guides on here to show you the proper sequence and carbs are last on the list

 

[Ccanady]

You can click on the thumbs up symbol if you wish to acknowledge and thank someone for their help and advice if it is useful most folk appreciate this.

I would suggest you check your valve settings before you do anything else .
If you hadn't even thought to replace your spark plugs before asking for help then I suggest you need to do a proper full service and check of everything. There are plenty of guides on here to show you the proper sequence and carbs are last on the list

Thanks..I had already liked multiple reply’s from this thread.

I had already done a “full service”. I rebuilt the entire engine to include valve settings. They are all to proper specs.

Only thing is, I reused the original plugs that came with the engine when I bought it. PO had only had them for a few miles before selling me the bike. (And they are very clean) this is why I was led to believe it was my carb settings. I’m not a carb guru...this is why I asked for help with my carbs...

 

[highboy_coupe]

It might be that the plug caps aren't on the leads properly and swapping the plugs wobbled the connection enough to be ok temporarily.

Take the caps off if it's green in there spray some contact cleaner and get a brush in there. Screw them back on well.

 

[5twins]

If the wires are original, or even just older replacements, it's a good idea to trim a quarter inch off the end to get to some fresh wire (if the length allows it). I go one step further and trim another 1/8" of the insulation off then fan the strands of the wire core out in a radial pattern like so .....

I think this better insures a good connection to the coil or plug cap. Oh, I brush some dielectric grease on the end of the wire before installing it too.

 

[Ccanady]

It might be that the plug caps aren't on the leads properly and swapping the plugs wobbled the connection enough to be ok temporarily.

Take the caps off if it's green in there spray some contact cleaner and get a brush in there. Screw them back on well.

I actually doubled checked this the other day but no issues. All my wires are brand new. Along with new ignition and charging system.

 

[Ccanady]

I would suggest you check your valve settings before you do anything else .list

You made me second guess my work...just went out to the shop and checked my valve spacing. It was set perfect. lol

 

[peanut]

You made me second guess my work...just went out to the shop and checked my valve spacing. It was set perfect. lol

biggest mistake made in troubleshooting is to assume that new components have been correctly manufactured and are in full working order , closely followed by the assumption that because something has been previously checked ok and was working it must still be so.

If you have checked everything and not found the cause of the issue................ then you have to have missed something.
At some point we have to swallow our pride and admit this and then starting from scratch ,.... recheck everything in a proper sequence of elimination .

what are your valve lash settings ?

 

[Ccanady]

biggest mistake made in troubleshooting is to assume that new components have been correctly manufactured and are in full working order , closely followed by the assumption that because something has been previously checked ok and was working it must still be so.

If you have checked everything and not found the cause of the issue................ then you have to have missed something.
At some point we have to swallow our pride and admit this and then starting from scratch ,.... recheck everything in a proper sequence of elimination .

what are your valve lash settings ?

Exhaust is .004 and intake is .002