Exhaust Swap: Carb Adjustment or ReJet?

[RustyTank]

Hello folks
I have a 1979 GS850G. I've recently cut the exhaust off the original pipes and welded on new. It's still 4 into 2, same headers, just shorter exhausts on either side of the bike. I figured the carbs would need to be adjusted after the swap and sure enough the bike doesn't run quite right since. I'm under the impression that a carb adjustment is needed, not a re-jetting. I'd be interested in hearing others thoughts on that. The bike ran great before this change, now I can get it to idle but there's a lot of popping, and, once warmed up, when I give it some gas it'll bog down and quit.

[Guest]

I’m confused. You modified your exhaust and you think that has changed the synchronization of the carbs? I suppose it’s possible that vacuum has been affected if the flow has been changed through the cylinders and it doesn’t hurt to check. However, I’m thinking the balance of your carbs is the same as it was before the modification and what you need to do is jetting. You can’t sync an exhaust. Syncing refers to making each carb butterfly open together with the same gaps. I think you need to do more reading.

[salty_monk]

If you've still got a stock airbox changing the pipe doesn't usually make a huge difference. I would suggest you try a main jet that is 2.5-5.0 bigger than stock, shim the needle up a washer or two higher (you actually take away the big plastic washer & replace with tiny metal ones. The source for those used to be radioshack but no doubt you can get elsewhere) & wind the pilot screws out. Usually you'll need an extra 1-1.5 turns out (to correct a hanging idle which would be your main symptoms as well as popping on overrun).

What is your stock main jet size? I may have the stage 1 Dynojet jets for that bike that you can use... I guess I may also have some washers somewhere too as I remember the Radioshack pack being way more than you need

I ran my 1000G that way (with a K&N inbox filter) and a V&H 4:1

[RustyTank]

I’m confused. You modified your exhaust and you think that has changed the synchronization of the carbs? I suppose it’s possible that vacuum has been affected if the flow has been changed through the cylinders and it doesn’t hurt to check. However, I’m thinking the balance of your carbs is the same as it was before the modification and what you need to do is jetting. You can’t sync an exhaust. Syncing refers to making each carb butterfly open together with the same gaps. I think you need to do more reading.

EDGECRUSHER, I really, really appreciate your comment. One of the things about owning old motorcycles, for me, is the continuous humbling it forces me to feel, it reminds me that I have so much to learn. Allow me to explain. When I wrote my post I was thinking, in all my wide eyed optimistic amateur-ness, that syncing the carbs was the same things as adjusting the carbs. After reading your comment I had an "oh *@%^" moment, did a quick google search and realized I had been equating two things that are very different. So here's where my head is at currently, I'm under the impression that the carbs need to be adjusted. You're right bud, I have more reading to do as well as more practice to do working on old bikes.

Additionally, years ago I had a 78 Honda CB550K, which is a four into four stock. I replaced the stock headers with a four into 1. With the help of a friend I re-jetted the carbs. Here's my point in telling this short story, taking a bike with a stock four into four exhaust and modifying it to be a four into 1 is an extreme change which, in my head at least, makes total sense that the carbs needed a re-jetting. But, what I've done with the GS is taken a four into two and left it a four into two, with slightly shorter exhaust pipes. I guess I'd just be surprised if the carbs needed to be re-jetted after such a minor change. Or, at least what I perceive to be a minor change.

And perhaps what I should do is scrap this entire post and start fresh with the question, do I need to re-jet or adjust the carbs?

[RustyTank]

If you've still got a stock airbox changing the pipe doesn't usually make a huge difference. I would suggest you try a main jet that is 2.5-5.0 bigger than stock, shim the needle up a washer or two higher (you actually take away the big plastic washer & replace with tiny metal ones. The source for those used to be radioshack but no doubt you can get elsewhere) & wind the pilot screws out. Usually you'll need an extra 1-1.5 turns out (to correct a hanging idle which would be your main symptoms as well as popping on overrun).

What is your stock main jet size? I may have the stage 1 Dynojet jets for that bike that you can use... I guess I may also have some washers somewhere too as I remember the Radioshack pack being way more than you need

I ran my 1000G that way (with a K&N inbox filter) and a V&H 4:1

Hey man. Nice to hear from you. So, my original post was a bit of a screw up. I was equating syncing the carbs with adjusting the carbs. What I know now is that those are two different things. To the point you made about the stock air box and not making a huge difference, yes, I'm keeping the stock air box so nothings changed there, the exhaust is still a four into two but with shorty pipes. Not super short, just shorter. I was under the impression that a carb adjustment would be necessary, but not a re-jetting. Your thoughts?

[Rob S.]

If it doesn't run optimally now, try just removing the airbox lid. That was the only mod recommended in many aftermarket exhaust instructions.

[RustyTank]

If it doesn't run optimally now, try just removing the airbox lid. That was the only mod recommended in many aftermarket exhaust instructions.

Uhm, there's no air box lid.

I suppose a video of what's happening while it's running would help. Maybe I'll try and put one together this weekend.

[Rob S.]

So, what is the air box lid?

You'll know it when you see it.

[salty_monk]

It's a G.. he's right there is no airbox lid. There are two side covers & you wouldn't want to leave them off.

Trust me... if it's not running right you'll need to wind the pilot screws out & look to re=jetting.

Been there... done that. It shouldn't be that difficult as you're running the stock airbox.

[RustyTank]

It's a G.. he's right there is no airbox lid. There are two side covers & you wouldn't want to leave them off.

Trust me... if it's not running right you'll need to wind the pilot screws out & look to re=jetting.

Been there... done that. It shouldn't be that difficult as you're running the stock airbox.

Salty, I'll PM you about those jets and washers.