Dies on Throttle

[BeerMonger]

When I give it throttle, it bogs and dies.

'82 GS850G
PO installed 4 to 1 after market exhaust. I don't know if he rejetted mains.
PO installed aftermarket coils.

Things I've done:
Carb rebuild (tear down, dip 24 hours, new o-rings from Robert)
New carb boots to air box
New carb boots to engine (complete with new O-rings)
Air box seal with weather stripping
Valve adjustment
Carb Sync

Engine starts and idles. Every 10 seconds it sounds like 1 cylinder doesnt fire (sounds like it misses a beat). Every 20 seconds there's a loud pop from the exhaust, like a backfire but not as loud. It sounds like the loud pops from harley's after you let off the throttle, if that makes sense.

13+ V across battery
12V across coils (relay coil mod)
4.0 ohms across primary side coils
20K ohms from plug cap to plug cap(1-4 and 2-3)
5k ohms thru plug caps

What would you do next?

 

[TheCafeKid]

When you dipped and reassembled the carbs what did you set the mixture adjustment screws at? And not to bust your balls but while you were in thee you really should have checked the jet sizes. A four into one exhaust will at least require a bump of two sizes on the mains usually.

 

[BeerMonger]

I adjusted the mixture screw using a colourtune. I got them bunsen burner blue, then richened them a 1/4 turn.

I'll do a search on jets now.

 

[almarconi]

Try giving it a little choke at idle and see if this lessens the popping. It is very possible that your idle circuit is still plugged (maybe why #1 keeps dropping out). Also check for air leaks.

Are all four pipes hot? Drip some water on each pipe, it should sizzle if that cylinder is firing.

 

[BeerMonger]

I tightened the muffler bolts when I put them on several months ago, but hadn't re-tightened them since I got the engine running and after a few test drives. All the bolts just tighted probably a half turn or more with little resistance, initially.

I also found that pipe #2 was not hot to touch. Very odd, since this morning I verified they were all firing. So I swapped the plug.

Now the engine purrs, got no pops, and the throttle response is great, as is the deceleration.

Thanks for the help guys. I still will investigate rejetting.

 

[TheCafeKid]

I tightened the muffler bolts when I put them on several months ago, but hadn't re-tightened them since I got the engine running and after a few test drives. All the bolts just tighted probably a half turn or more with little resistance, initially.

I also found that pipe #2 was not hot to touch. Very odd, since this morning I verified they were all firing. So I swapped the plug.

Now the engine purrs, got no pops, and the throttle response is great, as is the deceleration.

Thanks for the help guys. I still will investigate rejetting.

What colour was the plug you removed?? That would go along way in telling you what is/was going on in there to foul out the plug..

 

[BeerMonger]

Blacker than black. It's a really old plug, 3 of the 4 B8ES I bought have crapped out (in the midst of getting it running and tuned). So in desperation I put in an old BR8ES, this is the really black one I just pulled out.

For what it's worth, I should just get another set of B8ESes, and start again.

 

[TheCafeKid]

Blacker than black. It's a really old plug, 3 of the 4 B8ES I bought have crapped out (in the midst of getting it running and tuned). So in desperation I put in an old BR8ES, this is the really black one I just pulled out.

For what it's worth, I should just get another set of B8ESes, and start again.

If its black, is it shiny black or fluffy black? Could be oil fouled but more than likely carbon fouled, which means one circuit or more is probably running a little rich, or your float levels are off.. might check it, figure out whats going on or it simply may happen again..

 

[BeerMonger]

If its black, is it shiny black or fluffy black? Could be oil fouled but more than likely carbon fouled, which means one circuit or more is probably running a little rich, or your float levels are off.. might check it, figure out whats going on or it simply may happen again..

It's more fluffy black.
If the PO put on the 4-to-1 without changing jets, would you expect I need to go to bigger jets than stock, or smaller? I know you don't have all the facts to judge, just looking for a guess. I'm new to this stuff.

I'll order a carb float level gauge.

 

[TheCafeKid]

It's more fluffy black.
If the PO put on the 4-to-1 without changing jets, would you expect I need to go to bigger jets than stock, or smaller? I know you don't have all the facts to judge, just looking for a guess. I'm new to this stuff.

I'll order a carb float level gauge.

You will likely need to go to a larger than stock jet size assuming your current jets are stock sizes. You'll want to check that out for sure. As well as pilot jet sizes. I've seen POs do some silly things in order to get the bike to "run". Dunno off the top of my head what the stock sizes on your bike are (112.5 maybe?) but buy the next size up and the next size up from that. They're cheap so that way you don't have to wait if one size up isn't enough.
As far as a "float gauge" you can pick up a set of digital calipers from autoparts store that will get you in the ballpark. Floats should be 22.4mm +\- 1mm from the bowl gasket mating surface when measured with the carb upside down. Do a search on float measurement to make sure your measuring the float from the right spot as well.