Nipples! VM Carbs 2, 3, and 4

[iammulva]

Ok - first of all, thanks to everyone on this board whose input helped me get my carbs rebuilt. I just reinstalled them and went through the synch process using a CarbTune.

Some observations....

1. When going through the idle adjustment process (set idle to 1100, turn air screw until engine revs stop increasing, move to next carb) carbs 2 and 3 air screw adjustment did not change the engine rpm at all for either a full turn in or out from the starting position (which was 1 1/2 out for all four). Carbs 1 and 4 air screws did affect the RPM's as expected and were adjusted accordingly.

2. Carb 2, 3, and 4 have an air (vacuum?) nipple on the top. Carb 3 has a return air hose attached to it that runs to the petcock. Carbs 2 and 4 are capped with a rubber cap.

3. When running the synch I screwed the idle adjust spring in to get the RPM's into the 2500 - 3500 range. The engine was not staying at a steady RPM during the synch process - it was bouncing around +/- 500 RPMs.

4. Notwithstanding the issues above, I got the CarbTune markers to even out pretty well. When I took it for a test drive, I noticed the engine felt like it was surging when I was doing about 30-35 (normal neighborhood driving) in 3rd? gear. Did not pay super close attention because I had to stop working on it and will have to start troubleshooting it another time anyway. I also noticed popping in the carbs during the synch (Carbs 1 and 4 got the CarbTune tubes blown off the nipples several times).

5. I have not yet checked valves nor have I adjusted the fuel screws. I set the air/fuel screws as follows when I reassembled it. Air 1 1/2 turns out. Fuel 3/4 turns out.

6. I have stock airbox and 4 into 1 exhaust with Muzzy muffler.

Questions....

1. Is the fact that the air screw on Carb 2 and 3 did not affect engine RPM indicative of anything?

2. Should the nipples on Carbs 2 and 4 be capped or should there be a hose attached and routed down and out of engine area?

3. I've read on this forum that surging could be (among other things) a lean situation (or intake leak). Before I "declare" what my problem is, I'd like to know whether I should even be attempting to tune the carb without first having checked the valve clearances. I've got the feeler gauges, but haven't yet ordered gaskets to check them yet. Also, school is starting Mon so not sure when I'll get to the valves.

4. I have Dyna coils and ignition. Should I put that on before continuing to troubleshoot the carburetor?

As always - all input is greatly appreciated. Thanks again for getting me this far!

Chris Warren
'79 1000E

 

[Nessism]

Is the airbox on the bike and well sealed? These GS bikes don't run well without it unless you rejet the carbs. Also, did you replace the intake boot O-rings between the head and carb boots? They are very prone to leakage and need to be replaced if you have not already done so.

 

[iammulva]

Yes, airbox on. Air filter box too. Everything fully assembled/clamped. I replaced intake o-rings.

 

[SqDancerLynn1]

did you bench sync the carbs ?

 

[Tarbash 27]

Don't cap those vents on 2 and 4. Leave them open, some say its even better not to run the hoses on them when your running pods.

 

[iammulva]

Yes, bench synched.

 

[iammulva]

So with stock filter setup I should definitely un-cap?

 

[Nessism]

Yes, bench synched.

Bench synch is not good enough. You need to vacuum sync.

If you do a search there are some home made carb sync tools you can build for cheap. I've never messed with them but it's worth a try. Or better yet, find a sync gauge to borrow.

 

[iammulva]

I did both. Bench synched then synched again with CarbTune.

 

[Nessism]

I did both. Bench synched then synched again with CarbTune.

Sorry, I just reread your first post and clearly you mention the carb tune.

My suggestion is to adjust the valves before going too much farther. The shim and bucket valve design is very easy to adjust as long as you have the tool and some spare shims.

Since your bike has a header you are going to have to tweak the jetting to suite if it hasn't already been done. That said, doubt this is the problem at low speed like you describe.

One quick thing you can try is to open the fuel screws to 1 full turn. This will richen up the idle and just off a fair bit. Can't hurt to try.

 

[iammulva]

Just warmed up the bike and removed nipple caps. No appreciable difference. Behavior is surging around 2500RPMs some muffled popping from exhaust on decel at slow speeds (15mph or less) and some louder backfire-like popping when closing throttle quickly at higher RPMs and speed. Also some loud backfire/popping on harder acceleration/holding higher RPM and speed.

Turned fuel screws out 1/4 on carbs 1 and 4 (didn't do 2 and 3 bc I have big hands and no gloves and engine is hot). Seems like surging decreased some but still there around 2500RPM. Also seems like the softer popping has decreased but still getting the louder bacfires on closing throttle at speed. To me it seems like increasing fuel on the two carbs helped, but I thought backfire/pops from exhaust indicates too rich so that is contradictory in my mind, but then again I have no experience here.

Am planning on opening fuel screw on 2 and 3 when bike cools off.

Thoughts?

Is it worth it (or required) to re-check the synch with CarbTune?

<Scratching head>

 

[Guest]

I would throw on your new coils and ignition before you continue with the carbs. I had a similar problem with my 850 VM's and found a short in the ignition system to be part of the problem. Also check you exhaust system for any leaks at the head. Any you never know, I had to rebuild my carbs several times before they seemed to work right. Keep up the good work.