VS 1400 Can't Sync and Idle

[akramerb]

Hi all, looking for help with my 1990 VS1400 cable adjusments.

The brief story; I've owned and maintained this bike for about 15 years. The bike has always started and ran reliably with good power but last seasons I have experienced some stalling while coming to a stop and light popping/ coughing while at idle once the bike was warmed up.

I noticed the front cylinder sounds strong out the back and the light popping/coughing was coming from the rear cylinder. I checked the adjustment on both idle needle valves to confirm not to lean and obtained best rpm on both cylinders. I hooked up my old motion pro mercury filled sync gauge and my tach to the rear coil. The rear pressure read much lower than the front at 1200 rpms but more or less even at 4000. I adjusted the "sync" cable (rear cable front carb) and by slacking the cable quite a bit, I could obtain equal vacuum at both cylinders and stop the light popping/coughing but at this sync cable adjustment, could not get the bike to idle even with the idle stop knob fully tightened, the rpms are so low the bike eventually stalls and I see the idle stop screw doesn't even contact the stop.

Other clues to offer; I did a compression test and both cylinders are at 145 psi with decompression engaged. I also confirmed that the throttle grip cable and both carb throttle cables were slack and not interfering while adjusting the sync cable.

If there is anyone out there that has an idea of what the problem might be, your reply would be appreciated.

[Bear]

Did you ever find a cure for this issue? I’m having a similar issue where I have cleaned the carbs, replaced a few parts, and replaced the throttle cable. Now I cannot get the bike to idle with out tension on the throttle cable. I can’t get the bike to idle on the idle circuit but only at partial throttle.

[hillsy]

I don't recall seeing the first post - but if your idle adjustment is working OK then a bike that simply slows down and dies can indicate incorrect idle mixture adjustment amongst other things.

[Tbeck]

Carbs can be a real pain, and even more so if you are just throwing random crap in to see what works.
If it was me here's what I would do.
First I would rejet with oem parts, and I would replace the needle with an oem needle. If I recall there were some issues with aftermarket needles screwing up carbs, particularly on the LC's. Basically I would be doing a rebuild. As far as syncing the carbs? When you take it up to 4-4.5k you need to lock the throttle in place and make sure they read the same. Do the same at idle. Idle to just off idle is or should be pulling off the slow jet. So if your sync is off there you know where to start fixing.

[hillsy]

Carbs can be a real pain, and even more so if you are just throwing random crap in to see what works.
If it was me here's what I would do.
First I would rejet with oem parts, and I would replace the needle with an oem needle. If I recall there were some issues with aftermarket needles screwing up carbs, particularly on the LC's. Basically I would be doing a rebuild. As far as syncing the carbs? When you take it up to 4-4.5k you need to lock the throttle in place and make sure they read the same. Do the same at idle. Idle to just off idle is or should be pulling off the slow jet. So if your sync is off there you know where to start fixing.

The LC's had both carbs together linked by a single sync screw (not a cable) which makes sync adjustment a lot simpler. They also had the carbs laying almost horizontal which is where the problems with the needles ovaling out the seats / emulsion tubes happened. The seats are pressed into the carb bodies and Gary at GMan has the tooling to replace these.

The front carb on the 1400 is positioned almost horizontal as well - but I believe the needle seat / emulsion tube is bolt in (not pressed) so it is DIY to replace.