carb problems RPM hang

87 VFR700F2 stock air filter, stock jetting, stock exhaust.

Rebuilt carbs, set A/F 2 1/2 turns out. Double checked passages today with compressed air, pilot circuit and jet are clean. New carb to cylinder head boots.

Bike starts up with little to no choke, its running rich (can smell it) , but has RPM hang like its lean. Seemed to be over fueling, jumped the fuel pump to make sure the new float vavles are holding (tested good) so changed float hight to 10mm. Did the sync but still have RPM hang. Is 7mm the correct float hight? And what else could I be over looking?

 

cable :vtr2:

 

sorry it does it with the cable removed from the throttle wheel on the carb, sorry should of stated that. I can blip the throttle using thw cable wheel on the side of the carb, with the throttle cables disconnected, and she will hang at 3k or 4k then maybe come back down, like a lean hang, but at idle I can smell the unburnt fuel, its a head scratcher. I am using float valves from cheap cycle parts, they were not listed for the bike but seem to be working IDK maybe I'll just keep playing with float level, right now I'm at 10mm and it seemed to help a little.

 

This is a tough one as gen 2 does not suffer from this like the gen 1 does.......my first thought is to put a heat gun on the headers, looking for a difference. All I got from this distance as stated. Rebuilt a set last weekend that had chemicals applied to them and the air fuel mixture seals were destroyed as well as one slide diaphram. Could be as simple as damaged or missing air fuel screw seals.

 

Hmmm, I did try to reuse the little A/F orings, not the best of shape. You wouldn't have a size for those would you?

 

did the lil springs go back where they were supposed to on the linkages did you remove the entire carb assy from the air filter plate..

 

All linkages are reinstalled correctly and work correctly. yes I did take them all apart.

 

Sounds like an air leak of some kind. I know it can be a major pain to get these carbs back on. I'm sure you have double checked that they are seated good. What kind of condition were your diapharms on the slides? And are you sure tose are also seated well. But if those little bity o-rings are leaking on your fuel screws that would be enough to hang it up. If you took them all off the base plate did you use new seals when you re-attached them? I would be checking for anywhere that air can be coming into the mixture that is not supposed to be because that is what it sounds like? Your floats are probably fine so long as your jets are able to get fuel. But I would set them at the right level to be sure.

 

Set at 10mm you'd be leaner throughout the rev range, and it doesn't make alot of sense to compromise higher rpms to adjust the idle circuit.

Factory sez 7mm or 9mm, so take your pick !

Maybe someone replaced idle jets or drilled them. Try shutting off each cylinder with its mixture screw and see what happens. If you find one screw that won't stop its cylinder firing, your problem has been localized to that carb.

Your problem could be carb synch, so maybe try again.

 

Well I did some checking today. Seems the after market float valves are way different than OEM. Cleaned up the OEM float valves, set the float height and ran the fuel pump to make sure the float valves held. Started the bike up sounded a tad better, but still idle is rich but have the hang. Ordered up some A/F orings, also going to make sure the A/F screws are seating, one didn't seem to go in as far as the others, starting to wonder about crud or something in the threads. Driving it though other than the idle and a little rpm hang (Drops down after a few when its warmed up) seems to get up and go threw out the rest of the throttle range.

 

If idle was rich it would blub blub and stumble to get power. Like Taylor said it sound like you have an air leak, have you changed the carb insulators? If not they are surely old and crusty by now.

 

In my limited experience, persistant hanging idle problems are lean pilot circuits due to clogged passages and/or clogged pilot jets. Set the fuel screws at about 2 1/2 turns out, then turn each one in one at a tme........there should be a change in idle speed. If no change, that particular idle circuit is not working.

 

Got around to looking the carbs over again today. What I'm thinking is the A/F screws are not seating. I've glass blasted about 6 sets of carbs so far, but I think it might have been a bad idea with this setup. I notice looking into the idle port I didn't see any tip of the A/F needle tip. I closed them all and ended up with different amounts of the needle tips into the idle port. I chased the threads again with a tap and flushed out and with carb clean and blow gun, they managed to go a little further, but have no idea if the PO did something of if I F'd them up some how.

So anyway after that, I left them all closed, ran the bike and she still started with no choke... #3 carb throttle plate starts to get wet with fuel (only while running, but don't over fill if just running the fuel pump). Has anyone noted how far the A/F needle tip should be into the idle port? I notice one looked about half way across the port, IDK if it should go more, or if the needle tips are broken or what (they all looks the same so I doubt that). I might just try to track down a used set and start over. But as it stands I think most of the issue is the A/F idle adjustment and the fact the needles are not seating, might sound crazy I know, but looks like the issue to me.

 

Yeah, you should not have blasted these with glass. At the very worst you should have soda blasted them if you wanted to clean them.

 

Never had a problem before, but oh well such is life lol. I looked them over again today and the A/F needle seat inside the carbs look kinda out of round. Again no telling if it was me or the PO. So going to get a used set of carbs and try again (with out glass blasting).

 

Figured it out, I ended up getting a used set on ebay. Still went threw the carbs, cleaned everything, but when I started the bike, pretty much the same issue. Bad idle, raw fuel smell, open the throttle and it would rev up and hang, till you turn the idle speed screw and bring it down, then the rpms would be to low and it would almost stall. So started looking at them and noticed
1 the idle speed adjustment almost maxed out
2 really low vacumme on the mercury carbstix.

Powered up the fuel pump with the bike not running and sure enough fuel starts pooring out the vents. Don't understand how it even ran like that lol

Soaked the float needles in silcon spray for a little bit. Got it all back together, ran the fuel pump to prime and tested the carbs (made sure they didn't over fill again). Started the bike, and huge difference, no hang, no raw fuel smell, and engine vac much higher. Drove it around then went back to the shop to sync it up. All is good, minus the finishing touch on the A/F idle mix.

 

Jman, I'm a little confused. You say you figured it out, and that means the problem was the float valves weren't seating, but that was on a replacement set of carbs not the originals?
What about the vacuum leak theory?
I had a similar problem to yours on a CB750K (1980). Bike not running without choke. Rebuilt the carbs and then bike runs but I get this real bad idle hangup. Re-cleaned several times, replaced intake boot gaskets, better but still hanging up. The Motion Pro Sync tool showed me low vacuum across all four units. Couldn't find the leak. I finally gave up, cleaned a new set of carbs from a parts bike I bought, installed them and voila, no more vacuum problems. And BTW, the carbs now sync's dead on, really stable. Never found out what was wrong with the old carb set. Maybe it's the A/F screws/gaskets not seating. This thread is giving me ideas...

 

Low vac on the manometer is a good indicator of running rich on that carb or all 4 carbs in my case, that and wet throttle plates.

Both set of carbs did the same thing for me. I was trying to use an aftermarket float valve kit that wasn't correct on the first set, which seem to cause issues.

So the run down.

First set of carbs (original set) blasted, soaked jets, replace float valves(with wrong ones), and bowl gaskets. Ran like crap, idle adjustment would be almost at max then the bike would shoot up to 4k, then would turn the idle down and it wasn't linear to adjustment, it would just drop and almost die, then would turn it back up and then it would jump up again. Low vac on all 4 carbs and couldnt' sync with the manometer. I was going insane, and just doing the same thing over and over (never looked at the floats though, idk why). I was thinking I damaged the carb bodies some how or the A/F needles lol

Purchased second set of carbs, and they did the same thing. This time I soaked the float needles in silicone spray (sprayed into a cup and let the needles soak) installed on bike and bam, perfect

So now I have the first set, with correct aftermarket float valve, just waiting to try them again.

 

Nice Grand National. I got the OEM fack-tory carburetor re-build kit for my junk. Was not cheap but then again, you dont do it every day. $25.00 per carb as I recall.

 

As i was reading thru this i was thinking that the throttle butterfly valves were opened up and causing the rpm hang--and sure enough it turns that was the case, but i was too late to the party...

The root cause of the problem relates to how idle speed is properly adjusted, that is to say with the pilot mixture screws not the throttle stop screw. Notice that it is not called the "idle speed adjustment screw". The butterfly plates will be nearly totally closed at idle--all idle mixture goes thru the pilot circuit, not the transition ports. The throttle stop adjuster screw is just a handy way to bump up the entire idle speed range on all 4 carbs and keep the engine running while doing the 50-rpm drop adjustment of the pilot mixture screws--you open the butterfly valves so the engine keeps running above 1200 while you lean out the mixture, then back them down closed once all the mixtures have been set. When the pilot jets or mixture screw ports are clogged, then folks will resort to opening up the "idle speed adjuster" to get/keep the bike running...