No idle at all after carb rebuild

[avinitboys]

Just coming to the end of a very long rebuild/resto on a 76a. The engine has been out to get soda blasted and painted and other than having the head off to take a quick peek and building back up with new gaskets, valve clearances done etc, nothing else internally has been done. Mechanicaly everything seems pretty much spot on.
The bike had been sitting for several years under a sheet of plastic next door to Gatwick but I did get it running before stripping it to the frame. Now it's almost finished I'm close to chucking it on the f*****g runway as I just cant get it to idle at all. It was fitted with SP carbs and loom so I decided to keep it that way despite needing to track down a new TPS. Carbs were professionally ultrasonically cleaned by a friend who was amazed how much rubbish came out and yes, I did have to break them down to get them in his £2K machine! Anyway, they were built back up with new orings etc with pilots set to the usual 1.5 turns out. I didn't fall into any of the usual traps (we all know what they are) whilst rebuilding them so I'm confident everything is spot on. Once the engine is running, above 2K rpm it revs cleanly with no apparent flatspots but just fades if the revs go below. On starting it doesn't seem to make a difference if the choke is open or closed, it will simply refuse to fire unless a quick shot of Easystart is fired up the filter so I'm sure there's a clue there.
I intend to whip the carbs off again and try a more accurate bench balance with a caliper gauge but beyond that I'm a little stumped. Obviously cant get anywhere near balancing on the bike as it just wont run without throttle input, even if the idle screw is turned in.
It's a real shame as I'm almost there and have spent an age blasting, powdercoating and zinc plating every component so it looks like a new machine ( or as it's a Suzuki.....better than factory!!!!!)
Any ideas guys?

[Maelstrom]

I am no expert on FCRs but I have learnt a lot lately. If your carbs have a mid-body then this might be the cause of your pain.
If it is not then it will be in the future. See my contribution to this thread.
viewtopic.php?f=37&t=42858
The mid-body joint is circled in red in the image.

[rgvsp]

Have you cranked up the idle screw to see what difference it makes?


I'm just kind of guessing here. Possibly a venting/pressurizing issue since you aren't running the TPS system? Do you have the vent piece that attaches to the top of the airbox?

[avinitboys]

Hi there,

I have tried the pilot screws in every position with little joy. With regard to the sp carbs, I am running the tps as I managed to track down all the bits I needed to make everything work. The vent and sensor on the top of the airbox are connected, although I have no way of telling if everything is working correctly.
I've had the carbs off again and re-seated every o ring but that hasn't made any difference either. I am now considering trying another set of o rings in everything just to rule out any rogue ones.
The only gasket that I didn't change was the internal one between the main body and the jet block. Could that be the cause?
As I said, above 2k rpm it revs clean and drops back no problem without hanging so I'm still thinking something on the idle circuit somewhere. I know everything is spotless internally after so long in the ultrasonic tank!

These carbs are infuriating!

[rgvsp]

I'm not sure if that would be the cause of your problem but if you've gone to all the trouble getting them cleaned and changing all the other o rings, I'd swap those out as well. It can't hurt it!

[Variablevalves suck]

If its clean once on throttle it points towards the idle/pilot system.
Do you have the pilot screws rebuilt correctly with the spring then washer and finally the o'ring, also you have no old o'ring stuck up in the hole.

[avinitboys]

I've checked everything I can think of on the idle system. Definitely counted out four old o rings before I fitted the new ones and yes, in the correct order.
I'm beginning to doubt the o rings themselves.
As you suggest, everything points to the idle circuit somewhere. I'll whip the carbs off again and swap the entire idle screw assembly with some from a set of carbs I know work well, although they aren't SP carbs. I assume the idle circuits are the same?
One thing I have noticed as I've been fiddling about is that its very reluctant to start after its been running for a minute or two. Almost like some sort of fuel starvation. Float heights are all spot on so think I can rule that out but nevertheless, this again must indicate something's not quite right. Once it's running though, it appears very responsive although I've not actually ridden it yet. It simply refuses to idle in any way and dies within a second of dropping below 2k rpm.

[avinitboys]

Evening guys,

Well I've had another look at these carbs today and replaced every oring again, except the slide holders as I simply forgot about hem. Doh!
I decided to go for a 1.5mm thick oring for the pilot screws after trying two sets of 1mm ones with little joy.
Bike again refused to fire up without a bit of cold start up the Intake but, once running was really very crisp on the throttle, even better than before. It picks up amazingly well but still refuses to idle. I screwed the thumb wheel adjuster in a fair way to keep the bike at around 3k for ten minutes or so without any hint of a stutter but as before, as soon as you try to make adjustments to lower the idle it just away slowly. Again tied the pilot screws in every position from 1/2 a turn out to 3 turns out with no difference. No air leaks either.
After the old girl was showing a decent temperature I pulled open the cold start circuit expecting it to stutter but instead it just revved itself to death. Wouldn't have thought that was normal. Could the seals around the cold start pistons be the culprit. They were severely corroded when I stripped the carbs and I polished them before rebuilding with the old seals and retainers.
Could the slide holder rings be giving these symptoms? I doubt it as they are not part of the idle circuit
Really at a loss now

[Variablevalves suck]

When you pull the choke the bike should rev up to around 4000/5000rpm so sounds ok.
You say it will run sweet above 3000rpm so this is still pointing to the pilot system.
Need to check the pilot jets are clear but more importantly the pilot air jeta at the front of the carb inlet as these get pretty dirty.
I'd also check HT leads are tight etc as these sort of electrical faults can give idle or hi rev problems.

[dobbslc]

Have you checked to see if the spark is dropping out when it dies? It could be a duff pick up coil or did, I'm not certain with the gsxr but some bikes have both high and low speed coils. Low speed works up to about 1500 rpm then the high speed side cuts in. My LC did this, it would run fine when started and reved up but died on idle.
Cam timing could be out giving a low vacuum at slow engine speeds.....