77 GS550 won't start cold

[craze1cars]

Hello...long time listener, first time caller. Here's a brief introduction of my bike:

She's a 1977 GS550 I scored for nearly nothing from the original owner, 100% stock with only a bit over 16,000 miles on it. Bike had been sitting unused for about 8 years. I drained tank, fully rebuilt carbs, replaced air filter, oil/filter, got a new battery and got 'er fired up with little problem. Have been gradually getting it tuned in by reading plugs. In about 400 miles so far it runs like an absolute champ with no bogging whatsoever, getting about 50 mpg...seems just as happy idling smoothly at 1100rpms as it is ripping wide open at 9,500 rpms. A fun little bike and just what I needed to help me get my sea-legs back after 20 years out of the saddle.

My only gripe is EVERY morning she won't cold-start (cold engine, not cold air....air temps here are in the 80's most of the times I'm cold-starting). Best I can get is at full choke with throttle closed...it IMMEDIATLY fires up and runs at about 3,000 rpms right out of the hole, then falls flat on its face after about 2 seconds, never to restart again. While my thumb sits on the starter I can hear it firing on 1 or 2 cylinders, but not enough to keep it going. I've experimented with leaving full choke on as long as possible, shutting full choke down to 1/2 choke, down to no choke, giving various levels of throttle, etc...SAME result EVERY time. 2 seconds of 3,000 rpms, and she shuts down. Then if I give it just one very short hit of starting fluid and she rolls right into a beatiful idle and runs perfect all day long...warm restarts are perfect as long as I don't let it sit more than about 4 hours. But I just CAN'T get it to fire when the engine's fully cooled off without the almighty ether cheater. Setting to petcock to prime gives me a FEW more hits on the cylinders...like maybe hitting on 2 instead of just 1....but still not enough fuel getting in to let 'er run. Yet there is obviously plenty of fuel is running thorough the lines....if it can spin 9,500 Rpms without a single burp it's definitely flowing enough fuel to start.

As I've spent a lot of time searching these forums, I assure you I have checked thoroughly for intake leaks with a fine-tooth comb and it is absolutely air tight. When it's warmed up and running, no matter what I spray all around the carbs and intake boots I get absolutely NO variation...not a hint of a leak anywhere. My plug reads after rebuild and initial setup were a very light tan...almost white. So I suspected it was maybe too lean to fire up. So I now have richened things up so each plug is a very symmetrical crayola brown color. The machine still runs outstanding, but it's still not getting enough fuel when cold to fire without the brief shot of ether. Plugs are new, B8ES, gapped to specs. Points look real good, and properly gapped. Spark looks normal to me on all 4. While my stator is weak (only putting out about 12.4 volts), I ALWAYS start with a fully charged battery which is kept topped of daily with my Optimate battery charger. Starter spins the motor FAST and STRONG. My thinking is that there should be plenty of juice in a fully charged brand-new battery for a simple cold start regardless of my known weak stator. I haven't done much further electrical/ignition testing but I'm thinking that's where my next round of troubleshooting will need to focus. Please advise if you disagree.

Sorry so long, but I like to share exactly where I'm starting so we're all on the same page. Any suggestions for curing my cold-start woe would be much appreciated. Everything else about this bike it oustanding...\\/

[chef1366]

Try blowing carb cleaner through the brass choke tube (jet) in the float bowl. It sounds like a choke circuit issue to me.

[craze1cars]

Thanks, but they're spotless so I can't imagine what the carb cleaner would accomplish....when I rebuilt the carbs last month I disassembled and rebuilt the choke plungers as well as thoroughly cleaning all the tubes and orifices in there...they have all new O-rings and have been reassembled squeaky clean.

[chef1366]

When the bike dies after running it with the choke until it dies pull the plugs and see if they are wet. Could be a weak spark. If they are dry it's a fuel issue.

[craze1cars]

They are dry. It's definitely not a flooding issue, I'm largely convinced it's not getting enough fuel, which is why I enrichened the mixture on the idle/pilot circuit, but have no idea how to give it any more without making the plugs turn soot-black. I also don't want to be so focused on what I THINK the problem is that I can't see what the ACTUAL problem is....which is my whole reason for posting here. Hoping you guys can help me think outside my box.

Keep the suggestions coming...I sincerely appreciate it. FWIW, I just did a compression test to make sure the motor was healthy. All 4 cylinders warm, WOT, and dry are between 112 and 114 on my gauge (which I've learned reads a bit low). I didn't bother with a wet test as I don't see a problem there either.

[Nessism]

Not sure if this is a meaningful suggestion or not but how are your pilot fuel screws (the ones that come up from the underside)? Very common to snap off the tip in the small orifice. This is not likely the issue though since the bike runs so well when warm (as you stated). Another comment is to say that intake air leaks sometimes don't show up when spraying fluids on the outside (didn't on my old 450 at any rate).

Good luck.

[DimitriT]

Check valves.

Although it sounds like a fuel problem..

the choke circuit works by giving an initial spray of fuel (the contents of the tube) but then is limited by the choke jet (which sits in the bowl).

It takes time to refill the tube.

My guess is that your bike can run cold on that initial spray but once that runs out its not able to run on the choke circuit.

[KEITH KRAUSE]

Just to mention, your VM carbs choke circuit is bypassed if you open the throttle at all during starts. Yes it still effects mixture, but you're forcing the carbs to feed through the pilot circuit then.
Sounds mixture or vacuum related to me. Also sounds like the choke chambers have fuel upon start up or it couldn't "jump" to a 3K cold start.
Since you specifically said the carbs are clean and fresh inner o-rings, plugs, etc...
Unbalanced or poor carb synch can cause this. At start up, vacuum issues can be more noticable than after it's running, even if running only a few seconds. A quick shot of starter fluid is that little boost that compensates for uneven vacuum drawing on the pilot jets unevenly and feeding uneven mixtures into each cylinder.
The carbs must be bench synched and then follow that with a vacuum tool synch.
Before initial start up, I generally set the side air screws on your carbs to approx' 1 1/2 turns out from lightly seated. It's obvious you've been tinkering with the pilot fuel screws underneath. They can range about 1/2 to 1 1/4 turns out on a stock bike straight from the factory. I generally set them at 1 turn before initial start up. Fine tuning both screws will be needed later.
After warm up is complete, place the bike on the centerstand and set idle to 1,000 rpm. Adjust side air screws using the highest rpm method. Start at any carb, slowly turn an air screw in either direction until you hear the rpms max out. Fine tune it. They generally end up 1 1/2 to 1 3/4 turns out. Now re-adjust idle to 1,000 by adjusting the idle control knob. Repeat to remaining carbs.
Now you can vacuum tool synch.
Keep in mind the ignition timing and valve clearances must be correct for an accurate vacuum tool synch. I synch at approx' 3K but it's up to you. The pulsing levels at factory idle rpm can be difficult if not impossible to get very close on older bikes. A 3K synch works for me. Use fans to avoid over-heating.
After the synch, you can test the pilot circuit by riding around at steady/minimal throttle opening and chop off to read the plugs. Adjust the pilot fuel screws if needed. It's OK to set them a little differently from each other if needed for uniform plug color since they are for fine tuning each cylinder and allowing for small differences in each cylinder. They should end up within a 1/4 turn of each other under normal conditions, if not uniform. If you find that you moved the pilot fuel screws beyond 1/4 turn as a set, then I suggest re-adjusting the side air screws a final time, just to be as close as possible. Hopefully, your problem is gone after next cold start up.
I don't know what you've done but the synch and screw adjustments are critical to good starting and I'd do the above first because it's basic tuning.

[snowbeard]

well this one's not a root cause fix, but it seems to work amazingly well as a patch. I have the same issue as you (but with a much less thurough cleaning) I needed a shot of ether after a few days not running or on cold cold mornings. If you want to really fix this, keith has given you the path to enlightenment!! [-o<

pull those two breather tubes from the carbs that are routed back and into the engine. blow just a little puff into each one, sometimes I do one each and repeat. now start it.

this is not even my trick, it came from tips and tricks, but man it works wonders! has gotten me to work all sorts of times that she shouldn't have even started (I like to say it has a safety feature that it won't start wet or cold)

I've even considered putting a bulb on the end of the two hoses so I can just give it a squeeze prime.

OH: don't forget to replace the hoses, as the wind at speed pulls a vacuum on the bowls and you'll get weird lagging... ask me how I know.

[craze1cars]

All right! Thanks everyone for some GREAT info, some education on how the choke circuit works, and some ideas on how to fix the problem. My fuel pilot screws are not broken, I have indeed messed with them on a very limited basis with careful note taking so as not to get too far out of whack. Keiths recommendations especially hit home as I have NOT yet fully and properly synched the carbs as frankly it intimidated be a bit...plus it really is running great except for the start-up. But now I'm really guessing that's where my problem may lie and I need to get at that.

Sadly, I am now sick as a dog...and per doctors orders I will probably lay low in a drug-induced state for a few days, and try to recover before attempting any of this. But I will get around to it and will post my findings if I manage to get over the hump.

I really appreciate the time you guys took to write back, and if others have more suggestions, keep 'em coming. Thanks!

[KEITH KRAUSE]

OK. Hope you're better soon.
If you need VM synch info, I posted a detailed bench synch awhile back but since I don't remember how to post a link, you need to search it out.
Best I can help is to find "Hoomgars" technical section thread titled something like "GS1K bad fuel economy, rejet attn Keith Krause"? It's reply number 36 I believe, once you find the thread.
I wish some computer savvy member would help out and maybe we could apply a "sticky" to that info and it would be easy to find/read. I know many aren't aware of a basic VM bench synch and can use the info.
Only edit I can think of to that info is that some models have carbs that have a "master" carb. If that's the case, the master carb (#3) isn't adjustable and you simply synch the other carbs to match it. On carbs without a master carb, you can start with any carb you want to. Just get the slides all even. Don't use the "drill bit method" with VM carbs. The VM slides fully closed position is too low to use a drill bit and the synch won't be correct.

[craze1cars]

Thanks again. I will say this much...if bench synching involves little more than making sure the slides are all at exactly the same height at open and closed positions, then I did confirm that before reinstalling the carbs on the bike and they are all visually even. My bike did not have a master carb...all were adjustable.

I've worked on snowmobiles with multiple-Mikuni slide carbs for decades, and I do understand how important it is to have them synched so that slides are all in the SAME fully closed position, as well as a the same height in the WOT position. So this I can confidently say I checked this prior to installation, and they did not have the traditional drill bit gap you referred to either as the manual I followed did not recommend it...they are indeed closed when closed. They were all perfect, so I saw no reason to make any adjustments. When closing, they all hit bottom at the same level at the same time, and at WOT the slides all cross the top of the bore at the exact same time.

But if there's more to bench synching than that, I didn't do it. And I definitely did NOT do the final vacuum synch, as this was the more confusing part to me, I don't have a 'manometer' or whatever special tool I kept hearing about in most procedures I breezed through, and I have no experience with the process.

In fact, in an effort to hopefully avoid the whole need to synch, I left all the slide assembly linkages assembled during my cleaning/rebuild (just cleaned the needles thoroughly, quick sprayed the rest of the slides, and reinstalled...the componants above the needles never see enough fuel to get dirty anyway). So the linkages all still have the factory "glue" sealing the screws up top that perform this adjustment...have never been disturbed in its 30 year lifespan. But, maybe it's about that time.

I'll do some more searching. But I'm starting to think about just trying to find a competent bike mechanic here to do this final tuning step for me and get it dialed in. But finding a good mechanic is often like finding the needle in a haystack for me, which is why I learned to do most things myself. So if I can go it alone, this is still my preference.

But on that note, can anyone recommend someone VERY competent, who can do this to my 30 year old bike in or near Indianapolis, IN? Thanks!

[KEITH KRAUSE]

OK. Just trying to mention anything that wasn't mentioned in your list of things done.
Carbs will lose their synch under normal riding conditions. With the VM carbs, you must re-synch anytime the jet needles are disturbed. Even if the slide assembly isn't taken apart, just separating the throttle shaft from the slide arms and lifting the slides will compromise the previous synch. Anyone can get lucky and bench synch the carbs close but almost every time the vacuum tool synch will be a significant improvement.
Before getting into true troubleshooting, I always try to mention all basic maintanance and tuning first to fix any problem. If it's a fact that everything is adjusted correctly, clean, etc, then you know it's not tuning and you have a bad/wearing or incorrect part.
Typically, poor cold starting (only) is a mixture issue. Since your bike starts on the pilot circuit, with the assistance of the choke circuit, a pilot circuit/choke circuit problem is the most likely cause. But you say you've checked everything. That's why I suggested the bench AND vacuum tool synch. After all, if the vacuum drawing in each cylinder isn't close enough, you will have uneven mixtures. The vacuum tool synch may or may not fix your problem but it's a part of tuning that must be done before you can say "I don't know what could be wrong".
Your bike obviously needs just a little help in richening the mixture under cold starts. The pilot fuel and side air screws need to be adjusted like I said too because if they're not tuned correctly, a problem like yours is common. Tuning the pilot fuel screws can take a little time. They are sensitive to adjustments. Getting trustworthy pilot circuit plug reads takes some effort and has to be done with clean plugs and a completely warmed up motor.

[Steve Lowe]

Cold Start Problem

Has this cold start issue been resolved? I am having exactly the same symptoms on my 79GS750. I've tried everything in the post with no luck. The only new information I can provide is that I used an aftermarket jet kit since it was impossible to clean the old jets. I spent a lot of time cleaning the carb and had air through every passage after the rebuild.

The bike runs great when it's warm. It runs a bit rich so I still need to tune in the pilot jet. I've adjusted each between 3/4 to 1 1/4 turn to try starting but it doesn't make a difference.

Also, blowing in the vent tube as was suggested does work. I'm not sure if that points to anything more specific.

[Planecrazy]

I know you mentioned using the prime setting on the petcock without success, but how long did you leave it on prime? There was a thread last spring relating to this where people were having the same problem you do ... difficult starting when cold, and it was suggested that the float bowls slowly drain out as the bike sits. Putting the petcock on prime for ten seconds before starting and leaving it there until the bike starts seems to help a LOT of GS'rs (myself included) who have one or more bikes that run beautifully but are difficult to start cold.

You may want to revisit this solution again and see if waiting ten to fifteen seconds before cranking it over does the trick. By the way, DON'T forget to switch the petcock back to "run" once the engines running!

Regards,