Troubleshooting rough cold running and carb backfire

[john.berg]

Hi,

Been working thru troubleshooting a rough running cold ST1100/96. It runs fine once warmed up, when cold it sounds like it's running on a couple of cylinders, disappears once warm and I mean at least close to operating temp.

I have been thru the carbs twice now, float level, cleaned twice, replaced low speed jets and tubes, resynced a few times, slide diaphragms all look good. I get a popping thru a carb or two when it's cold also.

What next?

Valve adjustment? I saw one thread where someone had a similar problem and disappeared after a valve adjustment. Coil maybe, but it runs strong thru redline. I acquired the ST with 39k and couldn't tell you if the valve clearance has ever been checked.

 

[BuddingGeezer]

Did you remove the carb starter valve (I call it an enrichener)?? each carb has one connected to an assembly that opens them via the 'choke' lever. If they are clogged the fuel mixture will be too lean for cold operation on that carb.

A backfire through the carb is an indication of an exhaust valve open when ignition occurs. caused by out of timed valves, burnt valve or out of spec valve clearance.

 

[john.berg]

I did remove the enrichers and have also run some berrymans thru the carbs with the choke on, it made a slight difference.

The carb backfiring is fairly random when it's cold and doesn't occur after it's warmed half way to normal.

Could I have an exhaust valve that is too tight and disappears after the cylinders expand once warm? It runs real well on the road once it reaches temp so I'm thinking it's not a burned valve

 

[BuddingGeezer]

Could I have an exhaust valve that is too tight and disappears after the cylinders expand once warm? It runs real well on the road once it reaches temp so I'm thinking it's not a burned valve

Only one way to know. check 'em.

 

[Mark]

Backfiring? Has the timing belt been done?

California emissions... check for air leaks in PAIR system.

 

[BuddingGeezer]

.

A backfire through the carb is an indication of an exhaust valve open when ignition occurs. caused by out of timed valves, burnt valve or out of spec valve clearance.

My mistake, I ment to type intake valve.

 

[john.berg]

Hi,

I'm 99% sure the timing belt has not been done, right now it has ~40k miles. I disconnected the vacum lines this weekend to the PAIR valves and reconfigured the single vacum line supplying them. I assume this was the correct method, ? I left the PAIR valves and lines in place for now and really just disabled the vacum supply.

Wouldn't an overall cam timing issue appear as a problem throughout all ranges, seems to run fine on the road when warm but I have noting to compare to since this is my first ST.

I took it out yesterday and rode for an hour, still doesn't seem right, the idle seemed low from where it was set the day before and seemed to "hunt" at idle. One note, I did add the second half of the bottle of Berrymans to the full tank of gas. I did notice yesterday that the fast idle when pulling the choke seems to function now, it really did not in the past it just stumbled when pulling the choke on.

I'm close to throwing the towel in and maybe take into the stealer although CA Sport Touring is about an hour ride away, that might a better option. At the minimum I'm thinking of having the valve adjustment done, I assume the cam timing can be checked at the same time.

 

[Bigmak96]

With only 40K the belt is fine. they go well past the recommended change point of 90K. The valves in my 01 are an unknown as far as adjustments go. The PO said his guy always said"runs too well to need anything done". I just turned 97K so I will look at them this winter. I will put my money on a plugged orifice. I have not messed with my PAIR so I can't say.

 

[Rode2Nowhere]

Signing on to this one...
I've had my '95 w/112k for a month now, and have the identical carb-backfire experience, as well as the "hunting at idle" described. My backfiring is very infrequent, however, like the intake valve only sticks occasionally.

Haven't gotten into the carbs or checked valve lash clearances yet. I'm opening 'er up in a couple of weekends for timing belt change and steering head bearing replacement, so I'll do it all at once.

 

[Bigmak96]

Jonathan, are you planning on replacing your valley plumbing? Sounds like it could need it if it has not been done.

 

[john.berg]

Hi Jonathan,

I'm sort of glad someone else is experiencing this, sorry it's you.

I'm somewhat hopeful that part of the problem is something is plugged as Mark said, the fact that the Fast Idle is now working after a run yesterday with the 1/2 can of Berrymans in the tank is some indication. I actually ran a few miles with the choke on the get some fuel running thru enrichment circuit, maybe this is related to idle hunting and will work it self out with the carb cleaner sitting for a few days or running the entire tank full thru, crossing my fingers.

I'm guessing the Carb backfire is not related to this though, still searching.....I'm not real comfortable on doing the valve adjustment myself and my wife isn't too happy I've been working on the bike the entire Thanksgiving weekend although she hinted at letting me buy a new ST. I'm thinking about going out in the garage tonight and start throwing some wrenches and swearing to see if I can push her over the line on that one. What she doesn't know is I wouldn't sell this one, I'd just make her ride it and sell her bike.

 

[BuddingGeezer]

"Hunting at idle" When the throttle is closed and the idle stays high and then drops to the set idle, this is a lean condition. When the throttle is closed and the idle falls lower than the set rpm and then climbs to the set idle, you guess it, rich.

If the idle goes everywhere it's probably valves.

 

[John OoSTerhuis]

I disconnected the vacum lines this weekend to the PAIR valves and reconfigured the single vacum line supplying them. I assume this was the correct method, ? I left the PAIR valves and lines in place for now and really just disabled the vacum supply.

Hi John. "Reconfigured"...? Disabling the vacuum supply to the Air Suction Valves (clamping it off?) insures that the valves don't function like they're supposed to: cut off air to the exhaust manifold during throtttle roll-off (prevent afterfire). If your intention was to disable them completely, you need to block the exhaust ports. The ASVs are passive, providing air to the exhaust manifold ports all the time, except when abrupt roll-off provides high vacuum which closes them, cutting off the filtered air from the port on the front of the carb body. See chapter 1 of the Service Manual for an explantion.

John

 

[john.berg]

Hi John,

Thanks for the clarification, I guess what I did then enables them 100% of the time.

Any idea where I can get block off plates? Thought I would sandwich them between the head and the flange so I don't have the remove the feed tubing. From what I did is there any ill affects for the short term?

We I said reconfigued the vacum lines I eliminated the tee on the left side since the tee feed the PAIR valves. I replaced the exisiting lines to extend them so I could reach them more easily and on the right side added tee's with a cap to connect the sync hoses.

 

[BuddingGeezer]

Hi John,

Thanks for the clarification, I guess what I did then enables them 100% of the time.

Any idea where I can get block off plates? Thought I would sandwich them between the head and the flange so I don't have the remove the feed tubing. From what I did is there any ill affects for the short term?

We I said reconfigued the vacum lines I eliminated the tee on the left side since the tee feed the PAIR valves. I replaced the exisiting lines to extend them so I could reach them more easily and on the right side added tee's with a cap to connect the sync hoses.

When I removed my pair system, I took the chrome tubing and cut it where it bends to go behind the engine, flattened the tubes closed and mig welded the flattened ends. looking at the engine it appears that the pair system is in place.

 

[John OoSTerhuis]

First of all, I see no reason to remove a properly functioning, bulletproof system that reduces emissions (ill effects), and doesn't affect performance. Why pollute? That said, you can make your own blocking plates from thin aluminum sheet stock; or search here, at ST-Riders.net or my-mc for commercial plates.

I like the chrome tubes so would leave them in place. Even if you don't remove the ASVs, I'd still pull the filtered air hose on the front of the carb body and plug the nipple.

John

 

[BuddingGeezer]

I'd still pull the filtered air hose on the front of the carb body and plug the nipple.

John

If one doesn't plug this port, your gonna suck dirty air into the carbs.

 

[Rode2Nowhere]

Jonathan, are you planning on replacing your valley plumbing? Sounds like it could need it if it has not been done.

Ok... really showing my n00b-ness here (remember, had the bike a month). What's valley plumbing? :

Hi Jonathan,

I'm sort of glad someone else is experiencing this, sorry it's you.

No need to be sorry. This is the beauty of forum communities. Compare problems, find a solution.

"Hunting at idle" When the throttle is closed and the idle stays high and then drops to the set idle, this is a lean condition. When the throttle is closed and the idle falls lower than the set rpm and then climbs to the set idle, you guess it, rich.

If the idle goes everywhere it's probably valves.

Thanks! My "hunting" is so minimal, I can't even tell which direction it's going. It runs almost perfectly with the enricher fully engaged. When it's warmed up, and I bring it to 2k-2500, the backfiring and VERY OCCASIONAL carb-popping happens. It's not out-of-time, I'm sure. It's acting like an intake valve is sticking VERY OCCASIONALLY, but how would that happem? Swollen valve seal?

Like I said, a thorough carb cleaning, valve adjustment and belt change is happening in 2 weekends. We'll see how she goes. So far, the deepest I've gone is finding out that she came with a K&N (woo hoo!), but check out where I found some torn rubber gasketry in this picture...

Sorry John... I've completely hijacked your thread!

 

[Bigmak96]

but check out where I found some torn rubber gasketry in this picture...

None of my bikes has had that on them. I am guessing they all go away the first time the filter housing is removed. I wouldn't worry about that.

 

[Bigmak96]

Ok... really showing my n00b-ness here (remember, had the bike a month). What's valley plumbing?

Under the carbs there are two aluminum elbows that have had a history of cracking. They should be checked and replaced at any signs of weakness. They are only around $9 so many guys just R and R them as a matter of coarse.
There are water lines that run from those to the T-stat, the rad hoses, and several O-rings. The ones in the elbows will not resemble O-rings, just a dark, hard circle. My 01 has 97K on it and no leaks but when they do leak, it is generally from under the carbs.
If you do not have a service manual, I would suggest getting one, they are a great help.
Check out this site, you will find lots of good reading on the STs. http://www.st-riders.net/index.php
Good luck and don't worry, just dive in. There is always help here.