Bizarre, funky issue when rolling up to a stoplight

[Catbus]

Hey folks,
I got a 1979 550E and while it's a blast and I love the bike there are a few quirks it has relating to the idle

it cold starts absolutely fine, first crank or first kick and holds a nice idle with the choke on, and after riding it or a bit (like going into a store and coming back, it starts fine with the choke closed)

but after I get partway down the road pulling out of my driveway and flip the choke off the first time, the next time I coast with the clutch in or stop, the bike slowly dies, as in the idle is too low, unless I quickly crank the idle screw a few turns til it gets back up to a stable idle

THEN, when I get a few hundred feet til the next stoplight, about 75% of the time when I downshift, roll off the throttle and coast to the stop, the RPM climbs up to about 3000RPM until I move the idle adjust screw about a half turn or so, then it goes back down to a stable idle, until the next time I decelerate to a stoplight. It feels like it happens more when I come to a stop quite fast and brake quickly and abruptly.

Once the bike is moving there are zero issues, no backfiring, no hesitation or loss of power when I crank the throttle, it's just when slowing to a stop.


I'm by no means experienced in mechanics or carburetors so bear with me here

Thanks,
Mason

 

[BigD_83]

I went through your other posts, and I did not see that you have actually checked the valve clearances, cleaned the carbs or done a carb synchronization.

A combination of those three maintenance items should get you closer to where the bike needs to be in order to avoid the stated problem.

 

[Catbus]

This is true. I live somewhat in the boonies and have found a a bike mechanic somewhat close-by, as I am not comfortable with valve adjustments or taking apart the carb jets, etc, so am just clicking my heels hoping its a simpler issue. Wishful thinking I suppose but I have put those things off.

I went through your other posts, and I did not see that you have actually checked the valve clearances, cleaned the carbs or done a carb synchronization.

A combination of those three maintenance items should get you closer to where the bike needs to be in order to avoid the stated problem.

 

[jlyon40]

I had that same thing happen to my bike, I fixed it by fixing all the little air leaks I had. Normally the air leak would be because of an intake boot O-ring, but in my case all I did was get a new foam air filter and put new weather stripping on the filter and on the air filter side covers. That fixed my issue. I also put some Seafoam in and when I started up I had to initially adjust to idle screw because it was so messed up from the way it was running before. Once I got the idle screw adjusted properly the bike was idling perfectly even after being completely warmed up. I'm not sure if this will help with your issue, it could be a number of things from air leaks to the carbs needing tuned. Good luck!

 

[1948man]

To try something cheap and easy, leave the choke partially on for a while longer (experiment). Might not help but won't hurt anything. Does this symptom go away if you ride the bike for a significant amount of time and get it well warmed-up? I have bikes that will die on decel if I start riding too soon after startup with premature cancel of the choke.

 

[Redman]

Are you saing that when adjust the idle when engine cold/warm and then when engine hot the idle is too high.... and when adjust the idle when engine hot then the when engine is cold/warm the idle is too low.... that what you are saying...?
Classic sign of air leak around intake boots.... maybe the o-rings in the boots up against the engine, or where the boots seal agains the carbs, or thru the o-riongs around the choke/enricher rod, or cracks in the intake boots.
Inspect the boots, replace the intake boot orings.

Check out Cycle O-Rings site.

.

.

 

[Steve]

I'm by no means experienced in mechanics or carburetors so bear with me here.

This is true. I live somewhat in the boonies and have found a a bike mechanic somewhat close-by, as I am not comfortable with valve adjustments or taking apart the carb jets, etc, so am just clicking my heels hoping its a simpler issue. Wishful thinking I suppose but I have put those things off.

Hate to be blunt about it, but there is really no other way: if you are not comfortable with mechanical things, your choice of a 38-year-old bike that has not been YOURS for its entire life is highly questionable.

There have been rare exceptions, but usually, the best mechanic for your bike is the guy that looks back at you from the mirror.

If you are interested in learning and are willing to follow instructions, there is no better place to be than right here.

.

 

[JTGS850GL]

Hate to be blunt about it, but there is really no other way: if you are not comfortable with mechanical things, your choice of a 38-year-old bike that has not been YOURS for its entire life is highly questionable.

There have been rare exceptions, but usually, the best mechanic for your bike is the guy that looks back at you from the mirror.

If you are interested in learning and are willing to follow instructions, there is no better place to be than right here.

.

Agreed with Steve here. Anyone owning a 38 year old bike MUST either work on their own or have a ton of $$$ to throw at a mechanic. Finding one to work on these bikes is a whole nother matter. Learning to work on these guys is half the fun.

 

[BigD_83]

BTW, exactly how far into the boonies are you? Are we talking Sunshine Valley, or the real sticks such as bordering on Tweedsmuir, or tucked into a northern corner of Vancouver Island?

 

[1948man]

Hey folks,


it cold starts absolutely fine, first crank or first kick and holds a nice idle with the choke on, and after riding it or a bit (like going into a store and coming back, it starts fine with the choke closed)

but after I get partway down the road pulling out of my driveway and flip the choke off the first time, the next time I coast with the clutch in or stop, the bike slowly dies, as in the idle is too low, unless I quickly crank the idle screw a few turns til it gets back up to a stable idle

THEN, when I get a few hundred feet til the next stoplight, about 75% of the time when I downshift, roll off the throttle and coast to the stop, the RPM climbs up to about 3000RPM until I move the idle adjust screw about a half turn or so, then it goes back down to a stable idle, .

Thanks,
Mason

I guess this thread is dead but if you are readjusting your idle upward before the bike is warmed up enough to truly come off choke, when it does get warmed up the idle will then be too high. Only adjust your idle after the bike is well warmed-up. Up 'til that time just wean off choke a little slower. Sometimes the bike seems ready to come off choke but if the engine is put under load it is not really ready to pull yet.

 

[Guest]

VM carburetors will do that on deceleration especially on a down slope if the choke Is not totally closed. I believe the '79's had a cable setup. Might want to check that, it could lead to some unnecessary tail chasing.

 

[Catbus]

Sorry folks, I chedked back after the first couple days and assume no one else would reply further.
The part about immediately flipping the choke down the road is interesting, because the bike does die with the clutch pulled in after closing the choke maybe 100 feet down the road... which means while the RPMs slowly go down til it dies I wildly twist the idle screw til it goes back alive. Perhaps between that time and maybe half a kilometer it warm enough that when I reach the first stop sign near my house, the idle speed goes up to 3000 because of how I adjusted it earlier. I think this might be the problem. I'm not letting it properly warm. This is a tidbit of info I have not heard yet. I will go out for a ride in a bit and keep the choke on for longer, and then after it jumps to 3000, I'll set it once and see if it happens again.

Are you saing that when adjust the idle when engine cold/warm and then when engine hot the idle is too high.... and when adjust the idle when engine hot then the when engine is cold/warm the idle is too low.... that what you are saying...?
Classic sign of air leak around intake boots.... maybe the o-rings in the boots up against the engine, or where the boots seal agains the carbs, or thru the o-riongs around the choke/enricher rod, or cracks in the intake boots.
Inspect the boots, replace the intake boot orings.

Check out Cycle O-Rings site.

.

.

 

[Catbus]

Hate to be blunt about it, but there is really no other way: if you are not comfortable with mechanical things, your choice of a 38-year-old bike that has not been YOURS for its entire life is highly questionable.

There have been rare exceptions, but usually, the best mechanic for your bike is the guy that looks back at you from the mirror.

If you are interested in learning and are willing to follow instructions, there is no better place to be than right here.

.

Agreed with Steve here. Anyone owning a 38 year old bike MUST either work on their own or have a ton of $$$ to throw at a mechanic. Finding one to work on these bikes is a whole nother matter. Learning to work on these guys is half the fun.

Yes I am learning a lot as I go, it's a very fun experience and I find it quite fascinating, thankfully I have a friend who knows a reasonable amount about bike engines but he's getting somewhat tired of helping me every time.

 

[Catbus]

Agreed with Steve here. Anyone owning a 38 year old bike MUST either work on their own or have a ton of $$$ to throw at a mechanic. Finding one to work on these bikes is a whole nother matter. Learning to work on these guys is half the fun.

I guess this thread is dead but if you are readjusting your idle upward before the bike is warmed up enough to truly come off choke, when it does get warmed up the idle will then be too high. Only adjust your idle after the bike is well warmed-up. Up 'til that time just wean off choke a little slower. Sometimes the bike seems ready to come off choke but if the engine is put under load it is not really ready to pull yet.

This is great info, something I have not heard of yet. I'll try keeping the choke on longer, and wait a while before I touch the idle screw. I'll try going well up the hill and around the bend near my house before I touch the choke. What I did before was just ride about 100 feet down the road to warm it a bit, then choke off, but I suppose that is not enough time, since the next time I stop after taking the choke off, it always dies, unless, like I said, I wildly spin the idle screw. And then I assume after it warms by going about half a kilometer and I find the stop sign it's warmed more and that previously set idle speed is too high, thus going to 3000. I really hope this is the problem!

I'm shopping for petcock replacements that aren't $80+ before shipping, (the bike only runs on PRI or dies on ON or RES after whatever gas is in the carbs runs out) wondering if my replacement HAS to be a vacuum-actuated petcock or can I use any old generic chrome fuel on/off switch?

 

[Catbus]

VM carburetors will do that on deceleration especially on a down slope if the choke Is not totally closed. I believe the '79's had a cable setup. Might want to check that, it could lead to some unnecessary tail chasing.

It looks like my Choke is a flippy switch, not a long cable

 

[Catbus]

Well I went and took the bike for a warm-up. Followed advice and kept the choke active for as long as it would let me.... until I got halfway up the hill and it bogged and stalled lol I did not touch my idle screw until I rode around for a couple KM and then stopped, it still does the idle hanging, but this time only to about 2K. After riding it around for about an hour and I was riding it hard, the issue can still be replicated every time. I can force it back down by letting the clutch out to the biting point, but that doesn't solve the problem does it? Once I burn though this tank (still half full and I filled it a while ago, darn bike MPGs!), I will rip off the petcock and take it all apart, brush everything with gas, and soak it overnight. With the tank off I'll wiggle every carb tube and boot seal and check everything is sealed nice.

If none of that help, I suppose it's time to dis-assemble the carbs and pray I don't lose anything lol
oh, and buy all the little tools for dealing with the jets.... yiiippeeee :grumpy:

 

[Big T]

If your petcock is not working, is there a hose between the petcock and the #3 carb?

 

[Catbus]

Agreed with Steve here. Anyone owning a 38 year old bike MUST either work on their own or have a ton of $$$ to throw at a mechanic. Finding one to work on these bikes is a whole nother matter. Learning to work on these guys is half the fun.

If your petcock is not working, is there a hose between the petcock and the #3 carb?

I just checked and yes, there is a hose hooked up to the petcock and to a carb, not sure which side starts as 1. there's 2 vacuum hoses [edit: a fuel overflow and a vacuum hose] too for what its worth, too. I made sure to move them to a spot where no pockets of wind or anything can disturb this sensitive black magic.

 

[Catbus]

holy crap have I had the actual vacuum hose and the vacuum hose for operating the petcock diaphram switched the whole time? If carb #3 is the one with the idle screw.... that one is a freehanging "vacuum hose"

i'm going to look at more youtube videos....

 

[steve murdoch]

Not sure which carbs you have but hose locations can be found here.
http://members.dslextreme.com/users/storagecliff/images/hose_locations.html