Engine dies when throttle is closed.

I've gone through a series of different problems as I brought this old bike back to life, and I need some advice on where to go next.

I finally got a color tune and used it a couple of days ago (thanks Chacal!) and resolved some of the problems that I was having but not all. Before I used it, the bike would stumble of idle when cold and was very hard to start and keep running. This is where I am now:

I went out this morning (42F), installed new plugs and thumbed the starter. The bike started immediately but I had to coax it into idling by bliping the throttle and playing with the enrichment a little. I haven't run it enough since I color tuned it to know how much enrichment it will want but it was much easier to keep running than before. The bike idled well and all cylinders seemed to be heating at about the same rate. I took it out for a short ride and when I close the throttle to come to a stop, the bike will die. It starts back up very easily and after a blip or two from the throttle will idle well. It will not die unless its been under a load. I didn't ride it long enough to be able to read the new plugs, but I've had this problem all along, and after long rides they have been a nice suntan color. I added fuel with the mixture screws when I did the color tune - a bunch actually. The float levels are even across the board and are right at the lower spec.

I checked the pick up coils and they are within spec for resistance and are very clean. The ignition coils read good on the secondary side but are high on the primary (3.2 & 3.3 Ohms), the caps are good as is the alternator output. I just went out to look at the spark and there's a fair amount of white in a largely blue spark. The bike will usually stop this nonsense after its been ridden for 20 -30 miles, well past what I'd normally think it needs to warm up.

The only idea I have is the coils, but I'd appreciate the opinions of those of you that have more experience with these engines before I spend the money on them.

Thanks for reading.

 

did you ever check the valve clearance ? tight valves will do that
whats the idle set at?

 

I haven't checked the valves. I suppose I should try to find the time to do that. I would suspect if anything that they would be a bit loose rather than tight though. The bike has 13K miles and I've owned it since it had 5K and I doubt the valve covers have been off.

After I saw your post last night, I went out and did some experimenting. I have been running my idle @ 1000 RPM warmed up with no enrichment. I raised it in about 4 steps and it stopped the dying stuff at 1800-2000 RPM. This morning it was 38 and the bike started quickly, but I wasn't able to keep it running and get a steady idle for a short while. Finally, with about 3/4 enrichment it idled, but I took it out and it died at the first stop. I had to work today and when I came in, I tried starting it again. @ 48 deg with 3/4 enrichment it started easily and seemed to want to stay running @ about 1200 RPM with only a few quick throttle blips. I didn't try to ride it.

Thanks,

 

hold on the valve clearance for now, it's more of a hard run down the four lane, pull off a exit to a red light and stall kind of thing
1000 rpm idle should be very doable
have you had the carbs off and cleaned ? checked the intake tubes ?
clunk tested the slides ?
run some seafoam thru it ?
i'am not a color tuner, but how well warmed up was it when you did the color thing, i think five miles to get to operating temp then start tuning

 

The carbs have been off and I replaced the idle and main jets, needles and seats. The lower end was thoroughly cleaned, the top end not so much. The top ends really looked pretty good. The original tubes (from carb to engine) show evidence of cracking, but I've sprayed them with WD-40 & carb cleaner and can't identify any leaks.

No, and I've wondered if a sticking slide could cause this. Any chance they would stick more cold than when hot? Would they open much on a short slow run down the street?

Ran a bottle through it when I put it back on the road.

I think it was pretty well warmed up.

Thanks,

 

Hey mbrew welcome ,,,,just a quick observation,,I don't see were you synced the carbs? has this been done?

 

Only a bench synch using the paper clip method.

 

first things first
read this clunk test

hot or cold they must be free, they move almost every time you give it gas

a bench sync is really just a rough guess, you can do better
then get that color tuner back out

 

I'm just closing out old threads and I knew I left this one hanging. Since I stopped posting to this, I have done a full sync and I've done several chops with the plugs to get the idle mixture dead on. I'm not quite there, but this bike now has some pretty cold days that it starts real easy. I'm certain that the carbs are not to Ricks and some of the others standards but the bike runs pretty well if I do say so myself.

I did not describe my the most basic problem very well in that I didn't make it clear that it died when I rode off with the enrichment on. The OP of this thread described the problem much better and based on what was said in his thread, I've accepted it as normal and I give the bike a bit more warm up time.

Thanks,
Mike