High Hot Idle

[Prairiehammer]

I have a 1990 VR that has been sitting for eight years. After putting a new battery in, it started right up, but didn't idle. I drained the gas tank (the fuel in it was amber) and refilled with BP Premium spiked with 4 oz. Sea Foam.

 

I've cleaned the carbs and checked the slides and diaphragms, and adjusted and cleaned til I'm blue in the face. The bike will idle and has decent throttle response until it gets hot. When it gets into mid temperature gauge territory, the idle begins to climb until it is at 4.5K and I can't stand it any more, where upon I shut it down. This scenario happens whether sitting in the garage on the center stand or while riding. Sooner or late the idle will climb to the 3-4.5K range and make stop lights rather embarrassing and shifting very ugly.

 

I've been running Sea Foam through and have adjusted the pilot screws to 2.5-3 turns out (notably, turning the pilot screws in all the way or out all the way has NO effect with idle). I have synchronized the carbs (using mercury sticks) and they are very nearly perfect and never seem to change synchronization. I have tested the pressure sensor (vacuum sensor, boost sensor) and find voltages well within spec (2.18 when at atmosphere, .65 volts at .90kpa on a mityvac vacuum pump). Pulling vacuum on the pressure sensor causes an increase in rpm...sometimes. Idle must be over 1000 rpm for pressure sensor vacuum to effect it, apparently. I removed the pressure sensor and it's harness and connectors and cleaned them. I removed the TCI connectors and cleaned them. Some of the spade connectors were corroded, but not bad. Still, though an increase in vacuum always caused a voltage drop, this voltage drop did not always/usually result in an rpm climb ON A COLD TO WARM engine.

 

I checked for vacuum leaks at all the usual suspects; the pressure sensor vacuum hose and restricter = AOK, the sync plugs = AOK, the carb to head boots = not AOK. Number 3 and 4 seemed to be leaking at the carb bases. I checked for leaks with carb cleaner spray. Regarding the leaks at #3 and #4 carb boots: I removed the clamps and ground off the tabs that seemingly prevent the clamps from tightening too tight (?), and reinstalled the clamps on the boots and re-tightened. Spraying with carb cleaner around the newly tightened boots did not result in idle change, IE, no leaks. Curiously (and significantly ?), pulling the sync plugs off the idling bike, one at a time, has little effect on the idle, EXCEPT for #2. Opening the sync port on that cylinder, slows the idle significantly.

 

I have checked the float bowl fuel level and found all to be at 19mm below the reference line. I have monitored the fuel level on the running bike and it has remained very stable at this level. (I have rigged four fuel level gauging tubes attached to the carbs for testing). I realize that the fuel level is lower than spec, but that also indicates that an overfull fuel chamber is not causing the hot high idle, doesn't it? Or, could it be that the hot engine is running leaner and leaner and perhaps the lean condition is causing the high idle?

 

I changed the spark plugs (NGK DPR8EA-9). The old plugs (NGK DPR8EA-9) looked new, including the rather white porcelain. Perhaps running a bit lean? (However, during the course of testing and fixing, I ran through a tank full of premium and got just 25 mpg. Granted there was a lot of stationary running, testing, full throttle "testing", etc.

 

The mechanical parts of the throttle system are up to snuff: cables free and lubed, idle stop screw allowing for fully slow idle, carb butterflies all equally closed, and though I had a slide hanging initially, all slides/diaphragms are AOK.

 

As the bike sat there idling nicely away at about 1000 rpm, I was beginning to feel like I had fixed this vexing problem of high hot idle, but NO!, soon the rpm began to climb; first quickly to 3000, but then soon was at 4500 and I couldn't take it anymore. About 10-15 minutes has elapsed. I pulled the pressure sensor vacuum line from #2 and immediately the rpm dropped to about 1500 rpm. I reattached the vacuum hose and it zoomed to 4500 again.

 

I shut the bike down and had a beer.

 

I have been chasing this for days and am discouraged. I am seeking some new insight from someone out there. I have reviewed all pertinent threads, especially the one from warthogcrewchief this year, where he had a similar problem, but his solution doesn't seem to fit my problem.

[MiCarl]

You are probably lean. Lean mixture burns better hot than cold so idle climbs as engine warms up.

 

If you have no air leaks it'll be the carburetors. The fact that the mixture screws have no effect is an indicator here. Likely the idle jets or passages are plugged and you've compensated by having the idle stop screw in too far thus "idling" on the main jets - which the mixture screws do not affect.

 

You need to use carburetor cleaner (spray can) and compressed air to make sure every last passage and jet is open. Also, the air holes in the side of the pilot jet need to be clear.

 

While you have them off fix the float level.

[Prairiehammer]

Thanks, Carl

 

I was hoping that I wouldn't have to take the carbs off the bike and separate them and get inside, but you are probably right. Sigh! There's no way to open those jets and passages from the outside?

 

Kevin

[Yammer Dan]

Mine did the same thing and there are a lot of good answers for this. See last post. I for a lot of reasons didn't want to tear carbs down. Pulled line from intake on fuel pump and ran mix of 1/3 Sea-Foam 1/3 Marvel Mystery Oil and 1/3 Fresh gas. Intended to leave sit overnight but didn't get to get back to it for 3 days. Still ran crapy when I first started but the longer I ran it the better it got. I did put a new set of NGK's after I did the soak.

I get a lot of comments on this but I have the Fastest 1st Gen in West Virginia!! And it now idles Great.

[Flyinfool]

As long as there is at least a tiny bit of flow the seafom may help but if it can not get to the blockage then there is not much it can do. I would certainaly try a heavy dose of seafom and soaking before tearing anything apart.

 

Dan you only have the fasted 1st gen in WV, cuz you bought all the rest of them.

[Prairiehammer]

Thanks Dan and Jeff,

 

Dan as a matter of fact I had read your post sometime back about taking the pump and sticking the hose in a can of Sea Foam, er, at least I think it was you. Anyway, after draining and flushing the tank, i put a full can of Sea Foam into the tank, and got the bike started with a lot of choke and actually idled on Sea Foam for several minutes. I let everything sit overnight, drained the tank and carbs, and filled with fresh gas. I have been running the recommended dose of Sea Foam in the tank since.

 

With the air box off, I rigged an extension to the carb spray tube and have sprayed with force down through all the jets and orifices available from the top of the carbs. I have removed the diaphragms and slides and have forcefully spray carb cleaner through any and all orifices in those areas. I have removed the pilot screws (don't lose the tiny spring, washer or O-ring!) and have sprayed through them until I see the cleaner coming out at other remote locations. I have not used compressed air yet, though. Maybe next.

 

Kevin

[Yammer Dan]

Run it and keep fingers crossed. While mine was acting up I kept idle screw backed off til it wasn't a factor and it still did it sometimes. I just kept pushing it and a lot of Sea-Foam and it finally cleared up??

[escheue]

Pull the carbs and soak the hole thing over night. then use carb cleaner to blast through the idle scew openings and you should see it come through the carb throat. If that doesnt work try spraying through the ports inside the carb throat and observe it coming through the idle screw opening. The ports are very small and you may have to open the buterfly to see the idle ports in the carb. Put it back and sync and adjust and you should be good to go.

[6m459]

I don't like doing unpaid commercial endorsements but........

 

I like Techron, have you tried that?

 

Brian H.

[Prairiehammer]

Thanks for the replies:

Erik, as I stated in my last post, I have done what you recommend---several times.

Brian, I will try the Techron, if I can find it.

Currently, the hot fast idle is not happening. I am not absolutely sure as to the cause since I did several things in an effort to remedy the problem; spray cleaned the carbs yet AGAIN, removed the manifold clamps and modified all the clamps to allow tighter clamping, sprayed silicone spray up into the pressure/vacuum switch, continued to run Sea Foam through with the fuel, air/vacuum leak test with PROPANE and discovered a small leak on #4 that spraying carb cleaner around did not find, reseated the diaphragms and slides, synchronized the carbs AGAIN, making sure to start with the synch screws backed off, etc. It finally started to behave normally. Well, at least it didn't race as it warmed. It is still near impossible to modulate the cold engine with the "choke". It is either too fast or too slow while trying to warm the engine. Once warmed up, it is mighty fine, except for an annoying tendency to fall on it's face as I attempt to smoothly take off from a dead stop. I have to spool up the rpm and feather the clutch to make sure it don't stall. I believe the carbs are running lean overall, because of the low fuel level, but I don't have the gaskets and o-rings needed to remove and disassemble the carbs to make the fuel level adjustments and those parts are not scheduled to arrive this week before I go south. It is getting 38 mpg consistently through several tanks of BP and Shell Premium fuel.

Leaving on a 3000 mile trip to Florida and back this weekend. Wish me luck.

[Yammer Dan]

Good Luck!! I just came back from the Sunshine State. It is hot down there. Did you see the Startron thread on here? I would just keep a little Sea-Foam in it for a while. But the Startron is something to think about. Chevron gas has Techron in it. I use it every chance I get.

[escheue]

Good luck on the trip. I get Techron at Walmart all the time. I like to use it on the bike when I buy cheap gas. Shell and Chevron gas comes with it. It seams to keep things clean pretty well.

[Condor]

Gotta ask the question. You may have mentioned it and if you did I missed it. When the idle started to increase could you get it back down using the idle adjustment knob?? Or was the knob ineffective?? What sometimes happens is the previous owner will attempt to adjust the idle and mistakenly screw in the #1 sync screw. It actually will increase the idle, but what happens is #1 becomes the benchmark carb and really screws things up. I just ran into a bike that even though I backed off all the sync screws by what I felt was enough, #1 was still way in, and the idle still could not be adjusted with the knob. I finally backed the screw way off in order to get the idle knob to work, then went ahead and sync'd the carbs and everything balanced out nicely. Be forewarned. I wouldn't recommend using mercury sync sticks as the balance on the right side gets so far out you'll suck the mercury out of them.