Erratic RPMs after 20m ride, vacuum leak?

[macsinger]

Let me first say upon my virgin post that I very much love owning and riding my 1986 ZL 600 eliminator, and I am really happy to see such a great online presence for other people who own them and are enthusiasts!

Unfortunately I come to this forum with a problem with which I need the opinions and advice of other people who own this same motorcycle.

My baby is running well for most of the time. However, after about a 20 mile ride out into the country, as well as getting about five stoplights back into town, the RPMs accelerating from a stop will become extremely erratic, with the needle jumping from 1.5 thousand to 4 or 5 thousand RPMs until the engine just quits altogether. Occasionally there has been a rare, but very loud backfire just before the engine quits. The odd thing is that I can start it back up it will idle just fine. I can can even ride it very gently and slowly in first and second gear as long as I keep the RPMs down around 3 thousand. When I get home and let the bike sit for a while it will start up and the problem seems to be gone.

Now, some history. This happened to me about a week ago after the I enacted the exact same circumstances. I took taken a 20 mile ride into the country and about 5 stop lights back into town the above symptoms occurred. It also happened again last summer. Every time this has happened it has been after a long ride. Always in town, always after about 20 miles.

Now, my hypothesis... My first thought was that it could be a vacuum leak. After the first episode I found a spring clamp on a rubber boot connecting #1 carb to #1 cylinder. However I tightened it and the problem occurred again. The circumstances tend to suggest that regardless of what is actually occurring inside the engine, lots of heat seems to be triggering the problem. Even the fact that this has happened only in town once I've slowed down seems to suggest that the lack of airflow through the radiator and therefore more heat buildup could be contributing. Perhaps it is a vacuum leak that is happening when something gets hot enough to expand a rubber piece to the limit of creating a leak.

I am currently doing a complete flush of my coolant system. My reservoir tank for coolant was empty when I opened it up, but there was coolant all the way up to the to the radiator cap when I took it off.

The radiator fan seems to be working fine whether the engine switch is on or off, or whether the bike is running or not. I am now starting to suspect that if something is wrong with the coolant system it could very well be the water pump. Perhaps the gear that runs the pump is stripped. Although I have no idea how to test this as the water pump should only turn on when it is activated by the thermostat.

Any ideas on what could by causing this problem given the above circumstances. Any methods of isolating the vacuum leak? If you agree with me that the problem is caused by too much heat, could the water pump be the problem? If so, how do I test the pump to see if it is working. The manual(s) say nothing about the water pump.

 

[golddrakul]

Sounds like fuel starvation. At least two things to check; is the vent line clear all the way from the start in the gas cap to the end [maybe in canister or could be just open hose] and is there a blockage letting fuel trickle but not flow?

Venting on the bigger bikes [900/1000] has been an ongoing issue; fairly easy way to find out is take a spare key, do your ride [by the way, you're very lucky this is easily repeatable] and when the bike starts acting up open the gas cap and see if that helps. Wouldn't recommend doing this right after a fill-up, too much chance of slopping gas, I'd recommend about 1/2 tank.

Second issue can be a little trickier, I believe your bike has the same fuel petcock as the big ones, where the plastic filter deforms and lets garbage into the system. Drain the fuel bowls using a hose and glass jar to catch the fuel, see if it looks clean. If crud in it, pull fuel bowls, pull tank, clean both as well as the fuel line and put an inline fuel filter on it.

For what it's worth [internet trouble shooting tends to be hit and miss], I'd lean towards the venting problem. Bike pulls fuel, vent partially plugged, creates vacuum in tank, fuel won't flow [think finger over end of a straw holding water]; once stopped the restricted vent lets in enough air [or the rider opens the cap to check the gas!] and it runs again.

 

[rkerg]

I think that Wyldstreak had a kind of similar problem with his 600 and it turned out to be his CDI. I wouldn't rule out a bad coil.

 

[paulfun]

The problem you describe sounds like electrical to me. If the tach is being errettic that would be the indicator as with fuel problems the tach may be slightly jumpy but not erratic.
I would follow the test procedures in the manuel for the coils, cdi and check all plug wieres and boots for corrosion.
As golddrakul says its hard to diagnose over the internet, so you should always start with the basics just to ensure they are good anyway. In this case the tank vent, and petcok should be at least checked to ensure they are good.

Aslo just for a basic check on a water pump if you start the bike with the tank and cap off you can see the water level rise and fall when you rev the bike even if its cold and the thermostate is closed. The pump is always spinning.

 

[killer]

I tend to agree with Paulfun. Is the tach eratic and jumping from 1.5k to 3-4k, or is the engine actually jumping too? If it is just the tach needle acting eratically and the engine is only jumping a little,,, i would definetly think electrical. The long ride heats up the coils.

 

[golddrakul]

Paulfun has a good point, I was interpreting the tach jumping as erratic engine speed; on re-reading the original post it does sound like the tach is jumping around independent of the engine speed. That would point to electrical.

 

[macsinger]

don't think it is electrical, I can hear the RPMs...

I do not believe that the problem is electrical. I can tell from the sound that the engine is doing the RPMs that the tach is indicating. So when the tach is jumping between 3 and 5 thousand, you can hear the bike doing just that.

I have noticed another symptom... It follows the theory that I'm currently acting upon that something is wrong with the coolant system. I flushed my system and opened up the water pump to make sure it is functioning, which it is. However, idling the bike, with new coolant, for only about five minutes it got very hot, and the fan kicked in almost immediately. I could feel lots of heat radiating off the engine itself but not much heat coming off the radiator. Even with the fan on.

I am led to believe that something is inhibiting the circulation of coolant through the bike, and therefore the radiator is becoming ineffective.

How long do your bikes take to get hot if they are just idling? Seems like the bike shouldn't get this hot until at least ten or fifteen minutes of idle.

Thanks for all the posts!

post script...
I've also realized that my thermostat switch could be stuck in the closed position and directing coolant back to the water pump and not sending it to the radiator at all.

 

[golddrakul]

The thermostat itself could be stuck [the switch controls the fan turning on or off], if the bike is overheating you should see water going to the overflow bottle. Also, not sure overheating would make it run erratically like your describing. May have multiply issues [cooling vs running]; don't focus on only one possibility to the exclusion of the others.

 

[paulfun]

Usually an overheating engine will lack power and slowly get worse, not run irratic like you describe. Also in an overheat condition you would be getting fluid rapidly transfering into the overflow bottle and a steam burst at the left muffler where the pressure would bl;eed out of the system making a streak on the muffler.

 

[gosupes]

If you flushed the coolant and the engine heats up and you don't think the coolant is circulating thru the radiator, you could have an air bubble trapped in the system. Since it seems you think it is cooling related, do the simple things like replaced the t-stat and the cap, and bleed the air from the system. However, I have to agree with most everyone else and my gut instinct will say that it's electrical, possibly aggravated by the overheating conditions. Either coil or CDI.

 

[paulfun]

You can test your vacume leak thery simply by disconecting the vacume and emmisions from the carbs and air box and ensuring the intake boots are tight. Use a nickel to cover the hole on top the air box and regular vacume caps on the carbs. You will have to have one vacume line from the #2 carb to the petcok in place in order for it to work. Also the two vents for the float bowls must remain open.

When you change the fluid you must bleed the system or you will get an air pocket inside that prevents proper circulation. I would also recomend engine Ice instead of regular antifreeze as it will help your bike to run cooler and it also will remain cleaner inside the water ways and radiator.
A new radiator cap is always a good idea and will in most cases need replaced long befor the factory thermostate will. (for some reason the factory themostate seems to be good for about 100,000+ miles just ask the connie riders as its not usually a part that needs replaced. Nor do you see a lot of the zl riders having difficulty with them.

You can aslo check to see if the thermostate is working properly by removing it and testing it on the kitchen stove. Start a pot of water boiling to see at what temp it opens or if at all. (Just dont let the wife catch you using her good pots and pans for this)

I must agree with golddrakul and gouspes that this problem you describe is not truelly heat related. My gut is with electrical, but with your second post we may have read or misinterperated you incorrectly and it might simply be carb/fuel problems. Without a definate destinction in the terms used by erratic the best I can say is to use thye procces of elimination.
disconnect the emmisions
if it happens again
clean carbs and tank-PROPERLY
if it happens again
its electrical

dont forget that almost everyone here has at one time or an other had corrosion build up on the plug bwires and experianced some type of erratic running due to it and once cleaned or replaced it was gone! So checking for this is a must and should be done first.

I also always recomend checking and adjusting the valves before any other work is done unless you personally have done this just prior to having the said problems. This dosnt sound like a valve related problem but just knowing they are right is a piece of mind for me. AND cheap insurance as well.

 

[jeffzl1000]

this is definately the pick up coils if it ever happens to anyone else check that first i would suggest. and happy memorial day to all you vets