Idling problems when engine is hot

Hello everyone!

So I'm on my 2nd maxim now - this time I got a neglected beauty for $300. Only 5k miles, but it had not been ridden for several years.

The carbs had to be cleaned and many parts replaced - they were so gunky that the float bowl needles were almost cemented into their bowls.

Anyway, after putting in new jets and stuff I started having problems having the bike run well. I was able to sync the carbs, but looking through the colortune it seemed like cylinder 1 had no fuel in it - just the spark with black background. Surely, spark would appear if I try to rev up. The brass caps on the ignition screw are still there and I did not move them, since I had nice blue flame in the other cylinders.

Anyway, here are the problems I'm having:

1. The bike will start very well while cold, but as it warms up it will start having hard time revving up and eventually die.

2. Once again, when it becomes hot, I cannot seem to rev the bike up. The only way I can control the RPMs is using choke, and then the engine will suddenly accelerate to 5-6k, and will slowly wind down over 30 seconds if I close the choke again.

I have been playing around like this for the past month and I'm running out of options. What do you guys have to say?


Thanks!

 

Well, despite lack of replies I have some progress to report.

The problems seem to be not in the carbs. I took those carbs off, put them on my other 1980 non-YICS yamaha and after 2 minutes of sync the bike ran beautifully, rode around a few miles with it with no stalling, excellent acceleration and just lots of fun.

Anyway, so what should I be looking for in my other bike? Any input would be greatly appreciated. Thanks!

 

Hot engine and stalls that sounds like tight valve clearances to me. You might want to look at that.

 

I was hoping not to hear this answer, but I had a feeling I might have to dig in there.

Just one minute ago I started to look here http://www.xjbikes.com/Forums/viewtopic/t=14827.html

Is this what I have to do? Thanks!

 

Try running on PRI.
The diaphram in the petcock might be perished, make sure the fuel tank & filters are clean.

 

Not a bad idea, I'll try to swap fuel tanks and see how it goes.

However, the bike does start and run, but will not idle well - at first it will idle normally with choke on, eventually go up to 4-5k, and will stay high until the engine gets warm. Choke is now off, the engine will slowly start winding down and eventually stop. Then it won't even start until it cools off. Hard to believe this could be caused by petcock, but its certainly a better and easier place to look at than the shims. Thanks for the hint.

 

If your willing to muck with carbs, checking shims is not a big deal. (replacing them is not much worse.)

 

Ever since I got this bike I became a carb pro. If there was a competition where you had to take the carbs off, put them back on it the least amount of time, I'd win. Taking carbs apart, cleaning, putting them back together became so easy that I can do it with my eyes closed (well, almost!) Sync of carbs also takes about 3 minutes and I learned that owning a colortune is kinda useless in many cases, since the factory setting always gave me a blue light (thus those brass caps are still on!)

Anyway, tomorrow the valve cover is coming off, and I will check the clearances. The painful part will be ordering the shims and then waiting to receive them. Also, is that part that I see in the pictures really necessary? Thanks!

 

Drill Sargent: Gump Why did you put that (carb) back together so fast?

Gump: Because you said to Sargent!

Drill Sargent: Gosh Darnit gump you are a GD genius...


So if you'll send my your engine, I'll send you my carbs. You do my carbs and I'll do your shims...

 

The part is not necessary to check them. It or one of the work arounds in necessary to change them. Something has to hold the thing down so you have room to remove the shim.

 

Lithdoc, I can't see that your valve clearences could be out after only 5k.

 

Well, that was my thoughts as well. Although in the XJ service manual they do write that valve clearances should be checked at 5k miles.

If not the valves, what else could it be then?

 

Well, this adds some logistical complexity to my project then. First, I will have to order the tool, and only then I will be able to see what shims I need and order those! That will take at least 2 weeks for everything. Arrgh!

 

If you don't have one yet, install an external, inline fuel filter. That way you can observe fuel flow to the carbs which will aid in determining if your petcock is faulty.

You could be getting enough flow to feed the engine initially, but not enough to keep it running.

 

[quote="lithdoc"/]
Well, this adds some logistical complexity to my project then. First, I will have to order the tool, and only then I will be able to see what shims I need and order those! That will take at least 2 weeks for everything. Arrgh![/quote]

Yes, that is how valve adjustment works. However, for diagnostic purposes, you can atleast see if any are out. Who, knows, after 5k they may not be. That would then eliminate them as a source of the problem...

 

You can check a few things when it gets hot and starts running poorly. Start pulling plug wires. See which cylinders are working vs not working. Before it has a chance to cool down. Do a compression test to see how those cylinders check out (if the pressures are low recheck when cold). Last check for vacuum leaks. Use an unlit propane tourch and flood the area around each carb and see if the idle picks up. If it does its sucking air from that area. Lets see if that helps

 

The problem with non-working cylinders is an interesting one. I don't even need to do that - I can simply touch the exhaust manifold and feel if its hot or just luke warm. There is almost always one luke warm one - however, I am unable to replicate for the same cylinder to be not working - they alternate between 1st, 3rd and 4th. I'm yet to have the 2nd cylinder cold, but that is little of a relief.

 

Okay guys, seems like the problem is finally solved. Believe it or not, these awkward symptoms were caused by the faulty petcock. I simply switched the fuel tanks with my other maxim and the bike ran beautifully after spending a couple minutes on the sync. Having two identical bikes is a big help, since I can eliminate problems one by one.

Thus, I developed my new moto: "Why have one maxim when you can have two?"

Anyway, now I'm off to get a new petcock. I also need the right muffler - either patch the big hole or get another one. Where could I get one? Thanks!

 

As for valves - they are no big deal at all. I thought I was going to die looking at the pics, but it is REALLY easy.
Valves actually could be your problem... the initial valve adjustment is the most important, and if never done 25 some odd years ago, they need it bad.
The inital tolerances change, otherwise known as 'break in' and they do this more the first 5000 miles than they will any 5000 miles after that.

As for your valve parts etc - Crack that valve cover off, check your shims. If you are out of spec on any, order correct shims and a new gasket, as well as new donuts all around.

If you order from Chacal on here, get the aftermarket shims, some silcon grease, the gasket, and the donuts. Get a can of gasket spray at a local auto parts place (Len can't ship it air, so it will take a while to get)
With my experience, get Priority mail with Len, and you'll have it 3-4 business days (including saturday!) Far cry from 2 weeks, and you'll be ready to rock.