idle fine for 5 min, then sudden roar ...

This bike is idling along fine - as close to 1050 as I can get by eye on the tach - then after 5 min or so, suddenly it will go vrooooooooom up to 5+ K (I don't know how high it would go if I let it. It is somewhere around 5-6K by the time I get to the kill switch.) Fuel is running from an auxillary tank (open bucket directly overhead) through a fresh filter. You do not need to touch, or even look at the bike, just sudden vrrrrrrooooommmmmm!

Air leaks have been checked for, carbs are *clean* and "clunky", new plugs, fresh oil ...

Suggestions?

 

Is the auxilary tank a closed one? Usually if the RPM's go up suddenly it indicates a lack of fuel in the carburators, could be a tank vent issue.


grz Thijs

 

The aux tank is a 1 gal bucket hanging over the bike without any cover.

Why would a *lack* of fuel in the carbs cause the RPMs to go up? Seems counter intuitive, one would think it would go down, lacking fuel to drive the system?

It actually a possibility that it was fuel starved though. I had a fuel filter on that was somewhat clogged, it turned out.

 

She leans out and burns faster and hotter with less fuel.

 

4K seems like a big jump on account of a sudden leaning of mixture... I know you said you checked for air leaks, but my bike did this exact same thing as a result of the manifolds. The rubber had to get warm before it was pliable enough to open up the hairline cracks that were seemingly all over the place.

 

That is the same condition that will happen when your float needles are not letting enough fuel in the bowls. The engine will start sucking air and the rpms will go up. If you just rebuilt your carbs and bench synced them you will have to turn down the idle after it warms up. it may take a bit to get it where it needs to be. Sounds to me like that may be the problem. If it was lack of fuel in the bowls you would think it would start the high idle as soon as you start it.

 

I have a similar problem no mater what I can not get it to idle @ 1050 if it does it just starts to drop till it dies. so i adjust the idle up but when i blip the throttle (3000 RPMs)it takes awhile to settle down 5-10 seconds or so any ideas?

 

All the above. Seems this is part of the charm of the beast to me. I'm lucky as mine only went up to an average of 2200. I took the advice of another forum member (Sorry Buddy I don't remember who to give credit to) and adjusted it when it was warm AND idling at the high RPM. Dropped it to 1050 and it worked. BTW wear gloves or use pliers if you try this. There WAS a problem with it dying when it was cold and couldn't hold the 1050. I found by leaving the choke on a bit 'til it was fully warmed cured that. I've mentioned before that I buy and sell Honda Rebels. They are by nature one of the coldest bikes on the planet. It's not unusual to run them a mile or two with some choke. A little extra choke doesn't hurt them a bit and it won't hurt an XJ either. JMOOC

 

If their is one thing that I have figured out on the hitachi carbs is 95% of the issues that people are having is that the carbs are not clean. I went thru the whole process and traced the rpm slow to fall reving and fluctuating random rpm's to the one of the carb passages being dirty. The pilot circuit is actually a mini carb that lives inside of the main carb. The choke circuit also acts like a carb and obviously the carb is also a carb. The big distinction that I have found is that they all three allow both air and fuel into the engine. If any of the passages can not meter them correctly then it effects the total air fuel range across the rpm band. A clogged air pilot will make the bike lean burn thru out the range so then you start to play with the jets and chase the issue in a blind direction instead of finding the problem at the root . This means that you have 4 carbs with 3 circuits or any one of 12 passages can be dirty and causing the issues. The good news is that you can with a little testing isolate the trouble carb and passage and clean it one more time.



Art

 

My BIL pulled the tops off the carbs on WyldMax. First thing he said was these rubber seals (diaphrams) aren't working right, they're sticking. He took Vaseline and LIGHTLY lubed each one and it immediately became supple and pliable again. WyldMax hasn't missed a beat in the week since he did it. Now I'm no expert on these things, but my BIL is an old poor boy who had to learn to fix things on his own. NOW he's a certified HD mechanic and he knows more tricks than anyone I've ever known.

 

Folks,

I have put wire, fluid and 90psi air through every orifice that is exposed in those carbs.

Float levels have all been set using caliper and level setup. One was off by about .5 mm, but it was close enough that I wasn't going to open them up again.

Diaphragms were treated with RNR and a light layer of silicone grease in the groove to seat the diaphragm.

Passed the clunk test quite well. I polished the pistons with 1200 grit paper and WD40, and wrapped a green scrubby around a drill bit to polish carb part.

Carb boots are actually soft and whole!

New fuel line, new plugs, new fuel filter. (Replacing the filter did not affect anything.)

Synched with home made manometer.

The manifold boots looked good. I will warm it up and retry the propane, but no issues previously. The air filter is a new uni, and oiled.

All the pilot screws are open 2.5 turns, no colortune. I wish I had seen Ricks "tune by ear" before now. I may have time to try tomorrow, but probably not, I don't have a lot of time before I have to drop it off to the guy.

How long does the bike need to run before the plugs will show the "colors" they are supposed to show?

 

Is the filter installed backwards? I know sometimes its easy to overlook the obvious...)

 

are we sure all 4 are firing when it's cold (first few minutes) could it be running on 2 cylinders then the other 2 wake up and up goes the idle ?

 

Filter is installed correctly.

I'll double check the cylinder piars. but from all I have seen so far, they were fine.

I used an infrared thermometer on the manifold pipes and 1&4 are cooler than 2&3 ...

 

what are the temps of the pipes after 15 sec of first starting it up? After starting does the bike respond to the throttle crisply? If you remove the plug wires does the rpms drop?Did you bench sync them before you installed them? If you remove the sync caps and let air in does the rpms increase on the 4 cylinders? With the cap off spray a little carb cleaner in the nipple did the rpms increase? I still think you are chasing an imbalance of fuel/air ratio either from an air leak or fuel restriction.


Art