Hesitation below 2.5k rpm

Hi all

As the topic reads I have an issue with the bike hesitation below 2.5k rpm

The carbs have been rebuilt
Valve clearances in spec
Compression numbers in order
Vacuum synced the carbs


I am Having some issues with colortuning the carbs and I figure that clogged Idle circuits could be making the bike hesitate before it gets the going on the main jet. It only hesitates when under load.
My plug chop revealed three lean running cylinders and one rich.
I colortuned the rich one correctly but the flames in the lean cylinders didnt change color when trying to adjust the idle mixture.

Any other ideas on what could lie behind this lean running condition?
Ps: i had few backfires from one cylinder as well (guessing because it is running so hot)

Thank you for any input

 

If the color isn't changing when you adjust the mixture screw, something went wrong at the 'carb rebuild' stage.

Did you neglect the jet that goes through the carb bowl, by chance?

Sounds like something is obviously clogged, no fuel is coming through when you're pulling out the screw.

Backfire is from running those cylinders lean. I wouldn't be riding it like that much if at all avoidable, it's not kind to the engine to not be lubricating the upper half of the cylinder when running it at higher temps.

 

I pulled the carbs today and made sure that the pilot passages were good as gold!

However;
Cyl #3-4 are not responding much to colortuning but the backfiring from #4 has become more moderate. Its not too bad now. The butterfly valve shaft on #4 rattles shakes significantly more than the others and this is the cyl that produces the popping.
Can it be so that the ratteling shaft is due to backfiring? It is quite continous and every now and then the flame changes color and the backfire is more distinct. I opened the mixture screw a bit and this helped the popping.

But the problem persists...
I did a thorough leak check around the carbs and boots which came out without any leaks!
I am starting to suspect some kind timing issue, perhaps i messed something up when i renovated the head earlier this spring.

 

I forgot to mention that it returns to idle significantly slower than before as well!

 

Check your vacuum caps on the intake manifold boots. I had a few that were leaking and the propane torch test didn't reveal that. it could change your idle mixture and would definitely contribute to the slow return to idle. if that doesn't fix it then your enrichment circuits are clogged.

 

Thanks for the advice. Have an good tips on how t seal them while still being able to take them off?
Also what does my enrichment circuits (which have been tested and are unclogged) have to do with idle? When the "choke" is not operated there is no fuel added via the fuel enrichment circuit. Correct?

I have an idea that when I put the head back together and checked the valve clearances were they were fine but now that I have run the engine a little the valves have been seated in and thus I might have some valves out of spec... Will have to check that tomorrow.

I also read somewhere about how it was difficult to detect leaks between head and intake manifold as they vary with revs. I am using starter spray to search for leaks and before (when i had leaks) it was quite easy to locate them with starter spray.

 

Use an unlit propane torch around the intakes and carbs. It does the same thing a starter fluid but without the risk of eating away at your rubber bits.

For the vacuum caps the best option is to replace them. Chacal carries them and I have heard reports Pep Boys and NAPA auto parts carries something that will work. Take one in and ask them to match them up. Replace your petcock vacuum line as well if it is old and cracked or brittle.

You can also try replacing the clamps that hold them on with some stiffer ones assuming the boots aren't dried out and stiff. I would recommend avoiding glues or adhesives as you will need to remove them later to do adjustments season to season.

If you are not getting any change in flame color at idle when turning your idle fuel mixture screws then the passages are not clean enough or you need to replace the washes and o-rings under the fuel mixture screws. If you get no change in idle with the enrichment on then those passages are in need of service.

 

Ok. Thanks for the input.
But like I wrote I have just tested the pilot passages and fuel/air flows freely through them...
What are thoughts on timing issues or valves becoming seated after rebuild and thus throwing the clearances off?
The o-rings and washers are new in all carbs

 

The valves seat slowly over time. More likely is after you reassembled the head the cams and valve buckets and shims may take a short time to re-settle and may need adjustment. Also the FIRST rule to carb adjustments is VALVES FIRST. If your valves are off then you will get nowhere with the carbs until the valves are in spec.

Now with that being said, I had a hell of a time getting a smooth idle with my enrichment circuits plugged. The general rule with these XJ's is if Yamaha put it in the carbs it needs to be spotless and allow free flow of fuel to its full capacity no matter how insignificant of little that may be.

Make sure the enrichment circuit passages in the carb bowls are clean. It is a small needle-point diameter hole that needs to allow fuel to flow through. Soak them in carb cleaner then use a spray can of carb cleaner to try to pressure blast the varnish out. You can also use (with extreme care) the high E string from an electric guitar to gently nudge the varnish out of the passage. When you are done you should be able to shine light down in from the hole inside the carb bowl and see a pinpoint of it through the enrichment circuit looking down into passage from above. This took me about an hour and a half to do all four when I finally got them right.

 

Update:
Checked clearances and all are withing spec except #4 which is 0.21 out and 0.16 in.
I am wondering if this is the main problem, I dont think so. If they were too tight then yes but now they are a little on the loose side (open for a shorter time)
I will remove the carbs again and recheck float levels and also make 100% sure that boots 3&4 are not leaking, regardless of changeing rpms.

Btw: guessing bigfitz will drop the hammer on me for not having all the valve clearances within spec

 

He shouldn't have to. You already know its a situation to be taken care of. So do it

 

Are you saying the exhaust clearance for #4 was 0.21mm, and the intaxe 0.16mm? If so, that's only 0.01mm out of spec. That's three ten-thousands of an inch. No, that's not going to cause any hesitation in your engine. If all the other valves are in-spec, but "on the loose side" of spec, that's not a problem, either.

 

2nd update

Float levels have been checked and corrected so that they are 3mm below bowl gasket face (+-1mm)
Intake boots have been planed and mate flawlessly to head (no leak before but just to make sure)

Still not much response from mixture screw. It is possible to cut flow completly and the flame goes away.
The engine has begun to have a haning idle. If i blip the throttle it sticks at around 4-5k. I have tried to find a leak but no luck, not at low or high rpms.
I am having a meal break right now but then I will do as you suggest bluebass and take another stab at the mixture passage. Air/fuel flown through but I guess just not freely enough.

What could cause this surge in the engine?
Main Idle screw set too high due to lack of fuel at idle -> blip the throttle -> piston moves -> increased rpms -> holds the piston up -> idle hangs at high rpm/slowly moves down?

Btw: i really appreciate that you guys are taking your time to help out! Thanks

 

Before removing the carbs, with the tank off get the bike running. Blip the throttle then attempt to push the throttle shafts back down with a screwdriver. It might be your butterfly valves not completely returning to idle.

High revs at idle are caused by one of two conditions: extra fuel or extra air. A combination of both can absolutely cause high idle. The fact that you have three lean cylinders and one rich one seems to indicate that cylinder is getting less air than the others, so maybe it is closing all the way and the others are not.

Now this brings another thought to mind. When you rebuilt your carbs did you replace the throttle shaft seals? Did you remove the butterfly valves? If you removed the butterfly valves (as you would during replacement of the throttle shaft seals) it is possible that they might have been mistakenly re-installed backwards or slightly skewed, permitting air to flow around what would appear to be 'closed' valves. Do a check of all four butterfly valves by shining a flashlight into one end of the carb and inspect from the other end for light around the sides or an uneven pattern around the valves. If you did not remove the throttle shafts and butterfly valves, you may discover a leak at the throttle shaft seals which could contribute to your idle woes.

 

3d update! - Great sucess! And fail...

Cleaned all the passages again and flow increased.
You are right about sync being sligthly off so I adjusted that and voila, the cylinders are responding fine to colortuning. They are all tuned in well and idle drops nice and quick to its steady 1k. Took it for a spin too and it has a lot more power below 2,5k now, its a smooth increase to where the xj gets interesting (6-7k). Very pleased with this and big thanks to you fellows and especially bluebass.

However, I got a little overly excited and decided to proceed to change the handlebars for the clip ons i have lying around. Long story short, I managed to tip the bike over on its left side and it was lying for about 30 sek while i did some damage assesment of my left hand which got injured a little. Possibly busted the fork legs as the PO has installed vents that are linked together at the top. did not run the engine afterwards but hoping it wont be an issue.Will probably take the clip ons off again since I think it didnt look too good. Fork is too long and has too much rake for clip ons to look good i think.

So all in all the problems with the engine is (still is i hope) fixed but now I have some issues with the forks. Perhaps pick up a pair of old forks and replace mine, or use until I have time to rebuild them

 

When you first fire it up don't be surprised if cylinder 1 or 2 are flooded. Every time I've dropped mine and not been able to upright it immediately I have had to crank it some and clear the gas from at least one cylinder. You may also have to wait for the carb bowls to level out too since they were at an angle more sideways than not.

These bikes are great fun when they're tuned right. I would recommend riding with your OEM bars to see how the bike feels then see if you need the aftermarket bars. You may find that the look grows on you if the bars are at a comfortable position.

 

I think you might be right bluebass.
Another project has emerged however: think i am going to buy this off a guy in stockholm. http://www.blocket.se/stockholm/Moto_Gu ... ?ca=11&w=1
32000 swedish krona I around 4500 USD.
I think its a very cool bike even though its not an xj