Motor mis-firing and overheating

The story so far:

Started restoring my 1980 XJ 650 about 10 months ago fully strip it to the frame and started rebuilding. Motor wise I have re lapped the valves, fitted new rings and new timing chain. I stripped the carbies down and cleaned them benched sync them and refitted to the motor. When I connected the tank and put it on prime fuel flooded everywere but once the petcock is on normal there is no problem. I presume float levels are not correct? Will need to pull carbies off and re-do. Come time to start it up it fired straight away and I had it running on the 5th or 6th time and it ran OK. I then checked the timing which was OK and then sync the carbies which was my first attempt and got them spot on. It was sounding and running great. On the first test drive it ran great around town and through the gears. Heading out of town once again accelerating gently through the gears is ok but at 80 -90 kms it started to miss badly to the point where it would not run at all. It fired up again a couple of minutes later and ran fine but would repeat this proceedure a few kms down the road. I ran it again the next day with the same result it seems that when it get hotter that the miss firing starts. It also seems to be overheating slightly not too bad but certainly runs hotter than my xj 750 that Im currently riding. The pilot screw were bench set at 2.5 turns I reset these to 3 turns to make it richer thinking it might make a difference to the overheating. I have ran checks on the ignition coils and pick up coils and they seem ok. I dont know what to check from here so any help would be appreciated

 

You could actually be flooding it, if those floats are not set properly or if your needle valves aren't performing up to snuff, you could be drowning the motor. Maybe its only happening once in a while or maybe caused by vibration at certain speeds. As far as the overheating, not sure. A bad seal between boots and carb or head could be letting air thin out your mixture, put gas in a spray bottle, warm the bike up and spray SPARINGLY on these connections while the bike is idling, if you are sucking air your idle should go up.I'm no expert on these bikes in particular, but have owned and worked on old up bikes since I was just a kid playing in the dirt.

 

You could actually be flooding it, if those floats are not set properly or if your needle valves aren't performing up to snuff, you could be drowning the motor. Maybe its only happening once in a while or maybe caused by vibration at certain speeds. As far as the overheating, not sure. A bad seal between boots and carb or head could be letting air thin out your mixture, put gas in a spray bottle, warm the bike up and spray SPARINGLY on these connections while the bike is idling, if you are sucking air your idle should go up.I'm no expert on these bikes in particular, but have owned and worked on old up bikes since I was just a kid playing in the dirt.

 

You could be right I have just taken the carbs off and there was gas lying on the inside if the intake boots which led be to think that petrol could be in the oil so dropped the oil and sure enough some petrol has contaminated the oil. Will re-visit the carbs and try and do it properly this time. With the oil been contaminated may have been causing the over heating?

 

The oil is there to reduce the friction between the piston assembly and the inside of the cylinder bore, therefore if fuel was getting in there then you have diluted the viscosity of the oil and increased the friction. I think you are lucky that the motor didn't grenade on you.

 

Idk why my post was duplicated. Most air cooled inline 4 engines are actually cooled by oil when it comes to the 2 inboard cylinders. Large oil galleys run up the front of the motor that cool the oil which then circulates through the head and jug. Gasoline in the oil is always bad, it thins it out and changes its properties when it comes to heat. Hoe do you know the engine is hot? Maybe take the oil temp or shoot the heads with an infrared thermometer to confirm. Are your pipes turning colors? It's always best to confirm there is a problem before trying to solve it. Sorry if this double-posts, I'm doing this from my Android phone.

 

When I started to think about it (petrol in the oil) it did make me feel a little ill. Luckliy I dont think that much got in there but it did make the oil a little thinner which I dont think is a good thing. I have just finished reading a thread on setting the floats with a clear plastic tube which I will now do. When I cleaned the carbs I just set the floats by measuring. its clear I only half did the job.

 

Idk why my post was duplicated. Most air cooled inline 4 engines are actually cooled by oil when it comes to the 2 inboard cylinders. Large oil galleys run up the front of the motor that cool the oil which then circulates through the head and jug. Gasoline in the oil is always bad, it thins it out and changes its properties when it comes to heat. Hoe do you know the engine is hot? Maybe take the oil temp or shoot the heads with an infrared thermometer to confirm. Are your pipes turning colors? It's always best to confirm there is a problem before trying to solve it. Sorry if this double-posts, I'm doing this from my Android phone.

 

Gas-diluted oil in a freshly rebuilt motor is NOT a good thing.

Get the float levels right (as you now know how) and do an oil change, w/filter. Get the motor warmed up (10 miles or so, max) and then change the oil again.

And pray nothing bad happened in the interim. After a couple hundred miles, do a compression test.

 

Have a look at your Spark Plugs.
IF you are running it Critically Lean, ... the Plugs will be CLEAN.

Remove the Carbs and do a THOROUGH job of Cleaning and prepping them for Tuning.

Don't run it Lean and cause damage ruining what you worked so hard to restore.

 

I am fairly confident that the carbs have been cleaned well as I followed your thread on "clean your own carb the whole 9 yards" Rick I think the only thing im guilty of is not setting the floats correctly although now that the carbs are off the bike i will double check everthing. I did checked the plugs at first they were a very light tan color and after turning the pilot screws out to 3 turns they went a mid tan color. I have already droped the oil and removed the oil filter and will change the oil as you sugested Fitz. Do either of you think that the engine miss is related to the carbs or could it still be something else?

 

Don't know if I can express myself clearly on this, but could excessive fuel mixed with the oil cause a pre-detonation or post-detonation in a hot motor that could be misdiagnosed as sounding/feeling like a miss?

 

One everything's back together, with clean and proper oil, proper float levels and a good sync; let's see what happens.

ANYthing is possible at this point, it sounds like you came close to blowing up a freshly rebuilt mill.

Get everything right, and see if it still "misses." I'm betting not. If so, we'll address it then.

 

No I dont think that you can be any clearer than that. Will get everythink right and let you know how it went

Thanks Brian

 

OK

So Ive pulled the carbs off and found No 3 float way to high fuel just poured out the throat when attempting the clear tube method for the first time. the other 3 carbs were all about 3-4 mm too high as well. Set them all to 3-4 mm below the body as shown in the thread on this site. Checked over the rest of the carbs while they were out and everthing looked OK and clunk test good. Refitted to bike, changed oil and filter and re-synced carbs. Fine tuned by ear to the best of my capabilities and bike is purring like a kitten at idle with a good quick responce on the throttle. From 0 to 100 kph with a steady throttle its great no missing nice and smooth but try and power on from 100 kms and it boggs down and seems to splutter and wont rev over 5000rpm. I have tested for air leaks around the carb boots and all seems OK. (I refurbished the original boots by coating them with 6 layers of liquid insulation tape) I have fitted a new kit in the petcock and have checked the vent in the tank cap. What have I missed or could this not be carb related. Appreciate any help

Brian

 

"Checked over" --- did you pull the mixture screws and clean those passages? Did you pull the mains, and the emulsion tubes and ensure all the holes are clear and open? Have you popped the c-clip and nylon washer off the two outer carbs, to have a look at the only two throttle shaft seals you can see to give you an indication of their condition?

"re-sync'ed the carbs" --- using which method?

What do the plugs look like?

 

Hi Fitz
Yes I pulled the mixture screws and cleaned the passages. Mains and emulision tubes were all remove and clean. i did not though inspect any throttle seals. I synced the carbs using the one vacumn gauge method and all 4 plugs are a mid tan colour.

 

Well, unfortunately, when all other carburetion aspects have been covered thoroughly and you're still having the symptom you described, the cause often turns out to be...

Throttle shaft seals.

Better have a peek at the outer ones. If they're shot, you can be reasonably sure the others are too. If they look OK, it's no guarantee as to the condition of the ones you can't get to, but if they're toast you know they all are.

 

Thanks Fitz

It looks like open heart surgery one more time!! this sure has been educational!!

 

I used to put a few drops of TEAC Rubber Conditioner in the Throttle Shaft and let it work down to the Seal to keep the Seals from aging.

TEAC Re-conditioner is no longer available.
(If you have a Stereo-audiofile type in the family, ... they might have some,)

This stuff might do the trick.
Comes with a Precision applicator.

Before knocking-down the whole Rack and getting into a Major Restoration, ... it might be worth the 10-Bucks to see if those Seals can be softened-up.

http://www.mcmelectronics.com/product/2 ... rm=200-200