Carburetor issues

I have a 1981 XJ750 RH Seca and recently had some fuel leaking into the oil. I thought I was going to have to remove the carbs because of a possible stuck float, and when I was loosening clamps I mistakingly turned sync screws (don't ask me how I did that :roll: ) and of course when I found it was the petcock giving the issue and repaired that, I tightened them all the way down. Well, a friend of mine came over with a manometer and helped me sync the carbs again, but it hasn't run right since. Seems to run fine at idle, but when under load gives almost a backfire. Have rebuilt the carbs and cleaned them and the levels are fine. I have been through the carbs twice and synced them with the new Carbtune Pro I purchased, but the issue persists. Sounds like it is bogging down sometimes, and sputters or stumbles. I am at my wits end here. I am wondering if it could be a possible exhaust issue, but I don't find a leak there.

 

Backfiring thru the carbs = not good.

Run a compression test on all the cylinders. Might have a burned intake valve, or, hopefully, just an intake or exhaust valve that is way to tight.........have you checked the valve shim clearances yet?

 

Is that what it sounds like to you, backfiring through the carbs?

No, I haven't checked the compression yet, but I was thinking that I might need to. I recently rebuilt the top end of the engine with rpgoerlich and we checked the valve shim clearances at that time, but the bike has been run for about 2000 miles since then. Ironically, the bike was running fine other than fuel getting into the oil, up until I made the mistake of being in too big a hurry and turning the sync screws on the bike.

 

I had a problem similiar and found a crazy cause. because you had gas in the oil and fixed the petcock doesn't mean you fixed the reason gas got by the float needle valve. in a perfect world, you could leave your petcock on prime and the needle should seat well enough to keep the fuel back. So, the problem I found was this; (not saying it's your problem but could be the needles, seats, or how the needle moves in it's cylinder) there was a slight etch in the perimiter of the brass seat allowing fuel to go by, on one carb only. once i replaced the needle and seat the backfiring stopped. But if the needle is sticking in there and not letting it close all the way and make a seal I'm assuming the same symptom could happen.

 

When I cleaned and rebuilt the carbs, I replaced the fuel valve seats and the needles and made the adjustments on the bench to the floats to get the levels right. I didn't investigate the new needles extremely close, but I would hope that new would mean there is no scoring on them.

 

well then i'm with you, good luck. well another thing that was making me stumble and backfire that I fixed at the same time was 2 plug wires. the wire sticking out of the end core, the wire that gets bent backwards, was broken off. in this case the spark would happen erratically whenever it could jump to the connector. This was on the coil end in my scenario, afternmarket Dyna coils

 

I have looked into spark issues, though minimal and have all new caps, but the coils are original on the bike. I guess I had better start at testing the compression on each cylinder to see where that leads me, but I have a feeling I am going to find that it will be fine since it has had the top end rebuilt. Lapped all the valves, new valve guide seals, new rings, cleaned pistons to look like new, cleaned glazing from cylinder walls and re-honed them and all new gaskets. Replaced the clutch frictions and steels with OEM parts also.

 

i had popping on a single cylinder once and it was from a bad exhaust header gasket. it was sucking air in at the gasket on the intake stroke and the air was allowing the unburnt fuel to get burnt. I guess that makes sense with our bikes since there is a 'wasted' spark on the intake stroke...

 

Here are some videos I have that show my float level checks today. I also have a video here of how it is running.

Carb 4 http://www.youtube.com/watch?v=b8BkpkaLfZM

Carb 3 http://www.youtube.com/watch?v=sslsFTOFlRg

Carb 2 http://www.youtube.com/watch?v=PJ0dYfmpixw

Carb 1 http://www.youtube.com/watch?v=h4K6PhahAHo

Bike Running http://www.youtube.com/watch?v=-8orjFOUh3o

I don't know if you will be able to see the smoke in the video of the bike running as I just shot it and haven't even watched it yet.

 

Well, I am not sure just yet, but initial results of a re-sync of the carbs seems to have resolved the issue. I have only taken it for a ride through the neighborhood and seem to have no further stuttering. I will say that carb 4 was quite high on the manometer when I started the sync, so I leveled it with 3 then a slight adjustment to level 1 & 2 and the same to sync 1 & 2 with 3 & 4. I am guessing that 4 was pushing too much fuel compared to the rest of them. I know that I did the sync before, so I am still going to pickup a compression guage and do that test.

 

Well, the problem is persisting . So, I purchased a compression guage and did a test on all 4 cylinders and here are the results:

:arrow: Cylinder 1: 105 PSI
:arrow: Cylinder 2: 115 PSI
:arrow: Cylinder 3: 110 PSI
:arrow: Cylinder 4: 120 PSI

That is prior to me going into the valve cover and checking my clearances on the valve shims however, so I am not certain I can draw any conclusions just yet. Any thoughts would be appreciated and I will post clearances once I have gotten them. Thanks for all the input.

 

Ok, I just checked all of the clearances on the valves and here are the measurements.

:arrow: Cylinder 1: Intake: .13mm
:arrow: Cylinder 1: Exhaust: .24mm (out of spec by .04mm)

:arrow: Cylinder 2: Intake: .15mm
:arrow: Cylinder 2: Exhaust: .19mm

:arrow: Cylinder 3: Intake: .15mm
:arrow: Cylinder 3: Exhaust: .19mm

:arrow: Cylinder 4: Intake: .14mm
:arrow: Cylinder 4: Exhaust: .20mm

Now I am truly baffled. I am not even sure what the next step is at this point, as I am still experiencing the same issue and the carbs seem to be fine and other that one exhaust valve being a bit out of spec. I don't know what could be causing this stuttering/stammering and running rich issue. :?

 

Your compression numbers are low. Did you test at wide open throttle? Be sure to ground the plug wire...or disconnect the TCI <- easier. Then check with oil in the cylinder (teaspoon or less) so see if there is a difference. Minimum compression should be 120 psi (of course the gauge could be off)

 

Compression test was done with no spark plugs in the bike with the engine hot and just turning the engine over until I reached the highest reading on each cylinder. I did not however ground the spark plug wires.

 

By the way, the ironic thing here is that the top end of the engine has been rebuilt recently and there was no problem until I made the mistake of turning the synchronization screws on the carbs. Since then I have rebuilt and cleaned the carbs, re-syncing the carbs once that was complete. Also, the cylinder I am having problems with is cylinder 4 which is fouling out the spark plug and running rich, yet it has the highest compression of all the cylinders. I am stuck.

 

Again you have to do the compression test holding the throttle open or your readings mean nothing.

Take the plugs out, take the connectors off the TCI, hold WOT and crank away.

Let us know what you get doing that.

 

Might be a pain in the A** to do, but can you switch the coils from side to side on this bike? If you can, try it and see if the plug problem follows the coil. I know this can be done on twins, but not sure about your bike.
Just a thought.

 

Ok, I will do that. I did not understand that it was necessary to do it with the throttle open, as I thought it was a question as to whether or not I had done that by mistake. I had to call it quits for the night and will work with it tomorrow more.

 

OK, did compression check again with wide open throttle and spark plugs ground. Here are the results:

:arrow: Cylinder 1: 115 PSI
:arrow: Cylinder 2: 125 PSI
:arrow: Cylinder 3: 130 PSI
:arrow: Cylinder 4: 120 PSI

So, what does that indicate? Is it possible that the lower compression reading on cylider 1 is due to the fact that the exhaust valve is slightly loose?

Also did float level checks on the carbs while the bike is running this time and all carbs are right where they need to be.

 

check them again with a tablespoon of oil down each plug hole & post the results.

 

OK, sorry it took so long. Here are the results, though I am not certain I got exactly a tablespoon of oil in each cylinder.

Here are the results:

:arrow: Cylinder 1: 185
:arrow: Cylinder 2: 155
:arrow: Cylinder 3: 155
:arrow: Cylinder 4: 185

 

If the shims are all in spec' it would indicate that 1 & 4 need attention to the bores / rings. Maybe you didn't get the same amount of oil down 2 & 3.

 

BUMP

 

Wadaya mean bump, Rick, did you unlock it?

 

I may have locked it in error meaning to click on: "Stop Watching This Topic"

Sorry.

 

OK, after all this I may have found my problem. I decided to go back through the carbs again and check the cold start needles as I hadn't checked them before. Well, long story short. I found that on carb 4 the cold start needle had the retaining nut backed out slightly and the spring was rusted and broken. So, it has to be inriching the fuel in that cylinder. Now, if I could only find the spring locally to repair it.

 

Problem solved! Rebuilt carb 4's cold start and no more stammer/stutter. Man, a $2.00 spring gave me all this trouble ! Well, on the upside, I now know the carbs like the back of my hand as I pulled them apart in every way except seperating them one from another.