Sputtering problem on XJ750 Seca

Working on a member’s 82 750 Seca and we are having problems with the engine sputtering around 5K rpm and up range. It only happened during ½ to full throttle opening, it does not happen with gradual throttle opening. This is only happening when the bike is being ridden. It responds to throttle nicely when on the center stand.

A little background on the bike;
It was purchased not running and broken down for a complete rebuild before the engine was started. Everything was dissembled except the engine. The bike was cleaned painted and reassembles over a two year process. It looks real nice.

Carbs;
Carbs were rebuilt with new needles, seats, throttle shaft seals and all gaskets and O-rings replaced. Carb bodies and all internals are clean, passages were cleaned and flushed with air. Bench sync, wet setting floats and pre-setting mixture needles to 2 ½ turns. Carb slides are free and diaphragms’ have no holes or tears in them.

Engine;
Valve adjustments were done and all valves are in spec.
Compression on each cylinder is close to 135
Stock air box installed with a new OEM style filter. Exhaust is a new MAC 4 to 2 system.
New ignition coils, caps and plugs as part of the rebuild. As was a new fuse box installed.
Engine runs good, starts easily and idles well. It responds to throttle nicely and does not give an indication of problems.

Things I have done to find the problem;
Changed main jets from the stock 120 to 125. The problem existed with the 120 jets and is no better with the 125. Plugs show a slightly rich mixture with this setup.
Inspected the TCI board and touched up a few “iffy” solder joints. No improvement after this. Ohmed the pickup coils, 810 ohms each. Cleaned the wiring connections for the TCI. Still no improvement.
We have gone through everything to find the cure for this sputtering problem and have run out of ideas. I am still not totally convinced it is a fuel delivery problem and also suspect the TCI might be at fault.
I also question the float levels. They were set to 3mm with the carbs level on the bench. When they are installed in the bike the carbs are angled forward and the fuel level is slightly above the bowl/carb body line in the front. Could this be a problem?
Any extra insight on what might be causing this is welcome.

 

Sounds like fuel to me..float levels perhaps.

See also

http://www.xjbikes.com/Forums/viewtopic ... ering.html

 

Like the idea of iffy enrichment or tci ??....but on the setting float level 'angle', at the moment mine are on the bench to be set on the level ..tried setting them angled forward and it's made it rich although wet level measure was right. I musta been trying to be too clever and yam' know better.

 

I'am sort of on the same path but mine started when I took plates from a super trap to tone down the noise.
Started with 6 plates went to 4, mains were 128, with 6 it was a little rich but not bad,with 4 it was real rich, took out the air cleaner and it was better, still rich, 126 jets no air cleaner is about right but no AC, bad.
Try removing the air cleaner and test that, and while it's sputtering quickly let off the gas, see if it speeds up for a moment, both say rich.

 

I'll try pulling the air filter and see if that makes a difference. You would figure that the jetting would not change much just by adding a exhaust.

 

Anyone know the proper position for the main and pilot air jets? The parts diagram is not specific where they go. Right now the large jet is tward the throttle plate and the small is at the rear.

 

Someone else can correct me if I'm wrong, but isn't the Main air jet (smaller hole) towards the centre of the carby (closer to the Main fuel needle) & the Pilot air jet (larger hole) towards the front of the carby (nearer to the mixture screw)??
This is shown opposite in some manuals I think.

Cheers,

Tim

 

I believe you are right but I am not 100% sure. That is what I have right now. I was just trying to cover all my possibilities.

 

i will be watching this thread. I have the same problem with my xj650

 

the larger air jet is for the smaller fuel jet, carb cleaner in the bigger air jet should come out the pilot jet

 

Thanks Polock, They are in the right place, thats one more thing out of the way.

 

Well it ran it with the airfilter out and the same results, a little better but not much. Tried again with the airbox cover off and no filter, no change.
I am running out of things to check.. whats next... any ideas.

 

Well its almost there but the solution is puzzling.
I removed the carbs again changed the main jets to 117.5. Just got back from a test ride and had a huge improvement. There is a slight stutter between 5k and 6k rpm at full throttle but is only takes a second to pass that and it runs great all the way to 9k.
Now the puzzling thing... The bike has stock airbox with a new filter and a MAC 4 to 2 exhaust. The stock main jets are suppost to be 120. You would think that adding a less restrictive exhaust would make it run lean. It does not make much sense to have to go down a jet size to make the combination work.
Would like to hear others input on why this happens.

 

maybe the new air filter is more restrictive than the old one? different paper in it?
maybe mac 4/2 pipes aint what their cracked up to be?
remember rule #1:
don't argue with success.
but keep a eye on the plugs

 

Sure.

Exhaust tuning has a lot more to do with sound waves than it does restriction or "backpressure." It has to do with attenuating the sound waves so they don't reverberate back up the system and "crash into" the fresh waves coming down the pipe.

The original system isn't as "restrictive" as it is convoluted and therefore quiet; while still providing good exhaust flow.

When you change the exhaust system without changing any of the intake components, you tend to "move" the powerband around in the rev range rather than actually increase it. It may not "scavenge" as well as the stock system at some RPMs. So it's quite plausible to need, for instance, smaller mains but "fatter" pilots to properly compensate. In your case, it looks like slightly leaner mains alone may have done the trick.

But as Mr. P says, keep an eye on the plugs.