1992 - 3LD Won't rev past 5000rpm

[stevep10]

Background to it is this;

Starts great off the button every time no problems but.......
Bike won't rev past 5k rpm. Pulls great up to that point.
New plugs, coil, ht leads, caps, replacement known good carbs. Already has new manifold rubbers, carbs are balanced and float heights are good.
Stator output is good, where it's supposed to be,
pick-up resistance is within spec
Fuel pump output flow is good, no air leaks in vac pipe
I can see the timing marks moving with the engine running and timing light, but obviously cannot see how far they are going back due to them disappearing behind the casing.
It seems to me that the timing is not advancing fully with the engine speed, and because it is cdi controlled I cannot adjust or check any further than I already have.
I have ordered a used cdi from fleabay today, see if that does anything. I also got one of those china ones, £12.95, fitted it, bike won't even try and start with that on it.

Any other ideas out there please?

[scottieh]

What is the size of your main jet? You could try going up a size and seeing if you get the same performance issues. On my '85 43F at wide open throttle on stock jetting my bike would sputter a little at 4500-5000 rpm especially when going up hills. On the same bike with a larger main the problem went away.

[stevep10]

Hi
Thanks for the input
Tried all that, re-jetted upto 155 in stages on the main jets, used open pod filters, then double foam pod filters, tried restricting the airflow then opening the airflow, still ended up with the same problem.
Haven't run it yet with no filters at all, but the plan is to do that tomorrow sometime, in between lacing and truing new rims for another project I have on the go at the moment.

[motorang]

The speedo may be a bit tired and not show the actual revs (it is a magnetic thing).
Is the top speed what it should be?
You can calculate the revs from knowing current speed and transmission ratios.

Andy

[stevep10]

Speedo and rev counter are fine, everything works as it should.
You don't need a speedo or rev counter to know when an engine is not doing as it should, and this engine is certainly not doing as it should.
Compression is at 150psi both barrels, no leak back either so I can rule out pistons and valves as a cause.
If I get a chance over the weekend I'll have the covers off and check the static timing, make sure nothing has slipped a tooth.

[scottieh]

A half-tooth off on the cam sprocket should reveal itself much sooner than 5,000 rpm. On a '93 XT350 I installed the cam chain a half tooth off and while it started fine it wouldn't rev past 3,000 rpm without sputtering like a maniac. Wild loss of power. I suppose it's possible in your case.

Does your bike have any kind of electrical air sensor at the airbox, carb or intake manifold? What about vacuum line hookups that run to those places? I don't know your bike's design but on cars if the sensors are clogged, shorting, or bypassed they can cause performance issues like not fully advancing spark.

[stevep10]

Replacement cdi hasn't solved the problem still exactly the same
The vac lines are all new, that was one of my first inspection points, along with new fuel lines as well just in case.
Then added a new in line fuel filter, then took it out again. All no difference.
No sensors or anything on the airbox.
Gonna do some serious digging this weekend me thinks..........
This little problem is stopping me from putting this up for sale!

[motorang]

What exactly happens at 5k?
Is it all the same when riding up a hill or down a hill?
Does it only occur at high load/speed?
Any stuttering, cutting out? Or does it just get sluggish like it would lack 10 hp?

Did it ever run great while you had it? Or is it a rebuild?

I bought a bike once which did the same thing.
Pre-previous owner had put in a racing camshaft, gigger carb, megafone exhaust.
Then sold it as spares in bits and boxes, engine complete.

Previous owner did put a bike together from the bits, did not get it running properly.
Tried everything stock, carb, exhaust, downpipe, airbox etc

I tried hard to find the problems cause, took me a few months before I finally measured the cams ...

With a stock camshaft, everything was fine.

Did you check ignition timing? Maybe a faulty/weak sensor? No spark advance hence?

Andy

[stevep10]

Doesn't matter what speed or what gear, hits 5k, nothing more. Yes, used to run great until I parked it up for cosmetic surgery. Ive never had the engine in bits, strangely for me, never had to, it's sweet, no squeaks or rattles. Checked the timing with the strobe, appears to advance but obviously the mark goes out of sight so cannot check the degree at full advance.
Just checked, literally just, the static timing, appears the inlet cam is out one tooth retarded.
So, about to drop the water and remove the pipe in the head, have the cam cover off and have a bloody good look. I shall report back later.

[stevep10]

Sorted.
Although the intake cam appeared to be one tooth out, corrected that, it didn't make any difference to the problem.
The problem appears to have been with the air flow.
Took the air box off and went for a run, no filters on at all, absolutely flew again, no problems.
So I've fitted a pair of double foam road pod filters, been out, the old girl is back to her previous self.
Happy little bunny again
I'll take it for a longer run this week sometime just to be absolutely sure it has cured it before putting it up for sale.