XTZ750 wont start

[gavcarter]

Ok old as hell xtz750, wouldnt run, failed compression test.

Stripped down, valves and seats full of carbon build up.

Sorted that got compression again, clearances were done aswell put new battery on and it still wont start, only backfires really loud, sometimes out of carb aswell as exhaust, ideas anyone?

[P.]

Silly thought, but HT leads round the right way? Unsure if it really matters on an IL2 but my B4 was a swine.

[gavcarter]

[P.]

Only other thought is timing out, like... enough

[gavcarter]

[gavcarter]

ok just been an looked ( not a short job - somehow managed to forget cooling systems contain water and soaked myself unplugging the radiator ) Timing is ok i think - manual says align the marks on camshaft sprockets with top of head - they were - but it also says punch marks on cams should be inline with lines on holders, they were slightly out.

So by juggling each cam one tooth on the chain I have the punch marks lined up but now not the sprockets ( only fractionally like) - My guess is its due a new chain......

any way decided to check spark again

1st go: both bright blue
2nd go: nothing
3rd go: weak yellow
4th go: bright blue

Obviously there is an electrical gremlin somewhere so looks like a day with the multimeter tomorrow

[gavcarter]

Sorted the spark issue, it seems to be a worn starter relay, every time it clicks on i get a different resistance - ive bridged it for testing purposes and measure no resistance now so have a consistent bright blue spark

but I still have the problem above - wont start just back fires and sneezes

Ideas on causes? all that I can think of no is carb overhaul and new inlet rubber ( well the clamps at any rate) just to eliminate fuelling issues.

I have cleaned most of the carbs out and there is one or two worn seals in there - but impossible to tell if their leaking or not - its not like a leaky float bowl that is really obvious lol

[Robby]

Have you checked for water in the tank and/or carbs, and the rust that tends to go with it? A bit of water in the carbs can make starting tricky. It turns into a real ballache when it's a really small amount of water so it sometimes works.

I'm assuming this bike has been standing for a long time. If so, my usual list of things to work through are as follows (in no particular order, I just do all of them):

1. Carbs. Take them off, bring them inside, work on the living room table on several layers of cardboard. Strip them right down, get everything clean. This includes giving the carb pistons and their bores a bit of a polish with a small mop on a dremel. Set the mixture screws to factory settings, do a bench balance using a single strand of copper wire as a feeler gauge.
If there are worn or suspect seals, such as throttle shaft seals, replace them. Also give your inlet rubbers a very close inspection. If there is a lot of corrosion between the inlet rubber face and the cylinder head, you should clean the corrosion and replace any sealing O-rings to get a good seal. Silicon sealant is a bodge that I don't recommend, but have used. Very thin smear.

2. Fuel tank. If there's a hint of rust, de-rust it - https://www.realclassic.co.uk/techfiles/petrol_tank_cleaning_and_rust_removal.html

3. Electrics. Go over every connector and earthing point and clean it up, bright, clean metal is your friend. I tend to also put in an additional earthing strap from the engine back to the battery. If the engine is trying to earth to the frame over its frame mountings and they are rusty, a new strap will help. Also worth replacing HT leads and plug caps, they're cheap.

4. Fuse box. If it uses glass fuses, change the box for one that uses blade fuses. They cost a fiver off ebay. Glass fuses can be troublesome - failing to carry current even though they look fine.

5. Consumables. If the battery is suspect, replace it or at least use a known good one from another bike during fault finding. Fit new spark plugs. Change the air filter.


Once you get it running, even running a bit rough, ride it. Due to reasons of confusion and black magic, things tend to settle down after a bit of riding. Pulling it apart again is often not the solution. I like to get at least 100 miles on it before doing any fine tuning with carb balancing and mixture screws.

[davebike]

Have you now got compression ??
Is the cam timing correct ? you had the cams out to sort the valves
____________________
Dave
NC750Xdct + others at work !
davebike1@gmail.com

[gavcarter]

[Robby]

Bench balancing is balancing the carbs on the bench rather than on the bike. It isn't as good as a proper vacuum tool carb balance, and is intended to get them fairly close before fine tuning.

The intention is to get the butterflies opening and closing at the same point. I adjust the idle screw (counting the turns) until the master carb is lightly gripping a strand of wire. I then use the balancer adjusters to get the other carb(s) just gripping the wire, then go back and check the master. All carbs are now synched to each other. The final step is to wind the idle adjuster back to where it was.

This only matches the carbs to each other. In reality, each cylinder will pull a slightly different amount of vacuum so a proper balance is needed for fine tuning. You're not up to this level yet, seeing as it isn't trying.

I have two more guess, both timing related.

- Cam timing. You said it wasn't quite lining up, then you moved the camchain a tooth, and it still isn't quite lining up. You need to be checking the timing with the tensioner pushing against the chain, otherwise the chain will be slack and make your measurements appear to out by up to one tooth.

- Ignition timing. You said the pickup was fixed, what about the spinning bit that activates the pickup? I have come across bikes where the rotor also handles the advance by way of springs, and the whole thing can slide off the end of the crank and be fitted upside down. This throws the ignition timing off by 180 degrees, which can be enough to cause a backfire if you have an exhaust valve open and an exhaust full of mixture.

I am assuming that you have verified that fuel is getting to the carbs, the float level is somewhere near correct, and the mixture circuit has been cleaned out and the screw set correctly. Might be worth opening the mixture screws out a turn if the timing is right.

[gavcarter]

pilot screws I did earlier on today as an after thought, both two turns out.
Will check over flywheel and timing later on - when kids in bed lol

Will try the adjustment, but to be fair they look pretty close as is.

I have a vacuum gauge for when it eventually starts.

Also done the fuel level with clear tubes and a small ruler - they didnt need adjusting, plugs come out wet if i stop before i get a backfire.
Last back fire made the left hand air filter warm to the touch so I am a bit skeptical about it happening all the time when I attempt to start it

[gavcarter]

Solved, aftermarket ignitor set in wrong mode, was only firing one cylinder. Wasted spark system so only firing half as often as it should.

Rough as ducks backside but at least she goes now