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YZF750 poor tickover when warm and hard to restart.

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mpd72
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PostPosted: 15:26 - 08 Nov 2019    Post subject: YZF750 poor tickover when warm and hard to restart. Reply with quote

I'm recommissioning my 18,000 mile, '94 Boost Yamaha YZF750 after being stored for a few years. It's been a bit of a job, plenty of leaking gaskets including tank bottom, fuel repair tap, fuel pump, fuel hose, carbs off, new rubbers and cleaned twice, oil/filters/plugs/battery etc.

It starts spot on and will idle quite nicely within a few seconds. As it gets warm though, I get a little pop and backfire every so often if revving it and settling back to tick over starts revving a bit too high, then drops too low, needing choke to keep it alive. It's then quite difficult to restart.

I did put some Cataclean in the fuel to get at any bits of carb I couldn't clean in my 2 attempts, so wondered if this could be the cause?
l didn't strip the bank into 4 separate carbs due to seals being NLA, so it wouldn't fit in my ultrasonic cleaner. I've now put some fresh fuel in but it still did the same, although was maybe a bit better with fresh fuel.

So, do I..

Keep running it and hope it gets better with use, as it's been stood for 6 or 7 years and the Cataclean will eventually run through the system.

Accept that it may be valve clearances, even though these were done with new under bucket shims about 2,000 miles ago, albeit 10 years ago.

Something else?

Cheers folks.
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Last edited by mpd72 on 15:35 - 08 Nov 2019; edited 2 times in total
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Nobby the Bastard
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PostPosted: 15:30 - 08 Nov 2019    Post subject: Reply with quote

Sounds like an air leak, but I'd blame the mussies just to be on the safe side.
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Easy-X
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PostPosted: 16:17 - 08 Nov 2019    Post subject: Reply with quote

Nobby the Bastard wrote:
Sounds like an air leak, but I'd blame the mussies just to be on the safe side.


Not kewl, man!

(checks name of poster)

Oh, that's okay then... as you were Laughing

All cylinders firing okay? Might the backfire be some unburnt fuel from a cold cylinder?
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mpd72
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PostPosted: 16:52 - 08 Nov 2019    Post subject: Reply with quote

Easy-X wrote:

All cylinders firing okay? Might the backfire be some unburnt fuel from a cold cylinder?


After a couple of minutes running, the middle 2 pipes are half the temperature of the outside 2, but they do run different jets to compensate for heat build up in the middle.

It seems to be firing on all 4 though.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
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Tankie
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PostPosted: 16:55 - 08 Nov 2019    Post subject: Reply with quote

Balance the carbs with vacuum gauges
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Fizzer Thou
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PostPosted: 21:50 - 08 Nov 2019    Post subject: Reply with quote

Do a compression check when the engine is cold then once the engine is hot and the niggles start to appear.If the valve clearances are tight because of the seats becoming contaminated then it may well be that the centre two cylinders have reduced compression compared to when cold.

When you had the carbs off did you check the inlet gaskets between the cylinder head and the inlet manifolds?

Part 11 on the fiche https://www.cmsnl.com/yamaha-yzf750r-1996-4hd7-england-264hd-300e1_model38768/partslist/0011.html#.XcXiHriaLTQ

On my Exup1000 they are oval shaped 'O' rings and two out of the four on my engine had shrunk and stopped sealing correctly with the heat from the engine.No amount of carb balancing would compensate for leaking gaskets.I would expect to see fluctuating readings with gaskets that were compromised.
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mpd72
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PostPosted: 10:26 - 09 Nov 2019    Post subject: Reply with quote

Fizzer Thou wrote:


When you had the carbs off did you check the inlet gaskets between the cylinder head and the inlet manifolds?

Part 11 on the fiche https://www.cmsnl.com/yamaha-yzf750r-1996-4hd7-england-264hd-300e1_model38768/partslist/0011.html#.XcXiHriaLTQ

On my Exup1000 they are oval shaped 'O' rings and two out of the four on my engine had shrunk and stopped sealing correctly with the heat from the engine.No amount of carb balancing would compensate for leaking gaskets.I would expect to see fluctuating readings with gaskets that were compromised.


Ah, no. If things don't improve with the fresh fuel and a bit more running, I think I'll take the tank, airbox and carbs off again and have a look at the gaskets under the inlet manifold.

Still available through Yamaha too. Thanks for the suggestion, I;ve saved the part number. Thumbs Up
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
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Shaft
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PostPosted: 16:20 - 09 Nov 2019    Post subject: Reply with quote

Nobby the Bastard wrote:
Sounds like an air leak


I had almost identical symptoms on my last XJ900.

I was aware it hadn't been in regular use for a while before I bought it, so I did all that stuff too, plugs, injector cleaner etc.

Made it better for a while, but not totally sorted, then started to get worse again; didn't like to hot start, would drop a cylinder from time to time (although not properly, just seemed like it would miss every few cycles) sluggish revving, poor idle, the odd pop and bang.

I only spotted the problem when I was trying to start it in almost total darkness - hit the starter and noticed what I thought was a flash under the tank.

Bent down to have a look and there was an air pipe running from between carbs 2 and 3, which had obviously hardened over time and then split, the flash being a tiny flame that appeared through the hole in the pipe.

Replaced the pipe with some generic fuel line, problem solved.
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mpd72
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PostPosted: 14:11 - 10 Nov 2019    Post subject: Reply with quote

I popped the tank, airbox and carbs off earlier. The carb to inlet manifold rubbers have a few cracks around the top and bottom edges, so I've found some NOS items on Ebay and Amazon, to replace the 2 x rhs and 2x lhs ones.

The gaskets from inlet manifold to head are thin steel gaskets with a thin rubber coating, which has separated from the steel in places, so 2 X new gaskets and a tube of Threebond 1215 has been ordered as there was a smear of grey silicone on both faces too.

Hopefully this will eliminate any air intake leaks and get it running spot on again. Thumbs Up
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.
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