No power at low rpm

[Nlck]

Hello all,


I've been trying to get my 1990 Yamaha XTZ750 back on the road. Since I bought it its been non stop trouble. It's only really been carb trouble but I can't seem to fix this one issue. At around 1700-2000 rpm there is no power and the bike gets really jerky. It is almost unrideable in towns.


I have tried:

* stripping the carb down and giving everything a clean and taking it to a garage for them to ultrasonic clean
* replaced float and float seats
* run fuel system cleaner through
* spraying brake cleaner around intake boots to check for leaks

It is less noticeable when the bike is cold but it get much worse then the bike is warm. I'm also having a choke issue which I don't know if it could be linked. The choke doesn't really work that well. It only slightly raises the rpm and when its on the bike really struggles, it splutters and dies. I've got a new choke cable and I've checked the plungers and they look okay.


What would be the next steps to try to sort this issue out as I've drawn a blank on anything else I can try. This is the first bike I've ever had to do any mechanical work on so I am learning as I'm going.

I've found this video of an XT660z that is having a similar problem - https://www.youtube.com/shorts/Vg3bRbDYOho


Thank you!

[donnybach]

If you have a dyno place nearby, a quick run on there would show rich and lean conditions through the rev range.
Could be too rich at low revs, float height important. Choke not operating correctly might be a key indicator the carb isn’t set up as it should. Maybe the choke never fully turns off, and carb is adjusted to work around it.
Other options are spark generation, but I’d give the carb a base reset again to factory spec, might notice something obvious when you re-inspect it. I take it you have the correct manual/spec for model? New jets might help, unlikely to be worn, but will give you a methodical process for rebuilding, and force you to inspect in minute detail. Choke bogging symptoms def seem to show over-rich.

[Nlck]

My local dyno place doesn't want anything to do with my bike. When I've gone there hes basically told me to clear off.

The choke used to work fine and suddenly one day it stopped. The bogging has always been present even when the choke has worked. I might order new choke plungers even though the current ones look okay but you never know. I have a Haynes manual but its for the TDM, TRX and XTZ so it gets a bit confusing. Only other thing I could think of is the emulsion tubes are ovaled.

I have been having some electrical problems so it could be spark generation? My heated grips and my GPS have been randomly shutting off during riding. I have a phone charger connected that displays battery voltage and it drops down to 9V when riding but that seems to have magically fixed itself for now.

[donnybach]

Yeah, I had my 1vj tested years ago and they weren’t too happy trying to dyno an old 40ish hp motor. As to the electrics, before spending any money, I’d just go through the loom, and clean and lube all the connectors, and just check components to spec with a meter.
I’m sure with a little bit of tlc and she’ll be fine and ready for the summer.

There’s always dropping a 270degree TRX motor in her worst case scenario, which I’d love to ride.
I’m sure someone on here must have carb specs for the super ten.

[Nlck]

She's so close to being ready, it's just a couple of problems that I can't seem to figure out. Everything else I can deal with but I cant do any town riding which means I can't ride the bike to work so once that is sorted I'll be making up for the lost miles on her. I'll go over everything electric once the weather dries up a bit and I can get her out of the garage.

If I cant get this issue sorted or at least make some progress by the end of February I'm going to be doing research on putting the TRX or TDM motor in.

[James0541]

Nick,

I have exactly the same problem as you, and have been scratching my head for a while now trying to sort it. Where are you located?

[charliext1vj]

Chaps - no experience of XTZ750 or Tedium motors TDM :-) but found this video intersting maybe be a linked problem.

https://www.youtube.com/watch?v=vuWdonY33rc

I would say the fact it is loosing power equally over both carbs does sound like a set up problem? what does a quarter turn on the mixture screw achieve?

Do you get back firing on over run down a hill whilst holding 2K revs?

good luck

[Nlck]

Nick,

I have exactly the same problem as you, and have been scratching my head for a while now trying to sort it. Where are you located?

I was out the garage having another look last week and discovered a vacuum leak. The vacuum line running from below the right hand carb to the fuel pump had a small split in it. I had to spray brake cleaner at a certain angle to get the rpm to raise which is why I didn't discocover it earlier. Ive ordered vacuum line and hoe to be able to get time to fit it some time next week. Ill keep you updated on if that fixes it or not but it might be worth you having a look to see if that line is damaged.

I'm located in South Wales.

[Nlck]

Chaps - no experience of XTZ750 or Tedium motors TDM :-) but found this video intersting maybe be a linked problem.

https://www.youtube.com/watch?v=vuWdonY33rc

I would say the fact it is loosing power equally over both carbs does sound like a set up problem? what does a quarter turn on the mixture screw achieve?

Do you get back firing on over run down a hill whilst holding 2K revs?

good luck

I found that video yesterday and I'm going to give it a go when I get time. I also found a vacuum leak I previously missed so I've ordered vacuum line to see if that is the issue.

I used to get a misfire at idle and high rpm but never anything whike slowing down. That was down to an incorrect size float seats oring.

[James0541]

Nick,

I'm located in South Wales.

Ah, ok, I'm in the East Riding of Yorkshire. Unfortunately I don't have a garage to work in, I need a decent day to wheel the bike out of the shed to take a look.

Thanks for the pointers though.