GPz750 Fuel Problem?

[Grumpy Grandad]

Hi Everyone

I've just had my 85 Reg GPz750A3 Re-Commisioned after a 15 year lay up. A local freelance Mechanic did the spannering, as I didn't want to screw it up!

On the Fuel side he did the obvious Cleaned out the Fuel Tank and Relined it.
Stripped, Cleaned, Balanced and fitted a New Carb Service Kit.
Stripped, Cleaned and fitted a New Fuel Tap Kit.
Fitted New Spark Plugs.

Plus what else you'd expect. New Oil & Filter, New Air Filter, New Brake Fluid etc.

I've put 800 miles on it since Sept 24, just to restore my faith in it (although I've owned it since new) and to regain MY own confidence, as I've been off the road bike wise for the same period!

TBH it's performed pretty well. Recently however it seems to have developed a problem at lower revs. When coming down from higher revs/speeds it gets to about 4000 rpm, and then acts as if it needs a breather and doesn't want to rev, as I'm attempting to change down.

Previously it's struggled for a bit caught it's breath and then once it revs up into the higher revs it's fine.

Yesterday however it continued to slow down, refusing to rev, and eventually stopped. I switched off waited a bit then it started up and revved up ok. My first thoughts were, '' Something's not right with the carbs'' however I'm not a mecanical genius.

Any suggestions appreciated, and any recommendations for a suitable in-line fuel filter would be great.

Cheers. Ride Safe!

[Nobby the Bastard]

Whats the fuel flow to the carbs like?
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[RhynoCZ]

Sounds like fuel starvation.

1) You say the tank is relined. Did you check the tank breather is working properly, meaing not blocked?

2) Did you fit an inline fuel filter? If you did, get rid of it right now. I owned '88 GPz550 and the moment I put a filter there, thinking I'm doing the right thing, that's when all the fuel issues started.

3) Does your GPz run the vacuum fuel tap? If so, you either put it back together wrong or you have insufficient vacuum in the system to open the tap properly. This could be bypassed by using the PRI position of the tap.

EDIT: WARNING! Do not leave the tap in the PRI (prime) position when you don't ride the bike. It's like leaving a regular (non-vacuum) fuel tap in ON position = free flow of fuel. If the float valves fail, you'll leak the tank through the carbs on the ground when the engine is off. Don't ask me how I know.

4) Rebuild the carbs again, make sure to set correct float height.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[stinkwheel]

What was the air temperature like?

Could have been carb icing. What you described is exactly what would happen and air-cooled bikes from the 80s were quite prone to it. Even more so if they had pod filters. Later liquid cooled kawasakis had part of the coolant system plumbed through the carb bank specifically to prevent it.

Happens most in low air temperature, high humidity conditions. Not necessarily below freezing as the fuel vapourising off the needle tip drops the internal temperature a fair bit. So close to freezing is most likely, especially if humidity is high. Ice crystals form on the tip of the throttle needle due to high air flow which kills the engine. Usually happens when you close the throttle after a period of wide open throttle (and therefore high air flow). Engine stutters then cuts out. All returns to normal after about 30 seconds when the ice melts.

EDIT: Other thing that occurrs to me is sticking needle valves due to ethanol corrosion. Happened to my KLE500 in only a few months. Happily you can buy replacement ethanol safe ones with viton tips. The corrosion on the needle valve would be obvious on inspection, especially with a recently cleaned carb. That said, the service kit may have contained new needle valves.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kawasaki Jimbo]

[bypass2]

vacuum pipe may be split

[Grumpy Grandad]

Thanks for the replies everyone.

I wasn't ignoring you, I didn't realise I'd had any replies, just getting used to the site mechanics LOL.

I was looking at the ''No New Messages''

[Grumpy Grandad]

[quote="stinkwheel"]What was the air temperature like?

You could have hit the nail on the head SW

I've been out & about in 5-8 degrees and it's always happened after a good blast. Then I've got the sypthoms you describe.

Cheers!

[Grumpy Grandad]

[Grumpy Grandad]

[Grumpy Grandad]

[quote="stinkwheel"]What was the air temperature like?

Could have been carb icing. All returns to normal after about 30 seconds when the ice melts.

Update on the Fuel Situation:

I think it must have been Carb Icing as you suggested. Funnily enough as I continued to search OLD posts I saw one of yours suggesting the use of Silkolene FST so thought I'd give it a try.

That seems to have done the trick (or it's warmed up a bit!). Just got to figure out an easy way to add it to the fuel either after/before going for a blast and riding to the Petrol Station to fill up..... to get the correct mixture. Don't want to be riding around with a bottle of FST stuffed down my jacket
I'm sure I'm over thinking it

Not sure whether to stop using it as the weather really starts to warm up? I've read that it's pretty good for keeping the carbs and engine cleaner .... Any thoughts?

Cheers