Flat Spot on acceleration

[my02]

Thanks for taking the time to have a look.

My FJ73 has an impressive flat spot on acceleration mostly noticeable in 1st, 2nd and 3rd gear. Its most noticeable when the engine is cold.

I took the vehicle to my mechanic today who advised that the accelerator pump seemed to be working fine, however he did adjust the spray which he mentioned was off. He also replaced the points and reset the timing.

When I picked it up tonight, the problem was still there (couldn't see the mechanic as it was picked up after hours).

The problem can be mostly overcome by pulling the choke right out again but thats a PITA and isn't going to help with my economy.

My Gregory's manual ( Aussie owners guide) lists the accelerator pump linkage and plunger as possible causes however I assume that my mechanic has covered this. It also lists a sticking check valve ball.

So before I dismantle the carby tomorrow ( and risk bits flying everywhere) does anyone have any other ideas or areas to look at?
Also, is it the small check ball at the end of the accelerator pump plunger or the LARGE check ball that I need to inspect?

I haven't played with a carby before so I would like to know if there are any real problems to look for.

I also replaced the spark plugs 2 weeks ago to see if that would help. NUP!

I have looked through the SEARCH function here and in the 60 section but nothing seems to cover anything other than a possible plunger fault.

Thanks for your time.

 

[crushers]

float level?
fuel filter?
kink in the fuel line?
bad tank of fuel?

anything done just before this started happening??

 

[my02]

hi Wayne,

I'll have to check those things tomorrow.

The only thing that comes to mind is that I drove for 3 hours in driving rain before the problem arose but I cannot really see how that would be a problem.

I filled the tank with 98 octane fuel last week (normally its 91) and the difference is only marginal.

I can't imagine anything that would have caused the fuel lines to kink but again I will check. Will also replace the fuel filter tomorrow.

Thanks for advice.

 

[roscoFJ73]

What rpm does it happen at?
If the diapragm that opens the secondary stage gets a leak they run bad about 1700 rpm, about the same time when you change gear.
The good news is you dont have to dismantle the carb just replace that gadget on the side. Cost about $30 from memory.

 

[my02]

What rpm does it happen at?
If the diapragm that opens the secondary stage gets a leak they run bad about 1700 rpm, about the same time when you change gear.
The good news is you dont have to dismantle the carb just replace that gadget on the side. Cost about $30 from memory.

Hmmm, when I pull away from stop its bad straight away so I guess thats around what? 1000rpm ish.

When I'm moving, its flat straight after gear change so probably 1000 - 1200. Mind you with the choke open, its harder to tell.

I had a quick chat with the mechanic this morning and he thinks its definitely the secondary stage. I might try to nip along to REPCO to see if I can get a "fuel miser" carby kit which he recomended.

Looking at the Gregory's manual, the secondary diaprhagm seems to be as you say, removeable without dismantling the carb. Does that sound correct ( guess I should scoot downstairs and have a quick look eh?). Sounds like it may not require the mechanic if thats the case.

Cheers for the input thus far!

 

[roscoFJ73]

Hmmm, when I pull away from stop its bad straight away so I guess thats around what? 1000rpm ish.

When I'm moving, its flat straight after gear change so probably 1000 - 1200. Mind you with the choke open, its harder to tell.

I had a quick chat with the mechanic this morning and he thinks its definitely the secondary stage. I might try to nip along to REPCO to see if I can get a "fuel miser" carby kit which he recomended.

Looking at the Gregory's manual, the secondary diaprhagm seems to be as you say, removeable without dismantling the carb. Does that sound correct ( guess I should scoot downstairs and have a quick look eh?). Sounds like it may not require the mechanic if thats the case.

Cheers for the input thus far!

The problem I had was where it run ok till the 2nd stage began to cut in and it would run crap until the rpms got higher which sounds different to your problem.

It sounds like you have something blocking the jets. You should be able to look down and hand throttle it to see how the fuel is coming out.
Also check the window for fuel level,it shoul be halfway

 

[my02]

Ok, the problem seems to have been solved but not for the reasons we were expecting!

I went to have a quick look at the diaphragm and whilst in there noticed that the clamps holding the air hose together looked a little loose from where I had moved to get at the spark plugs

I turned the car on, had a fiddle here and there ( just getting familiar with the diaphragm) then tightened the clamps. On one tightening, the revs suddenly picked up ever so slightly. I have taken her for a run and everything seems back to normal, however I would like to know why that is. Is it due to too much air getting into the system and making it run lean?

Appreciate your attention and help so far Rosco!

 

[roscoFJ73]

Is it due to too much air getting into the system and making it run lean?

Appreciate your attention and help so far Rosco!

Yes ,but if it was going to the distributor it may had something to do with the vacuum advance.