Mystery - Flat spot in acceleration (1 Viewer)

[funk49]

I've got a mystery issue that I could use some help identifying.

I have a '72 FJ40 with the original F engine that runs really well but when I step down hard on the gas, it seems a little rough at the very beginning of acceleration. If I ease into the acceleration slowly from the beginning, it's fine but if I forget and stomp on the gas, acceleration is kinda wonky until I get past it (if that makes sense). I took it to my Toyota guy and he said that the carb needed to be rebuilt but this issue has persisted through a) a complete engine rebuild and b) the carb was totally rebuilt by Mark (@65swb45), so I honestly don't think the carb is the issue.

Any ideas/comments are totally appreciated, I'm completely stumped as to what would cause this because otherwise, this truck runs great.

 

[fjc-man]

What are you using for a distributor? Original with points?

 

[funk49]

I'm running the OEM replacement one from SOR...the silver one. This was also replaced during my rebuild. The one thing I'm wondering is if it could be a fuel pump issue because I think it may be original (and wasn't replaced during the rebuild). Fuel filter is new. It almost seems like a fuel advance problem that is happening when the gas pedal is engaged too rapidly.

 

[pb4ugo]

What's fuel advance? Check you timing, see if the mechanical and vacuum advance is working. Just because things are new doesn't mean there's not a problem with them.

 

[DesertFJ40]

I have a 71 and use to experience this. Could be a lot of factors. So yes, check timing, points gap, and no vaccum leaks. Pretty easy to check..... But my guess, as for how the issue was with me is either the jet size on your carb is to small (lean) and often results in hesitation when you first start to accelerate; this issue seems to resolve itself if you use ethenol free gas, but probably best to just change the jet a size or two up.

Second issue that I had was the valve gap size on the engine head. The exhaust valves would tighten up on my Fj40 every several hundred miles. I belive this was due to the build up of sludge my engine had from the previous owner. Eventually everything broke loose and runs as it should but, every time I noticed the hesitation issue, I would take the valve cover off and check the gap sizes and sure enough a few of them were too tight.


Hope these suggestions help

 

[Pin_Head]

Sounds like an intake manifold vacuum leak. Can you turn the idle mixture screw all the way in without the engine stalling? If so, there is no fuel at the transition slot to smooth the transition from idle to accelerate.

 

[charliemeyer007]

I think its your accelerator pump. I had that happen once in my 2bbl Rochester back in the 80's. The leather pump cup bonded to the aluminum body. I broke it loose, soaked the cup in USGI neatsfoot oil and stuck it back in. Still there and still works.

Easy test - look down the bore of your carb, work the throttle - you should see raw gas spray in the bore; if not then the accelerator pump is bad.

 

[middlecalf]

Easy test - look down the bore of your carb, work the throttle …

Engine not running

 

[charliemeyer007]

The accelerator pump should work as long as there is fuel in the float bowl - engine running or not.

 

[fjc-man]

middlecalf is saying for safety purposes, check accelerator pump action with engine off. Don’t want the engine deciding to backfire while your peering down the carb throat.

 

[charliemeyer007]

Yes that would be a good safety measure. Always pays to have a proper fire extinguisher on hand, same with a bottle of saline eye wash.

On my carb I can look down the bore at an angle and see the stream of gasoline safely enough. Your watch crystal or the mirror on your compass can work for you too.

 

[funk49]

Thanks for the replies everyone, it's very much appreciated! There are definitely enough good tips in here that I should be able to figure out the issue.

 

[Mark W]

I'm running the OEM replacement one from SOR...the silver one. This was also replaced during my rebuild. The one thing I'm wondering is if it could be a fuel pump issue because I think it may be original (and wasn't replaced during the rebuild). Fuel filter is new. It almost seems like a fuel advance problem that is happening when the gas pedal is engaged too rapidly.

Fuel advance? WTF is Fuel advance? In any case, it is not a problem with the fuel pump. The fuel pump merely keeps the bowl full. if it was a fuel pump problem, it would not evidence itself by acting up on initial acceleration and then smoothing out.

The first place I would look based on what you describe would be the accelerator pump. It is not uncommon for tiny piece of debris to find it's way into the accel pump and clog the orifice. Work the linkage and watch for a strong and steady stream of fuel being squirted into the primary bore.

The reason to do this when the engine is off is not for safety or anything like that. It is because you will not be able to see the stream of fuel if the engine is sucking air through the carb at the same time.

Mark...

 

[funk49]

Fuel advance? WTF is Fuel advance? In any case, it is not a problem with the fuel pump. The fuel pump merely keeps the bowl full. if it was a fuel pump problem, it would not evidence itself by acting up on initial acceleration and then smoothing out.

The first place I would look based on what you describe would be the accelerator pump. It is not uncommon for tiny piece of debris to find it's way into the accel pump and clog the orifice. Work the linkage and watch for a strong and steady stream of fuel being squirted into the primary bore.

The reason to do this when the engine is off is not for safety or anything like that. It is because you will not be able to see the stream of fuel if the engine is sucking air through the carb at the same time.

Mark...

Thanks Mark, this is good information. In terms of me typing fuel advance, not sure why I initially typed that. What I was trying to convey was that I've suspected a vacuum problem may be involved. Whatever is happening, it has to be an issue with something mechanical that hasn't been repaired/changed since I've been working on it for the last couple of years.

 

[middlecalf]

Have you done what @Pin_Head suggested, turn the idle mixture screw in and see if the motor starts to die or nothing happens?

 

[funk49]

Have you done what @Pin_Head suggested, turn the idle mixture screw in and see if the motor starts to die or nothing happens?

Haven't had a chance to get to it yet with the holidays quickly approaching but plan to this weekend.

 

[65swb45]

You should mention what your baseline timing and baseline vacuum are.

 

[fyton2v]

Ya. What everyone else is saying. It's not hard or expensive to ensure your timing is correct and your vacuum is good. And when those things are checked ... accelerator pump.