Monojet carb issues

[70STOVEBOLT]

My truck starts fine when cold, has a slight miss at idle, but otherwise runs good. The problem is when I shut it off and leave it sit for about 10 minutes I can smell gas. I opened the hood and found gas leaking out of the front of the carb. Above the idle mixture screw there is a linkage and it appears gas is running out of bowl there. I checked the float level and it is at spec. I took the needle and seat out and cleaned it, it doesn't look worn or damaged.
Also, the float is good, no holes and there is no gas in it. What else would cause this? I think the slight miss at idle is a plug wire or maybe the distributor cap needs replaced, been a while since a tune up. Any ideas would be appreciated.

Thanks,
Mark

[BMERDOC]

That is the accelerator pump. Mine leaks there too because the arm was bent and scuffed the bore. To my knowledge there is no real fix for it because it is on the main body. I could be wrong, have been before.

[Daner]

Your miss at idle could be a restricted fuel filter. I had a miss and couldn't figure out what it was. Changed filter and she idled smooth once again.

I don't have any ideas on the carb. I didn't even clean mine when i rebuilt the engine.

[Zeke's Garage]

Good luck with that voodoo infested mono jet business.. I ended up with 4 different carbs, swapping and cleaning and rebuilding, and still couldn't get one to run right on my 250... they were all just worn out and mangled.
So instead of buying a 'new' one and making life simple, I built a Megasquirt fuel injection system for it... Only took a year and now I can drive around with a laptop and impress the sport import guys. 'Cause that's important.

Yeah, probably not the best way to solve that little problem.


Good luck


Shawn

[70STOVEBOLT]

How much coin do you have in the injection system? I probably would not go that route, but you never know. I can live with the slight miss, but I really would be bummed if it burnt from fuel leaking onto the hot exhaust manifold.

[68gmsee]

Can you double check to see where the leak is coming from or maybe post a pic?

There shouldn't be any leaks outside the carb unless the bowl is cracked or flooding. One thing that we tend to overlook is the middle gasket. It may be just loose but they can deteriorate. Easy to fix. Just remove the two base screws and turn carb upside down and check the screws from the bottome.

They're very simple carbs to repair but if it's been rebuilt a few times they do get banged up quite a bit. Another thing you can do is to clean the outside real good with carb cleaner and let it idle as you watch the carb to see if you can isolate it.

[70STOVEBOLT]

I am sure the bowl is not cracked. It does not leak when running, only after it has been shut off and sits for more than 10 minutes. I replaced the exhaust manifold recently and I checked all the screws in the carb at that time. It is definitely leaking where the accele-
rator pump/power piston linkage come thru the base just below the bowl. It didn't start doing this until I rebuilt the carb and adjusted the float, that is why I think it could be the float level, but it doesn't seem to affect the way it runs, it doesn't flood or bog when I stomp it.

[Zeke's Garage]

Originally Posted by 70STOVEBOLT

How much coin do you have in the injection system? I probably would not go that route, but you never know. I can live with the slight miss, but I really would be bummed if it burnt from fuel leaking onto the hot exhaust manifold.

I probably have about $1k in the injection system, but that includes all the 4.3 TBI parts, Offy 4bbl intake, wiring and all the Megasquirt stuff plus the wideband o2 sensor and controller.

If I just wanted an injection system to install and run with less fiddling, I would have picked something different, but I honestly just wanted to learn the Megasquirt system so that I could understand more about engine management and tuning overall.

I'll be posting a build thread pretty soon, but if you have any other questions just let me know.

[70STOVEBOLT]

I'll be posting a build thread pretty soon, but if you have any other questions just let me know.[/QUOTE]


Cool, I look forward to the build thread.

[68gmsee]

Originally Posted by 70STOVEBOLT

I am sure the bowl is not cracked. It does not leak when running, only after it has been shut off and sits for more than 10 minutes. I replaced the exhaust manifold recently and I checked all the screws in the carb at that time. It is definitely leaking where the accele-
rator pump/power piston linkage come thru the base just below the bowl.....

10/4. The reason I asked to double check is that on Rochester monojets those links are not inside the fuel bowl part of the carb. They are separated by the casting. The only way fuel would leak out of there is if it was flooding or crack in the casting. In your case, I agree that it's probably the float adjustment.

[70STOVEBOLT]

Would it be set too high or too low? I would think if it were too high it would flood while idling or bog down on takeoff.

[68gmsee]

Originally Posted by 70STOVEBOLT

Would it be set too high or too low? I would think if it were too high it would flood while idling or bog down on takeoff.

Not necessarily. You may just be burning the excess and keeping the level down enough so it won't leak. Even if it did leak a little while driving, it would probably evaporate off with the heat and the fan blowing on it.

Check to make sure the float is set at least 1/4 inch from the top of the carb to the leading edge of the top of the float and making sure you keep slight pressure on the needle valve end while checking the adjustment. Also, if you remove the top and take the float out the fuel level should just be above the refill slot on the accelerator pump cylinder.

[70STOVEBOLT]

I will check it out. Thanks 68gmsee.

[70STOVEBOLT]

ttt

[BMERDOC]

Ahh, I see you're still working with this. Lets refresh with some pics and diagrams and see if we can't resolve your issue. Tell me if any of this helps.

[70STOVEBOLT]

It's either leaking from the power piston or the accelerator pump, not sure, but it is in that area right under the float bowl. Looks more like the power piston, I don't see fuel dripping from the accelerator pump linkage. Any idea why it would be leaking from there?

[cdowns]

is your gastank vented properly with the right gascap? or is there a noticable pressure when u remove the gascap??

[70STOVEBOLT]

Originally Posted by BMERDOC

That is the accelerator pump. Mine leaks there too because the arm was bent and scuffed the bore. To my knowledge there is no real fix for it because it is on the main body. I could be wrong, have been before.

I missed this post before. This may make sense, because I did not have this problem until I replaced the carb with a Holley rebuilt unit. The wierd thing is that it does not leak until the truck sits for about 10 minutes after shutting off the engine.

[70STOVEBOLT]

Originally Posted by cdowns

is your gastank vented properly with the right gascap? or is there a noticable pressure when u remove the gascap??

I have not modified the fuel plumbing at all so unless there is a restriction in the
return line, I don't think that is the problem. There is no pressure when the gas cap is removed, and it is the original cap as far as I can tell. It has been on there since I've owned the truck, about 12 years.

[BMERDOC]

Just got home from a long trip from Jersey. I'll do some research and see what I can dig up tomorrow. But for now it may be one of the places named, the upper gasket could be torn or maybe the casting is porous. The only reason I can think of that it would leak after sitting is heatsoak.

[Tom M.]

Mark, did you ever figure this out? I'm having the exact same issue you describe. I have my theories, but before I take apart the carb again, I hope you can give me a clue.

I rebuilt the carb with a kit, and also replaced the plastic float with a brass one. It runs great on my 292. I have three theories: 1. The float level is too high because the brass float is less buoyant than the plastic one. 2. Something wrong with the accelerator pump seal, although my bore is fine, perhaps the gasket isn't sealing right because of a funny incompatibility between my rebuild kit and the existing pump parts I had to reuse. 3. There's some strange heat pressure or vacuum going on in there. 4. For some reason the float needle isnt sealing. Anyway, I'd sure like to know whether I need to purchase a new, and different rebuild kit before I tear it apart.

Not wanting to go up in flames,
Tom
Posted via Mobile Device