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Old 11-04-2010, 09:22 PM
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Default Low vacuum and hesitation in first gear...

Hey guys,

I just rebuilt my carb and reinstalled it on my stock 1976 FJ40. It drives great down the highway and shifts great, but seems to have a dead spot when starting off in first gear (when I first release the clutch) from a dead stop. I have to rev up the engine more than necessary to get it to accelerate without dying.

Also, I only have 10 hg of vacuum at idle. There must be a vacuum leak, but this may have been there before the rebuild. I have checked and can't find any leakage at the base of the carb or intake manifold so far.

Any thoughts?

Thanks--
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Old 11-04-2010, 09:50 PM
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PCV circuit funkyness?

that's a big vacuum leak.
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Old 11-04-2010, 10:37 PM
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I haven't dealt with carburetors since working on mustangs with Holley carbs so excuse my ignorance if the parts don't carry over to your application. It sounds like the accelerator pump nozzle or diaphragm to me. A carb works by using air flow and vacuum to pull gas into the engine and the problem is that when you accelerate there is not enough air flow yet to pull in the gas that the engine is looking for so an accelerator nozzle is used to push extra gas in to compensate until the air flow is sufficient enough to pull in what is needed. You should be able to check it out by pushing the throttle and looking for a squirt of gas into the carb. It would either be the nozzle or the diaphragm that pushes the gas that would be the culprit if this is the problem. Usually it is pushing too much through the nozzle if it causes the vehicle to die and not enough gas if it just doesn't accelerate fast enough.

A better and more thorough explanation of what I'm talking about can be found in this video: http://www.youtube.com/watch?v=k-Nx5HEzvlY

Again, I don't know if this necessarily applies to LC carburetors, but I wanted to share my thoughts in case it does.

Last edited by satchel; 11-05-2010 at 04:15 AM.
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Old 11-05-2010, 06:51 AM
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On the rebuild, did you change the jets from what was in there? If you went smaller with the primary, it could be what is causing the slight stumble.

Do you have vacuum readings from before the carb rebuild? How does the current reading compare to them? The vacuum leak could be anywhere that is using engine vacuum; brake booster, emissions hoses, pcv system as well as the manifold connection to the engine block and carb. My initial guess would be that a vacuum line has become disconnected and or cracked.
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Old 11-05-2010, 09:21 AM
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Quote:
Originally Posted by RicardoJM View Post
On the rebuild, did you change the jets from what was in there? If you went smaller with the primary, it could be what is causing the slight stumble.

Do you have vacuum readings from before the carb rebuild? How does the current reading compare to them? The vacuum leak could be anywhere that is using engine vacuum; brake booster, emissions hoses, pcv system as well as the manifold connection to the engine block and carb. My initial guess would be that a vacuum line has become disconnected and or cracked.
Ricardo,

I did not run a vacuum check prior to the carb install, so I almost wonder if it already had low vacuum and that this is a separate carb issue. I kept the original jets, so no change there.

I put in a new accelerator pump that came with the carb kit, but swapped the old one back in thinking that it might have been the problem. It didn't make a difference at all. I do, however, need to make sure that it is pumping a visible amount of fuel into the barrel. I'll check that tonight.

Marco, what is the PCV? Does it have anything to do with this picture? This piece has been disconnected since I bought the 40...
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Old 11-05-2010, 09:30 AM
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PCV is positive crankcase ventilation

On most, it comes off of the valve cover, and is just a pressure release valve. It can become clogged, and then the pressure vents at whatever seal is weakest.
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Old 11-05-2010, 09:36 AM
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yep, that's the PCV hose (red thing)/valve (metal part in the hose and stuck in the rubber of the side cover tube).

make sure it is hooked up per the FSM. leaving it unhooked will certainly cause a big vacuum leak. Looks like yours might actually be broken since that metal piece has separated from the part in the side cover. Go to autozone or napa and see if you can find a new one and throw it on there.. make sure the hose headed to the intake manifold is nice and not broken and go from there.

here it is:

http://www.napaonline.com/Search/Det...+50070+2070028

you may want to get a new grommet too, they are cheap:

http://www.napaonline.com/Search/Det...+50070+2070028

the grommet holds it tight in the side cover tube
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Old 11-05-2010, 10:58 AM
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On the PCV valve and grommet. I've always picked mine up at the dealer. I have heard (but not experienced) that fitment with non-OEM has been an issue. Connecting the PCV will have a positive impact on your vacuum reading.
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Old 11-05-2010, 11:08 AM
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Thanks, guys. I will replace mine. When the engine is running, it seems to have a little "spring" poke out the end. Does that make any sense?
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Old 11-05-2010, 11:25 AM
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Did you end up finding the jets you needed? I actually brought a pocketful to the meeting last night in case you were there.
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