[rowling]

I have a stock 71 GTO, 400, TH 400, original Q jet that was rebuilt approx 4 years/7K miles ago. It has started to run rough and continuously surge after getting on the throttle. It appears to be when the engine starts to pull a lot of vaccum and about when the secondaries would start to open. I checked the timing and have also replaced the points with an electronic module, new cap and rotor. I haven't found any vaccum leaks and as some other threads suggested, put my hand over the carb to detect if the idle is affected -which it didn't appear to be.

I can drive around no problems if I stay out of the secondaries. Once I step on the throttle and it starts to run rough, it stays rough through all the power ranges including idle but only when in gear - in park it idles fine. After sitting overnight it will run fine again as long as I stay out of the throttle.

I have about four weeks of driving left so hopefully it can be a quick fix before the car goes into storage. And I'm out of ideas short of replacing the carb.

Any thoughts or ideas? As always, thanks for any input.

Glen

[mrpaul]

It sounds like the rods either fell off the hanger or the secondary bleeds are clogged.Try removing the hanger with the rods and make sure they are not bent.

[79firebird_99TA]

I started to think that the secondary rate is not set up right until I read on and saw that it runs rough after getting on it. Check and make sure that there are no linkages binding or sticking. Might just need to clean the exterior of the carb.

[sleepy]

If that carb has the stone filter in the inlet clean the filter and hook a hose to the gas line. run the hose into a CLEAN clear container. crank the motor to pump gas into the container. check the gas for garbage.
sometimes crap in the gas gets trapped in between the pump and the filter. By getting on it the pump pumps gas faster and the crap clogs(or partially clogs the filter. letting it set over night lets the crap settle back down the line. taking it easy the crap doesn't reach the filter with low flow(crap heavy enough to sink back down) but when you get on it the gas flows faster and brings the crap with it to the filter.
My dads ford PU had that problem ONLY when he pulled the camper. Had to stop and blow backwards threw the filter but JUST when pulling a load. No load it ran fine.

[Half-Inch Stud]

1. Does the Secondary Tip-In valve stay stuck-open?

2. Do the Secondary Throttle plates open & close smoothy?

---> Take a look down the Secondary Bores to see that all the Screw-heads are on the throttle plate screws.

[rowling]

Thanks for the replies. One thing I failed to do was cut open the filter when I changed it. So I opened it up and there was A LOT of gray rubber silicone bits in it. I mean a lot - I have never seen a filter that clogged. So I'm thinking some of the crud got past the filter and into the carb. Either that or there is still crud in the line like sleepy has suggested.

My next question - where the heck did that silicone come from? I never used any when I replaced the tank or on any fuel line connections. Is the diaphragm in the pump gray?

Glen.

[rowling]

One other question - mostly due to my lack of knowledge, but why would cleaning the carb exterior do anything?

Also - the secondary valves and plates operate smoothly and all the attach screws are secure.

Glen.

[sleepy]

Was the inside of the tank coated or sealed?