427/435 Fuel Pressure

[Domenic T.]

Re: 427/435 Fuel Pressure

Howard,
What Michael said is right.

Wht don't you leave the rod off and test drive it?

Also, I took a look down the manifold and you would need to have a borescope to see the inlet side of the exhaust crossover. BUT you would be able to see the presents of exhaust in the plenum.

From your last post it sounds like linkage. All the dueces I ever worked on HAD to have the end carbs COMPLETELY closed to idle right. I can only think that the bullitin that said the end carb was to be slightly open was in error.

Just think of it, its a massive vacume leak till you open the end carb enough for the venturi to suck fuel, That's why there is an accelerator pump on single carbs, (your center carb) it supplies the initial air input with fuel untill the venturi sucks fuel thru the jets.

DOM

[Domenic T.]

Re: 427/435 Fuel Pressure

Joe,
There shouldn't be idle screws on the out board carbs.

DOM

[Michael H.]

Re: 427/435 Fuel Pressure

Try disconnecting both outboard carbs and backing out the idle speed screws on them so that the throttles are closed. Work strictly off of the center carb............for now. If the condition persists, then you should suspect sticky/improperly adjusted flyweights in the distributor. Also, and very important: the vacuum advance diaphragm should not begin to deploy until engine vacuum drops at least 2 in-hg below idle vacuum. You should have connected vacuum advance to manifold vacuum and eliminate ported vacuum. I think that there is a better vacuum advance can that should be used with your L71. It might be the B20/B26 or B28. Check the archives.

Joe,

I don't think the rod adjustment had anything to do with the original problem. I think the latest rod adjustment created a new/2nd problem.
I think the rods should be correctly adjusted and then move on to find the real problem. The original problem only occured after the engine was at/near operating temp. End carburetor closed position adjustment would have been an issue all of the time.

If some adjustment of something doesn't cure the original issue, the adjustment should always be put back where it was originally. Randomly changine things is non productive.

The vacuum advance unit, or the way it's hooked up, has nothing to do with this problem. The car ran fine before so something changed.

We need more information.

[Mike R.]

Re: 427/435 Fuel Pressure

When you had the carbs apart did you remove the throttle plates? 3x2 carbs require unusual throttle plate gaskets. I had a similiar problem and found that the carbs had been rebuilt except for the throttle plate gaskets. I had similiar simptoms where it ran well cold (because it was on the choke and therefore rich, covering up the vacuum leak)

Good Luck

Thanks Ron.

The TI has a Lectric Limited replacement module installed that was in the unit when I bought the car nearly 4 yrs ago. The "263" coil was also replaced by the previous owner. It looks new that's all I can comment about it. Plug wires are new when I did the car (no signs of arching when lights are off).

I have just viewed a archived article on L71 carb set-ups and based on what the article indicates the rear carb is normally cracked slightly which leads me to think mine is correct with its low draw. I will further investigate with the carb cleaner though as perhaps a vacuum leak is developing under heated conditions.

Has anyone any experience with the latest TI electronic replacements from K & R out of Georgia ?

If I was to swap out the coil, is there any other aftermarket coil other than the "263" that can be used without harming the TI system ?

Thanks

[Michael H.]

Re: 427/435 Fuel Pressure

Wht don't you leave the rod off and test drive it?

From your last post it sounds like linkage. All the dueces I ever worked on HAD to have the end carbs COMPLETELY closed to idle right. I can only think that the bullitin that said the end carb was to be slightly open was in error.


DOM

Dom,

The adjustment of the rear rod isn't supposed to hold the carburetor partially open, and it won't if the adjustment is done properly.

The "half hole" specification for the rear rod insures that the slide bolt is not near/touching the bottom of the slot on the center carburetor.
With the rod "one half hole" away from being able to be inserted into the hole in the rear carburetor lever, the rod is then pulled rearward to align with the hole. This doesn't open the throttle plates on the carburetor. Instead, it pulls the small "T" bolt in the primary carburetor slot up, off the bottom of the slot.
That clearance insures that the end carburetor isn't being used as a throttle idle stop for the center carburetor.

My guess is that possibly Howard was pulling the rod rearward instead of pushing it forward when making the adjustment?

I don't think I would try driving the car with the rods on the end carburetors disconnected. That could be a VERY exciting ride!

The rods that connect the end carburetors to the center carburetor don't have anything to do with opening the end carburetors on acceleration. The rods CLOSE the end carburetors when the throttle is released.

[Scott S.]

Re: 427/435 Fuel Pressure

Howie, If you and Betty Anne come to "Vettes in the Valley" all will be well!
Scott

[Howard P.]

Re: 427/435 Fuel Pressure

Thanks for all the comments.

I have adjusted the linkage exactly as per the tech bulletin and agree with the comments made however I will now check base gaskets, possible cracks in the manifold at the centre carb etc. Though I don't think I have a crack in the manifold as I have sprayed liberally carb cleaner all around the area and have a no change in idle quality or level.

The distributor was build by Dave Fiedler and as he is the recognized expert in this area I don't feel the problem now lies with the distributor or vacuum advance. I've spoken with Dave by phone and I have run the various system checks he suggested and all checks out OK, that is what brings me back to the fuel side of the system.

I do have a spare centre carb (a parts carb essentially), I'm going to give it a go over and install it to see if I note any change. The rear carb has some possible throttle plate issue as the idle changes upward significantly when the linkage adjustment is performed.

I'll keep you fellows posted on the outcomes

[Dick W.]

Re: 427/435 Fuel Pressure

Michael Hanson has given you some very good advice

[Domenic T.]

Re: 427/435 Fuel Pressure

Howard ,
One last thing from me.

Carb cleaner will not test for a crack in the exhaust crossover under the center carb.

That leak pushes exhaust in the plenum along with the center carb sucking it.

That leak would be under the center carb and you would need to look for visual clues such as it being dirty from exhaust in the plenum or just seeing the crack.

Michael,

Thanks for the note, what I was getting at was for him to drive the car with the end carbs inop so they couldn't open. Removing them and puting plates on is a better idea but more work.

DOM

[Howard P.]

Re: 427/435 Fuel Pressure

Update to my original thread. We have resolved the issue !

I had done extensive testing/checking of fuel, ignition etc and tried everything as suggested in replied threads and had come up totally stumped. Went to the great expense of replacing the TI module, "263" coil, Cap, Rotor, Wires, even swapped out the centre carb with a spare. Checked fuel pump, wrapped the lines to eliminate any heat issue etc etc. Always ended up with the same result being that the car would simply die after running about 20 minutes or getting up to normal operating temp. I had the distributor completely overhauled by Dave Fiedler, during the restoration process, and I even spoke to him and he verified the distributor tested out fine when he did the original work.


As I was becoming increasingly frustrated I sought out an old school mechanic who was a GM dealer mechanic from the early 70's on. After taking the car to him and explaining all that I had tried and failed with he told me he was almost positive that my car was suffering from spark scatter. He said to leave the car with him and he would work on it and get back to me. Later that day I received a call and told me the car was fixed. I went to pick-up the car and we went for a 1/2 hour drive and the car run superbly. Very crisp, smooth and no hesitation at all, and most importantly no dying out !


He preceded to tell me that just as he suspected the car had one of the worst cases of spark scatter he had ever witnessed. This is a condition unique to TI ignition systems and is caused by a faulty distributor module. I told him that Dave Fiedler has verified that it tested good, and the mechanic then told me that good on the bench doesn't necessarily mean good in the car as once subjected to heat all kinds of things happen, which is exactly what was taking place in my case. The remedy in my case was we swapped out the TI distributor with a stock Corvette points distributor and very neatly rigged the wiring so that unless the shrouding is removed no one would know. The other option of course is to replace the distributor module and re-install the TI distributor. The mechanic told me that the TI system when it works is excellent but they are problematic and expensive to repair. That I'm already well aware of as the distributor was totally rebuilt.

So far so good. We can now drive an enjoy our Vette. Ay this point I have no intention of reinstalling the TI unit.

[Duke W.]

Re: 427/435 Fuel Pressure

He preceded to tell me that just as he suspected the car had one of the worst cases of spark scatter he had ever witnessed. This is a condition unique to TI ignition systems and is caused by a faulty distributor module.

Are you referring to the externally mounted TI amplifier or the pickup coil inside the distributor?

You said you replaced the "TI module". Do you mean the external TI amplifier?

Duke

[Howard P.]

Re: 427/435 Fuel Pressure

Are you referring to the externally mounted TI amplifier or the pickup coil inside the distributor?

You said you replaced the "TI module". Do you mean the external TI amplifier?

Duke

Duke

Yes the pick up inside the distributor and yes I did previously replace the external TI amplifier module with the Lectrics unit.

Howie

[Michael B.]

Re: 427/435 Fuel Pressure

I'd like to throw out one last experience that I had with my TI gremlin. The wiring connector from the magnetic pickup is a plastic "T" configuration. It connects the White wire at the top of the T and the White/Green to the vertical part of the T. I wouldn't think there would be a polarity issue but my experience was that it did. Making sure these wires connect properly at that connector fixed an intermittent problem I was having.


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