427/435 Fuel Pressure

[Howard P.]

Re: 427/435 Fuel Pressure

Update

Well I have taken each carb apart, cleaned/replaced all fuel filters, put a # 28 squirter in the centre carb (was #21), blown out the fuel blocks etc and re-assembled. Set mixture screws to 1-1/2 turns and replaced base gaskets on all three carbs. Car fired right up and once off choke held 900 RPM with 6 lbs of fuel pressure and temp gauge showing 200 degrees. Raytech indicated 191 at upper hose thermostat housing. Revs nice and crisp with excellent response.

After approx 10 minutes the car simply stumbles and dies. If I jockey the throttle it will stay running and and clears out for about 20 seconds then its starts all over again ??? I'm stumped, I have Pontiac Tri-Powers (Rochesters), and have no trouble setting them up at all but this is off the scale.

The car does seem to fire right back up and will idle properly for a minute or two before going through the same cycle again. I have loosened the fuel cap (which is vented), and it made no difference.

One thing to note ... the rear carb is pulling some air at idle, while the front is not. If I put my hand over the rear air horn the car stumbles badly. Disconnecting the linkage make no difference. I will next review the Vacuum canister on the rear carb and then explore the TI side of things.

If there are additional suggestions out there, please feel free to fire away.

Thanks !

[Ronald L.]

Re: 427/435 Fuel Pressure

These tri power set ups are a bit harder to diagnose and set up properly versus a 4 barrel, but you should spray some carb cleaner at your gasket areas carb to intake and intake to block/heads to see if you have vac leaks there. Then verify you have the three set up according to specs.

If it starts cleanly when cold, the you have something moving causing a leak when hot or your ignition system is failing under heat stress. That means you need to start through that entire system.
Coil original?
How old is the TI unit?
Plug Wires?

What do the plugs look like - any differences?

[Howard P.]

Re: 427/435 Fuel Pressure

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

[Domenic T.]

Re: 427/435 Fuel Pressure

Howard,
Sucking air at the rear carb doesn't sound normal to me even with a post that says they do.

I think you are on to something there.

In the winter I would take the end carbs off my 409 and put plates on the manifold to cap it off.

If the front or back pulled air on my set up it would not run right at all unless it was at a much higher RPM.

What Ron said about heat having something to do with it makes a lot of sence.

How about your intake manifold? are the bolts tight?

One other thing that I had happen was a crack in the exhaust cross over that is under the carb. It would open with heat and the car ran as you described.

That one took a long long long time for me to find. I was a kid then so I blocked the exhaust cross over and things were good again till winter.

DOM

[Timothy B.]

Re: 427/435 Fuel Pressure

Howard,

Are these Holley carburetors we are talking about, you mention Rochester? From your description IMO, the trouble is not in the ignition but in the carburetor.

The reason the car stumbles when covering the rear carb is because you are stopping the air flow and the internal leakage is creating a low pressure signal inside the carburetor and drawing fuel now that you have cut off the air by covering the carburetor top.

My suggestion would be to make a block off plate and bolt it to the intake for the front/rear carbs so only the main carburetor is on the engine. Make sure you plug vacuum hoses going to end carbs with golf tee and run the engine and see if the problem occures again, this will tell you where to start. I suspect there is leakage either on the metering plates on the end carbs or warped surface on the primary (middle) carburetor.

[Ronald L.]

Re: 427/435 Fuel Pressure

Howard, Tim and Dom have outlined tests where you'll probably find the problem.

If you get to replacing the coil any match from the corner autostore will be fine ot know you have a good one, usually the have a liftime guarantee and are cheap diagnostic tools.

[John H.]

Re: 427/435 Fuel Pressure

Here's the procedure from Chevrolet Service News (not shown in either Shop Manual) for adjusting the front and rear 3x2 carburetor closing rods; it will leave the rear carburetor throttle lever open by 1/2 the diameter of the rod where it enters the hole in the lever, with the front carburetor throttle plate fully closed.

I have no idea if this improves operation or not - just posting it for information.

[Howard P.]

Re: 427/435 Fuel Pressure

Thanks for all the information I will keep testing. I also spoke with Dave Fiedler yesterday who indicates that the pick-up coil in the distributor may have gone bad as when that happens symptoms very similar to mine take place.

I'll repost what I learn from all of this

[Joe C.]

Re: 427/435 Fuel Pressure

Thanks for all the information I will keep testing. I also spoke with Dave Fiedler yesterday who indicates that the pick-up coil in the distributor may have gone bad as when that happens symptoms very similar to mine take place.

I'll repost what I learn from all of this

Long time ago I owned a Datsun 280Z which would die when it got hot. After it cooled off for about 1/2 hour it would start and run again for about another 10 minutes and then die again. I could repeat this process all day long, and it was a hard one to diagnose because it was intermittent.

I put an ohmmeter across the pickup coil leads when the problem presented, and the needle would fluctuate wildly. After 1/2 hour, I put the meter across the leads and the needle was steady, and, of course, at that point I knew that she'd fire up..........which it did.

Although electronic problems sometimes begin as intermittent conditions before complete failure occurs, I don't think that this is your problem because it restarts and runs immediately after it stalls. It also keeps running if you give it fuel by using the accel pump. If it were an overheating/failing electronic part, then I don't think it would start again right away after a stall. It would need some time to cool off first.

[Carl N.]

Re: 427/435 Fuel Pressure

I agree with Dom:

"One other thing that I had happen was a crack in the exhaust cross over that is under the carb. It would open with heat and the car ran as you described."

I chased this problem for about a year on a brand "X" with 3 2's and the problem turned out to be the intake mainifold had a crack which would pull vacuum.

If you do not find the problem in the carbs or TI system, you may have to pull the intake and check very close for hairline cracks.

Good Luck,
Carl

[Timothy B.]

Re: 427/435 Fuel Pressure

After reading the procedure posted in John's post, I bet the rear carburetor throttle rod is one half hole short to keep the linkage from slaming the throttle blades shut against the throttle bores and sticking. This would keep the vacuum from opening them.

It probably balances out with the front rod but I can't tell from the picture. If it does, the throttle blades probably end up light tight but vacuum will pull them open.

[Howard P.]

Re: 427/435 Fuel Pressure

Update

Well, here's where we are at.

Checked the "263" coil with my Ohm Meter and it is testing within spec. I had a spare Lectric Ltd module for the TI Amp so I installed it and made certain the ground was good. Lastly, I reviewed the Tri-Power linkage set-up proceedure as provided by John Hinkley and set this linkage up.

Prior to setting up the linkage I ran the car for close to 20 minutes and it did not stall out when idling, however attempting to dive it caused a stall condition. It would immediately fire back up.

This is where things get interesting .... following the Tri-Power set-up tech bulletin I was certain the car had fully warmed up and the choke flap was straight up and down. I disconnected both front and rear linkage rods and adjusted the car idle to 850 RPM. It idled well at that level on the centre carb. I then set-up the rear carb rod per the bulletin and then did likewise with the front. The engine then idles at 1500 RPM which is way to high. It is crisp and responsive with no stumble but any attempt to lower the idle down cause the issue of no idle to take place again. Also odd, disconnecting front and rear linkages again cause a no idle condition to take place again and now the engine is at operating temp it will not idle at the former 850 RPM, in fact it won't idle at less than 1600 RPM !

I sprayed carb cleaner at the car base and at the base to body point and no difference was noted.

I think I'm safe to say I can rule out my TI system as being the problem and I am quite certain it is carb related. I am stumped though as these are rebuilt Holleys. I can only suspect the centre carb metering block or base plate is the problem.

Any opinions or suggestions ?

[Domenic T.]

Re: 427/435 Fuel Pressure

Howard,
Maybe with your center carb off you can check for the crack I was talking about in the exhaust crossover.

My carbs are not on and I will look and see if it can be done by eye or with a bore scope.

DOM

[Michael H.]

Re: 427/435 Fuel Pressure

This is where things get interesting .... following the Tri-Power set-up tech bulletin I was certain the car had fully warmed up and the choke flap was straight up and down. I disconnected both front and rear linkage rods and adjusted the car idle to 850 RPM. It idled well at that level on the centre carb. I then set-up the rear carb rod per the bulletin and then did likewise with the front. The engine then idles at 1500 RPM which is way to high.

Howard,

Something doesn't sound right. If the linkage is set properly on the end carburetors, I don't think the idle speed would/should change when the rods are connected. Sounds like one or both secondary carburetors are not closing down completely when the rods are connected.
If the idle speed picked up when the rods are attached, that means that one or both of the end carburetors are being held slightly open at idle.
Are you sure you have the procedure right on this adjustment?

[Joe C.]

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.