perplexing problem

2415 Views 24 Replies 12 Participants Last post by baz, Feb 11, 2004

BQS4

Discussion Starter · Feb 10, 2004

Need help with a problem.
Otto has offered advice on this, and sorry Otto, it has not cured the problem. I tried a few different types of fuel filters, even ran without a filter and the problem is still there. It's not the carb, as I have taken a known, good working weber from another car and the problem is still there. Same thing with the fuel pump. The problem is, when I accellerate moderate to hard, the car acts like it has hit a wall (imaginary). The motor wants to rev, and I know these motors will go beyond this point, but, the motor bogs. It's a stock 74 low-comp (7.6:1) with points, no Pertronix, as I have tried the Pertronix with it, and still the problem exsists, and a weber 32/36. The car starts, and idles like a champ, the best running one I have ever had, it's just under hard accelleration, it wants to bog. Normal accelleration is marvelous! It scoots a lot better than most cars I have ever had, but, not when you accellerate hard. It does this from a standing start, and running at speed, although it's not as bad, when at speed, but, it's still there. Any ideas?

1 - 20 of 25 Posts

garrymc

· #2 · Feb 10, 2004

Have you tested the vacuum advance/retard can on the distributor?

PROPEL

· #3 · Feb 10, 2004

Also check the throttle rod bushing against the firewall. Replace with OGTS poly bushing if needed.

madhatterpdc

· #4 · Feb 10, 2004

I'm sure that you have already tried these ideas, but...

You may want to check and see if there are any vacuum leaks. Could the timing chain be off a tooth or maybe stretched? Could a cam lobe be wiped out? Clogged exhaust? Clogged airfilter?

bear

· #5 · Feb 10, 2004

I'm sure you've checked a lot of things out on this, but from your description of the symptoms, there are only two things that will cause the car to act like that. Spark and fuel. It has been suggested you check the vacuum on the distributor, make sure you check both sides of the vaccuum module and that the lines are hooked up right. For the fuel problem, it sounds like the accelerator pump is not putting enough fuel in the venturi or the secondary is opening too fast and there is no fuel flow through the secondary fuel circuit, even though you have checked the carb out on another car, the carb has to be tuned for the application it's on. A vague response to be sure, but it's hard to long distance fix a problem like yours.

Ron

BQS4

Discussion Starter · #6 · Feb 10, 2004

Namba;
I see what you're saying about the fuel delivery, but, that's why I tried the other carb. I do know that the weber has to be tuned to each car, this I have done on many occasions, but, the exact same problem occured with both carbs. That's why I also referenced Otto in my first post. The motor is getting the gas. When it bogs down, it's still running, and the more you accellerate, it still gets fuel, but, it's like some tremendous hand holding back the car and it sounds like the motor is sucking itself inside out.

PROPEL

· #7 · Feb 10, 2004

Slipping clutch?

baz

· #8 · Feb 10, 2004

is the exaust old ? could a baffle have collapsed
some back presure is ok and would allow it to run ok up to a point

tekenaar

· #9 · Feb 10, 2004 (Edited)

Re: perplexing problem . . . Weber or Solex?

BQS4 said:
Need help with a problem.
Otto has offered advice on this, and sorry Otto, it has not cured the problem. I tried a few different types of fuel filters, even ran without a filter and the problem is still there. It's not the carb, as I have taken a known, good working weber from another car and the problem is still there. Same thing with the fuel pump. The problem is, when I accellerate moderate to hard, the car acts like it has hit a wall (imaginary). The motor wants to rev, and I know these motors will go beyond this point, but, the motor bogs. It's a stock 74 low-comp (7.6:1) with points, no Pertronix, as I have tried the Pertronix with it, and still the problem exsists, and a weber 32/36. The car starts, and idles like a champ, the best running one I have ever had, it's just under hard accelleration, it wants to bog. Normal accelleration is marvelous! It scoots a lot better than most cars I have ever had, but, not when you accellerate hard. It does this from a standing start, and running at speed, although it's not as bad, when at speed, but, it's still there. Any ideas?

Not clear from your original post, Weber or Solex? IF Solex, mechanical or vacuum secondary?

"Does the same thing with another carb" tends to eliminate the carb, though NOT entirely, but we'll get back to that.

You've also eliminated exhaust restriction, right?

All that's left is initial ignition timing and the centrifugal and vacuum advance curve, so let's discuss that for a moment. You say gradual acceleration isn't a problem, which tends to eliminate the centrifugal advance in the distributor.

Remove the distributor cap, twist the rotor in a clockwise direction and check for freedom of movement and "spring back" to static position . . . this checks the centrifugal action of the distributor. You should have some resistance (from springs) in the clockwise direction, but it should snap back to static position quickly when you let go of rotor.

All that's left on the ignition then is the vacuum controlled advance/retard part of the curve. With engine idling, disconnect the retard (small line) and plug it. Does the idle speed increase slightly? Test car on the road with just the vacuum advance connected (and retard line plugged). Same problem, right?

Disconnect advance side vacuum line and plug. Repeat road test with both vacuum lines disconnected and plugged. It should be a little more sluggish under gradual acceleration as well now . . . don't have the added vacuum advance in the distributor now.

Hook everything back up and try another coil. I suspect you won't discover any noticeble changes in your "rapid acceleration" problem, unless you're "overwhelming" the spark in the cylinder when dumping extra fuel into it suddenly . . . "extra fuel" SHOULD be happening, "overwhelming the spark" SHOULD NOT (weak spark, bad coil or plug wires).

I still think this is a fuel delivery problem of some sort. Are you sure the choke is fully open when the engine is at operating temp? Have you run the carb without any air cleaner whatsoever and with choke held open mechanically? Tell me results after you try ALL these things.

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Gary

· #10 · Feb 10, 2004

Fuel Tank sock??

tekenaar

· #11 · Feb 10, 2004

weak spark . . .

Just thought of one other thing . . . two, actually. Using points and original wiring to coil, right?

Is the small ground strap on the distributor breaker plate intact?

What is the voltage on the + side of the coil when the engine is running?

Old Hippie

· #12 · Feb 10, 2004

Hmm...sounds like timing/vacume to me. I had a like problem on a car last year. Recheck the timing and see how much advance you get with vac applied to the dist. That's the least problem. At the worst, the timing belt/chain may be off the mark. My 2c's

BQS4

Discussion Starter · #13 · Feb 10, 2004

Otto, et-al;
I'll look into each of these answers. It just a shame, as the car runs so good, and as you stated Otto, "gradual" accelleration is great, it's the hard accelleration where it comes in.

BQS4

Discussion Starter · #14 · Feb 10, 2004

BTW, it has a weber 32/36, and I swapped another weber onto itand as stated that each weber has to be tuned to the individual car, there should have been a difference, but, there was none.

bear

· #15 · Feb 10, 2004

Now there's a thought, Otto, a piece in the exhuast acting like a check valve. Nice easy acceleration and no problem. Rapid accel and the extra pressure closes the exhaust flow, similar to a deteriorated brake line. The only way to check it is to remove the muffler, or at least disconnect it. HTH.

Ron

BQS4

Discussion Starter · #16 · Feb 11, 2004

Well, I hate to have to shoot down the exhaust theory, but, the exhaust is new from the manifold back to the tip, with all new parts. This problem was happening before I got the exhaust redone.

nobody

· #17 · Feb 11, 2004

So far you have eliminated quite a bit. I would have to look at restrictions in the fuel line or tank, starving whatever carb is on it. Vacuum leak or bad advance unit is what I suspect. I would have to replace the advance unit and block off the brake vacuum line. You could have a small leak in either one or a bad line to either one.

Buck

· #18 · Feb 11, 2004

I had a very similar problem with a 67 Pontiac LeMans. It would idle great, cruise no problem, but when I punched the gas it would bog down. It turned out to be part of my fuel line would collapse under hard accelleration. Replaced the fuel line and all was good.

tekenaar

· #19 · Feb 11, 2004

carb jetting

BQS4 said:
BTW, it has a weber 32/36, and I swapped another weber onto itand as stated that each weber has to be tuned to the individual car, there should have been a difference, but, there was none.

OK Gene, how close are your mains, airs and idles to these:

Primary (32): Main 135; Air 165; Idle 50
Secondary (36): Main 140; Air 160; Idle 55

Check them regardless and let me know what you have in the carb now.

How does the carb from the "other car" run on the "other car" . . . ie do you have the same problem (bogs on hard acceleration) with that car?

kwilford

· #20 · Feb 11, 2004

I am just going to leave this thread under "Open Discussions" until we get to the answer...

Another thought. Have you checked your cam lobe lift? I had a similar problem on my GT a number of years ago. OK, just before I began this 18 year restoration. It would rev fine in neutral, and not bad in first and second gear. But as as soon as it was placed under "load", as in third or fourth gear, it would just power out, unless I kind of "snuck" it up to speed. After replacing all of the ignition system, and rebuilding the Weber about three times, I went to check the cam timing. Hmm, how come those two rocker arms aren't moving much? Well, I'll be darned. The intake lobes on the cam are all worn down!!! So it was like not being able to get "full throttle", since the barely-opening intake valves were severely restricting air/fuel flow.

Or....

1 - 20 of 25 Posts

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