Stumbling down the road
06-17-2018, 05:47 AM
#41
OK, it ran normal before the tune up right?
If you feel the ignition is good then think about the carburetor overhaul. Did you compare the old gaskets to the new gaskets to make sure all the holes are exactly the same. It sure will not be the first time something like this has happened, is there a rpm range that this condition starts or is it just when the engine is put under load.
If you feel the ignition is good then think about the carburetor overhaul. Did you compare the old gaskets to the new gaskets to make sure all the holes are exactly the same. It sure will not be the first time something like this has happened, is there a rpm range that this condition starts or is it just when the engine is put under load.
06-17-2018, 12:04 PM
#43
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As a mechanic, I'll say what I said several posts ago: getting this car on a DSO multimeter or old-school scope will point out your problem if it's ignition related. Simple as that. Let's say you put it on a scope and the pattern and distributor check out. Next step is a mechanical compression check and a leakdown check to verify engine mechanical integrity. You SHOULD install a vacuum gauge right now and tell us what you see. A steady needle at 15-21 is the goal. Ifthe needle is bouncing, you have mechanical issues............
06-17-2018, 01:25 PM
#44
Race Director
I am beginning to suspect a problem with adequate fuel delivery off idle.
Given that you have recently changed things on both the fuel pump and the carb it seems those are likely suspects. The car could be getting enough fuel to idle but not enough to accelerate and produce more power. I have seem that symptom from both weak fuel pumps and restricted carburetors.
I think you have received a couple good tips of things to check in posts 39 and 41.
Even a new fuel pump can have a bad lever/pin or diaphragm, allowing a small flow of fuel but not high volume, even if pressure is good.
A misplaced gasket in a carb can restrict fuel flow through the main jets, power valve or accelerator pump circuits.
That type of limited fuel delivery can produce the symptoms you describe. Ask me how I know.........
Good luck!
Given that you have recently changed things on both the fuel pump and the carb it seems those are likely suspects. The car could be getting enough fuel to idle but not enough to accelerate and produce more power. I have seem that symptom from both weak fuel pumps and restricted carburetors.
I think you have received a couple good tips of things to check in posts 39 and 41.
Even a new fuel pump can have a bad lever/pin or diaphragm, allowing a small flow of fuel but not high volume, even if pressure is good.
A misplaced gasket in a carb can restrict fuel flow through the main jets, power valve or accelerator pump circuits.
That type of limited fuel delivery can produce the symptoms you describe. Ask me how I know.........
Good luck!
06-17-2018, 05:13 PM
#45
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Update: 16:30pm Saturday. Put the Taylor 8mm plug wires back on. Car starts immediately. No pumping the pedal ... boom... starts as soon as you click the key. Starts immediately. Runs up and starts stumbling, misfiring as usual.
Checked with water spray on plugs and distributor in the dark... looking for cross-fire or arcing. Found nothing.
Tested around manifold and carb with carb cleaner to see if I could detect any kind of vacuum leak. Nothing. No change in RPMs.
Checked distributor wires... no shorts.
No smell of gas. Jets are delivering fuel. No flooding in carb.
Not sure where to go next. Tried everything I can think of. Suggestions?
Checked with water spray on plugs and distributor in the dark... looking for cross-fire or arcing. Found nothing.
Tested around manifold and carb with carb cleaner to see if I could detect any kind of vacuum leak. Nothing. No change in RPMs.
Checked distributor wires... no shorts.
No smell of gas. Jets are delivering fuel. No flooding in carb.
Not sure where to go next. Tried everything I can think of. Suggestions?
vacuum can (replaced)
no grounding failure on the secondary side of the coil & wires (with confirmation there is no external arcing).
no grounding of external wires on the coil primary side (the "-" terminal wire from the coil down to, and inside, the distributor is all good, and the breaker plate ground wire is good).
no vacuum leaks at the manifold (gas test).
fuel appears good (no obvious issues).
.
Do you have steady spark timing at a fixed rpm where it stumbles, at 3000 rpm? This can be tested with a timing light reading advance on the balancer, or with a scope.
.
The remaining ignition possibilities are:
one or more bad spark plugs (the idle test of rpm drop may pick this up, and lean or rich plugs will show a fuel problem, and it's easy to R&R with new plugs to eliminate the chance that one or more is cracked or fouled or have a nearly closed gap).
a bad electronic trigger (replace with points or a new trigger, or new distributor).
a very worn distributor with excessive lower bushing and shaft radial end play (that should show up as timing scatter at rpm with a timing light or on a scope, and requires a distributor rebuild or replacement).
.
Short of testing with an old fashioned timing light or scope, inspect and change the plugs, and if a friend has a spare distributor it may be worth the trouble to swap and test.
06-17-2018, 05:57 PM
#46
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He said it was missing at all speeds, now. Even idle. That's why I suggested a vacuum gauge and an oscilliscope.
06-17-2018, 06:58 PM
#48
My 2 cents we did the carb and fuel pump change ,
Cut the fuel line with a grinder the sediment in the line got into the jets and it acted same as your car.
Cut the fuel line with a grinder the sediment in the line got into the jets and it acted same as your car.
06-18-2018, 02:54 PM
#49
Stumbling down the road
I didn't notice a fuel filter suggestion. Just took the 2 out of mine after similar poor performance and found that I should have started my diagnostic journey with the easy inexpensive stuff!
06-18-2018, 03:05 PM
#50
Team Owner
GTO is spot on, you need to put a vacuum gauge (ported) on this engine and tell us what the readings are and what the needle is doing at idle.
06-18-2018, 07:20 PM
#51
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Manifold for the vacuum gauge would be best. Thanks for the support, Sir!
06-18-2018, 07:21 PM
#52
06-18-2018, 07:52 PM
#53
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2020 Corvette of the Year Finalist (appearance mods)
C1 of Year Finalist (appearance mods) 2019
have you done a compression check on all cylinders? 
.
.
Last edited by Kerrmudgeon; 06-18-2018 at 07:53 PM.
06-18-2018, 08:39 PM
#54
Team Owner
06-18-2018, 11:28 PM
#55
Le Mans Master
Thread Starter
OK. Monday evening...
Recap - Car had a leaking fuel pump. Changed the fuel pump... car ran great... then after a couple of weeks developed a growing misfire and noticeable stumble while under load accelerating only.
Changed to another new fuel pump. Rebuilt the carb. No changes. No dirt in the carb or blockages. Car fires immediately and idled perfectly.
But misfire and stumble at load persisted.
Checked fuel/air mix... Checked and set fuel bowl level. O.K. Steady vacuum at carb at 17 lbs. No bouncing.
New cap, rotor, inspected wires in carb. O.K.
New plug wires and plugs. Checked again - O.K.
Noticed some oil on #1 and #3. And blue smoke on that side.
Pulled off valve covers today. Will install new umbrella valve seals and
and O rings and do a leak down test this week.
Burnt valve? Cracked ring? Don't know at this point. Hoping it's just the seals are bad and oil is fouling one or more plugs.
That's all I got.
Recap - Car had a leaking fuel pump. Changed the fuel pump... car ran great... then after a couple of weeks developed a growing misfire and noticeable stumble while under load accelerating only.
Changed to another new fuel pump. Rebuilt the carb. No changes. No dirt in the carb or blockages. Car fires immediately and idled perfectly.
But misfire and stumble at load persisted.
Checked fuel/air mix... Checked and set fuel bowl level. O.K. Steady vacuum at carb at 17 lbs. No bouncing.
New cap, rotor, inspected wires in carb. O.K.
New plug wires and plugs. Checked again - O.K.
Noticed some oil on #1 and #3. And blue smoke on that side.
Pulled off valve covers today. Will install new umbrella valve seals and
and O rings and do a leak down test this week.
Burnt valve? Cracked ring? Don't know at this point. Hoping it's just the seals are bad and oil is fouling one or more plugs.
That's all I got.
06-19-2018, 09:14 AM
#56
Team Owner
Quick Tech: How to Read a Vacuum Gauge to Pinpoint Engine Problems
http://www.onallcylinders.com/2015/0...gine-problems/
http://www.onallcylinders.com/2015/0...gine-problems/
07-18-2018, 12:44 AM
#57
Le Mans Master
Thread Starter
Sorry to leave you guys hanging for so many weeks. We finally solved this thing. Took the car to an amazing mechanic here in the Bay Area. His name is Rob Muhleman... Think the red Corvette on the Fast and Furious movies.
He put the car on a scope and found one of the 6 month old plug wires arcing against the headers. Even he couldn't find it in the dark. Also found two of the exhaust valves a little tight... resulting in a slight loss of compression in both.
And... the distributor.... in need of replacement. Simply worn out.
Pick the car up tomorrow.
He put the car on a scope and found one of the 6 month old plug wires arcing against the headers. Even he couldn't find it in the dark. Also found two of the exhaust valves a little tight... resulting in a slight loss of compression in both.
And... the distributor.... in need of replacement. Simply worn out.
Pick the car up tomorrow.