Hesitates heavily at cruising speed
06-29-2010, 05:21 PM
#1
1st Gear Member
Thread Starter
Join Date: Sep 2009
Location: Arizona
Posts: 74
Hesitates heavily at cruising speed
Hello!
I have been trying to locate the problem for a long time now. My 1971 fastback hesitates and stumbles heavily during cruising speed when the secondaries are open. Fuel seems to be running back to the tank at idle.
This is what I have done:
1. New fuel pump
2. New fuel filter
2. Distributor checked and ok (tested)
3. Carburetor checked and ok (rebuilt)
4. Timing ok
5. Spark plug/gap ok (greyish)
6. Vacuum checked and ok
Thinking about a blockage in the gas tank. Should I invest in a new fuel sending kit unit? Spark wires are in good condition.
Any ideas? Help wanted!
Thomas
I have been trying to locate the problem for a long time now. My 1971 fastback hesitates and stumbles heavily during cruising speed when the secondaries are open. Fuel seems to be running back to the tank at idle.
This is what I have done:
1. New fuel pump
2. New fuel filter
2. Distributor checked and ok (tested)
3. Carburetor checked and ok (rebuilt)
4. Timing ok
5. Spark plug/gap ok (greyish)
6. Vacuum checked and ok
Thinking about a blockage in the gas tank. Should I invest in a new fuel sending kit unit? Spark wires are in good condition.
Any ideas? Help wanted!
Thomas
06-29-2010, 06:54 PM
#2
6th Gear Member
Join Date: Aug 2007
Location: Elk Grove, CA
Posts: 5,896
Question 1) why are your secondaries open while cruising? They shouldn't be, unless you have mechanical secondaries and have to lay into the throttle to keep it at cruising speed.
What kind of carb are you running? Sounds like it's running lean. The grey plugs support that. Normal plug color is a light tan.
What kind of carb are you running? Sounds like it's running lean. The grey plugs support that. Normal plug color is a light tan.
06-29-2010, 07:15 PM
#3
Moderator
Join Date: Dec 2005
Location: Hills of California
Posts: 5,208
a fuel flow test would be good.
Remove the fuel line from the carb and place it in a glass jar. Crank the engine over a few times maybe maybe a 2 to 3 second burst with the starter. Is the glass full?
-Gun
Remove the fuel line from the carb and place it in a glass jar. Crank the engine over a few times maybe maybe a 2 to 3 second burst with the starter. Is the glass full?
-Gun
06-29-2010, 07:45 PM
#4
2nd Gear Member
Join Date: Sep 2008
Location: AL
Posts: 413
Question 1) why are your secondaries open while cruising? They shouldn't be, unless you have mechanical secondaries and have to lay into the throttle to keep it at cruising speed.
What kind of carb are you running? Sounds like it's running lean. The grey plugs support that. Normal plug color is a light tan.
What kind of carb are you running? Sounds like it's running lean. The grey plugs support that. Normal plug color is a light tan.
how are the plug wires?
06-30-2010, 02:53 AM
#5
1st Gear Member
Thread Starter
Join Date: Sep 2009
Location: Arizona
Posts: 74
I have a Holley 4160 rebuild to a 4150. It has new gaskets, spring, jets, cam etc.
I also have a new ignition coil.
Plug wires are in good condition.
The hesitation only appears when I push the pedal to normal speed. It appears after a ****** and I always thought that was the secondaries. No problem at acceleration and very slow speed.
Theres fuel coming, but I will will try a fuel pressure gauge. I can se, in the fuel filter, the fuel running back during idle.
I also have a new ignition coil.
Plug wires are in good condition.
The hesitation only appears when I push the pedal to normal speed. It appears after a ****** and I always thought that was the secondaries. No problem at acceleration and very slow speed.
Theres fuel coming, but I will will try a fuel pressure gauge. I can se, in the fuel filter, the fuel running back during idle.
06-30-2010, 05:46 AM
#6
6th Gear Member
Join Date: Oct 2006
Location: Cork, Ireland
Posts: 6,417
Hiya,
just as the other's have said
If you converted from 4160, to 4150 I guess you just removed metering plate and added metering block. right?
a) secondaries must not be open at cruise or slight accelleration. unless you are using a double pumper carburetor and linkage is 1:1
b) the fuel pump should avoid gas flowing backwards. I mean if you have 6psi of fuel at carb, how can it possibly flow backwards. let's say the gas tank is not ventilated and pulling fuel out of it causes vacuum. then the vacuum get so high that pump can't overcome it and the car will stall. the same with the fuel pump conneted the wrong way around. carb empty->car stalls. What are you seeing> is it bubbles going towards the tank?
c) from your last update. do I understand correctly that during cruise it's fine, just when you give it a 'dab' on the accelleratior it stumbles?
if that's the case then either the mixture screws are too lean or the accellerator pump is not working (or misadjusted).
when you remove airfilter and look down the carb (engine off) and actuate the accellerator (throttle plate), do you see petrol spraying in both front bowls? nice spray, no dribble? and does the spray start _as soon as_ you actuate the throttle, no play in there?
just as the other's have said
If you converted from 4160, to 4150 I guess you just removed metering plate and added metering block. right?
a) secondaries must not be open at cruise or slight accelleration. unless you are using a double pumper carburetor and linkage is 1:1
b) the fuel pump should avoid gas flowing backwards. I mean if you have 6psi of fuel at carb, how can it possibly flow backwards. let's say the gas tank is not ventilated and pulling fuel out of it causes vacuum. then the vacuum get so high that pump can't overcome it and the car will stall. the same with the fuel pump conneted the wrong way around. carb empty->car stalls. What are you seeing> is it bubbles going towards the tank?
c) from your last update. do I understand correctly that during cruise it's fine, just when you give it a 'dab' on the accelleratior it stumbles?
if that's the case then either the mixture screws are too lean or the accellerator pump is not working (or misadjusted).
when you remove airfilter and look down the carb (engine off) and actuate the accellerator (throttle plate), do you see petrol spraying in both front bowls? nice spray, no dribble? and does the spray start _as soon as_ you actuate the throttle, no play in there?
06-30-2010, 07:13 AM
#7
1st Gear Member
Thread Starter
Join Date: Sep 2009
Location: Arizona
Posts: 74
First of all thanks for the help so far. I really appreciate it and I really want to solve this problem that started 1,5 years ago. It was not there when i bought the car and after a broken leg accident earlier this year itīs the first time in months that I can take the car out for a spin.
a) Yes, added a metering block and increased jets two sizes (69).
b) During idle the fuel decreases in the filter and sometimes there seems to be a lot of bubbling. I plugged the vapour canister sometime ago and maybe thats the problem, but I unplugged it and connected it to the air filter yesterday without results (actually was even worse) I had this problem with the old fuel pump so I donīt thinks itīs connected wrong.
c) Only stumbling during cruising and disapears if I push the pedal or going at really really slow speed. Petrol is spraying in both front bowls.
I will get the psi status later today. Maybe the gas cap isnīt vented enough?
a) Yes, added a metering block and increased jets two sizes (69).
b) During idle the fuel decreases in the filter and sometimes there seems to be a lot of bubbling. I plugged the vapour canister sometime ago and maybe thats the problem, but I unplugged it and connected it to the air filter yesterday without results (actually was even worse) I had this problem with the old fuel pump so I donīt thinks itīs connected wrong.
c) Only stumbling during cruising and disapears if I push the pedal or going at really really slow speed. Petrol is spraying in both front bowls.
I will get the psi status later today. Maybe the gas cap isnīt vented enough?
06-30-2010, 12:50 PM
#10
1st Gear Member
Thread Starter
Join Date: Sep 2009
Location: Arizona
Posts: 74
Itīs a 351W and I believe itīs an Edelbrock intake (Printed 351W on it). Nothing is original
No vaccum leaks because there is only new stuff in the carburetor. Tested vacuum last summer and it was 15 during idle. However, read somewhere that the steel ball in the passage that actuates the diaphrgam can get stuck and cause problems?
Idle is great and fuel level is correctly set in the carburetor..
Checked the fuel pressure and it was about 4 when I turned the key. Strangely, it was fuel in the filter today. Delayed effect after connecting to the vapour canister yesterday?
Changing the accelerator pump diaphragm tomorrow because thats the only thing that wasnt replaced.
What do you think?
No vaccum leaks because there is only new stuff in the carburetor. Tested vacuum last summer and it was 15 during idle. However, read somewhere that the steel ball in the passage that actuates the diaphrgam can get stuck and cause problems?
Idle is great and fuel level is correctly set in the carburetor..
Checked the fuel pressure and it was about 4 when I turned the key. Strangely, it was fuel in the filter today. Delayed effect after connecting to the vapour canister yesterday?
Changing the accelerator pump diaphragm tomorrow because thats the only thing that wasnt replaced.
What do you think?
