Engine stumbles/hesitation on heavy acceleration
Thread Starter
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
Hesitation on heavy acceleration qjet airdoor too tight?
I just noticed this yesterday, from 1st gear to 3rd when I hit around 3800 - 4000 RPM, rpm needle stops momentarily and moves up to higher rpm with car shuddering. It's like light bucking or slight hesitation and go (almost like transmission slipping feel). RPM needle and engine both are not smooth at this time. This is happening on heavy acceleration. It happens from stop and go climbing to 3800 - 4000 rpm and rolling start. On WOT, it shudders also than rpm picks up with little sputtering.
Inline fuel filter is new.
I thought fuel filter was clogged so I checked the filter and it's fine.
Until I hit 3800 RPM car runs fine and shifts fine.
Checked timing and it's 14/35. Maybe I thought it was too much vacuum advance and ran it without the vacuum advance and still same result.
Could this also be related to fuel starvation because I'm thinking ignition problem will show at any heavy acceleration, not just near 4K rpm.
I'm hoping it's not transmission (M21) problem.
Any ideas?
Thanks.
Inline fuel filter is new.
I thought fuel filter was clogged so I checked the filter and it's fine.
Until I hit 3800 RPM car runs fine and shifts fine.
Checked timing and it's 14/35. Maybe I thought it was too much vacuum advance and ran it without the vacuum advance and still same result.
Could this also be related to fuel starvation because I'm thinking ignition problem will show at any heavy acceleration, not just near 4K rpm.
I'm hoping it's not transmission (M21) problem.
Any ideas?
Thanks.
Last edited by jackwabbit703; May 26, 2013 at 12:44 AM.
Thread Starter
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
I forgot to add that, I've yet to play with the airdoor tension spring. Right now, spring tension on the airdoor is very tight. It takes little effort to push the airdoor open.
Perhaps, it's too tight for rushing air to overcome the airdoor spring? Could this cause momentary shudder/hesitation/slight bog on heavy acceleration and WOT?
Maybe I should try loosening the tension on the airdoor spring?
Perhaps, it's too tight for rushing air to overcome the airdoor spring? Could this cause momentary shudder/hesitation/slight bog on heavy acceleration and WOT?
Maybe I should try loosening the tension on the airdoor spring?
Burning Brakes
Joined: Apr 2013
Posts: 926
Likes: 2
From: Castle Rock CO
In 1974 there were 3 engine options so not knowing what yours is hard to research.
Could be just the carb is dirty and needs a good cleaning.
Have you done a tune up lately in may past experience the points needed adjustment about every 10,000 miles on a dwell meter and replaced just about as often.
One of the things that happened on my 1972 El Camino was the distributor got worn out went bad and I had to replace the whole unit at about 90,000 miles. After I installed a new distributor unit the hesitation went away. It was so sloppy I could not get the dwell to set right on it because it was inconsitant.
Could be just the carb is dirty and needs a good cleaning.
Have you done a tune up lately in may past experience the points needed adjustment about every 10,000 miles on a dwell meter and replaced just about as often.
One of the things that happened on my 1972 El Camino was the distributor got worn out went bad and I had to replace the whole unit at about 90,000 miles. After I installed a new distributor unit the hesitation went away. It was so sloppy I could not get the dwell to set right on it because it was inconsitant.
Last edited by MakoJoe; May 26, 2013 at 01:47 AM.
Thread Starter
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
Thanks for the reply. Spent 2 hours tinkering with the air valve springs and test runs.
Current default spring tension was 3/4 turns clockwise from where there's no spring tension.
I increased clockwise 1/8 turn from current 3/4 turn and it still bogged and stumbled around 4K. Went another 1/8 turn clockwise and it still bogged and spring tension was so tight from there, the car ran like crap and very stiff.
Went other way 2/4 turn from default 3/4 turns, still the same.
I went far as 1/4 turn from where spring has no tension and at 1/4 turn, air door was very easy to open but closed properly without sticking. Anything under this, air door got stuck in down position.
This is where I had no issue with bogging at 4K RPM or over.
I'm having little difficulty understanding why there's minimal spring tension, my car does not stumble or sputter at 4K. Isn't my air valve opening too early if spring is too loose and leaning the fuel too early?
This carb is commercial rebuilt jet p stage 2 800 cfm qjet. I bought it way before I found out Lars do rebuilds.
Bottom line is I was sweating a little thinking my tranny was slipping because it felt like it.
