Slight 'bucking' condition
Thread Starter
Pro
Joined: Nov 2001
Posts: 534
Likes: 6
From: Cave Creek AZ
Slight 'bucking' condition
On my 72 LT-1, when holding steady RPMs, I do not get smooth power, I can feel a slight bucking/hesitation. It is most noticable when cruising at 35-4000 rpm. I do not notice the engine itself hesitating or running poorly, it seems the car itself is slightly bucking. If I accelerate, the condition is gone and the car just goes. This occurs when the car is fully warmed up.
FYI - in the last 3 months - new front suspension & steering box, carb rebuilt, new MSD distributor/coil/wires, 170 t-stat, intake/valve cover/oil pan gaskets, all new rubber hoses/belts, added extra in-line fuel filter and had the fuel tank drained and cleaned. Prior to buying the car the rear suspension was all new incl driveshaft. Fuel pump is newer though not sure how old.
FYI - in the last 3 months - new front suspension & steering box, carb rebuilt, new MSD distributor/coil/wires, 170 t-stat, intake/valve cover/oil pan gaskets, all new rubber hoses/belts, added extra in-line fuel filter and had the fuel tank drained and cleaned. Prior to buying the car the rear suspension was all new incl driveshaft. Fuel pump is newer though not sure how old.
Burning Brakes
Joined: Jul 2004
Posts: 1,235
Likes: 0
From: Sick of these TOOLS & AZZHATS in DC
Carb running too lean will cause it to surge at cruise speeds. I'd check that first. If it's an automatic tranny or converter might be slipping, but you'd probably have other "symptoms" too if that were the case.
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
oh oh! I know! I know!! 
:
Lars has warned on many occasions that too much timing will cause that symptom , ie "bucking" at cruise. It is your vacuum advance that is making total timing excessive. On acceleration, no vac advance, but at cruise you are probably getting total timing of 55 degs or more. As a test, try disconnecting your vac advance. If symptoms persists then I am full of sheet and you move on to the next idea!
If symptom goes away consider replacing your vac can with a lesser amount of advance and/or retarding your timing.
:Lars has warned on many occasions that too much timing will cause that symptom , ie "bucking" at cruise. It is your vacuum advance that is making total timing excessive. On acceleration, no vac advance, but at cruise you are probably getting total timing of 55 degs or more. As a test, try disconnecting your vac advance. If symptoms persists then I am full of sheet and you move on to the next idea!
If symptom goes away consider replacing your vac can with a lesser amount of advance and/or retarding your timing.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
I had the same type of problem and it was excessive timing. I even have the 10 degree advance unit recommended by Lars. When I tried to set the car up with 36-38 degrees total mechanical advance it would feel like it was hesitating. After some experimenting I found that it liked 32-34 degrees advance depending on the weather. The car also felt stronger at cruise. Depending on your combo you may have to experiment until you find the sweet spot. Also, don't overlook fuel delivery. Check the sock on the pickup in the gas tank and check your fuel filters. Good luck.
Thread Starter
Pro
Joined: Nov 2001
Posts: 534
Likes: 6
From: Cave Creek AZ
Well I removed the vacuum advance from the distributor and Voila, no hesitating. Now the trick is to figure out how to make adjustments to the timing/vacuum.
Problem #1 - either my timing light sucks (doubtful) or the timing mark on my harmonic balancer is not aligned properly - can't get a reading on the true timing.
Problem #2 - can't adjust the distributor more than a couple degrees due to binding caused by the tach cable.
Question: What is the 10 degree advance unit? Never heard of this, could it be my solution?
Problem #1 - either my timing light sucks (doubtful) or the timing mark on my harmonic balancer is not aligned properly - can't get a reading on the true timing.
Problem #2 - can't adjust the distributor more than a couple degrees due to binding caused by the tach cable.
Question: What is the 10 degree advance unit? Never heard of this, could it be my solution?
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Originally Posted by DSchaefe
Well I removed the vacuum advance from the distributor and Voila, no hesitating. Now the trick is to figure out how to make adjustments to the timing/vacuum.
Problem #1 - either my timing light sucks (doubtful) or the timing mark on my harmonic balancer is not aligned properly - can't get a reading on the true timing.
Problem #2 - can't adjust the distributor more than a couple degrees due to binding caused by the tach cable.
Question: What is the 10 degree advance unit? Never heard of this, could it be my solution?
Problem #1 - either my timing light sucks (doubtful) or the timing mark on my harmonic balancer is not aligned properly - can't get a reading on the true timing.
Problem #2 - can't adjust the distributor more than a couple degrees due to binding caused by the tach cable.
Question: What is the 10 degree advance unit? Never heard of this, could it be my solution?
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Thread Starter
Pro
Joined: Nov 2001
Posts: 534
Likes: 6
From: Cave Creek AZ
I have an MSD HEI unit that has a max 10 degree vacuum advance, so I don't think I can change that out and get any better. My first inclination is to just disconnnect the Vac Advance and call it good. The MSD has adjustable mechanical advance but it is factory set at its slowest setting, I'll have to check witht he guy who installed it to see if he changed the springs/bushings from original.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
More than likely he did. If there is any question you can plot out the advance with a timing light. Just bring up the rpms 500 at a time from 1000 on up and write down how many degrees you have at each setting. Do this until you hit max and you will see what your curve is.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
if you are still having the bucking problem it could be related to a lean air/fuel mixture. i have had to tune a 72 lt-1 with the original holley #6239 and had to go up in primary jets, restrict the idle air bleed they were very lean from the factory because they were trying to make a high performance pass the smog laws of the time. on this lt-1 we went from 68 to 72 jets plus idle system work and the reading with both a wide band o2 based air/fuel meter and a 5-gas analyzer both showed it was VERY LEAN before the changes and that the air/fuel mixture was correct after the changes plus the car performed like it should (no ping, no surge and a lot more power)but we are in calif with reformulated gasoline. hope this helps
Thread Starter
Pro
Joined: Nov 2001
Posts: 534
Likes: 6
From: Cave Creek AZ
Great input from all - much appreciated.
I am going to try a new/better timing light tonite to see if I can get a read on the timing. My hunch is that, if anything, the car is running rich. I have had a bunch of trouble getting the carb to stay clean and adjusted properly. Finally installed an extra in-line fuel filter and had the gas tank cleaned out. The carb fuel filters were all fouled before this. I think it is still running rich based on periodic exhaust puffing and the smell of the exhaust (along with my gas mileage). I am going to pull a couple of plugs and take a look this weekend.
I am going to try a new/better timing light tonite to see if I can get a read on the timing. My hunch is that, if anything, the car is running rich. I have had a bunch of trouble getting the carb to stay clean and adjusted properly. Finally installed an extra in-line fuel filter and had the gas tank cleaned out. The carb fuel filters were all fouled before this. I think it is still running rich based on periodic exhaust puffing and the smell of the exhaust (along with my gas mileage). I am going to pull a couple of plugs and take a look this weekend.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
FYI a rich mixture has exhaust with a high CO (carbon monxide) content, carbon monxide has no smell! what you most likely smell is hydrocarbons (high HC) which is created by engine misfire or unburnt fuel in the exhaust, this can be from a lean or rich fuel mixture. high hc in the exhaust does have a smell and will burn your eyes and hurt as you breathe it. if you just had a rich mixture with no misfire you would never smell it but carbon monxide is a deadly gas that is hard to detect without a co meter (exhaust gas analyzer) or a wide bad o2 sensor based ir/fuel meter.
Melting Slicks
Joined: Sep 2002
Posts: 2,390
Likes: 2
From: New Castle DE
Originally Posted by PRNDL
oh oh! I know! I know!! :
Lars has warned on many occasions that too much timing will cause that symptom , ie "bucking" at cruise. It is your vacuum advance that is making total timing excessive. On acceleration, no vac advance, but at cruise you are probably getting total timing of 55 degs or more. As a test, try disconnecting your vac advance. If symptoms persists then I am full of sheet and you move on to the next idea! If symptom goes away consider replacing your vac can with a lesser amount of advance and/or retarding your timing.
:Lars has warned on many occasions that too much timing will cause that symptom , ie "bucking" at cruise. It is your vacuum advance that is making total timing excessive. On acceleration, no vac advance, but at cruise you are probably getting total timing of 55 degs or more. As a test, try disconnecting your vac advance. If symptoms persists then I am full of sheet and you move on to the next idea!
If symptom goes away consider replacing your vac can with a lesser amount of advance and/or retarding your timing.Did i ever mention how much i love this site!!!!! Changed the plugs and filter in my car over the weekend and had this same problem. It happened before but wasn't as noticeable. Did a search and found this, did exactly what was suggested above and sure enough that was the problem. checked timing and it was to high backed her down and now she runs like a champ at cruising speeds.