Buck or Stumble
Thread Starter
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
Buck or Stumble
Initially put this in the General forum but I thought it would be better here...
Wanted to get some input from some of those better at engine performance than I. My L46 seems to start and run well, but when not accelerating (on the road) will occasionally "buck" or stumble. It does not do this at idle or under acceleration. The carb was recently rebuilt and I have a new manifold and distributor. So I went to time the car and I noticed that the timing mark seems to jump about at times - and then I noticed that at idle the RPMs drop occasionally for bit as well (The idle RPM quickly drops and comes back up). When I connect a vacuum gauge, the needle sits solidly on about 16, drops a bit when I accelerate, and then goes back to a solid 16. So I assume there is no vacuum leak leaning out the engine. My next step was to change out the plugs in case there was a fouled one or one that was not firing cleanly. Does this sound reasonable? Any other suggestions to try and nail this down?
Wanted to get some input from some of those better at engine performance than I. My L46 seems to start and run well, but when not accelerating (on the road) will occasionally "buck" or stumble. It does not do this at idle or under acceleration. The carb was recently rebuilt and I have a new manifold and distributor. So I went to time the car and I noticed that the timing mark seems to jump about at times - and then I noticed that at idle the RPMs drop occasionally for bit as well (The idle RPM quickly drops and comes back up). When I connect a vacuum gauge, the needle sits solidly on about 16, drops a bit when I accelerate, and then goes back to a solid 16. So I assume there is no vacuum leak leaning out the engine. My next step was to change out the plugs in case there was a fouled one or one that was not firing cleanly. Does this sound reasonable? Any other suggestions to try and nail this down?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
You have too much total combined timing at cruise rpm with full centrifugal and full vacuum advance. You need to limit your vacuum advance to no more than 12 degrees (stock units will produce about 18) to limit your total combined timing at full advance to no more than 48 degrees. You can verify this situation by simpy yanking the hose off your vacuum advance unit and driving the car to see if the problem disappears. This issue is addressed in detail in my timing papers, available from me via e-mail request.
Lars
Lars
Thread Starter
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
You have too much total combined timing at cruise rpm with full centrifugal and full vacuum advance. You need to limit your vacuum advance to no more than 12 degrees (stock units will produce about 18) to limit your total combined timing at full advance to no more than 48 degrees. You can verify this situation by simpy yanking the hose off your vacuum advance unit and driving the car to see if the problem disappears. This issue is addressed in detail in my timing papers, available from me via e-mail request.
Lars
Lars
I will check that out this weekend. So I assume when I hit the accelerator the vac advance was minimal due to drop in vacuum, but when I was cruising the vacuum went back to maximum and the total advance was too much?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
You got it. Your vacuum advance control unit, if you have not modified it, probably pulls in anywhere from 16 - 24 degrees of vacuum advance on top of whatever your total timing is with full centrifugal advance. If your total timing is set up for the optimum 36 degrees at 2800-3000 rpm, you can have 36 + 20-ish degrees of advance at light throttle cruise rpm, making your actual total combined timing at cruise over 50 degrees. This does not work well with modern pump gas, which has a slightly faster burn rate than the gasoline formulas in use when these cars were new. It will, therefore, cause the "trailer hitching" bucking effect. Rather than retarding the timing to cure this, which wil result in a decrease in WOT power/performance, the cure is to limit the vacuum advance to no more than 12 degrees. With your 36 total mechanical timing, this will limit actual timing at cruise to no more than 48 degrees.
As noted, the quick test to see if this is your problem is to simply yank the hose off your vacuum advance, plug it, and go for a drive. If the problem is cured, limit your vacuum advance to 10-12 degrees.
Lars
As noted, the quick test to see if this is your problem is to simply yank the hose off your vacuum advance, plug it, and go for a drive. If the problem is cured, limit your vacuum advance to 10-12 degrees.
Lars
