1975 W100 360 Falls on its face under load

[Gordon Maney]

Moving it how, exactly?

[buzter]

side to side...turning...and lifting

[Gordon Maney]

Let me first say, I'm not messing with you here, we are trying to pick our way through this.

Based upon the language of your description, you are moving it in three different ways. I want to know about two different kinds of motion.

First of all, if we look down at the end of the distributor shaft, with the rotor off, can we move the distributor shaft in the direction that is at right angles to the length of the shaft — at right angles to the centerline of the shaft? My reason for wanting to know is that I wonder if the shaft is loose in the housing. Your distributor housing probably does not have separate bushings, rather the shaft runs directly in machined bores in the housing.

If the shaft is loose in the housing you will get excessive variation in dwell [angle] — that is the case whether it is a breaker point or electronic ignition system. So, if you look down at the end and move it that way — try moving it first to 12 o'clock, then 6 o'clock, then back to 12 o'clock, is there any motion?

Second, I want to know if — with the rotor in place on the end of the shaft — if you attempt to rotate the rotor in either direction, do you get motion in one direction with a spring-loaded effect bringing it back to its original position? There should be very noticeable motion when you do that. This is because of the action of the centrifugal advance mechanism. If you do not get that motion, your centrifugal advance is frozen up and will not work. If it does not work, you will have reduced power as you raise above idle.

Your comment below says you don't get any motion anywhere. Is that what we are to understand?



side to side...turning...and lifting

[buzter]

I totally understand and appreciate...mess away if you have to Gordon....and no I mean exterior of dist is what I was refereeing to as no movement...I will open it up and try as you described (very well actually, appreciate it) and see what my results are. I am glad you did describe as I was thinking exterior of dist...much appreciated

[buzter]

Gordon, working on the truck now..moving dist shaft 12-6 very..minute amount of play..rotating shaft does move and spring back

[Gordon Maney]

Probably OK then.

[buzter]

Well set timing...tuned carb...with Vac guage...has great power now...just still has hesitation under load...I guess next step may be to take intake off and check that...I can't believe it would have much carbon build up as it only gets run maybe 50 miles a year...but who knows...

[Gordon Maney]

Well set timing...tuned carb...with Vac guage...has great power now...just still has hesitation under load...I guess next step may be to take intake off and check that...I can't believe it would have much carbon build up as it only gets run maybe 50 miles a year...but who knows...

Tell me specifically what this means, please.

[buzter]

Hooked up Vac gauge to manifold on motor, loosened dist. turned both mixture screws on carb in all the way and turned both out 1-1/2 turns. Started truck up and turned dist for max vac and also listened to engine. then started with one screw turning in/out max 1/4 turn, let sit and checked vac and kept doing this till vac was at max. Screws (both) were pretty much set to 1-1/2 out both. Tightened down dist also. Set idle to 750 rpm. I did notice when I had my craftsman analyzer hooked up that the rpm seemed to want to dive down every once in awhile..but when I checked my tac in the truck it was steady, so I am not sure if it was the analyzer or the tac in the truck didn't read the dive in rpm...engine didn't seem to hesitate though.

[buzter]

Sorry Gordon or did you mean the hesitation...when I push the pedal to the floor it wants to die out...but if I slowly press the pedal it will go just fine. Issue I have is when I get to a pile of snow and I get into it to push it up, it wants to die out...but if I feather the pedal a bit before I get to the pile..it has all the power it always has had and will push snow right up the pile.

[Gordon Maney]

When you step on the gas pedal, changing the position of the throttle valve is not accompanied by an immediate response on the part of the engine.

It hesitates — hence the common use of that word. Also has been termed flat spot or stumble.



Possible causes:

Accelerator pump circuit not working, or not working properly.

Float level low.

Vacuum advance system not working properly.

Excessive required voltage due to problems in secondary circuit of ignition system.

Insufficient available voltage due to problems in primary circuit of ignition system.

Leakage to ground on secondary side of ignition system.

[DrPepper]

hesitation is in the carb...but not where you are looking

In the 70's and 80's Mopar and others were running "lean burn" engines to try and deal with emmisions. The Holly 2BBl's (and others) had a range of main jets that were available for different applications. We had an early 80's van, 318 2BBL 4 speed, did the same thing. At that time I went on the Holly web site and bought 4 sets of jets larger than the set in my carb. The jets are numbered with a tiny stamp on the edge. The second set I installed and it was like a different engine. Off idle response and pull up the range was night and day better.

If you find someone that has an exhaust sniffer to get mixture readings at various rpm's, (a dyno run is the best because it is under a load, $60 for 3 pulls here in Rockford with a reading of mixture at the bottom of the printout)then you will know what the carb is doing off-idle. Accelerator pump get's it started but the main jets is what the engine works off of.
Last thought is you can drill the jets, but that is a very final thing if you only have one set. I'll see if I can dig up some of the old jets in my box and post up a pix.. my $.02
DrPepper

[76WAGON200]

Make sure your carb base gasket is correct for the carb. There should be no areas uncovered or vacuum leaks will ruin throttle response. This issue definitely sounds fuel related since the problem occurs during rapid throttle movement but not at slow rates. The accelerator pump has been mentioned in here somewhere already but it is not just the pump volume that matters but the timing as well. Get the volume and timing wrong and the engine will either starve or flood when you stab the throttle. Install a clear inline fuel filter near the carb to visually confirm fuel is reaching the carb and isn't full of air. The rubber parts between the fuel pump and tank may have cracked and allowed air into the fuel line and that tends to wreak havoc on a carbs ability to function correctly. Strangely, the air can come in but the fuel doesn't leak out. Maybe try running out of a jerry can and see if there is any effect. A compression test is good but a leakdown test is better. This gives you an idea on valve seat/face and guide wear. Too much wear in those places can cause pulsations in the intake and crankcase that degrade carb control. If this engine has hardened valve seats, you should be ok but if not, unleaded fuel will have eaten your valves out already. My 73 D200 had this happen. I don't recall if the change was in 74 or 75 for the hardened seats. Just a few thoughts in case you run out of ideas.