1975 W100 360 Falls on its face under load

[buzter]

With it being a new factory rebuilt..I would hope all is good in the carb...

[Gordon Maney]

I did some testing tonight
Vac off carb @15 at 1500rpm and bout say 1-2 @idle
Timing bout 2 before
Rpm 7-800

15 in/hg vacuum at 1500 rpm and 1-2 in/hg at idle? That's quite low.

"Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg."

See useful reference from Motor Magazine here. Good diagnostic info included.

Also, perhaps a ring on the vibration dampener has rotated and the mark[s] have moved? Not sure if this is applicable on your engine.

[buzter]

Checked on vacuum today and around 2 in at idle is fine..when u step on the gas the carb produces vacuum to advance distributer..again this is carb vacuum

[Gordon Maney]

A reading of 2 in/hg at idle is waaaaay too low. When you crack open the throttle the vacuum should drop, not rise. When the vacuum drops from cracking the throttle the timing should retard, not advance.

[Gordon Maney]

Read through this:

http://www.motor.com/article.asp?article_ID=457

It has some good diagnostics.

[Paul in MN]

I found a big vacuum leak on my 88 360 on the intake manifold just to the left of the carb. There is a bolted on EGR valve which has a cast steel base. It looks bullet proof, but beneath it the gasket on the intake manifold has very thin sides. There are 2 ports in this gasket, one is manifold vacuum, and the other is the cross over exhaust or exhaust from the left bank of the engine. My son found the vacuum leak by spraying carb cleaner around that gasket (caused the engine to speed up). When I took the EGR valve off to replace the exhaust type gasket beneath it, I found that it had been failing long enough to allow enough exhaust to erode a good bit of steel on both the surface of the intake manifold and the base of the EGR valve.

The EGR is cheap enough to replace, but the erosion on the manifold with hot exhaust gasses passing through that area seemed to be too much of a challenge for JB weld or any other epoxy product. (I am not sure if that is true because I didn't try it). But I did solve the problem by removing the intake manifold (it needed a new gasket anyway) and having my son mill the EGR surface flat on a Bridgeport he uses at work. Lots of hassle, but no more vacuum leak!

As Gordon has said, the vacuum reading at idle should be high (near 20 inch Hg).

Another major loss of vacuum at idle could be your brake's master cylinder power booster. The diaphragm can fail internally, but then you can usually hear the sucking sound if you put your head and ears up under the dashboard near the steering column.

Keep trying!!

Paul in MN

[buzter]

That was next on my list..I actually picked up a set of gaskets last night to do the intake also..as I wanted to check carbon build up on crossover below carb..and just to seal it all back up.

What would u guys say the timing should be at with an rv cam...book states TDC stock..but I remember when I put the cam in it stated to do something diff. But cant find my info on the cam anymore..

[Gordon Maney]

Pulling up on the EGR diaphragm with your fingers as it idles is a simple test. Idle quality should become much worse, perhaps even killing it.

Let go to end the test.

[buzter]

I do not have an egr valve on the motor...there is a plate over that port. I will also check that for leaks

[buzter]

This weekend I will check compression, motor vaccum, hopefully get intake out, cleaned and replaced, and check egr plate for any leaks...anything else to add to the list guys

[Paul in MN]

Be sure to take that EGR delete plate off and check the gasket and condition of the steel beneath it. There is a very thin wall down the center of that gasket which separates engine vacuum and exhaust gas. If that thin wall of the gasket has been blown out the steel of the manifold will be badly eroded and a simple gasket replacement will fail again very shortly if not immediately.

There is a reason someone put that EGR delete plate there, it was not a factory installation, I do not believe. Check out the Rock Auto site for a picture of that gasket, and you will see what I mean. After finding the condition mine was in, I knew a replacement gasket would not have a chance of survival. I consider this a design flaw in this engine. The resulting vacuum leak can not be determined from checking along the outside of this gasket, as that might be still intact.

As I said before, I took the intake manifold to a milling machine and had that surface machined flat so a new gasket would have a chance of working. One of the problems of mounting the intake on the mill's table is the EGR surface is not parallel with the base of the manifold. So I.M. needs to be set on the mill table with step blocks and clamped tight. Not a big problem, just another annoyance.

There are a lot of vacuum hook-ups coming off the vacuum tree at the rear of this intake manifold, and a crack in any of the vacuum hoses can be the problem, and probably just adds to the other vacuum leaks.

Check very carefully and I think you will find some (hopefully ALL) of the problem(s).

Paul in MN

[buzter]

Did some measuring and overhauling this weekend..this is what I got

Vac advance opens with vac pressure
Engine Vacuum @ 18-19"
Compression test on all cylinders @ 125
Egr plate taken off and put back on with fresh silicone
Advanced timing to about 12 BTDC
Cleaned all plugs
Sprayed starting fluid around all vac connections and carb no increase in rpm
Sea foamed engine
Oh and a new seat cover..LOL
Tried bending carb fuel pump linkage a bit..didn't seem to help at all
All vacuum lines replaced prior to this year

Still falls on its face...have to feather the foot pedal...If I ease into it..works fine... and she has alot of power now...but still having issues under a load????

[712edf]

How many different carbs have you tried? Also a temporary inline fuel pressure gauge might to make sure the electric pump is legit.

Bucky

[buzter]

Last carb did it even after I put two kits in it..still did it..so bought a reman...so I've dealt with this on a couple carbs..and as far as fuel pressure I get a good pressure coming out of the line and two good squirts into the carb.

[Gordon Maney]

I am wondering if the timing chain might be loose.