Advice on engine vacuum...or lack of..

Bob Carriere · #1 20-09-07, 03:07

I am breaking the stride of the Hammond barn news 'cause I am frustrated.....

My engine won't suck!!!!

Here is the situation.

My rebuilt 261 was started by Dan.. the Wizard Man....

since then I have tweak the timing and it starts easy...but.. after all efforts I can only get about 9 inches of vacuum at idle and ONLY with the choke on full.....

Mixture screw is at 1 1/2...... further adjustments + or - made no difference.....

Comment by Dan was that I might be overcarburated..... yeah like having too much sex.....

I was running at the time Dan was here a NOS 7015011 Rochester B carb listed as suitable for a 261. In fact the same carburator on Stew's 261 works fine.

I checked the fuel pump for sufficient delivery of fuel..... squirts like the devil..... I checked every vacuum line and even removed the PCV in case it allowed too much air into the manifold... no changes.

Then I changed the carburator for the original 261 Rochester B carb that I rebuilt and got the same no idle response.

Now I can get a reasonable idle speed with a very steady 9 inch vacuum but only with full the choke on... with that setting it runs reasonably good.... racing the engine raises the vacuum to the 15 - 18 inch easily.... but again only with the choke on.

I do have a small ( I think) leak in the front intake manifold at port 1&2..... engine races in RPM when I squirt raw gas with a syringe near that port and an exhaust gas leak underneath at the rear port 3&4......

After engine was hot the bolts were retightened from center outward as recommended.

New one piece special thick manifold gasket is ordered and on the way....

I almost seems that, without the choke on, the engine does not develope enough vacuum to run......

Help.....What did I overlook....????

BooB

cletrac (RIP) · #2 20-09-07, 03:57

The vacuum leak will be the problem. It doesn't take much of a leak to keep an engine from idling. Do you have the little pipes that go in the ports to align the manifold?
Starting fluid works good to spray around gaskets to check for leaks.

Lynn Eades · #3 20-09-07, 11:06

Have you checked your valve timing?( are you a tooth out with your camshaft to crankshaft timing)

Alex Blair (RIP) · #4 20-09-07, 16:39

Quote:

Originally posted by Lynn Eades
Have you checked your valve timing?( are you a tooth out with your camshaft to crankshaft timing)

Bob..
The intake manifold leak should cure your vacouum leak but check the distributor centrifugal weights and the bellows in the advance..not with the starter fluid or you'll blow the dist.cap 50 feet in the air when the points arc..
pressurize the vacuum advance system with air and look for leaks that way...
the bellows may be cracked in the dist..
Clutching at straws but the timing does not seem to be advancing..

Bob Carriere · #5 21-09-07, 19:34

To David...... Yes the 3 little intake rings have been iinstalled.

I have found that a hypodermic needle/syringe filled with gasoline is an effective way to find leaks in specific areas.... I have traced the major leak to be at the front first intake runner....dripping gas causes the engine to pickup in RPM....... and by using a piece of sheet metal to deflect the air blast from the fan..... found that the second last exhaust port is also leaking underneath........

to Bluebell...... the timing of the valves was done by the machine shop..... I am hoping that this woudl not be the problem. Engine runs relatively smoothly with no clacking or missing...... also the vacuum gauge needle is very steady at 9 inches and not jumping around as if valves were out ......

to Alex....... the distributor was totally disassembled, cleaned and back together.....making sure the bushings were good..... new points, caps,condenser, wires and plugs......lubrication cup cleaned and refilled. To the best of my knowledge the mechanical weights are working freely...... the vacuum advance is new..... I have run the usual test to check for diaphram leaks..... suck on it and hold it to see of it springs back leaking...... I have watched the distributor advance mechanism in action when I speed up the engine.....rpm picks up with no stumble....

Following a suggestion from the Stovebolt forum I have ordered a "special leaking manifold kit" from a US suplier..... gasket for exhaust and intake is all in one piece and thicker than usual to account for slight variations. According to some users it worked very well.

It may very well be that my biggest problem is the leaking intake.... so I will try to be patient until the parts arrive.

I may consider having the manifold milled at a machine shop if necessary......

One consolation....... I am glad I am finding this problem now and not after ALL the sheet metal is back in place....

Stay tuned....... any other comments suggestions are welcome.

BooBee

Alex Blair (RIP) · #6 21-09-07, 22:14

Quote:

Originally posted by Bob Carriere
To David...... Yes the 3 little intake rings have been iinstalled.

I have found that a hypodermic needle/syringe filled with gasoline is an effective way to find leaks in specific areas.... I have traced the major leak to be at the front first intake runner....dripping gas causes the engine to pickup in RPM....... and by using a piece of sheet metal to deflect the air blast from the fan..... found that the second last exhaust port is also leaking underneath........

to Bluebell...... the timing of the valves was done by the machine shop..... I am hoping that this woudl not be the problem. Engine runs relatively smoothly with no clacking or missing...... also the vacuum gauge needle is very steady at 9 inches and not jumping around as if valves were out ......

to Alex....... the distributor was totally disassembled, cleaned and back together.....making sure the bushings were good..... new points, caps,condenser, wires and plugs......lubrication cup cleaned and refilled. To the best of my knowledge the mechanical weights are working freely...... the vacuum advance is new..... I have run the usual test to check for diaphram leaks..... suck on it and hold it to see of it springs back leaking...... I have watched the distributor advance mechanism in action when I speed up the engine.....rpm picks up with no stumble....

Following a suggestion from the Stovebolt forum I have ordered a "special leaking manifold kit" from a US suplier..... gasket for exhaust and intake is all in one piece and thicker than usual to account for slight variations. According to some users it worked very well.

It may very well be that my biggest problem is the leaking intake.... so I will try to be patient until the parts arrive.

I may consider having the manifold milled at a machine shop if necessary......

One consolation....... I am glad I am finding this problem now and not after ALL the sheet metal is back in place....

Stay tuned....... any other comments suggestions are welcome.

BooBee

Hi Bob..
Those old intake manifolds are so long that any kind off heat will play hell with the machine surfaces for warpage..Were the manifolds dne or sent with the short block to the machine shop...??
If they were they would have been milled true and mated to the head..If not ,pay me now...or pay me later...
You know the old saying..
Throw andother fat gasket on it..do not get any oil on the gasket and torque the sucker using the proper torking pattern and torque specs as laid out in the MB-C2...that old beer stained copy in your shop..
Good luck..