440 vacuum running a bit low on gauge

[MSN Member]

From: 67imp
Sent: 7/24/2008

on my dash is the vacuum driver minder gauge. when it is at idle it reads about 15 (I assume) inches of vacuum . the gauge says it should be between 17-21 inches at idle. when I raise the idle speed up to about 1000 rpm I do get a perfect 19 inch reading.
Is this an indication that I have a possible vacuum leak or my rings are leaking a bit? , or am I worrying about nothing

[denisondc]

Sent: 7/24/2008

Its more likely that your ignition timing is off somewhat, or that the gauge itself is not accurate, or that its not connected to a vacuum port in the actual intake manifold, below the throttle valve. I don't think the inexpensive vacuum gauges are more accurate than +/-%20 anyway.
Also the type of emissions equipment your vehicle has can make a big difference in the idle vacuum settings.
The higher you run the engine in neutral, the higher the vacuum reading is likely to be.

[MSN Member]

From: 67imp
Sent: 7/24/2008

It is connected to the intake port behind the carb . Original timing was 7 1/2 degrees according to the spec sheet on the valve cover. it is now set at 9-10 deg. advance . I has a slight hesitation when you try to get on it but runs good otherwise. When I got it and checked timing it was set at 22 deg. advance. i thought that maybe it was a tooth off on the chain or distributor but i checked T.D.C. and it is correct .

How far should/can it be advanced?

Another possibility is that the chain is stretched which would throw off timing also.

Thanks

Mike

[denisondc]

Sent: 7/25/2008

I think it can be advanced a couple of degrees beyond that factory setting of 7.5°, as long as it doesn't ping (detonation). Mine is a 413, with lower compression of 8 to 1, and is fairly unlikely to experience ping even when over-advanced. Too much advance will possibly lower the MPG you get, but I don't think it would be dramatic. I like to have my spark plugs be a very light brown - not quite white. Then I figure my timing is good.

If there is a hesitation on first putting the pedal down, that often means the accelerator pump is getting feeble -that the tiny leather piston is worn, or the check valve is sticking. You may be able to adjust the accel pump for a slightly larger squirt, but the hesitation wont hurt anything.

I don't think the timing chain is likely to be a problem on motorhomes. Timing chains wear out on things like delivery trucks, where the engine rpm is changing all the time. With RVs though, most of the engines life is spent at a fairly steady rpm, pushing the motorhome down the road. I replaced the timing chain on my 413 when it had about 94,000 miles on it. I was not able to tell ANY difference in the 'slack' between the old and the new chain/gear set. I only changed it because I already had the new chain, and had removed the timing cover to replace the 29 year old front crankshaft seal. That old seal was starting to ooze oil.

[toddabney]

From: toddabney
Sent: 7/25/2008

hi mike,have you tried to adjust your carburetor air/fuel mixture? lots of times you can bring up those numbers by adjusting the needle valve. if that doesn't help then try the harder  stuff. i have a "vac-o-meter"in my 1973 with the 440-3.that is a fancy vacuum gauge that helps to get the most mpg.i keep a eye on my vacuum numbers.i read somewhere that those type of gauges (mile-o-meter,vac-o-meter,ect.) were the only thing proven to help with better mpg.if you drive as to keep the vacuum as high as possible. i have never checked my mpg,but it makes me feel better.....happy trails,todd