Thread: Low Vacuum in 390

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Hi,

I got my 390 running well finally. There is just one slight problem, its not changing gear well at all. I took the car to my local tranny specialist and found that the tranny (FMX) and the modulator works fine. He tells me I must sort out my low vacuum issue first before we fine tune the tranny.

Its running a fairly large cam, about .406 lift if that makes any sense. When the motor was at idle I sprayed the manifold and carby base plate with carby cleaning to check if there was any fluctuation but there was none, So no leaks by the look of it..

The reading at idle is 150 on my weird aussie gauge?.. which is in the red but I think you guys work in inches?

have any of you guys had the same drama?

What can I do..

Radical cams ruin engine vacuum, you can put a vacuum storage tank in the system , that might solve the problem.If not a storage tank and small 12volt vacuum pump is in order. hope this helps jmdjr67

Yes I have heard of that. The bizaar thing is, the power brakes are fine! Plenty of vacuum there. I think the tranny specialist may be nuts...

I thought the camshaft was not that extreme to completely abolish all the manifold vacuum, it does have an edelbrock dual plane manifold. This could be a contributing factor as well I guess.

vacume to the modulator should be straight manifold vacume not carb venturi vacume. If thats correct I would measure vacume at the modulator end of the vacume line,maybe it has a leak in the line or it may be restricted.

Looking at my vacuum gauge 150 mmHg equals about 6 inches of vacuum,
Not near enough. I'd start looking to make sure you did not miss the timing marks when you installed the timing chain,

Originally Posted by HWORRELL

Looking at my vacuum gauge 150 mmHg equals about 6 inches of vacuum,
Not near enough. I'd start looking to make sure you did not miss the timing marks when you installed the timing chain,

yep thats away low how is his power brakes working

Originally Posted by Mark@Brisbane

Yes I have heard of that. The bizaar thing is, the power brakes are fine! Plenty of vacuum there. I think the tranny specialist may be nuts...

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i think he could be on most trannys have a set screw you ajust in the modulator .pull hose off there may be in there or you can buy one you can tune in .that helps the shifting yes need a good vac pull off in take or base plate no ventury vac

yes it is E Z to do that why i degree all cams .i have had more then one try to slip me up lucky i did catch it .the ones that can play hell with you are the 3 or 9 key ways they are not stamp the best some times

I was hoping that the cam would not be responsible! I guess im just not keen to pull it all off again and check. Im going to do some final checks with my vacuum gauge today because its such a glorious saturday morning to slave over a 390!

Hworrel, you were saying that you lost a championship once because the cam was out a tooth.. Was it still running ok or was it really noticable?

Ive attached my cam specs to this post just to show you all becuse alot of other blokes reakon Ive got a really bad combo thats why there's really low vacuum.

Cheers guys.

crap!! I cant even see that!! Oh well you get the idea.

Mark it sounded wonderfull in the shop and putting it on the trailer, With the choppy idle it was hard to tell till I hit the track and the dang thing would'nt rev. The points battle was close enough that I went ahead and ran it anyway (very poorly I might add) We pulled it apart the next day and found my mistake along with 8 burn't exhaust valves and 1 cooked piston. Lost the championship that season by 17 points....

Hworrel, Gee that is bad! So you basically had to rebuild your engine again?.. Well I see what you mean about it sounding lumpy and normal but it was only when you got it on the track it was very noticable! Well, my engine seems to run fine even when I stand on it. It does tend to ping a fair bit when you hold it flat! I havent even had it tuned yet, the rocker arms are rattleing a fair bit too. I do want to get it all adjusted right but there's no point if I have to pull it all down again.

I checked it again today more acuratley and it measured 200. It also was 200at the modualor. Something's got me scatching my head though.. I took off the pcv valve from the base plate of the carby and blocked it and it ran like shit and the manifold vacuum dropped 50???? Is this normal?

I really am having doubts about the timing marks being out only because it really goes well. Heaps of power and it revs out fine and idles nicely. Do you think it could be just a really bad cam, intake manifold, etc combo?..

If it will pull to 5000 rpms strong,you are not out of time at the cam. From what i can read on your cam sheet it isnt radical enough to kill the vacuum, plus you say the booster works. I would start over checking for a leak ,or a stopped up fitting.

Have you found your vacuum problem. I have a similar or same problem. I am running a elgin cam HYD .536 int and .562 on exh 75deg overlap. I am only reading 8-9inhg at the manifold and my c6 doesn't shift very good out of park. A big clunk. It shifts ok while driving. I have 20 inhg at higher rpm 1500 or above. I don't think I have any leaks but haven't check timing or valve adjustments yet.