Skip to main content

Folks,

A colleague and I have recently rebuilt my engine and having had a bit of an issue when cold which goes away when the engine heats up, I thought having replaced just about everything we would take some vacuum readings.

The engine has 4V closed chamber heads that have been fully refurbished/rebuilt (I wanted to go standard rather than alloy heads). The camshaft is a Comp Cams 292H (part No. 32-234-4; grind number FC 292H-10) and has the matching comp cam springs. It also has Crane 316T roller rockers fitted.

The manifold is a Ford racing dual plane set up and I have a new Holley 600 with vacuum secondaries.

With the PCV blanked off and also the distributor advance, I get about 10 psi vacuum at the manifold steady at idle (800 RPM), this increases to about 14 when throttle is blipped. At the base of the carb the vacuum is about 9-10 but fluctuates.
I have tried to get some data from Comp Cams but no reply to my note, so now wonder if this is due to overlap or actually a vacuum leak. The rough idle goes away when warmed up, but the vacuum reading does not change.

Any views?

Regards

Adrian
Original Post

Replies sorted oldest to newest

Disconnect the vacuum hose to the power brakes and see if this helps.

I think you should have around 14-15 inches with that cam but if the power brake diaphragm is leaking, you would wind up with a low vacuum like this.


OH! Don't forget about the power valve in the Holley carb. If it is blown out it will have the same symptoms and with the hassle of breaking in the new cam often gets overlooked and just one backfire will blow it out.

These things are absolutely horrendous now and even will blow out with the check valve installed. Genuine Holley parts are best but it is now a comparison between just plain crap and complete s hit.

Just because it is a new carb doesn't mean that you didn't blow out the power valve on first start up?

You need a power valve tester to know for sure. Otherwise you are just chasing dillusions.
Last edited by panteradoug
At one point I installed installed port plates on the intake side of the manifold and unknowingly blocked the exhaust crossover ports. The result was that the car took 20 minutes or more to warm up. Eventually I isolated the issue, fixed it, and the car returned to warming up very quickly. I have since added a phenolic spacer under the carb to mitigate any heat related issues.

I do not think this will have any affect on your manifold vacuum at idle when the engine is warmed up but it may help with your fine tuning efforts because the engine gets up to temperature faster.

My engine has a 292 solid lifter cam from Clay Smith Engineering and we have been able to get just shy of 13 hg vacuum at idle.

You may be able to get higher vacuum by increasing the initial distributor advance. Try gradually increasing it and you will notice higher RPM's. When you lower the idle to approximately 900 RPM's via the set screw on the carb then you may see higher vacuum. Keep doing this until you have achieved maximum vacuum. You may also notice that the engine sounds better at idle.

In turn this will affect your distributor advance curve. The above process increases initial advance and thus changes the advance curve. I am using the MSD 8477 distributor which has a readily configurable advance curve and total advance. Duraspark and other distributors may need tuning by a specialist.
Last edited by stevebuchanan
quote:
this increases to about 14 when throttle is blipped

Are you sure you get manifold vacuum and not ported vacuum? Maybe partly shrouded ported vacuum? To make sure, maybe source the vacuum for the gauge from the manifold not the carb?

Despite that, 10 is a bit low. But if e.g. your timing is off, that can do it. The whole process is in my book (sorry), I'll be happy to send you the relevant paragraphs, where you adjusst the ignition and the carb to obtain "highest steady vacuum". Maybe 10 is max you can get, often a proper tune and a nice 4 corner idle can give you higher vacuum. A totally stock engine up to 20, performance engine often 12-15
OK guys,

Thanks for the information and a few things to go at. Indeed we did get a backfire through the carb, so will investigate this and the power valve (probably swap out for a new one to start with). I clamped off the brake servo and it had no effect unfortunately.

I think there may be a point about the cam in that with the characteristics it has, it may be part of the reason but will see if I can do the timing and set up as mentioned above (the 600 has only 2 idle screws on the primaries, so only a two corner tune capability. I would not mind a read through the relevant paragraphs to see if it is the way I do things! The timing is set at 15 degrees initial with my new Petronix distributor, but would have to look to a specialist to get the curve changed (maybe to 38 degrees all in at about 2800rpm?). As mentioned, I am not running the vacuum advance as yet.

I have now used two different vacuum gauges and taken of the ported vacuum on the carb and also the ports on the intake manifold - both read the same.

I will keep trying and hopefully I can get to the 14 psi (g) soon...

Regards

Adrian
Most likely is the power valve at this point.

If you are using an automatic choke, make sure that the choke is set right in the middle of the dial on "index" and that it is working correctly.

If you try to start the engine by blocking the choke open it will raise the possibility of the engine backfiring, particularly if the air temp is cold.

If it does backfire it is probably from that, i.e., the choke making the starting mixture too lean and depending on the timing of the cam events, make it backfire just a little, or a lot.

Don't dump just gas into the carb instead of starting fluid.


You can confirm a bad power valve by carefully draining the fuel from the carb through the lower fuel bowl screw holes, then removing the primary fuel bowl and metering body.

If there is evidence of fuel in the vacuum chamber, if it is wet, between the metering block body and the main body of the carb, then it is leaking through the power valve diaphram.

That is a blown power valve.


Verify that you do have a checkvalve in this carb. It will be press fit into the throttle body in the vacuum hole that provides vacuum to that chamber from the intake manifold.

It is most likely that you do not have one BUT these check valves are so sensitive to backfire that even if you do, it can still blow out the power valve.

OR you can just try another Holley carb that you know is ok?


The best vacuum at idle with that cam is going to be in the 12-14 inch area, depending on how much initial timing the engine has.

It probably is going to need to be 16 to 18 degrees. That you need to decide using a vacuum gauge once you are up and running dependably.

Also verify the fuel level in the carb. Even new Holley carbs can have that set too high. I set that so that the fuel is right at the bottom of the vision port.

Fuel with even just 10% ethanol will cause the float to be set too high. Holley unfortunately does not take that into consideration and sets the floats like it was 1970 with leaded gas.

Why? I don't know but it just makes this all more complicated.

You need lots of experience with Holleys which means usually lots of grey hair. Too many mechanics are "fuel injection" guys and don't know this stuff until they wind up with a carb fire. Then that burns off their eyebrows and their grey hair.

Once you have confirmed that the power valve is good, I would recommend that you use a spray can of engine "starter fluid" sprayed into the carb.



In my cars that tend to sit for too long to retain fuel in the carbs (which is most of them) I've installed and electric fuel pump IN ADDITION to the mechanical pump.

What this does is prime the carb (or carbs). It just takes about 20 seconds to fill the bowls and makes it much easier, less stressful and safer to start a "dry engine".

I recommend that to everyone. It helps as a safety factor also.
quote:
OR you can just try another Holley carb that you know is ok?


You can pull out the power valve and suck on the metal top. If it sticks to your tongue, the diaphragm isn't split.

quote:
Once you have confirmed that the power valve is good, I would recommend that you use a spray can of engine "starter fluid" sprayed into the carb.


I use much less than a full spray can. I use a about a 2-3 second burst.

Cool

quote:
In my cars that tend to sit for too long to retain fuel in the carbs (which is most of them) I've installed and electric fuel pump IN ADDITION to the mechanical pump.


Agree there. I have this on another car of mine, and it does make starting easier. It has to be "engineered" correctly, though.
Allow me to try to get us back on track again

quote:
Most likely is the power valve at this point.


I wouldn't say this is "most likely", even though we all know that the Holley PVs are lousy quality. But Adrian writes it's a new carb. So PV should not be top of the list. Besides, a hardened power valve usually stays open instead of being pulled closed by vacuum, so it's more likely to create too rich than too lean. And therefore unlikely to create backfire in the carb.

quote:
If you are using an automatic choke, make sure that the choke is set right in the middle of the dial on "index" and that it is working correctly.

If you try to start the engine by blocking the choke open it will raise the possibility of the engine backfiring, particularly if the air temp is cold.

If it does backfire it is probably from that, i.e., the choke making the starting mixture too lean and depending on the timing of the cam events, make it backfire just a little, or a lot.


Adrian hasn't named the choke, maybe it's electric, maybe it's manual, maybe it's missing. But of course, if hard to start when cold, proper choke action should be checked

quote:
You can confirm a bad power valve by carefully draining the fuel from the carb through the lower fuel bowl screw holes, then removing the primary fuel bowl and metering body.

If there is evidence of fuel in the vacuum chamber, if it is wet, between the metering block body and the main body of the carb, then it is leaking through the power valve diaphram.

That is a blown power valve.


The simple test when you get it out is as follows:
Create vacuum with your mouth on it, close the hole with your tongue, see if it stays there or seeps
If that's OK, test if it's going hard by pushing it gently against a new PV of same value, they should move in a synchronized way and with no "jumps"

quote:
I think you should have around 14-15 inches with that cam

quote:
The best vacuum at idle with that cam is going to be in the 12-14 inch area

Which is it?Wink Neither actually, it's very difficult to tell what the max vacuum can be based on engine configuration

quote:
Fuel with even just 10% ethanol will cause the float to be set too high

What? Never heard that before, defies physics I think. Regardless, set the floats, has to be done on any new carb because the fuel pressure varies in every car

quote:
Once you have confirmed that the power valve is good, I would recommend that you use a spray can of engine "starter fluid" sprayed into the carb.

I would not recommend that. Using starter fluid is just to mask a problem, so only use it if you've tried everything else to start it

quote:
In my cars that tend to sit for too long to retain fuel in the carbs (which is most of them) I've installed and electric fuel pump IN ADDITION to the mechanical pump.

Rocky too I see. Is that added complexity worth it? Yes it takes 10 seconds on the starter to start mine if it's been sitting long, annoying. But on the other hand it allows the oil in the engine to get to every cavity before it fires, maybe that's not so bad
--
So back to what Adrian actually asked about. A cold start problem and a low vacuum problem on a recently rebuilt engine with a new carb. I suggest fixing the vacuum issue first, that is probably only due to the ignition and carb not adjusted right. When that's fixed it'll probably start better. Had a friend's friend's Mustang by, ran poorly, very hard to start. Timing was way off, set it right, both problems gone.
If it still starts fine hot but not cold, work on the choke first.

Just my $0.02

Add Reply

Post
×
×
×
×
Link copied to your clipboard.
×