Hi Guys,

I did a rebuild and put in a Comp Cam extreme cam, and changed to Stock Dove heads(mild port/polish & gasket matched) which put me at around 9.5 CR , new pre 71 timeing gears,and I am able to run on 87 Oct gas with the timing set back some.

I do get a bit more get up and go but not as much as I thought I was going to get. I do occationally get a back fire up through the carb-- I assume is a running too lean condition in the carb?

I suppose I really need to change from my 600+ cfm motorcraft carb to a 750 or 850 cfm carb and put in a aftermarket intake and exhaust manifold. That is my thoughts at this point. I believe from everything I have read on building up this is the way to go.

This all brings me to my question about ignition control. With the cam I am running I seem to have lost significant vacuum(I assume due to the duration the I/E valves are now open with the new cam) This lack of vacuum has affected my advance timing ability on my stock distributor.

There is no vaccum adjustment in the pot on the side of the distribtor and I have not been able to find a vaccum booster module.(Direct manifold vaccum was too hi to be able to use also) So I must go to an electronic ignition system which I should have gone to already. Any suggestions hear would be apprietiated.

Greg

 

If I am not mistaken you should have a DuraSpark II ignition system. With that being said, your system will normally run the rpm's to roughly 5500 before it starts to lose spark strength. You can go inside the distributor and change the mechanical advance springs to be stiffer. Do a google search for recurving the DuraSpark. There are a few sites that show you with pictures how to do it. Also use a vacum gauge to find the ported vacum lead on the carb. If you are still getting vacum at idle you will need to adjust your carb settings until you have 0 vacum then reset your ignition timing.

 

Hi Mark,

Thanks for the reply. I had found the vaccum and it was 0, at idle. The problem was that the vacumm was not coming up as it should. I did have a vacuum gauge attached.(Also, had a complete Bear diagnostic system hooked up at the time.) I would think I would need to weakin the springs for easier advance with less vacuum available?

I am not sure I have a Duraspark as I had an old MSD module that was origanally on this engine/distributor.

Greg

 

Does this help ?

 

Which cam? The extreme line has several choices.

I've got a sneaky feeling you may have too much cam and it's being choked off by your factory intake and exhaust manifolds. Not to mention the factory carb, which is probably in dire need of a dirt nap.

Backfiring through the carb is usually one of two things - a) a vacuum leaks, or b) the timing is backed off too far. Vacuum leaks are more a more common issue. I bet that's your problem. Another possibility is a misadjusted valve. Not very common with hydraulic lifters, but possible if you have the adjusters screwed in a little too tight.

At 9.5:1 CR you need to run mid-grade or premium and bump the timing as far as you can to get the most out of a hotter cam. Pulling back the timing to compensate for lower octane fuel defeats the purpose of all your hard work.

Brad

 

Check out what Crane Cams is releasing this week, its a Dizzy that has a vac port that you may...or may not use at all, but it has a computer biult in that will do a RPM dependant advance curve. It comes with the programs in, you just select the advance curve you want and your ready to rock and roll! I got mine last week, it looks sweet!

www.cranecams.com


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Hi Brad,

The Cam is RPM Range 1400, Part# 34-235-4, Grind # X4262H, advertised in/ex 262/270, @.50 218/226, Duration .514.524, Duration 111 deg

I purposely went with no more than 9.5 CR to be able to run on 87 Oct. I don't always get a back fire. Once in a while when I slowly bring in the secondaires on the carb. The Carb does not like the transition point of bringing in the secondaries(This carb was giving me nearly 15MPG before the block rebuild) If I just put the pedal staight to the metal--usually no back fire. I just did a 4 hour drive and no back firing and no pinging. (I set the timing on the road just getting rid of the ping). (Dwell is set at 29) I am running this truck at around 2000 to 3000 feet in elevation most of the time.

I am presently getting around 11 to 13 MPG.

I agree with you I believe I need some more flow with this cam.
And yes it really loves that 92 octane

This summer I am going to go with some water injection to help cool my heads and be able to set my timing more advanced.

Any tricks to find vacuum leaks other than a good ear?

 

Thanks for the pic Mil1ion,


My distriburtor does quite look like that but by that schematic I may be able to get the MSD system hooked in like it should be.

Greg

 

Vacuum or mechanical secondaries? Also, the at 9.5 CR you will probably want to bump your fuel octane requirements a point or two. 87 octane is marginal for 9.5:1.

As far as the vacuum leak goes, a can of carb cleaner is about the quickest way. You can also used the "rubber hose to the ear" technique, but it's less reliable (especially if you have poor hearing like I do).

Also, if you have access to a known good carb, try swapping it over to your truck and seeing if that cures the some of your ills.

Brad

 

Hi Brad,

I have C9VE heads. Yep 87 octane is marginal. I may only be at 9.3 CR. Don't know. But I am on a tight budget.

How do you check for vacuum leaks using the carb cleaner?

Greg

 

That cam is none to big. You should have plenty of vacuum. I've never measured the vacuum on my engine, but I have the same exact cam and very similar engine. I actually had to buy a different vacuum advance so I could decrease the amount of advance the vacuum was causing. I was getting some spark knock with tip in at part throttle. My overall timing was fine but the initial advance caused by the vacuum was excessive. All my power accessories work just fine, brakes, AC controls, etc.

 

Bigsnag,

I don't have the bigger valves in just the stock valve size that came with the C9VE heads.

I don't have a the updated carb, intake or exhuast or that nice cold air intake you have.

 

Take the carb cleaner and spray around the carb and intake gasket sealing areas. If you notice a change in RPM or idle quality when you hit a specific area, you've found your leak. It's like using soapy water to check for a tire leak, just in reverse.

Keep in mid that carb cleaner is pretty rough stuff and can take off paint if you're not careful. You don't have to give the engine a bath, just a squirt in the cracks will do. It helps to use the little plastic tube that comes with the can. You can be much more precise with your "squirt".

Brad