Guess we can't cut and paste from Word.

Anyway I was hoping to get some help getting my truck to pass DEQ in Oregon. I have a 78 F250 2wd standard cab with 400/C6. It was rebuilt a year ago using Tim's pistons, Comp cams DEH265, a Holley street dominator open plenum with EGR, a rebuilt Edelbrock 1406, and a rebuilt distributor. It has new plugs, wires, cap and rotor, 195 deg thermostat, PCV valve, headers, and dual exhaust. The engine now has just over 3000 miles on it.

The emission limits in Oregon are 300 for HC and 2.5 for CO and I am getting ~345HC and ~5.4CO out of both pipes. The plugs are gapped correctly and good for color. Compression is ~165 on all eight. Timing is at 10 deg, goes to 34deg with centrifugal advance, and another 10 deg vacuum advance. The shop that rebuilt my distributor had me run the vacuum advance off of manifold vacuum for better throttle response and so the idle timing is actually 17 deg. Vacuum at idle is ~17" with vacuum advance on manifold and ~15" if attached to ported vacuum. The vacuum moves up tp 17.5" and as low as 16" and is erratic. Idle screws have been adjusted for the best vacuum. It acts like it has an intermittent miss and has always acted this way.

I have tried blocking off all of the vacuum lines one at a time and the lines are all new. When I block the PCV valve the idle and vacuum drop, but vacuum is still unstable. I have pulled a vacuum on the EGR valve to open it up and the engine starts running really ruff, I guess there is a possibility that it is not fully sealing but I did check it before installing. I noticed when I pulled off the air cleaner that the intake for the PCV system has no noticable flow through it even though the PCV valve is pulling strong. I have tried a different ignition module. I thought about changing the coil, but shouldn't that be more of an at speed issue and not at idle? I don't have a lot of experience troubleshooting this type of issue and could use some suggestions before I start throwing parts at it. I would also like to know what people with a similar setup are getting for vacuum at idle and how smooth the idle is. Sorry about the long post, and the number of them, but I wanted to make sure I got all the info in.

Thanks,
Jeff

 

i think it's your initial timing and the manifold vacuum, as far as passing the emissions test.

what i used to do is run as little initial advance as possible and disconnect the vacuum. ignition advance is not your friend at the emissions test. if you need to have the vacuum hooked up and working, use the ported vacuum. give it just enough timing to limp it to and from the test. you'll probably need to raise the idle speed to keep it running with less advance.

maybe turn the mix screws in a 1/4 turn also, but not too much. too lean will actually send the readings off the charts.

 

Your EGR valve should run to ported vacuum; it should never open at idle, otherwise it will stall the motor. 17 degrees is way too much timing at idle. Your vacuum advance should run to ported vacuum, such that it does not advance the timing at idle, only when you begin to accelerate. If your vacuum advance is pulling the timing up at idle when connected to ported vacuum, your idle speed is too high as the throttle plates are too far open at idle. I'm glad to see you're using a vacuum gauge to set your idle mixture, as that helps a lot.

 

"It acts like it has an intermittent miss and has always acted this way. " My iddle does the same thing specially when cold, I use a similar cam to yours, 4 degrees bigger actualy, and i think its big enough to cause a loopey iddle. So i always thought it as the cause of the intermittent miss....

 

not at all, that's the way vacuum advance used to be set up, ported vacuum is the 'solution' to a 'problem' (govt regs)

Tom Custom uses a bit more intial, here's his run-down on how he sets it up

"The vacuum advance unit is adjustable. You need to limit the total advance under vacuum. You adjust the unit by inserting an allen key (I do not recall what size, try several until one fits) into the front (some have two) hose nipple. Turn it one way or the other (been a while, obviously) until total additional vacuum advance is about 10 degrees. Your engine will like this.

In my daily butt hauler I run manifold vacuum advance. The car is a 1973 Mercury Montego four door. It's a pretty big car, about 4200 lbs. The engine is a 351 Cleveland 2V, it has a PAW/SSI 11339 (194/204 @.05, .458/484), a Performer intake, a 600 Holley and duals. I use a Pertronix Igniter in a Ford dizz with a Ford Duraspark coil. The Holley has a light sec spring, so it cuts in quickly.

I run 12 degrees initial advance, use the stock curve and use manifold vacuum for the vacuum advance unit which is adjusted to provide only 10 degrees additional advance. The result is that at idle, the engine runs smoother with the ac on (York comp that does not cycle, unless you count when I have to turn it off when the core freezes. I could store meat in that car). The vacuum provides an increase in efficiency at cruise (18 MPG with 2.75s). Being that it is manifold vacuum, the amount of additional advance is reduced when I push into the throttle. Acceleration is easy and effortless, especially in the 2-5K range where Clevelands run great. The greatest advantage of this set-up is the reduction of advance under acceleration. The US 2V Cleveland heads have an open chamber. That chamber coupled with the dished pistons of 1973 create a great deal of surface area. Detonation is a real problem here, but I am able to run the car on 87 octaine with no problems, even on hot SoCal days with the AC on. When I was trying ported vacuum, I could get it to run well but detonation on acceleration was a problem. As it is, the car runs very well."
Ford 335 "Cleveland" Series Engine Forum: Vacuum Advance questions...

 

If you state testing is at idle, you will probably have to ditch the manifold vacuum at idle when testing (someone else said that ) My state dosnt check----for now!

I think performance and driveability is much better with manifold vacuum, but lets not start that pizzin contest again. Some dont agree with that. In essence, you must "detune" your engine at idle for emissions, which includes the changing of the correct mixture at the idle screws.

You may have to find someone with a "sniffer" (a friend, not the gov'nt) to get an idea of how to tune when headed for inspection-----can get complicated if they do random tests

 

Thanks for the feedback guys. I will try hooking the vacuum advance to a ported source and redo the mixture and see how it impacts the results On Wednesday.

It looks like the Oregon test is at idle. They take an initial reading, then run the engine at a higher RPM for a minute, and then check the idle reading again- I will verify with DEQ next time I go through.

fmc400, I only opened the EGR to test the impact on the engine at idle and it is attached to a ported source. My worry is that the valve may not fully seal and allow some exhaust gas to slip by. I had pulled a vacuum on the output side of the EGR valve before I installed and it held well, but it is an old part.

hollenjoe, I thought the miss might be attributed to the overlap of the cam as well, but shouldn't it be more cyclical and not erratic?

Was also wondering if a high energy coil would help with a larger plug gap to get a more complete burn?

Thanks again,
Jeff

 

Those government regulations you're mentioning are for reduced emissions which is what we're dealing with here. All of the information you have provided, while it is verifiable and correct, is used in terms of making the engine run smoother. That's not his problem; his problem is that his emissions are too high. I think he should do what Ford did in the early 70s which was lower the base timing and run the advance from ported vacuum.

Also, Jodell, you might try dialing in the carburetor with a jet and metering rod kit from Edelbrock. Unfortunately a performance-built motor like yours is going to be tough to pass with.

 

Too much timing at idle, even though it increased performance and efficiency, increased noX levels. Even if it was used earlier, ported vacuum was a consideration for emissions standards as was the idle screw limiter caps. Both kept the engine detuned for idle emissions. Thats why I think that you will have to tune with both ported vacuum and probably slightly lean (rich?) idle mixtures when being tested. Later you can tune for efficiency and performance by whatever means you want. Smoothness is not a part of emissions considerations , at least what I think of as "smooth".

That means that you will only have what the mfg recommended setting for initial timing for advance at idle, ~10-12 deg which is too low. I think both 400 and 351 are on the same track and I agree.

 

yeah, that was my suggestion in post #2

Jodell, what did you gap the plugs at? if they're at .045" i'd leave em alone. i recall one of the major plug makers saying that you can open the gap an additional .005" but i suspect they were reffering to a stock gap of .032" or .035" ? more than .045" and you could have other problems.

you didn't miss it by much the first time, i'm pretty sure if you run ported vac and retune the mix 1/4 turn lean from high idle you'll make it. there used to be a product called 'Pass' that you add to the gas, they had a money back guarantee? you could also do the 'water dribble' cleanout method. basically you just dribble water into the carb while slightly revving to keep it running. it'll steam clean the combustion chambers. and make sure you have the engine good and hot when you get to the test.

BTW, after the test if you set it back to run manifold vac, i wouldn't let it sit and idle for long periods.

 

I went with the lower spec for gap, I think it is ~.044, since the gap will grow a little as the electrode wears.

I am curious why I should not let it idle for long with the advance on manifold vacuum?

 

To tell you the truth, i dont know, ive never had it as trouble cuz everything in the engine is perfect(oil preasure, compression,etc), although every engine ive heard has the choppy constant and not eratical as you say, so something maybe wrong? If it is is probably a cable or a jumping spark..

 

the reason to not idle for extended periods is because the cats will get hot. aftermarket cams with more overlap and manifold vacuum advance will contribute to slightly more HC's passing the exhaust valves. the cats burn any unburnt HC's that get that far. emissions years engines were all tailored to reduce stress on the cats, EFI controls fuel much better than carbs do, cams were weaned to control overlap and unburnt HC's and ignition curves were castrated at idle. all this helps the cats live a long life with no complaints from consumers about expensive replacement costs. it'd be a shame to burn up your cats at idle for no reason.

 

No cats on my truck; I had actually thought about throwing some on to clean up the emissions, but do not have an airpump to get them to work correctly.

 

Well I passed the emissions test today on the fourth try. Ended up putting vac advance on ported vacuum, timing at 8 deg, and leaned out pretty far. HC ended up ~200 and CO was at 2.1. Thanks again for the help and hopefully this can help others.