Timing

[highdesertexplorer]

I've had this issue I just can't seem to resolve. I have a 78 with an 1800, a weber 32/36 carb and a crane electronic ignition. The engine is a fresh rebuild with about 350 miles on it. Stock cams (for now), high compression pistons, larger valves. Running premium fuel.

When I set the timing to 10° advanced it's a little hard starting (but turns over freely), idles pretty decent with a slight miss until warm. when accelerating from 2500 RPM's I get a hesitation and it start's missing. Runs great at higher RPMs. When I tune the timing by ear (the way I always did my '68 mustang) the timing shows about 25° advance at idle, but it runs smooth with no hesitation. But then it can be very hard starting due to pre-ignition...yet runs like a raped ape. But I'm too worried about detonation, and won't run it like that.

I've checked everything over a dozen times and cannot seem to figure out why it wants to run with so much advance timing. TDC on the belt cover is correct (verified with dial indicator), valve timing is correct, ignition timing is correct. The distributor advance appears to work properly. I checked the carb jetting and it appears to be as recommended.

Any ideas?? I'm completely out of any myself. :-/

[dmwhiteoak]

I am having the same issue.What is a dial indicator? Im thinking my crank may be off a tooth or two ? Its hard to get it spot on with the marks being on the cover. Looking foward to others ideas along with ya.

[racydave]

Obtaining the proper timing spark event is very important to tuning (nano seconds). At best all you will achieve is a compromise. I have messed with dizzys for a long time. That's why I run the mega-jolt totally programmable system. And with a crank sensor, I do not deal with the mechanical imperfections created in the timing belt and camshaft. The wear in the dizzy is a problem also, as the bushings and backing plate can wear. With the cam and possible vaccum problems, I think I would set it up without the vaccum advance. The best way to hold the backing plate in place is to keep the advance attached. If you want to check the backing plate, apply vaccum to the advance, and play with the plate. Any slop or wobble noticed is a problem. Buy, borrow or steal a timing light with a dial to tune the total advance. With the centrifugal advance working, you should achieve total advance around 3K. Set the total advance to around 36 BTDC, and then check the base timing. I run around 17BTDC at idle, but my system will crank at 10 BTDC and increase at idle due to RPM's. Some adjustments can be made with different springs on the centrifugal advance. I had my cam's dialed in when my head work was done, on the bench. I don't think you will be totally happy until you tune them to the sweet spot. Anyway, I hope this may be some help, as I seem to ramble on a bit. Happy trails!

[highdesertexplorer]

I rebuilt the distributor and don't have a vacuum advance. I made my own springs for the mechanical advance. I tried to set the curve to advance smoothly through 5K. You say it should be full advance at 3K? That seems too soon to me, but I can put some weaker springs in to give it a try. However, I don't have any top end (4000+) issues, it's the low-mid range that is giving me fits. I am hitting around 30-35 deg total advance.

I disagree about the timing needing to be so precise...this is a '78, not a brand new computer controlled high performance engine. Setting the timing to 10° advanced using a conventional timing light should be sufficient and keep the car running fairly well. It ran well on dual mechanical points for 40 years with zero electronic control, It should run just as well with a small upgrade, if not better. But I'm not hitting the "just as well" point yet.

I also set the cams on the bench. Afterward they lined up perfectly with the marks on the housing. Stock cams shouldn't need all that much tweaking. Maybe with the high compression pistons they'll need some...I don't really know.

I should also mention that the spark plugs are black. It's running rich, that's why I checked the jets. I was wondering if a too rich condition means that the fuel will burn better with a higher advanced spark, because closer to TDC the mixture is too saturated with fuel to burn properly. Or perhaps not vaporizing enough under compression. So perhaps dropping the jet size a bit might help...but as I mentioned they seem to fit what I've read is recommended.

I do need a vacuum gauge, as I haven't been checking it since initial start up.

I appreciate your input, it has definitely given me some things to think about.

[opus10583]

Hi,

Sorry to glom-on, but I've a similar problem, discovered last night: 1800, rebuilt 320miles ago on a NOS shortblock, lightened flywheel, stock cams, 32/36 DFEV, 4-2-1 manifold, rebuilt dual-point distributor with Pertronix ignitor swap, Pertronix 3ohm coil, timed 10deg btdc.

In my case there's a stumble/miss/roughness under load at speed with gentle acceleration into the secondary, 3-4,000, full throttle acceleration either masks the issue or eliminates it.

Anyone have any thoughts?

Thanks,
Mark

[Fi8spider]

Always tough trying to fine tune over the net though I think you've touched on it here.

I should also mention that the spark plugs are black. It's running rich, that's why I checked the jets. I was wondering if a too rich condition means that the fuel will burn better with a higher advanced spark, because closer to TDC the mixture is too saturated with fuel to burn properly. Or perhaps not vaporizing enough under compression. So perhaps dropping the jet size a bit might help...but as I mentioned they seem to fit what I've read is recommended.

Is the carb still jetted for stock engine? Dropping main jet size now you've bumped up valve size and comp ratio doesn't sound right. Air corrector jets are only a few bucks each and playing around with them during fine tuning can deliver some good results. The primary idle jet plays a bigger part the than the name suggests too.

[vandor]

Are you sure your timing marks are correct? Might be worth checking.

The stock 1800 distributor had 36 degrees of centrifugal advance, so if you are starting with 10 degrees at idle you have 46 degrees of total advance. One would think it would ping like crazy if it had that, so maybe your springs are limiting total advance?
Either way you need to check the advance at 1000 rpm intervals to really know if it's correct.

[highdesertexplorer]

Thanks for the responses guys.

Fi8spider: I would like to make some changes to the jets, but wasn't sure where to start. I'll try as you suggested and see where that takes me.

vendor: I verified TDC is correct. I'm using the stock cover, I'd assume that the marks are accurate, maybe not?? I'm also visually guessing at the total advance based on what I see with the timing light. How advance should I be seeing per 1000 rpms? So far my only preignition issues are while cranking the engine over. That's what is really confusing me. If I tune by ear, I end up with tons of advance timing and hard starting, but it still doesn't ping at high RPM's, and I can't see how that's possible. Timed at 10btdc I'm missing on the low end, but she still loooooves the top end.

Thanks again...your help is appreciated!

[sptcoupe]

The problems sound they could very well be a dirty idle jet, which controls the carb circuit from zero to about 3500 to 4000 rpms. When I have a miss in the lower rpm range or under steady throttle below about 4000 rpms, that's what I check first.

[highdesertexplorer]

Everything was clean when I checked the jet sizes. I took her for a run around town today. When I got home I readjusted the idle per weber (again). I am now less than 1 turn on the mixture screw, and maybe a 1/4+ turn on the speed screw. According to the manual, that means it's too rich and I need a smaller idle jet. Apparently I didn't have it adjusted as well as I thought, but I've also been messing with it in far cooler weather.

Before I go buying and changing jets, is there anything else I should check? I do know the butterflies are closing fully, so that shouldn't be an issue. I also added about 4° advance to the timing before I left...seems to run pretty sweet there. Still a miss at idle though; presumably due to the rich condition before my adjustment, but it did seem to pull better under load.

I'm pulling the rear end this weekend to clean/paint, change seals/gaskets, replace all the arms, replace the springs and shocks, replace brake seals, replace U-joints and the drive shaft bearing. Once that's done, my suspension front and back and drive-train will be 100% rebuilt and/or new.

[bradartigue]

IIRC the DFEV comes with a 50 idle jet and I've almost always replaced them with a 45 on the 1438/1608 and a 47 on the 1800/2000. That was for cars with stock cams.

[highdesertexplorer]

Mine has a 60 idle jet. According to http://www.webercarbsdirect.com/v/vspfi ... 36DFEV.pdf it's correct. Could that be a 57 and 55 perhaps? I've only seen 55's so far, and that's what I'm considering getting. Also considering upping the air corrector from 165 to 170 per Fi8spider's recommendation. I figure it can't hurt.

[garion]

This a pure guess...

Could the cams be swapped? Is that even possible? WOuld the car run if they were swapped?

[toplessexpat]

I'm assuming your setting everything up with #4 TDC not #1?

I always do my timing without the cover on. The vaguely accurate piece of 35+ year old plastic and fittings vs general wear and tear means popping it off and doing it is more appealing, and an appropriately applied set of white-out markings makes it almost a joyful process.

Brad makes a good suggestion on the correct jetting for you.

A

[highdesertexplorer]

@garion: the distributor run on the exhaust cam, so no...on all counts.

@toplessexpat: yes, tdc #4. I'm considering making my own timing indicator and losing the cover. But I'm positive the tdc mark is right on. You also think I should go from 60 to 47 on the idle jet (change of 13) or 50 to 47 (drop by 3, which in my case means 57)??

Thanks guys. I really appreciate you all helping me figure this out!