Cold start miss, then eureka?!

[351 celi]

OK, so I have searched the forum for an entire day, no golden nugget to fix my current issue. Here is the background, current situation and troubleshooting to date:

'73 250 shortblock 106k miles suspected rebuild, '77 head 69k miles, 6-1 header, YF carb, duraspark2 ignition running ported vacuum, fresh plugs, gapped .045", Taylor race wires, MSD blaster coil, static timing 12 degrees, new fuel pump and filter, all new gaskets and seals, dissassembled and cleaned head and valvetrain when on original shortblock (which ate #6 bearing and destroyed block). No milling or decking, stock cam and valvetrain, felpro headgasket, 10W-30 penzoil crap until she is all straightened out and make sure no leaks, 93 in the tank, stock C4.

Upon startup, she has a horrible rough idle, vibration, and my PCV valve is clicking open and closed (not caused by pressure in the valve cover), as if it is loosing vacuum or seeing pressure on the intake side. Under load, I have spuratic backfire through the intake. You can pull #4 and #6 wires without effecting the idle. Replaced the PCV valve, I verified I had good spark, even replaced the plugs, and ohmed and swapped wires. I will do a compression test tomorrow, but I did pull the plugs on each of those cylinders, put my thumb over the plug hole and did feel compression, and each felt the same as #5, but this tells me little. I pulled the valve cover, and have good lubrication the length of the rocker shaft out all orifices. Each valve and pushrod appears to be operating correctly and to the same degree as much as an eye can tell visually at idle and higher RPM. Once I put about 5+ miles on her, and then come to a stop and idle a moment, the car suddenly smooths out and she runs well at idle and load. The idle is like silk, power is restored, the PCV valve stops clicking, and the symptoms to not return until the car cools back down. If you then pull the plugwires, as before, the idle stumbles as it should. I pulled the valve cover off in this condition, and the valvetrain appears to be operating the same. I thought I likely had a stuck intake valve(s) or lifter(s) colapse, but cannot confirm this visually. There is no smoke to speak of, she runs very strong for a stock 250 when she finally quits missing. Any ideas, I am going nuts here?! I really do not think this is a carb or timing problem, and cannot confirm my suspicion of the valvetrain...

 

[66200i6]

Is the carb iceing?

Ric.

 

[351 celi]

No, I live in the South.

 

[tony1963]

I would say that the problem sounds mechanical, like a sticking valve.


Are the valve guides too tight on one or two of the valves?

 

[MPGmustang]

try playing with the choke, mabye it's not getting enough air, i had this problem sounds the same as described... it doesn't sound like a ignition problem, but i could be wrong... also check the heater tube from the exhaust to carb (i'm sure it's good as the choke opens when hot like it's supposed to) but i think the choke needs to be opened more, if it runs fine warmed up and bad cold then open or close the choke by finger to see if it's sufficating... the down side is i don't know what to think about this backfire problem of yours, check to see the advance it is 8-12* my 200 likes it @ 12* don't know about a 250...

i would check to see if you have vacum when cold to the distrbitor too, can't remember waht it means but you something with the timeing... (correct me if i'm wrong) but if you do then advance the timing some (this is if the choke didn't fix your problem...)

but all in all i think your headed in the right track on carb or timing related... i'd first check the carb as that is what performs differently when cold as opposed to the dsII any temp...

if you think it's valves, i've read to run 1qrt of ATF in the engine (not at HIGH revs), or mystery oil.... both might help but check the basics first...

Richard

edit: spelling/corrections

 

[351 celi]

I will check the choke today, and also the ported vacuum again too, thanks. I too thought it was the valves or lifters, but again, visually it looks to be operating the same on all cylinders, is this something I would not be able to observe? I will post back any results I find this evening. Thanks for some ideas!

 

[351 celi]

OK, choke is not even operating, always full open, no fast idle operation either. No vacuum advance at idle, it is ported vacuum. only at higher RPM, and that all checked out. Veried static timing, even bumped it up to 14 degrees. Did a compression check, dry, on #4, 5 and 6, and 138, 148, 122, respectively. Watched the valvetrain some more, no obvious issues. What else should I look at guys?

 

[MPGmustang]

the choke should be closed (or almost closed... about 1/8th of an inch open) when cold... if the engine is cold and the choke is open then hit the gas peddle, see if the choke closes... if yes idk if not then you need to replace the spring... others can shed more light on this subject....

basics

cold, choke = 1/8th inch open (ish)
hot, choke = fully open

did you check when hot or cold? when you checked was it sputtering like you described?

 

[Frankenstang]

Upon startup, she has a horrible rough idle...Once I put about 5+ miles on her, and then come to a stop and idle a moment, the car suddenly smooths out and she runs well at idle and load. The idle is like silk, power is restored

OK, choke is not even operating, always full open

the choke should be closed (or almost closed... about 1/8th of an inch open) when cold

I'd get the choke operational. A cold engine needs choke to create a richer a/f mix till the engine warms and fuel is atomized more efficiently. You've basically got a lean condition at cold idle without it...you can check this by manually holding the butterfly more closed (not closed all the way) at cold idle start, and the idle should smooth. No real reason to run without a choke...you could just richen the mixture but that would cause you to be too rich after warmup and majority of driving. Running a choke will use more fuel just till the engine warms, and you could also up the thermostat to 195* to minimize warmup time and save fuel.
Good luck!

 

[351 celi]

I do not think it is the choke, really. I have manually placed the choke in the engaged position with no effect. While nearly warmed up in the garage tonight, she would start fine, but if I blipped the throttle or pulled a plugwire, she would begin to run rough, and not return to smooth operation again unless I shut the car off and restarted the engine. Starting to think it is a carb issue. I put a vacuum gage on, when running well at idle, she pulls 18", and when rought, the gauge is very eratic as if it were going to 0 but then sees vacuum again, but then again losing it, so it appears to read between 6-16" bouncing very quickly. I checked my ported vacuum, and it goes from 0 slowly down to 14" max. On the YF carb, what is the vacuum port on the driver side (US) above the throttle plate originally supposed to connect to? I could not ever get it to read anything other than atmoshere. I think tomorrow I will have to hook up the wideband and oil pressure gauge, maybe air to fuel will reveal something. Keep the suggestions coming all, thanks again!

 

[351 celi]

Well guys, I am still fighting this issue. Since April I have replaced the carb with an RBS that I rebuilt. I have upgraded the ignition with a new coil and Accel 300+ module. I have installed a temperature gauge and vacuum gauge, and plan to install the wideband once I weld the bung into my header, maybe this weekend. I also replaced my rockers with an adjustable set I pulled from the junkyard after careful disassembly and cleaning, and new pushrods from Classic Inlines. I have disassembled my duraspark 2 for inspection, and while I was at it, installed the 925D advance kit, and added 1 qt. of marvel mystery oil. For testing, running no vacuum advance as I think my canister may be damaged, ran all new vacuum lines and caps, and have tested with all vacuum hookups bypassed. Since I upgraded the ignition, I noticed my dizzy is a little noisy, it is as though you can hear the arching. I swapped caps and rotor, with no traces of arching and no change. I can no longer trace the miss to any one cylinder, and now that it is summer, the car stabilizes sooner (my theory is oil temp/ viscosity changes how the lifters are functioning) Letting off the throttle after she reaches operating temp or coming to a stop to idle helps make the transition to ideal operation easier. Once she is running good and you let off and coast, vacuum will stabilize go to about 23", and at Idle, I have it tuned to 19". I am still suspecting lifter or valve issues. Can someone suggest an approach how to utilize my adjustable rockers if it is a lifter issue? If this does not fix it, I plan to pull the head, have a valve job with 302 springs, new cam, and lifters, and if I keep it NA, mill for 2V 1.08 autolite and 9.0:1-9.4:1 compression. The more I fight this little six, the more I think about finishing the 351W for the car if I have to do some much to get this one right again.
-Joe

 

[351 celi]

Fixed it. I completely backed the rocker off the valve and then turned it down until it had no more up and down play, but could still easily spin the pushrod between my fingers. I then adjusted the rockers while idling by backing them off until they ticked tightened until they stopped + 1/4 turn. That was it. Our springs are so light when the valve is closed, the lifter preload has to be spot on or they will pump up and leave the valves open.