So, in a prior thread I had thought the "stutter" on the 292 chevy 6 was due to carb calibration stuff...upon further testing, that turned out not to be the case. One suggestion was to look at the disti advance curve...it was bone stock smog-era HEI. I went in at this point and checked base timing..disconnected vac advance and plugged the port on the offy manifold...and found the timing to be waaaay advanced- I'd guess maybe 30*...not even on the timing tag. Adjusting it back to 10*, the car ran like hell...no power, and lots of that "stutter". I then got a HEI curve kit, and following Leo Santucci's recommendations, set initial timing to 10* and used the lightest springs in the kit so that all the advance was in by 2000RPM. Better, but still not great. I advanced the timing to where it was prior, and the car ran great...but still has that stutter. WTF? So, since I bought the motor as a rebuild (I've put about 2500 miles on it since install, and this stutter showed up about 1000 miles ago), I'm wondering: 1. is it possible the distributor is off by a tooth? this wouldn't make sense to me since the problem seems to have not been there from day 1) 2. Could this be a valve adjustment issue? 3. Failing distributor components? 4. Something else? Has anyone else run into something like this, regardless of motor? if so, what did you do? Any help is greatly appreciated.
Just updated my post ,very similar problems to yours,but mine will stutter/brake up and stall,give it a few hours and then it starts and runs fine(in the drive way),I'm lost to man-If I find a solution I will let you know
The dist. being "off by a tooth" won't matter if you can get the timing on spec by turning it. Did you do anything to the engine just before it started the stutter? Something you didn't tell us? maybe got some wires crossed on the cap, or changed plugs? Did you check the valve timing? It could be out a tooth too. 30 degrees advance will certainly make a 6 run bad, they don't like a lot of lead. Who did the rebuild? A pro shop or what? I've seen engines that wouldn't rev up due to the wrong valve springs, no tension at closed position. Pull the cover and test for that just by pushing down on the valves with the heel of your hand, or a small hammer.
Have you tried an adjustable vaccum advance? Or try swapping the vacuum source to a different location ( as in if it's ported now, trying manafold instead? ) Also have you checked the coil's output, and have a full 12 volts running into the system? If it originally had a points distributor, and factory wiring there would be a built in resistance factor, thus lowering the voltage. Hope you figure this out before I try building up a 250. Good Luck!
63Biscuit; Like RPope said. Distributor can't be 'off a tooth', if the timing light lights up the mark where it's supposed to be. I can only offer this. I ain't an expert by any stretch, but I have a 250 and a 292, both running up to my expectations. You say it 'stutters'. Under a load, I presume? That sounds like classic detonation to me, due to air fuel mixture being too lean at that moment/rpm. But that said, the valve timing and ignition timing come first. I think 'all of the advance by 2000' (as you put it), is too much. I would stick with stock advance, in a stock points distributor, if I suspected it something was amiss I'd switch the distributor. Fact is I am using the same distributor and advance bellows on both the 250 and the 292. Set the timing around 10 BTDC with vacc plugged off. Observe that the vacc advance is working/shifting the timing, by watching chalk mark dance around the pulley with a timing light. I run point gap at .015 to .018 which results in adequate dwell to get me up to valve float (with stock springs and cam). Don't expect a 292 to wrap like a 250 would. Even with a performance cam, the torque curve starts to drop off over 4000rpm anyhow, therefore turning higher rpm just isn't practical for street driving, time to shift into a higher gear. Check coil output by pulling a wire (or more) and verifying a sharp blue spark that jumps about an inch. Don't forget that spark plug wires break down with age, if in doubt put on new ones. That's good enough. If it still ain't running good by now, it's something else. As far as deviating from posted timing advance, I have never done that. Set it right, if the motor don't run right it's something else. Fucking around with the timing from nominal masks what the real problem is................... When you've got it running like a bat out of hell with stock timing setting, experiment with whatever you feel froggy about, one thing at a time so you can evaluate how any one change is working out. Since it seems you have never got it running good from day one, you need to back up and get it running, stock settings, etc., before delving into the experiments. I have one question. You ask if it might be valve adjustment. Yes, of course it could. I always do a prelim adjust on the valves before running for the first time and leave the cover bolts out during the initial run, pull the cover with motor at idle and check them before shut down. I don't trust anybody else's. That done, you know it's right, then you don't have to wonder any more...................... If you suspect the timing gear is off a tooth, line the timing mark on the dampner at TDC, on exhaust stroke. Look at #1 rocker arms, they should both be in overlap, that is Exhaust has started closing, Intake is starting to open, height of lift will be equal using a straightedge. If one is lower than the other, the timing gear to crankshaft gear mesh is off a tooth.(assuming you don't have a flat cam lobe). That all out of the way, only leaves carburetor, bad fuel, fuel pump, float level, intake leak or obstruction, obstruction in the exhaust pipe, etc...........
Have you verified that the timing mark on the balancer is correct? If not that is the first thing I would look at. Bring it up on compression stroke on #1 cyl. and when piston is at TDC, your timing mark and pointer should be at zero.If you don't know for sure,using the light is still just a guess. If that checks alright I would check the integrity of the distributor bushings, or swap in another distributor.
Might check for distr. housing bushing wear if this is a later model distributor. Early Chevy straight sixes used bushings at both the top and bottom of the housing, while the later ones had a single, slightly longer bushing further down in the middle of the housing. Probably saved them 2 cents a unit to build this way, but they didn't wear as well and the I.D. of the bushing took on a kind of hourglass shape. Maybe someplace like Bubba's can rebush it for you. All GM ever did was offer a new housing with bushing.
Thanks for the input! Engine is currently running HEI, and I ran the wiring direct to the fuse panel for this. The adjustable vac advance is something I'm considering.
To clarify, the "stutter" sounds like a big, fat, nasty "rap" out of the tailpipes, with a loss of power. If I back off a bit, and roll onto the throttle a little slower, it seems to catch up to itself. I agree that it sounded like classic lean burn condition, but richening the carb didn't solve the problem. Changes in timing seem to be the main thing that makes any difference. But that said, the valve timing and ignition timing come first. I think 'all of the advance by 2000' (as you put it), is too much. I would stick with stock advance, in a stock points distributor, if I suspected it something was amiss I'd switch the distributor. Fact is I am using the same distributor and advance bellows on both the 250 and the 292. Set the timing around 10 BTDC with vacc plugged off. Observe that the vacc advance is working/shifting the timing, by watching chalk mark dance around the pulley with a timing light. I run point gap at .015 to .018 which results in adequate dwell to get me up to valve float (with stock springs and cam). Plugs and wires are brand spanking new. Good point about timing getting set and then figuring out what else is causing the issue - but I'll be damned if it "feels" like I can't get enough advance throughout the rev range - which certainly doesn't make any logical sense given the fairly narrow range of a 292. It ran decent when I first put it in, and in the course of breaking it in and driving it over the last few thousand miles since winter, this stutter issue has cropped up. I agree with and follow the scientific method, and only changing one thing at a time to control the experiments. Good point. I'll look to get this done in the next little bit. I'll be able to determine this when I do the valve adjustment. I suppose it's possible for the valves to have come out of adjustment as the motor breaks in? One update here - I drove it to work yesterday. In the morning,with upper 60's for temps, it ran pretty well. By the afternoon, with temps in the 90's, it was "stuttering" again...only difference is temperature. The only timing thing I can guess that would be affected by really hot weather would be the soft vacuum lines to the disti. Otherwise...would warmer weather and running conditions point at the oil thinning out more...and could that affect the lifters?
You don't say what you are using for carburetor or exhaust. If you have removed carb heat, yesterday was an ideal day for carb icing. The hot, humid air of the afternoon will ice quicker than the cooler morning air. Dual exhaust?? Headers with no heat to intake??
Carb is a 500CFM Edelbrock running 2 stages rich. Intake is a 4bbl Offy with water heat. Exhaust is Langdon's cast split manifolds to 2" pipes with porters...no crossover.
HEI control module - original or replacement? Heat sink paste between the module and the distributor housing?
The module could be either...it looks clean like a new one would...but I don't know for sure. The heat paste is a good point...will look tonight.
QUOTE=63Biscuit;5407155] One update here - I drove it to work yesterday. In the morning,with upper 60's for temps, it ran pretty well. By the afternoon, with temps in the 90's, it was "stuttering" again...only difference is temperature. The only timing thing I can guess that would be affected by really hot weather would be the soft vacuum lines to the disti. Otherwise...would warmer weather and running conditions point at the oil thinning out more...and could that affect the lifters?[/QUOTE] I have never had a motor, any motor, with stock timing run bad, where changing the timeing 'by ear' was necessary or effective. It's my belief that people that think that are just imagining that they can hear better than............. That said, with the additional information you've offered, it's my opinion that if you took off that Eglebrock and put on something with around 390CFM, in my opinion your problem would be solved. Question; Water heated Offy? My Offys are exhaust maniflod heated, the Cliffords are water heated. Just curious because, of course, that's not your problem since it gets worse as things warm up................
As far as valve adjustment goes; If they were set up too tight at the beginning, even if it ran pretty good, and it's getting worse, I'd be looking for burnt exhaust seats/valves. Compression test should show that of course...... I guess I missed it if you said, I presume you are running solid lifters? Of course even with Hyd lifters they can be set too close..... I'm thinking you have gone full circle on the timing, ignition etc. If you pull a plug wire and observe the spark jumping that is about as good as you can get.
The ice forms at the bottom of the venturi in the carburetor due to expansion, not in the manifold. It takes a lot of heat to heat the carburetor. A water heated manifold may prevent condensation, or puddling, of gas in the manifold but it does not put exhaust temperature heat on the carburetor. Hot, humid weather is ideal for ice formation. Lots of moisture in the air.
Yep, OldBuzzard; I will never try a water heated intake........... My Offys with exhaust heated work great, so why would I take a chance? A friend of mine has a 250 with the Clifford and it gets warmed up about the time he gets where he's a-goin! Around here we icing conditions aren't as likely as in the land of lakes, and like you say, that don't mean he is immune from icing-up anyhow........
When 'Biscuit comes back we'll get clarification on that. The Offys I know about, old and the new ones sold by Summitt and Speedway are exhaust heated............... There has to be enough heat to migrate to the carburetor and keep it warm. People just don't realize an L6 ain't comparable to a V8, on a V8 the carb has to get warm, just because of where it's located! But I don't read 'Biscuit's problem as ice. If it was it'd be stalling at the first traffic light out of the yard...............
Looks like some good advise on the prior posts. ( btw i wouldnt always say that) Sometimes we just need a clear head and start all over. Step one i would have to know where number 1 cylinder TDC is actually at. Make a positive piston stop with a bolt and old spark plug. Install with all plugs out and turn engine over BY HAND until piston lightly contacts bolt. Mark that position . Then turn engine backwards untill piston hits again. STOP and mark that position. Then measure the exact distance between those two marks , THATS EXACT TDC !!!! mARK THIS POSITION PERMANTLY. Now we know where tdc is really at adjust timing to 10 degrees and check advance with light. The other issue could be the control module as well , the hei's are known to retard under a no load situation and can be a problem depending which part of asia the control came from. A new delco ( made in tawain) might fix the issue as well. If you would like send me the distributor and i will run it up on machine in the spirit of just plain helping a hamber out . Can turn it around the same day i get it............no charge just shipping. The HEIs are more complicated than some think ,pickup coil and coil must be a matched polarity etc............in correct coil will retard and cause a miss as well.......... lots going on here as well........ Glad to help.........
Maybe it leans out after it warms up and the choke fully opens. I have a 250 with a 390 holley on it and I had to bump up the accelerator pump and put a bigger squirter on it to get it to quit stumbling when you punch it.
It may not be ice, but one old 216 I had took about 4 miles to build up enough ice to choke it off most of the times it happened..
I had a problem like that with my 292.I am running dual Carter/Weber carbs on a Offy intake with headers and HEI distributor.Right after I installed the carbs and took it down the road a few miles it started to miss and stutter. It gas fouled the plugs so I blamed the carbs.After installing several sets of plugs and a fuel pressure regulator still the same problem.I decided my coil was not sending enough fire to burn the fuel. So I replaced my coil , module and pick up coil (to play it safe) and that took care of my problem.No more fouled plugs,missing or stuttering.It runs great.
Heading out to the shop here shortly to get the TDC issue solved. Will post more as I can. Sasrod - I've tried that with the Edelbrock (richening up the mix with rod/jet combos, spring changes, and moving the accel pump to the biggest shot possible)...all to no avail.
Got out to the shop tonight. TDC is confirmed accurate. Only surprise at all was that the plugs weren't gapped consistently - they were between .30 and .40...I opened them all up to .50. I pulled the HEI control module to check for heat transfer goop. Only stuff there sure seemed like dielectric grease to me, but maybe that was factory? Will find a place local to check the module tomorrow. If that comes up unsatisfactory, I'll send the distributor to GMC Bubba (thanks!) for a checkup. Will keep you posted.
When that engine built ice, was it more likely in warmer or cooler weather...or maybe humidity dependent. This would really suck to be the case here, as there's no provision with the cast headers to run exhaust manifold heat.
I had similar issue's with a ChevyII 4 cyl, I found the distributor was worn enough to allow the shaft to move and I found a rebuilt and it fixed alot of the problem. the other thing is todays gas. There was a tech article in the back of a Good Guys Gazette about a year ago that talked bout timing and todays gas. The short of the story is you can run a bunch more timing and it makes all the difference in the world how your car runs. But you need to know how much advance is in your distributor and what the grind is on your cam. There was a chart in the article that allowed you to set your timing accordingly. But read the article 1st! there is alot of info in there.
OK...had the HEI module tested yesterday and it came back "bad". What that exactly means, I'm not sure, but I have a new one. I'll get in the car int he next couple of days and report up. *crosses fingers*
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