View Full Version : Basic engine question - missing / rough running
08-12-2010, 10:31 PM
OK -I have been using my 57 pretty regularly for the past couple of weeks so the bugs are starting to come out. I'd like some opinions from you guys since I have a LOT to learn. (one thing I have learned is you can save a lot of time & energy if you ask questions)
I seem to have an intermittant miss or something. Feels and sounds like it is bogging down, then as the rpms climb it will clear up and run smoother. Also sounds like it misses some at idle but not always.
Truck is a 57 automatic w/a 350 - year unknown. Has an edelbrock intake and carb, headers, dual exhaust. Electronic ignition. Over abt the past year it has had the carb rebuilt, new plugs,wires, cap, rotor, coil. Tank and lines were replaced as well. Temp wise it runs btw 160 and 180. Keep in mind I did most of the repairs myself so there is a good chance I am the cause of my own problems :rolleyes:
Not really sure what my timing should be set at or my plug gap should be. If I wasn't gapped right would that cause them to foul/not fire right/miss?
Thought I may have stressed or have a bad connection on one of my wires but I checked them and they seem OK. They are not that old although they look a little rough, maybe from the heat from the motor?
Comments/ suggestions are welcomed and appreciated :tu
08-12-2010, 10:46 PM
Tom sounds like you might have a vacuum leak.
08-12-2010, 11:38 PM
Tom sounds like you might have a vacuum leak.
That's a good possibility. Does it start easy? Cold/Hot, It could be timing
08-13-2010, 09:36 AM
Thanks for the suggestion, I'll check for that first. and let you know.
It starts really easily. Hot or cold( even easier when hot)Always has. I don't even have my elec choke hooked up cause I thought that was causing a batt drain a while back. But that is a whole other thread......:)
08-13-2010, 10:27 AM
Could be a lotta things, But vacuum leak is the first guess. I would spray around your intake and carb base and see if you get a rpm change. You should be able to look up your plug gap on the internet I would guess .035 would put you in the ballpark but it depends on the plug. Timing is a function of the quality of gas you are using. advance it till you get hard starting/detonation and then back it off a bit. whatever the numbers come out to is what you can get away with with that fuel. Having said that, I doubt that is your problem. Let us know what you find.
08-13-2010, 11:02 AM
:confused0006: believe Gary has some very good points/thoughts, and things to check, I was just wondering ?Electronic ignition, what exactly are u running? msd ? if u are put plug gap at 45.
:anim_25: and keep us posted ,
08-15-2010, 11:37 AM
Snugged down carb and sprayed around it. No RPM change so I don't think I have a vacuum leak. May run a little better but that wasn't the culprit.
Gonna pull, check, clean my plugs next and see what that does.
Ignition is just a basic GM elc ign from autozone for a stock motor. Took the old stuff w/me when I got it so it is whatever the PO had done. Pretty sure he never went into the motor and did anything, think it is stock except for intake/exhaust. Other than this miss, it runs and sounds pretty good.
09-17-2010, 05:15 PM
Try with and without the Vacc Advance connected. Be sure to plug it when disconnected.
First thing I do with any "new" toy is identify the engine. I have bought three trucks in the last 4 year that had a "350" in it - none were. The 305 looks the same and yes a lot of the parts physically interchange - but not always the right thing. Get the VIN from the pad on the pass side of the block right in front of the cyl head, and you can determine what size, year and model it was installed in (sometimes there are conflicts that are resolved by the casting number on the bell).
Then compression test\tune up. Read the vacc as you warm it up - it should stay steady at around 18" hg. Look for fluctuations (rapid jumping of the needle) at various steady rpms - this will tell a lot about the valves lifters etc) As this is an all plugs out process - mark the wires first. Keep the plugs in order for later inspection. With all the plugs out crank the engine while watching the compression guage - it should be at peak within three hits, and the first hit should give at least 75% of total. When done you should see no less than 110 peak and all cyls should be close to same.
Then inspect your spark plugs. You can search google for "reading spark plugs" and see some good pics of what you are looking for. Any plug that is either extra black, extra shiny or extra white (compared to the others) is the likely cylinder thats giving a miss (if you even have a miss - later), check that wire and the cap inside (yes its new - but they do crack). Gapping will drastically effect the flame kernal when the fire first lights off - however I have seen gaps anywhere from .030 to .075 and it only showed on a scope - saw and felt no real immediate difference. The SB Chevy with the HEI like .045 - unmodified, higher compression or more cam usually means more gap but not always - you can step up voltage in the coil sometimes to meet the mods.
If the plug wires look bad and\or you have no clue how old they are - change em. I have found that the new silicone can be decieving when judging wear and tear - they can look good and cross or ground fire like crazy, or look horribly darkened and burnt and perform well (run the engine at night in a dark garage and you will see any major spark losses).
SBC timing is anywhere from 5-14 degrees btdc (more cam = less vacc = more need for static timing adv) @ 800 RPM and 32 degrees max by around 3200 RPM (with vacc adv disconnected). If you are within that with as much advance as you can run with no ping and no hard start - you are there.
Use the vacc guage to set idle mixture - highest attainable vacc with EQUAL AMOUNT OF SCREW (if you are off by 1/4 turn from equal thats not too bad - but more than that you should go in half the diff on one and out half the diff on the other).
One other good use of the Vacc guage - run it into the cab and watch it as you accelerate - if the problem comes in at a specific vacc level (vacc decreases as throttle is applied to load) - you can trace which circuit of the carb is off (at wide open throttle no vacc, fuel is coming in through all five circuits - any one of em could be off a bit and effect performance). this is mainly for the enrichment circuit tip-in point, which is VERY hard to tune in any carb other than the Holley.
Now lets discuss why it's important to know which engine you have. The compression ratio, heads, cam, and manifold create whats called volumetric efficiency. Thats basically how true to displacement potential the engine actually operates at. One would think a 350 ci engine would displace 350 inches of air per two revs - this is not the case. Stock engines are around 70% while normally aspirated race engines can hit 125% (blower can hit well over 200) AND VE itself changes with RPM due to fluid property of the airflow. The edelbrock carb is very easily over carburated - to much CFM rating for the engine and VE can make its metering and responses to the signal quite lazy. The Q-jet is most forgiving - in fact a Q-jet for a 79 350 olds and 79 455olds are the same. The q-jet is air flow DEMAND sensitive while the Holley and the Edelbrock are force feeders. Now you can run with too much carb- but it is a TON harder to tune.
An engine is an air pump - if you are not changing the air pump itself (long block which conribute compression, cam and head\valve flow to the VE formula) you will not really be moving more air - just changing the velocity of it - by manifold carb and headers changes! With bigger ports in the manifold and a bigger carb with no matching cam, valve or head porting - you could slow the air down and have fuel falling out of suspension! This would present as a miss - even though you're firing all 8. You HAVE to match what ya got in order for all aspects to work as a team.
One of the "350" trucks I bought had the low VE\Compression 305 smog motor (with a camel hump 2.02 head on one side and an 886 1.94 head on the other) with the Edelbrock manifold and 750carb kit for a 350 on it - it never ran right. I replaced with a Holley 680 Vacc secondary and got MUCH closer (and 680 was too big - the math said 470) - by the time I was done power tuning it ran quite decent (although the mis matched heads were obvious in the Vacc readings and idle quality). When the mains went out - during teardown I was kept laughing for a while over THAT combo.
Other things that can cause your issue:
A worn lifter can be overpumped, a worn cam lobe, a sticky valve (all show in Vacc reading as pulsy needle), or a restricted exhaust.
11-29-2010, 01:38 PM
Hey gang. Finally back on here, just been real busy. Wanted to give you an update for anyone interested and thank you all for the input. It is REALLY appreciated even thought I am not on here a lot!
Been driving the trk pretty often and just dealing with the rough running since it didn't do it all the time and I was hoping for a revelation or something while driving it. Well the other night I changed the oil, started it up afterwards and was standing there in the garage looking at the truck running w/the hood up. It was gettng dark and I happen to notice this little spark once in a while. Turns out I had a bad/melted/worn out spot on one plug wire and it was arcing over to the header. Only sometimes, and had it not been dusk out I probably would not have seen it. Changed that wire and rerouted to make sure nothing was touching anything hot. Guess what? FIXED!
Of couse this is only one in a long list of things that need addressing, but it is nice when something goes your way instead of something else needing fixed. Maybe I'll tackle heat next since the temps are dropping. :)
11-29-2010, 02:20 PM
Tom that is good news. Glad that is one less thing to have to figure out.
vBulletin® v3.8.7, Copyright ©2000-2013, vBulletin Solutions, Inc.