73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Performance => Topic started by: LTZ C20 on June 07, 2014, 08:16:28 PM
-
Hey all. So here's my problemo, my truck doesn't wanna stay running. I have a new engine that I built and now a new transmission. A lot of you, like Capt, Bake, Rich W and some others know I have had some issues with the engine.
So know I got everything all hooked back up again. She is just about ready to drive around, just minus a exhaust pipes. Open header at the moment. When I try to start it, it either starts right up or struggles and then starts. Then it will either cough and sputter and die OR I can give it a little gas, it will pick up and run and as soon as I let off the pedal, chokes and dies.
This morning, when it was cold, I actives the choke, started it and she ran. She ran good for about 10 min at 2 grand. When at op temp, I bumped the choke to the second stage and it ran there for about 5 min, then gave it a little gas and then eased of the pedal and it went to idle and idled no problem. After that I could shut it off, start it again, run fine. Let it sit for 5 min same thing, ran fine. I left it sit for 10-15 min, it doesn't wanna run, just struggles all round.
I have taped on the screw heads for the valves in the bowls of the carb to make sure they aren't sticking. I adjust the mixture in and out 1/4 to 1/2 turn both in and out with no change, still struggles. When it was running good and the choke was off I crept the idle down to 500 or so and it idled for prob 10 min easily.
It's like it's lethargic lol. Unless it's at 3,000 rpm it don't wanna stay running. It's a Holley 600 cfm electric choke, model number 80457S. While it sat for 6 months putting the transmission in, I never adjusted anything.
I need to make this start and run every time cuz as soon as exhaust is done and it's registered it becomes my DD again and my brother who just got his license will be Getting the Tahoe I've been doping around in for half a year. I miss the truck and I need to be able to drive it around the block to make sure the new trans is working. I haven't even been able to pull out of my driveway cuz the motor is being a poop.
-
What's your base timing set at? Have you checked to see that the centrifugal advance is coming in? How much centrifugal advance comes in when it's all in? Are you running your vacuum advance on timed vacuum or manifold vacuum? How much vacuum advance comes in? Are you sure the choke is closing to the right setting? Should be a gap between the choke plate and the bore even when the choke is all the way in, and usually, that is adjustable -- not sure about the Holley.
-
Base timing is at 12-14 I believe. I have not checked the centri advance. Not sure how much comes in either. It's a brand new GM performance HEI compete distributor that went in when I built the motor. I read that vacuum advance article on here that a member posted awhile ago. It's hooked to mani vacuum. I set the choke cap to be right in the middle. I believe there was 9 or 11 marking on the cap and I set it to the middle so it wasn't too soon or too far. At choke full on there is a small gap still between the plate and bore yes.
When I was driving it before the trans went out it ran pretty darn good. Wasn't perfect but I know it has to be fine tuned for max performance and efficiency. Orient get that done before the trans quit.
-
Correction on last sentence. "DIDN'T get that done before the trans quit."
-
Here's the curves on the distributor I think you have.
http://paceperformance.com/i-6251715-93440806-gm-performance-hei-distributor.html
Compared to the stock distributor, the centrifugal advance comes in late (mechanical advance should be all in by 3000 rpm). Also, that's a ton of vacuum advance, 20* where I think stock is more like 8*, and that 12-238-2 cam pulls a lot of vacuum, which means it's going to come in early.
Do you still have your old distributor around? Might be interesting to swap in a stock distributor and see what you get.
Oh, and you did check to see that the distributor is still where you set it, right? That it didn't wander because you were too light on the clamp?
Another test would be to retard the base timing to compensate for that huge vacuum advance and see what you get. A further test would be to connect it to ported vacuum (per the mfr recommendation for this distributor) to see how it idles with that huge vacuum advance cut off. (BTW, I'm the guy who posted that article about ported vs. manifold vacuum, but I still think it would be interesting to see how it idles without that big vacuum advance in.)
This article is interesting. Read forward from this point:
http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Ignition_advance
-
Yea that looks about right. I got it from work. (Work at Chevy Dealership)
I don't know where the old distributor is. I don't know if I have it either. Might have given it to a friend, might be in the garage somewhere. But it's old, crappy, doesn't work well and it's disgusting dirty and is not getting near my new engine lol.
The vac advance use to be at ported and I moved it to mani after I read that article. Is it possible that's my problem? I might be sucking it to far advanced. Forgot to mention at high rpm with open header I'm hearing a pop every once in a while. Not sure if that's normal with open header or timing to far??? It sounds kinda like if u got to cylinders swapped on the distributor and they are firing out of order but no where near as loud and scary. Maybe I should put it back to ported vac???? Should I drop my timing down??? This thing would pull hard if I jumped on it when it was running beige trans crap out. I would love to see how she runs fine tuned and with this new 700.
-
You could try ported vacuum, then you could try backing the timing down to maybe 4-6 degrees base timing. That at least would tell you if timing is the problem.
A lot of "performance" products are just that: if you are going to circle-track race the vehicle, or do drags or something, then it's the part to have, but they are not really made for street use.
You know that our engines are very similar in the bottom half of the rpm range, until those high-compression, oversize valve heads you have kick in with that cam's longer duration. I have the same torque peak at the same rpm. I run the stock distributor, though with Accel high-voltage coil, cap, and rotor. But I have the stock advance curves both mechanical and vacuum, and it starts and runs great.
So try ported vacuum and see how it idles. Then try backing off the timing to about 4* BTDC and see how it runs. If things clean up a lot, then you know that's the problem. You will need to put a stock distributor in it to really tune it for best street performance: if the advance curves on that distributor are the problem, you will never get it to the best street performance until you change it out or rebuild the advance curves.
-
Ok I will try that. Not sure how I'm gonna get the timing adjusted if it won't idle. Lol gotta try ported vac first. Could that cause poor transition throttle too? A couple times when I tried to rev it to get it to run it would hesitate for a second then catch and rev up.
-
Usually poor off-idle transition, if it's timing related, is from running on ported vacuum, but you have so much vac advance in that curve, you're in uncharted territory for me. 14*+22*+20* is 56* of total advance, which is too high. That's why I think backing off base timing 10* is a good idea.
To get it to idle, you could just loosen the disti and twist it CW a 1/4", and work from there.
-
Ok. So I'm still kind of confused on how to tell which advance is working when. When is mech advance activated, when is centri activated and when is vac activated?
Also if I turn the disti Clockwise, isn't that advancing? Or am I backwards. I can never remember which way is which. I should, I've had to do it enough freakin times.... I struggle with the finer points of tuning and such. I'm no pro yet haha.
-
Mechanical advance and centrifugal advance are the same thing. If you have the vacuum to the distributor disconnected, at idle what you see is "base timing". Rev the engine up to 3000 RPM, and on most distributors the mechanical (centrifugal) advance will be "all in", and you will see what is often called "total timing", which is base timing plus mechanical advance. Reconnect the vacuum advance to a manifold vacuum port, and what you will see at idle, when the vacuum is high, is base timing plus vacuum advance. At cruise, at highway speeds, you will actually get base timing plus mechanical advance plus vacuum advance all in at once. This number should not be higher than 50, some say 52. Right now you are at about 56.
Note that when starting, the engine is not yet pulling any vacuum, and you are starting the engine on base timing. As soon as it starts, the vacuum advance pulls in. With that base timing and that much vacuum advance, you can't keep it running. If you back off the base timing 10*, that will compensate for your vacuum advance being 20* instead of 10*. So if timing is the problem, backing off to 4* BTDC for your base timing should make it idle really nice.
The rotating pole piece on the distributor shaft moves clockwise, that means if you rotate the distributor body clockwise, the rotating pole piece will line up with the stationary pole piece on the distributor body later in its rotation, giving a later spark, i.e. retard.
-
Ahh ha! I see. So I first tried plugging both vac sources as run on base time, worked ok for about 1 min. Then put it to port vac, it didn't like that. So I went to adjust the disti and found the bolt was LOOSE! So I tightened it just to the point I could barely turn it with a small amout of force. Rotated it CW with no change. 1/4 inch retard and advance with no change. Went about 1/2 inch retard and it started to struggle so I went back to the middle then crept slowly CCW and it started to pick up and run better. I then tightened the disti clamp and put the vac advance back to ported vac, it was disco'd and the port vac was plugged while adjusting disti. I then revved it a few times and it's running good it seems. Idle was a little idle, small tweak, in park idle is about 550 or so and in gear idle is about 450. I thought that was slow but sat in gear for while, at 450 and it never struggled or popped or wanted to die or acted like it was starving for fuel. It just say there and idled at 450. So I shut it off. Fired right back up and ran. Then I closed my hood and slowly crept down my driveway, onto the street, went about 20 ft, turned around and came back into driveway. NO PROBLEM RUNNING OR MOVING WITH JUST OFF IDLE ACCEL!!!!
Choke seems to be working good. I don't own a timing light so I will have to borrow one and recheck but since I went to 1/2 inch or so CCW. Be running good. Now it's been sitting while I'm writing this. Gonna let it sit for prob 15-20 min total, then start her and see how she does.
My concern now is, yea I must be close, gotta check it with a light to be sure. But above 2500 rpm I think, maybe it's between 2 and 3 grand, when I hold it there, I'm stilling hearing that popping noise. Like I said when to plug wires are crossed on the disti cap. Could my CCW rotation be just a little to far causing popping from the exhuast? Is that normal for open header?
-
OK, so loose distributor, the rotating shaft dragged it way retarded.
Spec is 550 RPM idle in gear. Usually about 700 in Park.
Not sure on the popping. Quick google search says it's probably the headers.
https://www.google.com/?gws_rd=ssl#q=popping+open+headers
-
Yea I figured. I know spec is 550, but at 550 in park and at 450 in gear it sounds good, runs good. 700 sounds like it's idling to high. Maybe it's the header over stating it because it's so loud?? Not sure. I just drive up to the stop sign and it really seams to like 450. No problems idling there.
I hope that's all it is. Won't know till get pipes and muffs on it.
-
Any ideas how to take the slack out if the throttle cable? Seems a bit stretched in it's old age and isn't going quiet all the way to wide open.
-
So here we are! Truck is all tuned in and running like a champ. I borrowed a timing light from my girl friends brother and a vacuum gauge from a guy at work. Got timing set to just bout 4.5 deg BTDC. Then I removed all items plugged into vac ports on the carb and capped everything off. Brake booster, PCV valve, vac advance, even plugged the vac line to the HVAC control vacuum pod thingy on the firewall above the blower. That was after I discovered that the vac port on the mani was loose just a bit and the vac line to that HVAC pod was completely open!!!!
No wonder I'm having trouble! That's the FATEST vacuum leak in the history of mankind lol. I looked at the line and thought, "ok it's connected, wait what about the pod side?" Then I looked over at the line and went "SANTA MARIA!!" I'm an idiot!! Hahaha. Been trying to get it goin with my head up my butt this whole time.
So anyway, highest vac has been achieved, timing and curb idle set. I even adjusted the choke to run at 1500 at full on, 1000 at half on. Not high but elevated idle and it's been working great. Now park idle speed is about 700 and in gear idle speed is floating nicely at 550 or so. Allowing for fluctuation and the tach being not exactly a perfect number of course. Now she purrs like a kitten, sounds great, and runs AMAZING. I'm soo happy ad proud of myself lol.
I also, disconnected the tv cable for trans and then reinstalled it and readjusted it to make sure it was right. Truck is shifting into all gears, trans is working nicely, shifts are solid and positive, returns to idle every time, no hesitations, misses, or stumbling at all. Even got on it a little on my little test drive and she really goes. Open header makes a nice sound at 3 grand too. ;D
Thanks everyone for your input and guidance. And thanks to Rich for being patient with me and explaining everything in technical and in easy examples. I have been well educated. Thanks again all! Love this site. :)
-
And I love posts that people got their truck to where they want it!
What was your peak vacuum, BTW? I imagine you're pulling 18" or so with that cam and those heads, but I could be surprised.
-
Glad to hear you got it running again, nice to have your truck running where you want it.
-
Oh yes it's very nice.
Rich, with everything plugged and running at idle in gear it's about 8". I'de guess it's probably higher once the vac advance is plugged in. She runs soooooo good.
-
Vacuum will be highest at idle in Park. Vacuum is an inverse measure of the load on an engine: more load, less vacuum. Least load is idle in Park.
Your torque curve below 3000 is about the same as mine. It really is sweet to have 420+ lbft on tap coming out of the hole!
I think the big issue, other than plugging your massive vacuum leak and tightening down the distributor, was to get the ignition dialed in, taking into account that massive vacuum advance. At idle you are running 4.5 base plus 20 vacuum advance. I'm running 17 base plus the stock HEI for 1978 light trucks, which is 7.5 degrees vacuum advance. Notice anything about those numbers? :)
-
Oh ok. Did not know it was an opposite thing. I'm gonna run her for awhile like that and see how she does. Last night her overall performance was outstanding I would say. What I expect from a brand new engine.
Yes 420+ lbft is very nice. ;)
I would say it sounds like our pulling less vac advance then I am. As for as I know, the GM Performance disti is not much different than the stock ones. They had 2 different ones in the catalog. Mine is basically a stock I think, except for the higher advance of course and maybe different weighs and springs on the mech advance. Also the aluminum housing is a little nicer.
The other I believe is the same inside, however the housing is a BILLET finish. Which I do not like and is way more $$$. If I did my current vac advance is to much I'll do a some adjustments like put a stock advance diaphragm or something like that.
-
It's not so much an opposite thing as it is the total. You and I are both running 24.5 degrees advance at idle (base plus vacuum advance), we just get there a different way.
-
Ahhh, I get it. Ok cool. So I should be good then. :) thanks Rich.
-
Ahhh, I get it. Ok cool. So I should be good then. :) thanks Rich.
Yeah, you're good. That big advance is cool for the 9.5:1 compression ratio you are running. When I get on it hard at low rpm, my vacuum advance goes away and I am running at the base timing of 17 degrees. When you get on it hard at low rpm, your vacuum advance goes away and you are running at the base timing of 4.5 degrees. Which means you are *never* going to get knocking, and won't need premium gas, whereas at 9.5:1 with my 17 degrees of base timing, you might.
-
Sweet. Now I know what to expect when I jump on it. Haven't really seen what that motors got yet. Just a few tastes of a little power.
I'm running 91 octane anyway. I like it better with good fuel and should help keep insides a tad cleaner than regular juice.
-
Sweet. Now I know what to expect when I jump on it. Haven't really seen what that motors got yet. Just a few tastes of a little power.
I'm running 91 octane anyway. I like it better with good fuel and should help keep insides a tad cleaner than regular juice.
Not correct. 91 is not better, it's different. It's slower burning. I don't think it will do anything to help keep your engine clean, and it may cost you a bit in horsepower, torque, and mileage.
I calculate your dynamic compression ratio at about 7.9:1, and anything lower than 8:1 is fine for 87 octane. Particularly with 4.5 BTDC static timing. Running premium is almost certainly not necessary.
Those heads with that cam and distributor, the more I look at it, is really a sweet setup. I'd run regular and just keep your ears open for a while when you pull hard up a hill or anything like that, but I think you are pretty far away from the pre-detonation wall.
-
Mmmmm. Ok. That's interesting. I was told with these heads I would need 91 octane. So let me ponder you this.
Would be ok to run on mid grade? 99.9% of the time I always use Arco gasoline. Knock on wood, I've never had a problem with the quality of fuel. So would it more beneficial to run regular 87 OR would it be better to switch to a better 87. Say slowly work her over into Chevron 87 and just stick with Chevron regular?
I haven't spent 8 grand on and engine and transmission to dump crappy fuel in her. I wanna use what's gonna be best.
-
I think with those heads you might need 91 octane if you were pushing the timing, but that distributor running base timing at 4.5 degrees because it has a 20 degree vacuum advance helps a lot. I think that's why that distributor is set up that way. That said, you should be paying attention when you go to low vacuum (high engine load) at low rpms -- slugging up a hill or whatever. If you hear any knocking, go up one level, like to 89, and pay attention again. So no loud stereo for a while, so you can hear the thing if it knocks. With the stock dished pistons in there, though (You didn't change pistons, did you? As I recall you left the bottom alone.) I would be surprised if you had problems running 87. Flat-top pistons, different story. 91 octane all day long.
As for fuel, I always buy from a name-brand retailer. I like BP, Shell, Mobil. Cheap gas may not even meet spec, or may have other problems. I think you are better off running 87 from a major than 91 from Lester's Bait, Beer & Gas.
-
Ok so that means that 87 from Arco should be fine. They are name brand and I never get gas from scooter's stop n rob.
The block is a GM Performance fast burn 385 originally. Now it's bored 30 over, has all new Clevite bearings, the original crank to match the block, new rods and pistons are stock style, with 4 valve relief but larger diameter obviously for the over bore. They aren't stock made tho, they are Speed Pro pistons and rods, and the rings are either Clevite or Speed Pro, can't remember.
-
And the block ha been completely hot tanked, bored honed and the main and cam bearing perchs and caps have been line honed I believe it's called. Wen they bolt it together and run the cutter tegu and make it all uniform and equal. I think the only thing that wasn't done was a blueprint and balance.
-
I mean yea it's balanced but not like you get when you do balanced and blueprinted engine.
-
If the Speed Pro pistons are flat, run 91 octane. If they are dished (Is that what you mean by "stock style pistons"?), then the 87 octane should be fine. Like I said, test it. And fill it only half a tank with 87, so you can add 91 and get to 89 if you get knocking.
-
If you turned the piston upside down and put it on the table, it would rest flush on the table. Without the middle being higher than the edges. So it's flat across the top but with 4 valve reliefs.
-
I have used 91 in it since the very beginning of this engine.
-
If you turned the piston upside down and put it on the table, it would rest flush on the table. Without the middle being higher than the edges. So it's flat across the top but with 4 valve reliefs.
Use 91 octane, then. Your compression ratio is higher than I thought.
-
So I was on the right track anyway.
-
So I was on the right track anyway.
Yup. And with that higher compression ratio, the large-vacuum-advance distributor with a low base timing is looking smarter and smarter. Means that the spark will be way back from the detonation wall when you pull hard at low rpms, even with that compression ratio. Nifty.
-
Sweeeet. That sounds great to me!! Thanks Rich! I will have to clean up the engine bay, take some good pics and maybe a video and post it up.
-
Put your full setup in that post, please. Block, pistons, cam, heads, distributor, etc. With part numbers. I think you have a pretty sweet build there, and other people will want to know.
-
Oh don't worry I will. I started one but it kinda got post bones and disappeared. I have to dig out all the paper work and receipts. Go thru and collect all my pictures from the build, organize and sort thru them. Then I will build a complete post. It's going to be everything I have done to the power train. From trans cooler all the way to rear diff. Engine, trans, driveline, axle, cooling system, a/c setup, all that good stuff.
heck, I gonna have to break out the laptop and do it on there cuz that's just to much for my phone and thumbs to handle. Lol