Momar

Well, I just replaced my cc quadrajet w/ a 750 demon dp. I also had to go w/ a mechanical/vacuum dist obviously. Well, my brother had stock one that he used for a little while so I am using it. The problem is, that I seem to have very little mechanical advance. I checked it, and it is only getting about 8* mech advance. It has a curve kit in it and the advance comes in pretty quick, but shouldnt I be getting more than 8*?

The reason I was even checking is that it seemed that the car pulled pretty good down low, but when I took it up above 4000, I got a poping/backfiring noise. I have not heard detonation before, but thought that in case that was what that was I would retard the timing some. That made it run worse even lower rpms. I tried advancing it and it got better. I tried advancing it some more and it seemed to run better up high in with just a pop here and there. When I turned the car off, the starter would hardly turn the motor because of how much base timing I had set. I am not sure the exact timing because my timing tab has mysteriously dissappeared. I marked around 0* on my timing cover and I think I started w/ about 8ish degrees, and I know I was running about 12 w/ my cc carb, and it didnt have a hard time turning over so it was set higher than that to even come close to smoothing out the high rpms.

The dist has a curve kit in it, and both springs are the mediums. The timing comes in pretty soon but like I said, it only has 8*. I took it appart, and modified the slots in the advance plate or whatever it is called to allow it to advance more, but it is catching on the bottom of the pegs where the springs attach so I will have to do some more grinding. Is there anything else I can do? Why would the dist only have 8* of mechanical advance in the first place? Isnt that kind of low?

I am planing on going back to the track this coming saturday so I wont be able to afford an aftermarket dist by then, but will probably end up getting one. I have run a best of 9.410 in the 1/8th mile, but that was without the demon carb, or my spohn lca's. I was having some kind of problem up high before that felt like I was running out of fuel, so i was hoping w/ my new pump and carb that I would do better, but if I cant get this problem fixed I am liable to do worse. Am I on the right track by lenghtening the amount of travel those slots allow? On the upside, it feels more powerful and responsive then it did w/ the other carb, and even shifting at 4000 or so I chirped them into third just shifting quick, and it didnt do that before. I think once I get the timing issue figured out it will do pretty well, but this is the last test and tune night of the season at the track.


Ben

Momar

ttt

five7kid

iTrader: (14)

Without having an exact known TDC, it's a little hard to say you've got things right.

But, some options.

1st, do you have the vacuum advance disconnected while you're doing the initial setting? Even a little vacuum on the advance can throw you off.

2nd, are you sure you have zero mechanical advance at idle? That's a tough one, and is best determined on a distributor machine, but it's important. If the weights are swinging out partially at idle, lengthening the slots isn't the answer. Try stiffer advance springs and see what happens (again, w/o the vacuum advance hooked up).

A last desperate resort would be to remove the advance springs, disconnect the vacuum advance, start the engine & run the RPMs up to about 2500 and while holding the throttle absolutely steady set the timing for maximum RPMs. Shut it down, reinstall the advance springs & vacuum advance, run it that way.

Momar

Well, I put a mark on my timing cover where tdc is on a motor I have setting in the garage is so it is close. I also marked where I had my base timing set at even though I dont know exactly what it is that way I could check my mechanical advance, so I know that the amount of mech advance I was getting was right. I will try stiffer springs to make sure it isnt advancing at idle though. I plan on getting a timing tab on it soon, but I just havent been able to yet. Oh, and I did have the vacuum advance disconnected.

Ben

Damon

Real common for an old HEI to get the centrifugal mechanism "gummed up" or otherwise just not capable of giving full advance any more. It's not usually the springs and weights- it's the advance plate trying to rotate around the distributor shaft- it gets gummed up in between the two of them and you get slow or restricted advance. It's also possible somebody before you got the weights or centerplate installed upside-down or backwards. It'll never work correctly like that. Also, if the centrifugal weights have "eaten into" their fulcrum pins it will, likewise, do bad things to the advance curve.

A properly working HEI will usually give about 20-22* of centrifugal advance, depending on the centerplate used, but most are right around that number.

For as cheap as they are to buy at a junkyard.............

Momar

Well, I am pretty sure it is advancing as far as the slots will let it. I dont know for sure that it isnt advancing at idle though. The reason I believe it is fully advancing is because I did lenghten the slots(before I even got a reply here) and got a bit more advance, but still nowhere near enough. The advance plate seems to turn freely, so I dont think it is the problem. The weights, I am not sure about though. I will have to check into those.

I am pretty sure that this timing issue is what is causing my drivability issue, but will the lack of timing make it run like crap up high? It seems to get a bit better the more advance my base timing, but if I advance it enough for the higher rpms to even seem somewhat close to what they should be my starter can hardly turn the motor over.

There has been a curve kit put in the dist. This is the first I have ever messed w/ one, so how will I know if everything is in there right. It seems that if you turn them over it wont let it hardly turn at all so I am "pretty sure" that the weights are in there right. There are plastic "bushings" where the wights pivot. They seem kind of tight and I kind of wondered if that could be the problem.

Thanks for the help guys I really appreciate it.

Ben

RB83L69

Check the cam that the weights pivot against; if it's upside-down, it won't work right.

The cam plate is the biggest variable from one junkyard dist to another. You can replace all the other calibration parts (weights, springs, vacuum can) off the shelf; but I know of absolutely nowhere to get that stupid little cam plate from. You just have to keep looking around until you find one that suits your engine's wants.

Momar

By the cam plate, you are refferring to the piece that goes between the weights right? This one looks like it is new and probably came with the weights if it is what I am thinking about.

Ben

RB83L69

Ues, it is the piece that's about an inch and a half long and 3/8 inch wide, that the arms of the weights work against.

Try turning it upside-down from the way it is now, and see if it works any better.

Momar

Sounds like a plan.

Thanks

Ben

rjmcgee

Can't you check the timing while just cranking the engine over with the starter? You would have to unhook all of the sparkpkug wires or something so it wouldn't start with out disabling the ignition. That would eliminate the mechanical advance at idle, I think. Yeah, unhook all spark plug wires except #1 and hook your timing light up to it. Have someone else crank the starter and you should see your base timing and rule out your mechanical advance as the problem, or pin point it. Somebody correct me if I'm way off base here.

Momar

Well, it will probably be friday before I get a chance to try any of this, but no one ever answered about how a motor will normally act up high if it isnt getting enough timing. I can see that there is a prob w/ my timing, but I just wondered if it causes an noticable/audible problem or just a loss of acceleration. I have the new carb, and different dist, so I just wondered if you guys think that the timing is the problem i am hearing/feeling?

As far as the carb, it is setup how it came out of the box so far other than I tuned the idle mixture for best vacuum. Another problem I have been having, is for the first little while when I start the car after it has been sitting for a few hours it kind of surges. Once it warms up it runs fine? Any idea's on what this is?

Thanks

Ben

RB83L69

It will act lazy. As it climbs through the RPMs it will have no particular desire to speed up any more.

=lean

Momar

I will get the stuff and tune the carb at some point(hopefully soon), but right now I have some other places where my money needs to be. I figure I would rather get my timing set up right first anyway, so I can know what I am working with when I tune it.

You say that surging is lean? What would that mean if it only does it during warm up? Do you know what would need changed?

Thanks

Ben

RB83L69

the choke

Momar

OK, I will mess with that some.

Ben

Momar

Well, I dont think that the amount of advance is my problem. I think I am getting proper advance now. I set my full timing w/ base, and mechanical to 36. It still had the problem. It got worse if I retard it more and better if I advance it more. I am running pretty rich at least when I am in the gas. My friend rode behind me and said it put black smoke out when I got on it. Well, I pulled the plugs out today, and they were kind of tanish, but had some black to them. I think that the idle mixture is good, but when I get on it it is running rich. I am running an 8 psi fp. I have a regulator so I can turn it down if you guys think that may help. It is not coming out of the bossters, or anything while and the idle mixture seems fine and I pull about 15" of vacuum at idle.

I did notice that if I rev it from under the hood I can hear the poping under the hood. it is very rapid like it is every cylinder. I tried putting the stock coil back in, and that didnt help. Could it just be running so rich that it would cause this problem? Like I said, the more I advance it the smoother it gets. Does this sound like an overly rich mixture? Other than idle mixture I have done no tuning to the carb yet. I have heard that the power valve can make quite a bit of drivability difference in these, but I still need to check the vacuum at a roll in gear to know what to get. I also plan on getting a jet and pump cam kit and tuning that stuff eventually, but I will be kind of short on funds for the next couple weeks.

Any ideas are greatly appreciated.

Thanks

Ben

five7kid

iTrader: (14)

That old HEI is sounding pretty suspecious. The factory modules can be very weak (don't charge up the coil well, especially at higher RPMs), and the trigger wheel (sometimes called the reluctor) can lose its magnetism. The pickup coils have been known to go weak as well.

If you could get your hands on a known-good distributor - that has everything different between the cam gear and spark plug wires - that would isolate the ignition from all other possible sources of the problem.

I believe you were having problems similar to this with the CC system, though. Are you certain you don't have the valves adjusted too tight?

Momar

You are right. I just reset the valve lash, and 90% of the problem went away. With the cc carb, you didnt really get a poping like this, just a kind of funny feeling. Maybe it is because of the open element that I can hear it, I dont know. Anyway, it still seems dead up high, but runs much better now. I am also getting about 14 initial on the timing w/ 36 total so that is cleared up. I am going to put back on my accel coil and see if that helps up high. I have 416's w/ minimal port work so I am sure that they are part of my high end problem, but I will take care of that when the sportsman's go in. I got an air fuel gauge that I am going to hook up temporarily to help out when I go to do some tuning so that should help out. To bad today was the last test and tune at the track here for the season. Oh well, I will get it eventually.

Thanks

Ben

Momar

Ok, it seems that the problem was mostly gone after setting the valve lash, and resetting the timing. The thing is, that it came back after a couple of days. It was never completely gone but much better. The car still doesnt seem to pop through the carb when you rev it from the engine compartment like it did before we reset the valve lash. I kind of wondered if maybe I have a bad valve train component or something. I dont get a problem just driving around normally, but if I go above 3000 or 3500 it starts to pop. Can detonation be a pop or does it always have pinging noise to it. Its not as distinct a pop as a backfire.

I have kind of wondered if it could be a weak ignition component also. I know the spring in my dist cap is bent pretty bad, and I have another sitting around so I was planning on trying it. This doesnt really make since to me since it seemed to get better temporarily with the valve lash. I will probably clean the plugs again, because until I get this carb tuned during idle and normal cruize the air fuel gauge is showing one bar from the richest mark(this couldnt cause the prob could it). I plan on tuning the carb soon also. I dont want to put new plugs in till after I do because I dont want to just foul out another set of plugs.

Another thing is that my brother talked to summit on the phone, (he was asking about a cam for a 455 in his 77) and they told him that you couldnt run over 9.5:1 on an extreem series cam without detonation. My combustion chambers have been smothed to help, but they are 58 cc 305 heads so I think I should have at least around 10:1 if not a touch higher. This is why I want to find out exactly what kind of noise detonation makes. I have tried retarding the timing, and if anything it gets worse but it definately does not get better. I am thinking about trying some 110 in it today especially if you guys think that might be my problem. If that fixes it I will probably just stay out of the gas until I get he sportsman's on.

Momar

Well, I just drove past the station in town that used to sell 110, and it is closed down so I dont think I will really be able to try that out. Any suggestions will be greatly appreciated.

Ben

Momar

ttt

SSC

Yep, comp told me the same thing about the compression, well they said these cams were designed for under 9.5:1 and max power would be 9:1. Thats why We had to use dished pistons when we went back through the engine for the corvette. It did smooth out a bit with the lower compression (same xe268). It deteonated like a crazy set @10* with just the stock pistons with 55cc 601 heads. That SUCKED! We actually messed with the timing quite a bit when our combo was similiar to yours and couldent really get good power without detonation starting. Thats why I like the old magnum series cams.

Detonation if really bad, I mean really bad can casue a stuble in the engine that could backfire back up the carb. Is it only when you cob it past 3k or will lightly accelerating pop too?

Momar

Well, I will have to try to see if it still does it under slow accel or not. It seems like it at least doesnt do it as bad, but I dont remember. I know that I started having this problem when I put this carb and distributer on. If it is detonation, the computer controlled carb and dist setup used a knock sensor so it may have been retarding it. I have a hard time believing that it was able to retard it enough to stop this though because when the dist was messed up, and I was only getting 8* of mechanical and running it about 10 initial I was still having the same problem. That was only giving me about 18* total. If the computer setup was retarding it far enough to stop this though, that would explain my dissapointing 1/8 times. My best was a 9.410 @74 w/ a 2.015 60 ft. The thing is, I gained about 1.5 mph by upping my timing from about 8 to about 12 one night at the track. I went from 74.xx to 75.98. This was on street tires where as the other was on drag radials though.

Anyway back on subject. I was told that detonation will make a actual pinging noise. I have never heard a ping sound. It is kind of like a thunk. Its hard to describe. I have a set of sportsman II's that will be going on it. I think that those will only bring it down to around 9.8:1 even w/ normal gaskets and I was considering using thin gaskets to try to get about 10:1. I may buy another cam if this is the problem, but I want to eleminate other stuff first. What still doesnt make since is that it got mostly better for a day or 2 when I readjusted the valve lash. It never went completely away, but seemed to get better.

Thanks for the help.

Ben

Momar

Well I tried the slow acceleration thing a couple times today. Well, it at least doesnt do it as bad. As of the first time I did it today, I was going to tell you it didnt, but I just tried again on the way home, and it did a little unless I was hardly touching it at all. When just at a cruize at high rpm it doesnt do it, but if I acclerate much at all it does it. I will not say for sure, but it also seems as if it is working its way lower in the rpms. It also seemed to work its way lower in the rpms before, and then we reset the valve lash, and it seemed mostly better, then it came back after a day or 2 and seems to be working its way down again. Like I said, I cant gaurantee that but I will pay attention to it. It could be in my head(pun intended). I am thinking about changing all of the pushrods out for a different set, and resetting the valve lash again and seeing how it does. If it seems to go away again but not all the way, I will have to look further into the valve train I guess. If it doesnt make a difference, then i can probably rule it out for the most part.

Thanks

Ben

Momar

Well, I switched to another set of stock pushrods, and a set of roller tiped rockers I had and reset the valve lash. It smoothed out and almost totally got rid of that problem just like before. It only lasted about 5-10 minutes though. When I was cruizing down my road toward home it just all of a sudden started poping real loud even under light accel. I parked it and it was even poping at idle. It sounds kind of like a knock, but I took the air cleaner off, and it was poping through the carb. It sounded kind of like an intake valve not closing all the way or something, but why would it have ran fine for a few minutes and then all of a sudden started doing this?

It seems odd to me that both times I have reset the valve lash it has seemed to get rid of the problem mostly, but it has came back both times. Quicker this time than last. I dont know if it makes a differene or if it is just tuning, but it felt sluggish up high even when it was running smooth.

Thanks again.

Ben