Persistent Detonation

[bobs66440]

Hi Guys,

This isn't for my T Bucket, rather for my '55 Chevy. You guys seem very knowledgeable about such things so I figured I would ask you if you don't mind. If it's not appropriate for this forum, please feel free to delete.

Anyhoo, I am running a 396 Chevy, bored .030", oval port iron heads, balanced assembly, 10:1 or 10.5:1 compression, I can't remember (according to the PO), aftermarket HEI dist, Holly 750 double pumper, single plane hi-rise intake, TH400 trans with a 2500 stall converter (according to the PO), um, I think that's it. Very choppy idle and this engine has almost no power below 4000rpm... then it revs like a 2 stroke dirt bike...I assume because of the cam & intake. Here's the cam card...



The problem I'm having is it pings like crazy under load if I try to set the initial timing any more than 10BTDC. I've had two different HEI distributors in it and it does the same thing. The one that's in it currently has about 15 degrees mech advance, so I'm getting a total of about 25 (no vacuum advance) at 2500rpm. At idle, it's already into the mechanical advance a little. The engine will idle fine in gear at about 900-1000 rpm but if I try to connect the vacuum advance it will not idle in gear at all, so I run with it disconnected. When I put it in gear, the idle drops quite a bit and the car wants to creep forward, so I assume the converter doesn't have enough stall? I guess that's why it won't idle with the advance connected? I'm at the limit of my knowledge on this. I really don't fully understand the relationship between all the components and a setup that will make them all work in harmony. The car runs and drives fine, but I know there is a lot of HP left on the table due to the possibly mismatched or mal-adjusted parts. I just don't know what to do about it. Any help would be greatly appreciated.

 

[2old2fast]

pings like crazy=too much timing.... 10 *initial isn't much for that cam , I'd think you'd be better 14 -16..... you're sure there's only 15 mech in the dist ? most have 20 - 25... sounds like you need to re-curve the dist ...14* initial plus20 mech all in dy 22-2500 rpm.... the vacuum adv. should add timing....depends on the unit but 10-15*..... you're sure it's double PUMPER and not a dual -feed ? idle drops a lot could be he converter's too tight or you're lowering the rpm right where the mech adv kicks in/out.... talk to me,
dave ...... that much compression needs 92-93 octane....
sorry about the typing errors , i'm on my tablet

 

[bobs66440]

pings like crazy=too much timing.... 10 *initial isn't much for that cam , I'd think you'd be better 14 -16..... you're sure there's only 15 mech in the dist ? most have 20 - 25... sounds like you need to re-curve the dist ...14* initial plus20 mech all in dy 22-2500 rpm.... the vacuum adv. should add timing....depends on the unit but 10-15*..... you're sure it's double PUMPER and not a dual -feed ? idle drops a lot could be he converter's too tight or you're lowering the rpm right where the mech adv kicks in/out.... talk to me,
dave ...... that much compression needs 92-93 octane....
sorry about the typing errors , i'm on my tablet

Hi, thanks for the reply. Yes, definitely a double pumper. If I connect the vacuum advance it adds around 12* if memory serves. There's only about 15* mechanical advance with this dist. With the previous dist I could run 20* initial because the dist only had 5* mechanical advance. That's why I changed it. Bottom line is I can only run a total of about 25* all in without it pinging. That's why I can't use the vacuum advance. And I meant to add that I always run at least 91 octane.

 

[one finger john]

First thing I would do is find out how many degrees advance there are in the distributer. How do you KNOW how many degrees are in the mechanical advance? Somebody just tell you that or have you had it checked and verified (preferably on a Sun dist. machine). Then, once you know what the total mechanical timing is, setting the INITIAL timing should be easy. Bring the balancer mark up to 0*, firing, and align the rotor with the no. one spark post on the dist. cap. The vacuum advance should be facing forward.
Timing adjustments are easier with two people. Have someone sit in the drivers seat, put their foot on the brake with the trans in neutral or park and start the car. It will start with zero timing advance. Have your helper idle the engine up until it has reached operating temperature. Make sure all vacuum ports on the carb are plugged. No vacuum to the vacuum advance can. For now work in 5* changes. Start by advancing the initial setting by 10* to 10* advanced. Car should be idling fairly smoothly on it's own. Adjust idle richness screws first to get a better handle on idle mixture (usually 3/4 to 1 turn out, screw in the screws till the engine just starts to die and then out a quarter of a turn). If you have four corner idling adjusting screws, do the same. Then adjust idle speed. Shoot for 1000 rpms.
Once the car is idling smoothly OUT of gear, have your helper drop the trans into gear (WITH HIS FOOT ON THE BRAKE) and see if the idle speed needs to be adjusted up or down. Lower rpms in gear the better. This theoretically should give you a good base line for further ignition changes.
If you put an inexpensive timing tape on the balancer you can also verify the TOTAL timing by having your helper put the car in park and bring the rpms up to 2500, 3000 and 3500 rpms to see what the total timing is. Should be done advancing by 3200 rpms or or so. This will all be done with the vacuum advance disconnected. Do not get the timing light tangled up with fan or fan belt. Take it for a test drive and see how it runs. If it is a little listless give it 5 more degrees initial for a total 35*.
Again, with the vacuum advance disconnected. And verify TOTAL timing thru the timing tape method.
We can hook the vacuum advance up later.
Do all of this with the gas you will normally run.
And do this outside of the garage.

Lets us know how it runs and then we will tackle the dreaded vacuum advance.

Thats it for now, I'm sure someone will speak if I am too far off.

John

 

[bobs66440]

First thing I would do is find out how many degrees advance there are in the distributer. How do you KNOW how many degrees are in the mechanical advance? Somebody just tell you that or have you had it checked and verified (preferably on a Sun dist. machine). Then, once you know what the total mechanical timing is, setting the INITIAL timing should be easy. Bring the balancer mark up to 0*, firing, and align the rotor with the no. one spark post on the dist. cap. The vacuum advance should be facing forward.
Timing adjustments are easier with two people. Have someone sit in the drivers seat, put their foot on the brake with the trans in neutral or park and start the car. It will start with zero timing advance. Have your helper idle the engine up until it has reached operating temperature. Make sure all vacuum ports on the carb are plugged. No vacuum to the vacuum advance can. For now work in 5* changes. Start by advancing the initial setting by 10* to 10* advanced. Car should be idling fairly smoothly on it's own. Adjust idle richness screws first to get a better handle on idle mixture (usually 3/4 to 1 turn out, screw in the screws till the engine just starts to die and then out a quarter of a turn). If you have four corner idling adjusting screws, do the same. Then adjust idle speed. Shoot for 1000 rpms.
Once the car is idling smoothly OUT of gear, have your helper drop the trans into gear (WITH HIS FOOT ON THE BRAKE) and see if the idle speed needs to be adjusted up or down. Lower rpms in gear the better. This theoretically should give you a good base line for further ignition changes.
If you put an inexpensive timing tape on the balancer you can also verify the TOTAL timing by having your helper put the car in park and bring the rpms up to 2500, 3000 and 3500 rpms to see what the total timing is. Should be done advancing by 3200 rpms or or so. This will all be done with the vacuum advance disconnected. Do not get the timing light tangled up with fan or fan belt. Take it for a test drive and see how it runs. If it is a little listless give it 5 more degrees initial for a total 35*.
Again, with the vacuum advance disconnected. And verify TOTAL timing thru the timing tape method.
We can hook the vacuum advance up later.
Do all of this with the gas you will normally run.
And do this outside of the garage.

Lets us know how it runs and then we will tackle the dreaded vacuum advance.

Thats it for now, I'm sure someone will speak if I am too far off.

John

Hi, thanks for the detailed explanation. The way I know what the mechanical advance is, when it's idling I set the initial timing at 10*. Then I rev the engine until the mark stops moving. Take the reading (25*) and subtract the initial. I always thought that's how it's done? This was all done with the vacuum advance disconnected as mentioned. I can't use it anyway because the car will not idle in gear with it connected. I already went through the process of setting the idle adjustment on the carb using a vacuum gauge, but it's not so easy hearing much difference (or seeing it on a vacuum gauge) due to the super choppy idle because of the cam. But it's as good as it's going to get now, which is pretty good.

 

[one finger john]

Bob, this sounds like another case of your not listening to what we are offering to fix the problem. Without ALL the info on one piece of the puzzle the puzzle will not be solved.

1. What is the number of degrees of mechanical timing in the distributer and has this been verified by an independent source ??? DO YOU KNOW FOR A FACT HOW MANY DEGREES ARE IN THE DISTRIBUTER ? NO, YOU PROBABLY DON'T.
2. Is the balancer degreed so that you actually know how many TOTAL degrees timing you have ?
3. A cam so "super choppy" that it is uncontrollable, blurs the operators vision (and probably his thought processes) and can not idle in gear with the vacuum can hooked up. Were you drawing from the ported vacuum or the manifold vacuum ?
Doesn't matter. In essence hooking up the vacuum advance to the manifold will ADD advance (however many degrees are in the advance can) and idle will speed up. Hooked up to the ported source, NOTHING will be effected, idle will not speed up. No vacuum will be drawn until the butterflys are cracked. Hooking up the vacuum advance will not cause the car to die. Something else, in addition to the original problem, is active.

Sigh. This is not rocket science. This thread has Oino's finger prints all over it.

Mike, esplain to Bob why I can't participate any more. I'm toast, I'm done. I'm outta here.


John

 

[bobs66440]

Well, thank you for your condescending reply. I said initially that I'm at the limit of my knowledge and don't fully understand, and I answered your questions to the best of my ability. I apologize if I didn't instantly become an expert. It may not be rocket science to you, but we are all not so lucky to have such a gift. Have a good day.

 

[ellis8500]

I really don't fully understand the relationship between all the components and a setup that will make them all work in harmony. The car runs and drives fine, but I know there is a lot of HP left on the table due to the possibly mismatched or mal-adjusted parts. I just don't know what to do about it.

I think you might have said it all there. Cam, heads, valves, intake, carb, pistons, stroke, all of it have to match for what you want the egine to do. If you put a high RPM cam in a stock engine it just will not run very well at all. The wrong intake on a stock engine will not work very well. Get my point? So instead of trying to solve an issue as many here do, I suggest you contact UltraDyne and let them help you solve your problem. You may have the wrong cam shaft for the engine as it is. You talk to them, tell them what you have and what you want the engine to do and they will recommend a cam. Start there and see what they say.

 

[bobs66440]

I think you might have said it all there. Cam, heads, valves, intake, carb, pistons, stroke, all of it have to match for what you want the egine to do. If you put a high RPM cam in a stock engine it just will not run very well at all. The wrong intake on a stock engine will not work very well. Get my point? So instead of trying to solve an issue as many here do, I suggest you contact UltraDyne and let them help you solve your problem. You may have the wrong cam shaft for the engine as it is. You talk to them, tell them what you have and what you want the engine to do and they will recommend a cam. Start there and see what they say.

Thank you. I will do that.

 

[2old2fast]

I'm guessing here , but somethings hokey I've never heard an engine ping at idle ?? especially w/only 10* initial.... unless someone has limited the advance , I find it strange that you are showing only 15* mech. ......have you verified that the balancer is accurate ???[marks line up at tdc] have you run a compession test ?? GENERALLY , over 200 psi cranking compression is getting above 10:1...... are these iron heads ???? keep coming w/info so we can try to figure this out ...
dave

 

[PotvinGuy]

Bob, did this combo ever run OK, or is it a new combo? Are you sure that you have TDC nailed? I only ask because I've seen so many motors mismarked. Pinging is almost always due to too much advance. Or could the cam be off a tooth or two? That grind doesn't look too radical. The idle shouldn't be choppy. Do you trust the card? And the builder? I've gotten some really bad parts from hinky builders. Ultradyne is now part of Bullet Cams. Let us know what they say.

 

[bobs66440]

I'm guessing here , but somethings hokey I've never heard an engine ping at idle ?? especially w/only 10* initial.... unless someone has limited the advance , I find it strange that you are showing only 15* mech. ......have you verified that the balancer is accurate ???[marks line up at tdc] have you run a compession test ?? GENERALLY , over 200 psi cranking compression is getting above 10:1...... are these iron heads ???? keep coming w/info so we can try to figure this out ...
dave

As I mentioned in my first post, it pings under load and the heads are oval port iron heads. It idles fine unless I have the vacuum advance connected while in gear. Then it wants to die. I haven't done a compression test on it, I suppose that would be a good idea. I didn't know the relationship between cranking pressure and compression ratio. That's good to know so I can check that out.I can also double check marks on the balancer, that's a good idea. I can't explain the lack of mechanical advance. the distributor I had in it previously would only advance 5 degrees beyond initial while watching the mark move with the timing light as I rev it.. But maybe I'm doing it wrong.

 

[bobs66440]

Bob, did this combo ever run OK, or is it a new combo? Are you sure that you have TDC nailed? I only ask because I've seen so many motors mismarked. Pinging is almost always due to too much advance. Or could the cam be off a tooth or two? That grind doesn't look too radical. The idle shouldn't be choppy. Do you trust the card? And the builder? I've gotten some really bad parts from hinky builders. Ultradyne is now part of Bullet Cams. Let us know what they say.

It has been like this since I got it. Not 100% positive about TDC. I suppose the balancer could have moved. I will check that...

Most of the info I have is based off what the PO told me and also the guy who built the engine. Whether it's gospel or not is hard to tell...I'll have to call Bullet, and I think I need to do some homework, thanks!

 

[T-Test]

Well, thank you for your condescending reply. I said initially that I'm at the limit of my knowledge and don't fully understand, and I answered your questions to the best of my ability. I apologize if I didn't instantly become an expert. It may not be rocket science to you, but we are all not so lucky to have such a gift. Have a good day.

Carry the car to a qualified mechanic and have him/her check it out and you will have spent less time and money trying to fix it. JMTCW

 

[2old2fast]

Where's the fun in that...
dave

 

[bobs66440]

Carry the car to a qualified mechanic and have him/her check it out and you will have spent less time and money trying to fix it. JMTCW

I wish I knew one. Most shops around here only know cars with computers. Guys like me have to learn on our own. Most things I can figure out, but this has me stumped. But I realize I need to get some baseline info to move forward. Thanks

 

[T-Test]

Where's the fun in that...
dave

REMEMBER--- It's only $$$$$ when you have a busted engine!!!

 

[2old2fast]

Just thinking out loud , is /was the carb new w/this build , is it possible there's been a lot of low rpm running whereby you have excess carbon buildup, this can cause pre-ignition..... has some well -meaning soul removed & plugged the power valve & then not jetted up the carb so it's lean untill the sec. start flowing , this can cause pre-ignition...
random thoughts....dave
what's the timing at 4000 rpm where it "goes" ??

 

[bobs66440]

Just thinking out loud , is /was the carb new w/this build , is it possible there's been a lot of low rpm running whereby you have excess carbon buildup, this can cause pre-ignition..... has some well -meaning soul removed & plugged the power valve & then not jetted up the carb so it's lean untill the sec. start flowing , this can cause pre-ignition...
random thoughts....dave
what's the timing at 4000 rpm where it "goes" ??

I thought about carbon buildup...and it could be possible I suppose, but the engine only has maybe 3000 miles on it. Though, when I got it, it went through 0ne quart of oil in 100 miles and smoked like crazy! I traced it to a poorly placed and unbaffled PCV. It was sucking the oil right out of the valve cover. I relocated it to the upper portion of the valve cover and installed a baffle. Now it's fine. Burning that much oil, I suppose it wouldn't take long to build up carbon. Maybe try some Seafoam? Has anyone used it?

And it also almost always runs on when I shut it off. I have to shut it off in gear most of the time. Carbon would cause that also, no?

 

[PotvinGuy]

What do the plugs look like? I got one of those carbs with the plugged power valve (from a big name) with no warning. After I took it apart and called them, they said it was "one way to make a blower carb." As opposed to the right way, I guess.