would carbon knock be my problem ?
 

Still on the hunt for my knocking noise as I posted about maybe a cracked flex plate. It is a knocking noise not a ticking noise. http://www.67-72chevytrucks.com/vboa...newthread&f=24 I have the engine apart and see nothing wrong with bearings, no bad cylinders etc. Cylinders still have cross hatch ,everything looks nice ,only about 10,000 miles or less on rebuild. Only thing is there alot of carbon on piston tops and inside the heads on the valves. When I got the engine he talked me into a eddy 1405 carb which I was always fighting to get it to run good ,too rich then too lean. Was going to switch back to the Rochester when the noise happened. Do you think the carbon is my smoking gun for noise. There is no marks on heads or piston to show it was hitting. I,ve done everything . I had the motor running with no torque converter or pullies and belts. Only other place would be mech fuel pump which showed no noise with scope when running, timing chain showed no marks on cover, replaced dist. oil pump I don,t know if that would make that kinda noise. Will get to heads and also re-check bearing clearance's after I clean up my mess and make some room. Just looking for others insight and opinions. I just want to know what the heck is making this noise.

Re: would carbon knock be my problem ?
 

other pic. Also remembered this noise happened twice within a year but only for 10 seconds or so

Re: would carbon knock be my problem ?
 

That much carbon could indeed create hot spot(s) that could cause preignition (knock). It has been known to happen. I'd definitely want to track down the source of so much carbon in the cylinders (stuck PCV, ultra rich mixture, too cool of a thermostat, etc.)

Just some ideas. I think you're heading down the correct road in your investigation, though. Good luck! :)

Re: would carbon knock be my problem ?
 

I blame the carb and me too. I was getting bad gas mileage for awhile until I started f**cking with it. It,s just frustrating trying to fix something but don,t what it is I,m supposed to fix.

Re: would carbon knock be my problem ?
 

Judging from the looks of that piston the short answer is yes, a chunk of carbon can and will make a knocking or hammering sound. An old school fix for carbon build up is drizzling water down the carb while revving the motor. Then take a rawhide hammer and smack the valve stems to clear the seats. That carb needs to be leaned way out! Clean everything up and button her back up with new plugs and take a close listen. I'd also double check your timing, maybe advance it to 14 or 15 degrees BTDC?

Re: would carbon knock be my problem ?
 

Thanks, makes me relieved. I,m this far so I,ll check bearing clearances and put new ones in while I,m here and send the heads out for a check and a clean up and get rid of that eddy carb

Re: would carbon knock be my problem ?
 

As much carbon that I see, you might look at the quench distance.....figure the distance, piston to top of the cylinder + the gasket. If you a a bunch more than say .055, Then it will tend to detonate. Longhorn:chevy:

Re: would carbon knock be my problem ?
 

Thanks, some of the piston tops that are closer to the center of intake where it is the richest ( correct ? I,ll have look ) had close to 1/8 inch of carbon in spots and who knows what fell off during disassemble

Re: would carbon knock be my problem ?
 

Looks like your eddy carb was flooding a bit.

Re: would carbon knock be my problem ?
 

It does look on the top left picture that carbon may have been hitting the head. It must have been burning quite black or if it was from oil, then blue.

Yeah, the water trick probably would have worked. I pulled my six cylinder only to find that the oil rings were stuck. Water would have cleaned that up OK. Seals were needed too though.

Was your Rochester a quadrajet? I used to think they were all gas hogs but am finding ours to work well and it is not giving negative surprises on fill ups. On the other hand yours should work and there must be something wrong with it like wrong jets or a missing piece or assembled wrong or something. Have you got an overhaul manual for it?

Press on and you will get it figured out.

Re: would carbon knock be my problem ?
 

That seems like a lot of carbon, even in the intake ports. I think it was sucking oil past the intake gaskets. Oh well, clean it up and button it up and keep tuning.

Re: would carbon knock be my problem ?
 

Is a quadrajet which I,m going get rebuilt or maybe try it myself ,have all winter now. The edilbrock had too much stuff to make it run good, needles and spring rates etc. always had a flat spot here or there , change something and something else would change and all the money and time to change all this stuff for a daily driver. Quadrajet was always good to me. Live and learn , O well

Re: would carbon knock be my problem ?
 

That does indeed look like an oil consumption issue. Can't get that much coke from only gasoline, ime. OP should look at the rings, for sure.

Re: would carbon knock be my problem ?
 

That's a lot of piston top carbon for 10k miles.

A little story...
Back in the mid-1990s an unnamed manufacturer had a V-6 that had an unusually tight quench clearance. It was prone to having CCD (combustion chamber deposit) noise after not many miles. It was physical interference between the piston top, cylinder head, and the carbon chunks trapped between them. I can't blame them for that design decision, as it should give better fuel economy. It's just that on some gasolines, that were formulated with certain lower cost detergents, they had more CCD. When all that plus manufacturing tolerance stack up went the wrong way, it was a noise problem.

The short term fix to clean it up was a bottle or two of Techron Concentrate. It's a persistent steady gentle cleaner that cleans up intake valve deposits, combustion chamber deposits, and also injector deposits. And after that cleanup use only Chevron gasoline to keep things clean. Techron is based on a polyether amine molecule that is not prone to making additional CCD like many (most?) of the others were back then.

The long term fix was to open up the quench clearance in the engine design.

If the noise comes back, it's probably worth running a couple of tanks treated with Techron Concentrate at the recommended dosage of 1 oz per gallon of tank size.

I have lots of dyno test experience with that product. I've been away from that kind of lab testing for almost 15 years now, but my friends still there assure me it's still high quality powerful stuff.

Re: would carbon knock be my problem ?
 

Here's a paper on it. I guess it's ok to name names, since Mazda shared their results publicly. https://www.sae.org/publications/tec...ontent/950741/

Re: would carbon knock be my problem ?
 

Looking @ the heads, I dont see oil....all carbon/dry? I would have to ask, did the engine use oil? I will say that in 100,000 miles, I have never seen that build up on the pistons. You could have an issue with the rings, or intake seal.....but I have to agree with the poster that said that you cannot make that much coke on gasoline. I still say also, check the Quench....IMHO, you cannot make that much carbon on the pistons, if the quench is right. That said I do feel that you found you knock;) Longhorn:chevy:

Re: would carbon knock be my problem ?
 

Thanks for all the replies , engine is all torn apart and will replace rings and bearings if clearances check out good . So far every thing is nice and shiny on the crank and the block looks nice an fresh, cross hatch looks good.

Re: would carbon knock be my problem ?
 

I don,t see why I would have a quench problem with a stock 350 L-48 which has 8.5-1 comp. or could I ? Valves are real nasty on top of some of the valves inside the head. No oil consumption , I will look at all the old gaskets ,I did save them. Also maybe gasket problem was my trouble. I remember looking for leaks but found none because the best I could get from a vacuum gauge on the engine was 10 HG (if that is what it is called ) so I looked for leaks. Another thing is machine shop never bored the engine out. It still has the stock pistons in it. Maybe it didn,t need it or he used oversize rings if they still do that. I was happy knowing what made the noise , now I want to know why. Will it never end LOL

Re: would carbon knock be my problem ?
 

What kind of gas do you run? I didn't think gas could cause that much carbon but according to Matt's post #15 it can be caused by inferior gasoline. I know high test gas can carbon up motorcycles but didn't think it could be anything like this.

With your stock pistons about .025 in the hole with a .015 gasket would give you around .040 of quench. You probably do have .040 of carbon on some of those pistons. Very weird.

Re: would carbon knock be my problem ?
 

I just run cheap reg. gas. The poor truck very seldom seen the hiway, just around town ina small town of 60,000. No freeways or nothing. tried to make a effort to get on the hiway now and then but far and few between. Did use good oil, Lucas hot rod oil with zinc.

Re: would carbon knock be my problem ?
 

Maybe you need to do the 6 ounce water dribble tune up on your vehicles every once in awhile.
Dribble a little water into your carb at 1200 rpm. The water turns to steam and cleans the chambers nicely.
An old shady mechanic trick from way back in time.
Charge 50 bucks for a tuneup and use only water.

Re: would carbon knock be my problem ?
 

I,ll think I will try that . I have another truck with a nice 454 in it which should get some hi-way miles . Was thinking of buying a cheap bore scope just to see if I,m getting bad carbon build up, any suggestions.

Re: would carbon knock be my problem ?
 

Posts 13 and 16.
Www.Cliffshighperformance.com for Quadrajet parts and advice.
His parts are not cheap, but are the best, and ethanol resistant. You don’t get even half the parts in a parts store carb kit, Cliff can send you ALL the parts you need.

Re: would carbon knock be my problem ?
 

Thus trick is one of my favorites but I bring the rpm up between 2-3k rpm since the motor tends to try and stall.

OP. Double check your intakes fit. If the block was decked it's possible that the intake isn't sealing correctly. Make sure the intake isn't bottoming out at the china walls. I am leaning towards this since you said the vacuum was so low. That can also come from timing but I think that it's a leak that sucks oil from the crank case.i run a solid lifter circle track cam and get 8-11 inches of vaccum, just for comparrison.

Another thing to check is the coil. I was fighting fouling plugs and finally realized that the coil was dieing. It finally got to the point that I was blowing grey smoke at idle.

Re: would carbon knock be my problem ?
 

A stock L-48, should pull 18-20 hg @ idle.....if it does not, you have ign issues or an air leak;) We can go into ign 101....basically , you will want 10-12 intial, read with the lite on the balancer.....rev to 3000-3200, to give ALL in from 32-36 degrees. The vac advance, is not part of the total timing, but you will want to add 10-12 degrees on vac timing Full manifold source. This will give you the timing that you need @ cruise speeds (slow burn times). With the vac advance plugged in, you may see 18-22 degrees on timing...if you buzz it a little on A lite load, you will see the 48-52 that I talked about. Just understand,that when you open the throttle, the vac advance drops out....it is a componsater, for the slow burn times during cruise. It will also make your engine run cooler.:chevy: longhorn