chevman24

iTrader: (1)

I have a stock 71, 350 with headers, Demon Carb(road demon jr 625), Performer rpm intake. Initial Timing is 12 degrees BTDC because the carb requires 10-12 initial advance. This engine was running perfectly when it was in the truck, but now its in my 88 camaro with mechanical fuel pump.
The problem im having is getting it to run right.
Ive switched out the main jets( secondaries are not jetted) I run 82 or 83 jets, 3,5,4,6 plugs turn white, plug damage occurs and a popping sound occurs under heavy acceleration(detonation?).
Then i run 84s, numbers 3,5,4,6 plugs turn a brown color but 1,8,7,2 stay black. Do i increase jet size till they are all the same color?
It has good oil pressure weather its warm or cold. It also has strong vacuum.
Is this an indication that the rings are shot? Or should i keep running it this way till the issue solves itself. I have an air fuel guage but no matter what i do it says its running rich.
I just want to get the car running right until i build a new engine.

Thanks for the input

F-BIRD'88

You are way off on the carb jetting. The out of the box jetting should be right within 4 jet sizes either way. You are way beyond that.
If this is the carb model 6282010V reinstall the stock #70 main jets.
Check the air bleed sizes against the barry grant specifications.
You have a manifold vacuum air leak or a fuel system fuel flow restriction or internal carb fuel passage blockage causing a severe fuel leanout condition. Could be a ignition system fault as well. Start over. Ignore the spark plug colour as an indiction of proper AFR. Only look for a overheated chamber (ground strap colour) detontation (silver/black pepper) or carbon fouling, oil fouling black (ignition, flooding, PCV, oil rings) {danger signs} when looking the the plugs.
If the exhaust system is not leaking air into it, the AFR gauge will read correctly unless it is malfunctioned or mis calibrated. Are you using a NB or Wide band AFR meter?

You need to check and verify proper vacuum and mechanical advance function. Not just the inital base idle timing. Get a balancer timing tape or dial advance type timing light. Do you have a ground strap connecting the engine to the chassis/firewall?

Atilla the Fun

After thinking it over since yesterday, I've reached the same conclusion: start over. Strange it should be the center 4 cylinders versus the outer 4. I was thinking about the intake manifold, then moved on to rings, valve seals, vacuum leaks, coolant getting into the intake ports, it all comes back to pull the carb off the intake, then pull the intake off the heads.

chevman24

iTrader: (1)

The carb number you posted is correct with vac secondaries. Thats what i dont understand, it ran perfectly fine on the truck. No its not a wide band monitor.
Plug straps are black.
It must be a vaccuum leak cause the vac guage reads 22 inch pounds. Everyone else says it should be lower (14hg). It also idles better with the vac guage connected. I have two vac canisters one adjustable, the other is not. It idles the same with both and both are new.
There is a ground strap connected to the back of the block to the firewall.
I have a brand new timing light. I time it so as i turn the distrib the light blinks when its on the 12 degree mark on the balancer with timing light. But im not sure if thats right.

Atilla the Fun

The stock '71 350 low-performance cam should pull around 21 inches of vacuum at sea level.

F-BIRD'88

Adjust the timing with the vacuum advance disconnected. Then reconnect it.
Ported vacuum source.
Only the end of outer metal shell and end threads of the spark plug should collect black carbon.
Either AFR meter will work. You want the idle adjusted for best idle quality and idle manifold vacuum and the AFR gauge should read 14.7:1 or slightly richer 13:1 to 14.5:1 ish AFR at idle.
It should be very close to this while cruising at party throttle and go rich and stay rich when you rug it. WOT as long as you have your foot in it.
What ever vacuum hose you are disconnecting from the carb to connect the vacuum gauge to the carb, must be leaking.

If the porcelin part the spark plug is carbon black (fouled) it is finished. Replace and correct the cause.

chevman24

iTrader: (1)

I did adjust timing with the vac advance disconnected. But its connected to full manifold vacuum port on carb during normal operation. Is that whats causing this?
I wanted to buy the wide band AFR but didnt want to spend the 300.
How do i determine whether its the low performance or high performance?
its a 397010 block and heads are the 333882's. Would the 75cc heads indicate the low performance engine?

chevman24

iTrader: (1)

I switched to the ported vacuum on the carb and back to the 70 jet setting. It idles better and there isnt any stumble or hesitation in the rpm band. The afr meter still says its rich but ill run it this way for a while and see what happens.
Changing jets too often i think the threads on the main body, for mounting the main float bowl, are starting to strip out. I wonder if they make a heli-coil that size? I dont want to buy any carb parts since the current setup will be unusable with the new engine combo that is in the future. Which is likely to be topped with a Demon or Holley 750 cfm carb.

I appreciate the assitance

F-BIRD'88

The 882 heads are definately low perf.
A cylinder head upgrade will be like bolting on a after burner and should be high on your wish list for vastly improved power and perf.

ibmtech

iTrader: (1)

I know the 882's are low perf, but would they be 75cc ??

chevman24

iTrader: (1)

If the engine had forged or newer internals a head change would have happend already. I have a set of 461s with 64cc. Too bad they need to be rebuilt. The cost to rebuild is greater than the cost of buying new aluminum heads. What would the gains be if i used them and what compression would they have?

I know the 882s are that cc or are close because i had them off.

F-BIRD'88

Yes the 882's are 76cc, unless someone shaved them.

Can't tell you the cr cause I don't know whats in your motor for parts.
if the motor has stock dished pistons the cr could be as low as 8:1
depending on the head gasket used.
If flat top pistons it is around 8.5 9:1 again sepending on the gasket.

I picked up 80hp when swapping 882's for vortecs, on a 350.
The track et was reduced by 1 full second.

chevman24

iTrader: (1)

They are stock 882s and it has dished pistons. Like i said this engine is just in there for now. It has about a year left on it before its sold or sits in storage waiting for the future vehicle its going to sit in. Exchanged for a forged short block with AFR heads. Not sure about cubic inches but its between a 350, 383, or 402 small block.

chevman24

iTrader: (1)

Ok here's an update. I put the 70 jets back in and it started running better. Then after a while it started to have starting issues. It would start and stay running if i held my foot on the throttle. (Yes the choke is setup right) It wouldnt idle until it warmed up a bit.
I switched to 73 jets and it would start right up and it ran well for a couple days. I then checked the plugs. Some were black while others were white and one (number 1 cylinder) had plug damage. Not mechanical plug damage but i could freely rotate the ceramic part in the metal shell (detonation? Too lean fuel mix?).
I switched to 76 jets and it runs as good as it did when i first put the smaller baseline jets in. I checked the plugs and most of them looked the same.
I'll see where that goes.
Another question is why did GM change the design on the r45ts plugs? The old ones looked more like the r44 plug.The new design is longer, so with headers its a bitch to get at. The new ones are made in the U.S. and the previous design was from france or mexico.

thanks

F-BIRD'88

Put the stock jets back in the carb and leave them there. You do not tune the idle by changing the main jets. Get a clue. Replaace the spark plugs and install them correctly this time. Fix the ignition problem. A motor will not idle or run right with fouled spark plugs.

chevman24

iTrader: (1)

I put brand new spark plugs(used autolite ap 25, same as r45ts) in it when i changed the jets back to the baseline setting. I gapped them at .35 and installed them in the heads. The engine started hesitating under hard acceleration which would explain the jet change.

F-BIRD'88

If this motor ran fine and pulled hard in the truck and it faulters now under hard accel then you have a fuel flow problem.

I had the same problem with my mechanical pump in my car with two motors.
The pump was huge and fine but still could not feed the motor at high rpm.

Replaced with a rear mounted Carter P4594 electric and new 3/8" line from tank to carb and it is a whole different car.
The factory fuel line picks up way too much heat and so does the pump.
The pump cannot move fuel that is full of bubbles.
Put a fuel pressure gauge on it that you can read while driving.

chevman24

iTrader: (1)

Electric Fuel pressure guage is the next on my list. I have a guage in the engine compartment but its one of the liquid filled ones. I learned shortly afterwards not to buy the liquid ones cause heat can mess up the liquid in the guage.
With 71 jets in the carb, all of the plugs are either a brown color, or white. Too much or not enough fuel?
I turned up the fuel pressure and it began driving without any hesitation. then i noticed my fuel filter was backwards( the pump was flowing into the out side not the in side) and fuel pressure went up a bit.
Setting total timing. I have a digital inductive timing light. It is model number 3568 made by EQUUS if anyone is familiar with it. Mechanical/Initial advance is 12 degrees on the engine. how do I set/ figure out total timing? in other words. Where do i set the flashing light in relation to the timing mark on the pointer and balancer?

chevman24

iTrader: (1)

Bump

chevman24

iTrader: (1)

So i FINALLY got the right part that i needed. Third time is the charm i guess.
Now i cant adjust the secondary needle and seat. What i mean is if i adust the float a little bit up on the secondary side, the bowl will fill up and overflow. but if i adjust it down at all, it will shut the fuel off completely. Is it the float adjustment itself? It has a brand new float and needle and seat. The main float operates perfectly so im thinking its not related to the fuel pressure. Any ideas?

Rolling Thunder

Couple things for starters i noticed that you gapped your r45ts equivalents to .035? The gap should be .045. Next i assume your probably running a dual plane intake. Its not horrably uncoommon to see your condiditon where half the plugs ar rich and half are fine or half lean and half fine or some combination like that. The reason usually is an unbalanced carb and the cylinders fed by thoes barrels wiht a fault will be rich or lean or something different than the other side. I would deffinetly look for vacuum leaks with a propane torch or something and then you may want to concider takeing a deeper look into that carb. Unfortunetly i have no experiance with demon carbs or how there put together internally so icant really give specifc areas to check but if its not a vacuume leak that sounds like your problem.

chevman24

iTrader: (1)

The problem is/was all with the carb. But all the past information was solved with exception to the new issue.
The Secondary needle and seat being non-adjustable. What i mean is that the secondary bowl is either overfilling or it hardly has any fuel in it at all. The primary float is perfectly fine so im thinking its not pressure related. It has a .110 needle and seat in the primary and a .97(thats all the auto place had at the time) in the secondary, would that cause the problem? I cleaned all the lines and checked for any debris and couldnt find any.

chevman24

iTrader: (1)

[quote=Rolling Thunder;4152815]Couple things for starters i noticed that you gapped your r45ts equivalents to .035? The gap should be .045. Next i assume your probably running a dual plane intake.

I originally thought the engine was the 250 hp(requires the .35 gap) model but have since noticed that its the 215 hp model that uses r44t plugs( not sure about the gap). Ill keep the r45ts in there cause they worked well in the previous vehicle without any issues. I will try the .45 gap and see what that does.
I also put a new model HEI distrib(coil in cap) in it instead of the old points system. The r45ts will probobly work better anyways since they are recommended for the later model engines with the HEI.
The backfiring that i was noticing was because i had the vac advance connected to full manifold vac instead of ported vac on the carb. It starts, runs and drives much better now that i switched that. That was also probobly where the vac leak was as well.

Rolling Thunder

Well i cant say if useing the .97 over the .110 is causeing you a problem but i can say that i cannot say that its not causeing you a problem eather. I would say order the right one if you cant pick it up locally because its deffinetly a potential problem. As for the float adjustment again not being familiar with demon carbs im having a hard time with the idea that its not adjustable as every other carb ive worked on has some means of adjustment as it does not adjust itself. Again it could be caused by the needle valve not being right or a bad float or is binding or something but ide first look into adjusting the float level first. The instruction manual that came with the carb or if you dont have it it should be available online should have the procedure for adjusting the floats.

chevman24

iTrader: (1)

I ordered the correct needle and seat yesterday. Basically a Holley and a demon are the same thing. The only difference is that a demon holds more fuel volume and is actually a larger carb.
the problem has been fixed. The needle and seat wasnt lining up correctly with the float. I had an older brass float lying around so i used that. Ive had better luck with brass floats anyways. they seem to be more accurate. Just my .02 cents
Thanks for the help