Can the cam itself be the cause of dieseling?

[Guest bastaad525]

Okay, I've been over this subject 100 times, and have tried everything I can think of to solve it, along with trying a ton of suggestions from you guys on these boards, and still my dieseling won't go away. After getting a valve job on the head and installing a new cam (regrind, 480 lift 270 duration), and also cleaning the carbon off the piston tops (of which there wasn't that much anyways) I tuned the SU's at a smog place on their exhaust gas computer, with the car under load, so I'm sure the carbs are as in tune as they're gonna get. I've adjusted the timing everywhere from 2 degs to 20 degs BTDC, adjusted the idle as low as it will go stably, about 550rpm, have checked for intake leaks with the car running and found none, and have tried running richer and leaner than the baseline setting I got at the smog place. I've tried killing the car by disconnecting the MSD ignition. About the only thing I haven't tried yet is going with a 160 thermostat in place of the 180 in there now. At any rate the car doesn't run warmer than it used to before the cam and head. Oh before I forget, the carbs are Ztherapy rebuilt SU's less than 6 mos old, and I always run 91 octane gas (best I can get in CA). I have no problems at all with detonation, though I did run into it when I bumped the timing up, right now I have it set at 13 degs, and it will start pinging if I bump it up to around 16-18 or so. I have also tried running stock, one step, and two steps colder heat range of spark plugs. The fact remains that until having the head taken off, given a valve job, and put back on with the new cam, the car never dieseled. So it's gotta have something to do with the head or the cam. Can a hotter cam somehow cause dieseling itself???

[Lockjaw]

Have you checked the cam timing yet?

 

I have never had a dieseling issue, no matter what cam I have run, and I bet I have run 7 or 8 different one's, not counting the stock one's.

[Pop N Wood]

Usually dieseling is a result of excessive compression and low octane gas. On an older car it can be caused by carbon build up in the combustion chambers. The carbon both increases the compression ratio and heats up adding to the premature ignition. But since you have just had the head done I guess we can rule out carbon build up.

 

My guess is when the heads were done you somehow bumped up the compression ratio. Did you have the head milled/resurfaced? Did you use the correct thickness head gasket?

 

Maybe someone else on the board can tell you a way to determine your compression ratio and whether it might be excessive. If it is too high then your solution might be something as simple as a thicker head gasket or seating the valves further into the head.

 

Solutions? Try race gas. Obviously not a long term solution for a street car but might be interesting to see whether a tank or two will make the problem go away.

 

Water injection?

 

Fuel cut off. Killing the spark will not help a dieseling problem since the fuel is self igniting. My Ford Fiesta had a carb with a solenoid attached to the idle circuit. Turning off the ignition cut off the idle circuit in the carb so no fuel would flow. Absolutely sure fire way to stop dieseling. Fuel injection will do the same thing.

 

The curious thing to me anyway is that you are not experiencing detonation. If it is an excessive compression problem like I am speculating then it would seem like you would get some detonation under heavy acceleration.

[Guest bastaad525]

Actually the head didn't need to be shaved or milled at all this time, as the head had already been rebuilt 20,000 miles ago, and was straight when I sent it in this time. So the compression should not have changed. I used a thicker-than-stock felpro gasket this time, I think 2mm? My compression should be between 10.3 and 10.5:1. I know a few other guys that run this compression on 91 octane w/o problems. I do want to try a fuel or idle cutoff but I'm not sure how to do this with SU's.

 

Yes Lockjaw after the last time I posted I did double check the cam timing and it is perfectly lined up for the #2 position, V groove is spot on.

[yo2001]

I think your cam is too small. Post the spec and may be we can go from there. You also should try retarding the cam timing. you might be building too much cylinder pressure. Mine is couple degree retard also.

[Guest bastaad525]

It's a 480 lift 270 duration. Lockjaw asked me for some of the other specs but I dont' know what they are. I'm thinking of putting the cam back on the #1 mark and seeing what happens... what do you think?

[yo2001]

that might be good. make sure that you don't have too much slack in the timing chain. before that, check your compression. That should tell you some problem.

[Lockjaw]

If you have an electric fuel pump, you could wire it manually and shut it off. If the car is an auto, shutting it off in drive would solve your problem too.

 

I am a little befudled by the whole thing. I just don't recall ever having a problem like that.

 

Maybe you just got a bad tank of gas?????? I suppose you could dump a bottle of octane boost in there and see if that helps.

[Pop N Wood]

Killing the fuel pump won't solve your problems because the idle circuit will still draw fuel from the carb bowls. Also I wasn't proposing a fuel cut off solenoid as a potential solution. Maybe it is possible but my guess it would be an impossible mod to the carbs. Just throwing out ideas and past experiences.

 

Now if you wanted to go to fuel injection....

[Guest bastaad525]

Hey Tim. Actually no pulling the choke doesn't 'help' the problem, what I was asking about is actually using the choke to stall the engine. If I open the choke fully the engine will usually stall in about 3 seconds. This is without turning the ignition off. I did do this a few times since last I was at your house but sometimes it makes the car really hard to get started, even if I hold the throttle full open. So I can only guess this method floods the engine out pretty bad. Letting the car idle before shutting down usually seems to make the dieseling worse, if anything.

[Guest bastaad525]

See that's what I was thinking also... even if you cut off the fuel there is still some in the float bowls to get sucked in, enough to diesel still I'm sure. I was thinking more like a solenoid that could fully shut both throttle butterflies, but would not interfere in any way with regular idle adjustment and syncing. It's definately not just a bad tank of gas, as this has been going on for like 2 months now, and I've tried gas from different places. I also tried a few different brands of octane boost but nothing helped.

[Tim240z]

Brian,

I seem to remember you saying that the runon was solved if you pulled the choke prior to shutdown?

If that is the case, I still think that the problem is leaness.

Does it help if you let the engine idle for a while before shutting down?

If you're planning on coming on Sunday, we can take another look.

Tim

[Guest Anonymous]

ok you know you base timing

what you total timing

not on paper

get a timming light that has the knob for setting total timing

sears sells one

is it under or about 45 deg?

next thing in my mind is the rubber in your balancer sliped

that will drive you NUTS

pull #1 plug spin the motor to what it says is tbc stick a 3/8 drive extion in the hole

turn the motor a littel the tool should move down with the pistion and up it may jam going up so keep your eyes open

 

good luck

[rags]

Did this just start after the valve job? From what I've read so far the problem has to be with something you have changed since the valve job. Did you happen to change heat ratings on the plugs? Try colder ones. Are you sure the head gasket did not hang into the combustion chamber? Something is glowing in there, plug electrode, part of the gasket, something. I don't think the cam would cause this problem unless the overlap was really small, like zero!

Joe

[Guest bastaad525]

Yep Rags that's the problem... this DID start only after getting the valve job. My mechanic did end up losing my NGK's, which were one step colder than stock, and replaced them with stock heat range Denso's. This was one of the first things I tried to fix the problem, was to replace the denso's with the one step colder NGK's. When this didn't help I then tried NGK's that were two steps colder, and still no luck. I'm thinking you may be right about the head gasket maybe overhanging a little into the cyl. chamber. Unfortunately I didn't put the head on myself, so I'll never know for certain until I pop it off again some day, but that answer certainly seems to make the most sense.

[Pop N Wood]

You need to measure your actual compression ratio and see if it is in bounds. Maybe you ended up with a much higher ratio than you thought. I am not experienced enough to tell you how to do it or exactly how to interpret what you see. But it sounds to me like you have tried everything else. If you bought the car used then there is no telling what you have in terms of former head work and pistons.