Poor performing 383

Guest · Post by **Guest** » Fri Sep 10, 04 7:53 am

OK where has my query gone? ? I put it on, on Thursday evening and it has vanished.
Anyway I?ve got a 68 Plymouth GTX that has had it?s 440 replaced with a standard performance ?72 383 somewhere along the way. The problem is when trying to accelerate any way quick, especially when cold, the engine will go like a dog, spitting and missing. The severity of the missing gets worse the heavier I plant the pedal ? If I suddenly plant my foot it will totally bog then after a second or so it will sputter and backfire continuously, seemingly running on about 4 cylinders until I lift off. The problem lessens as the engine warms but as I?m driving along at a steady speed of around 60 I can hear the engine fluttering (missing). There are a couple of potential problems ? The fuel pump is only putting up 4psi and drops to around 2psi after 5 or 6 seconds of constant acceleration as the fuel level drops and this is then obviously followed by the engine dying off (I am planning on replacing this over the winter). (The carb is a Carter 625cfm 4 barrel and everything else on the engine is stock ? I have the float level set so that the top of the float is 3/8? off the top inside face of the carb when the inlet seats are closed.)
The cold cranking pressure ranges from 125psi to 145psi across the cylinders when cold and from 140 to 160psi when hot. The pressure is slow to rise on the gauge ? requiring 10 ? 15 revs before the gauge levels off ? indicating worn rings. What is the cranking pressure supposed to be for this engine?
I pulled three plugs (60thou gap) the other night and to my surprise they were the ideal light tan colour, with no sign of oil or sootiness ? which is probably largely due to the MSD Digital 6 ignition, MSD HVC coil and MSD leads.
I have changed the metering rods richer, leaner, changed the metering rod springs from light to heavy ? all had little or no effect.
Can someone please tell me what I?m missing? ? I?m guessing at lack of compression being a big factor.

Anonymous · Post by **Anonymous** » Fri Sep 10, 04 8:01 am

I had something similar on me old Honda Dave, turned out to be cross firing through the leads. Check the wires while reving the engine with the lights out and watch for sparks or an eeirie glow

Dave-R · Post by **Dave-R** » Fri Sep 10, 04 8:44 am

Sounds like spark plug leads to me too. Most likely the coil lead. Make sure they are good, clean and seperated. Check the coil is clean on the face and the dizzy cap is clean inside and out. Check the points if it has any with a dwell meter.

Guest · Post by **Guest** » Fri Sep 10, 04 9:10 am

Sorry meant to say it has brand new MSD super stock 50 ohm leads which are seperated with MSD seperators. The possible short fall is the stock electronic distributor - I replaced one of the advance springs with an aftermarket one and left the original light spring in place. I have a brown cap covering the stock rotor but the rotor tip and the contacts inside the cap are being quickly burnt away with the energy of the MSD. I have also noted a grey hazieness inside the cap possibly from crossfiting - If I closed the gaps down from 60 thou to 40thou maybe that would reduce the crossfiring. I may buy an MSD pro distributor - If I buy one for a 440 and use an o-ringed spacer collar between it and the body would there be any issue (I intend on building a 440 and don't want to buy two distributors)

Dave-R · Post by **Dave-R** » Fri Sep 10, 04 10:23 am

No problem using a 440 dizzy and spacer. MP sell a spacer for that very purpose.

Not sure that a new dizzy will cure the problem unless there is something wrong with your dizzy like a bent shaft or worn bearings? Something happen when you swapped the advance springs?
Not sure that reducing the plug gap will help either unless the gap is greater than the rotor-to-cap gap.

I know several people using a stock electronic dizzy with MSD set-ups and they have no problems. I myself use a Jacobs system with a stock dizzy and have no probs.

Maybe it is fuel after all? You do know the fuel supply is faulty so I would fix the known problem first and see what you have left.

Anonymous · Post by **Anonymous** » Fri Sep 10, 04 12:02 pm

I have a spare spacer ring, the outer is not o-ringed but i've never seen it leak (it seals on the lip not the sides)

Guest · Post by **Guest** » Fri Sep 10, 04 12:45 pm

Thanks very much for the offer - I can make my own spacer though as I have access to a lathe. Does anyone think my problem might be compression related as I think it sounds very low even for a std performance motor.

Dave-R · Post by **Dave-R** » Fri Sep 10, 04 12:55 pm

I just realised something.

You can't have a 1972 383. They stopped making them in the 1971 model year! Doh!

Anyway compression ratios were well down from 1971 so I don't think your readings are too low.

Guest · Post by **Guest** » Fri Sep 10, 04 1:24 pm

I just thought that such a wide variation from cylinder to cylinder (approx 20psi) was a sure sign of an engine that had too many birthdays.
I had planned on buying new ali Edelbrock heads and dropping them on to the engine and then using them on my 493 when I get around to building it - however the way this engine is going I'm not sure it is worth the bother - as I'd probably lose so much down by the rings - I might just knock 40 thou off the face of these heads and see what it does to improve the situation in the mean time.
As far as a replacement fuel pump is concerned I think I'll go electric with a regulator, as I'd like to run nitrous at some stage, and from what I read into nitrous manufacturers they seem to prefer electric pumps

Dave-R · Post by **Dave-R** » Fri Sep 10, 04 4:11 pm

Yeah sure it is worn. But it will still run Ok and not give the sort of problems you have. Some of that variation may be due to poor valve seat sealing. It might just need a de-coke and valve lap.

If you use NOS you need an electric pump over a certain hp levels. If it was a stock engine with a small squirt you can get away with a good mechanical pump (over 100gph at a good pressure). If you have a lot of power without the NOS or want a big shot you need the electric pump for sure.

So what year engine is this anyway now we know it is not a '72? If it is a '72 you are the proud owner of a 400 b-block.

Guest · Post by **Guest** » Sun Sep 12, 04 7:36 am

I've got the engine numbers somewhere, and I found a post on this site somewhere that I used to decipher the numbers as being a '72 but I'll have to search the numbers out again, I know for sure that it is just a std performance engine.
I added Restore engine treatment to the engine 150 miles ago (you're supposed to run it for 500 miles before it reaches it's peak) - this stuff really works - I originally did a cold compression test months ago, and then a hot compression test some time after which obviously had higher readings. I did another cold compression test yesterday, and the compression is up on average 12.5psi over the original cold compression test - now averaging 155psi.
The car was last run from cold for about 3 miles - When I pulled the plugs yesterday they weren't as ideal as the first evening with some plugs looking slightly black and a couple had a slightly glazed look which suggests that the car is running too rich when cold.
I am going to pull the intake manifold off this week and check the intake ports for coking - I am also going to block the heat risers with some steel shim.
Dave on the topic of Watts linkages I haven't forgotten - I am in the process of getting some software to allow me to reduce the size of the photos.

Dave-R · Post by **Dave-R** » Mon Sep 13, 04 9:50 am

Just go to the tucows website and download Irfanview. It is a very small program and it is FREE!

Guest · Post by **Guest** » Sat Sep 18, 04 9:52 pm

The choke is still connected but it does open up normally after a few minutes of operation - but the problem still remains even after the choke is opened fully - although not as bad.
I took the intake off today - believe it or not someone had fitted the valley pan and intake manifold at one time without using a single gasket. I noticed that the intake faces and the runners downstream of the gasket face were coated with oil, however the runners on the intake were pretty dry. So it looks like i've got an intake leak on the inside of the engine!! The vacuum level is always around 16-17" at idle, however I've just realised that it is connected to a single runner (the same one the brake servo is connected to), however this runner may be sealing fine so I won't be getting a representative vacuum level for the whole engine. I know there is a port at the rear of the Carter below the butterflies - this was blocked off by the previous owner - with a bolt!! - I take it this is the best place to get a true reading for the engine.
Tell me if I'm wrong but this intake leak sounds like a major root of my problems.

Alex · Post by **Alex** » Sat Sep 18, 04 10:13 pm

The valley pan is the gasket

usually a very small amount of RTV smeared around the ports helps sealing but the gasket (pan) is a compression gasket and needs no paper or other type of gasket to seal it.

Clean everything up as clean as you can get it, put a very fine smear of RTV around the ports on the head and the manifold then bang it back together, follow the torque sequence so you pull the manifold down evenly and you should be ok.

Check the base gasket under the carb as that is a major source of air leaks as well.

I would set the plugs at 50 as well, 60 is a bit big for my liking and check the timing it sounds like it may be seeing a bit much when it is cold?