400 Big Block running issues

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GJUK

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Hi from over in the UK.

I have a 68 dart with a 400 big block, running a high (ish) lift cam and a 750 holly.

The car seems to be miss firing a bit every so often, you can start the car and it will be missing. Another time you can start the car and it runs really well.

It's had new spark plugs and plug leads.

The car came from Kansas in late 2013 and I've not changes any of the tuning on the carb/dizzy at all, perhaps it needs changing to run on our 98 Ron fuel?

I took it to a local drag stip here (Santa Pod) and it ran well in 1st, 2nd but in top gear towards 4k rpm it just started missing again and I had a huge shake from the engine and had to back off.

Does it sounds like a timing issue to you guys? Or maybe a poor spark?

Thanks,

Jon
 
Hey Jon!

Cover the basics on the ignition system first since you suspect an issues with it. Start at the plugs and work your way back on the system. While you have the plugs out for an inspection, look at the plugs for a rich condition, black soot, or a lean condition, white plug.
Double check wires for cracks. Make sure there not cross firing. Be sure the cap and rotor contacts are clean. Adjust timing. Use as much initial advance as you can get. Until the starter kicks or has a hard time turning when you start it. Then Back it off. Limit total with vacuum line attached to 52*'s.

Then As above, check the carbs float levels. Tune the carb if need be. The miss at a higher rpm leads me to believe your running lean up top in the rpm band. Inspect the fuel filter.

Now some questions for you;

Do you have/know the cam specs or what cam it is?
What size fuel line is on the car?
Does it remain that size to the pick up in the gas tank?

To me, the on and off miss firing could be in the carb. If nothing is found on the ignition.
The carb may need to be opened up and cleaned.
 
Choke sticking and/or fuel supply issues? floats or needle valves sticking? rust or crud in the float bowls?

Loose connection at a plug wire end?

Still have points distributor or electronic ignition? Points could be dirty or out of adjustment.
 
like someone else said FUEL SYSTEM VOLUME PROBLEM , good luck.
 
Are your fuel filters clogged?

Where is your fuel filter located, before of after the fuel pump?

I prefer to mount the fuel filter after the pump, as the fuel is gravity fed to the pump, and you don't want the restriction on that side of the pump, after it is better.


Could you have a clogged pick up tube filter in the gas tank? Sometimes they get old and fall apart and clog the pick up tube.


Check your fuel supply and then start looking at the carb next.
 
Even though you put new plug wires on I would check them again.
Just because they are new doesn't mean they aren't bad.
replacement parts now have a 20% failure rate.
These stats are from ATRA.
 
Try borrowing a different ECU first. (Different brand)
Most likely these are giving issues at higher rpms.
 
If it's run fine until recently, and being how it's intermittent, I'd suspect a ignition problem. As "rumble" said, work your way thru that first. Make sure you have a good ground on the ign. box and it could even be a coil that's no longer performing up to the task.
 
GJUK said:
Hi from over in the UK.

I have a 68 dart with a 400 big block, running a high (ish) lift cam and a 750 holly.

The car seems to be miss firing a bit every so often, you can start the car and it will be missing. Another time you can start the car and it runs really well.

It's had new spark plugs and plug leads.

The car came from Kansas in late 2013 and I've not changes any of the tuning on the carb/dizzy at all, perhaps it needs changing to run on our 98 Ron fuel?

I took it to a local drag stip here (Santa Pod) and it ran well in 1st, 2nd but in top gear towards 4k rpm it just started missing again and I had a huge shake from the engine and had to back off.

Does it sounds like a timing issue to you guys? Or maybe a poor spark?

Thanks,

Jon
Click to expand...


Hey, we need you to report back with what you have tried and how it worked so we can help you more.

And if you do find the problem, let us know. This is how we learn and can help others with the same problem later.
 
Hi,

Thanks for the replies, sorry i am not ignoring you all!

I'm out of hte UK at the moment so can't really try anything on the dart. That said I have already done the following:

new spark plugs 100 miles ago
New HT leads.
New King lead from dizzy to coil
New battery cable from battery to starter
New battery cable from battery to shell (negative)
New Earth strap for the engine

Checked cleaned and pushed hom the bulkhead connections


... Now I've ordered a carb rebuild kit for my 750 double pumper, perhaps something is stuck or there is some crud in there.

Thank you all, will report back in about a week!
 
krazykuda said:
Are your fuel filters clogged?

Where is your fuel filter located, before of after the fuel pump?

I prefer to mount the fuel filter after the pump, as the fuel is gravity fed to the pump, and you don't want the restriction on that side of the pump, after it is better.


Could you have a clogged pick up tube filter in the gas tank? Sometimes they get old and fall apart and clog the pick up tube.


Check your fuel supply and then start looking at the carb next.
Click to expand...

Hi

New fuel filter is fitted, after the pump (which is an engine mounted mechanical type). Fuel filter is new and seems to be flowing well. Though I can not tell the pressure its coming in at.

Fuel lines are 8mm at the moment, though I've been told to go to 10.

Also I did notice the fuel boiling on two drives out, I had a glass (I know I've changed this now) fuel filter where you could see the fuel boiling. It does seem to run better when its cold (once it gets going).

Starts on the button everytime though - which is why I think the problem is either carb related or timing related.

Thanks

Jon
 
OldmanRick said:
If it's run fine until recently, and being how it's intermittent, I'd suspect a ignition problem. As "rumble" said, work your way thru that first. Make sure you have a good ground on the ign. box and it could even be a coil that's no longer performing up to the task.
Click to expand...

Hi. Thanks

Ignition box is well mounted to the shell of the car.
King lead from the coil has a really good spark.
Dizzy has been cleaned and all contacts brushed up. Rotor arm is clean also and set in position well.
 
rumblefish360 said:
Hey Jon!


Do you have/know the cam specs or what cam it is?
Click to expand...

The cam is a 'high lift' hydraulic cam.

What size fuel line is on the car?
Click to expand...

Stock steel lines from the tank then to an 8mm fuel hose to the pump, 8mm hose to a filter and 8mm to the carb.

Does it remain that size to the pick up in the gas tank?
Click to expand...

It's all about the same size from what I can see, yes.

To me, the on and off miss firing could be in the carb. If nothing is found on the ignition.
The carb may need to be opened up and cleaned.
Click to expand...

Yeah, I'm going to rebuild this as my next port of call!

Thank you.
 
Holleys do best with a rebuild every couple years. If it were me I'd plan for that, and then pull each plug and read it. If any are fouled or look bad, replace them all after the carb is rebuilt. I'd also do a simple compression test whiel the plugs are all out just to make sure there's nothing wrong with the lower end. It's probably just simple wear. These cars required constant maintenance when they were new - nothing has changed that much except we don't use them as much.
 
Big update!

The carb has now been fully rebuilt by someone that knows them very well. So I know this has been done correctly.

(Long update here, still have a problem)

So, spent the whole weekend on the car.

Saturday

Got the carb running okay with a friend, kind of got the timing there abouts though when you put the car in to D or under any load its just stopped.

Stripped the dizzy, seems it has very light springs in it - the kit I bought from Turnip on here will not fit the dizzy (my one has a screw adjuster in it already for limiting the timing total advance). The lightest spring in the kit was much heavier than the springs on the dizzy (so I assume the timing kicks in very quickly).

Sunday.

Mick kindly popped over.

Got the car running even better.
Idles at 1050 rpm, seems happy there, does not like going any lower.
15 16 inches of vac on idle
in drive its 800rpm and holds well

Test driver 1 and 2:

Took the car out, idles well, pulls much better than ever before. 1-2 flat out to 5k rpm really great. Did another full run into third. Pulls really well then off the throttle the car chokes up, doesnt like it and dies.

You can't start the car at all, unless you have the throttle wide open?! Then it runs like a bag of ****, nurse it home. On the drive its idling and running fine again ?!?!?! It only seems to run like a pig after giving it beans.

changed parts

known working coil changed
rotor arm changed
New HT leads
New king lead from coil to dizzy
New carb gaskets fitted
Carb fully rebuilt by someone that knows what they are doing


checks
Plugs all cleaned and checked firing all okay. All 8 checked all 8 HT leads checked
plugs are now a nice brown
Fuel pressure is 6.7 psi
no fuel leaks
no air leaks
no Vac in use
Vac pipes blocked off

carb checks:
fuel bowls correct, primary and secondary
jet size okay (stock for 700 dp)
blade openings okay


Further ideas I've been given

ballast check for cracks (top of the engine bay)?
drilling the throttle blades?
change the ignition module?
fuel boiling?

Timing was an issue and mostly done by ear as TDC was impossible to find on the markings on the damper, I am going to have to find TDC and check the timing. That said the car runs a DREAM now, apart from when you come off the gas after booting it through the gears and just decides to go in to special mode and stops.

So, any more ideas guys?
help.gif


From what I call tell the timing is spot on. The carb is spot on. The car is happy, something else is causing this problem is my gut feeling... I've had bulk head connector issues before now... Not sure.

Another forum I am on have suggested fuel pressure being an issue. I disconnected the fuel pipe and put a gauge in there, cranked the car over on the starter and it showed 7psi. I've just bought an in line guage to have on while it's running also to give a more real reading

J
 
Is the new gauge one that can be seen in the passenger area or is it under the hood? This sounds like a fuel SUPPLY issue. If the gauge is inside, have someone watch it when you run through the gears at WOT to repeat the problem. Have you looked at the mesh sock on the fuel pickup in the tank? Is the tank vented?
 
cletus said:
Is the new gauge one that can be seen in the passenger area or is it under the hood? This sounds like a fuel SUPPLY issue. If the gauge is inside, have someone watch it when you run through the gears at WOT to repeat the problem. Have you looked at the mesh sock on the fuel pickup in the tank? Is the tank vented?
Click to expand...


It's an under the hood type, I think that the psi might be getting a little high during high rpms...


Tank venting = unknown.

Not checked the sock pickup in the tank, could that be blocked?

Cheers

Jon
 
GJUK said:
It's an under the hood type, I think that the psi might be getting a little high during high rpms...


Tank venting = unknown.

Not checked the sock pickup in the tank, could that be blocked?

Cheers

Jon
Click to expand...

If you are able to repeat the problem with full throttle runs I would be willing to bet one of two scenarios. I'm not really familiar with the Holley carbs, someone help me out here, I think it may be possible that at WOT you are dumping enough fuel on the plugs that they are getting fouled, making it run like **** and tough to start back up, after you get it started, it could be clearing them after some running.

I bet it's either that or it's gobbling all the fuel out of the float bowls and starving at the top of 3rd gear, then the pump has to catch up.

Once you KNOW your carb and timing are correct, if it still does this same thing, I bet the pressure is dropping to the point where the float bowls are out of fuel at the top of 3rd gear. The first things I would look at would be to make sure the floats are set as high as you can get away with on that carb, pull the sending unit out of the fuel tank to check the mesh sock to be sure it's not gunked up. If it looks good you will want to get a new gasket for it anyway, the gasket likely came out in pieces. Make sure the fuel line hasn't been crushed somewhere on the bottom of the car. The tank should be vented to be sure when the fuel level is dropping it doesn't try to stop the flow of fuel with vacuum - picture dunking a glass in a full sink of water, turning it upside down, and pulling the bottom of the glass above the level of water in the sink, the water stays in the glass until the top of the glass clears the level of water in the sink. I know the tank is different from '68 to '73, but they may have had a similar setup, mine had a 2nd line from the tank that went up to the engine bay (this part I know for sure), was somehow hooked into a charcoal canister (I think, it's been years since I dealt with it) that I think was tied into the vacuum system on the engine. I removed all that and added a breather filter to where the 2nd line tied into the tank.

Those are the possible scenarios I would try to eliminate first, hopefully this helps.

Good luck! Please keep us posted on what you find.

- Steve
 
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