Technical 283 runs rich, seems to misfire, gets bad mileage

The 283 in my T Bucket seems to misfire at full throttle. It doesn't hesitate or bog, just sounds a little rough, and seems down on power a bit. It also seems to get under 10 mpg. On the good side, the car starts very easily and cruises well.

The car has a brand new Quick Fuel Slayer 450 cfm, which is the same as a Holley 4160. It doesn't run much different than it did with the 750 cfm 4160 it replaced. I'm quite sure the carb isn't the problem.

I've checked compression, vacuum, timing chain, plug wires, everything is good. The plugs are correct and gapped properly, but of course they are very sooty.

I ran the car with one of the plugs out and grounded against the engine so I could see the spark. It was orange, and seemed to be a decent size... I wasn't really sure what to look for, other than a really obviously weak spark.

The distributor has a Pertronix conversion kit in it. I think the coil is stock, it's just a plain black thing. Those are what I now suspect, mostly because I can't think of anything else. Is it unusual for the ignition system to work fine starting, idling and cruising, but poorly at full throttle? I'm going to check the resistance on the coil, but I don't know if it'll do any good because I'm not sure what its origin is or what the spec should be. Anything else anyone can think of? Thanks for any advice.

 

Try another coil-like to see a blue spark.

 

Are you running a resister or no resister?
Get a coil that matches what the Pertronix wants in Ohm's.
Pertronics don't get along well with some Spark plug wires.................................
Timing?

 

I'd also check that the timing is advancing like it should.
Pete

 

Pertronix - when I put ours in, it was suggested to run a Petronix coil, I think the 'stock' ones don't have enough juice, I think.

 

Put a good distributor it & try it ...

 

I would have said look at what has changed, hence, look at the carb first.

 

Cam lobes worn?

 

Is the engine fairly stock? What's the manifold vacuum at idle and at steady cruising speed? What's the initial and total ignition timing? Does it have a vacuum advance, and is it connected and working properly?

 

Totally stock. Vacuum at idle is 17. Initial is 4 btdc, which is the factory spec. Don't know what total is, but I did order some timing tape so I should know soon. It has a vacuum advance, which seems to be working ok, but can't be entirely sure.

 

To begin with, an orange spark is a weak spark. Not only do you "like to see a blue spark", it's the only spark you want to see. A good hot blue spark. They are getting weak before they get to actually being orange colored

Even if your carburetor is a brand new one out of the box (better said especially if) you can be about 100% sure that it needs to be tuned for your application. Very unlikely you will ever be able to just bolt any carburetor on any engine and then get top notch results. Unless the carb you would be installing was just recently running great, came off an engine nearly identical to the engine you have, in a car just like your car.

What you are describing sounds exactly like a combination of the two. Poor spark and a carburetor set up way too rich.

It also sounds like you have tried to use two carburetors, one after the other, without tuning either one of them. Which is why they both ran about the same. They didn't run the same because they were both good. Opposite.

 

When the tape shows up find true TDC using the piston stop method in relation to the harmonic TDC......................................................

 

I'd do that before the tape showed up. You need to know that your timing mark is exactly on with the actual TDC.
You should still be able to tell if the vacuum advance and mechanical advance are working right or close to right just by using the timing light and watching the mark move if you don't have a light with the built in advance check system.
I'd agree with getting the correct Pertronix coil, having all the pieces match and jive in the ignition should solve some issues.

 

If it is running rich, you might have to change jets in the carburetor. Sooty plugs mean too much fuel. go down a couple of sizes. The carb must have some type of power valve that might have to be replaced. Most carbs are set up for the most common engines which would likley be a 350 cubic inch.

If it was misfiring it would probably be popping back through the carb or backfiring.

Another thing to consider is that rodents like small tubes like exhaust systems and sometimes build a nest in them.

 

It doesn't seem like the carb and 450 CFM should be okay for a light car on the street. Any modifications to the SBC, cam, intake, heads, etc? It may want more CFM.

Do you have another distributor to swap in? Even a working points type would be good. I would also adjust the float height and adjust the idle mixture.

 

I had to go read the specs on the carb SL-450-VS - Slayer series Vac Sec, Electric Choke, Side Hung Single Inlet Fuel Bowls and see that it has an electric choke.
Is the choke opening up right? They will finally open if the electric choke is hooked or almost open up but it will open a lot slower than it would otherwise.
Reading a bit more shows that they are designed to run not more than 6.5 lbs fuel pressure with 5 to 5.5 being better . Have you checked the fuel pressure?
I'm going to ***ume that you didn't build the car and following behind someone else you often have to backtrack through the systems to get it all squared away. If the fuel pump is the stock 283 mechanical pump you should be fine but if it is an aftermarket mechanical or electric pump it's important to know the specs on it and what it is actually putting out pressure wise.
One thing at a time though, don't get caught up in making several changes at one time.
Timing tape is good as you will bring the engine up on TDC and make sure that the 0 on the tape is in the correct spot.
Coil to match the ignition is a good move just to help the longevity of the ignition plus better spark.
Compression test that you did should show that the cylinder pressures are even meaning that the internals are in reasonably good shape.
As I said earlier make sure that the choke is opening fully in a short time frame. Most of those are supposed to open in under 2 minutes with around 90 seconds being the aimed for mark with later carburetored engines.

 

Does it have more than one vacuum port? If so, be sure it is on full vacuum not ported. hat's what your problem sounds like.

 

No, it is not unusual at all. In fact it is a real good clue. That is exactly what should be expected given a weak spark and an overly rich fuel mixture. Get a good coil that matches up with the Pertronix you have and be sure it is installed properly. Get your timing set right, be certain your distributor is operating as it should, then start tuning your carburetor.

Get the information you need to do it right. Or take it to someone who will do it right. You won't be disappointed. The 450cfm on a stock 283 in a T-bucket should be a nice combination and perform real well. A blast to drive. T-buckets can easily be overdone, many of them are.

 

8 out of 10 carb problems are ignition related.

 

Leaner fuel mixtures are harder to ignite. Higher compression, higher loads, will expose weak ignition. If ignition is defective it might only run halfway decent when carb is running pig-rich.

I spent a lot of time chasing a blue spark. Bubba sez electronic ignition doesn't necessarily produce a blue spark. It's true.

But orange or red, is no good. Should be able to jump a 1/2" air gap to ground with a fat bluish-white spark from the coil wire. When the ignition gets squared away, then the engine/carburetor will respond best to tuning.

 

Well yeah, whitish, whitish-bluish, bluish-whitish, blue, whatever it's all good. But when it starts going yellowish on down to orange there are bound to be some problems already showing up. Or red, never seen a red spark myself. My experience is even at orange there is no way the engine is running well, if at all.

 

Thanks for all the response to this thread. Today I did some tests Pertronix recommended to check the coil. It p***ed all of them.

I also replaced the four hole squarebore carb to spreadbore manifold adapter with an open one. The car didn't run any differently. I figured it was a long shot, but the new adapter was only $25, and I'm running out of ideas.

That brings me back to the coil. I know I'm just throwing parts at it, but I figure I'll toss a new coil on there, despite all tests indicating that the one on there now is good. (It just occurred to me as I was typing this, I wonder if it's the cap or rotor. I didn't take a close look at them, but they look fairly new. I'll take a closer look at those tomorrow. It would be great if all I needed was a rotor.)

I don't think its the carb or fuel system. The carb might not be ideally set up, but I don't think it would be too far off, since it's intended for small V8s that are stock or mildly modded. Also, the car runs nearly identically with either the old or new carb, and it seems if they were both really screwed up they'd be screwed up in different ways. Not ruling out the carbs entirely, but I think spark is the thing to check for now.

 

...I had a problem some what like your having,...make a long story short I found the coil to go bad when it was hot and when the rpms were up it would start to break up/miss, but it would idle ok. when the coil was cold it tested good get it hot and it started to go bad.. I changed the coil and the motor ran great. I am not saying this is your problem but it is something to look at that does not cost money. miller

 

Remember, if the coil wire is defective all 8 are going to show some deficiency also. There only as good as the main coil wire.
Humor me plug your vacuum advance can, go for a ride..............................

 

Using a square-bore carb on a spread-bore manifold or vice versa through an adapter is NEVER a good idea; but I don't think that is your major problem.

Electronic ignitions require the stable voltage of an alternator; you have not posted (or I missed it) if you have an alternator or generator.

Modern resistor type spark plug wires can really cut the firing voltage.

Agree with all of the previous posters about the orange spark. Find out WHY the spark is orange, fix it to white or blue, and your major problem with go away. Then you can start playing with the carburetor mis-match.

But remember, an engine with a "perfect" carburetor and low firing voltages will run extremely rich!

Keep plugging (no pun intended), you will find the problem.

Jon.

 

It doesn't matter one bit what you think about a problem. What fixes problems is what's actually done to get to the solution. So far you have checked the coil. If the recommended tests were done correctly then it appears your coil is OK. But what you think you should do is replace it.

YOU might be running out of ideas. What you might try now is implementing the ideas leading to a solution that you have been given from a number of members here. Many of us have been doing this a very long time. My own case 50+ years. The fundamentals of tuning an engine haven't changed much in that time. You (or a computer) do this and this and this, etc., and it will run real well. But you HAVE TO DO this and this and this, etc.

What one thinks or hopes or wishes for is not going to get the job done. When you know what all the facts are and what has been done about them it's very likely the problem will be solved. Provided what has been done is the correct thing, done correctly.

Finally, if you think this or that carburetor is OK by looking at the outside of it, you are not making life easier for yourself. Quite the opposite. You absolutely have to go inside and be sure of what is going on in there. Or take it to someone who can actually (proven) do it for you. Not someone who thinks they can. New carburetors are no more likely to be setup correctly than anything else. My experience the last 5 or 10 years is that they are very likely to be a ****ed up mess inside. Of course a used carburetor is also suspect. You can't ***ume anything.

But then maybe you think you are lucky. Maybe you are. I wouldn't make that bet.

 

I had a similar problem and it was the magnetic pick up ring on the Pertronix conversion. The magnets were loosening up under the plastic adhesive. Same motor, only with a 500 cfm edelbrock that I did have to tune for the motor. To shorten my reply, I put points back in and she purred like always.

Sent from my KFDOWI using The H.A.M.B. mobile app

 

Are you still trying to use the sooty plugs? If so they could easily be firing off to the side and all manner of weird stuff. Ive even seen them affect the color of the spark since you’re not seeing the whole spark at the electrode i.e. some of it’s traveling across the carbon in the soot back down to the body of the plug instead of jumping the gap.

 

I would suggest new plugs and a new Pertronix coil that MATCHES the series of module you have. There are 3 different coils and 3 different systems.

I know a lot of guys running the 450 Holley on 292, 300 and 302 engines and they were fairly close for them. Yes, they could use fine tuning but not nasty rich. You did set the floats didn't you?

SPark

 

I may have missed it but during your ownership has this car ever run well,the reason I asked if it did, did this happen all of a sudden, or did you change something? if you changed something go back to there. and I think the best advice is install a known good dist and see how it runs Tom p.s. all the advice you have been given is good now it is up to you to implement it