Carburator Conundrum

[Jollystang]

Im posting this in the hope I can once and for all get to the bottom of a running issue I have had since my car arrived in Blighty...............................

I havent posted this in a specific Gen section as its more engine Displacement v CFM thats the confusion here!

My 302 would of originally been a lowly 140hp yes just 140 from a V8 but hey it was the 70's and primitive emmisions control systems restrictive manifolds etc.

Move forward 30 years in the cars history and the original engine threw a rod then the car got pushed into a farmers barn for a further 10 years.

When it was rescued by the guy I bought it from he just ordered a 302 crate engine not specifically for a Mustang II one that was listed as covering a wide range of years and suggestions are this is around 200hp in stock form

So we have the baseline the original carb was a 2 barrel off the original engine of no more that 300cfm with a restrictive intake to match.

This Crate engine was supplied with all new internals but you added your own intake and carb etc.

So the car arrives here topped off with a Summit 8600vs 4 barrel of 600cfm flavour on top of a matching Summit dual plane intake,so far so good.

The Summit 8600vs is basically a Holley Double Pumper copy ;)

And here is the offending article!



Only thing is it has never ran right on startup it throws coaldust out the rear suggesting its not burning all the fuel its being supplied?

Last area meet I attended I very nearly never got there,it was a pig to start and instantly flooded I then had to remove all the plugs and clean them and let the excess fuel evapourate before I got it running again.

I just about made it to the meet on the motorway it was ok though in town I prayed for green lights everywhere as it was dying at slow speeds.

At present its cunsuming roughly 7mpg!!! constantly fouling my plugs and stinks of jungle juice :(

So I come to the decision to change the carb I think its too large? All the online CFM calculators suggest I need smaller?

They suggest around 370cfm nearest is a Holley 390cfm.

I ask advice in the Mustang II groups only to find that virtually everybody with a 302 is using a 600cfm and recommend 600cfm?

Those using 600cfm are not experiencing my appauling MPG they are getting around 15mpg,no fouled plugs,no soot,no stench of jungle juice....................................

So I come here today for a rolecall,who here has a 302 thats fairly stock with an aftermarket 4 barrel?

What brand is the carb and what CFM is it and whats your mpg and do you suffer from any of the above?

Fouled plugs,soot,excessive fuel smell etc?

At this point I have no idea which way to jump?

Its been suggested a 390cfm wont cut the mustard and everybody is suggesting 600cfm but I am having soo much trouble from the one I have!

Answers on a postcard please...................................

Jo

[richard]

It's either flooding (float level incorrect, too much fuel pressure) or the jets are way too large.

600 is a little large for a stocker but not so it should be a huge problem.

These things usually come in the box reasonably jetted, so I'd start with the float level, if it's a Holley clone then you can do it running at your own risk of course-fuel will invariably come into contact with a hot engine with dodgy 40 year old electrics, frowned on now but that's how we did it.

[Jollystang]

It's either flooding (float level incorrect, too much fuel pressure) or the jets are way too large.

600 is a little large for a stocker but not so it should be a huge problem.

These things usually come in the box reasonably jetted, so I'd start with the float level, if it's a Holley clone then you can do it running at your own risk of course-fuel will invariably come into contact with a hot engine with dodgy 40 year old electrics, frowned on now but that's how we did it.

Thanks Richard

Spoke to my mate who is a mechanic this morning he has said when he took it apart for me to try and solve it the floats were set perfect he is looking into it for me to see what would be best for a carb size? Id rather go with Holley as he knows them well its all he has ever used but it appears theres a 390 then 570 and not much inbetween :(

What size do you think is best for a stock motor?

Thanks

Jo

[mad4v8]

Apparently the summit cab IS a Holley which is no longer in production and just has the Summit name on it. It takes Holley jets. Your problem seems like the jets are too big, fitting another carb could put you into exactly the same position. It must be worth changing jets in the existing one.
There's a Summit video on how to determine the main jet; https://www.youtube.com/watch?v=0MOVRTqrbag

[pmustang]

500 is probably perfect. 600 fine

I've switched to Holleys as have had issues with Edelbrocks

[cobraboy]

If the carb was sold as suitable for a 302 then the jets should be OK. I don't have Holley jet data, but I just looked up the jetting for 1.12 Autolite 4100 which is the larger of the two carbs used on a 289 and the Primaries are 48 and the secondaries 56. This gives you a guide - see what you have.

The other thing is that damned auto choke !

There are 3 screws around the black cover on the side, loosen them and look at the end of the cap, there will be an arrow -> pointing to LEAN. Turn the cap until the choke flap opens all the way, this will keep it open.

Start the car by 3 or 4 pumps on the throttle and then hold open a crack, hold on the throttle until it will idle.

If you are fouling plugs and getting 7mpg something is very wrong, likely the choke staying on or wrong jetting.

[ALLAN R]

You have got two problems here for a street engine. Firstly the carb is clearly not working correctly if its flooding the engine. This could be a whole raft of things and to flood to the extent that the engine is stopping running is unlikely to be just an over jetting problem. I suspect an internal fuel leak.

The second problem for a street car is that you would do a lot better with a vacuum secondary carb rather than a double pumper. Quite why you would put a double pumper on top of a dual plane manifold defeats me. I would get this carb. https://www.holley.com/products/fuel.../parts/0-81570

Its 570cfm with a manual choke. Vacuum secondary carbs sort of size themselves for the engine as the secondary choke opening is controlled by the flow through the primary chokes. Its dead simple and will work really well on your 302 engine. A manual choke may seem old hat but you have total control. All you need to fit is a simple coke cable. I fitted one of these to my 289 and it runs like a Swiss watch and gives pretty good mpg 17-23mpg urban/motorway.

[sportsroof]

If it's a double pumper, why am I seeing what appears to be a vacuum can?
Martin

[Jollystang]

Thanks Guys

Taken on board everything you have said and my mate who does my spannering has got back to me and said 600cfm but that said he did say 500 originally but holley seem to no longer do a 500 :(

Think your right Stuart my mate is changing the jets for me just to see if theres any improvement? As you said I feared I may end up in the same position because the jets are similar size so with that in mind and bearing in mind as Cobraboy pointed out the Autolite jets were in the latter 40's to later 50's range and just at the point you posted Allan I just happened to come across the 570 as you posted it :)

At 54 and 65 the jets are much nearer the old Autolight's sizes unlike mine which are 67 Primary 73 secondary!

My Summits are getting jetted down nearer that of the 570 even if it works as the dragons say "Im Out!" im in the long run going to get a Holley probably the Street Avenger 570 now ;)

[Jollystang]

If it's a double pumper, why am I seeing what appears to be a vacuum can?
Martin

Its not a Double Pumper that kind of copied it and made a hybrid or as Stuart said it appears it was a failed design by Holey that Summit took on ...................................

So it looks like a double pumper but its like a hybrid clone ;)

[Mach1 Dude]

Its not a Double Pumper that kind of copied it and made a hybrid or as Stuart said it appears it was a failed design by Holey that Summit took on ...................................

So it looks like a double pumper but its like a hybrid clone ;)

What is a hybrid clone? I too can see a vacuum canister, which in book makes it a four barrel Vacuum seconderies carb, if it were a double pumper it would not have a vacuum cansiter, it can't work both ways.

[MeZapU]

Those are very good carburetors and are copies of the Autolite 4100's but with Holley jets, needle and seats and power valves making them very easy to service. It is indeed a vacuum secondary carb and not a double pumper. I had one of thee 600VS Summit carbs on my F100's old 352ci FE for a while and managed about 18mpg's with it on a run with my 5th gear transmission. They are very economical. Even found it to originally be a bit too lean and actually jetted it up and still achieved those mpg numbers.

It sounds to me like you have a leaking or stuck open power valve. How many turns out are the idle mixture screws?

And like others have said, make sure the choke is opening all the way.

If yours is truly a 600cfm unit, this is it
https://www.summitracing.com/int/par...00vs/overview/

[mike67fb]

Clue is in the part number 8600VS....VS as in vacuum secondary.

https://www.summitracing.com/parts/sum-m08600vs

[phil k]

I'd personally start with setting up the auto choke. As a side note with all due respects to your friend he will never in a million years get your carb set up correctly, at idle yes but through the rev range he's got no chance unless he has access to a rolling road. Why not sae your money on a new carb and spend it on having the thing set up correctly. A few years ago I built an engine for a fella and he had the car (Holley) set up on a rolling road, it took them 2 days of swapping jets etc to get the carb set up perfectly across the rev range and I can categorically say it was the best money he ever spent. I drove the car and it was probably the best carb equipped Mustang I have ever driven.

Phil

[rsk289]

If you're set on a different carb, it's hard to beat a properly-rebuilt 4100. It's a fantastic carb, very easy to work on, and was designed specifically for these engines. There's always one or two rebuilt ones on ebay.
As to your problems, first thing I'd do is get rid of the autochoke. You do not need a choke on a cast-iron V8 in this country. Neither of my cars has a choke - cold start method is to turn it a few times to get fuel in the bowls (both have mechanical fuel pumps and sit for long periods). Then, 5 pumps on the throttle, turn the key and just keep a little throttle on for maybe 20-30 seconds, depending on engine. The block starts to warm so quickly it'll probably run OK from there. Nothing to affect mixture then - and if it's a bit harder to start for the uninitiated, there's an additional security element, I suppose. If memory serves (don't count on it), those autochokes require a 12V feed to keep them OFF, which seems bonkers to me...

Oh, and I completely second what Phil has suggested. A rolling road is the only way to set up a carburetted engine. My Mustang has a stroked 351W with an Edel 750 which was setup by John Sleath up near Doncaster - 2hr drive there, a day on the rollers, 2hrs home, and worth every single penny.