Thirsty engine Lousy carb

[mustang_sam]

Hello everyone. I have a 1972 coupe with a 351W engine in it. It originally had the straight six w/ a manual transmission but upon purchase, the previous owner ( a very good friend of mine) also gave me a 351W and a C4 tranny (both in very good condition) for free!!! So I swapped the straight six with the 351.

The problem I am experiencing with the new set up is that when you rev up the engine, at about 3500 rpm and higher it coughs horribly out of the carb (not a backfire). Same applies @ 3500 rpm and more when driving. The distributor, wires, and spark plugs have been checked and are fine. From what I have gathered talking to the car electrician and tune-up guy here in my home town, they both believe that the carb is too small (low cfm carb) and fails to deliver enough fuel and air mixture to the cylinders at higher rpms. Btw the intake is for a two barrel carb (since it has only two inlet holes on top of it). The carb on the 351 is a Holley (pix are attached) but I can't find the model on it. For now due to my restricted budget I can't change the intake and go four barrel. I was wondering if first, there is any hope doing something to the current carb to solve the problem and if not, what would be a good two barrel carb for this engine to make it at least a decent daily driver???

Thanx to all!!

 

[gpierce]

Sounds like fuel delivery to me, one way or another. How is the fuel pump, filter, etc?

 

[Tubo]

Sounds like fuel delivery to me, one way or another. How is the fuel pump, filter, etc?

+1

I seriously doubt it's an insufficent CFM issue. The 2bbl CFM is more than the primaries on a comprable 4bbl carb. (Thats why a 4bbl can deliver better fuel economy than a 2bbl)

That Holley 2bbl has been sitting on a shelf for how long?? Previous fuel in bowls has turned to varnish & gunk. Carb probably needs to be rebuilt or throughly cleaned, along with gpierce's check FP & fuel filter advise.

 

[astikhossw]

what type of distributor do you have vacuum advance or mech.it could be backing the timing off so check for sticking and put a light on it to see if it is backing off before messing with the carb.

 

[hyena429]

+2 to above...I serously dout it is a CFM issue either...I would check the fuel filter and fuel pump too...And the carb just mite be full of gunk "or not" But easy to check the fuel filter and pump before you have to rebuild the carb....Rebuild kit for that carb would be pretty cheap...But you may wanna save for a 4 barrel if that carb is shot....Be better off in the long run...Keep you're eyes peeled for a cheap 4 barrel manifold for a 302 or 351 or 289...They come up cheap used on craigs list alot.

 

[Don65Stang]

The carb model is stamped on the front right air horn. You can see it in the first pic you posted.

I agree that a carb rebuild is a good first step.

 

[Tubo]

+2 to above...I serously dout it is a CFM issue either...I would check the fuel filter and fuel pump too...And the carb just mite be full of gunk "or not" But easy to check the fuel filter and pump before you have to rebuild the carb....Rebuild kit for that carb would be pretty cheap...But you may wanna save for a 4 barrel if that carb is shot....Be better off in the long run...Keep you're eyes peeled for a cheap 4 barrel manifold for a 302 or 351 or 289...They come up cheap used on craigs list alot.

While I agree entirely with the sentiment of Heyena Man's post, IIRC the 289-302 intake manifolds, are not compatible with the wider head placement of a 351. Gotta have a 351 intake.

 

[72HCODE]

well here is my take.

insufficient accelerator pump shot.

it sounds like you are right on edge and a slight adjustment might help

you have a bunch of options and that will allow you to keep the same 2 barrel holley

Option 1: buy a holly accelerator pump cam kit.

from the picture i can see you have a 30 cc standard pump shot with a brown cam.

different cams give different pump shot profiles.

http://www.summitracing.com/parts/hly-20-12?seid=srese1&gclid=CPHXkvWEgrgCFVKf4AodSzAAiw

https://www.holley.com/20-12.asp

they also give different fuel amounts depending on throttle plate angle.

So you may find a fast solution just swaping the cams out until you find one the engine likes.

assuming that failed.

option 2:

go back to the brown cam.

now inspect the shooter you have on the 2 barrel. it might be a .24 or a .26 size.. go to a .28 or .32. or .34 size that will deliver more pump shot at all throttle angles regardless of cam.

basically what you want is keep upping the shooter size until the engine hesitates and some black smoke comes out the back, then you back off .02 shooter size, so if you came from a .24 to a .30 and black soot flew out the tailpipe and the engine hesistated, then go to a .28 and you are about perfect.

you tried both and neither worked.

option 3:

well you may need a .50 cc accelerator pump kit.

https://www.holley.com/20-11.asp

fits almost all holleys ups the 30cc accelerator pump shot to a 50cc shot. I had to do this on my car and additionally play with the shooter size.

These 3 options give tremendous tuning options for a 2 barrel and will save you money. you could swap out a 300cfm 2 barrel for a 550cfm 2 barrel and have the same problems. basically for the most part the larger CFM gives more air and not much more fuel and drops vacuum signal. many higher cfm holleys have the same jetting and power valve and shooter sizes as the smaller cfm carbs.

is is possible for a carb to be too small or too large for an engine, but i think in this case you are just on edge for a tune to match the carb fuel delivery to what the engine wants.

-------

other things to think about.

option 4:

Timing: too much timing too early can cause a lean backfire through the carb. you could try backing off initial timing 2 degrees and see if things improve.

if somebody modified the mechanical advance in the distributor that can cause major problems. a lot of guys get a mr gasket mechanical distributor tuning kit and put 1 or 2 very soft springs on the mechanical the timing comes on too fast for the fuel provided and you get a carb backfire. you could hear the engine pinging but not know what it sounds like until you hear the engine when it is fuel rich sounding.

you could inspect the machanical advance in the distributor and if somebody messed around with it go back to stock with heavier springs to hold back the timing as rpm rises and keep the engine from going lean holding it back as the fuel is delivered.

option 5:

exhaust: how big the exhaust is can have a massive effect on the engine running lean mid throttle or accelerating. if you have no exhaust right now or open headers the engine will run super lean without enough back pressure. if somebody put a REALLY big tube exhaust on the car that could cause major issues.

so you have a bunch of things you could try. this is why "car guys" have like 5 carburetors sitting on a shelf because you kept swapping them out hoping to get one that worked out of the box at 300-400$ a pop.

another option you could up the JET size inside the carb, that could help above 3500 rpms as well.

 

[hyena429]

While I agree entirely with the sentiment of Heyena Man's post, IIRC the 289-302 intake manifolds, are not compatible with the wider head placement of a 351. Gotta have a 351 intake.

Ohh ya.....They have a taller deck dont they..Kinda like 351m's vs 351c i guess..lol

 

[MotoArts]

Does is stumble when driving, or just in neutral/park?

I'd need to know if it regurgitates when creeping up on 3500, or just when stabbing the throttle from idle in park.

There's a big difference.

With the problem (supposedly) basically beginning at 3500R's, I'd tend to believe it's not an accel pump issue. The pump shot works to get it off of the idle circuit and overcome the big "air dump" when the butterflies are tipped.

Jets, yes, maybe... but doubtful.

Keep in mind a possible timing chain or valvetrain/timing issue... bent pushrods, flat lifters/cam lobes, worn rockers...

Need more specifics...

 

[caspianwendell]

+2 to above...I serously dout it is a CFM issue either...I would check the fuel filter and fuel pump too...And the carb just mite be full of gunk "or not" But easy to check the fuel filter and pump before you have to rebuild the carb....Rebuild kit for that carb would be pretty cheap...But you may wanna save for a 4 barrel if that carb is shot....Be better off in the long run...Keep you're eyes peeled for a cheap 4 barrel manifold for a 302 or 351 or 289...They come up cheap used on craigs list alot.

While I agree entirely with the sentiment of Heyena Man's post, IIRC the 289-302 intake manifolds, are not compatible with the wider head placement of a 351. Gotta have a 351 intake.

+1 they will not fit

 

[mustang_sam]

First of all, sorry AGAIN for the long post but please bear with me. Thank you everyone for their valuable input. Regarding the carb, it was brand new and never used when installed and we did up the jets and it did not solve the problem. Distributor has vacuum advance which is connected to a vacuum inlet on the intake not the carb. That is because no one could find any vacuum inlet on the carb itself and the same problem occurred before and after connecting it!!! What comes to mind though is the mechanical fuel pump. Although it appeared to be brand new when initially installed, the rocker arm would not connect/sit on the camshaft lobe (I apologize for my poor technical terminology) since it was not pumping any fuel. As a result to achieve a temporary solution, the mechanic changed the rocker arm angle by cutting and rewelding it at a different angle. I suspect the new angle although making the pump work seemingly ok, is not correct. So I believe the pump should be my starting point toward fixing this problem considering this summer the pump also overheated multiple times and died on me when the weather was hot and I was stuck in traffic (42 + Celsius). I do have a new electric pump that is for V8 range rovers. Does anyone think installing that is a good idea???? The reason I am asking this question is that for now I am temporarily living in Iran (which I failed to mention and that’s where the car is) and finding parts here is almost impossible. The only two options are either calling the only Ford store in Tehran (I don’t live there) which usually don’t have it and if they do (new or used), they charge whatever price they like, or buying it online in the United States and begging a friend or family member to bring it over when travelling back to Iran which also limits the part size. So you can imagine how much running around and work has been done to get the car to this state. Just to post this message on the board, I had to go out and buy 6 vpn and proxifier programs (only one worked) in order to get through the government’s internet filtering system. I am sure in the near future they will block this one too!!! Other than two older mechanics and two tune-up guys here in my hometown (Shiraz) no one will get near American cars!!!! Everyone else here are part changers and not mechanics!!! Upon returning to the United States, I will get a huge box and fill it up with all the parts I want, take it to Oakland port, ship it to Dubai, and swim it to Iran but for now…..

Thanx to all and sorry for very long post and sob story!!!!!

Sincerely

Sam

 

[gpierce]

Hi Sam. Shiraz is a great all around red wine and is a staple in Australia and I find it really cool there is a town called Shiraz. Do you guys know how to party or what??

From what you describe it really sounds like a fuel pump problem. I have no idea what kind of drugs that mechanic was on, but I wouldn't trust someone that did that kind of workaround unless they told you: "This is to get you to the nearest town before you die of thirst, fix it properly when you have time/money".

I'm sure there are some Range Rovers with Carburetors, but if it is from one with fuel injection do not use it, far too much pressure for a carb. I personally don't like electric pumps without a regulator on it, and if I were to install an electric pump, it would have one inline. If that pump is from a carbureted LR, then it might work OK but you need to find out it's max pressure and match that up to what Holleys can take.

Your profile says you are in Northern California, but it sounds like you are not which might explain your mechanic's advice Where are you???

Greg

 

[mustang7173]

mustang_sam,

I noticed that the Holley has two large, possible 3/8 size ports that are open, on each side. Also, It looks like your PVC hose is not connected either. I suspect that one of the those open ports should be the PCV connection. If you can determine which one is the PVC connection, connect it, and then you could cap off the other one. This might or might not help. I have seen in the pass where leaving the PCV disconnected can cause some issues. Please let the forum know what the model number. Look on the front of the air horn. With the model, the forum can provide you with more information.

Thank You

mustang7173

 

[pegasus]

You could try Summit Racing they ship world wide,

Something like this, as long as the postage isn't too dear,

http://www.summitracing.com/parts/crt-m6962/overview/make/ford

 

[Tubo]

Sam,

New 2bbl eh??

So, I got to thinking....Hmmm, a chancy operation at times.

Anywho...For a "outside the box" answer. In the late-60's I had a V-8 that would Cuhff-Chuff-Chuff (like a backfire butt not a backfire) when pushed past 65mph. As in passing. Turned out to be a burned valve.

May get some other methods, however. IIRC trouble shooting this would be a compression bled down test. If nothing else a cheap test.

 

[Totalled]

That's a weird holley 2 barrel. Almost looks like the outboard carb for a tripower setup.

I'd get a new 350 cfm holley, and the correct 351W fuel pump like pegasus posted. Buy a spare, since sourcing them is difficult for you.

http://www.summitracing.com/parts/hly-0-7448/overview/

 

[BigPinoyHunk]

if it is the carb and when budget allows it, getting a edelbrock performer rpm or equiv and a 600cfm carb would spice it up some and run better.

 

[pegasus]

That's a weird holley 2 barrel. Almost looks like the outboard carb for a tripower setup.

I'd get a new 350 cfm holley, and the correct 351W fuel pump like pegasus posted. Buy a spare, since sourcing them is difficult for you.

http://www.summitracing.com/parts/hly-0-7448/overview/

Hmmmm, now you mention It, Why would a 2brl Holley have a secondary Vacuum Diaphragm, unless it was part of something else? I have 2 - 500 holleys and they look nothing like that one, Mate if you PM you your address I'll send you one, they both proberly need a kit through them as they have been sitting for at least 8-10 years, one came of a 302 Cleveland the other of a Holden 308, both worked fine when removed...