The Stovebolt Site Forums The Shop Area Engine Shop Got my Fentons on now need Carb tuning advice

I have added Fenton exhaust with intake manifold heat pipes, I have kept the standard intake manifold and single carb (Zenith).

The Fentons are definately doing something , as now I have the horrible bog off idle. It was running great with the original exhaust manifold.

Is there something tuneable in the Zenith to cure this, or do I need dual carbs also.

Did you adjust the carb's air/fuel mixture?

If not, do you have a vacuum gauge?
Set the idle speed and then adjust the mixture to maximum vacuum.
Then, if necessary, reset the idle speed, and then the mixture.

Did you set the timing (to maximum vacuum at idle)?

That's the price you pay for eye candy. The dual manifolds can't provide enough heat to the intake to keep the mixture atomized, even with the "heat" tubes. Unless you're planning to run wide open throttle and high RPM most of the time, the dual exhausts are a big mistake. You could jet the carb way over-rich and hope you keep enough fuel atomized to cure the bog, but gas mileage will suck and you'll carbon-foul spark plugs regularly. Dual carbs will make the problem worse, not better.
Jerry

The heat tubes and plate work fine on the three engines that I have used Fenton dual headers on. My carbs (duals or single) seem to be well matched to the engines and my gas mileage is better, unless I step down on the accelerator.

Before you give up, Dusty, how did you adjust your carb and timing?

What size engine do you have?
What size engine did the Zenith come from?

My truck runs just fine with the heat tubes installed,too. If you need more heat,Patrick's,(in the "Bolt" "Links" section), sells a kit for water heat,that's plumbed to the water pump. Their kit instructs how to plumb the intake.You may have over-looked a vacuum line,or maybe didn't get it tight enough?? Sounds more like a vacuum leak than a carburetor problem,if it ran good before the "Fenton's" were installed. I agree with Jerry 99% of the time,but I LIKE the sound of the "Fenton's",PLUS I gained better gas mileage,(1 Rochester B),and better performance! Hope this helps.

Are you sure you're thinking of Patrick's? I'm pretty sure that Patrick's kit is for exhaust heat, and that Langdon's is the one that has the water heat kit.
For the record, I'm using exhaust heat for mine. Made my own plate and tube kit using a stainless plate and 3/8" brake line. Plus, the headers and tubes are wrapped with Kevlar header wrap to keep the heat in. I don't have any issue with a bog on acceleration.

Thanks for your thoughts guys.

tc , I adjusted the idle mixture and the timing a tiny bit on my 216 and have 20" vac the engine starts perfect and the hot tubes get REALLY hot , maybe I could try lagging the tubes. I have no idea what the Zenith is off. I just threw it on the engine (4 yrs ago) 'cause the original W-1 was weeping bad .

A pic of it is here http://www.jalopyjournal.com/forum/threads/dustys-truck-stuff.908428/page-3

Jerry , I do so hope you are wrong about this issue.

OK- - - -I see part of the problem- - - - -the tubes between the manifolds and the heater plate are far too small, unable to flow enough volume of exhaust gas to keep the intake plenum hot. Try to increase the tubing size to 1/2" (12 MM maybe?) so enough gas can flow under the carb to keep the mixture atomized. If the Zenith carb was performing OK with the stock exhaust there's no reason to believe it can't function as well with duals. Unless you can find some VERY small carbs for a dual setup, the bog will get much worse. Most people seriously over-carburete a twin carb engine.
Jerry

Dusty,

Your tubes look like the 1/2" malleable metal tubes on one of my engines. I use same sized 1/2" copper tubing on another engine.

Did you adjust your timing and carb mixture to maximum vacuum, using a vacuum gauge?

What size engine are you working with?

A Zenith carb from a 228 would be significantly different from a 270 or a 302 (and maybe even a carb from a 248 would be too much carb).

Thanks for your thoughts Jerry , TC.

The engine is 216 standard.

The heat tubes are 3/8" dia, the manifolds came threaded for the brass fittings I have used.

I will see if I can increase the size to 1/2"

Yes TC, I used a vac gauge to get max vacuum.

The only ID on the Zenith is 20 on the throttle housing.

I have always run the timing on the ball , but today I advanced it 2 deg on the octane selector (91 octane fuel) got another 1" on the vac gauge , it has improved a bit , but I think the heat makes more difference. I will try to find some 1/2" fittings and tube.

The Zenith should have a round tag roughly the size of a dime, which is riveted on to the top casting. On the tag, there should be at least one set of stamped numbers oriented in a circle. There are probably two sets, an inner circle and an outer circle.

If two sets, the inner circle is the one that identified the carb.

Jon.

Thanks for your time Jon, I have spent some time carefully cleaning that tag, there are definitely a series of numbers around the outer and some inner.

It is very hard to distinguish what they are , but maybe 778 .

Dusty - 778 is not all of the number. Try cleaning the tag with a pencil eraser. Then hold a strong light to shine on the tag about 2 degrees above horizontal from the edge. It throws shadows in the stamped number.

If 9778, then it would be from a GMC 270, and probably is going to be difficult to tune well on a 216.

But not to worry. There are lots of dudes that frequent this forum that would swap you a GMC 228 carb for the 270 carb.

Jon.

Finding a carb to swap might be a lot simpler than getting it to New Zealand. However- - - -I have a friend there who sends me things courtesy of an airline pilot friend who mails the items to me once he lands in the USA. It might be possible to arrange for a similar situation for a carb headed down under.
Jerry

Thanks for your help Jon , the eraser was good.

If I hold my mouth right and use my fertile imagination , the first number may very well be a 9.

The outer numbers show good as 2135998

So the chances of it being a 270 carb are good as I have never seen another sized GMC engine in this country.

I have still got the original W-1 and perhaps I should make the effort to stop it leaking (especially from the copper plugs) but the local carb shop is a challenge to deal with and my chemical sensitivity makes dealing with fuel risky. Such is life, eh.

Oh, if it's a 270 carb , that would explain the mpg change

Jon, what about the suitability of a 7492D Zenith on a 216 or a 235? There's one available on Ebay right now at a bargain price.
Jerry

Jerry - it would work fairly well (better than a Rochester ); but that is the older style Zenith, and parts are pretty much only available from that grouchy old hillbilly in Missouri. Originally a GMC 230.

Dusty - the 2135998 definately corresponds to the 9778, which would be for a GMC 270.

As far as the W-1 leaking at the copper plugs. If the drive plugs, put a drift punch on the plug, and hit it with a hammer. If the threaded plugs with a screw slot, remove the copper "gasket" from beneath the plug, apply 1/2 drop of BLUE Loctite to the thread. Screw in the plug, and allow to cure for 24 hours before placing in gasoline. DO NOT USE RED LOCTITE FOR THIS APPLICATION!!!

Jon.

Jon, thank you very much for your guidance , I will work on cleaning up the leaks on the original W-1 this week , it was always a good carb on this truck and hopefully will be again.

I'm not all that skilled on these older engines, but on my 292 I installed a clifford water heated intake, headers, and a 600 cfm edelbrock and hei. No bog....pulls hard..almost burn the tires without dumping the clutch. Pulled a hill in 3rd from almost a dead stop towing a 2500 pound trailer and accelerated all the way up no issue. Course this is a much larger engine, but what I'm saying is that the 600 cfm edelbrock is used on 383 v8's all the time...as of now my 292 was supposed to be way "overcarbed" but she runs fine and cruises wonderfully. Bogging has something to do with the A/F mixture and means yer either getting way to much fuel or not enough fuel. I work on small engines and same the same principal applies. Check your plugs...if black yer mixture is to rich and will cause bog...if to lean you'll have hesitation and that won't show on a plug. I saw a guy use headers and tripple single barrels on a 216...he's on youtube. And the thing runs fine. A 235 ain't to much bigger and people have used dual carbs on them so....You might consider checking ignition as well...if the timing is off or the ignition isn't advancing then that will cause bog as well. TJ

I have yet to delve into the W-1 leakage issue , mostly just because I am scared of the toxic reaction I know I will have , so as an interum test I fiddled with the manifold heat some more;

OK , so this is interesting ..........

I changed the 3/8" steel line for 1/2" copper . Thought the 1/2" would supply a larger quantity of hot gas .
But no! ....... The copper seems to be such a great conductor that the air from the engine fan is cooling it enough for the hot box effect to be less, yep , truck runs a little worse now.

Gonna find something to wrap the copper with next

Then if that doesn't work , water heat , then bite the bullet and go for the W-1 fix .

Updates as they happen

Update , I procrastinated on fixing the fuel weepy carb and went and changed the metal heat lines for water heating . 1/2" radiator hose from the water pump, thru the intake hot box to the temp sender fitting.
In a way it is an improvement , once the engine is up to temp, but , of course , it takes WAYYY too long to happen and so is generally unsatisfactory.
So , I decided to endure the toxic aftermath and change the carb.
The 20min job took 4 hrs !!
First carb would not idle , dismantled it , cleaned (air blast) reassembled , still no idle.
2nd carb, idled perfect , no throttle responce . dismantled , cleaned , replaced the accel pump leather, reassembled , still no throttle responce.
3rd carb , ugly , pored fuel down the throat , like the float was stuck down.
Getting pretty done in by now ,
4th carb , stripped it first , cleaned , again just blew high pressure air , not optimum I know , but soaking in thinners etc just is not possible for me. The petrol its self is more than I can cope with.I resized the idle tube end , sealed the copper washers , bumped the copper plugs per Jons instructions (thanks Jon) the originally leaky carb now seems to hold fuel mostly where its supposed to . Idles perfect , instant off idle throttle response . Very happy , take it for a drive .

NO POWER ! up to about 1200 rpm all is good , then it is just flat , pulling the choke a bit helps, but I am over it now. gotta repair myself . Sleep this [censored] off.

Dusty, you need to adjust the metering rod. It sounds like you are running way lean. The rod coming off the throttle shaft runs up the back side of the carb and works the accelerator pump and metering rod. There is a tool for setting proper depth, but in your case not much help since the engine is not stock. To set mine, I used a dial caliper and measured the hight of the metering rod as it sets at idle. I measured from the small shaft the rod slips over to the base of the top, the dial caliper was used like a depth gauge. I then raised the metering rod, bending the long rod at the base where it makes the 90 degree bend, .040" and went for a drive. I kept raising the metering rod till the flat spot went away, it took three test drives till I was happy with it. Fuel milage will suffer if you keep raising it, so stop when it feels right, you can always go back the other way. If by chance you can't get there, you may need to open up the jet or change to a richer metering, rod.
http://quadrajetparts.com/carter-ca...r-parts-metering-rods-c-299_312_395.html

http://old-carburetors.com/Chevy/pages/001-Chevy.htm, this link has the original Carter files, go to the home button at the top left to find your carburetor if its listed. Under the parts section it will list the OEM part number of the metering rod and jets. I use these numbers to search e-bay for what I want.

Joe

Thanks for your suggestion Joe, I did that and got some improvement , but it seemed like I was only achieving a modest improvement after 0.110" adjustment , because I know nothing , I thought that was a lot out of the original adjustment for just an exhaust change.
So I found several other metering rods and measured them all (all were a little different) I eventually chose one and polished it down a few thou in diameter, in doing so , I tapered the step at each dia change , lovely response, smooth , modest power. Gonna leave it like that for a few days , then the camper is going back on and I can test its pulling power under full load again.

Regards