Spitting back thru carb

Update: Spitting back thru carb

I have a 67 CJ5 with a dauntless, Rochester carb freshly rebuilt, GM HEI ignition, fenderwell headers and new plugs and wires. The motor showed 30,000 miles on the speedo which was broken, but the inside of the pan had practically no sludge so that might be about right. The Dauntless starts great, doesn't smoke or even drip oil, has good torque, idles great--just a great motor.

However, it does spit back out of the carb in certain situations. When hot the motor will run strong for a couple of miles then start spitting which can only be stopped by taking load off the engine. This occurs when in overdrive around 2200 to 2500 rpm. I can shift out of overdrive and run it up to 3000 rpm and the spitting goes away. I run about 5* of advance to avoid pinging.

The headers are tight to the manifold and when coasting down hill with the throttle closed there is a very mild backfire on the passenser side, but nothing very bad.

What do you guys think could be causing this? I haven't replaced the fuel pump or pressure tested it, but it looks good from outside. I also haven't read the plugs yet.

Any help would be appreciated.

UPDATE 5-30-11

Well, I got some 1/8" hi-temp gasket material from McMaster Carr, and built a shield from 1/8" aluminum to solve my spitting through carb problem. Summit Racing has various carb heat insulators that run 1/2" thick-should I fab one from this thick material? I also changed the t-stat to a 180*. The problem seems better now, but still not capable of going 55mph on a 95* day after the Dauntless gets hot. I think the timing is a little too early so I'll check that next.

What Holley carb is a bolt-on replacement for the Rochester?

I have the heat-shield that goes near the fuel pump that holds the fuel filter but can't figure out how it attaches. I looked in the FSM but don't see anything there. Anybody have a picture or description thay can post?

Thanks again,

 

I would also check the vacuum advance..

 

Happened to me. My cause was heat transfer to the carb from the intake manifold causing boiling over of fuel in the carb, causing hard starts (flooding and vapor lock I assumed). I am using a heat gasket now, which seems to have corrected that part. I have talked to others with similar issues. They have used electric fuel pumps, heat shields for the carbs, and buying gas at some gas stations that have minimal to no etoh in the fuel. The problem seems to be the etoh content of the fuel causing it to boil more easily. While running, there may not be enough pressure in your fuel pump at lower Rpms when hot to overcome any developing vapor in the lines or carb (excuse the run on sentence).

 

Isn't 5* a little low for a HEI ignition? That's what I was using with standard ignition. Now with the HEI I'm running 10* with .045 plug gap and no problems.

 

I appreciate the help. I have been wondering about vapor lock too so I use the return line from the pump to the tank. I am using a 195* t-stat so I might need to go lower than that--it is 95* here already and will only get hotter. It is a manual choke, Ive set the carb using the vacuum gauge and I'll check that again. I really did try more advance on the timing but it pings. What psi should the pump be operating at during idle? When I shift out of overdrive to take the load off, the motor runs at 3000 rpm and the problem seems to go away.

I'll have more questions later I'm sure, and once again thanks. I don't know any mechanics around here that could solve anything without an OBD.

Where do you get the E3 plugs, or what do you ask for at the parts house? Where do you get the carb insulating flange or block--Summit Racing maybe?

 

I got mine through Summit Racing..

 

it sounds EXACTLY like what my OF sounded/acted like when i stuck an EF HEI distributor in it. it idled great until you wound it up to about 2500, then it popped and hissed, backfired and did generally ugly things.let off the gas and everything went away.
where did you get the HEI? are you sure its OF HEI and not an EF? was it running prior?

 

I got my distributor from a source listed on Merl's Garage website. I can find out which one later. I'm going to switch t-stats to either a 160 or 180. It gets hot here in Central Texas usually near 90 at dark--do you think the 160 would be hot enough? The Dauntless has a flex a lite fan and high flow water pump. I don't see a carb insulator gasket or spacer on Summit Racings' site yet.

 

Seems like a small hassle but it would weed out the distributor if you put the old distributor on(if it was running good with it) and seen what it did. Just a suggestion if you did not toss the thing like a lawn dart when you switched.

You could run a heat shield between the intake and the carb made from a piece of 7/16 aluminum (or thicker but 7/16 is enough to divert the heat away if you bend it up away from the carb) wedged between 2 gaskets. Its a really old circle track trick for preventing vapor lock.

Both these won't cost you much time/money(gaskets mainly) but may be able to tell you whats going on.

 

To update what's going on I changed the t-stat to a 180 tonight and measured the fuel pump pressure which was between 5 and 6 psi. I'll see if that makes a difference but would still like to put an isolator between the carb and manifold. Cerial-Did you really mean 7/16" for the thickness of the aluminum, and how do you bend something that thick with normal shop tools? Would something thinner work between 2 gaskets?

 

Well, the 180* thermostat didn't solve it but I'm glad I put it in anyway. As far as off the shelf carb insulators I can't find anything at Summit Racing so I'll try Speedway I guess. I may end up making my own insulator with some material from McMaster Carr.

 

Bump

 

Check the cam lift. Sound like a bad lobe on the exhaust side. I had the same problem and turn out to be a lobe on the cam. I think it because of no zinc in the new oils.

 

Some good advice thrown out here by others.. when I read your first post - I started thinking of common issues that could be the problem, vacuum leak, low float level(lean condition), etc.. then now I'm thinking, what about your rochester ? The throttle shaft can wear out and cause a vacuum leak. Just something to check if you still cant figure it out.

 

I built a carb shield out of 1/8" aluminum and used some 1/8" hi-temp gasket material on the manifold side. Is this too thin? I can get some thicker aluminum, 1/2" or so, but cant imagine being able to bend it around the edges with what I have available.

I can try putting the stock distributor back on--I only ran it about a minute to get it off the trailer before I started the refurb, but it, the distributor, appears ok. When I got the Jeep it had an Autolite carb on it with the Rochester in the glove-box. It too was only run a minute or so before rebuilding the Rochester. Maybe the PO had carb trouble or he wouldn't have changed it no?

What is a Holley direct bolt-on carb?