The Stovebolt Site Forums The Shop Area Engine Shop Lousy Acceleration, Surging, Popping - HELP!

Several of you have been helping me with this car for some time and I appreciate it so much - but here's where I am now...

We're talking about a '54 Coupe 235 6 volt - Rochester BC carb. just professionally rebuilt. Have replaced plug wires, points, condenser, rotor, cap. Have adjusted the dwell, timing and idle. Idle mixture set at 1 3/4.

Seems to run fine at idle and have good power, but as it accelerates into the upper RPM's it begins to surge and pop and almost cut out.

I've checked for vacuum leaks to wiper motor, the vac. advance seems to be working fine (how would one know if there is a leak there?)

I've tried various timing settings with no improvement or worsening of the problem.

In earler stages of this work Automark helped me with a flooding issue. I read somewhere that flooding can ruin the plugs - any thoughts?

I would check the fuel pump volume and pressure.At least 5 psi.Pull the fuel pump outlet line off and see if it will fill a coffee can before the engine uses up the fuel in the carb and dies out.Let me know what you find.

Also check/replace any and all fuel filters.

How about the fuel delivery system. Check for clogged fuel lines or crimped hoes. Fuel pump & filters. If all that is good how about the coil. The points may not be installed right. Flooding will carbon up the plugs. Pull 1 or 2 out & see if they are black with soot. They can be cleaned up. The vacuum advance would not cause this problem but you can see if it,s working or not by watching it as you accelerate the engine . you should see it turn the dist.

revted47,

You corrected part of your previous problem by adjusting something on the carburetor so that the truck didn't flood out after being run for awhile...what did you do? If you sit in the driveway with the truck running in neutral and very slowly increase the throttle, does all heck break loose consistently and at the same relative RPM.

If the float was set to high and this was the previous problem..which was corrected by dropping the float level, you could be starving for fuel as you come off idle. Do you have the phenolic spacer under your carburetor? There is a small notch in the carburetor base gasket that lines up with small hole in the carburetor base. This allows correct operation of the power valve. The gasket must not cover that hole.

Did you time the engine to the ball on the flywheel? This must be done with the vacuum advance line pulled and carburetor port plugged.

Just for grins, if you can bring the RPM's up quite high, slam the palm of your hand over the carburetor throat, mash the throttle to full bore and don't remove your hand until the truck dies. Let it sit for a few minutes and start it and see if the magically fixed the problem.

How old was the gentleman that rebuilt your carburetor? Don't give up!


Stuart

Stuart,

I corrected the fuel mixture problem by resetting the idle mixture to 1 1/2 turns out. It begins to run rough at about 1100-1200 RPM and really lousy at or above 1600. Tried the hand over the carb throat trick - all I got was a hand full of gas and now it REALLY backfires at high RPM!

I did time the engine to the ball on the flywheel with vac. disconnected and carb. plugged - set it to 8 deg. advanced as was recommended in a previous post. (Have an advanced Snap-On timing light able to dial in degrees of advance.)

For those who asked: It's pumping a can full of gas and then some - good flow. I have new coil and believe the points and timing are set correctly.

I checked the plugs - #3 is carboned, the rest have some carbon around the edge but the center electrode and prong are clean.

Oh, BTW - the Carb. guy is at least 70 years old! And you'll be glad to know that he marked the gasket for the carb "UP" just so bozos like me would not get it mixed up (course that assumes I put the plastic spacer in correctly too!).

revted47,

Wonderful work...great explanations. If the truck has a functioning choke in place, I think I would do the 'up to speed' drill in the driveway. As it gets to the 1100/1200 threshold where it starts to run rough, hold the RPM's there and see if pulling the choke slightly improves the situation. If you don't have a choke, holding a cupped hand over the carburetor to restrict the flow of air can do it too.

When it backfires, is this through the carburetor? With the engine at an idle, you can use a spray can of carb cleaner with the little extension on the spray tip and squirt around likely vacuum leak locations. These would be carburetor base, manifold to head junction...anywhere the system might be pulling in unwanted air.

It sure sounds like it is starving for fuel when if comes into the midrange RPM's. This could be a plugged jet or a float that was set to high.

As this rig is a car, I am assuming it has hydraulic lifters..maybe this isn't correct. Have you verified the firing sequence 100%...153624. The carb to base gasket can be installed two ways and still read "up"..one is correct and one ain't! I sure hope you are enjoying this workout...success is just a heartbeat away.

Stuart

Thanks for the encouragement. I'm on my way to try the recommended prodedures. It does not have hydraulic lifters and I adjusted the valves back when things were running well - maybe that #3 hole needs to be readjusted? It is backfiring out the tailpipe and I caught a glimpse of a puff of black smoke on the last one. Was a real bang. I'll bet some of the neighbors wonder what the heck is going on.

Backfiring out the tailpipe sounds like retarded timing to me but you've checked that at idle. How about this - put your timing light on with vacuum advance still connected, rev the engine up and see if your timing is advancing to keep up with the RPM. That BB should move away from the pointer (actually will appear to be moving counter-clockwise if your centrifical advance is working correctly. It should move clear down into the sight hole until you can't see it anymore.

Stuart - and other stovebolters...I've made adjustments to the choke and yes, that did clear up the backfiring. Firing order confirmed. I reset the choke by adjusting the automatic choke disk to run richer - hope that was what was intended. It did seem to hold the choke in a slightly more close position. However, I checked the timing again and it was off after resetting the choke, so I reset and that reintroduced some small popping at higher RPM. I checked the timing/vac. advance by watching the BB drift counter clockwise as the RPM is increased. Again, set the timing to 8 degrees advanced.

Ok, here's the problem that still remains - the car coughs and sputters on accelleration or when putting a load on it. Runs at a constant speed, say 40MPH in 2nd (High RPM) but quite rough.

It feels like I'm getting closer, maybe almost there, but it ain't right yet! Your help is appreciated.

revted47,

I forgot this was a car with automatic choke. I would set it to factory recommended specs. It's interesting that this choke change altered the timing so much. When you had the distributor cap off did you grab the rotor and turn it to make sure the centrifugal advance mechanism was free and springing back.

Normally I would say that a backfire out the tailpipe was a over-fuel condition, but your plugs aren't black and sooty. Can you disregard the flywheel marks and time the engine so it runs well and doesn't exhibit the problems. Don't use a light, just tweek and check by ear and feel. Did you get a chance to look at the carburetor base gasket and its orientation?

Stuart

Disconnect the vacuum advance line & try it that way. Some engines will run rough when you have to much spark advance. Popping through the exhaust can be caused by a bad plug. Take a real hard look at them. Sometimes you can crack one putting them in. Especially Champions. I have been putting plugs in for 55 years & I cracked 2 Champion plugs on a V-8. They broke open & there was nothing imside.

Took Sunday off, but am working on the '54 car again - Here's my report of what I've done per recommendations thus far - and the remaining problem. Plugs check out good except for too much carbon on #3 (could that be valve timing?) Centrifugal advance mechanism is working correctly as is the vac. advance. I did try "tweeking" it without light - by ear and feel - got it to run real smooth, pretty much even at higher RPM - though not perfect, close though. I think I've pretty much eliminated the backfiring. Engine has good acceration and power. Exhaust is smooth, clean and cool.

Here's the remaining problem - and it's pretty bad. The moment I begin to drive i.e. put a load on it, it surges terribly as RPM's increase - like a kid learning to drive a stick shift. Since it only happens when I'm driving I'm guessing it the surge starts around 1200-1500 RPM (about 15mph in 1st, 25-30 in 2nd). As noted above the gas flow is excellent, the carb has been rebuilt professionally as part of this job.

revted47,

There is a deal in the carburetor called a power valve. It is held closed by engine vacuum that it senses through the little hole in the base and base gasket. When you goose the throttle and vacuum drops, a small spring under the power valve pushes it up and gives the engine a little extra fuel. This valve is a precision sliding fit and the springs is small and delicate. If this isn't free to move or sticks open when it shouldn't, I think it will give you symptoms like you are experiencing.

Would it be possible to talk to your carb guy and share these symptoms with him. Between a incorrect float setting and/or a funky power valve, it sure sounds like it's carburetor related.

Have you double checked for vacuum leaks..maybe with the spray can of carb cleaner or a unlighted (?) propane torch. The line to the vacuum wipers is good, right. Maybe plug it for now.

I'm grasping at straws...lets blame the old carb guy, maybe we can outrun him.

Stuart

Stuart,

Thanks so much for your prompt response. I did check for vac. leaks, even pluged the wiper line at the manifold throat, tightened bolts, sprayed carb cleaner all around the intake manifold and base of carb. Tried to detect any change of engine rpm. There was none so I take it that there is no leak. I understand how the power valve works - I've replaced several in my years with my truck - so I think I'll have a talk with the carb man as you suggested. I'll let you know.

with all you say you have done look at the carb sound,s like lean fuel,,,fuel pump,,,gas filter,,,gas tank vent,,plugged fuel pickup line in tank,,,hole or leak in fuel line from tank to pump(sucking air),,,rubber line sucking together on tank side of pump,,,float level,,jetting,,power system....doc

I'd like to take you in a diffent direction...I had similar performance problems...these Rochester's (I hate them) tend to load-up when there is too much fuel pressure. I put an adjustable regulator on my fuel line dialed it down to 4.5 lbs and performance immediately improved.

Stuart - and all who have been so helpful - I should have listened to Stuart earlier when he kept asking about the carb rebuild. Took it back to the shop where it was rebuilt, guy wasn't in so I left it there - and on the way home it occured to me - why don't you swap out the carb. from your truck, it's a Rochester B and the Car is a BC, but hey, I can rig the choke and make it work. Did the swap, drove the car this morning and it runs beautifully - great accelleration, lots of power, no missing.

Obviously I'll have a chat with Joe the carb man today, but I think the problem, at least from my end, is solved. With a good carb. I should be in business.

Again, thanks so much to all of you guys, especially Stuart (Automarc) who has hung in there with me for a period of several weeks on this thing.

the carbs are pretty basic on them so maybe you should look at who you have doing the work...doc

Revted....Since you got your problem solved..I am going to hijack this thread..I can get a rebuilt Carb for my truck from Checker..for about 80$..has anyone dealt with there rebuilts..Performance?? M/G