Engine surge at speed

Your tranny nose mount is lose or broken. It will bump when you shift the car from forward to reverse also

Please explain. By surging I ment that you can feel the motor gain and lose momentum while the rpm's  appear on the tack to stay the same.

what spark plugs are you running? Is your dwell set correctly? 

Not sure about the plugs, builders choice. Timing set at 30 degrees tdc. Surging just started, ran great for 4-5 thousand miles. Stationary it runs up great without a miss while holding it at a constant speed. Any speed. Under load driving it surges. Will check tranny mount but don't quite understand the relationship to my problem.

Sounds like the carbs might have an issue involving the transition  ports......  One carb is possibly leading the other by a very few degrees......   I'd try re-syncing the carbs as a starting point....  Contact Danny P. for possibly the best way to do this....   I'm assuming that you are just getting started in this hobby.... 

Carbs snyc out exactly the same using a german snail. I am relately new to getting deeper into problems but have been keeping valves and carbs adjusted since the late '60's.

Jim

I had something similar going on, not sure if it's exactly the same.

For me, i traced the surge to my accelerator. There was a glitch in the pedal/cable where it hung up and caused a sticking point when depressing the pedal. I couldn't feel it until I pressed the pedal with my hand. I disconnected the cable from the carburetor linkage and the glitch was gone.

In my case I ripped out the whole throttle assembly and linkage and installed a SyncLink system.

 If your transmission is moving up  at the nose, when you give it the gas  it will cause a petal surge.

I just had the very same problem  I also thought it was  the transition from idle to main jets. Instead I poked around with a mirror and thought I noticed something that was not right.  I have Dells and one of the screws that seals a channel that controls some kind of flow----I believe to the float chamber had come loose.  IT idled fine.  Revved up fine while stationary but surged under load.  Took my shortest flat sd and tightened and problem solved.  Engine builder left it loose.  No one is perfect. Running great now but you know Dells.  That may change tomorrow. 

BluzBrudda, good thinking. On Webers, make sure the main jet stack is fully snug as well as the idle jets and yes, even the stud and nut that holds the venturis in. If that is all good, take the tops off and ensure the accel jets are tight.

 

Trans mount is a good one to check. Did you tighten the accel cable? No play could cause that too. 

 

Also check ignition(loose Pertronix) and fuel pump volume/pressure.

What rpm does it surge at? All? Load: part or full?

The surge is suttle. You can feel it at 3000 rpm's on up in 2nd through  4th gear. From what I have read it might be a problem with a clogged primary jet as the idle circuit seems smooth. The problem is getting to the adjustments. It's like the antique boats that I restore. You can see it or you can touch it but you can't do both.

Main jets are on top and should be easy enough in a Speedster, just take off your air cleaners. Maybe one is loose, I've never seen a main jet clog, that would be a big chunk of crap! There are no adjustments above 3000, do you know what A/F ratio is when on the mains? May be one size too lean, which might cause surging. Really, check trans mounts first though.

Checked the tranny bolts and they are tight. Problem began after I changed oil, adjusted and double checked the valves to 0.006 and fine tuned the carbs with the linkage off with a snail. Runs and sounds great until I get to 3000 then it starts to buck slightly like it's starved for gas. New gas filter didn't help. Run 93 octane non- ethanol all the time.

 AHHHH! I suspect the stalk screen in the tank may be cruddy  and is collapsing  when the fuel suction is higher.. This is a pain because you have to drain the tank an   undo the nut at the bottom of the tank to replace the screen.. If it is full of mess?  You may want to pull and flush the tank. I've had this problem before ..

years ago on one of my beetles, i had a tiny rock stuck in the little tube on the gas tank.  similar symptoms.

 

PITA to fine.  i used a piece of wire to poke it out

 

any of your fuel lines squished?

None are collapsed. Fuel runs fine to the filter from the fuel pump. Filter stays relatedly full at all RPM's. I know it's on the wrong side of the pump but that's the way it came AND it has worked fine for 10,000 miles. I will move it after this get figured out.

That should read relatively.

^ I hate when 'auto-correct' messes with my shirt! 

Even though the fuel filter seems to stay "relatively full", you may have a partially clogged filter and that could be the cause of your fuel starvation, especially as you demand more fuel as you open up the throttle plates.  If it's partially clogged it will look full, but that's more because not enough can get out to satisfy demand and it backs up at the filter.

 

Simple (and cheap) fix, too......just swap it out with a new filter and see if it improves.

Gordon, I did the new filter last weekend. Didn't help. The old one did have some small flakes of crud but really not that bad fot 10k miles.

If you have some money to spend, take it to a chassis dyno and run it til it starts acting up.  They should be able toread the AF ratio and tell you if it is a fuel or ignition problem.  Trouble is most dyno operators are young guys who know all about computer cars but nothing about webers or dellortos.  

I have resolved to get professional help at the end of the week. I wanted to rule out as many possible causes that I could to save myself a few dollars. It's really not about the $, I really like the idea that I can do alot of the maintenance myself. It makes owning a classic so much more enjoyable. I will report back what the trouble was if it can be found. Thanks for all the suggestions and if you can think of more let me know. I enjoy trying something new after work every day.

Hey Jim , I do not know where you are located but if you are close to me ( North Carolina)  I would be happy to hook up my LM1 and help you figure it out , might cost you a Budweiser for labor though. Mike McCarthy